Method of Protecting a Component Against Hot Corrosion

Abstract

A method of protecting a component against hot corrosion comprising the steps: (a) applying a chromium diffusion coating to the component; and (b) applying a further coating to selected regions of the chromium diffusion coating, the selected regions being chosen dependent on subsequent use of the component.

Description

This invention relates to a method of protecting a component against hot corrosion.

The invention finds particular application in the protection against hot corrosion of a gas turbine engine rotor blade.

It is known that chromium provides excellent protection against so called Type I and Type II hot corrosion. In this regard, diffusion coatings produced by the diffusion of chromium and aluminium into the alloy substrate have long been used to provide this protection. MCrAlY overlay coatings (where M is Ni or Co or a combination of the two) have been used as an alternative to diffusion coatings at higher temperatures to protect against oxidation. The use of diffused chromium alone (chromising) is known to provide excellent protection against relatively low temperature Type II hot corrosion, and further to be strain tolerant (to have no effect on the fatigue properties of the substrate).

According to the present invention there is provided a method of protecting a component against hot corrosion comprising the steps: (a) applying a chromium diffusion coating to the component; and (b) applying a further coating to selected regions of the chromium diffusion coating, the selected regions being chosen dependent on subsequent use of the component.

Preferably, the selected regions are regions not subject to higher physical stress in the subsequent use of the component.

In a first method according to the present invention described below, the further coating comprises an aluminium diffusion coating.

In a second method according to the present invention described below, the further coating comprises an MCrAlY overlay coating, where M is Ni or Co or a combination of the two.

In a third method according to the present invention described below, the further coating comprises an MCrAlY overlay coating, where M is Ni or Co or a combination of the two, and the method further comprises the step (c) applying an aluminium diffusion coating to the selected regions coated with the MCrAlY overlay coating.

In the second and third methods the MCrAlY overlay coating applied in step (b) suitably comprises: 30 to 70 weight % Nickel; 30 to 50 weight % Cobalt; 15 to 25 weight % Chromium; 5 to 15 weight % Aluminium; and up to 1 weight % Yttrium.

In the second and third methods the MCrAlY overlay coating applied in step (b) may additionally include one or more elements selected from the group consisting of rhenium, silicon and hafnium.

The chromium diffusion coating applied in step (a) suitably comprises 15 to 30 weight % chromium and is 5 to 25 microns thick.

Methods according to the present invention find particular application in the protection against hot corrosion of nickel based superalloy components.

Methods according to the present invention find particular application in the protection against hot corrosion of gas turbine engine rotor blades.

The present invention also extends to components protected against hot corrosion by means of a method according to the present invention.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows a gas turbine engine rotor blade and the coating of this blade using a first method in accordance with the present invention;

FIG. 2 is a view of a side of the rotor blade of FIG. 1 hidden in FIG. 1 but to be seen when looking from the right in FIG. 1;

FIG. 3 shows a gas turbine engine rotor blade and the coating of this blade using a second method in accordance with the present invention;

FIG. 4 is a view of a side of the rotor blade of FIG. 3 hidden in FIG. 3 but to be seen when looking from the right in FIG. 3;

FIG. 5 shows a gas turbine engine rotor blade and the coating of this blade using a third method in accordance with the present invention; and

FIG. 6 is a view of a side of the rotor blade of FIG. 5 hidden in FIG. 5 but to be seen when looking from the right in FIG. 5.

In each of the first to third methods the rotor blade coated is a nickel based superalloy rotor blade. The rotor blade may be produced by conventional or directionally solidified (including single crystal) casting techniques. Typical alloys are MarM247, IN6203 and CMSX-4.

Referring to FIGS. 1 and 2, the blade coated comprises an outer shroud part 1 (above dotted line A), an aerofoil part 3 (between dotted lines A and B), a platform part 5 (between dotted lines B and C), and a root part 7 (below dotted line C). The blade includes an internal cooling passage 9 which commences as shown in FIG. 1, loops within the blade, and exits (exit not shown) via the top side of shroud part 1.

In a first stage of the first method, all surfaces of all parts of the blade, including internal cooling passage 9, are chromised, i.e. chromium is diffused into the surfaces. This diffusion is achieved by any suitable method, e.g. pack cementation or chemical vapour deposition (CVD). This results in a surface layer rich in chromium. The layer should typically contain 15 to 30 weight % chromium and be 5 to 25 microns thick.

In a second stage of the first method, an aluminium diffusion coating is applied to all external surfaces of the blade above dotted line M. This diffusion is again achieved by any suitable method, e.g. pack cementation or CVD. Masking is employed below dotted line M to prevent stray aluminium depositing below this line. If such stray depositing does occur, this is acceptable between dotted lines M and S, but not below dotted line S, i.e. not on the so called fir tree root of root part 7. Thus, a chromium modified aluminide coating results on all external surfaces of the blade above dotted line M. The so called outer beta layer of the chromium modified aluminide coating should typically contain 15 to 30 weight % aluminium and 5 to 15 weight % chromium. The total thickness of the chromium modified aluminide coating, including inter-diffusion zone, should typically be 25 to 100 microns.

Finally, the blade is heat treated to ensure that it maintains its optimum mechanical properties.

It is to be noted that in the final blade, all external surfaces above dotted line M are chromised plus aluminised, whereas all external surfaces below dotted line M and internal cooling passage 9 are chromised only.

Referring to FIGS. 3 and 4, the blade coated comprises an outer shroud part 1 (above dotted line A), an aerofoil part 3 (between dotted lines A and B), a platform part 5 (between dotted lines B and C), and a root part 7 (below dotted line C). The blade includes an internal cooling passage 9 which commences as shown in FIG. 3, loops within the blade, and exits (exit not shown) via the top side of shroud part 1.

In a first stage of the second method, all surfaces of all parts of the blade, including internal cooling passage 9, are chromised, i.e. chromium is diffused into the surfaces. This diffusion is achieved by any suitable method, e.g. pack cementation or CVD. This results in a surface layer rich in chromium. The layer should typically contain 15 to 30 weight % chromium and be 5 to 25 microns thick.

In a second stage of the second method, an MCrAlY overlay coating (where M is Ni or Co or a combination of the two) is applied to the following parts of the blade: outer shroud part 1, aerofoil part 3, and the top face 11 of platform part 5. The coating suitably comprises 30 to 70 weight % Nickel, 30 to 50 weight % Cobalt, 15 to 25 weight % Chromium, 5 to 15 weight % Aluminium, and up to 1 weight % Yttrium. The coating may additionally include one or more elements selected from the group consisting of rhenium, silicon and hafnium. The coating is applied by any suitable method, e.g. by thermal spray techniques such as vacuum plasma spraying (VPS), low pressure plasma spraying (LPPS), and high velocity ox-fuel spraying (HVOF), or by electroplating. Masking is employed to ensure that MCrAlY is not deposited on the remainder of platform part 5 below top face 11, and on root part 7.

Finally, the blade is heat treated to ensure that it maintains its optimum mechanical properties.

It is to be noted that in the final blade, outer shroud part 1, aerofoil part 3, and the top face 11 of platform part 5 are chromised plus have an overlay coating of MCrAlY, whereas the remainder of platform part 5 below top face 11, root part 7, and internal cooling passage 9 are chromised only.

Referring to FIGS. 5 and 6, the blade coated comprises an outer shroud part 1 (above dotted line A), an aerofoil part 3 (between dotted lines A and B), a platform part 5 (between dotted lines B and C), and a root part 7 (below dotted line C). The blade includes an internal cooling passage 9 which commences as shown in FIG. 5, loops within the blade, and exits (exit not shown) via the top side of shroud part 1.

In a first stage of the third method, all surfaces of all parts of the blade, including internal cooling passage 9, are chromised, i.e. chromium is diffused into the surfaces. This diffusion is achieved by any suitable method, e.g. pack cementation or CVD. This results in a surface layer rich in chromium. The layer should typically contain 15 to 30 weight % chromium and be 5 to 25 microns thick.

In a second stage of the third method, an MCrAlY overlay coating (where M is Ni or Co or a combination of the two) is applied to the following parts of the blade: outer shroud part 1, aerofoil part 3, and the top face 11 of platform part 5. The coating suitably comprises 30 to 70 weight % Nickel, 30 to 50 weight % Cobalt, 15 to 25 weight % Chromium, 5 to 15 weight % Aluminium, and up to 1 weight % Yttrium. The coating may additionally include one or more elements selected from the group consisting of rhenium, silicon and hafnium. The coating is applied by any suitable method, e.g. by thermal spray techniques such as VPS, LPPS, and HVOF, or by electroplating. Masking is employed to ensure that MCrAlY is not deposited on the remainder of platform part 5 below top face 11, and on root part 7.

In a third stage of the third method, those parts of the blade to which the MCrAlY overlay coating was applied (outer shroud part 1, aerofoil part 3, and the top face 11 of platform part 5) are over-aluminised, i.e. an aluminium diffusion coating is applied to these parts. The diffusion is achieved by any suitable method, e.g. pack cementation or CVD. Masking is employed to ensure that stray aluminium is not deposited on the remainder of platform part 5 below top face 11, and on root part 7. The result of the over-aluminisation should be that the outer surface of the MCrAlY overlay coating has an aluminium content of typically 15 to 30 weight %. The total thickness of the over-aluminised MCrAlY coating, including inter-diffusion zone, should typically be 100 to 200 microns.

Finally, the blade is heat treated to ensure that it maintains its optimum mechanical properties.

It is to be noted that in the final blade, outer shroud part 1, aerofoil part 3, and the top face 11 of platform part 5 are chromised plus have an overlay coating of MCrAlY, which MCrAlY overlay coating has been over-aluminised, whereas the remainder of platform part 5 below top face 11, root part 7, and internal cooling passage 9 are chromised only.

It is to be appreciated that in the above described first to third methods the application of further coating(s) in addition to the initial chromium diffusion coating is restricted to regions of the rotor blade not subject to higher physical stress in use of the blade. In the first method, the diffused aluminium coating is restricted to all external surfaces above dotted line M. In the second method, the MCrAlY overlay coating is restricted to outer shroud part 1, aerofoil part 3, and the top face 11 of platform part 5. In the third method, the MCrAlY overlay coating plus over-aluminisation is restricted to outer shroud part 1, aerofoil part 3, and the top face 11 of platform part 5. This restriction ensures that those regions of the blade that are subject to higher physical stress are coated with a chromium diffusion coating alone which is strain tolerant, and that the strain tolerance of this coating is not degraded by the application of further coating(s). The purpose of the application of the further coating(s) is to provide additional protection against hot corrosion. The approach taken therefore with regard to the application of the further coating(s) is as follows. It is first determined where on the blade there will be a region of relatively high temperature. Further coating(s) are then applied to this region provided it is not also a region that will experience higher physical stress.

Claims

1-20. (canceled)

21. A turbine blade comprising:

an aerofoil;

a shroud arranged at a first end of the aerofoil;

a platform arranged at a second end of the aerofoil;

a root arranged adjacent the platform; and

a protective coating system for protecting the turbine blade against hot corrosion, wherein the coating system has: a first layer with a chromium diffusion coating applied to the turbine blade, and a second layer arranged on top of the first layer having an aluminium diffusion coating applied to the aerofoil, the shroud, the platform and the root, wherein the second layer covers the root only in part.

22. The turbine blade according to claim 21, wherein the first layer covers the entire turbine blade.

23. The turbine blade according to claim 21, wherein the chromium diffusion coating comprises 15 to 30 weight % chromium and is 5 to 25 microns thick.

24. The turbine blade according to claim 21, wherein the turbine blade is a nickel based superalloy turbine blade.

25. A turbine blade comprising:

an aerofoil;

a shroud arranged at a first end of the aerofoil;

a platform arranged at a second end of the aerofoil, the platform comprising a top face arranged adjacent the aerofoil;

a protective coating system for protecting the turbine blade against hot corrosion, wherein the coating system comprises: a first layer, the first layer comprising a chromium diffusion coating applied to the turbine blade, and a second layer, the second layer arranged on top of the first layer and comprising a MCrAlY overlay coating, wherein M is selected from the group consisting of Ni, Co and a combination of Ni and Co, and the overlay coating is applied only to the aerofoil, the shroud and the top face of the platform.

26. The turbine blade according to claim 25, wherein the first layer covers the entire turbine blade.

27. A turbine blade according to claim 25, wherein the turbine blade comprises a third layer, wherein the third layer is an aluminium diffusion coating, arranged on top of the second layer.

28. The turbine blade according to claim 25, wherein the MCrAlY overlay coating comprises:

30 to 70 weight % Nickel,

30 to 50 weight % Cobalt,

15 to 25 weight % Chromium,

5 to 15 weight % Aluminium, and

up to 1 weight % Yttrium.

29. The turbine blade according to claim 25, wherein the MCrAlY overlay coating additionally includes at least one element selected from the group consisting of rhenium, silicon and hafnium.

30. The turbine blade according to claim 25, wherein the chromium diffusion coating comprises 15 to 30 weight % chromium and is 5 to 25 microns thick.

31. The turbine blade according to claim 25, wherein the turbine blade is a nickel based superalloy turbine blade.

32. A method of protecting a turbine blade against hot corrosion, comprising:

providing the turbine blade comprising: an aerofoil, a shroud arranged at a first end of the aerofoil, a platform arranged at a second end of the aerofoil, and a root arranged adjacent the platform;

applying a first layer comprising a chromium diffusion coating to the turbine blade; and

applying a second layer comprising an aluminium diffusion coating to the aerofoil, the shroud, the platform and the root, wherein the second layer covers the root only in part.

33. The method according to claim 32, wherein the chromium diffusion coating comprises 15 to 30 weight % chromium and is 5 to 25 microns thick.

34. The method according to claim 32, wherein the turbine blade is a nickel based superalloy turbine blade.

35. A method of protecting a turbine blade against hot corrosion, comprising:

providing the turbine blade, wherein the turbine blade comprises: an aerofoil, a shroud arranged at a first end of the aerofoil, and a platform arranged at a second end of the aerofoil, the platform comprising a top face arranged adjacent the aerofoil;

applying a first coating layer including a chromium diffusion coating to the turbine blade; and

applying a second coating layer on top of the first coating layer to the aerofoil, the shroud and the top face of the platform, wherein the second layer is an MCrAlY overlay coating, and wherein M is selected from the group consisting of Ni, Co and a combination of Ni and Co.

36. The method according to claim 35, further comprising applying a third coating layer on top of the MCrAlY coating, wherein the third coating is an aluminium diffusion coating.

37. The method according to claim 35, wherein the MCrAlY coating comprises:

30 to 70 weight % Nickel;

30 to 50 weight % Cobalt;

15 to 25 weight % Chromium;

5 to 15 weight % Aluminium; and

up to 1 weight % Yttrium.

38. The method according to claim 35, wherein the MCrAlY overlay coating includes at least one element selected from the group consisting of rhenium, silicon and hafnium.

39. A method according to claim 35, wherein the chromium diffusion coating comprises 15 to 30 weight % chromium and is 5 to 25 microns thick.

40. A method according to claim 35, wherein the turbine blade is a nickel based superalloy turbine blade.