GB2322383A

GB2322383A - A coated superalloy article

Info

Publication number: GB2322383A
Application number: GB9703737A
Authority: GB
Inventors: Mehar Chand Meelu
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1997-02-22
Filing date: 1997-02-22
Publication date: 1998-08-26
Also published as: GB9703737D0

Abstract

A superalloy article 20 has a multilayer coating 22 comprising a chromium, titanium and/or tantalum and silicon enriched layer 24, a platinum aluminide coating 26, an alumina layer 28 and a ceramic thermal barrier coating 30. The platinum aluminide coating 26 is a very good corrosion and oxidation protective coating for the superalloy article 20. The chromium and silicon enriched layer 24 prevents silicon in the platinum aluminide-silicide coating diffusing to the superalloy article 20 to minimise the formation of silicon rich topologically close packed phases at the interface between the superalloy article 20 and the multilayer coating 22. The system forms a good bond coating for the ceramic thermal barrier coating 30. The chromium, titanium and/or tantalum may be applied by chromising or sputtering. Additional steps include depositing yttrium, zirconium or erbium.

Description

A COATED SUPERALLOY ARTICLE AND A METHOD OF COATING A SUPERALLOY ARTICLE The present invention relates to coated superalloy articles and to methods of coating superalloy articles, particularly nickel and cobalt superalloy turbine blades or turbine vanes.

It is known to produce aluminide-silicide protective coatings on superalloy turbine blades or turbine vanes to extend the service lives of the turbine blades or turbine vanes.

It is known to produce aluminide-silicide coatings on a superalloy article by depositing a silicon filled organic slurry on the superalloy article and then pack aluminising as described in US4310574. The aluminium carries the silicon from the slurry with it as it diffuses into the superalloy.

Another method of producing aluminide-silicide coatings is by depositing a slurry containing elemental aluminium and silicon metal powders on a superalloy article and then heating to above 760 C to melt the aluminium and silicon in the slurry, such that they react with the superalloy and diffuse into the superalloy article as described in US3248251. A further method of producing aluminide-silicide coatings is by repeatedly applying the aluminium and silicon containing slurry and heat treating as described in US5547770. Another method of producing aluminide- silicide coatings is by applying a slurry of an eutectic aluminiumsilicon or a slurry of elemental aluminium and silicon metal powders on a superalloy article and diffusion heat treating to form a surface layer of increased thickness and reduced silicon content, and a layering layer which comprises alternate interleaved layers of aluminide and silicide phases and a diffusion interface layer as described in published European patent application No. 0619856A.

It is also known to produce platinum aluminide-silicide coatings on a superalloy article by coating the superalloy article with platinum, then heat treating to diffuse the platinum into the superalloy article and then simultaneously diffusing aluminium and silicon from the molten state into the platinum enriched superalloy article as described in published International patent application No. W095/23243A. Another method of producing platinum aluminide-silicide coatings on a superalloy article is by coating the superalloy article with platinum, then heat treating to diffuse the platinum into the superalloy article, a silicon layer is applied and is then aluminised as described in published European patent application No.

0654542A. It is also possible to diffuse the silicon into the superalloy article with the platinum as described in EP0654542A. A further method of producing platinum aluminide-silicide coatings on superalloy articles is by electrophoretically depositing platinum- silicon powder onto the superalloy article, heat treating to diffuse platinum and silicon into the superalloy article, then electrophoretically depositing aluminium and chromium powder and then heat treating to diffuse the aluminium and chromium into the superalloy article as described in US5057196.

It has been found that if a platinum aluminide- silicide coating is produced on a superalloy article that the silicon tends to diffuse to the interface between the substrate and the coating and this results in the formation of very brittle phases, topologically close packed phases (TCP phases), with the superalloy substrate. These brittle TCP phases are needle like silicon rich phases which extend into the substrate. The TCP phases are undesirable because they destroy the mechanical properties of the substrate. Also these silicon rich phases may have lower melting points than the superalloy substrate. Thus the use of a platinum aluminide-silicide coating directly onto a superalloy article is not practical because these TCP phases increase the stress concentrations in the superalloy substrate leading to premature failure of the superalloy article.

The invention therefore seeks to provide a platinum aluminide-silicide on a superalloy article without the formation of the brittle phases.

Accordingly the present invention provides a method of coating a superalloy article comprising the steps of : (a) applying a coating comprising at least one of chromium, titanium and tantalum to the superalloy article and diffusing at least one of the chromium, titanium and tantalum into the superalloy article to produce a layer on the superalloy article which is enriched in at least one of chromium, titanium and tantalum, (b) and depositing a platinum aluminide-silicide coating on the layer on the superalloy article which is enriched in at least one of chromium, titanium and tantalum.

Preferably step (a) comprises chromising the superalloy article.

Alternatively step (a) may comprise sputtering titanium, tantalum or titanium and tantalum onto the superalloy article and diffusing into the superalloy article.

Preferably step (b) comprises depositing the platinum onto the layer on the superalloy article which is enriched in at least one of chromium, titanium and tantalum, heat treating to diffuse the platinum, simultaneously diffusing aluminium and silicon from the molten state into the coated superalloy article.

Alternatively step (b) may comprise depositing the platinum onto the layer on the superalloy article which is enriched in at least one of chromium, titanium and tantalum, heat treating to diffuse the platinum, depositing silicon and then depositing aluminium and diffusing the silicon and aluminium into the coated superalloy article.

Preferably the platinum is deposited by electroplating.

Preferably the platinum is heat treated at a temperature greater than 10000C. More preferably the platinum is heat treated at a temperature of 11200C for 1 to 2 hours to diffuse the platinum. Preferably the platinum is deposited to a thickness between 5 and 15 micrometers.

Preferably the chromising is at a temperature of 11000C for 5 hours.

Preferably the diffusion of aluminium and silicon is at a temperature in the range 7500C to 11200C. Preferably the simultaneous diffusion of aluminium and silicon from the molten state is repeated at least once more.

After step (b) there may be an additional step of: (c) applying a ceramic thermal barrier coating.

Alternatively after step (b) there may be additional steps of: (c) forming alumina on the platinum aluminide- silicide coating, (d) applying a ceramic thermal barrier coating by physical vapour deposition.

Preferably the ceramic thermal barrier coating is deposited by electron beam physical vapour deposition.

After step (b) and before step (c) there may be an additional step of : depositing at least one of yttrium or yttrium and zirconium.

Alternatively after step (b) and before step (c) there may be an additional step of : depositing at least one of chromia, silica or alumina.

Preferably the method comprises depositing the chromia, depositing the silica onto the chromia and depositing the alumina onto the silica.

After step (b) and before step (c) there may be an additional step of : depositing erbium.

Preferably the erbium is deposited by ion implantation.

The present invention also provides a coated superalloy article comprising a layer enriched in at least one of chromium, titanium and tantalum on the superalloy article, the layer enriched in at least one of chromium, titanium and tantalum having silicide phases and a platinum aluminide coating on the layer enriched in at least one of chromium, titanium and tantalum.

There may be a ceramic thermal barrier coating on the platinum aluminide coating.

Preferably there is an alumina layer on the platinum aluminide coating and a ceramic thermal barrier coating on the alumina layer.

Preferably the ceramic thermal barrier coating has columnar grains.

The superalloy article may be a nickel based superalloy article. The superalloy article may be a high rhenium containing nickel based superalloy article. The superalloy article may be a single crystal superalloy. The superalloy article may be a turbine blade or a turbine vane.

The platinum aluminide coating may comprise some silicide phases, the layer enriched in at least one of chromium, titanium and tantalum comprises some platinum aluminide and the bond coating comprises some silicide phases.

The present invention will be more fully described by way of example with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view through a coated superalloy article according to the present invention.

Figure 2 is a cross-sectional view through a further coated superalloy article according to the present invention.

A superalloy article 10, for example a gas turbine engine turbine blade or turbine vane, has a multilayer coating 12 as shown in figure 1. The multilayer coating 12 comprises a chromium and silicon enriched layer 14 on the superalloy article 10 and a platinum aluminide coating 16 on the chromium and silicon enriched layer 14. The chromium and silicon enriched layer 14 generally has a greater percentage of chromium than the superalloy article 10 and also has the silicon present in the form of chromium silicide phases, additionally the chromium and silicon enriched layer 14 may have some platinum aluminide. The platinum aluminide coating 16 comprises platinum aluminide with some chromium silicide phases.

It is believed that the provision of the chromium and silicon enriched layer 14 between the superalloy article 10 and the platinum aluminide coating 16 prevents the silicon in a platinum aluminide-silicide coating diffusing to the superalloy article 10. It is postulated that this is due to the chromium in the chromium enriched layer 14 reacting with the silicon to form chromium silicides which prevents the silicon diffusing to the superalloy article 10 and thus minimises the amount of the deleterious brittle TCP phases formed in the superalloy article 10. Preferably the chromium enriched layer 14 prevents silicon diffusing to the superalloy article 10. Thus the chromium and silicon enriched layer 14 may be considered to be a chromium silicide layer as this forms the majority of the layer 14. Chromium is a very strong silicide and forms a very stable chromium silicide intermetallic.

The multilayer coating 12 is produced on the superalloy article 10 by depositing a chromium coating onto the superalloy article 10 and the chromium is then heat treated to diffuse the chromium into the superalloy article 10 to form the chromium enriched layer 16 on the superalloy article 10. The chromium is deposited and diffused preferably by pack chromising or out of pack vapour chromising. The chromising process requires a temperature of 11000C for 5 hours.

A platinum layer is then deposited on the chromium enriched layer 14 and the platinum is then heat treated to diffuse the platinum into the chromium enriched layer 14.

The platinum is deposited to a thickness of 5 to 15 micrometers by electroplating, physical vapour deposition or other suitable means. The platinum is heat treated at a temperature greater than 10000C, for example 1 hour at 1120 0C followed by gas fan quenching and ageing for 24 hours at 8450C.

Aluminium and silicon are then deposited and the aluminium and silicon are heat treated to diffuse the aluminium and silicon into the platinum to form the platinum aluminide-silicide coating 16. The aluminium and silicon is deposited using a slurry comprising aluminium and silicon powders dispersed in a chromate/phosphate binder and the slurry is cured to a solid matrix which holds the metal pigments in contact with the metal surface during the heat treatment. The aluminium and silicon is heat treated at a temperature in the range 750 0C to 1120 0C to simultaneously diffuse the aluminium and silicon from the molten state as described in US3248251 incorporated herein by reference. The silicon may be deposited first by spraying a silicon filled slurry and then pack aluminising. The aluminium diffusing into the platinum carries the silicon with it as described in US4310574 incorporated herein by reference. Other suitable methods of depositing and diffusing the aluminium and silicon may be used.

It may be beneficial to repeat the deposition of the aluminium and silicon and the diffusion heat treatment steps to provide a plurality of bands rich in silicon and a plurality of bands rich in aluminium with the silicon rich bands and aluminium rich bands arranged alternately through the depth of the platinum aluminide-silicide coating. This technique is described more fully in published International Patent application No. W093/23247.

During the diffusion of the silicon and aluminium into the chromium and platinum coated superalloy article the aluminium reacts with the platinum to form a platinum aluminide coating 16 and the silicon diffuses through the platinum to the chromium enriched layer to form the chromium silicide coating 14. Thus the chromium enriched layer stops the silicon diffusing to the superalloy article 10.

Another superalloy article 20, for example a gas turbine engine turbine blade or turbine vane, according to the present invention has a multilayer coating 22 as shown in figure 2. The multilayer coating 22 comprises a chromium and silicon enriched layer 24 on the superalloy article 20, a platinum aluminide coating 26 on the chromium and silicon enriched layer 24 and an alumina layer 28 on the platinum aluminide coating 26 and a ceramic thermal barrier coating 30 on the alumina layer 30. The chromium and silicon enriched layer 24 generally has a greater percentage of chromium than the superalloy article 20 and also has the silicon present in the form of chromium silicide phases, additionally the chromium and silicon enriched layer 24 may have some platinum aluminide. The platinum aluminide coating 26 comprises platinum aluminide with some chromium silicide phases.

It is believed that the provision of the chromium and silicon enriched layer 24 between the superalloy article 20 and the platinum aluminide coating 26 prevents the silicon in a platinum aluminide-silicide coating diffusing to the superalloy article 20. It is postulated that this is due to the chromium in the chromium enriched layer 24 reacting with the silicon to form chromium silicides which prevents the silicon diffusing to the superalloy article 20 and thus minimises the amount of the deleterious brittle TCP phases formed in the superalloy article 20. Preferably the chromium enriched layer 24 prevents silicon diffusing to the superalloy article 20.

Thus the chromium and silicon enriched layer 24 may be considered to be a chromium silicide layer as this forms the majority of the layer 24. Chromium is a very strong silicide and forms a very stable chromium silicide intermetallic.

The ceramic thermal barrier coating comprises for example yttria stabilised zirconia, although other suitable ceramics may be used. Preferably the ceramic thermal barrier coating comprises columnar grains.

The multi layer coating 22 is produced on the superalloy article 20 using the same processes as described to produce the multilayer coating 12 on the superalloy article 10.

The alumina layer 28 is formed on the platinum aluminide coating 26 by heating the article in an oxygen containing atmosphere to a sufficiently high temperature to oxidise the surface of the platinum aluminide coating 26 to form alumina.

The ceramic thermal barrier coating 30 is deposited onto the alumina layer 28 by placing the superalloy article 20 into an electron beam physical vapour deposition chamber and depositing the ceramic onto the alumina layer 28 by electron beam physical vapour deposition to form a columnar grained ceramic thermal barrier coating 30. It is likely that the alumina layer 28 forms on the platinum aluminide coating 26 while the superalloy article 20 is heated in the electron beam physical vapour deposition chamber as some oxygen is present.

Alternatively it is possible to deposit a ceramic thermal barrier coating onto the platinum aluminide coating by vacuum plasma spraying or argon shrouded plasma spraying and an alumina layer may not be formed on the platinum aluminide coating.

In the case of a multilayer coating comprising a ceramic thermal barrier coating it may be beneficial to deposit yttrium or yttrium and zirconium onto the platinum aluminide coating before depositing the ceramic thermal barrier coating. It is believed that the yttrium or yttrium and zirconium diffuses into the platinum aluminide to form fingers of yttria or yttria stabilised zirconia in the subsequently formed alumina to key the ceramic thermal barrier coating onto the platinum aluminide coating. The yttria or yttria and zirconia fingers match the yttria stabilised ceramic thermal barrier coating. The yttrium or yttrium and zirconium may be deposited by ion implantation or sputtering.

Also in the case of a multilayer coating comprising a ceramic thermal barrier coating it may be beneficial to deposit mixed oxides of chromia, silica and alumina onto the platinum aluminide coating before depositing the ceramic thermal barrier coating. It is believed that the addition of chromia, silica and alumina will key the ceramic thermal barrier coating onto the platinum aluminide coating because the chromia and silica beneficially alter the activity of the alumina. The chromia, silica and alumina may be arranged in a single layer or in three layers with a chromia layer on the platinum aluminide coating, a silica layer on the chromia layer and an alumina layer on the silica layer. The mixed oxides may be deposited by plasma spraying a balanced mixture of the oxides or sequentially plasma spraying the three oxides.

Additionally if the multilayer coating comprises a ceramic thermal barrier coating it may be beneficial to deposit erbium onto the platinum aluminide coating before depositing the ceramic thermal barrier coating. It is believed that the addition of erbium will oxidise to form erbia to key the ceramic thermal barrier coating onto the platinum aluminide coating. It is believed that the erbia beneficially alters the activity of the alumina. The erbium may be deposited by ion implantation or sputtering.

The ceramic thermal barrier coating 30 adheres well to the alumina layer 28 formed on the platinum aluminide coating 26. It is believed, in operation, that when all the available aluminium, for formation of an alumina layer, in the platinum aluminide coating 26 is exhausted that the silicon in the platinum aluminide coating 26 and silicon in the chromium silicide layer 24 forms a silica layer and that the ceramic thermal barrier coating 30 adheres well to the silica layer. It is believed that the silica reacts with the zirconia in the ceramic thermal barrier coating to form zirconium silicate which increases the adhesion of the ceramic thermal barrier coating 30.

We believe that the platinum in the platinum aluminidesilicide coating increases the diffusion activity of the silicon making the silicon seek out strong silicide forming elements and thus the silicon diffuses into the superalloy article to find these elements.

In tests we have deposited a platinum aluminide-silicide coating onto a low rhenium containing nickel based single crystal superalloy, for example CMSX4, and found that the silicon rich TCP phases are formed at the interface with the superalloy article. CMSX4 is produced by the Cannon-Muskegon Corporation of 2875 Lincoln Street, Muskegon, Michigan, MI 49433-0506, USA. CMSX4 has a nominal composition of 6.4wt% tungsten, 9.5wit cobalt, 6.5wit% chromium, 3.0wt% rhenium, 5.6wt% aluminium, 6.5wit% tantalum, l.Owt% titanium, 0.lwt% hafnium, 0.6wt% molybdenum, 0.006wt% carbon and the balance is nickel.

In tests we have deposited a platinum aluminide-silicide coating onto a high rhenium containing nickel based single crystal superalloy, for example CMSX10, and found that the silicon rich TCP phases are formed at the interface with the superalloy article. CMSX10 is produced by the Cannon Muskegon Corporation of 2875 Lincoln Street, Muskegon, Michigan, MI 49433-0506, USA. CMSX4 has a nominal composition range of 3.5 to 6.5wt% tungsten, 2.0 to 5.0wt% cobalt, 1.8 to 3.0wt% chromium, 5.5 to 6.5wit rhenium, 5.3 to 6.5wit% aluminium, 8.0 to 10.0wt% tantalum, 0.2 to 0.8wit% titanium, 0.25 to 1.5wt% molybdenum, 0.02 to 0.05wt% hafnium, o to 0.03wt% niobium, 0 to 0.04wt% carbon and the balance is nickel.

In tests we have deposited a platinum aluminide-silicide coating onto a low rhenium containing nickel based single crystal superalloy, for example MAR-M002, and found that the silicon rich TCP phases are formed at the interface with the superalloy article. MAR-M002 is produced by the Martin Marietta Corporation of Bethesda, Maryland, USA. MAR-M002 has a nominal composition of lOwt% tungsten, 10wit% cobalt, 9wt% chromium, 5.5wt% aluminium, 2.5we tantalum, 1.5wt% titanium, 1.5wt% hafnium, 0.015wt% carbon and the balance is nickel.

Thus the use of the chromium enriched layer prevents the silicon in a platinum aluminide-silicide coating diffusing to the superalloy article by forming stable chromium silicide intermetallic phases. Thus the formation of deleterious silicon rich TCP phases at the superalloy article interface is minimised. A further benefit of using the chromium coating is that this further improves the hot corrosion resistance of the coating.

The invention has been described with reference to a chromium and silicon enriched coating on the superalloy article, it is also possible to have a titanium and silicon, a tantalum and silicon or a titanium, tantalum and silicon enriched layer on the superalloy article. In these cases the titanium and tantalum form stable silicide phases with the silicon to minimise the diffusion of the silicon to the superalloy article. However, the chromium enriched layer is preferred because chromium is a stronger silicide former than titanium and tantalum. The thickness of the titanium and tantalum layers may need to be relatively thick to provide sufficient concentration to react with the silicon after diffusion into the superalloy article. The titanium and tantalum are preferably deposited onto the superalloy article by sputtering, but other suitable processes may be possible.

The superalloy article may be a nickel based superallty article or a cobalt based superalloy. The nickel based superalloy may be a high rhenium content nickel based superalloy for example CMSX10, allow rhenium content nickel based superalloy for example CMSX4 or any other nickel based single crystal superalloy. The superalloy article may be a gas turbine blade or gas turbine vane.

Claims

Claims:

1. A method of coating a superalloy article comprising the steps of : (a) applying a coating comprising at least one of chromium, titanium and tantalum to the superalloy article and diffusing at least one of the chromium, titanium and tantalum into the superalloy article to produce a layer on the superalloy article which is enriched in at least one of chromium, titanium and tantalum, (b) and depositing a platinum aluminide-silicide coating on the layer on the superalloy article which is enriched in at least one of chromium, titanium and tantalum.

2. A method as claimed in claim 1 wherein step (a) comprises chromising the superalloy article.

3. A method as claimed in claim 1 wherein step (a) comprises sputtering titanium, tantalum or titanium and tantalum onto the superalloy article and diffusing into the superalloy article.

4. A method as claimed in any of claims 1 to 3 wherein step (b) comprises depositing the platinum onto the layer on the superalloy article which is enriched in at least one of chromium, titanium and tantalum, heat treating to diffuse the platinum, simultaneously diffusing aluminium and silicon from the molten state into the coated superalloy article.

5. A method as claimed in any of claims 1 to 3 wherein step (b) comprises depositing the platinum onto the layer on the superalloy article which is enriched in at least one of chromium, titanium and tantalum, heat treating to diffuse the platinum, depositing silicon and then depositing aluminium and diffusing the silicon and aluminium into the coated superalloy article.

6. A method as claimed in claim 4 or claim 5 wherein the platinum is deposited by electroplating.

7. A method as claimed in claim 4, claim 5 or claim-6 wherein the platinum is heat treated at a temperature greater than 10000C.

8. A method as claimed in claim 7 wherein the platinum is heat treated at a temperature of 1120 0C for 1 to 2 hours to diffuse the platinum.

9. A method as claimed in claim 4, claim 5, claim 6, claim 7 or claim 8 wherein the platinum is deposited to a thickness between 5 and 15 micrometers.

10. A method as claimed in claim 2 wherein the chromising is at a temperature of 11000C for 5 hours.

11. A method as claimed in claim 4, claim 6, claim 7, claim 8 or claim 9 wherein the diffusion of aluminium and silicon is at a temperature in the range 7500C to 11200C.

12. A method as claimed in claim 4, claim 6, claim 7, claim 8, claim 9 or claim 11 wherein the simultaneous diffusion of aluminium and silicon from the molten state is repeated at least once more.

13. A method as claimed in any of claims 1 to 12 comprising after step (b) an additional step of: (c) applying a ceramic thermal barrier coating.

14. A method as claimed in any of claims 1 to 12 comprising after step (b) the additional steps of: (c) forming alumina on the platinum aluminide- silicide coating, (d) applying a ceramic thermal barrier coating by physical vapour deposition.

15. A method as claimed in claim 14 wherein the ceramic thermal barrier coating is deposited by electron beam physical vapour deposition.

16. A method as claimed in any of claims 13 to 15 comprising after step (b) and before step (c) an additional step of : depositing at least one of yttrium or yttrium and zirconium.

17. A method as claimed in any of claims 13 to 15 comprising after step (b) and before step (c) an additional step of : depositing at least one of chromia, silica or alumina.

18. A method as claimed in claim 17 comprising depositing the chromia, depositing the silica onto the chromia and depositing the alumina onto the silica.

19. A method as claimed in any of claims 13 to 16 comprising after step (b) and before step (c) an additional step of : depositing erbium.

20. A method as claimed in claim 19 wherein the erbium is deposited by ion implantation.

21. A method of coating a superalloy article substantially as hereinbefore described with reference to the accompanying drawings.

22. A coated superalloy article comprising a layer enriched in at least one of chromium, titanium and tantalum on the superalloy article, the layer enriched in at least one of chromium, titanium and tantalum having silicide phases and a platinum aluminide coating on the layer enriched in at least one of chromium, titanium and tantalum.

23. A coated superalloy article as claimed in claim 22 wherein there is a ceramic thermal barrier coating on the platinum aluminide coating.

24. A coated superalloy article as claimed in claim 22 wherein there is an alumina layer on the platinum aluminide coating and a ceramic thermal barrier coating on the alumina layer.

25. A coated superalloy article as claimed in claim 24 wherein the ceramic thermal barrier coating has columnar grains.

26. A coated superalloy article as claimed in any of claims 22 to 25 wherein the superalloy article is a nickel based superalloy article.

27. A coated superalloy article as claimed in claim 26 wherein the superalloy article is a high rhenium containing nickel based superalloy article.

28. A coated superalloy article as claimed in claim 27 wherein the superalloy article is a single crystal superalloy article.

29. A coated superalloy article as claimed in any of claims 22 to 28 wherein the superalloy article is a turbine blade or a turbine vane.

30. A coated superalloy article as claimed in any of claims 22 to 29 wherein the platinum aluminide coating comprises some silicide phases, the layer enriched in at least one of chromium, titanium and tantalum comprises some platinum aluminide and the bond coating comprises some silicide phases.

31. A coated article substantially as hereinbefore described with reference to and as shown in the accompanying drawings.