WO2014126633A2

WO2014126633A2 - Spallation-resistant thermal barrier coating

Info

Publication number: WO2014126633A2
Application number: PCT/US2013/072558
Authority: WO
Inventors: Brian T. Hazel; David A. Litton
Original assignee: United Technologies Corp
Current assignee: RTX Corp
Priority date: 2012-12-26
Filing date: 2013-12-02
Publication date: 2014-08-21
Anticipated expiration: 2015-06-26
Also published as: WO2014126633A3

Abstract

In a method for coating a substrate (22), a ceramic first layer (40) is applied by electron beam physical deposition (210). A ceramic second layer (42) is suspension plasma sprayed (220).

Description

SPALLA ION-RESISTANT THERMAL BARRIER COATING

BACKGROUND

[0001] The disclosure relates gas turbine engines. More particularly, the disclosure relates to thermal barrier coatings for gas turbine engines.

[0002] Gas turbine engine gaspath components are exposed to extreme heat and thermal gradients during various phases of engine operation. Thermal-mechanical stresses and resulting fatigue contribute to component failure. Significant efforts are made to cool such components and provide thermal barrier coatings to improve durability.

[0003] Exemplary thermal barrier coating systems include two-layer thermal barrier coating systems. An exemplary system includes NiCoCrAlY bond coat (e.g., low pressure plasma sprayed (LPPS) ) and an yttria-stabilized zirconia (YSZ) thermal barrier coat (TBC) (e.g., air plasma sprayed (APS) or electron beam physical vapor deposited (EBPVD) ) . Prior to and while the barrier coat layer is being deposited, a thermally grown oxide (TGO) layer (e.g., alumina) forms atop the bond coat layer. As time-at-temperature and the number of cycles increase, this TGO interface layer grows in thickness. An exemplary YSZ is 7 weight percent yttria-stabilized zirconia (7YSZ) .

[0004] US2003/0152814 discloses a thermal barrier coating wherein a strain-tolerant columnar grain ceramic (e.g., 7YSZ) is applied by EB-PVD followed by air plasma spray or low pressure plasma spray of an insulative layer (e.g.,

yttria-ceria) . US7306859 discloses EB-PVD of YSZ to form a columnar layer followed by plasma spray to form a non-columnar layer that is relatively thick along the platform surface of a blade .

[0005] Exemplary TBCs are applied to thicknesses of 1-40 mils ( 0.025-1. Omm) and can contribute to a temperature reduction of up to 300°F (167°C) at the base metal. This temperature reduction translates into improved part durability, higher turbine operating temperatures, and improved turbine

efficiency .

SUMMARY

[0006] One aspect of the disclosure involves a method for coating a substrate. A ceramic first layer is applied by electron beam physical vapor deposition (EB-PVD) . A ceramic second layer is applied by suspension plasma spray.

[0007] In additional or alternative embodiments of any of the foregoing embodiments, during said spraying of the second layer, an environmental pressure remains at least 95kPa.

[0008] In additional or alternative embodiments of any of the foregoing embodiments, a metallic bondcoat is applied prior to the electron beam physical vapor deposition of the first layer .

[0009] In additional or alternative embodiments of any of the foregoing embodiments, the second layer is applied directly atop the first layer.

[0010] In additional or alternative embodiments of any of the foregoing embodiments: the substrate is a substrate of a turbine element having an airfoil and a gaspath surface transverse thereto; a characteristic thickness of the first layer is greater along the airfoil than along the gaspath surface; and a characteristic thickness of the second layer is greater along the gaspath surface than along the airfoil.

[0011] In additional or alternative embodiments of any of the foregoing embodiments: the characteristic thickness of the first layer along the gaspath surface is less than 65% of the characteristic thickness of the first layer along the airfoil; and the characteristic thickness of the second layer along the airfoil is less than 50% of the characteristic thickness of the second layer along the gaspath surface. [0012] In additional or alternative embodiments of any of the foregoing embodiments: the first layer has a thickness of 0.013-0.076mm; and the second layer has a thickness of

0.025-0.48mm.

[0013] In additional or alternative embodiments of any of the foregoing embodiments: the electron beam physical vapor deposition of the ceramic first layer is to a first layer depth of at least 0.013mm; and the suspension plasma spraying of the ceramic second layer is to a second layer depth of at least 0.025mm.

[0014] In additional or alternative embodiments of any of the foregoing embodiments, the second layer comprises

yttria-stabilized zirconia or gadolinium-stabilized zirconia.

[0015] In additional or alternative embodiments of any of the foregoing embodiments, the substrate comprises a nickel-based superalloy .

[0016] In another aspect, a coated article may comprise: a substrate; an EB-PVD ceramic first layer; and a suspension plasma sprayed ceramic second layer above the first layer.

[0017] In additional or alternative embodiments of any of the foregoing embodiments: the first layer comprises material selected from the group consisting of yttria-stabilized zirconia or gadolinium-stabilized zirconia or combinations thereof; and the second layer comprises yttria-stabilized zirconia or gadolinium-stabilized zirconia.

[0018] In additional or alternative embodiments of any of the foregoing embodiments, the substrate comprises a

nickel-based superalloy.

[0019] In additional or alternative embodiments of any of the foregoing embodiments, a bondcoat is between the first layer and the substrate.

[0020] In additional or alternative embodiments of any of the foregoing embodiments, the article consists essentially of the substrate, the bondcoat, the first layer, and the second layer .

[0021] The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a partially schematic sectional view of substrate having a thermal barrier coating (TBC) .

[0023] FIG. 2 is a partially schematic view of a vane bearing the TBC.

[0024] FIG. 3 is a partially schematic view of a blade bearing the TBC.

[0025] FIG. 4 is a flowchart of a process for coating the substrate of FIG. 1.

[0026] Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0027] FIG. 1 shows a thermal barrier coating system 20 atop a metallic substrate 22. In an exemplary embodiment, the

substrate is a nickel-based superalloy or a cobalt-based superalloy such as a cast component (e.g., a single crystal casting) of a gas turbine engine. Exemplary components are hot section components such as combustor panels, turbine blades, turbine vanes, and airseals. One particular alloy is PWA 1484.

[0028] The coating system 20 may include a bondcoat 30 atop a surface 26 of the substrate 22 and a thermal barrier coating (TBC) system 28 atop the bondcoat. A thermally grown oxide (TGO) layer 24 may form at the interface of the bondcoat to the TBC.

[0029] The TBC is a multi-layer TBC with at least two layers. A first layer 40 is a lower layer. A second layer 42 is over the first layer. In the exemplary system, the TBC consists of or consists essentially of the first and second layers (e.g., subject to relatively small gradation/transition with each other and with the bondcoat (if any) as noted above) .

[0030] FIG. 2 shows a vane 50 comprising the cast metallic substrate 22. The vane includes an airfoil 52 having a surface comprising a leading edge 54, a trailing edge 56, a pressure side 58, and a suction side 60. The airfoil extends from an inboard end at a platform or band segment 62 to an outboard end and an outboard shroud or band segment 64. The segments 62 and 64 have respective gaspath surfaces 66 and 68. These are essentially normal to the airfoil surfaces. The TBC system extends at least along the surface of the airfoil and the surfaces 66 and 68.

[0031] The exemplary first layer 40 is applied in such a way as to be thicker along the airfoil surface than along the surfaces 66 and 68. For example, it may be applied via

electron beam physical vapor deposition (EB-PVD) or other PVD. During deposition, the airfoil may be rotated and tilted relative to the vapor source to provide full circumferential coverage of the airfoil surface. However, during this process, the surfaces 66 and 68 are relatively parallel to the flow direction and receive only a fraction of the coating thickness achieved on the airfoil. Thus, the thickness of the first layer along the surfaces 66 and 68 is substantially less than along the surface of the airfoil. Thus, the EB-PVD process preferentially coats the airfoil surface relative to the surfaces 66 and 68. Exemplary average EB-PVD thicknesses along the surfaces 66 and 68 are less than 75% of average EB-PVD thickness along the airfoil.

[0032] To compensate for this, the second layer 42 may be applied via a process which preferentially coats the surfaces 66 and 68 relative to the airfoil surface. In particular, a suspension plasma spray process may be used. Suspension plasma spray, similar to other thermal spray processes, uses high velocity plumes focused by the nozzle of the plasma spray gun, which enables coating of targeted surfaces. In this case, the platform surfaces would be targeted and the airfoil surfaces would be avoided. If it is critical to ensure no coating of certain surfaces, masks can be affixed onto those surfaces during suspension plasma spraying. The result is that SPS may produce greater thickness along the platform than along the airfoil. In this example, a relative thickness is such that an average SPS thickness (if any) along the airfoil is less than 75% of an average SPS thickness along the surfaces 66 and 68.

[0033] Traditional plasma sprayed coatings as defined in US7306859 typically require a rough interface for mechanical bonding (>200 microinch Ra (>5 micrometer) and maybe up to 600 microinch Ra (15 micrometer)) . A standard EB-PVD surface is closer to 100-150 microinch Ra (2.5-3.8 micrometer) putting the strength of the bond between the two layers in question. SPS on the other hand has been demonstrated to bond well to much smoother surfaces down to -60 microinch Ra (-1.5 micrometer) . Secondly, SPS has the optional potential to produce either a porous low-K microstructure or a

pseudo-columnar strain tolerant microstructure depending upon the demands of the component to be coated.

[0034] Exemplary materials for the layers 40 and 42 may be of similar nominal composition (e.g., 7YSZ) or may be of

differing nominal compositions.

[0035] The exemplary bondcoat 30 is a metallic bondcoat such as an overlay bondcoat or a diffusion aluminide. An exemplary MCrAlY overlay bondcoat is PWA 1386 NiCoCrAlYHfSi . This may be applied by low-pressure plasma spray (LPPS) among several possibilities. Alternative bondcoats are gamma/gamma prime and NiAlCrX bondcoats and may be applied via processes further including cathodic arc and ion plasma. Exemplary bondcoat thicknesses are 2-500 micrometers, more narrowly, 12-250 micrometers or 25-150 micrometers on average.

[0036] An exemplary first layer 40 thickness along the airfoil is 0.010 inch (0.25mm) with an exemplary range of 0.002-0.016 inch (0.05-0.41mm) , more narrowly, 0.003-0.012 inch

( 0.076-0.305mm) . Such exemplary layer thickness may be a local thickness or an average thickness (e.g., mean, median, or modal as may be thicknesses of the second layer discussed below) . A similarly-measured first layer thickness along the surfaces 66 and 68 may be substantially lower on average

(e.g., less than 75% or less than 65% of the airfoil first layer thickness, more particularly, 30-70% or 40-60%) .

[0037] An exemplary thickness of the second layer 42 along the surfaces 66 and 68 is at least 0.001 inch (0.025mm) (more particularly, 0.001-0.019 inch (0.025-0.48mm) or 0.004-0.015 inch ( 0.1-0.038mm) . An exemplary total thickness of both ceramic layers is from 0.002-0.020 inch (0.05-0.5mm) (more particularly, 0.005-0.016 inch (0.13-0.41mm) ) . A

similarly-measured second layer thickness along the airfoil surface may be substantially lower on average (e.g., less than 75% or less than 50% or less than 40%, more particularly, 0-30% or 1-10% or 0-10%) . An exemplary first layer composition is a YSZ or a gadolinia-stabilized zirconia (GSZ) or a mixture thereof .

[0038] An exemplary second layer may also be in such families. A more specific first layer material would be YSZ or a YSZ/GSZ combination. A more particular second layer material would be YSZ. Another situation would be the first layer as the

combination of a thin (0.0002 inch to 0.004 inch

(0.005-0.01mm) ) YSZ layer (sublayer) deposited by EB-PVD, followed by a thicker GSZ layer (sublayer) deposited by EB-PVD (0.003 inch to 0.015 inch (0.08-0.38mm) ) . Those two sublayers would then be topped by suspension plasma spray coating - either YSZ or GSZ - on the platforms (primarily) as the second layer 42.

[0039] FIG. 3 shows a blade 100 having an airfoil 102

extending outward from a platform 104. The blade includes an attachment root 106 inboard of the platform. The platform 104 has an outboard gaspath surface 108 which may be subject to similar coating considerations relative to the airfoil 102 as the surfaces 66 and 68 are relative to the airfoil 52.

[0040] FIG. 4 shows an exemplary process 200 for coating the substrate. After initial substrate manufacture (e.g., casting, finish machining, cleaning, and the like) the bondcoat is applied 202. This may be done by LPPS (e.g., as described above) . This may be performed in a first chamber (not shown) whereafter the substrate (s) are transferred 204 to a second chamber. A surface preparation 206 may comprise further cleaning and/or grit blasting (e.g., in yet other chambers) . There may also be thermal conditioning via heater (not shown) . The first layer 40 may be applied 210 via EB-PVD in the second chamber. A further surface preparation (not shown) may follow.

[0041] After application of the first layer, the substrate is transferred 218 to a third location (e.g., a plasma spray booth) where the second layer 42 is then applied 220 by SPS. Exemplary SPS deposition and apparatus are disclosed in US Ser. No. 13/408,460 entitled "Spallation-Resistant Thermal Barrier Coating" and filed February 29, 2012, the disclosure of which is incorporated by reference in its entirety herein as if set forth at length. As noted above, the traversal of the spray may be controlled (e.g., manually or automated) to target surface areas transverse to the airfoil.

[0042] Additional layers may be deposited (whether in the aforementioned chambers or otherwise) . The exemplary

embodiment, however, terminates coating after the second layer is applied.

[0043] Relative to EB-PVD, the SPS columnar microstructure may have different column structure/microstructure. The SPS coating will be polycrystalline, typically free of distinct lamellar features common in historic plasma spray coatings. The SPS coating is characterized by columns separated by vertical cracks or defined gaps (e.g., the column diameter is such that the coating is characterized by greater than 100 gaps per inch (40 gaps/cm), more narrowly >80 gaps/cm or 80- 160 gaps/cm (characteristic "diameters" being the inverse thereof) ) . In contrast, a typical EB-PVD coating has

characteristic single crystal columns with a determined crystallographic texture with individual column diameters of about 10-20 micrometers. The SPS coating typically contains porosity ranging from 10 to 40% by volume, more particularly 15% to 25% by volume. The SPS coating typically has a thermal conductivity ranging from 0.7 to 2.5 W/mK, more particularly, 0.8 to 2 W/mK.

[0044] The exemplary SPS process uses relatively fine

particles as raw material (e.g., <10ym, more particularly, 20nm-2ym or 200nm-lym average particle size) that have

relatively high spallation lives when applied to relatively smooth surfaces (contrasted with conventional air plasma spray) . The interface between the SPS layer 42 and the EB-PVD layer 40 may be superior to prior art bi-layers in TBC

spallation lives (e.g., prior art using APS instead of SPS) . Because SPS ceramic coatings have columnar morphology, they are more strain tolerant. Thus, the strain energy release rate on propagating a crack at the interface, which is the driving force for spallation, is lower. The strain energy release rate during interface crack propagation arises due to the

difference in coefficient of thermal expansion (CTE) of the metallic bondcoat and substrate and the ceramic topcoat layers .

[0045] For APS coatings, the lower strain tolerance of the ceramic coating causes strain energy release rates that are sufficient to drive spallation of ceramic coatings that have smooth interfaces with the metallic substrate. However, if the interface is made rough, mechanical interlocking forces increase the resistance of the interface to strain energy release during crack propagation, so cracks don not propagate and spallation does not occur.

[0046] Optionally, the SPS process may make columnar type structures similar to those of EB-PVD in addition to providing superior interface that will retain the strain tolerance desired. The result is a low in-place stress during thermal cycling that produces low strain energy at the interface and therefore high resistance to spallation.

[0047] The use of "first", "second", and the like in the following claims is for differentiation within the claim only and does not necessarily indicate relative or absolute

importance or temporal order. Similarly, the identification in a claim of one element as "first" (or the like) does not preclude such "first" element from identifying an element that is referred to as "second" (or the like) in another claim or in the description. [0048] Where a measure is given in English units followed by a parenthetical containing SI or other units, the

parenthetical ' s units are a conversion and should not imply a degree of precision not found in the English units.

[0049] One or more embodiments have been described.

Nevertheless, it will be understood that various modifications may be made. For example, implemented in the remanufacture of a given article for the reengineering of the configuration of such article, details of the baseline and its use may

influence details of any particular implementation.

Accordingly, other embodiments are within the scope of the following claims.

Claims

CLAIMS What is claimed is:

1. A method for coating a substrate (20), the method

comprising :

electron beam physical vapor deposition (210) of a ceramic first layer (40); and

suspension plasma spraying (220) of a ceramic second layer (42) over the first layer.

2. The method of claim 1 wherein:

during said spraying of the second layer, an

environmental pressure remains at least 95kPa.

3. The method of claim 1 further comprising:

applying (202) a metallic bondcoat (30) prior to the electron beam physical vapor deposition of the first layer.

4. The method of claim 1 wherein:

the second layer is applied directly atop the first layer .

5. The method of claim 1 wherein:

the substrate is a substrate of a turbine element (50; 100) having an airfoil (52; 102) and a gaspath surface (66, 68; 108) transverse thereto;

a characteristic thickness of the first layer is greater along the airfoil than along the gaspath surface; and

a characteristic thickness of the second layer is greater along the gaspath surface than along the airfoil.

6. The method of claim 5 wherein:

the characteristic thickness of the first layer along the gaspath surface is less than 65% of the characteristic

thickness of the first layer along the airfoil; and the characteristic thickness of the second layer along the airfoil is less than 50% of the characteristic thickness of the second layer along the gaspath surface.

7. The method of claim 1 wherein:

the first layer has a thickness of 0.013-0.076mm; and the second layer has a thickness of 0.025-0.48mm.

8. The method of claim 1 wherein:

the electron beam physical vapor deposition of the ceramic first layer is to a first layer depth of at least 0.013mm; and

the suspension plasma spraying of the ceramic second layer is to a second layer depth of at least 0.025mm.

9. The method of claim 1, wherein:

the second layer comprises yttria-stabilized zirconia or gadolinium-stabilized zirconia.

10. The method of claim 1, wherein:

the substrate comprises a nickel-based superalloy.

11. A coated article (50; 100) comprising:

a substrate (22);

a physical vapor deposition deposited (210) ceramic first layer (40) ; and

a suspension plasma sprayed (220) ceramic second layer (42) above the first layer.

12. The article of claim 11 wherein:

the first layer comprises material selected from the group consisting of yttria-stabilized zirconia or

gadolinium-stabilized zirconia or combinations thereof; and the second layer comprises yttria-stabilized zirconia or gadolinium-stabilized zirconia .

13. The article of claim 11 wherein :

the substrate comprises a nickel-based superalloy.

14. The article of claim 11 further comprising:

a bondcoat (30) between the first layer and the

substrate .

15. The article of claim 14 consisting essentially of the substrate, the bondcoat, the first layer, and the second layer .