WO2025136780A1

WO2025136780A1 - Chemically-strengthened substrate and methods of chemically strengthening a substrate

Info

Publication number: WO2025136780A1
Application number: PCT/US2024/059730
Authority: WO
Inventors: Fang-Yu Hsu; Yuhui Jin; Aize LI; Qiao Li; Tao Liu; Vitor Marino Schneider; Yu Cheng
Original assignee: Corning Inc
Current assignee: Corning Inc
Priority date: 2023-12-21
Filing date: 2024-12-12
Publication date: 2025-06-26
Anticipated expiration: 2026-06-21

Abstract

Methods of chemically strengthening a substrate with a thickness from 10 micrometers to 100 micrometers include contacting the substrate with a molten salt solution maintained a from 350°C to 400°C for from 10 minutes to 90 minutes. In aspects, the molten salt solution includes at least two anions associated with at least two potassium salts. A concentration of a first potassium salt and a second potassium salt is at least 2 wt% of the molten salt solution. In aspects, methods include transferring the substrate to a cooling chamber with a temperature that decreases at a cooling rate from 4°C/min to 20°C/min. In aspects, methods include contacting the substrate with an acidic solution with a pH from 3.5 to 4.5 for from 10 seconds to 3.5 minutes. Chemically-strengthened substrates with a thickness from 10 micrometers to 100 micrometers has a maximum compressive stress from 650 MegaPascals to 1200 MegaPascals.

Description

CHEMICALLY-STRENGTHENED SUBSTRATE AND METHODS

OF CHEMICALLY STRENGTHENING A SUBSTRATE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C. § 119 of China Application No. 202410330020.9 filed on March 21, 2024, which in turn, claims the benefit of priority of China Application No. 202311773622.3 filed on December 21, 2023, the contents of each of which are relied upon and incorporated herein by reference in their entireties.

FIELD

[0002] The present disclosure relates generally to chemically-strengthened substrates and methods of chemically strengthening a substrate and, more particularly, to chemically-strengthened substrates comprising a thickness of 100 micrometers or less and methods of chemically strengthening a substrate comprising a thickness of less than 100 micrometers.

BACKGROUND

[0003] Glass-based substrates are commonly used, for example, in display devices, e.g., liquid crystal displays (LCDs), electrophoretic displays (EPD), organic light-emitting diode displays (OLEDs), plasma display panels (PDPs), or the like.

[0004] There is a desire to develop foldable versions of displays as well as foldable protective covers to mount on foldable displays. Foldable displays and covers should have good impact and puncture resistance. At the same time, foldable displays and covers should have small minimum bend radii (e.g., about 10 millimeters (mm) or less). Plastic displays and covers with small minimum bend radii tend to have poor impact resistance and/or puncture resistance. Furthermore, conventional wisdom suggests that ultra-thin glass-based sheets (e.g., about 75 micrometers (pm or microns) or less thick) with small minimum bend radii tend to have poor impact resistance and/or puncture resistance. Still further, thicker glass-based sheets (e.g., greater than 125 micrometers) with good impact resistance and/or puncture resistance tend to have relatively large minimum bend radii (e.g., about 30 millimeters or more). Consequently, there is a need to develop foldable apparatus that have low minimum bend radii, good impact resistance, and good puncture resistance.

SUMMARY

[0005] There are set forth herein chemically-strengthened substrates (e.g., foldable substrates) and methods of chemically strengthened substrates (e.g., to make the same). Providing a glass-based substrate and/or a ceramic-based substrate can provide good dimensional stability, reduced incidence of mechanical instabilities, and/or good impact and puncture resistance. Methods of the aspects of the disclosure can increase a pen drop height that the foldable apparatus and/or foldable substrate can withstand, increase a survival rate of substrates folding to a parallel plate distance of 5 mm, 3 mm, 2 mm, and/or 1 mm, and/or increase a foldability of the substrate.

[0006] In aspects, the substrate can be chemically strengthened with a molten salt solution comprising at two anions associated with at least a first potassium salt and a second potassium salt. Providing the first potassium salt with multiple (i.e., two or more) potassium atoms per anion can increase an effective concentration and/or activity of potassium in the molten salt solution, which can facilitate increased maximum compressive stress in the resulting chemically strengthened foldable substrate. Providing a first potassium salt in the molten salt solution with a pKa of about 9 or more and/or a pH from about 9 to 12 of the molten salt solution can improve the strength and/or foldability of the resulting chemically strengthened foldable substrate, for example, by selectively etching flaws inherent in the foldable substrate that might otherwise be magnified by the chemical strengthening treatment. As discussed with reference to the Examples here, potassium carbonate (K2CO3) has a more pronounced and unexpected increase in compressive stress than other components in molten salt solutions. Additionally, without wishing to be bound by theory, it is believed that the carbonate anion can facilitate precipitation of other cations (e.g., lithium, sodium) exchanged out of the foldable substrate, which can increase a longevity of the molten salt solution (e.g., by removing components from the solution phase that could otherwise “poison” the molten salt solution). As demonstrated by the Examples discussed herein, providing a first temperature of the molten salt solution less than 400°C can increase a maximum compressive stress developed for a predetermined depth of layer and/or depth of compression. Also, for some of the molten salt solutions discussed herein, a temperature of 350°C or more may be used to ensure that salts are molten. [0007] It has been observed that foldable substrates with a thickness of about 50 pm or less (e.g., from about 10 gm to about 50 gm or from about 10 gm to about 30 gm) are unexpectedly sensitive to what happens after the foldable substrate is removed from the molten salt solution. For these thin foldable substrates, even relatively small difference in compressive stress across the surface thereof can result in waviness and/or warp that can produce optical distortions that can be visible to a user of a consumer electronic product that the foldable substrate may be incorporated in. Consequently, the controlled temperature of the cooling chamber can facilitate a relatively even compressive stress across the surface of the foldable substrate. Also, providing an initial temperature of the cooling chamber of 180°C or more (e.g., 200°C or more or 220°C or more) can facilitate the removal of a residual portion of the molten salt solution before it solidifies. Without wishing to be bound by theory, the first potassium salt can have a higher melting temperature than the second potassium salt, which means that incorporating the first potassium salt in the molten salt solution can increase a viscosity of the molten salt solution and/or cause the molten salt solution to solidify at higher temperature than a molten salt solution without the first potassium salt. Consequently, allowing a residual portion of the molten salt solution on the foldable substrate after it is removed from the molten salt solution can be especially useful when the molten salt solution includes the first potassium salt. Reducing the temperature of the cooling chamber to a final temperature of about 100°C or less (e.g., from about 25°C to about 100°C or from about 60°C to about 90°C) can enable the foldable substrate to be subsequently treated (e.g., relatively quickly or immediately) thereafter using aqueous solutions (e.g., rinsing with water or an alkaline detergent solution, contact with an aqueous acidic solution). Providing a cooling rate from about 4°C/min to about 20°C/min can quickly reduce a temperature of the cooling chamber (and foldable substrate) while being able maintain a relatively consistent temperature throughout the cooling chamber (and/or foldable substrate), for example, to produce a relatively consistent compressive stress across the surface of the foldable substrate.

[0008] Providing an etching rate of about 1 pm/min or less (e.g., about 1.0 pm/min or less) can facilitate a substantially uniform removal of material from the surface(s) of the foldable substrate. As discussed above, foldable substrates with a thickness of about 50 pm or less (e.g., from about 10 pm to about 50 pm or from about 10 pm to about 30 pm) are quite sensitive to differences in compressive stress and thickness variation across its surface. Consequently, providing an etching rate of about 1 pm/min can remove a relatively uniform thickness and portion of the compressive stress from the surface(s) to reduce an incidence of waviness and/or warp that can produce optical distortions that can be visible to a user of a consumer electronic product that the foldable substrate may be incorporated in. Without wishing to be bound by theory, providing a relatively low temperature of acidic solution (e.g., from about 20°C to about 40°C or from about 20°C to about 25°C) can decrease the concentration of SiFe' anions since the reaction from FhSiFe and 2 H⁺ + SiFe' is endothermic. Decreasing a concentration of SiFe' anions can be associated with decreased deposition (e.g., redeposition) of silica or silica-like materials on the surface that could otherwise produce variation in the thickness and/or compressive stress across the surface of the foldable substrate. Providing a relatively high pH (e.g., from about 3.5 to about 4.5, from about 3.6 to about 4.3, or from about 3.7 to about 4.0) can decrease an etching rate that can help produce a relatively uniform compressive stress and thickness across the foldable substrate. Providing a combined concentration of HF and NH4F of about 4.0 wt% or less, about 3.5 wt% or less, about 3.0 wt% or less, about 2.5 wt% or less, or about 2.0 wt% (e.g., from about 1.25 wt% to about 4.0 wt%, from about 1.3 wt% to about 3.5 wt%, from about 1.35 wt% to about 3.0 wt%, from about 1.4 wt% to about 2.5 wt%, from about 1.5 wt% to about 2.0 wt%) can provide relatively controlled and even etching of the foldable substrate and/or reduce deposition of material (e.g., silica, silica-like material, ammonium fluoride crystals) on the foldable substrate that could impair the optical properties of the foldable substrate.

[0009] In aspects, the substrate thickness of the substrate can be about 50 pm or more (e.g., from about 50 pm to about 100 pm, from about 50 pm to about 90 pm, or any of the corresponding subranges therebetween discussed above) and associated with one or more of (1) a depth of compression as a percentage of the substrate thickness 209 from about 10% to about 30%, from about 16% to about 26%, or any of the corresponding subranges therebetween discussed above, (2) a depth of layer (e.g., first depth of layer and/or second depth of layer) of potassium in a range from about from about 3 pm to about 20 pm, from about 10 pm to about 15 pm, or any corresponding subrange discussed above, and/or (3) a maximum compressive stress (e.g., first maximum compressive stress and/or second maximum compressive stress) can be in a range from about 650 MPa to about 1,200 MPa, from about 800 MPa to about 1,100 MPa, from about 850 MPa to about 1,200 MPa, or any corresponding subrange discussed above. In aspects, the substrate thickness can be about 50 pm or less (e.g., from about 10 pm to about 50 pm, from about 10 pm to about 30 pm, or any of the corresponding subranges therebetween discussed above) and associated with one or more of: (1) a depth of compression as a percentage of the substrate thickness from about 10% to about 30%, from about 12% to about 19%, or any of the corresponding subranges therebetween discussed above, (2) a depth of layer of potassium in a range from about from about 3 pm to about 20 pm, from about 5 pm to about 9 pm, or any corresponding subrange discussed above, and/or (3) a maximum compressive stress (e.g., first maximum compressive stress and/or second maximum compressive stress) can be in a range from about 650 MPa to about 1,200 MPa, from about 750 MPa to about 1,100 MPa, from about 750 MPa to about 1,000 MPa, or any corresponding subrange discussed above.

[0010] Some example aspects of the disclosure are described below with the understanding that any of the features of the various aspects may be used alone or in combination with one another.

[0011] Aspect 1. A method of chemically strengthening a substrate, the substrate comprising a thickness from 10 micrometers to 100 micrometers defined between an existing first major surface and an existing second major surface opposite the existing first major surface, the method comprising: contacting the existing first major surface of the substrate with a molten salt solution maintained a first temperature for a first period of time, the molten salt solution comprising at least two anions associated with at least a first potassium salt and a second potassium salt, a concentration of the first potassium salt and a concentration of the second potassium salt is 2 wt% or more of the molten salt solution, the first temperature is in a range from about 350°C to about 400°C, and the first period of time is in a range from about 10 minutes to about 90 minutes.

[0012] Aspect 2. The method of aspect 1, wherein the first potassium salt comprises two or more potassium atoms per anion, and a pKa of the potassium salt is 9 or more, and a concentration of the first potassium salt is in a range from about 2 wt% to about 12 wt% of the molten salt solution.

[0013] Aspect 3. The method of any one of aspects 1-2, wherein the first potassium salt is potassium carbonate K2CO3, and a concentration of the first potassium salt is in a range from about 2 wt% to about 12 wt% of the molten salt solution.

[0014] Aspect 4. The method of any one of aspects 1-3, wherein the concentration of the first potassium salt is in a range from about 2.5 wt% to about 5.0 wt%.

[0015] Aspect 5. The method of any one of aspects 1-3, wherein the concentration of the first potassium salt is in a range from about 5 wt% to about 12 wt%.

[0016] Aspect 6. The method of aspect 5, wherein the concentration of the first potassium salt is in a range from about 8 wt% to about 12 wt%.

[0017] Aspect 7. The method of any one of aspects 3-6, wherein the molten salt solution further comprises from 0 wt% to 5 wt% of a third potassium salt associated with a third anion that is different from the anions associated with the first potassium salt and the second potassium salt, and the third potassium salt comprises two or more potassium atoms per anion.

[0018] Aspect 8. The method of aspect 7, wherein the third potassium salt comprises potassium sulfate K2SO4, and a concentration of the third potassium salt is from about 0.5 wt% to about 5 wt%.

[0019] Aspect 9. The method of any one of aspects 1-8, wherein the second potassium salt is potassium nitrate KNO3, and a concentration of the second potassium salt is in a range from about 50 wt% to about 98 wt% of the molten salt solution.

[0020] Aspect 10. The method of aspect 9, wherein the concentration of the second potassium salt is in a range from about 88 wt% to about 98 wt%.

[0021] Aspect 11. The method of any one of aspects 1-10, wherein a pH of the molten salt solution at the first temperature is in a range from about 9 to 12.

[0022] Aspect 12. The method of any one of aspects 1-11, wherein a presence of the first potassium salt increases a compressive stress imparted by the contacting the existing first major surface with the molten salt solution by about 5% or more relative to immersing the substrate in a comparative molten salt solution with the same composition as the molten salt solution with the absence of the first potassium salt.

[0023] Aspect 13. The method of any one of aspects 1-12, wherein the thickness of the substrate is in a range from about 15 pm to about 50 pm. [0024] Aspect 14. The method of any one of aspects 1-12, wherein the thickness of the substrate is in a range from about 50 pm to about 90 pm.

[0025] Aspect 15. The method of any one of aspects 1-13, wherein the method further comprises, before the contacting the existing first major surface with the molten salt solution, heating the substrate at a temperature in a range from about 250°C to about 350°C for a period of time from about 10 minutes to about 4 hours.

[0026] Aspect 16. The method of any one of aspects 1-13 or 15 inclusive, further comprising, after the contacting the existing first major surface with the molten salt solution: transferring the substrate from the molten salt solution to a cooling chamber, a temperature of the cooling chamber decreases from an initial temperature to a final temperature at a cooling rate in a range from about 4 °C/min to about 20 °C/min, the initial temperature is in a range from about 180°C to about 300°C, and the final temperature is in a range from about 25°C to about 100°C.

[0027] Aspect 17. A method of chemically strengthening a substrate, the substrate comprising a thickness from 10 micrometers to 50 micrometers defined between an existing first major surface and an existing second major surface opposite the existing first major surface, the method comprising: chemically strengthening the substrate in a molten salt solution maintained at a first temperature for a first period of time, the first temperature is in a range from about 350°C to about 400°C, and the first period of time is in a range from about 10 minutes to about 90 minutes; and transferring the substrate from the molten salt solution to a cooling chamber, a temperature of the cooling chamber decreases from an initial temperature to a final temperature a cooling range in a range from about 4 °C/min to about 20 °C/min, the initial temperature is in a range from about 180°C to about 300°C, and the final temperature is in a range from about 25°C to about 100°C.

[0028] Aspect 18. The method of any one of aspects 16-17, wherein the final temperature is in a range from about 60°C to about 90°C.

[0029] Aspect 19. The method of any one of aspects 16-18, wherein the initial temperature is in a range from about 180°C to about 220°C.

[0030] Aspect 20. The method of any one of aspects 16-19, further comprising, after the cooling chamber reaches the final temperature, rinsing the substrate with water, an alkaline detergent solution, or combinations thereof. [0031] Aspect 21. The method of any one of aspects 1-20, wherein an initial maximum compressive stress of the substrate after the contacting the existing first major surface with the molten salt solution is from about 800 MegaPascals to about 1500 MegaPascals.

[0032] Aspect 22. The method of aspect 21, wherein the initial maximum compressive stress is from about 900 MegaPascals to about 1200 MegaPascals.

[0033] Aspect 23. The method of any one of aspects 1-20, further comprising: contacting the existing first major surface with an acidic solution for a second period of time to remove an outer layer from the existing first major surface to form a new first major surface, a pH of the acidic solution is in a range from 3.5 to 4.5, and the second period of time is from about 10 seconds to about 3.5 minutes; and then rinsing the new first major surface with water.

[0034] Aspect 24. A method of chemically strengthening a substrate, the substrate comprising a thickness from 10 micrometers to 100 micrometers defined between an existing first major surface and an existing second major surface opposite the existing first major surface, the method comprising: chemically strengthening the substrate in a molten salt solution maintained at a first temperature for a first period of time, the first temperature is in a range from about 350°C to about 400°C, and the first period of time is in a range from about 10 minutes to about 90 minutes; contacting the existing first major surface with an acidic solution for a second period of time to remove an outer layer from the existing first major surface to form a new first major surface, a pH of the acidic solution is in a range from 3.5 to 4.5, and the second period of time is from about 10 seconds to about 3.5 minutes; and then rinsing the new first major surface with water.

[0035] Aspect 25. The method of any one of aspects 23-24, wherein a second temperature of the acidic solution is from about 20°C to about 40°C.

[0036] Aspect 26. The method of aspect 23, wherein the second temperature is in a range from about 20°C to about 25°C.

[0037] Aspect 27. The method of any one of aspects 23-26, wherein the acidic solution comprises a buffered HF solution.

[0038] Aspect 28. The method of any one of aspects 23-27, wherein the acidic solution comprises, as a wt% of the acidic solution: from about 0.5 wt% to about 1.5 wt% HF; and from about 0.75 wt% to about 2.5 wt% NH4F.

[0039] Aspect 29. The method of aspect 28, wherein the acidic solution comprises, as a wt% of the acidic solution: from about 0.5 wt% to about 0.75 wt% HF; and from about 0.9 wt% to about 1.5 wt% NH4F.

[0040] Aspect 30. The method of any one of aspects 23-29, wherein the acidic solution removes the outer layer at rate of about 1.0 micrometers per minute or less.

[0041] Aspect 31. The method of any one of aspects 23-30, wherein the substrate comprises an initial maximum compressive stress before the contacting with the acidic solution, the substrate comprises a final maximum compressive stress after the contacting with the acidic solution, and the final maximum compressive stress is less than the initial maximum compressive stress, as a percentage of the initial maximum compressive stress, by from about 10% to about 25%.

[0042] Aspect 32. The method of aspect 31, wherein the final maximum compressive stress is less than the initial maximum compressive stress, as a percentage of the initial maximum compressive stress, by from about 15% to about 20%.

[0043] Aspect 33. The method of any one of aspects 31-32, wherein the final maximum compressive stress is in a range from about 700 MegaPascals to about 1200 MegaPascals.

[0044] Aspect 34. The method of any one of aspects 23-33, further comprising, after contacting the existing first major surface with the acidic solution, rinsing the substrate with water or another acidic solution.

[0045] Aspect 35. The method of any one of aspects 23-34, wherein 95% or more of samples of the substrate can withstand a parallel plate distance of 5 millimeters.

[0046] Aspect 36. The method of any one of aspects 23-35, wherein the substrate exhibits a pen drop threshold height of 10 centimeters or more in a Pen Drop Test.

[0047] Aspect 37. The method of any one of aspects 23-35, wherein the substrate exhibits a pen drop threshold height of 20 centimeters or more in a Pen Drop Height.

[0048] Aspect 38. The method of any one of aspects 23-37, wherein about 30% or more samples of the substrate can withstand a parallel plate distance of 3 millimeters when the thickness is in a range from about 50 micrometers to about 100 micrometers.

[0049] Aspect 39. The method of any one of aspects 23-36, wherein the thickness is from 10 micrometers to 50 micrometers.

[0050] Aspect 40. The method of aspect 39, wherein the thickness is from 10 micrometers to 30 micrometers.

[0051] Aspect 41. The method of any one of aspects 39-40, wherein 90% or more of samples of the substrate can withstand a parallel plate distance of 2 millimeters.

[0052] Aspect 42. The method of any one of aspects 39-40, wherein 10% or more of samples of the substrate can withstand a parallel plate distance of 1 millimeter.

[0053] Aspect 43. The method of any one of aspects 1-42, wherein the substrate is a glass-based substrate.

[0054] Aspect 44. The method of aspect 43, wherein a composition of the substrate comprises, as a mol% of the substrate: from about 60 mol% to about 70 mol% SiCh; from about 8 mol% to about 16 mol% AI2O3; from about 12 mol% to about 18 mol% Na?O; from about 2 mol% to about 6 mol% MgO; and from about 0.1 mol% to about 2.0 mol% CaO.

[0055] Aspect 45. The method of aspect 44, wherein the composition comprises, as a mol% of the substrate: from about 64 mol% to about 69 mol% SiCh; from about 9 mol% to about 15 mol% AI2O3; from about 14 mol% to about 17 mol% Na2O; from about 2.5 mol% to about 5.5 mol% MgO; from about 0.3 mol% to about 1.2 mol% CaO; and from 0.0 mol% to about 0.5 mol% K2O.

[0056] Aspect 46. A chemically-strengthened substrate comprising: a thickness defined between a first major surface and a second major surface opposite the first major surface, the thickness is from about 10 micrometers to about 100 micrometers; and a first compressive stress region extending to a first depth of compression from the first major surface, a first depth of layer of potassium is about 5 micrometers or more, a maximum first compressive stress is from about 650 MegaPascals to about 1200 MegaPascals, wherein the chemically-strengthened substrate comprises a glass-based material, 95% or more of samples of the chemically-strengthened substrate can withstand a parallel plate distance of 5 millimeters, and the substrate exhibits a pen drop threshold height of 10 centimeters or more in a Pen Drop Test.

[0057] Aspect 47. The chemically-strengthened substrate of aspect 46, wherein a composition of the chemically-strengthened substrate, as a mol% of the chemically-strengthened substrate: from about 60 mol% to about 70 mol% Si O2; from about 8 mol% to about 16 mol% AI2O3; from about 12 mol% to about 18 mol% Na?O; from about 2 mol% to about 6 mol% MgO; and from about 0.1 mol% to about 2.0 mol% CaO.

[0058] Aspect 48. The chemically-strengthened substrate of aspect 47, wherein the composition comprises, as a mol% of the wherein a composition of the chemically-strengthened substrate, as a mol% of the chemically-strengthened substrate: from about 64 mol% to about 69 mol% Si O2; from about 9 mol% to about 15 mol% AI2O3; from about 14 mol% to about 17 mol% Na2O; from about 2.5 mol% to about 5.5 mol% MgO; from about 0.3 mol% to about 1.2 mol% CaO; and from 0.0 mol% to about 0.5 mol% K2O.

[0059] Aspect 49. The chemically-strengthened substrate of any one of aspects 46-48, wherein the maximum first compressive stress is from about 800 MegaPascals to about 1100 MegaPascals.

[0060] Aspect 50. The chemically-strengthened substrate of any one of aspects 46-49, wherein 90% or more of samples of the substrate can withstand a parallel plate distance of 3 millimeters. [0061] Aspect 51. The chemically-strengthened substrate of any one of aspects 46-50, wherein the thickness is from about 10 micrometers to about 50 micrometers.

[0062] Aspect 52. The chemically-strengthened substrate of aspect 51, wherein the thickness is from about 10 micrometers to about 30 micrometers.

[0063] Aspect 53. The chemically-strengthened substrate of any one of aspects 51-52, wherein the maximum first compressive stress is from about 750 MegaPascals to about 1100 MegaPascals.

[0064] Aspect 54. The chemically-strengthened substrate of any one of aspects 51-53, wherein 90% or more of samples of the substrate can withstand a parallel plate distance of 2 millimeters.

[0065] Aspect 55. The chemically-strengthened substrate of any one of aspects 51-54, wherein 10% or more of samples of the substrate can withstand a parallel plate distance of 1 millimeter.

[0066] Aspect 56. The chemically-strengthened substrate of any one of aspects 46-50, wherein the maximum first compressive stress is from about 850 MegaPascals to about 1200 MegaPascals.

[0067] Aspect 57. The chemically-strengthened substrate of any one of aspects 46-50, wherein the maximum first compressive stress is from about 1000 MegaPascals to about 1200 MegaPascals.

[0068] Aspect 58. The chemically-strengthened substrate of any one of aspects 46-50 or 56-57 inclusive, wherein about 30% or more samples of the chemically-strengthened substrate can withstand a parallel plate distance of 3 millimeters.

[0069] Aspect 59. The chemically-strengthened substrate of any one of aspects 46-50 or 56-57 inclusive, wherein about 50% or more samples of the chemically-strengthened substrate can withstand a parallel plate distance of 3 millimeters.

[0070] Aspect 60. The chemically-strengthened substrate of any of aspects 46- 59, wherein the chemically-strengthened substrate exhibits a haze of about 1% or less.

[0071] Aspect 61. The chemically-strengthened substrate of any one of aspects 46-60, wherein the chemically-strengthened substrate exhibits a transmittance from 90% to 95%. [0072] Throughout the disclosure, the drawings are used to emphasize certain aspects. As such, it should not be assumed that the relative size of different regions, portions, and substrates shown in the drawings are proportional to its actual relative size, unless explicitly indicated otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

[0073] The above and other features and advantages of aspects of the present disclosure are better understood when the following detailed description is read with reference to the accompanying drawings, in which:

[0074] FIG. 1 is a schematic view of an example foldable apparatus in a flat configuration according to aspects, wherein a schematic view of the folded configuration may appear as shown in FIG. 5;

[0075] FIG. 2 is a cross-sectional view of an example foldable apparatus consisting of a foldable substrate taken along line 2-2 of FIG. 1 according to aspects;

[0076] FIG. 3 is a cross-sectional view of an example foldable apparatus along line 2-2 of FIG. 1 according to aspects;

[0077] FIG. 4 is a schematic view of example foldable apparatus of aspects of the disclosure in a folded configuration wherein a schematic view of the flat configuration may appear as shown in FIG. 1;

[0078] FIG. 5 is a cross-sectional view of a testing apparatus to determine the minimum parallel plate distance of an example modified foldable apparatus and/or foldable substrate along line 5-5 of FIG. 4;

[0079] FIG. 6 is a schematic perspective view of a pen drop apparatus;

[0080] FIG. 7 is a schematic perspective view of a foldable consumer electronic product;

[0081] FIG. 8 is a schematic plan view of an example consumer electronic device according to aspects;

[0082] FIG. 9 is a schematic perspective view of the example consumer electronic device of FIG. 8;

[0083] FIG. 10 is a flow chart illustrating example methods of chemically strengthening a substrate to form a foldable substrate and/or foldable apparatus in accordance with aspects of the disclosure;

[0084] FIG. 11 schematically illustrates a step in a method of chemically strengthening a substrate comprising heating the substrate; [0085] FIG. 12 schematically illustrates a step in a method of chemically strengthening a substrate comprising contacting the substrate with a molten salt solution;

[0086] FIG. 13 schematically illustrates a step in a method of chemically strengthening a substrate comprising decreasing a temperature of a cooling chamber and/or allowing the molten salt solution to drip off of the substrate;

[0087] FIG. 14 schematically illustrates a step in a method of chemically strengthening a substrate comprising rinsing the substrate;

[0088] FIG. 15 schematically illustrates a step in a method of chemically strengthening a substrate comprising contacting the substrate with an acidic solution;

[0089] FIG. 16 is a cross-sectional view of a foldable apparatus after the step shown in FIG. 12 and/or before the step shown in FIG. 15;

[0090] FIG. 17 schematically illustrates a compressive stress in MegaPascals (vertical axis - y-axis) for Examples 1-6 and Comparative Examples AA-DD;

[0091] FIG. 18 schematically illustrates a depth of layer in micrometers (vertical axis - y-axis) for Examples 1-6 and Comparative Examples AA-DD;

[0092] FIG. 19 schematically illustrates a compressive stress in MegaPascals (vertical axis - y-axis) as a function of a concentration in wt% of K2CO3 (horizontal axis - x-axis) in the molten salt solution;

[0093] FIG. 20 schematically illustrates depth of layer in micrometers (vertical axis - y-axis) as a function of a concentration in wt% of K2CO3 (horizontal axis - x-axis) in the molten salt solution;

[0094] FIG. 21 schematically illustrates a survival rate (in percent on the vertical axis - y-axis) as a function of parallel plate distance in millimeters (horizontal axis - x-axis);

[0095] FIG. 22 schematically illustrates a survival rate (in percent on the vertical axis - y-axis) as a function of parallel plate distance in millimeters (horizontal axis - x-axis);

[0096] FIG. 23 schematically illustrates a survival rate (in percent on the vertical axis - y-axis) as a function of parallel plate distance in millimeters (horizontal axis - x-axis);

[0097] FIG. 24 schematically illustrates a survival rate (in percent on the vertical axis - y-axis) as a function of parallel plate distance in millimeters (horizontal axis - x-axis); [0098] FIG. 25 schematically illustrates a survival rate (in percent on the vertical axis - y-axis) as a function of parallel plate distance in millimeters (horizontal axis - x-axis); and

[0099] FIGS. 26A-26C schematically illustrate reflections visible from chemically-strengthened substrates exhibiting different levels of waviness.

[00100] Throughout the disclosure, the drawings are used to emphasize certain aspects. As such, it should not be assumed that the relative size of different regions, portions, and substrates shown in the drawings are proportional to its actual relative size, unless explicitly indicated otherwise.

DETAILED DESCRIPTION

[00101] Aspects will now be described more fully hereinafter with reference to the accompanying drawings in which example aspects are shown. Whenever possible, the same reference numerals are used throughout the drawings to refer to the same or like parts. However, claims may encompass many different aspects of various aspects and should not be construed as limited to the aspects set forth herein.

[00102] FIGS. 1-5 illustrate schematic views of foldable apparatus 101, 301, and/or 401 comprising a foldable substrate 201 in accordance with aspects of the disclosure. Unless otherwise noted, a discussion of features of aspects of one foldable apparatus and/or foldable substrate can apply equally to corresponding features of any aspects of the disclosure. For example, identical part numbers throughout the disclosure can indicate that, in some aspects, the identified features are identical to one another and that the discussion of the identified feature of one aspect, unless otherwise noted, can apply equally to the identified feature of any of the other aspects of the disclosure.

[00103] As shown in FIGS. 1-3, example aspects of foldable apparatus 101 and/or 301 can comprise the foldable substrate 201 in accordance with aspects of the disclosure in an unfolded (e.g., flat) configuration while FIG. 5 demonstrate a foldable apparatus 401 comprising the foldable substrate 201 in accordance with aspects of the disclosure in a folded configuration. In aspects, as shown in FIGS. 4-5, the foldable apparatus 401 can comprise and/or consist of the foldable substrate 201. In aspects, as shown in FIG. 3, the foldable apparatus 301 can comprise a layer (e.g., PET sheet 321) attached to the foldable substrate 201 by an adhesive layer 311 with the understanding that other layers (e.g., release liners, display devices, additional substrates) can be used in addition or instead of the shown layer. [00104] Throughout the disclosure, with reference to FIG. 1, the width 103 of the foldable apparatus 101 and/or 301 is considered the dimension of the foldable apparatus taken between opposed edges of the foldable apparatus in a direction 104 of a fold axis 102 of the foldable apparatus, wherein the direction 104 also comprises the direction of the width 103. Furthermore, throughout the disclosure, the length 105 of the foldable apparatus 101 and/or 301 is considered the dimension of the foldable apparatus 101 and/or 301 taken between opposed edges of the foldable apparatus 101 and/or 301 in a direction 106 perpendicular to the fold axis 102 of the foldable apparatus. In aspects, as shown in FIGS. 1-3, the foldable apparatus of any aspects of the disclosure can comprise a fold plane 109 that includes the fold axis 102 and a direction of a substrate thickness 209 when the foldable apparatus is in the flat configuration (e.g., see FIG. 2). The fold plane 109 may comprise a central axis 107 of the foldable apparatus positioned, for example, at a second major surface 205 of the foldable apparatus 101 and 301 (see FIGS. 2-3). In aspects, the foldable apparatus can be folded in a direction 111 (e.g., see FIG. 1) about the fold axis 102 extending in the direction 104 of the width 103 to form a folded configuration (e.g., see FIGS. 4- 5). In aspects, as shown in FIGS. 2-3, the foldable apparatus 101 and/or 301 and/or the foldable substrate 201 can comprise a first major surface 203 and/or a second major surface 205 that are substantially planar, where a central portion of the foldable apparatus can be indistinguishable from adjacent portions. As shown in FIGS. 1-5, the foldable apparatus may include a single fold axis to allow the foldable apparatus to comprise a bifold wherein, for example, the foldable apparatus may be folded in half. In further aspects, the foldable apparatus may include two or more fold axes, for example, with each fold axis including a corresponding central portion similar or identical to the central portion discussed herein. For example, providing two fold axes can allow the foldable apparatus to comprise a trifold wherein, for example, the foldable apparatus may be folded with the first portion, the second portion, and a third portion similar or identical to the first portion or second portion with the central portion and another central portion similar to or identical to the central portion positioned between the first portion and the second portion and between the second portion and the third portion, respectively.

[00105] Foldable apparatus 101 and/or 301 of the disclosure comprise the foldable substrate 201. In aspects, the foldable substrate 201 can comprise a glassbased substrate having a pencil hardness of 8H or more, for example, 9H or more. In aspects, the foldable substrate 201 can comprise a glass-based substrate. As used herein, “glass-based” includes both glasses and glass-ceramics, wherein glassceramics have one or more crystalline phases and an amorphous, residual glass phase. A glass-based material (e.g., glass-based substrate) may comprise an amorphous material (e.g., glass) and optionally one or more crystalline materials (e.g., ceramic). Amorphous materials and glass-based materials may be strengthened. As used herein, the term “strengthened” may refer to a material that has been chemically strengthened, for example, through ion exchange of larger ions for smaller ions in the surface of the substrate, as discussed below. However, other strengthening methods, for example, thermal tempering, or utilizing a mismatch of the coefficient of thermal expansion between portions of the substrate to create compressive stress and central tension regions, may be utilized to form strengthened substrates. Exemplary glassbased materials, which may be free of lithia or not, comprise soda lime glass, alkali aluminosilicate glass, alkali-containing borosilicate glass, alkali-containing aluminoborosilicate glass, alkali-containing phosphosilicate glass, and alkali- containing aluminophosphosilicate glass. In one or more aspects, a glass-based material may comprise, in mole percent (mol %): SiCh from about 40 mol% to about 80 mol%, AI2O3 from about 5 mol% to about 30 mol%, B2O3 from 0 mol% to about 10 mol%, ZrCh from 0 mol% to about 5 mol%, P2O5 from 0 mol% to about 15 mol%, TiCh from 0 mol% to about 2 mol%, R2O from 0 mol% to about 20 mol%, and RO from 0 mol% to about 15 mol%. As used herein, R2O can refer to an alkali metal oxide, for example, Li2O, Na2O, K2O, Rb2O, and CS2O. As used herein, RO can refer to MgO, CaO, SrO, BaO, and ZnO. In aspects, a glass-based substrate may optionally further comprise from 0 mol % to about 2 mol % of each of Na2SO4, NaCl, NaF, NaBr, K2SO4, KC1, KF, KBr, As₂O₃, Sb₂O₃, SnO₂, Fe₂O₃, MnO, MnO₂, MnO₃, Mn2O3, Mn3O4, Mn20?. “Glass-ceramics” include materials produced through controlled crystallization of glass. In aspects, glass-ceramics have about 1% to about 99% crystallinity. Examples of suitable glass-ceramics may include Li2O-A12O3-SiO2 system (i.e., LAS-System) glass-ceramics, MgO-AhO3-SiO2 system (i.e., MAS- System) glass-ceramics, ZnO x AI2O3 ^x nSiO2 (i.e., ZAS system), and/or glassceramics that include a predominant crystal phase including P-quartz solid solution, P- spodumene, cordierite, petalite, and/or lithium disilicate. The glass-ceramic substrates may be strengthened using the chemical strengthening processes. In one or more aspects, MAS-System glass-ceramic substrates may be strengthened in Li2SO4 molten salt, whereby an exchange of 2Li⁺ for Mg²⁺ can occur.

[00106] In aspects, the glass-based substrate (e.g., foldable substrate 201) has SiC>2 is the largest constituent and, as such, SiCh is the primary constituent of the glass network formed from the glass-based composition. Pure SiCh has a relatively low CTE. However, pure SiCh has a high melting point. Accordingly, if the concentration of SiC>2 in the glass-based composition is too high, the formability of the glass-based composition may be diminished as higher concentrations of SiCh increase the difficulty of melting the glass, which, in turn, adversely impacts the formability of the composition. If the concentration of SiCh in the glass-based composition is too low the chemical durability of the glass-based material may be diminished, and the glassbased material may be susceptible to surface damage during post-forming treatments. In aspects, the glass-based substrate can comprise SiCh in an amount of 60 mol% or more, 61 mol% or more, 62 mol% or more, 63 mol% or more, 63.5 mol% or more, 64 mol% or more, 70 mol% or less, 69 mol% or less, 68 mol% or less, 67 mol% or less, 66 mol% or less, or 65 mol% or less. In aspects, the glass-based substrate can comprise SiCh in a range from 60 mol% to 70 mol%, from 61 mol% to 70 mol%, from 62 mol% to 69 mol%, from 63 mol% to 69 mol%, from 64 mol% to 69 mol%, from 65 mol% to 69 mol%, from 64 mol% to 68 mol%, from 65 mol% to 67 mol%, or any range or subrange therebetween. In preferred aspects, the glass-based substrate comprises SiCh in an amount from 60 mol% to 70 mol% or from 64 mol% to 69 mol%.

[00107] The glass-based substrate (e.g., foldable substrate 201) can include AI2O3. AI2O3 may serve as a glass network former, similar to SiCh. AI2O3 may increase the viscosity of the glass-based composition due to its tetrahedral coordination in a glass melt formed from a glass-based composition, decreasing the formability of the glass-based composition when the amount of AI2O3 is too high. However, when the concentration of AI2O3 is balanced against the concentration of SiC>2 and the concentration of alkali oxides in the glass-based composition, AI2O3 can reduce the liquidus temperature of the glass melt, thereby enhancing the liquidus viscosity and improving the compatibility of the glass-based composition with certain forming processes. The inclusion of AI2O3 in the glass-based compositions can enable the high fracture toughness values described herein. In aspects, the glass-based substrate comprises AI2O3 in a concentration of 8 mol% or more, 9 mol% or more, 10 mol% or more, 11 mol% or more, 12 mol% or more, 16 mol% or less, 16 mol% or less, 15 mol% or less, 14 mol% or less, or about 13 mol% or less. In aspects, the glass-based substrate can comprise an amount of AI2O3 in a range from 8 mol% to 16 mol%, from 9 mol% to 15 mol%, from 10 mol% to 15 mol%, from 11 mol% to 14 mol%, from 12 mol% to 13 mol%, or any range or subrange therebetween. In preferred aspects, the glass-based substrate comprises AI2O3 in an amount from 8 mol% to 16 mol% or from 9 mol% to 15 mol%.

[00108] The glass-based substrate (e.g., foldable substrate 201) can include Na2O. Na2O may aid in the ion-exchangeability of the glass-based composition, and improve the formability, and thereby manufacturability, of the glass-based composition. However, if too much Na2O is added to the glass-based composition, the CTE may be too low, and the melting point may be too high. Additionally, if too much Na2O is included in the composition relative to the amount of Li2O the ability of the glass-based substrate to achieve a deep depth of compression when ion exchanged may be reduced. In aspects, the glass-based substrate comprises Na2O in an amount of 12 mol% or more, 13 mol% or more, 14 mol% or more, 15 mol% or more, 18 mol% or less, 17 mol% or less, 16 mol% or less, or 15 mol% or less. In aspects, the glass-based substrate comprises an amount of Na2O in a range from 12 mol% to 17 mol%, from 13 mol% to 18 mol%, from 14 mol% to 17 mol%, from 15 mol% to 16 mol%, or any range or subrange therebetween. In preferred aspects, the glass-based substrate comprises Na2O in an amount from 12 mol% to 18 mol% Na2O or from 14 mol% to 17 mol%.

[00109] The glass-based substrate (e.g., foldable substrate 201) may include K2O. The inclusion of K2O in the glass-based composition increases the potassium diffusivity in the glass-based material, enabling a deeper depth of a compressive stress spike (DOLSP) to be achieved in a shorter amount of ion exchange time. If too much K2O is included in the composition the amount of compressive stress imparted during an ion-exchange process may be reduced. In aspects, the glass-based substrate can comprise K2O in an amount of 0.0 mol% or more, 0.1 mol% or more, 0.25 mol% or more, 1 mol% or less, 0.75 mol% or less, 0.5 mol% or less, or 0.3 mol% or less. In aspects, the glass-based substrate can comprise an amount of K2O in a range from 0.0 mol% to 1 mol%, from 0.0 mol% to 0.75 mol%, from 0.0 mol% to 0.5 mol%, from 0.1 mol% to 0.3 mol%, or any range or subrange therebetween. In preferred aspects, the glass-based substrate can comprise an amount of K2O in a range from 0.0 mol% to 1 mol% or from 0.0 mol% to 0.5 mol%.

[00110] The glass-based substrate (e.g., foldable substrate 201) can include MgO. MgO may lower the viscosity of a glass, which enhances the formability and manufacturability of the composition. The inclusion of MgO in a glass-based composition may also improve the strain point and the Young’s modulus of the glassbased composition. However, if too much MgO is added to the glass-based composition, the liquidus viscosity may be too low for compatibility with desirable forming techniques. The addition of too much MgO may also increase the density and the CTE of the glass-based composition to undesirable levels. The inclusion of MgO in the glass-based composition also helps improve the fracture toughness. In aspects, the glass-based substrate can comprise MgO in an amount of 2 mol% or more, 2.5 mol% or more, 3.0 mol% or more, 3.2 mol% or more, 3.5 mol% or more, 4.0 mol% or more, 6 mol% or less, 5.5 mol% or less, 5.0 mol% or less, or 4.9 mol% or less. In aspects, the glass-based substrate can comprise an amount of MgO in a range from 2mol% to 6 mol%, from 2.5 mol% to 5.5 mol%, from 3.0 mol% to 5.0 mol%, from 3.2 mol% to less than or equal to 4.9 mol%, from 3.5 mol% to 4.9, or any range or subrange therebetween. In preferred aspects, the composition comprises MgO in an amount from 2 mol% to 6 mol% or from 2.5 mol% to 5.5 mol%.

[00111] The glass-based substrate (e.g., foldable substrate 201) described herein may include CaO. CaO may lower the viscosity of a glass, which may enhance the formability, the strain point, and the Young’s modulus. However, if too much CaO is added to the glass-based composition, the density and the CTE of the glassbased composition may increase to undesirable levels and the ion exchangeability of the glass-based substrate may be undesirably impeded. The inclusion of CaO in the glass-based composition also improves the fracture toughness. In aspects, the glassbased substrate can comprise CaO in an amount of 0.1 mol% or more, 0.2 mol% or more, 0.3 mol% or more, 0.4 mol% or more, 0.5 mol% or more, 0.7 mol% or more, 2.0mol% or less, 1.5 mol% or less, 1.2 mol% or less, 1.1 mol% or less, 1.0 mol% or less, or 0.9 mol% or less. In aspects, the glass-based substrate can comprise an amount of CaO in a range from 0.1 mol% to 2.0 mol%, from 0.2 mol% to 1.5 mol%, from 0.3 mol% to 1.2 mol%, from 0.4 mol% to less than or equal to 1.1 mol%, from 0.5 mol% to 1.0 mol%, from 0.7 mol% to 1.0 mol%, or any range or subrange therebetween. In preferred aspects, the glass-based substrate comprises CaO in an amount from 0.1 mol% to 2.0 mol% or from 0.3 mol% to 1.2 mol%.

[00112] In aspects, the glass-based substrate can be substantially free or free of one or more of P2O5, B2O3, TiCh, ZnO, ZrCh, Ta20s, HfCh, La2Os, and/or Y2O3. As used herein, the term “substantially free” means that the component is not purposefully added as a component of the batch material even though the component may be present in the final glass-based composition in very small amounts as a contaminant, such as less than 0.1 mol%. For example, the inclusion of ZrCh in the glass-based composition may result in the formation of undesirable zirconia inclusions in the glass-based material, due at least in part to the low solubility of ZrCh in the glass-based material. Also, the inclusion of Ta20s, HfCh, La2O3, and/or Y2O3 may increase the cost of raw materials associated with the glass-based substrate.

[00113] In aspects, the glass-based substrate (e.g., foldable substrate 201) can comprise from about 60 mol% to about 70 mol% SiCh, from about 8 mol% to about 16 mol% AI2O3, from about 12 mol% to about 18 mol% Na2O, from about 2 mol% to about 6 mol% MgO, and from about 0.1 mol% to about 2.0 mol% CaO. In aspects, the glass-based substrate (e.g., foldable substrate 201) can comprise from about 64 mol% to about 69 mol% SiO2, from about 9 mol% to about 15 mol% AI2O3, from about 14 mol% to about 17 mol% Na2O, from about 2.5 mol% to about 5.5 mol% MgO, from about 0.3 mol% to about 1.2 mol% CaO, and from 0.0 mol% to about 0.5 mol% K2O.

[00114] The foldable substrate 201 can comprise a glass-based substrate, and the first major surface 203 and/or second major surface 205 can comprise one or more compressive stress regions. In aspects, a compressive stress region may be created by chemically strengthening. Chemically strengthening may comprise an ion exchange process, where ions in a surface layer are replaced by-or exchanged with-larger ions having the same valence or oxidation state. Methods of chemically strengthening will be discussed later. Without wishing to be bound by theory, chemically strengthening the foldable substrate 201 can enable good impact and/or puncture resistance (e.g., resists failure for a pen drop height of 20 centimeters). Without wishing to be bound by theory, chemically strengthening the foldable substrate 201 can enable small (e.g., smaller than about 10 mm or less) bend radii because the compressive stress from the chemical strengthening can counteract the bend-induced tensile stress on the outermost surface of the substrate. A compressive stress region may extend into a portion of the first portion and/or second portion for a depth called the depth of compression. As used herein, depth of compression means the depth at which the stress in the chemically-strengthened substrates and/or portions described herein changes from compressive stress to tensile stress. Depth of compression may be measured by a surface stress meter or a scattered light polariscope (SCALP, wherein values reported herein were made using SCALP-5 made by Glasstress Co., Estonia) depending on the ion exchange treatment and the thickness of the article being measured. Where the stress in the substrate and/or portion is generated by exchanging potassium ions into the substrate, a surface stress meter, for example, the FSM-6000 (Orihara Industrial Co., Ltd. (Japan)), is used to measure depth of compression. Unless specified otherwise, compressive stress (including surface CS) is measured by surface stress meter (FSM) using commercially available instruments, for example the FSM-6000, manufactured by Orihara. Surface stress measurements rely upon the accurate measurement of the stress optical coefficient (SOC), which is related to the birefringence of the glass. Unless specified otherwise, SOC is measured according to Procedure C (Glass Disc Method) described in ASTM standard C770-16, entitled “Standard Test Method for Measurement of Glass Stress-Optical Coefficient,” the contents of which are incorporated herein by reference in their entirety. Where the stress is generated by exchanging sodium ions into the substrate, and the article being measured is thicker than about 400 pm, SCALP is used to measure the depth of compression and central tension (CT). Where the stress in the substrate and/or portion is generated by exchanging both potassium and sodium ions into the substrate and/or portion, and the article being measured is thicker than about 400 pm, the depth of compression and CT are measured by SCALP. Without wishing to be bound by theory, the exchange depth of sodium may indicate the depth of compression while the exchange depth of potassium ions may indicate a change in the magnitude of the compressive stress (but not the change in stress from compressive to tensile). The refracted near-field (RNF; the RNF method is described in U.S. Patent No. 8,854,623, entitled “Systems and methods for measuring a profile characteristic of a glass sample”, which is incorporated herein by reference in its entirety) method also may be used to derive a graphical representation of the stress profile. When the RNF method is utilized to derive a graphical representation of the stress profile, the maximum central tension value provided by SCALP is utilized in the RNF method. The graphical representation of the stress profile derived by RNF is force balanced and calibrated to the maximum central tension value provided by a SCALP measurement. As used herein, “depth of layer” (DOL) means the depth that the ions have exchanged into the substrate and/or portion (e.g., sodium, potassium). Through the disclosure, when the maximum central tension cannot be measured directly by SCALP (as when the article being measured is thinner than about 400 pm) the maximum central tension can be approximated by a product of a maximum compressive stress and a depth of compression divided by the difference between the thickness of the substrate and twice the depth of compression, wherein the compressive stress and depth of compression are measured by FSM. Throughout the disclosure, an absolute value of compressive stress is reported as compressive stress, and an absolute value of central tensile stress is reported as central tensile stress.

[00115] In aspects, as shown in FIG. 2, the first major surface 203 of the foldable substrate 201 can comprise a first compressive stress region 212 extending to a first depth of compression 216 from the first major surface 203. Although not shown, the first compressive stress region 212 can also comprise a first depth of layer of one or more alkali metal ions (e.g., potassium) associated with the first compressive stress region. In aspects, as shown in FIG. 2, the second major surface 205 of the foldable substrate 201 can comprise a second compressive stress region

214 extending to a second depth of compression 218 from the second major surface 205. Although not shown, the second compressive stress region 214 can also comprise a second depth of layer of one or more alkali metal ions (e.g., potassium) associated with the first compressive stress region. As shown in FIG. 2, dashed lines 213 and

215 correspond to a location where a stress in the foldable substrate switched from compressive to tensile (or visa versa) corresponding to a boundary (i.e., depth of compression) of the corresponding compressive stress region. It is to be understood that the first compressive stress region 212 and/or the second compressive stress region 214 shown in FIG. 2 for the foldable substrate 201 of the foldable apparatus 101 can also be present in other foldable apparatus (e.g., foldable apparatus 301 and/or 401 shown in FIGS. 3-5 even though not explicitly labeled in FIGS. 3-5).

[00116] In aspects, the first depth of compression 216 and/or the second depth of compression 218 as a percentage of the substrate thickness 209 can be about 5% or more, about 10% or more, about 12% or more, about 14% or more, about 16% or more, about 18% or more, about 20% or more, about 30% or less, about 26% or less, or about 22% or less, about 20% or less, about 19% or less, about 18% or less, about 17% or less, or about 16% or less. In aspects, the first depth of compression 216 and/or the second depth of compression 218 as a percentage of the substrate thickness 209 can range from about 5% to about 30%, from about 10% to about 26%, from about 12% to about 22%, from about 14% to about 20%, from about 16% to about 19%, from about 16% to about 19%, from about 16% to about 18%, or any range or subrange therebetween. In aspects, the first depth of compression 216 and/or the second depth of compression 218 as a percentage of the substrate thickness 209 can be about 15% or more, for example, in a range from about 16% to about 30%, from about 16% to about 26%, from about 18% to about 24%, from about 20% to about 22%, or any range or subrange therebetween. In exemplary aspects, the first depth of compression 216 and/or the second depth of compression 218 as a percentage of the substrate thickness 209 can be in a range from about 10% to about 30%, from about 12% to about 19%, or from about 16% to about 26%.

[00117] In aspects, the first depth of compression 216 and/or the second depth of compression 218 can be about 1 pm or more, 3 pm or more, about 4 pm or more, about 5 pm or more, about 6 pm or more, about 7 pm or more, about 10 pm or more, about 11 pm or more, about 12 pm or more, about 13 pm or more, about 30 pm or less, about 25 pm or less, about 20 pm or less, about 17 pm or less, about 15 pm or less, about 14 pm or less, about 13 pm or less, about 12 pm or less, about 10 pm or less, about 9 pm or less, about 8 pm or less, or about 7 pm or less. In aspects, the first depth of compression 216 and/or the second depth of compression 218 can be in a range from about 1 pm to about 30 pm, from about 3 pm to about 25 pm, from about 3 pm to about 20 pm, from about 4 pm to about 17 pm, from about 5 pm to about 15 pm, from about 6 pm to about 14 pm, from about 6 pm to about 13 pm, from about 7 pm to about 12 pm, from about 7 pm to about 10 pm, or any range or subrange therebetween. In aspects, the first depth of compression 216 and/or the second depth of compression 218 can be about 10 pm or less, for example, in a range from about 3 pm to about 10 pm, from about 5 pm to about 10 pm, from about 6 pm to about 9 pm, from about 7 pm to about 8 pm, or any range or subrange therebetween. In aspects, the first depth of compression 216 and/or the second depth of compression 218 can be about 10 pm or more, for example, in a range from about 10 pm to about 20 pm, from about 10 pm to about 17 pm, from about 11 pm to about 15 pm, from about 12 pm to about 14 pm, from about 12 pm to about 13 pm, or any range or subrange therebetween. In preferred aspects, the first depth of compression 216 and/or the second depth of compression 218 can be in a range from about 3 pm to about 20 pm, from about 5 pm to about 9 pm, or from about 10 pm to about 15 pm.

[00118] In aspects, the first depth of layer and/or the second depth of layer of one or more alkali metal ions (e.g., potassium) as a percentage of the substrate thickness 209 can be about 5% or more, about 10% or more, about 12% or more, about 14% or more, about 16% or more, about 18% or more, about 20% or more, about 30% or less, about 26% or less, or about 22% or less, about 20% or less, about 19% or less, about 18% or less, about 17% or less, or about 16% or less. In aspects, the first depth of layer and/or the second depth of layer of one or more alkali metal ions (e.g., potassium) as a percentage of the substrate thickness 209 can range from about 5% to about 30%, from about 10% to about 26%, from about 12% to about 22%, from about 14% to about 20%, from about 16% to about 19%, from about 16% to about 19%, from about 16% to about 18%, or any range or subrange therebetween. In aspects, the first depth of layer and/or the second depth of layer of one or more alkali metal ions (e.g., potassium) as a percentage of the substrate thickness 209 can be about 15% or more, for example, in a range from about 16% to about 30%, from about 16% to about 26%, from about 18% to about 24%, from about 20% to about 22%, or any range or subrange therebetween. In preferred aspects, the first depth of layer and/or the second depth of layer of one or more alkali metal ions (e.g., potassium) as a percentage of the substrate thickness 209 can be in a range from about 10% to about 30%, from about 12% to about 19%, or from about 16% to about 26%.

[00119] In aspects, the first depth of layer and/or the second depth of layer of one or more alkali metal ions (e.g., potassium) can be about 1 pm or more, 3 pm or more, about 4 pm or more, about 5 pm or more, about 6 pm or more, about 7 pm or more, about 10 pm or more, about 11 pm or more, about 12 pm or more, about 13 pm or more, about 30 pm or less, about 25 pm or less, about 20 pm or less, about 17 pm or less, about 15 pm or less, about 14 pm or less, about 13 pm or less, about 12 pm or less, about 10 pm or less, about 9 pm or less, about 8 pm or less, or about 7 pm or less. In aspects, the first depth of layer and/or the second depth of layer of one or more alkali metal ions (e.g., potassium) can be in a range from about 1 pm to about 30 pm, from about 3 pm to about 25 pm, from about 3 pm to about 20 pm, from about 4 pm to about 17 pm, from about 5 pm to about 15 pm, from about 6 pm to about 14 pm, from about 6 pm to about 13 pm, from about 7 pm to about 12 pm, from about 7 pm to about 10 pm, or any range or subrange therebetween. In aspects, the first depth of layer and/or the second depth of layer of one or more alkali metal ions (e.g., potassium) can be about 10 pm or less, for example, in a range from about 3 pm to about 10 pm, from about 5 pm to about 10 pm, from about 6 pm to about 9 pm, from about 7 pm to about 8 pm, or any range or subrange therebetween. In aspects, the first depth of compression 216 and/or the second depth of compression 218 can be about 10 pm or more, for example, in a range from about 10 pm to about 20 pm, from about 10 pm to about 17 pm, from about 11 pm to about 15 pm, from about 12 pm to about 14 pm, from about 12 pm to about 13 pm, or any range or subrange therebetween. In preferred aspects, the first depth of layer and/or the second depth of layer of one or more alkali metal ions (e.g., potassium) can be in a range from about 3 pm to about 20 pm, from about 5 pm to about 9 pm, or from about 10 pm to about 15 pm.

[00120] In aspects, the first compressive stress region 212 can comprise a first maximum compressive stress and/or the second compressive stress region 214 can comprise a second maximum compressive stress. In further aspects, the first maximum compressive stress can be substantially equal to the second maximum compressive stress. In further aspects, the first maximum compressive stress and/or second maximum compressive stress can be about 500 MegaPascals (MPa) or more, about 600 MPa or more, about 650 MPa or more, about 700 MPa or more, about 750 MPa or more, about 800 MPa or more, about 850 MPa or more, about 900 MPa or more, about 950 MPa or more, about 1,000 MPa or more, about 1050 MPa or more, about 1,500 MPa or less, about 1,300 MPa or less, about 1,200 MPa or less, about 1,150 MPa or less, about 1,100 MPa or less, about 1,050 MPa or less, about 1,000 MPa or less, about 950 MPa or less, about 900 MPa or less, about 850 MPa or less, or about 800 MPa or less. In further aspects, the first maximum compressive stress and/or second maximum compressive stress can be in a range from about 500 MPa to about 1,500 MPa, from about 600 MPa to about 1,300 MPa, from about 650 MPa to about 1,250 MPa, from about 650 MPa to about 1,200 MPa, from about 700 MPa to about 1,150 MPa, from about 750 MPa to about 1,100 MPa, from about 800 MPa to about 1,050 MPa, from about 850 MPa to about 1,000 MPa, from about 900 MPa to about 950 MPa, or any range or subrange therebetween. In further aspects, the first maximum compressive stress and/or second maximum compressive stress can be about 700 MPa or more, for example, in a range from about 700 MPa to about 1,500 MPa, from about 700 MPa to about 1,300 MPa, from about 700 MPa to about 1,200 MPa, from about 750 MPa to about 1,150 MPa, from about 800 MPa to about 1,100 MPa, from about 850 MPa to about 1,100 MPa, from about 900 MPa to about 1,050 MPa, or any range or subrange therebetween. In further aspects, the first maximum compressive stress and/or second maximum compressive stress can be about 1,000 MPa or more, for example, in a range from about 1,000 MPa to about 1,500 MPa, from about 1,000 MPa to about 1,300 MPa, from about 1,000 MPa to about 1,200 MPa, from about 1,050 MPa to about 1,150 MPa, or any range or subrange therebetween. In preferred aspects, the first maximum compressive stress and/or second maximum compressive stress can be in a range from about 650 MPa to about 1,200 MPa, from about 750 MPa to about 1,100 MPa, or from about 850 MPa to about 1,200 MPa.

[00121] In aspects, the substrate thickness 209 can be about 50 pm or more (e.g., from about 50 pm to about 100 pm, from about 50 pm to about 90 pm, or any of the corresponding subranges therebetween discussed above) and associated with one or more of: (1) a depth of compression (e.g., first depth of compression 216 and/or the second depth of compression 218) as a percentage of the substrate thickness 209 from about 10% to about 30%, from about 16% to about 26%, or any of the corresponding subranges therebetween discussed above, (2) a depth of layer (e.g., first depth of layer and/or second depth of layer) of potassium in a range from about from about 3 pm to about 20 pm, from about 10 pm to about 15 pm, or any corresponding subrange discussed above, and/or (3) a maximum compressive stress (e.g., first maximum compressive stress and/or second maximum compressive stress) can be in a range from about 650 MPa to about 1,200 MPa, from about 800 MPa to about 1,100 MPa, from about 850 MPa to about 1,200 MPa, or any corresponding subrange discussed above. In aspects, the substrate thickness 209 can be about 50 pm or less (e.g., from about 10 pm to about 50 pm, from about 10 pm to about 30 pm, or any of the corresponding subranges therebetween discussed above) and associated with one or more of: (1) a depth of compression (e.g., first depth of compression 216 and/or the second depth of compression 218) as a percentage of the substrate thickness 209 from about 10% to about 30%, from about 12% to about 19%, or any of the corresponding subranges therebetween discussed above, (2) a depth of layer (e.g., first depth of layer and/or second depth of layer) of potassium in a range from about from about 3 pm to about 20 pm, from about 5 pm to about 9 pm, or any corresponding subrange discussed above, and/or (3) a maximum compressive stress (e.g., first maximum compressive stress and/or second maximum compressive stress) can be in a range from about 650 MPa to about 1,200 MPa, from about 750 MPa to about 1,100 MPa, from about 750 MPa to about 1,000 MPa, or any corresponding subrange discussed above.

[00122] Throughout the disclosure, a tensile strength, ultimate elongation (e.g., strain at failure), and yield point of a polymeric material (e.g., adhesive, polymer-based portion) is determined using ASTM D638 using a tensile testing machine, for example, an Instron 3400 or Instron 6800, at 23°C and 50% relative humidity with a type I dogbone shaped sample. Throughout the disclosure, an elastic modulus (e.g., Young’s modulus) and/or a Poisson’s ratio is measured using ISO 527- 1 :2019. Throughout the disclosure, the Young’s modulus of the glass-based materials and ceramic-based materials are measured using the resonant ultrasonic spectroscopy technique set forth in ASTM E2001-13, titled “Standard Guide for Resonant Ultrasound Spectroscopy for Defect Detection in Both Metallic and Non-metallic Parts.” In aspects, the foldable substrate 201 can comprise an elastic modulus of about 10 GigaPascal (GPa) or more, about 40 GPa or more, about 60GPa or more, about 70 GPa or more, about 100 GPa or less, about 80 GPa or less, about 60 GPa or less, or about 20 GPa or less. In further aspects, the foldable substrate 201 can comprise a glass-based portion comprising an elastic modulus ranging from about 10 GPa to about 100 GPa, from about 40 GPa to about 100 GPa, from about 60 GPa to about 100 GPa, from about 80 GPa to about 100 GPa, or any range or subrange therebetween.

[00123] The transmittance and haze values reported herein are measured using a BYK Haze-Gard Dual (BYK Gardner). In aspects, an “optically transparent material” or an “optically clear material” can have an average transmittance of 75% or more, 80% or more, 85% or more, or 90% or more, 92% or more, 93% or more, 94% or more, 95% or more, 96% or more in the wavelength range of 400 nm to 700 nm through a 0.7 mm thick piece of the material. The average transmittance in the wavelength range of 400 nm to 700 nm is calculated by measuring the transmittance of whole number wavelengths from about 400 nm to about 700 nm and averaging the measurements. In aspects, the foldable substrate can be optically transparent. In aspects, the foldable substrate 201 can comprise an average transmittance (averaged over optical wavelengths from 400 nm to 700 nm) of about 80% or more, about 90% or more, about 91% or more, about 92.0% or more, about 92.2% or more, about 92.5% or more, about 92.8% or more, about 93.0% or more, about 99% or less, about 96% or less, about 95% or less, or about 94% or less. In aspects, the foldable substrate 201 can comprise an average transmittance (averaged over optical wavelengths from 400 nm to 700 nm) can be in a range from about 80% to about 99%, from about 90% to about 96%, from about 90% to about 95%, from about 91% to about 95%, from about 92.0% to about 95%, from about 92.2% to about 94%, from about 92.5% to about 94%, from about 92.8% to about 93%, or any range or subrange therebetween.

[00124] As used herein, haze refers to transmission haze that is measured through the first major surface 203 in accordance with ASTM DI 003 -21 at 0° relative to a direction normal to the first major surface 203. Haze is measured using a BYK Haze-Gard Dual (BYK Gardner). A CIE D65 illuminant is used as the light source for illuminating the foldable substrate 201. Haze values reported herein are measured through a substrate comprising a thickness of 0.7 mm with the light incident on the first major surface 203 being measured as it exits the first major surface 203. In further aspects, the haze of the foldable substrate 201 can be about 5% or less, about 2% or less, about 1% or less, about 0.8% or less, about 0.5% or less, or about 0.3% or less. In further aspects, the haze of the foldable substrate 201 can be in a range from about 0.01% to about 5%, from about 0.05% to about 2%, from about 0.1% to about 1%, from about 0.1% to about 0.8%, from about 0.2% to about 0.5%, or any range or subrange therebetween.

[00125] As shown in FIGS. 2-3, the foldable substrate 201 can comprise a first major surface 203 and a second major surface 205 opposite the first major surface 203. In aspects, the first major surface 203 can extend along a first plane, and/or the second major surface 205 can extend along a second plane. In further aspects, the second plane (second major surface 205) can be parallel to the first plane (first major surface 203). As used herein, a substrate thickness 209 of the foldable substrate 201 is defined between the first major surface 203 and the second major surface 205 as an average distance therebetween. In aspects, the foldable substrate 201 can be an ultra-thin substrate, meaning that the substrate thickness 209 is about 100 micrometers or less. In aspects, the substrate thickness 209 can be about 10 micrometers (pm) or more, about 15 pm or more, about 20 pm or more, about 25 pm or more, about 30 pm or more, about 40 pm or more, about 50 pm or more, about 60 pm or more, about 70 pm or more, about 100 pm or less, about 95 pm or less, about 90 pm or less, about 85 pm or less, about 80 pm or less, about 75 pm or less, about 70 pm or less, about 60 pm or less, about 50 pm or less, about 40 pm or less, about 30 pm or less, or about 25 pm or less. In aspects, the substrate thickness 209 can range from about 10 pm to about 100 pm, from about 15 pm to about 95 pm, from about 20 pm to about 80, from about 25 pm to 75 pm, from about 30 pm to about 70 pm, from about 40 pm to about 60 pm, from about 40 pm to about 50 pm, or any range or subrange therebetween. In aspects, the substrate thickness 209 can be about 50 pm or more, which can exhibit greater impact resistance and/or puncture resistance than even thinner foldable substrates and reasonable foldability (e.g., at a parallel plate distance of 5 mm - discussed below), for example in a range from about 50 pm to about 100 pm, from about 50 pm to about 95 pm, from about 50 pm to about 90 pm, from about 60 pm to about 80 pm, from about 70 pm to about 75 pm, or any range or subrange therebetween. In aspects, the substrate thickness 209 can be about 50 pm or less, which can exhibit increased foldability (e.g., at a parallel plate distance of 3 mm or less or 2 mm or less - discussed below) than thicker substrates, for example, in a range from about 10 pm to about 50 pm, from about 15 pm to about 50 pm, from about 20 pm to about 45 pm, from about 25 pm to about 40 pm, from about 30 pm to about 40 pm, or any range or subrange therebetween. In aspects, as shown, a local thickness of the foldable substrate 201 can be substantially uniform (e.g., substantially equal to the substrate thickness 209) across the first major surface 203 and/or the second major surface 205.

[00126] As used herein, if a first layer and/or component is described as “disposed over” a second layer and/or component, other layers may or may not be present between the first layer and/or component and the second layer and/or component. Furthermore, as used herein, “disposed over” does not refer to a relative position with reference to gravity. For example, a first layer and/or component can be considered “disposed over” a second layer and/or component, for example, when the first layer and/or component is positioned underneath, above, or to one side of a second layer and/or component. As used herein, a first layer and/or component described as “bonded to” a second layer and/or component means that the layers and/or components are bonded to each other, either by direct contact and/or bonding between the two layers and/or components or via an adhesive layer. As used herein, a first layer and/or component described as “contacting” or “in contact with” a second layer and/or components refers to direct contact and includes the situations where the layers and/or components are bonded to each other. [00127] As shown in FIG. 3, the foldable apparatus 301 can comprise an adhesive layer 311. As shown, the adhesive layer 311 can comprise a first contact surface 313 and a second contact surface 315 that can be opposite the first contact surface 313. In aspects, as shown, the first contact surface 313 of the adhesive layer 311 can comprise a planar surface, and/or the second contact surface 315 of the adhesive layer 311 can comprise a planar surface. An adhesive thickness 319 of the adhesive layer 311 can be defined between the first contact surface 313 and the second contact surface 315 as the average distance therebetween. In aspects, the adhesive thickness 319 of the adhesive layer 311 can be about 1 pm or more, about 5 pm or more, about 10 pm or more, about 100 pm or less, about 60 pm or less, about 30 pm or less, or about 20 pm or less. In aspects, the adhesive thickness 319 of the adhesive layer 311 can range from about 1 pm to about 100 pm, from about 5 pm to about 60 pm, from about 10 pm to about 30 pm, from about 10 pm to about 20 pm, or any range or subrange therebetween. In aspects, as shown in FIG. 3, the first contact surface 313 of the adhesive layer 311 can face and/or contact the first major surface 203 of the foldable substrate 201. In aspects, as shown in FIG. 3, the second contact surface 315 of the adhesive layer 311 can face and/or contact another layer (e.g., PET sheet 321 discussed below).

[00128] In aspects, the adhesive layer 311 can comprise one or more of a polyolefin, a polyamide, a halide-containing polymer (e.g., polyvinylchloride or a fluorine-containing polymer), an elastomer, a urethane, phenolic resin, parylene, polyethylene terephthalate (PET), and polyether ether ketone (PEEK). Example aspects of polyolefins include low molecular weight polyethylene (LDPE), high molecular weight polyethylene (HDPE), ultrahigh molecular weight polyethylene (UHMWPE), and polypropylene (PP). Example aspects of fluorine-containing polymers include polytetrafluoroethylene (PTFE), polyvinylfluoride (PVF), polyvinylidene fluoride (PVDF), perfluoropolyether (PFPE), perfluorosulfonic acid (PFSA), a perfluoroalkoxy (PF A), fluorinated ethylene propylene (FEP) polymers, and ethylene tetrafluoro ethylene (ETFE) polymers. Example aspects of elastomers include rubbers (e.g., polybutadiene, polyisoprene, chloroprene rubber, butyl rubber, nitrile rubber), and block copolymers (e.g., styrene-butadiene, high-impact polystyrene, poly(dichlorophosphazene). In further aspects, the adhesive layer 311 can comprise an optically clear adhesive. In even further aspects, the optically clear adhesive can comprise one or more optically transparent polymers: an acrylic (e.g., polymethylmethacrylate (PMMA)), an epoxy, silicone, and/or a polyurethane. Examples of epoxies include bisphenol-based epoxy resins, novolac-based epoxies, cycloaliphatic-based epoxies, and glycidylamine-based epoxies. In even further aspects, the optically clear adhesive can comprise, but is not limited to acrylic adhesives, for example, 3M 8212 adhesive, or an optically transparent liquid adhesive, for example, a LOCTITE optically transparent liquid adhesive. Exemplary aspects of optically clear adhesives comprise transparent acrylics, epoxies, silicones, and polyurethanes. For example, the optically transparent liquid adhesive could comprise one or more of LOCTITE AD 8650, LOCTITE AA 3922, LOCTITE EA E- 05MR, LOCTITE UK U-09LV, which are all available from Henkel.

[00129] In aspects, although not shown, a coating can be disposed over the second major surface 205 of the foldable substrate 201. In even further aspects, a coating thickness of the coating can be about 0.1 pm or more, about 1 pm or more, about 5 pm or more, about 10 pm or more, about 15 pm or more, about 20 pm or more, about 25 pm or more, about 40 pm or more, about 50 pm or more, about 60 pm or more, about 70 pm or more, about 80 pm or more, about 90 pm or more, about 200 pm or less, about 100 pm or less, or about 50 pm or less, about 30 pm or less, about 25 pm or less, about 20 pm or less, about 20 pm or less, about 15 pm or less, or about 10 pm or less. In further aspects, the coating thickness of the coating can range from about 0.1 pm to about 200 pm, from about 1 pm to about 100 pm, from about 10 pm to about 100 pm, from about 20 pm to about 100 pm, from about 30 pm to about 100 pm, from about 40 pm to about 100 pm, from about 50 pm to about 100 pm, from about 60 pm to about 100 pm, from about 70 pm to about 100 pm, from about 80 pm to about 100 pm, from about 90 pm to about 100 pm, from about 0.1 pm to about 50 pm, from about 1 pm to about 50 pm, from about 10 pm to about 50 pm, or any range or subrange therebetween.

[00130] In aspects, the coating can comprise a polymeric coating. In further aspects, the polymeric coating can comprise one or more of an ethylene-acid copolymer, a polyurethane-based polymer, an acrylate resin, and a mercapto-ester resin. Example aspects of ethylene-acid copolymers include ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, and ethylene-acrylic-methacrylic acid terpolymers (e.g., Nucrel (DuPont)), ionomers of ethylene acid copolymers (e.g., Surlyn (DuPont)), and ethylene-acrylic acid copolymer amine dispersions (e.g., Aquacer (BYK)). Example aspects of polyurethane-based polymers include aqueous modified polyurethane dispersions (e.g., Eleglas (Axalta)). Example aspects of acrylate resins which can be UV curable include acrylate resins (e.g., Uvekol resin (Allinex)), cyanoacrylate adhesives (e.g., Permabond UV620 (Krayden)), and UV radical acrylic resins (e.g., Ultrabond windshield repair resin, for example, Ultrabond (45CPS)). Example aspects of mercapto-ester resins include mercapto-ester triallyl isocyanurates (e.g., Norland optical adhesive NOA 61). In further aspects, the polymeric coating can comprise ethylene-acrylic acid copolymers and ethylenemethacrylic acid copolymers, which may be ionomerized to form ionomer resins through neutralization of the carboxylic acid residue with typically alkali metal ions, for example sodium, and potassium and also zinc. Such ethylene-acrylic acid and ethylene-methacrylic acid ionomers may be dispersed within water and coated onto the substrate to form an ionomer coating. Alternatively, such acid copolymers may be neutralized with ammonia which, after coating and drying liberates the ammonia to reform the acid copolymer as the coating. By providing a coating comprising a polymeric coating, the foldable apparatus can comprise low energy fracture.

[00131] In aspects, the coating can comprise a polymeric coating comprising an optically transparent polymeric coating layer. Suitable materials for an optically transparent polymeric coating layer include, but are not limited to: a cured acrylate resin material, an inorganic-organic hybrid polymeric material, an aliphatic or aromatic hexafunctional urethane acrylate, a siloxane-based hybrid material, and a nanocomposite material, for example an epoxy and urethane material with nanosilicate. In aspects, an optically transparent polymeric coating layer may consist essentially of one or more of these materials. In aspects, an optically transparent polymeric coating layer may consist of one or more of these materials. As used herein, “inorganic-organic hybrid polymeric material” means a polymeric material comprising monomers with inorganic and organic components. An inorganic-organic hybrid polymer is obtained by a polymerization reaction between monomers having an inorganic group and an organic group. An inorganic-organic hybrid polymer is not a nanocomposite material comprising separate inorganic and organic constituents or phases, for example inorganic particulate dispersed within an organic matrix. More specifically, suitable materials for an optically transparent polymeric (OTP) coating layer include, but are not limited to, a polyimide, a polyethylene terephthalate (PET), a polycarbonate (PC), a poly methyl methacrylate (PMMA), organic polymer materials, inorganic-organic hybrid polymeric materials, and aliphatic or aromatic hexafunctional urethane acrylates. In aspects, an OTP coating layer may consist essentially of an organic polymer material, an inorganic-organic hybrid polymeric material, or aliphatic or aromatic hexafunctional urethane acrylate. In aspects, an OTP coating layer may consist of a polyimide, an organic polymer material, an inorganic- organic hybrid polymeric material, or aliphatic or aromatic hexafunctional urethane acrylate. In aspects, an OTP coating layer may include a nanocomposite material. In aspects, an OTP coating layer may include a nano-silicate at least one of epoxy and urethane materials. Suitable compositions for such an OTP coating layer are described in U.S. Pat. Pub. No. 2015/0110990, which is hereby incorporated by reference in its entirety by reference thereto. As used herein, “organic polymer material” means a polymeric material comprising monomers with only organic components. In aspects, an OTP coating layer may comprise an organic polymer material manufactured by Gunze Limited and having a hardness of 9H, for example Gunze’s “Highly Durable Transparent Film.” As used herein, “inorganic-organic hybrid polymeric material” means a polymeric material comprising monomers with inorganic and organic components. An inorganic-organic hybrid polymer is obtained by a polymerization reaction between monomers having an inorganic group and an organic group. An inorganic-organic hybrid polymer is not a nanocomposite material comprising separate inorganic and organic constituents or phases, for example inorganic particulate dispersed within an organic matrix. In aspects, the inorganic-organic hybrid polymeric material may include polymerized monomers comprising an inorganic silicon-based group, for example, a silsesquioxane polymer. A silsesquioxane polymer may be, for example, an alky-silsesquioxane, an aryl- silsesquioxane, or an aryl alkyl-silsesquioxane having the following chemical structure: (RSiOi.s)n, where R is an organic group for example, but not limited to, methyl or phenyl. In aspects, an OTP coating layer may comprise a silsesquioxane polymer combined with an organic matrix, for example, SILPLUS manufactured by Nippon Steel Chemical Co., Ltd. In aspects, an OTP coating layer may comprise from 90 wt% to 95wt% aromatic hexafunctional urethane acrylate (e.g., PU662NT (Aromatic hexafunctional urethane acrylate) manufactured by Miwon Specialty Chemical Co.) and 10 wt% to 5 wt% photo-initiator (e.g., Darocur 1173 manufactured by Ciba Specialty Chemicals Corporation) with a hardness of 8H or more. In aspects, an OTP coating layer composed of an aliphatic or aromatic hexafunctional urethane acrylate may be formed as a stand-alone layer by spin-coating the layer on a polyethylene terephthalate (PET) substrate, curing the urethane acrylate, and removing the urethane acrylate layer from the PET substrate. An OTP coating layer may have a coating thickness ranging of 1 pm to 150 pm, including subranges; for example, from 10 pm to 140 pm, from 20 pm to 130 pm, 30 pm to 120 pm, from 40 pm to 110 pm, from 50 pm to 100 pm, from 60 pm to 90 pm, from 70 pm to 80 pm, or any range or subrange therebetween. In aspects, an OTP coating layer may be a single monolithic layer. In aspects, an OTP coating layer may be an inorganic-organic hybrid polymeric material layer or an organic polymer material layer having a thickness in the range of 80 pm to 120 pm, including subranges. For example, an OTP coating layer comprising an inorganic-organic hybrid polymeric material or an organic polymer material may have a thickness of from 80 pm to 110 pm, 90 pm to 100 pm, or any range or subrange therebetween. In aspects, an OTP coating layer may be an aliphatic or aromatic hexafunctional urethane acrylate material layer having a thickness within one or more of the thickness ranges discussed above in this paragraph or for the coating thickness.

[00132] In aspects, the coating, if provided, may also comprise one or more of an easy-to-clean coating, a low-friction coating, an oleophobic coating, a diamondlike coating, a scratch-resistant coating, or an abrasion-resistant coating. A scratchresistant coating may comprise an oxynitride, for example, aluminum oxynitride or silicon oxynitride with a thickness of about 500 micrometers or more. In such aspects, the abrasion-resistant layer may comprise the same material as the scratch-resistant layer. In aspects, a low friction coating may comprise a highly fluorinated silane coupling agent, for example, an alkyl fluorosilane with oxymethyl groups pendant on the silicon atom. In such aspects, an easy-to-clean coating may comprise the same material as the low friction coating. In other aspects, the easy-to-clean coating may comprise a protonatable group, for example an amine, for example, an alkyl aminosilane with oxymethyl groups pendant on the silicon atom. In such aspects, the oleophobic coating may comprise the same material as the easy-to-clean coating. In aspects, a diamond-like coating comprises carbon and may be created by applying a high voltage potential in the presence of a hydrocarbon plasma.

[00133] In aspects, as shown in FIG. 3, a layer (e.g., PET sheet 321) can be disposed over the first major surface 203 of the foldable substrate 201, and/or the layer (e.g., PET sheet 321) can be attached to the foldable substrate 201 by the adhesive layer 311. In further aspects, the layer (e.g., PET sheet 321) can be disposed over and/or contact the second contact surface 315 of the adhesive layer 311. In further aspects, as shown, a first surface area 323 of the PET sheet 321 can face the first major surface 203 of the foldable substrate 201, face the second contact surface 315 of the adhesive layer 311, and/or contact the second contact surface 315 of the adhesive layer 311. A thickness 329 of the PET sheet 321 is defined as an average distance between the first surface area 323 and a second surface area 325 opposite the first surface area 323. As discussed below with reference to the Pen Drop Test, the adhesive thickness 319 of the adhesive layer 311 (e.g., Optically Clear Adhesive 8212 available from 3M) can be 50 pm and a thickness 329 of the PET sheet 321 can be 100 pm, although other materials and/or thicknesses are possible in other aspects of the foldable apparatus. For example, in other aspects, the layer (e.g., PET sheet) can comprise a polymeric material (not limited to PET) such as polyesters (e.g., polyethylene terephthalate (PET)) and polyolefins (e.g., low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP)). Also, in other aspects, the layer (e.g., PET sheet 321) can be replaced with a release liner, which can comprise a paper and/or a polymer. Exemplary aspects of paper comprise kraft paper, machine-finished paper, poly-coated paper (e.g., polymer coated, glassine paper, siliconized paper), or clay-coated paper. Additionally or alternatively, the layer can include and/or comprise a display device, for example, a liquid crystal display (LCD), an electrophoretic display (EPD), an organic light-emitting diode (OLED) display, or a plasma display panel (PDP). The display device can be part of a portable electronic device, for example, a consumer electronic product, a smartphone, a tablet, a wearable device, or a laptop.

[00134] Aspects of the disclosure can comprise a consumer electronic product. The consumer electronic product can comprise a front surface, a back surface, and side surfaces. The consumer electronic product can further comprise electrical components at least partially within the housing. The electrical components can comprise a controller, a memory, and a display. The display can be at or adjacent to the front surface of the housing. The display can comprise a liquid crystal display (LCD), an electrophoretic display (EPD), an organic light-emitting diode (OLED) display, or a plasma display panel (PDP). The consumer electronic product can comprise a cover substrate disposed over the display. In aspects, at least one of a portion of the housing or the cover substrate comprises the foldable apparatus discussed throughout the disclosure. The consumer electronic product can comprise a portable electronic device, for example, a smartphone, a tablet, a wearable device, or a laptop.

[00135] The foldable apparatus disclosed herein may be incorporated into another article, for example, an article with a display (or display articles) (e.g., consumer electronics, including mobile phones, tablets, computers, navigation systems, wearable devices (e.g., watches), and the like), architectural articles, transportation articles (e.g., automotive, trains, aircraft, sea craft, etc.), appliance articles, or any article that may benefit from some transparency, scratch-resistance, abrasion resistance or a combination thereof. An exemplary article incorporating any of the foldable apparatus 101 and/or 301 and/or foldable substrate 201 disclosed herein is shown in FIGS. 8-9. Specifically, FIGS. 8-9 show a consumer electronic device 800 including a housing 802 having front 804, back 806, and side surfaces 808. Although not shown, the consumer electronic device can comprise electrical components that are at least partially inside or entirely within the housing. For example, electrical components include at least a controller, a memory, and a display. As shown in FIGS. 8-9, the display 810 can be at or adjacent to the front surface of the housing 802. The consumer electronic device can comprise a cover substrate 812 at or over the front surface of the housing 802 such that it is over the display 810. In aspects, at least one of the cover substrate 812 or a portion of housing 802 may include any of the foldable apparatus disclosed herein, for example, the foldable substrate 201.

[00136] Also, FIG. 7 schematically shows a perspective view of a consumer electronic product 701 that is foldable. The consumer electronic product 701 can include the foldable apparatus 101 and/or 301 and/or the foldable substrate 201 in accordance with aspects of the present disclosure. As shown, the consumer electronic product 701 can include a front surface 703 and a side surface 705. The consumer electronic product 701 can include electronic components, including a display 702 that can be viewed through the front surface 703. In aspects, as shown, the consumer electronic product 701 can be folded in a direction 712 to form a folded configuration that brings a first end 727 and a second end 737 (opposite the first end 727) closer together (than in the unfolded configuration). Additionally, as shown, the consumer electronic product 701 can be folded so that the front surface 703 and/or display 702 faces itself, although the consumer electronic product could be folded opposite the direction 712 so that the front surface 703 is on the outside of the consumer electronic product in the folded configuration. The consumer electronic product 701 shown in FIG. 15 can be folded about the fold axis 102, where a central portion 781 is located between a first portion 721 including the first end 727 and a second portion 731 including the second end 737. A location of the fold axis 102 can determine a first distance 713 between the first end 727 and the fold axis 102 (e.g., in direction 106) relative to a second distance 715 between the second end 737 and the fold axis 102 (e.g., in direction 708). A total length of the consumer electronic product (e.g., length 105 in FIG. 1) can be the sum of the first distance 713 and the second distance 715). Also, as shown, the consumer electronic product is depicted as being in a folded or partially folded configuration with an angle A formed by front surface 703 about the fold axis 102.

[00137] Throughout the disclosure, refractive index is measured in accordance with ASTM E1967-19, where the first wavelength comprises 589 nm. In aspects, the first refractive index of the foldable substrate 201 may be about 1.4 or more, about 1.45 or more, about 1.48 or more, about 1.49 or more, about 1.50 or more, about 1.6 or less, about 1.57 or less, or about 1.55 or less, about 1.53 or less, or about 1.52 or less. In aspects, the first refractive index of the foldable substrate 201 can range from about 1.4 to about 1.6, from about 1.45 to about 1.57, from about 1.48 to about 1.55, from about 1.49 to about 1.53, from about 1.50 to about 1.52, or any range or subrange therebetween.

[00138] In aspects, the adhesive layer 311 can comprise a second refractive index within one or more of the ranges discussed above with regards to the first refractive index of foldable substrate. In aspects, a differential equal to the absolute value of the difference between the second refractive index of the adhesive layer 311 and the first refraction index of the foldable substrate 201 can be about 0.1 or less, about 0.07 or less, about 0.05 or less, about 0.001 or more, about 0.01 or more, or about 0.02 or more. In aspects, the differential can range from about 0.001 to about 0.1, from about 0.001 to about 0.07, from about 0.01 to about 0.07, from about 0.01 to about 0.05, from about 0.02 to about 0.05, or any range or subrange therebetween. In aspects, the second refractive index of the adhesive layer 311 may be greater than the first refractive index of the foldable substrate 201. In aspects, the second refractive index of the adhesive layer 311 may be less than the first refractive index of the foldable substrate 201. [00139] FIG. 5 schematically illustrates aspects of the foldable apparatus 401 comprising and/or consisting of the foldable substrate 201 in accordance with aspects of the disclosure in a folded configuration. As shown in FIG. 4, the foldable apparatus 401 is folded such that the second major surface 205 of the foldable substrate 201 is on the outside of the foldable apparatus 401 and the first major surface 203 is on the inside of the foldable apparatus 401. In the folded configuration although not shown, if a display device was positioned on the inside of the bend, a user would view the display device through the foldable substrate 201 and, thus, would be positioned on the side of the second major surface 205. Alternatively, if the display device was positioned on the outside of the bend, a user would view the display device through the foldable substrate 201 and, thus, would be positioned on the side of the first major surface 203. Alternatively, although not shown, the foldable apparatus can be folded such that the first major surface of the foldable substrate is on the outside of the folded foldable apparatus, where a user would view the display device through the foldable substrate and, thus, would be positioned opposite the display device.

[00140] As used herein, “foldable” includes complete folding, partial folding, bending, flexing, or multiple capabilities. As used herein, the terms “fail,” “failure,” and the like refer to breakage, destruction, delamination, or crack propagation. A foldable apparatus achieves a parallel plat distance of “X,” or withstands a parallel plat distance of “X”, has a parallel plat distance of “X,” or comprises parallel plate distance of “X” if it resists failure when the foldable apparatus is held at parallel plate distance of “X” for 10 minutes at about 25°C and about 50% relative humidity. Likewise, a foldable apparatus achieves a parallel plate distance of “X,” or has a parallel plate distance of “X,” or comprises a parallel plate distance of “X” if it resists failure when the foldable apparatus is held at a parallel plate distance of “X” for 10 minutes at about 50°C and about 50% relative humidity. In aspects, the foldable substrate and/or the foldable apparatus can be rollable. As used herein, a foldable substrate or a foldable apparatus is “rollable” if it can achieve a threshold parallel plate distance over a length of the corresponding foldable substrate and/or foldable apparatus that is the greater of 10 mm or 10% of the length of the corresponding foldable substrate and/or foldable apparatus. Throughout the disclosure, the “survival rate” or % of samples that can withstand a parallel plate distance of X mm refers to the percentage of at least 20 samples that withstand bending to the parallel distance of X mm.

[00141] As used herein, the “parallel plate distance” of a foldable apparatus and/or foldable substrate is measured with the following test configuration and process using a parallel plate apparatus 501 (see FIG. 5) that comprises a pair of parallel rigid stainless-steel plates 503 and 505 comprising a first rigid stainless-steel plate 503 and a second rigid stainless-steel plate 505. When measuring the “parallel plate distance”, the foldable apparatus or foldable substrate is placed between the pair of parallel rigid stainless-steel plates 503 and 505 as is (without modification). For example, as shown in FIG. 4, the foldable apparatus 101 shown in FIG. 2 consisting of the foldable substrate 201 is placed between the pair of parallel rigid stainless-steel plates 503 and 505 without modification with the second major surface 205 of the foldable substrate 201 contacting the pair of parallel rigid stainless-steel plates 503 and 505 as the foldable apparatus 401. For determining a “parallel plate distance”, the distance between the parallel plates is reduced at a rate of 1 millimeter per second (mm/sec) until the parallel plate distance 511 is equal to the “parallel plate distance” to be tested. Then, the parallel plates are held at the “parallel plate distance” to be tested for 10 minutes at about 85°C and about 85% relative humidity. As used herein, the “minimum parallel plate distance” is the smallest parallel plate distance that the foldable apparatus can withstand without failure under the conditions and configuration described above.

[00142] In aspects, the foldable apparatus 101, 301, and/or 401 and/or the foldable substrate 201 can achieve a parallel plate distance of 20 mm or less, 10 mm or less, 7 mm or less, 5 mm or less, 3 mm or less, 2 mm or less, or 1 mm or less. In aspects, the foldable apparatus 101, 301, and/or 401 and/or the foldable substrate 201 can comprise a minimum parallel plate distance of about 5 mm or less, 4 mm or less, about 3 mm or less, about 2 mm or less, or about 1 mm or less. In aspects, the foldable apparatus 101, 301, and/or 401 and/or the foldable substrate 201 can comprise a minimum parallel plate distance ranging from about 0.5 mm to about 5 mm, from about 0.5 mm to about 4 mm, from about 0.5 mm to about 3 mm, from about 0.5 mm to about 2 mm, from about 1 mm to about 2 mm, or any range or subrange therebetween.

[00143] In aspects, the foldable substrate 201 can exhibit a survival rate of about 90% or more, about 92% or more, about 95% or more, about 97% or more, about 98% or more, about 99% or more, or about 100% at parallel plate distance of 5 mm (i.e., about 90% or more, about 92% or more, about 95% or more, etc. of samples of the foldable substrate can withstand a parallel plate distance of 5 mm). In aspects, the foldable substrate 201 can exhibit a survival rate of about 90% or more, about 92% or more, about 95% or more, about 97% or more, about 98% or more, about 99% or more, or about 100% at parallel plate distance of 3 mm (i.e., about 90% or more, about 92% or more, about 95% or more, etc. of samples of the foldable substrate can withstand a parallel plate distance of 3 mm). In aspects, the foldable substrate 201 can exhibit a survival rate of about 30% or more, about 35% or more, about 40% more, 50% or more, about 55% or more, about 60% or more, about 65% or more, about 70% or more, about 75% or more, about 80% or more, about 85% or more, about 90% or more, about 95% or more, or about 100% at parallel plate distance of 3 mm (i.e., about 30% or more, about 35% or more, about 40% or more, etc. of samples of the foldable substrate can withstand a parallel plate distance of 3 mm). In aspects, the foldable substrate 201 can exhibit a survival rate of about 90% or more, about 92% or more, about 95% or more, about 97% or more, about 98% or more, about 99% or more, or about 100% at parallel plate distance of 2 mm (i.e., about 90% or more, about 92% or more, about 95% or more, etc. of samples of the foldable substrate can withstand a parallel plate distance of 2 mm). In aspects, the foldable substrate 201 can exhibit a survival rate of about 10% or more, about 15% or more, about 20% or more, about 25% or more, about 30% or more, about 40% or more, or about 50% at parallel plate distance of 1 mm (i.e., about 10% or more, about 15% or more, about 20% or more, etc. of samples of the foldable substrate can withstand a parallel plate distance of 1 mm).

[00144] In aspects, when the foldable substrate 201 comprises a substrate thickness 209 of about 50 pm or more (e.g., from about 50 pm to about 100 pm, from about 50 pm to about 90 pm, or any of the corresponding subranges discussed above) can exhibit (1) a survival rate of about 90% or more, 92% or more, and/or 95% or more at a parallel plate distance of 5 mm (e.g., from about 90% to about 100%, from about 92% to about 99%, from about 95% to about 97%) and/or (2) a survival rate of about 30% or more, about 35% or more, or about 40% or more at a parallel plate distance of 3 mm (e.g., from about 30% to about 50%, from about 35% to about 45%). In aspects, when the foldable substrate 201 comprises a substrate thickness 209 of about 50 pm or less (e.g., from about 10 pm to about 50 pm, from about 10 pm to about 30 pm, or any of the corresponding subranges discussed above) can exhibit (1) a survival rate of about 90% or more, 92% or more, 95% or more, 97% or more, 98% or more, or 99% or more at a parallel plate distance of 5 mm (e.g., from about 90% to about 100%, from about 92% to 100%, or from about 95% to about 99%), (2) a survival rate of about 80% or more, about 90% or more, or about 95% or more at a parallel plate distance of 3 mm (e.g., from about 80% to about 100%, from about 90% to 100%, or from about 95% to about 99%), (3) a survival rate of about 90% or more, about 92% or more, about 95% or more, or about 97% or more at a parallel plate distance of 2 mm (e.g., from about 90% to about 100%, from about 92% to 100%, from about 95% to about 99%, or from about 97% to about 99%), and/or (4) a survival rate of about 10% or more, about 15% or more, or about 20% or more at a parallel plate distance of 1 mm (e.g., from about 10% to about 50%, from about 15% to about 40%, from about 20% to about 30%).

[00145] The foldable apparatus and/or the foldable substrate may have an impact resistance defined by the capability of a region of the foldable apparatus and/or foldable substrate to avoid failure at a pen drop height (e.g., 5 centimeters (cm) or more, 10 centimeters or more, 20 cm or more), when measured according to the “Pen Drop Test.” As used herein, the “Pen Drop Test” is conducted such that samples of foldable apparatus and/or foldable substrate are tested with the load (i.e., from a pen dropped from a certain height) imparted to a major surface (e.g., second major surface 205 of the foldable substrate 201 and/or foldable apparatus 101 and/or 301) with the foldable substrate 201 configured as shown in FIG. 3 when PET sheet 321 with a thickness 329 100 pm attached to the adhesive layer 311 consisting of Optically Clear Adhesive 8212 (available from 3M) having a thickness of 50 pm attached to and contacting the first major surface 203 of the foldable substrate 201. As such, the PET sheet in the Pen Drop Test is meant to simulate a foldable electronic display device (e.g., an OLED device). During testing, the foldable substrate 201 bonded to the PET sheet is placed on an aluminum plate (6063 aluminum alloy, as polished to a surface roughness with 400 grit paper) with the PET sheet 321 in contact with the aluminum plate. No tape is used on the side of the sample resting on the aluminum plate.

[00146] A tube is used for the Pen Drop Test to guide a pen to an outer surface of the foldable apparatus. For the foldable apparatus 101, 301, and/or 401 and/or the foldable substrate 201 shown in FIGS. 2-3 and 5 (modified as described in the previous paragraph), the pen is guided to the second major surface 205 of the foldable substrate 201, and the tube is placed in contact with the second major surface 205 of the foldable substrate 201 so that the longitudinal axis of the tube is substantially perpendicular to the second major surface 205 with the longitudinal axis of the tube extending in the direction of gravity. Referring to FIG. 6, a pen drop apparatus 601 includes a ballpoint pen 603, which is a BIC Easy Glide Pen, Fine comprising a tungsten carbide ballpoint tip 605 of 0.7 mm (0.68 mm) diameter, and a weight of 5.73 grams (g) including the cap. The ballpoint pen 603 is held at a predetermined height 609 from an outer surface (e.g., second major surface 205 of the foldable substrate 201) of the sample (see foldable apparatus 301 shown in FIG. 3). A tube (not shown for clarity) is used as part of the pen drop apparatus 601 to guide the ballpoint pen 603 to the outer surface (e.g., second major surface 205 of the foldable substrate 201) of the sample, and the tube is placed in contact with the outer surface so that the longitudinal axis of the tube is substantially perpendicular to the outer major surface with the longitudinal axis of the tube extending in the direction of gravity. The tube has an outside diameter of 1 inch (2.54 cm), an inside diameter of nine-sixteenths of an inch (1.4 cm), and a length of 90 cm. An acrylonitrile butadiene (“ABS”) shim (not shown) is employed to hold the ballpoint pen 603 at a predetermined height 609 for each test. After each drop, the tube is relocated relative to the sample to guide the pen to a different impact location on the sample.

[00147] For the Pen Drop Test, the pen is dropped with the cap attached to the top end (i.e., the end opposite the tip) so that the ballpoint can interact with the test sample. In a drop sequence according to the Pen Drop Test, one pen drop is conducted at an initial height of 1 cm, followed by successive drops in 0.5 cm increments up to 20 cm, and then after 20 cm, 2 cm increments until failure of the test sample. After each drop is conducted, the presence of any observable fracture, failure, or other evidence of damage to the sample is recorded along with the particular pen drop height. Using the Pen Drop Test, multiple samples can be tested according to the same drop sequence to generate a population with improved statistical accuracy. For the Pen Drop Test, the pen is to be changed to a new pen after every 5 drops, and for each new sample tested. In addition, all pen drops are conducted at random locations on the sample at or near the center of the sample, with no pen drops near or on the edge of the samples. [00148] For purposes of the Pen Drop Test, “failure” means the formation of a visible mechanical defect in a laminate. The mechanical defect may be a crack or plastic deformation (e.g., surface indentation). The crack may be a surface crack or a through crack. The crack may be formed on an interior or exterior surface of a laminate. The crack may extend through all or a portion of the foldable substrate 201. A visible mechanical defect has a minimum dimension of 0.2 mm or more.

[00149] In aspects, the foldable substrate 201 and/or the foldable apparatus 101 and/or 301 can resist failure for a pen drop at a pen drop height of 10 centimeters (cm), 12 cm, 14 cm, 16 cm, or 20 cm. In aspects, a maximum pen drop height that the foldable substrate 201 and/or the foldable apparatus 101 and/or 301 can withstand without failure may be about 10 cm or more, about 12 cm or more, about 14 cm or more, about 15 cm or more, about 16 cm or more, about 18 cm or more, about 20 cm or more about 40 cm or less, or about 30 cm or less, about 25 cm or less, about 20 cm or less, or about 15 cm or less. In aspects, a maximum pen drop height that the foldable substrate 201 and/or the foldable apparatus 101 and/or 301 can withstand without failure can be in a range from about 10 cm to about 40 cm, from about 12 cm to about 40 cm, from about 14 cm to about 30 cm, from about 16 cm to about 30 cm, from about 18 cm to about 30 cm, from about 20 cm to about 25 cm, or any range or subrange therebetween. In aspects, when the substrate thickness 209 of the foldable substrate 201 is about 50 pm or more (e.g., from about 50 pm to about 100 pm, from about 50 pm to about 90 pm, or any of the corresponding subranges discussed above), the foldable substrate 201 can withstand a pen drop from a pen drop height of 15 cm or more or even 20 cm or more. In aspects, when the substrate thickness 209 of the foldable substrate 201 is about 50 pm or less (e.g., from about 10 pm to about 50 pm, from about 10 pm to about 30 pm, or any of the corresponding subranges discussed above), the foldable substrate 201 can withstand a pen drop from a pen drop height of 10 cm or more.

[00150] Aspects of methods of chemically strengthening the foldable substrate 201 (e.g., in methods of making the foldable apparatus 101, 301 and/or 401) illustrated in FIGS. 2-3 and 5, in accordance with aspects of the disclosure, will be discussed with reference to the flow chart in FIG. 10 and example method steps illustrated in FIGS. 12-15 along with the cross-sectional view illustrated in FIG. 16.

[00151] In a first step 1001 of methods of the disclosure, as shown in FIGS. 11-12, methods can start with providing a foldable substrate 1111. In aspects, the foldable substrate 1111 may be provided by purchase or otherwise obtaining a substrate or by forming the foldable substrate. In aspects, the foldable substrate 1111 can comprise a glass-based substrate. In further aspects, glass-based substrates can be provided by forming them with a variety of ribbon forming processes, for example, slot draw, down-draw, fusion down-draw, up-draw, press roll, redraw, or float. In further aspects, glass-based substrates comprising ceramic crystals can be provided by heating a glass-based substrate to crystallize one or more ceramic crystals. The foldable substrate 1111 may comprise an existing first major surface 1113 and an existing second major surface 1115 opposite the existing first major surface 1113. In further aspects, an initial thickness 1119 of the foldable substrate 1111 (defined as an average distance between the existing first major surface 1113 and the existing second major surface 1115) can be within one or more of the ranges discussed above and/or may be within 5 pm of the final thickness (e.g., substrate thickness 209) (i.e., greater than the final thickness by from 0.1 pm to about 5 pm or from about 0.5 pm to about 4 pm). In further aspects, the existing first major surface 1113 and/or the existing second major surface 1115 can extend along a plane. In aspects, the foldable substrate 1111 can have a composition within one or more of the ranges discussed above for the glass-based substrate (e.g., foldable substrate 201). In aspects, at the end of step 1001, the foldable substrate 1111 can be substantially unstrengthened. As used herein, substantially unstrengthened refers to a substrate comprising either no depth of layer, no depth of compression, a depth of layer in a range from 0% to about 5% of the substrate thickness, or a depth of compression in a range from 0% to about 5% of the substrate thickness.

[00152] After step 1001, as shown in FIG. 11, methods can optionally proceed to step 1003 comprising heating the foldable substrate 1111 at a predetermined temperature for a predetermined period of time. In aspects, as shown in FIG. 11, heating the foldable substrate 1111 can comprise placing the foldable substrate 1111 in an environment (e.g., oven 1101) maintained at the predetermined temperature for the predetermined period of time. In aspects, the predetermined temperature can be about 250°C or more, about 270°C or more, about 280°C or more, about 290°C or more, about 300°C or more, about 350°C or less, about 330°C or less, about 320°C or less, about 310°C or less, or about 300°C or less. In aspects, the predetermined temperature can be in a range from about 250°C to about 350°C, from about 270°C to about 330°C, from about 270°C to about 320°C, from about 280°C to about 310°C, from about 280°C to about 300°C, or any range or subrange therebetween. In aspects, the predetermined temperature can be less than a first temperature that the molten salt solution used in step 1005 (discussed below) is maintained at. In aspects, the predetermined period of time can be about 10 minutes or more, about 20 minutes or more, about 30 minutes or more, about 45 minutes or more, about 1 hour or more, about 4 hours or less, about 2 hours or less, about 1.5 hours or less, or about 1 hour or less. In aspects, the predetermined period of time can be in a range from about 10 minutes to about 4 hours, from about 20 minutes to about 2 hours, from about 30 minutes to about 1.5 hours, from about 45 minutes to about 1 hour, or any range or subrange therebetween. Heating the foldable substrate before the chemical strengthening treatment in step 1005 can reduce thermal shock to the foldable substrate and facilitate a more even compressive stress region across the surfaces of the foldable substrate.

[00153] After step 1001 or 1003, as shown in FIG. 12, methods can proceed to step 1005 comprising contacting at least the existing first major surface 1113 with a molten salt solution 1203 maintained at a first temperature for a first period of time to develop at least an initial compressive stress region. In aspects, as shown in FIG. 12, the molten salt solution 1203 can be contained in a molten salt bath 1201. In aspects, as shown in FIG. 12, the contacting at least the existing the existing first major surface 1113 with the molten salt solution 1203 can comprise immersing the foldable substrate 1111 in the molten salt solution 1203, for example with both the existing first major surface 1113 and the existing second major surface 1115 in contact with the molten salt solution 1203, although only a portion of the foldable substrate (e.g., existing first major surface) may contact the molten salt solution in other aspects. Chemically strengthening the foldable substrate 1111 by ion exchange can occur when a first cation within a depth of a surface of a foldable substrate 1111 is exchanged with a second cation within a molten salt solution 1203 that has a larger radius than the first cation. For example, a lithium cation within the depth of the surface of the foldable substrate 1111 can be exchanged with a sodium cation or potassium cation within the molten salt solution 1203. Similarly, a sodium cation within the depth of the surface of the foldable substrate 1111 can be exchanged with a potassium cation within the molten salt solution 1203 to develop compressive stress within the foldable substrate 1111. Consequently, the surface of the foldable substrate 1111 is placed in compression and thereby chemically strengthened by the ion exchange process since the lithium cation has a smaller radius than the radius of the exchanged sodium cation or potassium cation within the molten salt solution 1203.

[00154] In aspects, the first temperature of the molten salt solution 1203 can be about 350°C or more, about 360°C or more, about 370°C or more, about 380°C or more, about 400°C or less, about 390°C or less, or about 380°C or less. In aspects, the first temperature of the molten salt solution 1203 can be in a range from about 350°C to about 400°C, from about 360°C to about 400°C, from about 370°C to about 390°C, from about 380°C to about 390°C, or any range or subrange therebetween. As demonstrated by the Examples discussed herein, providing a first temperature of the molten salt solution less than 400°C can increase a maximum compressive stress developed for a predetermined depth of layer and/or depth of compression. Also, for some of the molten salt solutions discussed herein, a temperature of 350°C or more may be used to ensure that salts are molten. Without wishing to be bound by theory, the lower temperature of the molten salt solution (e.g., about 400°C or less, from about 350°C to about 400°C) is believed to improve the properties of the substrate by preventing stress relaxation and providing a more controlled and uniform compressive stress across the substrate.

[00155] In aspects, the first period of time that the foldable substrate 1111 (e.g., existing first major surface 1113) is in contact with the molten salt solution 1203 can be about 10 minutes or more, about 15 minutes or more, about 20 minutes or more, about 30 minutes or more, about 45 minutes or more, about 60 minutes or more, about 90 minutes or less, about 75 minutes or less, about 60 minutes or less, about 45 minutes or less, about 30 minutes or less, about 20 minutes or less, or about 15 minutes or less. In aspects, the first period of time that the foldable substrate 1111 (e.g., existing first major surface 1113) is in contact with the molten salt solution 1203 can be in a range from about 10 minutes to about 90 minutes, from about 15 minutes to about 75 minutes, from about 20 minutes to about 60 minutes, from about 30 minutes to about 45 minutes, or any range or subrange therebetween. In aspects, the first period of time that the foldable substrate 1111 (e.g., existing first major surface 1113) is in contact with the molten salt solution 1203 can be about 30 minutes or less, for example, in a range from about 5 minutes to about 30 minutes, from about 10 minutes to about 20 minutes, from about 10 minutes to about 15 minutes, or any range or subrange therebetween. [00156] In aspects, the molten salt solution 1203 can comprise at least two anions associated with different salts. In further aspects, the at least two anions can be associated with different potassium salts, and the molten salt solution 1203 can comprise potassium ions in addition to the at least two anions. In even further aspects, a concentration of the first potassium salt and a concentration of the second potassium salt in the molten salt solution 1203 can be 2 wt% or more (e.g., 2.0 wt% or more), 2.5 wt% or more, 3.0 wt% or more, 4.0 wt% or more, 5.0 wt% or more, 7 wt% or more, 8 wt% or more, or 10 wt% or more of the total 100 wt% of the molten salt solution 1203 (i.e., before immersing the foldable substrate 1111). Unless otherwise indicated, the composition of the molten salt solution 1203 refers to the composition before the foldable substrate 1111 is immersed therein and is based on a total 100 wt% of the molten salt solution. It is to be understood that the molten salt solution can comprise additional components beyond the components of the two potassium salts discussed herein, for example, a sodium salt, a lithium salt, silicic acid, or combinations here. For example, the molten salt solution can comprise silicic acid, as a wt% superaddition to the molten salt solution excluding the silicic acid, in an amount of 0.1 wt% or more, about 0.3 wt% or more, about 0.5 wt% or more, about 1.0 wt% or less, about 0.7 wt% or less, or about 0.5 wt%, for example, in a range from about 0.1 wt% to about 1.0 wt%, from about 0.3 wt% to about 0.7 wt%, from about 0.3 wt% to about 0.5 wt%, or any range or subrange therebetween.

[00157] In further aspects, a concentration of a first potassium salt in the molten salt solution 1203 be about 2 wt% or more (e.g., about 2.0 wt% or more), about 2.5 wt% or more, about 3.0 wt% or more, about 4.0 wt% or more, about 5.0 wt% or more, about 7 wt% or more, about 8 wt% or more, about 10 wt% or more, about 12 wt% or less, about 10 wt% or less, about 8 wt% or less, about 5 wt% or less (e.g., about 5.0 wt% or less), about 4.0 wt% or less, or about 3.0 wt% or less. In further aspects, a concentration of a first potassium salt in the molten salt solution 1203 be in a range from about 2 wt% to about 12 wt%, from about 2.5 wt% to about 10 wt%, from about 3.0 wt% to about 8 wt%, from about 4.0 wt% to about 5 wt%, or any range or subrange therebetween. In further aspects, a concentration of a first potassium salt in the molten salt solution 1203 be about 5 wt% or more, for example in a range from about 5 wt% to about 12 wt%, from about 7 wt% to about 12 wt%, from about 8 wt% to about 10 wt%, or any range or subrange therebetween. In further aspects, a concentration of a first potassium salt in the molten salt solution 1203 be about 5.0 wt% or less, for example in a range from about 2.0 wt% to about 5.0 wt%, from about 2.5 wt% to about 5.0 wt%, from about 3.0 wt% to about 4.0 wt%, or any range or subrange therebetween. In preferred aspects, a concentration of the first potassium salt in the molten salt solution (based on a total 100 wt% of the molten salt solution before the foldable substrate is immersed therein) can be from about 2 wt% to about 12 wt%, from about 2.0 wt% to about 5.0 wt%, or from about 5 wt% to about 12 wt%.

[00158] In further aspects, the first potassium salt can comprise two or more potassium atoms per anion. Providing the first potassium salt with multiple (i.e., two or more) potassium atoms per anion can increase an effective concentration and/or activity of potassium in the molten salt solution, which can facilitate increased maximum compressive stress in the resulting chemically-strengthened foldable substrate. Throughout the disclosure, a pKa of a potassium salt is measured in accordance with OPPTS 830.7370 “Dissociation Constants in Water” from the United States Environmental Protection Agency (August 1996) available through the National Service Center for Environmental Publications. In further aspects, the first potassium salt can comprise a pKa of about 9 or more, about 10 or more, about 10.5 or more, about 11 or more, about 20 or less, about 15 or less, about 13 or less, or about 12 or less. In further aspects, the first potassium salt can comprise a pKa in a range from about 9 to about 20, from about 10 to about 15, from about 10.5 to about 13, from about 11 to about 12, or any range or subrange therebetween. Providing a first potassium salt in the molten salt solution with a pKa of about 9 or above can improve the strength and/or foldability of the resulting chemically-strengthened foldable substrate, for example, by selectively etching flaws inherent in the foldable substrate that might otherwise be magnified by the chemical strengthening treatment. Exemplary aspects of potassium salts with more than two potassium atoms per anion and a pKa of about 9 or more include potassium carbonate (K2CO3) and potassium phosphate (K3PO4). A preferred aspect of the first potassium salt is potassium carbonate (K2CO3), and a concentration of potassium carbonate (as the first potassium salt) can be within one or more of corresponding ranges discussed in the previous paragraph (e.g., from about 2 wt% to about 12 wt%, from about 2.0 wt% to about 5.0 wt%, or from about 5 wt% to about 12 wt%). As discussed with reference to the Examples here, potassium carbonate (K2CO3) has a more pronounced and unexpected increase in compressive stress than other components in molten salt solutions. Additionally, without wishing to be bound by theory, it is believed that the carbonate anion can facilitate precipitation of other cations (e.g., lithium, sodium) exchanged out of the foldable substrate, which can increase a longevity of the molten salt solution (e.g., by removing components from the solution phase that could otherwise “poison” the molten salt solution).

[00159] In further aspects, the molten salt solution comprises a second potassium salt associated with the two or more anions, where the anion of the first potassium salt is different than the anion of the second potassium salt. In even further aspects, the second potassium salt can be or more or more potassium nitrate (KNO3) and/or potassium chloride (KC1). A preferred aspect of the second potassium salt is potassium nitrate (KNO3). In further aspects, a concentration of the second potassium salt (e.g., potassium nitrate) in the molten salt solution can be about 50 wt% or more, about 60 wt% or more, about 70 wt% or more, about 80 wt% or more, about 84 wt% or more, about 88 wt% or more, about 89 wt% or more, about 90 wt% or more, about 91 wt% or more, about 92 wt% or more, about 93 wt% or more, about 94 wt% or more, about 95.0 wt% or more (e.g., about 95 wt% or more), about 96.0 wt% or more, about 97.0 wt% or more, about 97.5 wt% or more, or about 98.0 wt% or more (e.g., 98 wt% or more). In further aspects, a concentration of the second potassium salt (e.g., potassium nitrate) in the molten salt solution can be in a range from about 50 wt% to about 98.0 wt%, from about 60 wt% to about 98 wt%, from about 70 wt% to about 98 wt%, from about 80 wt% to about 98 wt%, from about 84 wt% to about 98 wt%, from about 88 wt% to about 98.0 wt%, from about 89 wt% to about 97.5 wt%, from about 90 wt% to about 97.0 wt%, from about 91 wt% to about 96.5 wt%, from about 92 wt% to about 96.0 wt%, from about 93 wt% to about 95.5 wt%, from about 94 wt% to about 95.0 wt%, or any range or subrange therebetween. In further aspects, a concentration of the second potassium salt (e.g., potassium nitrate) in the molten salt solution can be about 88 wt% or more, for example in a range from about 88 wt% to about 98 wt%, from about 88 wt% to about 97.5 wt%, from about 88 wt% to about 97.0 wt%, from about 88 wt% to about 96.0 wt%, from about 88 wt% to about 95.0 wt%, from about 88 wt% to about 94.0 wt%, from about 88 wt% to about 93.0 wt%, from about 88 wt% to about 92.0 wt%, from about 89 wt% to about 91 wt%, from about 90 wt% to about 91 wt%, or any range or subrange therebetween. In further aspects, a concentration of the second potassium salt (e.g., potassium nitrate) in the molten salt solution can be about 95.0 wt% or more, for example, in a range from about 95.0 wt% to about 98.0 wt%, from about 95.0 wt% to about 97.5 wt%, from about 96.0 wt% to about 97.0 wt%, or any range or subrange therebetween. In preferred aspects, a concentration of the second potassium salt (e.g., potassium nitrate) in the molten salt solution can be in a range from about 50 wt% to about 98 wt%, from about 88 wt% to about 98 wt%, or from about 95 wt% to about 98 wt%.

[00160] In further aspects, the molten salt solution 1203 can comprise a third potassium salt associated with a third anion of the at least two anions, where the third anion is different from the anions associated with the first potassium salt and the second potassium salt (discussed above). In even further aspects, the third potassium salt can have two or more potassium atoms per anion (similar to the first potassium salt). An exemplary aspect of the third potassium salt is potassium sulfate K2SO4. For example, the molten salt solution 1203 can comprise K2CO3 as the first potassium salt, KNO3 as the second potassium salt, and K2SO4 as the (optional) third potassium salt. In even further aspects, a concentration of the third potassium salt (e.g., potassium sulfate) in the molten salt solution can be 0 wt% or more, about 0.1 wt% or more, about 0.3 wt% or more, about 0.5 wt% or more, about 0.8 wt% or more, about 1.0 wt% or more, about 1.2 wt% or more, about 1.5 wt% or more, about 1.8 wt% or more, about 2.0 wt% or more, about 2.5 wt% or more, about 3.0 wt% or more, about 3.5 wt% or more, about 4.0 wt% or more, about 5 wt% or less (e.g., about 5.0 wt% or less), about 4.5 wt% or less, about 4.0 wt% or less, about 3.5 wt% or less, about 3.0 wt% or less, about 2.5 wt% or less, about 2.0 wt% or less, about 1.5 wt% or less, about 1.0 wt% or less, about 0.8 wt% or less, or about 0.5 w% or less. In even further aspects, a concentration of the third potassium salt (e.g., potassium sulfate) in the molten salt solution can be in a range from about 0 wt% to about 5 wt%, from about 0.1 wt% to about 5.0 wt%, from about 0.2 wt% to about 5.0 wt%, from about 0.5 wt% to about 5.0 wt%, from about 0.8 wt% to about 4.5 wt%, from about 1.0 wt% to about 4.0 wt%, from about 1.2 wt% to about 3.5 wt%, from about 1.5 wt% to about 3.0 wt%, from about 1.8 wt% to about 2.5 wt%, from about 2.0 wt% to about 2.5 wt%, or any range or subrange therebetween. In even further aspects, a concentration of the third potassium salt (e.g., potassium sulfate) in the molten salt solution can be about 2.0 wt% or less, for example, in a range from about 0 wt% to about 2.0 wt%, from about 0 wt% to about 1.5 wt%, from about 0.1 wt% to about 1.0 wt%, from about 0.1 wt% to about 0.8 wt%, from about 0.2 wt% to about 0.5 wt%, or any range or subrange therebetween. [00161] Due to the presence of the first potassium salt (e.g., having a pKa or 9 or more, potassium carbonate), in aspects, the molten salt solution 1203 can be basic (i.e., have a pH greater than 7). In further aspects, a pH of the molten salt solution 1203 can be about 8 or more, about 9 or more, about 10 or more, about 10.5 or more, about 11 or more, about 15 or less, about 13 or less, or about 12 or less. In further aspects, a pH of the molten salt solution 1203 can be in a range from about 8 to about 15, from about 9 to about 13, from about 9 to about 12, from about 10 to about 13, from about 10.5 to about 12, or any range or subrange therebetween. In preferred aspects, the pH of the molten salt solution can be in a range from about 9 to 12 or from about 10 to 12. Providing pH from about 9 to 12 of the molten salt solution can improve the strength and/or foldability of the resulting chemically-strengthened foldable substrate, for example, by selectively etching flaws inherent in the foldable substrate that might otherwise be magnified by the chemical strengthening treatment. Additionally, in aspects, as discussed below and shown in FIGS. 13-14 and 16, the chemical strengthening treatment of step 1005 can create an initial first compressive stress region 1212 and/or an initial second compressive stress region 1214. For example, the presence of the first potassium salt can increase a compressive stress imparted by the contacting the existing first major surface (in at least step 1005) with the molten salt solution 1203 by about 5% or more (e.g., about 10% or more, from about 5% to about 20%, from about 5% to about 15%, or from about 7% to about 10%) relative to immersing the foldable substrate in a comparative molten salt solution with the same composition as the molten salt solution with the absence of the first potassium salt.

[00162] In aspects, after step 1005, as shown in FIG. 13, methods can proceed to step 1007 comprising transferring the foldable substrate 1111 to a cooling chamber 1301, and a temperature of the cooling chamber is decreased from an initial temperature to a final temperature. In further aspects, as shown in FIG. 13, the foldable substrate 1111 can still contain a residual portion of the molten salt solution (indicated by drop 1305) and/or deposits 1303 on the surface (e.g., existing first major surface 1113) from contact with the molten salt solution in step 1005. In even further aspects, as shown in FIG. 13, the foldable substrate 1111 can be suspended in the cooling chamber 1301, for example, to facilitate the remove the residual portion of the molten salt solution from the foldable substrate 1111 (as indicated by drop 1305) that can run off of the foldable substrate 1111 in a direction of gravity (not shown but presumed to be down in FIG. 13).

[00163] In aspects, the initial temperature of the cooling chamber 1301 (e.g., when the foldable substrate 1111 is placed therein) can be about 300°C or less, about 280°C or less, about 260°C or less, about 240°C or less, about 220°C or less, about 180°C or more, about 190°C or more, about 200°C or more, about 210°C or more, or about 220°C or more. In aspects, the initial temperature of the cooling chamber 1301 (e.g., when the foldable substrate 1111 is placed therein) can be in a range from about 180°C to a bout 300°C, from about 190°C to about 280°C, from about 200°C to about 260°C, from about 210°C to about 240°C, from about 210°C to about 220°C or any range or subrange therebetween. In preferred aspects, the initial temperature of the cooling chamber 1301 can be in a range from about 180°C to about 300°C or from about 180°C to about 220°C. In further aspects, a difference between the first temperature than the molten salt solution 1203 is maintained at in step 1005 and the initial temperature of the cooling chamber 1301 in step 1007 (i.e., first temperature minus initial temperature) can be about 50°C or more, about 75°C or more, about 100°C or more, about 120°C or more, about 140°C or more, or about 160°C or more. Providing an initial temperature of the cooling chamber that is lower than the molten salt solution (e.g., by about 50°C or more, about 100°C or more, or about 140°C or more) can decrease a residual chemical strengthening occurring from any residual portion of the molten salt solution or deposits from the molten salt solution on the foldable substrate after it is removed from the molten salt solution). In particular, it has been observed that foldable substrates with a thickness of about 50 pm or less (e.g., from about 10 pm to about 50 pm or from about 10 pm to about 30 pm) are unexpectedly sensitive to what happens after the foldable substrate is removed from the molten salt solution. For these thin foldable substrates, even relatively small difference in compressive stress across the surface thereof can result in waviness and/or warp that can produce optical distortions that can be visible to a user of a consumer electronic product that the foldable substrate may be incorporated in. Consequently, the controlled temperature of the cooling chamber can facilitate a relatively even compressive stress across the surface of the foldable substrate. Also, providing an initial temperature of the cooling chamber of 180°C or more (e.g., 200°C or more or 220°C or more) can facilitate the removal of a residual portion of the molten salt solution before it solidifies. Without wishing to be bound by theory, the first potassium salt can have a higher melting temperature than the second potassium salt, which means that incorporating the first potassium salt in the molten salt solution can increase a viscosity of the molten salt solution and/or cause the molten salt solution to solidify at higher temperature than a molten salt solution without the first potassium salt. Consequently, allowing a residual portion of the molten salt solution on the foldable substrate after it is removed from the molten salt solution can be especially useful when the molten salt solution includes the first potassium salt.

[00164] In further aspects, the final temperature of the cooling chamber 1301 can be about 25°C or more, about 40°C or more, about 60°C or more, about 70°C or more, about 100°C or less, about 90°C or less, or about 80°C or less, about 70°C or less, or about 60°C or less. In further aspects, the final temperature of the cooling chamber 1301 can be in a range from about 25°C to about 100°C, from about 40°C to about 90°C, from about 60°C to about 90°C, from about 60°C to about 80°C, from about 70°C to about 80°C, or any range or subrange therebetween. Reducing the temperature of the cooling chamber to a final temperature of about 100°C or less (e.g., from about 25°C to about 100°C or from about 60°C to about 90°C) can enable the foldable substrate to be subsequently treated (e.g., relatively quickly or immediately) thereafter using aqueous solutions (e.g., rinsing with water or an alkaline detergent solution, contact with an aqueous acidic solution).

[00165] In further aspects, a cooling rate of the temperature of the cooling solution can be obtained using sufficient ventilation and/or circulation of environment (e.g., air) through the cooling chamber. In further aspects, a cooling rate of the temperature of the cooling solution (e.g., from the initial temperature to the final temperature) can be about 4 °C per minute (°C/min) or more, about 6°C/min or more, about 8°C/min or more, about 10°C/min or more, about 12°C/min or more, about 14°C/min or more, about 20°C/min or less, about 18°C/min or less, about 16°C/min or less, about 14°C/min or less, or about 10°C/min or less. In further aspects, a cooling rate of the temperature of the cooling solution (e.g., from the initial temperature to the final temperature) can be in a range from about 4°C/min to about 20°C/min, from about 6°C/min to about 18°C/min, from about 8°C/min to about 16°C/min, from about 10°C/min to about 14°C/min, from about 12°C/min to about 14°C/min, or any range or subrange therebetween. Providing a cooling rate from about 4°C/min to about 20°C/min can quickly reduce a temperature of the cooling chamber (and foldable substrate) while being able maintain a relatively consistent temperature throughout the cooling chamber (and/or foldable substrate), for example, to produce a relatively consistent compressive stress across the surface of the foldable substrate.

[00166] In aspects, after step 1005 or 1007, as shown in FIG. 14, methods can proceed to step 1009 comprising (e.g., after removing the foldable substrate from the molten salt solution in step 1005 and/or after the cooling chamber reaches the final temperature) rinsing the foldable substrate 1111 with a solution 1403. In further aspects, the solution 1403 can be contained in a bath 1401 and/or the foldable substrate 1111 can be immersed in the solution 1403 (e.g., with the existing first major surface 1113 and the existing second major surface 1115 in contact with the solution 1403). In further aspects, as shown between FIGS. 13 and 14, the solution 1403 can remove (e.g., dissolve and/or displace) deposits 1303 from the molten salt solution remaining on the foldable substrate 1111. In aspects, the solution 1403 can be agitated (e.g., ultrasonicated) to further facilitate removal of deposits 1303 and/or contaminants on the surface that could interfere with a uniform treatment of the surfaces of the foldable substrate in subsequent steps. In further aspects, the solution 1403 can be water (e.g., purified, filtered, deionized, and/or distilled), an alkaline detergent solution, or combinations thereof. As used herein, a pH of a solution is measured in accordance with ASTM E70-90 at 25°C with standard solutions extending to a pH of at least 14. In even further aspects, the alkaline detergent solution (e.g., solution 1403) can comprise an alkaline detergent and a pH of about 11 or more, about 12 or more, about 12.5 or more, about 12.8 or more, about 14 or less, about 13.5 or less, or about 13.2 or less. In aspects, the alkaline detergent solution (e.g., solution 1403) can comprise a pH ranging from about 11 to about 14, from about 12 to about 14, from about 12.5 to about 13.5, from about 12.8 to about 13.2, or any range or subrange therebetween. In aspects, the alkaline detergent solution (e.g., solution 1403) can comprise an alkaline detergent in a concentration from about 0.5 wt% or more, about 1 wt% or more, about 1.5 wt% or more, about 2 wt% or more, about 4 wt% or less, about 3 wt% or less, or about 2.5 wt% or less. In aspects, the alkaline detergent solution (e.g., solution 1403) can comprise an alkaline detergent in a concentration ranging from about 0.5 wt% to about 4 wt%, from about 1 wt% to about 4 wt%, from about 1.5 wt% to about 3 wt%, from about 2 wt% to about 3 wt%, from about 2.5 wt% to about 3 wt%, or any range or subrange therebetween. An exemplary aspect of an alkaline detergent solution includes SemiClean KG (Yokohama Oils & Fats Industry Co.). Exemplary aspects of sonication include ultrasonication and megasonication. Without wishing to be bound by theory, sonication (e.g., ultrasonication, megasonication) can help remove contaminants (e.g., particles, oils) from a surface by forming microscale bubbles as the surface, by increasing circulation of the alkaline detergent solution through agitation, and/or by loosening contaminants through vibration directly. In aspects, the alkaline detergent solution and/or water can be substantially free of a rheology modifier. As used herein, a rheology modifier is a component other than a solvent or a listed component (e.g., acid, hydroxide-containing base, EESiFe, fluoride-containing compound) that modifies the viscosity of the solution or the shear-dependent behavior (e.g., dilatant, thixotropic). Example aspects of rheology modifiers that the solution can be substantially free of include one or more of cellulose, a cellulose derivative (e.g., ethyl cellulose, methyl cellulose, and AQUAZOL (poly 2 ethyl-2 oxazine)), a hydrophobically modified ethylene oxide urethane modifier (HUER), and an ethylene acrylic acid.

[00167] In further aspects, the solution 1403 can comprise a rinsing temperature and/or be in contact with the foldable substrate 1111 for a rinsing period of time. In further aspects, sonication can be applied for at least half of the rinsing period of time, for example, the entire first period of time. In further aspects, the rinsing period of time can be about 2 minutes or more, about 3 minutes or more, about 4 minutes or more, about 5 minutes or more, about 60 minutes or less, about 40 minutes or less, about 20 minutes or less, about 10 minutes or less, about 8 minutes or less, or about 6 minutes or less. In further aspects, the rinsing period of time can range from about 2 minutes to about 40 minutes, from about 2 minutes to about 20 minutes, from about 3 minutes to about 20 minutes, from about 3 minutes to about 10 minutes, from about 4 minutes to about 8 minutes, from about 4 minutes to about 6 minutes, or any range or subrange therebetween. Providing a rinsing period of time of at least 2 minutes can effectively remove contaminants and/or deposits from the surface. Providing a rinsing period of time of less than 40 minutes can keep a chance of damage or breakage withing acceptable ranges. In aspects, the first temperature can be about 20°C or more, about 25°C or more, about 30°C or more, about 35°C or more, about 65°C or less, about 60°C or less, about 55°C or less, or about 45°C or less. In aspects, the first temperature can range from about 20°C to about 65°C, from about 25°C to about 60°C, from about 30°C to about 55°C, from about 35°C to about 45°C, or any range or subrange therebetween. Providing the alkaline detergent solution may selectively act on surface flaws (e.g., removing, rounding, blunting) before removing material from other parts of the surface, which can increase the impact resistance of the substrate without removing a substantial thickness from the surface of the foldable substrate. In further aspects, the

[00168] As shown in FIGS. 14 and 16, after and/or at the end of step 1005, 1007, and/or 1009, the chemically-strengthened foldable substrate (i.e., foldable substrate 1111) can comprise (e.g., as a result of the chemical strengthening treatment described above) an initial first compressive stress region 1212 extending to an initial first depth of compression 1216 from the existing first major surface 1113 and an initial first depth of layer of one or more alkali metal ions (e.g., potassium) associated with the initial first compressive stress region 1212, and/or the foldable substrate 1111 can comprise an initial second compressive stress region 1214 extending to an initial second depth of compression 1218 from the existing second major surface 1115 and an initial second depth of layer of one or more alkali metal ions (e.g., potassium) associated with the initial second compressive stress region 1214. In further aspects, a maximum initial first compressive stress of the initial first compressive stress region 1212 and/or a maximum initial second compressive stress of the initial second compressive stress region 1214 can be within one or more of the ranges discussed above for the maximum first compressive stress. In further aspects, a maximum initial first compressive stress of the initial first compressive stress region 1212 and/or a maximum initial second compressive stress of the initial second compressive stress region 1214 can be about 800 MPa or more, about 850 MPa or more, about 900 MPa or more, about 950 MPa or more, about 1,000 MPa or more, about 1,500 MPa or less, about 1,300 MPa or less, about 1,200 MPa or less, about 1,100 MPa or less, about 1,050 MPa or less, about 1,000 MPa or less, or about 950 MPa or less. In further aspects, a maximum initial first compressive stress of the initial first compressive stress region 1212 and/or a maximum initial second compressive stress of the initial second compressive stress region 1214 can be in a range from about 800 MPa to about 1,5000 MPa, from about 850 MPa to about 1,300 MPa, from about 900 MPa to about 1,200 MPa, from about 950 MPa to about 1,100, from about 1,000 MPa to about 1,050 MPa, or any range or subrange therebetween. In further aspects, the presence of the first potassium salt can increase a compressive stress imparted by the contacting the existing first major surface (in at least step 1005) with the molten salt solution 1203 by about 5% or more (e.g., about 10% or more, from about 5% to about 20%, from about 5% to about 15%, or from about 7% to about 10%) relative to immersing the foldable substrate in a comparative molten salt solution with the same composition as the molten salt solution with the absence of the first potassium salt. In further aspects, a maximum initial first compressive stress of the initial first compressive stress region 1212 and/or a maximum initial second compressive stress of the initial second compressive stress region 1214 can be greater than the resulting maximum first compressive stress and/or the resulting maximum second compressive stress by about 5% or more, about 8% or more, about 10% or more, about 12% or more, about 15% or more, about 17% or more, or about 20% or more, for example, in a range from about 5% to about 30%, from about 8% to about 25%, from about 10% to about 22%, from about 12% to about 20%, from a bout 15% to about 18%, or any range or subrange therebetween.

[00169] After step 1005, 1007, or 1009, as shown in FIGS. 15-16, methods can proceed to step 1011 comprising contacting at least the existing first major surface 1113 with an acidic solution 1503 maintained at a second temperature for a second period of time to remove an outer layer (e.g., outer compressive layer extending to a first outer depth 1603 of the initial first compressive stress region 1212 shown in FIG. 16) to form a new first major surface (e.g., first major surface 203) and the first compressive stress region 212. In aspect, as shown, the existing second major surface 1115 can also be contacted with the acidic solution 1503 to remove an outer layer (e.g., outer compressive layer extending to a second outer depth 1605 of the initial second compressive stress region 1214 shown in FIG. 16) to form a new second major surface (e.g., second major surface 205) and the second compressive stress region 214. In aspects, as shown in FIG. 15, the acidic solution 1503 can be contained in a bath 1501 and the foldable substrate 1111 can be immersed in the acidic solution 1503, although the acidic solution can contact the foldable substrate (e.g., existing first major surface 1113) in other situations in other aspects. In further aspects, as shown in FIG. 16, the first outer depth 1603 and/or second outer depth 1605 of the outer layer removed by the acidic solution 1503 (see FIG. 15) can be about 3.5 pm or less, about 3.0 pm or less, about 2.5 pm or less, about 2.0 pm or less, about 1.5 pm or less, about 1.0 pm or less, about 0.8 pm or less, about 0.1 pm or more, about 0.3 pm or more, about 0.5 pm or more, about 0.8 pm or more, about 1.0 pm or more, or about 1.5 pm or more. In further aspects, as shown in FIG. 16, the first outer depth 1603 and/or second outer depth 1605 of the outer layer removed by the acidic solution 1503 (see FIG. 15) can be in a range from about 0.1 pm to about 3.5 pm, from about 0.3 pm to about 3.0 pm, from about 0.5 pm to about 2.5 pm, from about 0.8 pm to about 2.0 pm, from about 1.0 pm to about 1.5 pm, or any range or subrange therebetween. Consequently, as shown in FIG. 16, the first outer depth 1603 and/or second outer depth 1605 is less than the initial first depth of compression 1216 and/or the initial second depth of compression 1218, respectively, and the foldable substrate 201 after contact with the acidic solution 1503 (see FIG. 15) can comprise the first compressive stress region 212 and/or the second compressive stress region 214 with reduced compressive stress relative to the corresponding initial compressive region. In further aspects, an amount of compressive stress reduction (i.e., removed by the acidic solution) as a percentage of the maximum initial first compressive stress and/or maximum initial second compressive stress can be about 10% or more, about 12% or more, about 15% or more, about 17% or more, about 20% or more, about 22% or more, about 25% or less, about 22% or less, about 20% or less, about 17% or less, or about 15% or less. In further aspects, an amount of compressive stress reduction (i.e., removed by the acidic solution) as a percentage of the maximum initial first compressive stress and/or maximum initial second compressive stress can be in a range from about 10% to about 25%, from about 12% to about 22%, from about 15% to about 20%, from about 17% to about 20%, or any range or subrange therebetween. In further aspects, the resulting compressive stress regions can comprise a respective maximum compressive stress within one or more of the ranges discussed above with reference to the maximum first compressive stress.

[00170] An etching rate (i.e., rate of material removed from each surface - existing major surfaces - of the foldable substrate) of the acidic solution can be adjusted based on the second temperature, the contents of the acidic solution including the selection of components, concentration of components, and resulting pH of the acidic solution. In aspects, an etching rate of the acidic solution 1503 can be about 1 pm per minute (pm/min) or less (e.g., about 1.0 pm/min or less), about 0.9 pm/min or less, about 0.8 pm/min or less, about 0.7 pm/min or less, about 0.6 pm/min or less, about 0.5 pm/min or less, about 0.4 pm/min or less, about 0.1 pm/min or more, about 0.2 pm/min or more, about 0.3 pm/min or more, about 0.4 pm/min or more, about 0.5 pm/min or more, or about 0.6 pm/min or more. In aspects, an etching rate of the acidic solution 1503 can be in a range from about 0.1 pm/min to about 1.0 pm/min, from about 0.2 pm/min to about 0.9 pm/min, from about 0.3 pm/min to about 0.8 pm/min, from about 0.4 pm/min to about 0.7 pm/min, from about 0.5 pm/min to about 0.6 pm/min, or any range or subrange therebetween. Providing an etching rate of about 1 pm/min or less (e.g., about 1.0 pm/min or less) can facilitate a substantially uniform removal of material from the surface(s) of the foldable substrate. As discussed above, foldable substrates with a thickness of about 50 gm or less (e.g., from about 10 gm to about 50 gm or from about 10 gm to about 30 gm) are quite sensitive to differences in compressive stress and thickness variation across its surface. Consequently, providing an etching rate of about 1 pm/min can remove a relatively uniform thickness and portion of the compressive stress from the surface(s) to reduce an incidence of waviness and/or warp that can produce optical distortions that can be visible to a user of a consumer electronic product that the foldable substrate may be incorporated in.

[00171] In aspects, the second temperature of the acidic solution 1503 can be about 20°C or more, about 22°C or more, about 25°C or more about 28°C or more, about 30°C or more, about 40°C or less, about 35°C or less, about 30°C or less, about 28°C or less, about 25°C or less, or about 23°C or less. In aspects, the second temperature of the acidic solution 1503 can range from about 20°C to about 40°C, from about 20°C to about 35°C, from about 20°C to about 30°C, from about 20°C to about 28°C, from about 20°C to about 25°C, from about 22°C to about 23°C, or any range or subrange therebetween. Without wishing to be bound by theory, providing a relatively low temperature of acidic solution (e.g., from about 20°C to about 40°C or from about 20°C to about 25°C) can decrease the concentration of SiFe' anions since the reaction from FhSiFe and 2 H⁺ + SiFe' is endothermic. Decreasing a concentration of SiFe' anions can be associated with decreased deposition (e.g., redeposition) of silica or silica-like materials on the surface that could otherwise produce variation in the thickness and/or compressive stress across the surface of the foldable substrate.

[00172] In aspects, the second period of time that the foldable substrate 201 or 1111 (e.g., existing first major surface 1113 or first major surface 203) is in contact with the acidic solution 1503 can be about 20 seconds or more, about 30 seconds or more, about 45 seconds or more, about 60 seconds or more, about 75 seconds or more, about 90 seconds or more, about 120 seconds or more, about 3.5 minutes or less, about 3 minutes or less, about 2.5 minutes or less, about 2 minutes or less, about 1.5 minutes or less, or about 1.0 minute or less. In aspects, the second period of time can be in a range from about 20 seconds to about 3.5 minutes, from about 30 second to about 3 minutes, from about 45 seconds to about 2.5 minutes, from about 60 seconds to about 2 minutes, from about 75 seconds to about 1.5 minutes, or any range or subrange therebetween. In aspects, the acidic solution 1503 can be agitated (e.g., stirred, ultrasonicated) during the second period of time. Without wishing to be bound by theory, agitating the acidic solution can decrease a supersaturation of silica-like compounds near the surface.

[00173] As discussed above, a pH of a solution is measured in accordance with ASTM E70-90 at 25°C. In aspects, a pH of the acidic solution 1503 can be about 3.5 or more, about 3.55 or more, about 3.6 or more, about 3.65 or more, about 3.7 or more, about 3.75 or more, about 3.8 or more, about 4.5 or less, about 4.3 or less, about 4.0 or less, about 3.9 or less, about 3.8 or less, or about 3.7 or less. In aspects, a pH of the acidic solution 1503 can be in a range from about 3.5 to about 4.5, from about 3.55 to about 4.3, from about 3.6 to about 4.0, from about 3.65 to about 3.9, from about 3.7 to about 3.8, from about 3.75 to about 3.8, or any range or subrange therebetween. Providing a relatively high pH (e.g., from about 3.5 to about 4.5, from about 3.6 to about 4.3, or from about 3.7 to about 4.0) can decrease an etching rate that can help produce a relatively uniform compressive stress and thickness across the foldable substrate.

[00174] In aspects, the acidic solution can comprise a buffered HF solution and/or an aqueous acidic solution. As used herein, buffered HF means that the solution contains NH4F or a similar compound that produces F’ anions in the acidic solution. In aspects, the acidic solution can comprise HF, as a wt% of the acidic solution, in an amount of about 0.5 wt% or more, about 0.55 wt% or more, about 0.6 wt% or more, about 1.5 wt% or less, about 1.25 wt% or less, about 1.0 wt% or less, about 0.75 wt% wt% or less, about 0.7 wt% or less, or about 0.65 wt% or less. In aspects, the acidic solution can comprise HF, as a wt% of the acidic solution, in an amount in a range from about 0.5 wt% to about 1.5 wt%, from about 0.5 wt% to about 1.25 wt%, from about 0.5 wt% to about 1.0 wt%, from about 0.5 wt% to about 0.75 wt%, from about 0.55 wt% to about 0.70 wt%, from about 0.6 wt% to about 0.65 wt%, or any range or subrange therebetween. In aspects, the acidic solution can contain NH4F, as a wt% of the acidic solution, in an amount of about 0.75 wt% or more, about 0.8 wt% or more, about 0.85 wt% or more, about 0.9 wt% or more, about 0.95 wt% or more, about 1.0 wt% or more, about 1.1 wt% or more, about 2.5 wt% or less, about 2.25 wt% or less, about 2.0 wt% or less, about 1.75 wt% or less, about 1.5 wt% or less, about 1.3 wt% or less, about 1.2 wt% or less, about 1.1 wt% or less, or about 1.0 wt% or less. In aspects, the acidic solution can contain NH4F, as a wt% of the acidic solution, in an amount in a range from about 0.75 wt% to about 2.5 wt%, from about 0.8 wt% to about 2.25 wt%, from about 0.8 wt% to about 2.0 wt%, from about 0.85 wt% to about 1.75 wt%, from about 0.9 wt% to about 1.5 wt%, from about 0.95 wt% to about 1.3 wt%, from about 1.0 wt% to about 1.2 wt%, from about 1.0 wt% to about 1.1 wt%, or any range or subrange therebetween. In exemplary aspects, the acidic solution can comprise from about 0.5 wt% to about 1.5 wt% or from about 0.5 wt% to about 0.75 wt% HF and/or from about 0.75 wt% to about 2.5 wt% or from about 0.9 wt% to about 1.5 wt% NH4F. Providing a combined concentration of HF and NH4F of about 4.0 wt% or less, about 3.5 wt% or less, about 3.0 wt% or less, about 2.5 wt% or less, or about 2.0 wt% (e.g., from about 1.25 wt% to about 4.0 wt%, from about 1.3 wt% to about 3.5 wt%, from about 1.35 wt% to about 3.0 wt%, from about 1.4 wt% to about 2.5 wt%, from about 1.5 wt% to about 2.0 wt%) can provide relatively controlled and even etching of the foldable substrate and/or reduce deposition of material (e.g., silica, silica-like material, ammonium fluoride crystals) on the foldable substrate that could impair the optical properties of the foldable substrate.

[00175] In aspects, after step 1011, methods can further proceed to step 1013 comprise rinsing the foldable substrate with water, an alkaline detergent solution, or combinations thereof. For example, with reference to FIG. 12, step 1013 can comprise rinsing the foldable substrate (e.g., foldable substrate 201 here instead of foldable substrate 1111 in FIG. 12) with a solution 1203 (e.g., alkaline detergent solution, water) that can be contained in a bath 1201. In further aspects, step 1013 can comprise rinsing with water followed by rinsing with an alkaline detergent solution, the reverse order, or multiple rinses involving water and/or an alkaline detergent solution. In further aspects, step 1013 can comprise any one or more of the aspects discussed above with reference to step 1009. For example, providing an alkaline detergent solution in step 1013 can neutralize residual etchant from step 1011, which can prevent surface defects and/or produce a more uniform thickness of the foldable substrate. Providing the alkaline detergent solution in step 1013 can neutralize and/or remove hydrogen (e.g., hydronium) enrichment at the surface of the foldable substrate, which might otherwise lead to large flaws as a result of stress corrosion during the subsequent chemical strengthening. Providing the alkaline detergent solution may selectively act on surface flaws (e.g., removing, rounding, blunting) before removing material from other parts of the surface, which can increase the impact resistance of the substrate without removing a substantial thickness from the surface of the foldable substrate.

[00176] In aspects, after step 1009, 1011, or 1013, methods can proceed to step 1015 comprising assembling a foldable apparatus from the foldable substrate. In further aspects, step 1015 can comprise disposing the adhesive layer 311 or a polymer-based portion over the foldable substrate 201 (e.g., first major surface 203). In further aspects, step 1015 can further comprise disposing a layer (e.g., display device, another substrate, PET sheet 321) over the adhesive layer 311 (see FIG. 3) or the polymer-based portion disposed earlier in step 1015. In further aspects, step 1015 can further comprise disposing a release liner over the adhesive layer 311 (see FIG. 3) or the polymer-based portion disposed earlier in step 1015. In aspects, step 1015 can comprise disposing a coating over the foldable substrate (e.g., second major surface).

[00177] After steps 1009, 1011, 1013, and/or 1015, the method can be complete at step 1017. In aspects, step 1017 can comprise further assembling the foldable apparatus, for example, by disposing a coating opposite a release liner or display device, or by disposing a release liner or display device opposite a coating. At the end of step 1009, 1011, 1013, and/or 1015, the foldable substrate 201 can be similar to or identical to the foldable substrate 201 shown in FIGS. 2-3. In aspects, methods can proceed along the steps discussed above, for example, sequentially through steps 1001, 1003, 1005, 1007, 1009, 1011, 1013, 1015, and 1017. In aspects, methods can follow arrow 1002 from step 1001 to step 1005 if the foldable substrate 1111 is to be chemically strengthened without being pre-heated. In aspects, methods can follow arrow 1004 from step 1005 to step 1009, for example, if the chemically- strengthened foldable substrate is rinsed in step 1009 without placing the foldable substrate in a cooling chamber with a controlled temperature profile. In aspects, methods can follow arrow 1006 from step 1005 to step 1011, for example, if the foldable substrate is to go directly from being chemically-strengthened in step 1015 to being etched by the acidic solution (e.g., without rinsing and/or being placed in a cooling chamber with a controlled temperature profile). In aspects, methods can follow arrow 1008 from step 1007 to step 1011, for example, to if the foldable substrate is to be transferred from the cooling chamber to the acidic solution (e.g., without rinsing the foldable substrate therebetween). In aspects, methods can follow arrow 1010 from step 1011 to step 1017, for example, if methods are complete at the end of step 1011. In aspects, methods can follow arrow 1012 from step 1011 to step 1015, for example, if the foldable substrate is to be assembled as part of a foldable substrate after the etching with the acidic solution (e.g., without rinsing therebetween). In aspects, methods can follow arrow 1014 from step 1013 to step 1017, for example, if methods are complete at the end of step 1013. In aspects, methods can follow arrow 1016 from step 1009 to step 1017, for example, if methods are complete at the end of step 1009. Any of the above options may be combined to make a chemically-strengthened foldable substrate and/or foldable apparatus in accordance with aspects of the disclosure.

[00178] In aspects, methods in accordance with aspects of the disclosure may consist of the steps discussed above. For example, the foldable substrate may not be further treated between one or more (or even all of) the steps described above with reference to the flow chart in FIG. 10. Throughout the disclosure, the phrase “not further treated” or “not be further treated” excludes treatments to the first major surface other than the stated contacting with a solution and rinsing with water (e.g., purified, filtered, deionized, distilled). Exemplary aspects of treatments that can be excluded under “not further treated” or “not be further treated” include treatment with additional acidic solutions, basic solutions, fluorine-containing solutions, detergents, and mechanical polishing of the foldable substrate.

EXAMPLES

[00179] Various aspects will be further clarified by the following examples.

[00180] Examples 1-3, 21-24, 29-43, and 48-50 and Comparative Examples AA-BB, II-JJ, MM-NN, QQ, and TT comprised a glass-based substrate with Composition 1 (nominally, in mol% of: 68.9 SiCE; 10.1 mol% AI2O3; 4.9 mol% MgO; 0.5 mol% CaO; 15.5 Na?O; and 0.1 mol% SnCE). Examples 1-3 and Comparative Examples AA-BB and QQ comprised a substrate thickness of 80 pm. Examples 21-24 and 40-43 and Comparative Examples II-JJ comprised a substrate thickness of 75 pm. Examples 29-39 and 49-50 and Comparative Examples MM-NN comprised a substrate thickness of 30 pm.

[00181] Examples 4-20, 25-28, 44-47, and 51-72 and Comparative Examples CC-HH, KK-LL, OO-PP, and RR-SS comprised a glass-based substrate with Composition 2 (nominally, in mol% of: 65.0 SiCE; 14.1 mol% AI2O3; 3.4 mol% MgO; 1.0 mol% CaO; 16.4 Na?O; and 0.1 mol% SnCE). Examples 4-6 and Comparative Examples CC-DD and RR comprised a substrate thickness of 80 pm. Examples 14-20 and 25-28 and Comparative Examples GG-HH comprised a substrate thickness of 75 pm. Examples 44-47 and Comparative Examples KK-LL comprised a substrate thickness of 70 pm. Examples 7-13 and 51-72 and Comparative Examples EE-FF, OO-PP, and SS comprised a substrate thickness of 30 pm.

[00182] Tables 1-2 and FIGS. 17-18 present the treatment conditions and properties of Examples 1-6 and Comparative Examples AA-DD. The balance of the molten salt solution (after account for the second potassium salt ) is potassium nitrate (KNO3) with any silicic acid added by superaddition thereto. For example, Example 1 is 95 wt% KNO3 and 5 wt% K2CO3 with 0.5 wt% silicic acid by superaddition. In FIG. 17, the vertical axis 1703 (e.g., y-axis) corresponds to compressive stress in MPa, and the horizontal axis 1701 (e.g., x-axis) corresponds to the different Examples with the dashed line separating Examples using Composition 1 from those using Composition 2. In FIG. 18, the vertical axis 1803 (e.g., y-axis) corresponds to depth of layer in pm, and the horizontal axis 1801 (e.g., x-axis) responds to the different Examples with the dashed line separating Examples using Composition 1 from those using Composition 2.

[00183] Table 1 presents the properties for Composition 1 (Examples 1-3 and Comparative Examples AA-BB). Comparing Example 1 to Comparative Example AA, the further addition of K2CO3 at 420°C decreases the compressive and a depth of layer relative to Comparative Example AA. The molten salt solutions for Comparative Example BB and Examples 2-3 were maintained at 380°C instead of 420°C. As demonstrated by Comparative Example BB, a longer chemical strengthening treatment (69 minutes versus 30 minutes) at the lower temperature (380°C versus 420°C) can achieve about the same (or even slightly higher) compressive stress with a reduced depth of layer. As discussed above, the lower temperature of the molten salt solution is believed to improve the properties of the substrate by preventing stress relaxation and providing a more controlled and uniform compressive stress across the substrate. Adding a second potassium in salt in Examples 2-3 improves the compressive stress and (improves or maintains) the depth of layer relative to Example 1 and Comparative Example BB. Further, Examples 2-3 improve the compressive stress relative to Comparative Example AA. Table 1 : Treatment Conditions and Properties of Examples 1-3 and Comparative Examples AA-BB (Composition 1 with 80 pm thickness)

Table 2: Treatment Conditions and Properties of Examples 4-6 and Comparative Examples CC-DD (Composition 2 with 80 pm thickness)

[00184] Table 2 presents the results for Composition 2 (Examples 4-6 and Comparative Examples CC-DD). Comparing Example 4 to Comparative Example CC, the further addition of K2CO3 at 420°C decreases the compressive and a depth of layer relative to Comparative Example CC. The molten salt solutions for Comparative Example DD and Examples 5-6 were maintained at 380°C instead of 420°C. As demonstrated by Comparative Example DD, a longer chemical strengthening treatment (69 minutes versus 30 minutes) at the lower temperature (380°C versus 420°C) can achieve higher compressive stress with a reduced depth of layer. As discussed above, the lower temperature of the molten salt solution is believed to improve the properties of the substrate by preventing stress relaxation and providing a more controlled and uniform compressive stress across the substrate. Examples 5-6 add second potassium salts. For Example 6, the added K2CO3 improves both the compressive stress and depth of layer relative to Example 4 and improves the compressive stress relative to Comparative Examples CC-DD. However, the added K3PO4 increases the depth of layer but decreases the compressive stress relative to Example 4 (and has about the same properties of Comparative Example DD). Consequently, the addition of K3PO4 does not improve the properties of Composition 2 (comparing Example 5 to Comparative Example DD - although it does improve compressive stress for Composition 1) whereas K2CO3 improves the compressive stress (and depth of layer) for both Composition 1 (comparing Example 3 to Comparative Example BB) and Composition 2 (comparing Example 6 to Comparative Example DD).

[00185] Tables 3-4 and FIGS. 19-20 present the treatment conditions and properties of Examples 7-20. In FIGS. 19-20, the vertical axis 1903 or 2003 (e.g., y- axis) corresponds to compressive stress in MPa, and the horizontal axis 1901 or 2001 (e.g., x-axis) corresponds to a wt% of K2CO3 in the molten salt bath.

[00186] Table 3 and FIG. 19 present the results for Composition 2 with a substrate thickness of 30 pm. Curve 1907 corresponds to Comparative Example EE and Examples 7-9, respectively from left to right, that were chemically strengthened at 400°C for 12 minutes. As shown, a maximum compressive stress of 975 MPa in curve 1907 (greater than Comparative Example EE by about 4% or more) was unexpectedly obtained for Example 10 with 5 wt% K2CO3. Based on this result, it is expected that adding from about 2 wt% to about 5 wt% (e.g., from about 2.5 wt% to about 5.0 wt%) K2CO3 to the molten salt bath will also exhibit the unexpectedly increased compressive stress (when chemically strengthening a substrate with a thickness less than 50 pm - e.g., from about 10 pm to about 50 pm or from about 10 pm to about 30 pm - at 400°C).

[00187] Curve 1909 corresponds to Comparative Example FF and 10-12, respectively from left to right, that were chemically strengthened at 380°C for 18 minutes. As shown, curve 1909 (380°C) is above curve 1907 (400°C) for K2CO3 content of 10 wt% or more (e.g., 12 wt% or more or less than 15 wt%). As discussed above, chemically strengthening at a lower temperature (e.g., 380°C instead of 400°C) achieve the same or greater compressive stress with additional time (e.g., 18 minutes instead of 12 minutes). For curve 1909, a maximum compressive stress of 984 MPa was unexpectedly observed for 10 wt% K2CO3 (Example 11), although 5 wt% K2CO3 (Example 10) also had a high compressive stress (e.g., about 980 MPa or more). Based on this result, it is expected that from about 2 wt% to about 12 wt% (e.g., from about 2.5 wt% to about 12 wt%, from about 5 wt% to about 12 wt%, or from about 8 wt% to about 12 wt%) K2CO3 to the molten salt bath will also exhibit the unexpectedly increased compressive stress (when chemically strengthening a substrate with a thickness less than 50 pm - e.g., from about 10 pm to about 50 pm or from about 10 pm to about 30 pm - at 380°C). FIG. 19 also shows a difference 1917 between the compressive stress of Comparative Example EE (line 1905) and the compressive stress of Example 11 (line 1915) of about 50 MPa (i.e., 47 MPa) corresponding to an increase of about 5% or more in compressive stress going from Comparative Example EE to Example 11.

[00188] As shown in Table 3 (but not plotted in FIG. 19), Example 13 corresponds to Example 11 without the silicic acid, and Example 13 still exhibits an increase in compressive stress relative to Comparative Example EE (although not relative to Comparative Example FF or Example 11). Table 3 also shows the pH measured for a solution made by diluting 5 g of the molten salt solution (cooled to ambient temperature) in 100 grams of deionized water using the standard discussed above. As shown, the pH of pure KNO3 (Comparative Example EE) is 7.51, which is roughly neutral. In contrast, the pH of a 10 wt% K2CO3 and 90 wt% KNO3 (Examples 11 and 13) is about 11. The pH of the other molten salt baths was not measured. As discussed above, it is believed that the increased pH of Examples 11 and 13 (relative to Comparative Example EE) can improve the strength and/or foldability of the resulting chemically strengthened foldable substrate, for example, by selectively etching flaws inherent in the foldable substrate that might otherwise be magnified by the chemical strengthening treatment.

Table 3: Treatment Conditions and Properties of Examples 7-13 and Comparative Examples EE-FF (Composition 2 with 30 pm thickness)

* = pH measured by dissolving 5 grams of the molten salt solution cooled to ambient temperature in 100 grams of deionized water

— = not measured

Table 4: Treatment Conditions and Properties of Examples 14-20 and Comparative Examples GG-HH (Composition 2 with 75 pm thickness)

[00189] Table 4 and FIG. 20 present the results for Composition 2 with a substrate thickness of 75 pm (instead of 30 pm as discussed above for Table 3 and FIG. 19). Curve 2007 corresponds to Comparative Example GG and Examples 14-16, respectively from left to right, that were chemically strengthened at 400°C for 12 minutes. As shown, a maximum compressive stress of 1123 MPa in curve 2007 (greater than Comparative Example EE by about 5% or more - i.e., 4.75%) was unexpectedly obtained for Example 14 with 5 wt% K2CO3. Example 15 also exhibits a compressive stress of about 1120 MPa. Based on this result, it is expected that adding from about 2 wt% to about 12 wt% (e.g., from about 2.5 wt% to about 12 wt%, from about 5 wt% to about 12 wt%, or even from about 2.5 wt% to about 5.0 wt%) K2CO3 to the molten salt bath will also exhibit the unexpectedly increased compressive stress (when chemically strengthening a substrate with a thickness greater than 50 gm - e.g., from about 50 gm to about 100 gm or from about 50 gm to about 90 gm - at 400°C).

[00190] Curve 2009 corresponds to Comparative Example HH and 17-19, respectively from left to right, that were chemically strengthened at 380°C for 18 minutes. As shown, curve 2009 (380°C) is above curve 2007 (400°C) for K2CO3 content around 10 wt% (e.g., from about 8 wt% to about 12 wt%). This is a different trend than was observed for the thinner substrates in FIG. 19 and Table 3. For curve 2009, a maximum compressive stress of 1131 MPa was unexpectedly observed for 10 wt% K2CO3 (Example 18). Based on this result, it is expected that from about 5 wt% to about 12 wt% (e.g., from about 8 wt% to about 12 wt%) K2CO3 to the molten salt bath will also exhibit the unexpectedly increased compressive stress (when chemically strengthening a substrate with a thickness greater than 50 pm - e.g., from about 50 gm to about 100 pm or from about 50 gm to about 90 gm - at 380°C). FIG. 20 also shows a difference 2017 between the compressive stress of Comparative Example GG (line 2005) and the compressive stress of Example 18 (line 2005) of about 50 MPa or more (e.g., about 60 MPa or more - 59 MPa) corresponding to an increase of about 5% or more (i.e., 5.5%) in compressive stress going from Comparative Example GG to Example 18.

[00191] As shown in Table 4 (but not plotted in FIG. 20), Example 20 corresponds to Example 18 without the silicic acid, and Example 20 still exhibits an increase in compressive stress relative to Comparative Examples GG-HH (although not relative to Example 18). This suggests that the thinner substrates (Table 3) are more sensitive to silicic acid content while the thicker substrates (Table 4) are less sensitive.

[00192] Examples 1-20 and Comparative Examples AA-HH were relatively quickly cooled (e.g., quenched) in air before being rinsed in deionized water; however, it is not always possible to quickly cool the substrates in an industrial environment, especially when the molten salt bath can hold multiple tons and can represent a substantial thermal mass. Although not shown, it was observed that residual molten salt deposits and/or optical distortions formed on substrates that were allowed to cool in air under industrial conditions, where it is believed that the cooling rate was relatively slow. Consequently, controlled cooling conditions are described in Tables 5-6. [00193] Tables 5-6 present the treatment conditions and properties for Examples 21-28. In addition to the chemical strengthening details in Tables 5-6, Examples 21-28 were also subjected to the following treatment: the substrate was removed from the molten salt solution (at 380°C or 420°C) and transferred to a cooling chamber (that was initially physically located above the molten salt solution for 5 minutes to allow residual molten salt bath to drip from the substrate back into the molten salt bath as a way of transferring the substrate to a cooling chamber) and held at 285°C for 5 minutes before being cooled from an initial temperature of about 270°C to a final temperature of about 70°C (roughly linear cooling profile with a cooling rate of about 4°C/minute) before being rinsed in a bath of deionized water and allowed to cool to ambient temperature.

[00194] Table 5 presents the results for Composition 1 with a thickness of 75 pm (and subjected to the cooling treatment described in the previous paragraph). As shown, the molten salt solutions with 5 wt% K2CO3 (both with and without silicic acid - Examples 22-24) exhibit increased compressive stress of about 5% or more (e.g., 4.8% or more for Examples 22-24) with the chemical strengthening with K2CO3 at 380°C demonstrating greater increases in compressive stress of about 7% or more (Examples 22 and 24) relative to Example 21.

Table 5: Treatment Conditions and Properties of Examples 21-24 (Composition 1 with 75 pm thickness) with cooling treatment

Table 6: Treatment Conditions and Properties of Examples 25-28 (Composition 2 with 75 pm thickness) with cooling treatment.

[00195] Table 6 presents the results for Composition 2 with a thickness of 75 pm (and subjected to the cooling treatment described above). As shown, the molten salt solutions with 5 wt% K2CO3 (both with and without silicic acid - Examples 26- 28) exhibit increased compressive stress with the chemical strengthening with K2CO3 at 380°C demonstrating greater increases in compressive stress of about 3.5% or more (Examples 26 and 28) and Example 28 exhibiting an increase in compressive stress of 5% or more (i.e., 6.5%) relative to Example 25. Also, although not shown, a visual inspection of Examples 21-28 with the naked eye did not detect any residual from the molten salt solution or optical distortions. Examples 21-28 (Tables 5-6) demonstrate that the controlled cooling conditions do not lead to optical distortions (for a substrate thickness of 50 pm or more) and the addition of K2CO3 (e.g., from about 2 wt% to about 12 wt%, from about 2 wt% to about 5 wt%, or from about 2.5 wt% to about 5.0 wt%) still produces increases in compressive stress (especially at temperatures less than 400°C).

[00196] Table 7 presents the treatment conditions and properties of Examples 29-39 that explored cooling treatments (in combination with the second potassium salt) for thinner (30 pm thickness) substrates of Composition. FIGS. 26A-26C are included to convey what level of distortions noted in the “Visual Inspection” column of Table 7. In FIGS. 26A-26C, the contours of fluorescent tube lights is reflected from the substrates are schematically shown. FIG. 26A shows a substrate with “light distortion” 2601, where the contours 2603 and 2605 are relatively smooth with little to no local deviations in the contours from the general shape of the contours, which is the target situation. FIG. 26B shows a substrate with “medium distortion” 2611, where the contours 2613, 2615 and 1617 show ripples (e.g., ripples 2614) that are pronounced local distortions of in the general shape of the contours (e.g., compare to the smoothness of contours 2603 and 2605 in FIG. 26A) with other contours (e.g., contour 219) may be relatively unaffected. FIG. 26C shows a substrate with “high distortion” 2621, where the contours 2623, 2625, 2627, and 2629 exhibit extreme waviness, for example, with the contours appear to be disconnected (or nearly disconnected) and/or little to none of the local shape of the contour has a shape resembling that of a substrate with lesser distortions (e.g., compare with FIGS. 26A- 26B). Although the angle used for viewing the substrate in FIG. 26A is different from the angle used to view FIGS. 26B-26C, the general guidance discussed above is applicable.

[00197] In Table 7, compressive stress differences are relative to the properties of Example 29. In Table 7, Examples 32, 34-36, and 38 exhibit high distortion. Also, Examples 32, 34-36, and 38 exhibited waviness in the surface, which can be observed by viewing the surface at an oblique angle. Notably, Examples 34 and36 with high distortion were cooled in ambient air, which was relatively slow (e.g., much less than 4°Cmin) in the industrial conditions used (for Examples 29-39). Likewise, Examples 35 with high distortion was cooled at a rate less than 4°C/min (e.g., less than 3°C/min). The high distortions observed for Example 32 in combination with the waviness suggests that the chemical strengthening was too quick (e.g., to high a temperature for too short a time - 400°C for 8 minutes - for a thickness less than 50 pm) to achieve a uniform compressive stress layer. Examples 30 and 39 exhibited medium distortions. Examples 29, 31, 33, 37, and 39 exhibited light distortions. Notably, Examples 29-31, 33, and 37 with medium or light distortions had a cooling rate of 4°C/min or more. Examples 31, 33, and 37 suggest that a cooling treatment that quickly decreases the temperature by about 100°C or more (e.g., about 120°C or more) relative to the temperature of the molten salt solution is associated with decreased distortions. Compared to Examples 21-28 in Tables 5-6 with a thickness of 75 pm, Examples 29-39 in Table 7 demonstrate that thinner substrates (e.g., thickness of about 50 pm or less, from about 10 pm to about 50 pm, or from about 10 pm to about 30 pm) are much more sensitive to the conditions for the cooling treatment. As discussed above, it is believed that the small differences (e.g., non-uniformities) in the compressive stress developed in the thinner substrates can lead to optical distortions. Consequently, quickly decreasing the temperature when the substrate is removed from the molten salt solution (e.g., by about 100°C or more or about 120°C or more) can reduce residual chemical strengthening in addition to decreasing a temperature of the cooling chamber by about 4°C/min or more (e.g., from about 4°C/min to about 20°C/min).

Table 7: Treatment Conditions and Properties of Examples 29-39 (Composition 1 with 30 pm thickness) with cooling treatment

[00198] FIGS. 21-22 and Tables 8-9 present the behavior for (survival % — % of samples withstanding) various parallel plate distances for Examples 40-47 and Comparative Examples II-LL. For Examples 40-47 and Comparative Examples II-LL a sample size of 30 sheets was used for test. In FIGS. 21-22, the vertical axis 2103 or 2203 (e.g., y-axis) corresponds to the % of samples withstanding (i.e., % surviving) a parallel plate distance, and the horizontal axis 2101 or 2202 (e.g., x-axis) corresponds to the parallel plate distance in mm tested. It is to be noted that the horizontal axis 2101 or 2202 is not linear; rather the axis labels correspond to the different distances where samples were measured, which is roughly logarithmically spaced (but not quite).

[00199] Table 8 presents the treatment conditions and properties for Examples 40-43 and Comparative Examples II-JJ (with quick cooling in air for nonindustrial conditions) with Composition 1 and a substrate thickness of 70 pm. Table 8 presents the survival % at (i.e., % of samples withstanding) parallel plate distances of 5 mm and 3 mm to highlight the trend shown in FIG. 21 with additional points (e.g., about a dozen different parallel plate distances in total). In FIG. 21, curves 2105 and 2107 correspond to Comparative Examples II-JJ, respectively; and curves 2109, 2111, 2113, and 2115 correspond to Examples 40-43, respectively. Unless otherwise indicated, etching for the Examples involved a 2 wt% HF (non-buffered) solution. As shown, curve 2107 (Comparative Example JJ - 0% CS removed by etching) has 0% of samples withstanding a parallel plate distance of even 5 mm. Curve 2105 (Comparative Example II - 18% CS removed by etching) has 100% of samples withstanding a parallel plate distance of 5 mm but 3% of samples withstanding a parallel plate distance of 3 mm. Examples 40-43 that were chemically strengthened with 5 wt% K2CO3 and different amounts of compressive stress (CS) removed by etching have 90% or more (e.g., 95% or more, about 97% or more) of samples withstanding a parallel plate distance of 5 mm. Specifically, Examples 42-43 (with Example 42 having roughly the same amount of compressive stress removed as in Comparative Example II) have 10% or more (e.g., about 20% or more) of samples withstanding a parallel plate distance of 3 mm, which is much higher than that seen for Comparative Example II. Consequently, the addition of 5 wt% K2CO3 can improve the foldability of the substrates, as demonstrated parallel plate performance (e.g., at 3 mm for a substrate thickness of 70 pm - from about 50 pm to about 100 pm or from about 50 pm to about 90 pm).

Table 8: Treatment Conditions and Properties of Examples 40-43 and Comparative Examples II-JJ (Composition 1 with 70 pm thickness)

Table 9: Treatment Conditions and Properties of Examples 44-47 and Comparative Examples KK-LL (Composition 2 with 70 pm thickness)

[00200] Table 9 presents the treatment conditions and properties for Examples 40-43 and Comparative Examples II-JJ (with quick cooling in air for nonindustrial conditions) with Composition 2 and a substrate thickness of 70 pm. Table 9 presents the survival % at (i.e., % of samples withstanding) parallel plate distances of 5 mm and 3 mm to highlight the trend shown in FIG. 22 with additional points (e.g., about a dozen different parallel plate distances in total). In FIG. 22, curves 2205 and 2207 correspond to Comparative Examples KK-LL, respectively; and curves 2209, 2211, 2213, and 2215 correspond to Examples 44-47, respectively. As shown, curve 2207 (Comparative Example LL - 0% CS removed by etching) has 0% of samples withstanding a parallel plate distance of even 5 mm. Curve 2105 (Comparative Example LL - 18% CS removed by etching) has 97% of samples withstanding a parallel plate distance of 5 mm and 60% of samples withstanding a parallel plate distance of 3 mm. Examples 44-47 that were chemically strengthened with 5 wt% K2CO3 and different amounts of compressive stress (CS) removed by etching have 90% or more (e.g., 95% or more, about 97% or more, or about 100% for Examples 45 and 47) of samples withstanding a parallel plate distance of 5 mm. Examples 46-47 have 10% or more (e.g., about 20% or more, about 30% or more or about 40% or more) of samples withstanding a parallel plate distance of 3 mm. Example 47 has 63% of samples withstanding a parallel plate distance of 3 mm, which is much higher than that seen for Comparative Example KK. Consequently, the addition of 5 wt% K2CO3 can maintain or improve the foldability of the substrates, as demonstrated parallel plate performance (e.g., at 3 mm for a substrate thickness of 70 pm - from about 50 pm to about 100 pm or from about 50 pm to about 90 pm).

[00201] FIGS. 23-24 and Tables 10-11 present the behavior for (survival % - - % of samples withstanding) various parallel plate distances for Examples 47-52 and Comparative Examples MM-PP. For Examples 48-53 and Comparative MM-PP, a sample size of 30 sheets was used for test. In FIGS. z223-24, the vertical axis 2303 or 2403 (e.g., y-axis) corresponds to the % of samples withstanding (i.e., % surviving) a parallel plate distance, and the horizontal axis 2301 or 2402 (e.g., x-axis) corresponds to the parallel plate distance in mm tested. It is to be noted that the horizontal axis 2301 or 2402 are linear (corresponding to measurements every 0.2 mm decrease in parallel plate distance - as opposed to the scale used in FIGS. 21-22).

[00202] Table 10 presents the treatment conditions and properties for Examples 48-50 and Comparative Examples MM-NN (with quick cooling in air for non-industrial conditions) with Composition 1 and a substrate thickness of 30 pm. Table 10 presents the survival % at (i.e., % of samples withstanding) parallel plate distances of 2 mm and 1 mm to highlight the trend shown in FIG. 23 with additional points (e.g., about seven different parallel plate distances in total). In FIG. 23, curves 2305 and 2307 correspond to Comparative Examples MM-NN, respectively; and curves 2309, 2311, 2313, and 2315 correspond to Examples 48-50, respectively. As shown, curve 2307 (Comparative Example NN - 0% CS removed by etching) has 30% of samples withstanding a parallel plate distance of 2 mm and 0% of samples withstanding a parallel plate distance of 1 mm. Curve 2305 (Comparative Example MM - 18% CS removed by etching) has 100% of samples withstanding a parallel plate distance of 2 mm but 10% of samples withstanding a parallel plate distance of 1 mm. Examples 47-49 that were chemically strengthened with 5 wt% K2CO3 and different amounts of compressive stress (CS) removed by etching have 90% or more of samples withstanding a parallel plate distance of 2 mm. Specifically, Example 50 having 95% or more (e.g., about 100%) of samples withstanding a parallel plate distance of 2 mm and more than 10% (e.g., 15% or more or about 20% or more) of samples withstanding a parallel plate distance of 1 mm, which is much higher than that seen for Comparative Example MM. Consequently, the addition of 5 wt% K2CO3 can improve or maintain the foldability of the substrates, as demonstrated parallel plate performance (e.g., at 1 mm for a substrate thickness of 30 pm - from about 10 pm to about 50 pm or from about 10 pm to about 30 pm).

Table 10: Treatment Conditions and Properties of Examples 48-50 and Comparative Examples MM-NN (Composition 1 with 30 pm thickness)

Table 11 : Treatment Conditions and Properties of Examples 51-53 and Comparative Examples OO-PP (Composition 2 with 30 pm thickness)

[00203] Table 11 presents the treatment conditions and properties for Examples 51-53 and Comparative Examples OO-PP (with quick cooling in air for non-industrial conditions) with Composition 2 and a substrate thickness of 30 pm. Table 11 presents the survival % at (i.e., % of samples withstanding) parallel plate distances of 2 mm and 1 mm to highlight the trend shown in FIG. 24 with additional points (e.g., about seven different parallel plate distances in total). In FIG. 24, curves 2405 and 2407 correspond to Comparative Examples OO-PP, respectively; and curves 2409, 2411, 2413, and 2415 correspond to Examples 51-53, respectively. As shown, curve 2407 (Comparative Example PP - 0% CS removed by etching) has 60% of samples withstanding a parallel plate distance of 2 mm and 0% of samples withstanding a parallel plate distance of 1 mm. Curve 2405 (Comparative Example OO - 18% CS removed by etching) has 95% of samples withstanding a parallel plate distance of 2 mm and 25% of samples withstanding a parallel plate distance of 1 mm. Examples 51-53 that were chemically strengthened with 5 wt% K2CO3 and different amounts of compressive stress (CS) removed by etching have 90% or more of samples withstanding a parallel plate distance of 2 mm. Specifically, Examples 52-53 have 95% or more (e.g., about 100%) of samples withstanding a parallel plate distance of 2 mm. For Example 53, it is believed that handling issues contributed to a relatively low survival % at a parallel plate distance of 1 mm. In view of the results in Table 10 and the 2 mm survival rate in Table 11, the addition of 5 wt% K2CO3 can improve or maintain the foldability of the substrates, as demonstrated parallel plate performance (e.g., for a substrate thickness of 30 pm - from about 10 pm to about 50 pm or from about 10 pm to about 30 pm).

[00204] Table 12 presents the threshold pen drop height for Example 3 and Comparative Examples AA and MM for substrates with Composition 1 and a thickness of 80 pm. In Table 12, the samples were not etched to isolate the impact of the molten salt solution on the impact resistance, as evaluated in the Pen Drop Test. As shown, Example 3 has a threshold pen drop height of 16.6 cm, which is 3.1 cm higher than (23% increase compared to) Comparative Example MM and 2 cm higher than (14% increase compared to) Comparative Example AA. Consequently, the addition of 5 wt% K2CO3 can increase the impact resistance of the substrate, as measured by the pen drop threshold height.

Table 12: Threshold Pen Drop Heights for Example 3 and Comparative Examples AA and MM (Composition 1 and 80 pm thickness)

Table 13: Pen Drop Heights for Example 6 and Comparative Examples CC and NN (Composition 2 and 80 pm thickness)

[00205] Table 13 presents the threshold pen drop height for Example 6 and Comparative Examples CC and NN for substrates with Composition 2 and a thickness of 80 pm. In Table 13, the samples were not etched to isolate the impact of the molten salt solution on the impact resistance, as evaluated in the Pen Drop Test. As shown, Example 6 has a threshold pen drop height of 21.1 cm, which is 12.8 cm higher than (152% increase compared to) Comparative Example NN and 4.9 cm higher than (30% increase compared to) Comparative Example CC. Consequently, the addition of 5 wt% K2CO3 can increase the impact resistance of the substrate, as measured by the pen drop threshold height. Specifically, Example 6 is able to withstand a pen drop height of 20 cm or more.

[00206] For the rest of the Example, unless otherwise indicated, the Examples (and Comparative Examples) were chemically strengthened in 100 wt% KNO3 maintained at 400°C for 12 minutes. Also, unless otherwise indicated, etching treatments occurred at 22°c.

[00207] Table 14 presents the etchant composition and conditions for Examples 54-56 and Comparative Example SS. Comparative Example SS is a nonbuffered HF solution while Examples 54-56 are buffered HF solutions due to the addition of NH4F. As shown, the buffered HF solutions (Examples 54-56) have higher pH than Comparative Example SS. However, as indicated in the “visual inspection” column, these samples had various issues. Example 54 and Comparative Example SS had a blue color that disappeared when the concentration of HF was decreased in Examples 55-56 to less than 2 wt% HF. Example 55 exhibited noticeable warp. Example 56 was hazy, which is attributed to the formation of a precipitate that patterned the etching - possibly due to the higher NH4F concentration there.

Table 14: Etching Conditions and Properties of Examples 54-56 and Comparative Example SS (Composition 2 with 30 pm thickness)

[00208] Consequently, modifications to the etching conditions were investigated in Examples 57-61 shown in Table 15 (with Example 56 and Comparative Example presented again for comparison). For Examples 57-58, the substrate was rinsed halfway through the etching (after 50 seconds) and then etched for the remaining time (another 50 second) before being rinsed again - whereas Examples 56 and 59-61 were only rinsed after the full etching time. In Example 57 the rinse halfway through was with deionized water (DI), which decreased the haze but also imparted a slight wrinkle to the surface. Example 58 used HNO3 instead of DI for the rinse halfway through the etching, but Example 58 also exhibited low haze and low wrinkling. It is believed that either (1) transferring the substrate between the etchant and the rinsing solution multiple times allowed residual etchant on parts of the surface to continue etching while other parts were not etched, (2) there was still some precipitant buildup on the surface that effective masked the etching, or (3) both. Also, Examples 57-58 removed more compressive stress than Example 56 despite having the same total etching time (50 seconds x 2 cycles = 100 seconds). Instead, Examples 59-61 used a shorter overall etching time to reduce the ability of a precipitant to form on the surface that could mask the etching. In Example 59, the substrate was rinsed with DI; in Example 60, the substrate was rinsed with oxalic acid; and in Example 61, the substrate was rinsed with nitric acid. Still, Examples 59-61 exhibited noticeable wrinkling on the surface.

Table 15: Etching Conditions and Properties of Examples 56-61 and Comparative Example SS (Composition 2 with 30 pm thickness)

[00209] Table 16 further explored modifications to the etching conditions as well as modifications to the buffered HF composition for Examples 62-66 (with Example 56 repeated for comparison). In Examples 62-63, the etching temperature was increased, which decreased the haze, but there was still noticeable haze and some wrinkling of the surface (with Example 63 additionally be warped). Examples 64-66 had decreased etchant concentration (i.e., halved concentration relative to Example 56), which did not have any noticeable defects to the naked eye. This was true for etching times from 100 seconds to 200 seconds (that were investigated) as well as temperatures from 22°C to 30°C (that were investigated). Also, as noted in Table 16, Examples 64-66 had an etch rate of about 1.0 pm/min or less while Examples 56 and 62-63 had higher etching rates. Consequently, etching rates of about 1.0 pm/min or less unexpectedly provided etched substrates with no visible defects. It is to be noted that this is only an issue for thinner substrate (e.g., about 50 pm or less, from about 10 pm to about 50 pm, or from about 10 pm to about 30 pm) and certain compositions (e.g., Composition 2 but not necessarily Composition 1).

Table 16: Etching Conditions and Properties of Examples 56 and 62-66 (Composition 2 with 30 pm thickness)

[00210] Table 17 presents the etching compositions and rinsing conditions for Examples 65 and 67-68 and Comparative Example TT that all had a substrate thickness of 30 pm. Comparative Example TT had Composition 1 and demonstrates that the blue color seen with Composition 2 (Comparative Example SS) is not seen for Composition 1. For Example 67, the substrate was rinsed twice during the etching (evenly spaced) and again at the end (for effectively three etch-rinse cycles). Even with the lower etchant concentration (Example 67 compared to Example 62), Example 67 had noticeable haze and wrinkling of the surface. This suggests that some uneven etching was occurring when the substrate was transferred from the etching solution to the DI rinse. Example 68 was the same as Example 67 but the rinsing solution was 5 wt% nitric acid instead of DI, but Example 68 also has noticeable wrinkling on the surface. Even with the wrinkling seen in Examples 67-68, both Examples 67-68 had higher survival rates for a 1 mm parallel plat distance (68% and 23%, respectively) than Comparative Example TT. However, Example 65 (discussed above) is able to have greater than 20% (e.g., about 30% or more, about 40% or more) of sample withstand a parallel plate distance of 1 mm without any visible defects.

Table 17: Treatment Conditions and Properties of Examples 65 and 67-68 and Comparative Example TT (30 pm thickness)

Table 18: Treatment Conditions and Properties of Examples 70-73 and Comparative Examples AA and SS (Composition 2 with 30 pm thickness)

[00211] Table 18 and FIG. 25 present the properties of Examples 69-72 and Comparative Examples AA and SS. As noted above, Examples 69-72 and Comparative Examples AA and SS were chemically strengthened in 100 wt% KNO3 maintained at 400°C for 12 minutes before being quickly cooled (not in an industrial setting). The composition of the etching solution and time that the etching solution was in contact with the substrate are presented in Table 18. In FIG. 25, the survival rate (i.e., percent of samples withstanding a specific parallel plate distance) is presented on the vertical axis 2503 (e.g., y-axis) while the parallel plate distance in mm tested is presented on the horizontal axis 2501 (e.g., x-axis) on a linear scale. In FIG. 25, curves 2505 and 2515 correspond to Comparative Examples AA and SS, respectively. Curves 2507, 2509, 2511, and 2513 correspond to Examples 69-72 respectively. As shown in Table 18 and FIG. 25, Comparative Example AA had 35% of the samples withstand a parallel plate distance of 2 mm but 0% withstood a parallel plate distance of 1 mm. Comparative Example SS faired better with 93% of samples withstanding a parallel plate distance of 2 mm and 15% of samples withstanding a parallel plate distance of 1 mm.

[00212] Examples 69-72 used the lower concentration buffered HF solution of Examples 64-65 with different treatment times increasing from 50 seconds to 175 seconds going from Example 69 to Example 72. As shown, Examples 69-72 all had more than 90% (e.g., about 95% or more) of sample withstand a parallel plate distance of 2 mm. Examples 70-72 had more than 20% of samples withstand a parallel plate distance of 1 mm, which is better than Comparative Examples AA and SS. Further, Examples 71-72 had about 30% or more (e.g., about 40% or more or about 50% or more) of samples withstand a parallel plate distance of 1 mm, which is more than double the rate for Comparative Example SS. This demonstrates that the lower concentration buffered HF etching solution (e.g., with treatment times less than 3.5 minutes and with an etching rate of about 1.0 pm/min or less) provide unexpectedly improved foldability and reliability, as demonstrated by the increased survival rate at a parallel plate distance of 1 mm.

[00213] The above observations can be combined to provide chemically- strengthened substrates (e.g., foldable substrates) and methods of chemically strengthened substrates (e.g., to make the same). Providing a glass-based substrate and/or a ceramic-based substrate can provide good dimensional stability, reduced incidence of mechanical instabilities, and/or good impact and puncture resistance. Methods of the aspects of the disclosure can increase a pen drop height that the foldable apparatus and/or foldable substrate can withstand, increase a survival rate of substrates folding to a parallel plate distance of 5 mm, 3 mm, 2 mm, and/or 1 mm, and/or increase a foldability of the substrate.

[00214] In aspects, the substrate can be chemically strengthened with a molten salt solution comprising at two anions associated with at least a first potassium salt and a second potassium salt. Providing the first potassium salt with multiple (i.e., two or more) potassium atoms per anion can increase an effective concentration and/or activity of potassium in the molten salt solution, which can facilitate increased maximum compressive stress in the resulting chemically strengthened foldable substrate. Providing a first potassium salt in the molten salt solution with a pKa of about 9 or more and/or a pH from about 9 to 12 of the molten salt solution can improve the strength and/or foldability of the resulting chemically strengthened foldable substrate, for example, by selectively etching flaws inherent in the foldable substrate that might otherwise be magnified by the chemical strengthening treatment. As discussed with reference to the Examples here, potassium carbonate (K2CO3) has a more pronounced and unexpected increase in compressive stress than other components in molten salt solutions. Additionally, without wishing to be bound by theory, it is believed that the carbonate anion can facilitate precipitation of other cations (e.g., lithium, sodium) exchanged out of the foldable substrate, which can increase a longevity of the molten salt solution (e.g., by removing components from the solution phase that could otherwise “poison” the molten salt solution). As demonstrated by the Examples discussed herein, providing a first temperature of the molten salt solution less than 400°C can increase a maximum compressive stress developed for a predetermined depth of layer and/or depth of compression. Also, for some of the molten salt solutions discussed herein, a temperature of 350°C or more may be used to ensure that salts are molten.

[00215] It has been observed that foldable substrates with a thickness of about 50 pm or less (e.g., from about 10 pm to about 50 pm or from about 10 pm to about 30 pm) are unexpectedly sensitive to what happens after the foldable substrate is removed from the molten salt solution. For these thin foldable substrates, even relatively small difference in compressive stress across the surface thereof can result in waviness and/or warp that can produce optical distortions that can be visible to a user of a consumer electronic product that the foldable substrate may be incorporated in. Consequently, the controlled temperature of the cooling chamber can facilitate a relatively even compressive stress across the surface of the foldable substrate. Also, providing an initial temperature of the cooling chamber of 180°C or more (e.g., 200°C or more or 220°C or more) can facilitate the removal of a residual portion of the molten salt solution before it solidifies. Without wishing to be bound by theory, the first potassium salt can have a higher melting temperature than the second potassium salt, which means that incorporating the first potassium salt in the molten salt solution can increase a viscosity of the molten salt solution and/or cause the molten salt solution to solidify at higher temperature than a molten salt solution without the first potassium salt. Consequently, allowing a residual portion of the molten salt solution on the foldable substrate after it is removed from the molten salt solution can be especially useful when the molten salt solution includes the first potassium salt. Reducing the temperature of the cooling chamber to a final temperature of about 100°C or less (e.g., from about 25°C to about 100°C or from about 60°C to about 90°C) can enable the foldable substrate to be subsequently treated (e.g., relatively quickly or immediately) thereafter using aqueous solutions (e.g., rinsing with water or an alkaline detergent solution, contact with an aqueous acidic solution). Providing a cooling rate from about 4°C/min to about 20°C/min can quickly reduce a temperature of the cooling chamber (and foldable substrate) while being able maintain a relatively consistent temperature throughout the cooling chamber (and/or foldable substrate), for example, to produce a relatively consistent compressive stress across the surface of the foldable substrate.

[00216] Providing an etching rate of about 1 pm/min or less (e.g., about 1.0 pm/min or less) can facilitate a substantially uniform removal of material from the surface(s) of the foldable substrate. As discussed above, foldable substrates with a thickness of about 50 pm or less (e.g., from about 10 pm to about 50 pm or from about 10 pm to about 30 pm) are quite sensitive to differences in compressive stress and thickness variation across its surface. Consequently, providing an etching rate of about 1 pm/min can remove a relatively uniform thickness and portion of the compressive stress from the surface(s) to reduce an incidence of waviness and/or warp that can produce optical distortions that can be visible to a user of a consumer electronic product that the foldable substrate may be incorporated in. Without wishing to be bound by theory, providing a relatively low temperature of acidic solution (e.g., from about 20°C to about 40°C or from about 20°C to about 25°C) can decrease the concentration of SiFe' anions since the reaction from FhSiFe and 2 H⁺ + SiFe' is endothermic. Decreasing a concentration of SiFe' anions can be associated with decreased deposition (e.g., redeposition) of silica or silica-like materials on the surface that could otherwise produce variation in the thickness and/or compressive stress across the surface of the foldable substrate. Providing a relatively high pH (e.g., from about 3.5 to about 4.5, from about 3.6 to about 4.3, or from about 3.7 to about 4.0) can decrease an etching rate that can help produce a relatively uniform compressive stress and thickness across the foldable substrate. Providing a combined concentration of HF and NH4F of about 4.0 wt% or less, about 3.5 wt% or less, about 3.0 wt% or less, about 2.5 wt% or less, or about 2.0 wt% (e.g., from about 1.25 wt% to about 4.0 wt%, from about 1.3 wt% to about 3.5 wt%, from about 1.35 wt% to about 3.0 wt%, from about 1.4 wt% to about 2.5 wt%, from about 1.5 wt% to about 2.0 wt%) can provide relatively controlled and even etching of the foldable substrate and/or reduce deposition of material (e.g., silica, silica-like material, ammonium fluoride crystals) on the foldable substrate that could impair the optical properties of the foldable substrate.

[00217] In aspects, the substrate thickness of the substrate can be about 50 pm or more (e.g., from about 50 pm to about 100 pm, from about 50 pm to about 90 pm, or any of the corresponding subranges therebetween discussed above) and associated with one or more of: (1) a depth of compression as a percentage of the substrate thickness 209 from about 10% to about 30%, from about 16% to about 26%, or any of the corresponding subranges therebetween discussed above, (2) a depth of layer (e.g., first depth of layer and/or second depth of layer) of potassium in a range from about from about 3 pm to about 20 pm, from about 10 pm to about 15 pm, or any corresponding subrange discussed above, and/or (3) a maximum compressive stress (e.g., first maximum compressive stress and/or second maximum compressive stress) can be in a range from about 650 MPa to about 1,200 MPa, from about 800 MPa to about 1,100 MPa, from about 850 MPa to about 1,200 MPa, or any corresponding subrange discussed above. In aspects, the substrate thickness can be about 50 pm or less (e.g., from about 10 pm to about 50 pm, from about 10 pm to about 30 pm, or any of the corresponding subranges therebetween discussed above) and associated with one or more of: (1) a depth of compression as a percentage of the substrate thickness from about 10% to about 30%, from about 12% to about 19%, or any of the corresponding subranges therebetween discussed above, (2) a depth of layer of potassium in a range from about from about 3 pm to about 20 pm, from about 5 pm to about 9 pm, or any corresponding subrange discussed above, and/or (3) a maximum compressive stress (e.g., first maximum compressive stress and/or second maximum compressive stress) can be in a range from about 650 MPa to about 1,200 MPa, from about 750 MPa to about 1,100 MPa, from about 750 MPa to about 1,000 MPa, or any corresponding subrange discussed above.

[00218] Directional terms as used herein — for example, up, down, right, left, front, back, top, bottom — are made only with reference to the figures as drawn and are not intended to imply absolute orientation.

[00219] It will be appreciated that the various disclosed aspects may involve features, elements, or steps that are described in connection with that aspect. It will also be appreciated that a feature, element, or step, although described in relation to one aspect, may be interchanged or combined with alternate aspects in various nonillustrated combinations or permutations. [00220] It is also to be understood that, as used herein the terms “the,” “a,” or “an,” mean “at least one,” and should not be limited to “only one” unless explicitly indicated to the contrary. For example, reference to “a component” comprises aspects having two or more such components unless the context clearly indicates otherwise. Likewise, a “plurality” is intended to denote “more than one.”

[00221] As used herein, the term “about” means that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error, and the like, and other factors known to those of skill in the art. Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, aspects include from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. Whether or not a numerical value or endpoint of a range in the specification recites “about,” the numerical value or endpoint of a range is intended to include two aspects: one modified by “about,” and one not modified by “about.” It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint and independently of the other endpoint.

[00222] The terms “substantial,” “substantially,” and variations thereof as used herein are intended to note that a described feature is equal or approximately equal to a value or description. For example, a “substantially planar” surface is intended to denote a surface that is planar or approximately planar. Moreover, as defined above, “substantially similar” is intended to denote that two values are equal or approximately equal. In aspects, “substantially similar” may denote values within about 10% of each other, for example, within about 5% of each other, or within about 2% of each other.

[00223] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that any particular order be inferred. [00224] While various features, elements, or steps of particular aspects may be disclosed using the transitional phrase “comprising,” it is to be understood that alternative aspects, including those that may be described using the transitional phrases “consisting of’ or “consisting essentially of,” are implied. Thus, for example, implied alternative aspects to an apparatus that comprises A+B+C include aspects where an apparatus consists of A+B+C and aspects where an apparatus consists essentially of A+B+C. As used herein, the terms “comprising” and “including”, and variations thereof shall be construed as synonymous and open-ended unless otherwise indicated.

[00225] The above aspects, and the features of those aspects, are exemplary and can be provided alone or in any combination with any one or more features of other aspects provided herein without departing from the scope of the disclosure.

[00226] It will be apparent to those skilled in the art that various modifications and variations can be made to the present disclosure without departing from the spirit and scope of the disclosure. Thus, it is intended that the present disclosure cover the modifications and variations of the aspects herein provided they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. A method of chemically strengthening a substrate, the substrate comprising a thickness from 10 micrometers to 100 micrometers defined between an existing first major surface and an existing second major surface opposite the existing first major surface, the method comprising: contacting the existing first major surface of the substrate with a molten salt solution maintained a first temperature for a first period of time, the molten salt solution comprising at least two anions associated with at least a first potassium salt and a second potassium salt, a concentration of the first potassium salt potassium salt and a concentration of the second potassium salt is 2 wt% or more of the molten salt solution, the first temperature is in a range from about 350°C to about 400°C, and the first period of time is in a range from about 10 minutes to about 90 minutes.

2. The method of claim 1, wherein the first potassium salt comprises two or more potassium atoms per anion, and a pKa of the potassium salt is 9 or more, and a concentration of the first potassium salt is in a range from about 2 wt% to about 12 wt% of the molten salt solution.

3. The method of any one of claims 1-2, wherein the first potassium salt is potassium carbonate K2CO3, and a concentration of the first potassium salt is in a range from about 2 wt% to about 12 wt% of the molten salt solution.

4. The method of any one of claims 1-3, wherein the concentration of the first potassium salt is in a range from about 2.5 wt% to about 5.0 wt%.

5. The method of any one of claims 1-3, wherein the concentration of the first potassium salt is in a range from about 5 wt% to about 12 wt%.

6. The method of any one of claims 2-5, wherein the molten salt solution further comprises from 0 wt% to 5 wt% of a third potassium salt associated with a third anion that is different from the anions associated with the first potassium salt and the second potassium salt, and the third potassium salt comprises two or more potassium atoms per anion.

7. The method of claim 6, wherein the third potassium salt comprises potassium sulfate K2SO4, and a concentration of the third potassium salt is from about 0.5 wt% to about 5 wt

8. The method of any one of claims 1-7, wherein the second potassium salt is potassium nitrate KNO3, and a concentration of the second potassium salt is in a range from about 50 wt% to about 98 wt% of the molten salt solution.

9. The method of any one of claims 1-7, wherein a pH of the molten salt solution at the first temperature is in a range from about 9 to 12.

10. The method of any one of claims 1-9, wherein a presence of the first potassium salt increases a compressive stress imparted by the contacting the existing first major surface with the molten salt solution by about 5% or more relative to immersing the substrate in a comparative molten salt solution with the same composition as the molten salt solution with the absence of the first potassium salt.

11 The method of any one of claims 1-10, wherein the thickness of the substrate is in a range from about 15 pm to about 50 pm.

12. The method of any one of claims 1-11, further comprising, after the contacting the existing first major surface with the molten salt solution: transferring the substrate from the molten salt solution to a cooling chamber, a temperature of the cooling chamber decreases from an initial temperature to a final temperature at a cooling rate in a range from about 4 °C/min to about 20 °C/min, the initial temperature is in a range from about 180°C to about 300°C, and the final temperature is in a range from about 25°C to about 100°C.

13. The method of claim 12, wherein the initial temperature is in a range from about 180°C to about 220°C.

14. The method of any one of claims 12-13, further comprising, after the cooling chamber reaches the final temperature, rinsing the substrate with water, an alkaline detergent solution, or combinations thereof.

15. The method of any one of claims 1-14, wherein an initial maximum compressive stress of the substrate after the contacting the existing first major surface with the molten salt solution is from about 800 MegaPascals to about 1500 MegaPascals.

16. The method of any one of claims 1-15, further comprising: contacting the existing first major surface with an acidic solution for a second period of time to remove an outer layer from the existing first major surface to form a new first major surface, a pH of the acidic solution is in a range from 3.5 to 4.5, and the second period of time is from about 10 seconds to about 3.5 minutes; and then rinsing the new first major surface with water.

17. A chemically-strengthened substrate comprising: a thickness defined between a first major surface and a second major surface opposite the first major surface, the thickness is from about 10 micrometers to about 100 micrometers; and a first compressive stress region extending to a first depth of compression from the first major surface, a first depth of layer of potassium is about 5 micrometers or more, a maximum first compressive stress is from about 650 MegaPascals to about 1200 MegaPascals, wherein the chemically-strengthened substrate comprises a glass-based material, 95% or more of samples of the chemically-strengthened substrate can withstand a parallel plate distance of 5 millimeters, and the substrate exhibits a pen drop threshold height of 10 centimeters or more in a Pen Drop Test.

18. The chemically-strengthened substrate of claim 17, wherein a composition of the chemically-strengthened substrate, as a mol% of the chemically-strengthened substrate: from about 60 mol% to about 70 mol% SiCh; from about 8 mol% to about 16 mol% AI2O3; from about 12 mol% to about 18 mol% Na?O; from about 2 mol% to about 6 mol% MgO; and from about 0.1 mol% to about 2.0 mol% CaO.

19. The chemically-strengthened substrate of any one of claims 17-18, wherein 90% or more of samples of the substrate can withstand a parallel plate distance of 5 millimeters.

20. The chemically-strengthened substrate of any one of claims 17-19, wherein the thickness is from about 10 micrometers to about 50 micrometers, and 90% or more of samples of the substrate can withstand a parallel plate distance of 2 millimeters.

21. The chemically-strengthened substrate of any one of claims 17-20 inclusive, wherein about 30% or more samples of the chemically-strengthened substrate can withstand a parallel plate distance of 3 millimeters.