Method of depositing a MCrAIY bond coating

Description

FIELD OF INVENTION

[0001] The invention relates to a method of depositing the bond coating according to the preamble of claim 1.

STATE OF THE ART

[0002] Components designed for the use in the area of high temperature, e.g. blades or vanes of a gas turbine, are usually coated with resistant coatings. The coating protects the base material against corrosion and oxidation due to the thermal effect of the hot environment and consists of an alloy mostly using the elements Al and Cr. Most turbine components are coated for the protection from oxidation and/or corrosion with, for example, a MCrAIY 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, wherein M is selected from one or a combination of iron, nickel and cobalt. As an example US-A-3,528,861 or US-A-4,585,481 are disclosing such kind of oxidation resistant coatings. US-A-4,152,223 as well discloses such method of coating and the coating itself. Besides the γ/β-MCrAlY-coating, there is another class of overlay MCrAIY coatings which are based on a γ/γ'-gamma/gamma prime-structure. The advantages of γ/γ'-coatings is that they have a negligible thermal expansion mismatch with alloy of the underlying turbine article. For higher thermal fatigue resistance the γ/γ'-coating are more convenient compared to the γ/β-type of MCrAIY-coatings. A higher thermal fatigue resistance in coatings is most desirable since failure of the most turbine blades and vanes at elevated temperature is typically thermal fatigue driven.

[0003] Among γ/γ'-coatings and γ/β-coatings, the field of γ/β-coatings have been an active area of research and a series of patents has been issued. E.g. a Ni-CrAlY coating is described in US-A-3,754,903 and a CoCrAlY coating in US-A-3,676,058. US-A-4,346,137 discloses an improved high temperature fatigue resistance NiCoCrAlY coating. US-A-4,419,416, US-A-4,585,481, RE-32,121 and US-A-A-4,743,514 describe MCrAIY coatings containing Si and Hf. US-A-4,313,760 discloses a superalloy coating composition with good oxidation, corrosion and fatigue resistance.

[0004] In contrast to the γ/β-coatings, the γ/γ'-type of MCrAIY coatings, known e.g. from US-A-4,973,445, are relatively new. The unique feature of this type of γ/γ'-coatings is that their thermal expansion mismatch is close to zero in combination with a high ductility, what make these coatings more resistant to thermal fatigue. However the limitations are the low aluminum content and hence their low reservoir of aluminum.

[0005] Furthermore, in the state of the art Thermal-Barrier-Coatings (TBC) are known from different patents. US-A-4,055,705, US-A-4,248,940, US-A-4,321,311 or US-A-4,676,994 disclose a TBC-coating for the use in the turbine blades and vanes. The ceramics used are yttria stabilized zirconia and applied by plasma spray (US-A-4,055,705, US-A-4,248,940) or by electron beam process (US-A-4,321,311, US-A-4,676,994) on top of the MCrAIY bond coat.

[0006] One major disadvantage of γ/γ'-type of MCrAIY coatings is that due to the low aluminum content they do not form a continuous alumina film at temperatures below 1000°C what leads to a problem with the bonding adherence with the TBC. Therefore US-A-5,894,053 developed a process for applying a particulate metallic adhesion layer for ceramic thermal barrier coatings to metallic components. The essential content of the patent is a process of forming a roughened surface by applying particulate materials on the surface using binder, principally soldering power. The disadvantages of the process are the depression of the melting point of coating by soldering, a potential fatigue debits of the bond coating and the fluxing of the Thermally Grown Oxide (TGO) by the soldering material. In addition, there is no hint within US-A-5,894,053 of how to enhance the alumina forming capacity of a γ/γ'-type of MCrAIY coating.

SUMMARY OF THE INVENTION

[0007] It is object of the present invention to find a method of depositing a γ/γ'- bond coating resisting to crack during thermal cycling prevalent in the engine. Another object of the present invention is to provide a bond coating with an enhanced surface roughness for an increased TBC adhesion. Yet another object of the present invention is to provide a layer on top of the coating which forms an alumina TGO readily in the engine or by prior heat treatment.

[0008] According to the invention a method of depositing a bond MCrAIY-coating according to the preamble of claim 1 was found, wherein before the TBC is applied, an inner layer on top of the surface of the article consisting of γ/γ'-MCrAlY is deposited using powder in the size range from 5 to 65 µm and an outer bond coating layer on top of the inner layer, which outer layer is more coarse than the inner layer and consisting of β-NiAl or γ/β-MCrAlY or γ/γ'-MCrAlY, is deposited using powder in the size range from 30 to 125 µm. Preferably the inner layer on top of the surface of the article is deposited using powder in the size range from 15 to 50 µm, most preferable below 30 µm, and the outer layer on top of the inner layer is deposited using powder with a particle size from 35 to 90 µm.

[0009] Due to the fact that the outer bond coating layer is deposited using a powder which is more coarse then the underlying inner layer, the surface roughness and the TBC adherence is significantly increased.

[0010] For the formation of Al₂O₃ prior to TBC-deposition the deposited bond coating can be heat-treated at temperatures up to 1140°C, which is possible in air, argon, vacuum or an environment conductive to form the alumina scale, which further increases the TBC adherence. Beside that the heat-treatment stabilizes the coating. To form the alumina scale the outer layer can as well be aluminized using a pack or an out of pack gas phase diffusion process.

[0011] The coating can be applied by a galvanic or plasma spray or any other conventional Plasma Vapor Deposition (PVD) method used for deposition of overlay and bond coatings.

DETAILED DESCRIPTION OF INVENTION

[0012] It is disclosed a bond MCrAIY-coating of an article for the use within a high temperature environment for the protection of the base alloy of turbine blades and vanes.

[0013] According to the invention the bond MCrAIY-coating consists of two different layers. An inner layer on top of the surface of the article consisting of MCrAIY with a structure of γ/γ'. The inner layer is deposited with a powder in the size range from 5 to 65 µm. An outer layer on top of the inner layer consists of β-NiAl, γ/β-MCrAlY or even of γ/γ'-MCrAlY. But, in contradiction to the inner layer, the outer layer is deposited with a coarse powder in the size range from 30 to 125 µm. A ceramic coating such as TBC is deposited on top of the outer bond coating layer. Due to the fact that the outer bond coating layer is deposited using a powder which is more coarse then the underlying inner layer, the surface roughness and the TBC adherence is significantly increased.

[0014] Preferably the inner layer on top of the surface of the article is deposited using powder in the size range from 15 to 50 µm, most preferable below 30 µm, and the outer layer on top of the inner layer is deposited using powder with a particle size from 35 to 90 µm.

[0015] The technology disclosed in this invention directly translates lifetime improvement by increasing TBC adherence due to enhanced surface roughness of the external layer. The composition microstructure of the outer layer can also be independently adjusted to allow formation of an alumina scale beneath the TBC. An example is the use of β-NiAl or γ/β-MCrAlY or even γ/γ'-MCrAlY as the outer layer. addition the inner layer of γ/γ'-MCrAlY comprises one or a combination of Y, Hf, Zr and Si with (wt-%) 0.01 - 5% Y+Hf+Zr+Si and one or a combination of Ta, Fe, Ga, Mg and Ca.

[0016] The outer layer may consist of a β-NiAl outer layer and the β-NiAl may contain aluminum from about 20 to 33 wt.-%, which falls in the single phase range of NiAl phase diagram. The β-NiAl has a high oxidation resistance which can be further enhanced by a minor elemental addition. This is known e.g. from the patents US-A-4,610,736, US-A-5,116,438, US-A-5,516,380, US-A-5,116,691, US-A-4,961,905, US-A-4,478,791 or US-A-5,215,831. A possible content of the inner layer of β-NiAl is (wt-%) 0.001 - 0.5% Y, 0.001 - 0.5% Hf, 0.001 - 0.5% Zr, 0.1 - 1.5% Si, 0 - 1.0% Ca, 0 - 1.0% Mg, 0 - 4% Ga, 0 - 4% Fe, 0.1 - 4.0% Ta.

[0017] Optionally, for the formation of Al₂O₃ prior to TBC-deposition, the deposited bond coating may be heat-treated at temperatures of up to 1140°C, which can be done in air, argon, vacuum or an environment conductive to form the alumina scale, which further increases the TBC adherence. Beside that the heat-treatment stabilizes the microstructure of the coating. Thereby, the 1140°C heat-treatment has been found to be most advantageous to fully stabilize the microstructure. The 1140°C heat-treatment can also be used to pre-form alumina prior to TBC deposition. For the formation of the aluminum scale the outer layer can be aluminized using a pack or an out of pack gas phase diffusion process.

[0018] The coating can be applied by a galvanic or plasma spray or any other conventional PVD method used for deposition of overlay and bond coatings.

Claims

1. A method of depositing a bond MCrAIY-coating to a surface of an article for increasing the roughness for enhanced adhesion of a Thermal-Barrier-Coating (TBC), wherein before the TBC is applied,

- an inner layer on top of the surface of the article consisting of γ/γ'-MCrAlY is deposited using powder in the size range from 5 to 65 µm and

- an outer bond coating layer on top of the inner layer, which outer layer is more coarse than the inner layer and consisting of β-NiAl or γ/β-MCrAlY or γ/γ'-MCrAlY, is deposited using powder in the size range from 30 to 125 µm.

2. The method of depositing a bond MCrAIY-coating according to claim 1, wherein the inner layer on top of the surface of the article is deposited using powder in the size range from 15 to 50 µm and the outer layer on top of the inner layer is deposited using powder in the size range from 35 to 90µm.

3. The method of depositing a bond MCrAIY-coating according to claim 1, wherein the inner layer on top of the surface of the article is deposited using powder in the size range below 30µm.

4. The method of depositing a bond MCrAIY-coating according to any of the claims 1 to 3, wherein the deposited bond coating is heat-treated at a temperature up to 1140 °C prior to the TBC deposition.

5. The method of depositing a bond MCrAIY-coating according to claim 4, wherein the deposited bond coating is heat-treated in air, argon, vacuum or an environment conductive to form a alumina scale prior to the TBC deposition.

6. The method of depositing a bond MCrAIY-coating according to claim 1, wherein the outer layer is aluminized using a pack or an out of pack gas phase diffusion process.

7. The method of depositing a bond MCrAIY-coating according to claim 1, wherein the different layers are deposited by using a galvanic or a plasma spray or any PVD method used for deposition of bond coatings.