WO2005038074A1

WO2005038074A1 - Method of applying a thermal barrier coating system to a superalloy substrate

Info

Publication number: WO2005038074A1
Application number: PCT/EP2004/052542
Authority: WO
Inventors: Abdus Suttar Khan; Mohamed Nazmy
Original assignee: Alstom Technology AG
Current assignee: GE Vernova GmbH
Priority date: 2003-10-17
Filing date: 2004-10-14
Publication date: 2005-04-28
Anticipated expiration: 2006-04-17

Abstract

The invention discloses a process for deposition of plasma sprayed TBC without a bond coat. The process has the following steps: surface treatment by grit blasting followed by heat treatment in air or in an atmosphere with reduced oxygen partial pressure, and a second grit blasting followed by heat treatment upon which TBC is deposited by air plasma spraying. The coating is resistant to spallation during thermal cycling as well as after long term isothermal exposure and subsequent rapid cooling.

Description

METHOD OF APPLYING A THERMAL BARRIER COATING SYSTEM TO A SUPERALLOY SUBSTRATE

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Field of the invention

The invention relates to a method of applying a thermal barrier coating system 20 to a surface of a component that is made of a superalloy substrate according to the independent claim 1.

State of the art

25 Components designed for use in the area of high temperature environment, e.g. blades or vanes of a gas turbine, are usually coated with environmentally resistant coatings. The coating protects the base material against corrosion and oxidation due to the thermal effect of the hot environment. Most turbine 30 components are coated for protection from oxidation and/or corrosion with, for example, a MCrAIY (M = Ni, Co, or Fe, one or in combination) coating (base coat) and some are also coated with a thermal barrier coating (TBC) for thermal insulation. MCrAIY protective overlay coatings are widely known in the prior art. They are a family of high temperature coatings. As an example US- A-3,528,861 or US-A-4,585,481 disclose such kind of oxidation resistant coatings. US-A-4, 152,223 as well discloses such method of coating and the coating itself.

Furthermore, Thermal-Barrier-Coatings (TBC) are known in the state of the art. US-A-4,055,705, US-A ,248,940, US-A-4,321,311 or US-A-4,676,994 disclose TBC-coatings for the use in the turbine blades and vanes. The ceramics used are yttria-stabilized zirconia (YSZ). This material is applied by plasma spraying (US-A-4,055,705, US-A-4,248,940) or by an electron beam process (US-A-4,321 ,311, US-A-4,676,994) wherein the yttria stabilized zirconia is applied on top of the MCrAIY bond coat.

The plasma sprayed TBCs generally fail by delamination and a number of factors are thought to contribute to the failure of the TBC:

a) Unfavorable stress distribution at the TBC-bond coat interface due to thermal expansion mismatch between substrate and bond coats as well as the difference in physical and mechanical properties between the TBC and bond coat, b) The growth stress of thermally grown oxide (TGO), largely affected due to formation of mixed oxide TGO in preference to pure aluminum oxide, c) Coating process is not duly optimized which results in a low porosity in the TBC in preference high porosity.

There have been two distinct approaches of improving durability of TBC. The first approach has been to stress relief of TBC i.e. make the TBC more strain tolerant by optimization of process parameters producing a strain tolerant microstructure. The second approach is to promote formation of a pure and continuous alumina scale containing no or negligible mixed oxides.

Attempts to enhance durability of TBC by making the TBC more strain tolerant generally followed the following routes: a) Manufacturing of columnar structure TBC by electron beam physical vapor deposition (EBPVD): The columnar grain structure in TBC provides a mechanism for stress re- lief. However, the EBPVD process is not quite adaptable to industrial gas turbine (IGT) blades or vanes, firstly due to small deposition chamber size while the IGT component size is relatively large. In US-A-6, 180,184, US- A-5,830,586 and US-A-6,306,517 is described that depositing a columnar grained TBC provides yet another stress relief mechanism. This EBPVD TBC has higher thermal conductivity while lower conductivity is preferred in gas turbine industry. b) Vertical segmentation of TBC by depositing dense TBC (US 5,073,433), using a Praxair gun provided an alternative mechanism of stress relief of TBC. c) Other examples of segmented TBC are described in US-B 1-6,224,963 where a segmented TBC is produced by a laser drilling of a selected area of a TBC. US-A-5,681 ,616 describes segmented TBC by abrading a portion of the TBC with a high-pressure liquid jet. Another example of segmented TBC is described in US 5,840,434 wherein a segmented TBC is produced by control of EBPVD process parameters. A method of forming a macro-segmented TBC by placing a three- dimensional pattern or feature on the surface is disclosed in US-A- 6,316,078. The disclosed features could be either raised ribs or grooves on the substrate or on the bond coat. In US-A1 -2002/0146584 and US-A1 -2002/0146541 a surface is formed by a cast feature or rivets placed on the surface upon which the TBC is deposited, d) A different method to stress relief TBC is described in US-A-4,457,948 where a stress relief in TBC is provided by a post-coating heat-treatment by rapid quenching of TBC samples from elevated temperature which resulted in a cracking of the TBC . The other approach of improving the durability of TBC is to minimize expansion mismatch or promote a formation of pure alumina TGO with no mixed oxides. The technology is summarized here:

a) In order to reduce expansion mismatch, in US-A-5,863,668 and US-A- 6,093,454 are used two layer bond coats, the first layer is MCrAIX (X= reactive element, such as Y, Zr, Hf, Yb) and the second layer is MCrAIX mixed with chromia, alumina and other oxides. Reduction of expansion mismatch improves the durability of TBC. b) Attempts of forming a pure aluminum oxide TGO on a MCrAIY bond coating have not been very successful. Generally the bond coatings deposited by plasma spraying or electron beam process forms a mixed oxide TGO. For example one could take any given composition of coating from the Ni- CrAIY, NiCoCrAIY or CoCrAIY family, the first formed oxide in here will contain NiO, CoO, Cr₂θ₃ and spinel, besides, alumina. During degradation the amount of non-protective oxides in TGO becomes high. The oxides in TGO remain in place and continue grow until the TBC is let go. Practical approaches to form continuous alumina only TGO have been difficult. c) Typically, the oxidation of superalloys is poor and they are unable to sus- tain alumina growth with time and temperature. Due to limitation of the oxidation resistance of superalloy the industry is forced to use the bond coats (i.e. MCrAIY) with the TBC system with the full knowledge of its shortcoming.

The following state of the art is known regarding the development of applying a TBC without a bond coat to a surface of a superalloy substrate:

a) In situ formation of alumina scale upon superalloy: A durable TBC is described in US 5,262,245 by first forming a thin alumina scale upon the superalloy substrate directly in a chamber prior to TBC deposition. The TBC has a columnar grain structure deposited by EBPVD. The pre-oxidation was done by leaking oxygen in a controlled rate in the deposition chamber while the specimens were being heated by diffused electron beam. Subsequently TBC was deposited by vaporizing the target using focus beam. The alumina thickness was in the range 0.25 to 20 microns. The resulting columnar grain TBC had higher durability. One of the critical requirements is that the superalloy must have the ability to form adherent alumina scale, b) Pre-treatment of a superalloy substrate to remove sulfur from the superalloy substrate: Sulfur is known to adversely affect oxidation of superalloy. The sulfur free superalloy upon oxidation forms alumina scale.

For example, sulfur is removed from a superalloy by using magnesium salts at elevated temperature (US 5,346,563). Magnesium interacts with the existing scale and makes the scale porous to sulfur thus allowing sulfur to escape to the environment. Removal of sulfur allows formation of pure alumina scale upon which the columnar grain TBC is deposited by EBPVD process.

In a separate process, sulfur is removed from a superalloy by flowing pure hydrogen over the superalloy at elevated temperature (US 5,538,796). The substrate, subsequently oxidized formed alumina scale upon which columnar grain TBC was deposited by EBPVD process. Prior to alumina formation a portion of the substrate can be coated optionally with alumina or an environmental coating that form an alumina scale upon oxidation.

It is a disadvantage of these known methods for applying a no bond coat TBC that they require complex purification steps prior to TBC deposition. Both methods provide the no bond coat TBC with a columnar grain structure by EBPVD process. SUMMARY OF THE INVENTION

It is the aim of the present invention to create a simple method of applying a ceramic thermal barrier coating system to a surface of a component prefer- able for use in the area of high temperature environment, e.g. for blades or vanes of a gas turbine, wherein a durable TBC without a bond coat is deposited by plasma spraying process. As discussed above there is no prior art regarding the plasma spraying of TBC without a bond coat.

According to the present invention the method of applying a coating system to the surface of a component made of a superalloy substrate which is capable to form alumina scale comprises the steps of

- grit blasting of the surface of the substrate, then heat treating at elevated temperature for pre-oxidation of the surface, cooling to room temperature, - again grit blasting and repeating of the heat treatment followed by cooling and

- depositing TBC by thermal plasma spraying.

The advantage of the invention is that the process is simple and that TBC can be deposited by thermal spraying without a bond coat. The new method does not require complex purification steps prior to TBC deposition. There is no need for applying a bond coat to the surface of the substrate prior to the applying of the TBC. It is to be noted that in contrast to columnar grain structure TBC done by EBPVD process, the plasma spray TBC has lower thermal con- ductivity and therefore is preferred. Additionally, the plasma spray process is adaptable to large industrial gas turbine components.

The heat treatment is carried out in air or in an atmosphere with reduced oxygen partial pressure.

The heat treatment temperature and time can be adjusted based upon super- alloy oxidation behavior at elevated temperature. The heat treatment temperature and time are determined by the superalloy oxidation characteristics and the heat treatment temperature is in the range up to 1200°C for a period up to 100 hours in air or an gaseous atmosphere with reduced oxygen partial pressure. Often some superalloy substrate may require the above conditions to promote pure alumina scale. Reduced pressure may also be advantageous for the stated reasons.

When the heat treatment is carried out in reducing atmosphere the oxygen partial pressure in the environment is controlled at a temperature and time suitable to form a continuous and adherent alumina scale. This allows a good adhesion of the TBC without a bond coat.

It is an advantage to carry out the heat treatments at temperatures in the range of 1050-1080°C and to use such a grit blasting time and media that are able to remove the first formed alumina scale because that alumina contains NiO/Cr₂θ3. Because of the pure alumina formation at the surface of the superalloy substrate it is possible to coat the substrate with TBC by plasma spraying without a bond coat. Good resistance against spallation or damage of the TBC is achieved.

It is useful when the layer thickness of the scale formed after the second heat treatment is at least 50 nanometer. In addition, it is useful when the ceramic coating that is applied with a plasma spray process to that scale has a thickness in a range of up to 3 mm and a porosity of 10 to 25 %.

The superalloy substrate can be a nickel base, cobalt base or iron base alloy.

BRIEF DESCRIPTION OF DRAWINGS

Preferred embodiments of the invention are illustrated in the accompanying drawings, in which Fig. 1 shows a gas turbine blade; Fig. 2 shows a micrograph of the surface (the initial state) of two samples applied with a coating system according to the invention and Fig. 3 shows a micrograph of the surface (after isothermal and cyclic tests) of the two samples in Fig. 2.

The drawings show only parts important for the invention.

DETAILED DESCRIPTION OF INVENTION

The present invention is generally applicable to components that operate within environments characterized by relatively high temperature, and are therefore subjected to severe thermal stresses and thermal cycling. Notable examples of such components include the high and low-pressure vanes and blades, shrouds, liners for combustion chambers and augmentor hardware of gas turbine engines.

Fig. 1 shows as an example such a component 1 comprising a blade 2 against which hot combustion gases are directed during operation of the gas turbine engine. The blade 2 has cooling holes 4, which are on the external surface 5 of the component 1 as well as on the platform 3 of the component. Through the cooling holes 4 cooling air is ducted during operation of the engine to cool the surface 5. The surface 5 is subjected to severe attack by oxidation, corrosion and erosion due to the hot combustion gases. In many cases the component 1 consists of a nickel, a cobalt, or an iron base superalloy. The component 1 can be single crystal (SX), directionally solidified (DS) or polycrystalline. While the advantages of this invention are described with reference to a turbine blade or vane as shown in Fig. 1, the invention is generally applicable to any component on which a coating system 6 - ceramic thermal barrier coating (TBC) - may be used to protect the component from its environment. The blade 2 in this embodiment of the invention is made of a nickel base super- alloy with the following chemical composition (in weight %): 0.02 C 0.13 Si 5.0 Al 0.005 B 5.0 Co

7.7 Cr 0.12 Hf 2.0 Mo

5.8 Ta 1.4 Ti 7.8 W remainder Ni and unavoidable impurities.

This superalloy has an excellent oxidation behaviour in forming adherent AI₂O3 to deposit ceramic TBC by plasma spraying directly on the surface of the article without using a bond coat. Plasma sprayed TBC is the most suitable coating for industrial gas turbine engine due to its low thermal conductivity and low costs. According to the invention, the surface roughness is optimised by grit blasting to promote pure alumina TGO (thermally grown oxides) and TBC microstructure with optimum porosity.

The method of applying a coating system 6 to the surface 5 of the blade 2 comprises the following steps in this embodiment of the invention: - first grit blasting of the surface 5 of the blade 2 followed by a heat treatment of the blade 2 in air at elevated temperature for pre-oxidation of the surface 5, cooling of the blade 2, and - repeated grit blasting of the surface 5 of the blade 2 followed by a heat treatment in air at elevated temperature, cooling, and then - depositing a ceramic TBC (thermal barrier coating) 6 on top of said heat- treated substrate by air plasma spraying. The first grit-blasting step is necessary to induce an optimum surface roughness that should be in the range from 20 to 50 micrometer. Then follows the first heat treatment in air at 1050 - 1080 "C/1-4 h. The present example is heat treated in air at 1080 °C/4 h. This leads to a pre-oxidation of the surface 5. The formed oxides are Al 0₃, which unfortunately do also contain NiO/Cr₂0₃.This is a disadvantage because non-pure Al₂0₃ does not allow a good adhesion of the TBC 6. Therefore a second grit-blasting step is necessary to remove the formed AI₂O3 with NiO/Cr₂0₃. The grit-blasting process uses such time and media that are able to remove the first formed alumina scale. Cr (in the γ-phase) is enriched in the scale layer during the first heat treatment. Therefore the Cr concentration in the layers below the scale layer is lowered. After having removed the first scale layer by the second grit-blasting process there is less (or no) Cr in the material at the surface and the Cr activity is reduced so that almost only AI2O3 is formed during the second heat treatment because the Al concentration in the γ'-phase is constant. The formation of the stable alumina will minimize the formation of the less stable NiO. Therefore the formation of NiO/Cr₂O₃ is minimized. This second grit-blasting process is followed by a second heat treatment in air at 1050-1080 °C/1-8 h. The present example is heat treated in air at 1080 °C/4 h. This oxidation treatment forms rather predominantly alumina scale with a layer thickness of at least 50 nanometer.

According to a second example of the invention the second grit-blasting process is followed by a second heat treatment at 1080°C/4h in an inert gaseous atmosphere with reduced oxygen partial pressure. The only difference between the second and the first example of the invention is the reduced oxygen partial pressure in the second teat treatment which should be less than 10 ~⁴ atm. Under reduced oxygen pressure NiO is completely unstable, therefore the formation of AI2O3 only is promoted.

The next step is applying a ceramic TBC 6 with the well-known air plasma spray (APS) process directly to the surface 5 of the oxidized substrate. In the present example the used TBC is 7% yttria stabilized zirconia (Zr0₂ stabilized with Y₂0₃). It has a porosity of 10-25 % and a thickness of about 250-350 μm. The last step is an ageing at 870 °C for 16 h, which is needed for the base alloy.

The coating system applied with a method according to the invention has excel- lent properties with respect to the resistance against damages or spallation.

Spallation resistance was tested under cyclic and isothermal conditions and is documented in Fig. 2 and Fig. 3.

Fig. 2 shows a micrograph of the initial state of two samples of the above- described Nickel base superalloy component with TBC without a bond coat.

Fig. 3 shows a micrograph of the same samples after the cyclic and isothermal tests. The upper sample was tested at 1000 °C/1000 h/ air cooling

This experiment tests the effect of cumulative TGO built up followed by cooling to room temperature. The sample survived more than 1000 hours isothermal exposure followed air cooling to room temperature without damage or spallation of TBC.

The lower sample in Fig. 3 shows the result of a cyclic test at 1000 °C/1 h followed by 10 minutes cooling to room temperature (air cooling). The test simulates the effect of thermal shock on TBC adhesion. The sample survived 100 cycles at 1000 °C without showing any TBC damage or spallation.

The invention is not limited to the above-described embodiment. For example, different superalloys can be used, such as cobalt base or iron base alloys. Furthermore, the treatment temperature and time can be adjusted based upon superalloy oxidation behavior at elevated temperature. The temperature and time are determined by the superalloy oxidation characteristics and the heat treatment temperature can be in the range up to 1200°C for a period up to 100 hours in air or reduced oxygen partial pressure. The heat treatment can be carried out in reducing atmosphere to control oxygen partial pressure in the environment at a temperature and time suitable to form a continuous and adherent alumina scale.

List of reference numbers

1 component

2 blade

3 platform cooling hole

5 surface of component 1

6 TBC, coating system

Claims

Claims 1. A method of applying a coating system (6) to the surface (5) of a component (1) made of a superalloy substrate capable of forming alumina scale comprising the steps of - first grit blasting of the surface (5) of the component (1) followed by a heat treatment of the component (1) at elevated temperature for pre- oxidation of the surface (5), cooling of the component (1 ), and - repeated grit blasting of the surface (5) of the component (1) followed by a heat treatment at elevated temperature for oxidation of the surface (5), cooling, and then - depositing a ceramic TBC (thermal barrier coating) (6) on top of said heat-treated substrate by plasma spraying.

2. The method according to claim 1, wherein the heat treatment is carried out in air.

3. The method according to claim 1, wherein the heat treatment is carried out in an inert gaseous atmosphere with reduced oxygen partial pressure.

4. The method according to claim 1 , wherein the heat treatment temperature and time are determined by the superalloy oxidation characteristics and wherein the heat treatment temperature is in the range up to 1200 °C for a period up to 100 hours.

5. The method according to claim 4, wherein the first heat treatment is done at 1050-1080°C/ 1-4 h.

6. The method according to claim 4, wherein the second heat treatment is done at 1050-1080°C/ 1-8 h.

7. The method according to claim 1, wherein such a grit blasting time and media are used during the second grit blasting process that are able to remove the first formed alumina scale.

8. The method according to claim 1, wherein a layer thickness of the scale formed after the second heat treatment is at least 50 nanometer.

9. The method according to any of the claims 1 to 8, wherein the ceramic coating (TBC) is applied with a thickness up to 3 mm.

10. The method according to claim 9, wherein the applied ceramic coating (TBC) has a porosity of 10 to 25 %.

11.The method according to any of the claims 1 to 10, wherein the superalloy is made of nickel base, cobalt base or iron base alloys.