U.S. patent application number 11/168660 was filed with the patent office on 2007-01-25 for diffusion barrier and protective coating for turbine engine component and method for forming.
Invention is credited to Ramgopal Darolia, Mark Daniel Gorman, Bangalore Aswatha Nagaraj.
Application Number | 20070020399 11/168660 |
Document ID | / |
Family ID | 34314123 |
Filed Date | 2007-01-25 |
United States Patent
Application |
20070020399 |
Kind Code |
A1 |
Gorman; Mark Daniel ; et
al. |
January 25, 2007 |
Diffusion barrier and protective coating for turbine engine
component and method for forming
Abstract
A turbine engine component comprising a substrate made of a
nickel-base or cobalt-base superalloy, a non-metallic oxide or
nitride diffusion barrier layer overlying the substrate, and a
protective coating overlying the barrier layer, the protective
coating comprising at least one platinum group metal selected from
the group consisting of platinum, palladium, rhodium, ruthenium and
iridium. The diffusion barrier layer may be a deposited or
thermally grown oxide material, especially aluminum oxide. The
protective coating may be heat treated to increase homogeneity of
the coating and adherence with the substrate. The component
typically further comprises a ceramic thermal barrier coating
overlying the protective coating. Also disclosed are methods for
forming a protective coating system on the turbine engine component
by forming the non-metallic oxide or nitride diffusion barrier
layer on the substrate and then depositing the platinum group metal
on top of the barrier layer.
Inventors: |
Gorman; Mark Daniel; (West
Chester, OH) ; Nagaraj; Bangalore Aswatha; (West
Chester, OH) ; Darolia; Ramgopal; (West Chester,
OH) |
Correspondence
Address: |
HASSE & NESBITT LLC
8837 CHAPEL SQUARE DRIVE
SUITE C
CINCINNATI
OH
45249
US
|
Family ID: |
34314123 |
Appl. No.: |
11/168660 |
Filed: |
June 28, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10681433 |
Oct 8, 2003 |
6933052 |
|
|
11168660 |
Jun 28, 2005 |
|
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Current U.S.
Class: |
427/402 ;
427/299; 427/377; 427/446; 427/569 |
Current CPC
Class: |
F05D 2300/2112 20130101;
F05D 2300/2281 20130101; C23C 4/18 20130101; C23C 28/3455 20130101;
F05D 2300/21 20130101; Y02T 50/60 20130101; C23C 4/02 20130101;
F05D 2230/90 20130101; F05D 2300/143 20130101; F01D 5/288 20130101;
F05D 2300/228 20130101; C25D 5/48 20130101; C23C 28/345 20130101;
C23C 28/36 20130101; C23C 28/322 20130101; F05D 2300/2118 20130101;
F05D 2300/611 20130101; F05D 2300/22 20130101; C25D 5/34
20130101 |
Class at
Publication: |
427/402 ;
427/446; 427/569; 427/299; 427/377 |
International
Class: |
B05D 3/00 20060101
B05D003/00; B05D 3/04 20060101 B05D003/04; B05D 1/36 20060101
B05D001/36; H05H 1/24 20060101 H05H001/24; B05D 1/08 20060101
B05D001/08; B05D 3/02 20060101 B05D003/02 |
Claims
1. A method for forming a protective coating system on a turbine
engine component, the method comprising: a) providing a substrate
made of a nickel-base or cobalt-base superalloy; b) forming a
non-metallic oxide or nitride diffusion barrier layer on the
substrate; and c) depositing a protective coating comprising at
least two platinum group metals selected from the group consisting
of platinum, palladium, rhodium, ruthenium and iridium on the
barrier layer, said protective coating comprising at least about
40% by weight of platinum or rhodium, or mixtures thereof.
2. The method of claim 1 wherein the diffusion barrier layer is a
thermally grown oxide material.
3. The method of claim 2 wherein the thermally grown oxide layer is
promoted by depositing a layer of aluminum, aluminide, chromide, or
platinum group metal on the substrate, followed by an oxidation
step.
4. The method of claim 3 wherein the diffusion barrier layer is
aluminum oxide having a thickness of from about 0.05 to about 10
microns.
5. The method of claim 1 wherein the substrate surface is roughened
by grit blasting, etching, peening, grooving, or combinations
thereof, prior to forming the diffusion barrier layer by a low
pressure plasma spray, air plasma spray or high velocity oxy-fuel
process.
6. The method of claim 1 wherein the diffusion barrier layer is
aluminum oxide having a thickness of from about 0.5 to about 5
microns.
7. The method of claim 6 wherein the protective coating has a
thickness of from about 10 to about 60 microns.
8. The method of claim 6 wherein the protective coating comprises
at least about 50% by weight of platinum or rhodium, or mixtures
thereof.
9. The method of claim 8 wherein the protective coating comprises
at least three metals selected from the group consisting of
platinum, palladium, rhodium, ruthenium, and iridium.
10. The method of claim 9 wherein the platinum group metals are
sequentially deposited.
11. The method of claim 1 wherein the platinum group metal is
deposited using an electroplating step.
12. The method of claim 1 wherein the platinum group metal is
deposited by ion plasma deposition.
13. The method of claim 1 wherein the protective coating is heat
treated at a temperature of from about 900.degree. C. to about
1200.degree. C. for from about 1 to about 8 hours.
14. A method for forming a protective coating system on a turbine
engine component, the method comprising: a) providing a substrate
made of a nickel-base or cobalt-base superalloy; b) forming a
non-metallic oxide or nitride diffusion barrier layer on the
substrate; c) depositing a protective coating comprising at least
one platinum group metal selected from the group consisting of
platinum, palladium, rhodium, ruthenium and iridium on the barrier
layer; and d) forming a ceramic thermal barrier coating over the
protective coating.
15. The method of claim 14 wherein the diffusion barrier layer is a
thermally grown oxide material.
16. The method of claim 15 wherein the thermally grown oxide layer
is promoted by depositing a layer of aluminum, aluminide, chromide,
or platinum group metal on the substrate, followed by an oxidation
step.
17. The method of claim 14 wherein the diffusion barrier layer is
aluminum oxide having a thickness of from about 0.05 to about 10
microns.
18. The method of claim 17 wherein the protective coating comprises
at least two metals selected from the group consisting of platinum,
palladium, rhodium, ruthenium and iridium.
19. The method of claim 18 wherein the protective coating has a
thickness of from about 10 to about 60 microns, and comprises at
least about 50% by weight of platinum or rhodium, or mixtures
thereof.
20. A method for forming a protective coating system on a turbine
engine component, the method comprising: a) providing a substrate
made of a nickel-base or cobalt-base superalloy; b) forming a
non-metallic oxide or nitride diffusion barrier layer on the
substrate; c) depositing a protective coating comprising at least
about 40% by weight of a platinum group metal selected from the
group consisting of platinum, palladium, rhodium, ruthenium and
iridium, and mixtures thereof, on the barrier layer; and d) forming
a ceramic thermal barrier coating over the protective coating.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/681,433, filed on Oct. 8, 2003, incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a diffusion barrier layer
and protective coating for turbine engine components that are
exposed to high temperature, oxidation and corrosive environments.
More particularly, the invention is directed to forming a
non-metallic oxide or nitride diffusion barrier layer between a
superalloy substrate and a protective coating for the substrate.
The protective coating can be an environmental coating or a bond
coat for a thermal barrier coating on the turbine engine component,
and is formed by depositing at least one platinum group metal on
the diffusion barrier layer.
[0003] In aircraft gas turbine engines, the turbine vanes and
blades are typically made of nickel-based or cobalt-based
superalloys that can operate at temperatures of up to about
1150.degree. C. Various types of coatings are used to protect these
superalloys. One type of protective coating is based on a material
like MCrAl(X), where M is nickel, cobalt, or iron, or combinations
thereof, and X is an element selected from the group consisting of
Ta, Re, Ru, Pt, Si, B, C, Hf, Y and Zr, and combinations thereof.
The MCrAl(X) coatings can be applied by many techniques, such as
high velocity oxy-fuel (HVOF), plasma spray, or electron
beam-physical vapor deposition (EB-PVD). Another type of protective
coating is an aluminide material, such as nickel-aluminide. A
platinum-aluminide coating can be applied, for example, by
electroplating platinum onto the substrate, followed by a diffusion
step, which is then followed by an aluminiding step, such as pack
aluminiding. These types of coatings usually have relatively high
aluminum content as compared to the superalloy substrates. The
coatings often function as the primary protective layer (e.g., an
environmental coating). As an alternative, these coatings can serve
as bond layers for subsequently applied overlayers, e.g., thermal
barrier coatings (TBCs).
[0004] When the protective coatings and substrates are exposed to a
hot, oxidative, corrosive environment (as in the case of a gas
turbine engine), various metallurgical processes occur. For
example, an adherent alumina (Al.sub.2O.sub.3) layer ("scale")
usually forms on top of the protective coatings. This oxide scale
is desirable because of the protection it provides to the
underlying coating and substrate.
[0005] At elevated temperatures, interdiffusion of elemental
components between the coating and the substrate often occurs. The
interdiffusion can change the chemical characteristics of each of
these regions, while also changing the characteristics of the oxide
scale. In general, there is a tendency for the aluminum from the
aluminum-rich protective layer to migrate inwardly toward the
substrate. At the same time, traditional alloying elements in the
substrate (e.g., a superalloy), such as cobalt, tungsten, chromium,
rhenium, tantalum, molybdenum, and titanium, tend to migrate from
the substrate into the coating. (These effects occur as a result of
composition gradients between the substrate and the coating).
[0006] Aluminum diffusion into the substrate reduces the
concentration of aluminum in the outer regions of the protective
coatings. This reduction in concentration will reduce the ability
of the outer region to regenerate the protective alumina layer.
Moreover, the aluminum diffusion can result in the formation of a
diffusion zone in an airfoil wall, which undesirably consumes a
portion of the wall. Simultaneously, migration of the traditional
alloying elements like molybdenum and tungsten from the substrate
into the coating can also prevent the formation of an adequately
protective alumina layer.
[0007] A diffusion barrier between the coating and the substrate
alloy can prolong coating life by eliminating or greatly reducing
the interdiffusion of elemental components. However, very thin
layers of some materials may be insufficient for reducing the
interdiffusion at high operating temperatures. Also, there should
not be a substantial mismatch in CTE (coefficient of thermal
expansion) between the protective coating, the diffusion barrier
layer and a superalloy substrate. Otherwise, the overlying coating
may spall during thermal cycling of the turbine engine
component.
[0008] Thus, new diffusion barrier layers and protective coatings
that overcome some of the drawbacks of the art would be welcome for
high-temperature superalloy substrates. The barrier layer should
have relatively low "interdiffusivity" for substrate elements and
the protective coating. The barrier layer should also be chemically
compatible and compositionally stable with the substrate alloy and
any protective coating, especially during anticipated service lives
at temperatures up to about 1150.degree. C. Moreover, the barrier
layer should exhibit a relatively high level of adhesion to both
the substrate and the protective coating. The barrier layer should
also exhibit only a minimum of CTE mismatch with the substrate and
protective coating. Furthermore, the barrier layer and the
protective coating should be capable of deposition by conventional
techniques.
BRIEF DESCRIPTION OF THE INVENTION
[0009] In one aspect, the invention relates to a turbine engine
component comprising:
[0010] a) a substrate made of a nickel-base or cobalt-base
superalloy;
[0011] b) a non-metallic oxide or nitride diffusion barrier layer
overlying the substrate; and
[0012] c) a protective coating overlying the barrier layer, the
protective coating comprising at least one platinum group metal
selected from the group consisting of platinum, palladium, rhodium,
ruthenium and iridium.
[0013] In another aspect, this invention relates to a turbine
engine component comprising:
[0014] a) a substrate made of a nickel-base or cobalt-base
superalloy;
[0015] b) a non-metallic oxide or nitride diffusion barrier layer
overlying the substrate;
[0016] c) a protective coating overlying the barrier layer, the
protective coating comprising at least one platinum group metal
selected from the group consisting of platinum, palladium, rhodium,
ruthenium and iridium; and
[0017] d) a ceramic thermal barrier coating overlying the
protective coating.
[0018] Another aspect of the invention relates to a method for
forming a protective coating system on a turbine engine component,
the method comprising:
[0019] a) providing a substrate made of a nickel-base or
cobalt-base superalloy;
[0020] b) forming a non-metallic oxide or nitride diffusion barrier
layer on the substrate; and
[0021] c) depositing a protective coating comprising at least one
platinum group metal selected from the group consisting of
platinum, palladium, rhodium, ruthenium and iridium on the barrier
layer.
[0022] The invention also relates to a method for forming a
protective coating system on a turbine engine component, the method
comprising:
[0023] a) providing a substrate made of a nickel-base or
cobalt-base superalloy;
[0024] b) forming a non-metallic oxide or nitride diffusion barrier
layer on the substrate;
[0025] c) depositing a protective coating comprising at least one
platinum group metal selected from the group consisting of
platinum, palladium, rhodium, ruthenium and iridium on the barrier
layer; and
[0026] d) forming a ceramic thermal barrier coating over the
protective coating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a perspective view of a gas turbine engine
component; and
[0028] FIG. 2 is a sectional view through the component of FIG. 1
along line 2-2, showing one embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0029] This invention is directed to a diffusion barrier layer and
protective coating for a turbine engine component, such as a
turbine blade or vane, having a substrate made of a nickel-base or
cobalt-base superalloy. The term "superalloy" is intended to
embrace complex cobalt- or nickel-based alloys that include one or
more other elements, such as chromium, rhenium, aluminum, tungsten,
molybdenum, tantalum and titanium. Superalloys are described in
various references, e.g., U.S. Pat. Nos. 5,399,313 and 4,116,723.
High temperature alloys are also generally described in
Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd Edition,
Vol. 12, pp. 417-479 (1980), and Vol. 15, pp. 787-800 (1981). The
actual configuration of the substrate may vary widely. For example,
the substrate may be in the form of various turbine engine parts,
such as combustor liners, combustor domes, shrouds, blades,
nozzles, or vanes.
[0030] As used herein, "diffusion barrier layer" is meant to
describe a layer of material that prevents the substantial
migration of alloy elements of the substrate into the protective
coating. Non-limiting examples of alloy elements of the substrate
are nickel, cobalt, iron, aluminum, chromium, refractory metals
(e.g., tungsten, tantalum, rhenium, and molybdenum), hafnium,
carbon, boron, yttrium, titanium, and combinations thereof. These
elements, when diffused into the protective coating, can be
detrimental to the environmental resistance and/or thermal barrier
coating adhesion properties of the turbine engine component. The
diffusion barrier layer is relatively thermodynamically and
kinetically stable at the service temperatures encountered by the
turbine engine component.
[0031] Overlying the superalloy substrate is the non-metallic oxide
or nitride diffusion barrier layer. The barrier layer is formed
between, and typically is in direct contact with, the superalloy
substrate and the overlying protective coating. The barrier layer
is tightly adherent and substantially impermeable to diffusion of
atoms from the substrate and from the overlying protective coating.
It is thermally stable and has a low self-diffusion coefficient.
The diffusion barrier layer essentially creates a stable zone
between the underlying substrate and the protective coating that
prevents interactions, which are usually undesirable, between the
substrate and the coating.
[0032] The diffusion barrier layer herein can be an oxide of a
variety of elements, for example, aluminum, zirconium, hafnium,
yttrium, silicon, and chromium, and mixtures thereof. Of these,
oxides of aluminum and zirconium, or mixtures thereof, are
typically used. Such oxides can be deposited on the superalloy
substrate by various processes known in the art, such as thermal
spray, sol gel, laser deposition, physical vapor deposition,
chemical vapor deposition, or ion plasma (also known as cathodic
arc) deposition, or they can be formed on the substrate as a
thermally grown oxide.
[0033] In one embodiment, the oxide layer is alumina. A thin
alumina layer can significantly reduce the migration of elements
from the protective coating inwardly and the migration of elements
outwardly from the substrate into the protective coating, thereby
stabilizing the compositions of both the coating and substrate.
Alumina forms a strong bond with nickel-base superalloy substrates
used in airfoils. It also forms a strong bond with the protective
coatings herein.
[0034] In one embodiment, a thin, tightly adherent alumina layer is
formed on the surface of a Ni-base superalloy substrate by
subjecting the substrate to a controlled oxidizing heat treatment
at a temperature above about 980.degree. C. Aluminum, inherent in
commonly used nickel-base superalloy substrates for airfoil
applications, such as, for example, Rene N5 having a nominal
composition of 6.2% Al, oxidizes at the surface of the substrate
forming a tightly adherent alumina film. While the film thickness
will depend on the temperature and the length of time at the
temperature, the thickness typically is from about 0.5 to about 5
microns, more typically about 1 micron.
[0035] In another embodiment, a thermally grown oxide layer is
promoted by depositing a thin layer (e.g., about 1-25 microns
thick, depending on the choice of material and deposition process)
of aluminum, an aluminide (e.g., Ni or Co), a chromium-rich
material like Ni--Cr--Al, or a platinum group metal such as
platinum, on the substrate. A variety of deposition processes may
be used, but physical vapor deposition, chemical vapor deposition,
thermal spray, plating or diffusion coating processes are typically
used. The aluminum, chromium or platinum group metal promotes the
formation of the thermally grown oxide during a pre-oxidation step.
The oxide-promoting layer formed is typically thin enough to
diffuse to a low concentration and not be considered an additional
layer at the completion of the coating process.
[0036] If additional strength is required between the diffusion
barrier layer and the substrate, a mechanical bond can be generated
between the barrier layer and substrate by including fine oxides of
reactive elements, including at least one element selected from the
group consisting of Zr, Y, Ca, Ce and Hf, as disclosed in U.S. Pat.
No. 6,455,167. These reactive elements may already be present in
the substrate in sufficient amounts to cause the formation of
internal oxides during a subsequent heat treatment after
application of the diffusion barrier layer.
[0037] In another embodiment, the diffusion barrier layer in the
form of an alumina scale is applied directly to the substrate or to
a pre-bond coat applied to the substrate. If the above mechanical
interlocks are not employed, or if the substrate includes
sufficient reactive metals to form the requisite amounts of fine
oxides across the substrate/diffusion barrier interface during a
subsequent heat treatment, a pre-bond coat is typically not
applied. If the substrate does not include sufficient amounts of
reactive elements or if mechanical interlocks are desired across
both interfaces of the diffusion barrier, then a pre-bond layer
including the above reactive elements may be deposited over the
substrate. In this embodiment, a thin layer of alumina having a
thickness of less than about 10 microns, and typically about 1
micron or less, is directly deposited over either the substrate
surface or the pre-bond coat applied to the substrate surface.
Unlike the prior embodiment in which the alumina was thermally
grown over the underlying material, in this embodiment a thin layer
of oxide, alumina for example, is directly applied to the
underlying material by sputtering, organo-metallic chemical vapor
deposition or by electron beam physical vapor deposition. The
applied oxide layer may also include reactive elements that can
assist in the formation of oxides during subsequent heat treatment.
The protective coating can then be applied over the diffusion
barrier and fine oxides forming the mechanical interlocks, when
required, can be grown in a thermal treatment as set forth
above.
[0038] In other embodiments, the diffusion barrier layer comprises
a non-metallic nitride material, such as chromium nitride, aluminum
nitride and titanium nitride as disclosed in U.S. Pat. Nos.
5,484,263, 6,528,189 and 6,129,998. The diffusion barrier layer
herein may also comprise mixtures of oxides and nitrides (e.g.,
oxy-nitrides).
[0039] Methods for applying the diffusion barrier layer over the
substrate are known in the art. They include, for example, electron
beam physical vapor deposition (EB-PVD), electroplating, ion plasma
deposition (IPD), chemical vapor deposition (CVD), plasma spray
(e.g., air plasma spray (APS)), high velocity oxy-fuel (HVOF),
low-pressure plasma spray (LPPS), sputtering, and the like.
Single-stage processes can be used to deposit the entire coating
chemistry. Those skilled in the art can adapt the present invention
to various types of equipment. For example, the alloy coating
elements can be incorporated into the source material in the case
of physical vapor deposition. Thermal spray processes (e.g., APS,
LPPS, and HVOF) benefit from surface roughening (e.g., grit
blasting, etching, shot peening, surface grooving, or combinations
thereof) prior to deposition in order to improve adhesion of the
diffusion barrier layer.
[0040] The thickness of the barrier layer will depend on a variety
of factors, including the particular composition of the substrate
and the layer (or layers) applied over the barrier layer; the
intended end use for the turbine engine component; the expected
temperature and temperature patterns to which the component will be
subjected; and the intended service life and repair intervals for
the coating system. When used on a turbine engine blade or airfoil,
the barrier layer usually has a thickness in the range of about
0.05 to about 10 microns, typically from about 0.5 to about 5
microns, and more typically from about 1 to about 3 microns. These
ranges may be varied considerably to suit the needs of a particular
end use.
[0041] Sometimes, a heat treatment is performed after the diffusion
barrier layer is applied over the substrate. The purpose of the
heat treatment is to improve adhesion and to enhance the chemical
equilibration between the barrier layer and the substrate. The
treatment is often carried out at a temperature in the range of
about 950.degree. C. to about 1200.degree. C., for up to about 16
hours.
[0042] A protective coating comprising at least one platinum group
metal selected from the group consisting of platinum, palladium,
rhodium, ruthenium and iridium, and optionally comprising chromium,
nickel, hafnium, zirconium, aluminum, yttrium, or cerium, or
mixtures thereof, is then formed overlying the diffusion barrier
layer. The resulting protective coating system comprises the
diffusion barrier layer and the protective coating. The protective
coating can be used as an environmental coating or a bond coat for
a ceramic thermal barrier coating deposited on the overlay coating.
The thermal barrier coating system formed provides improved
resistance to oxidation, spallation, and hot corrosion as compared
to conventional bond coats such as aluminide diffusion coatings and
MCrAlY coatings. Additionally, conventional bond coats and
environmental coatings often have high levels of aluminum that can
diffuse into the base metal and create a secondary reaction zone
that reduces the mechanical strength of the component. This is
avoided in the present invention by forming a protective coating
comprising relatively inert platinum group metals. The protective
coatings have low oxidation rates, and are sometimes referred to as
inert coatings. The resulting coatings also have high strength and
durability, and a minimum of thermal expansion mismatch with
ceramic thermal barrier coatings used on turbine engine components.
The protective coatings herein typically replace conventional
environmental coatings and bond coats used on turbine engine
components. The invention thus provides an improved turbine engine
component that is protected against high temperatures and adverse
environmental effects by the diffusion barrier layer and protective
coating herein, and optionally further protected by an additional
ceramic thermal barrier coating.
[0043] The present invention is generally applicable to turbine
engine components that operate within environments characterized by
relatively high temperatures, severe thermal stresses and thermal
cycling. Such components include the high and low-pressure turbine
nozzles and blades, shrouds, combustor liners and augmentor
hardware of gas turbine engines. One such example is the
high-pressure turbine blade 10 shown in FIG. 1. The blade 10
generally includes an airfoil 12 against which hot combustion gases
are directed during operation of the gas turbine engine, and whose
surface is therefore subjected to severe attack by oxidation,
corrosion and erosion. The airfoil 12 is anchored to a turbine disk
(not shown) with a dovetail 14 formed on a root section 16 of the
blade 10. Cooling holes 18 are present in the airfoil 12 through
which bleed air is forced to transfer heat from the blade 10. While
the advantages of this invention will be described with reference
to the high pressure turbine blade 10 shown in FIG. 1, and
particularly nickel-base superalloy blades of the type shown in
FIG. 1, the teachings of this invention are generally applicable to
any turbine engine component susceptible to degradation from
diffusion of substrate elements into an overlay coating used to
protect the component from its environment.
[0044] FIG. 2 shows a thermal barrier coating system 20 of a type
that benefits from the teachings of this invention. Coating system
20 includes a ceramic layer 26 and a protective coating 24, which
serves as a bond coat to the ceramic layer 26. A diffusion barrier
layer 28 overlays blade substrate 22. Substrate 22 is typically a
high-temperature material, such as an iron, nickel or cobalt-base
superalloy.
[0045] The diffusion barrier layer 28 is employed to minimize
diffusion between the protective coating 24 and the substrate 22.
In one embodiment, the diffusion barrier is a thin layer (e.g.,
from about 0.05 to about 10 microns thick) of aluminum oxide, which
may be deposited on the substrate by processes such as thermal
spray, sol gel, laser deposition, physical vapor deposition, or
chemical vapor deposition, or formed as a thermally grown oxide, as
described above. For example, an aluminum oxide layer about 1
micron thick may be formed by oxidizing the surface of an aluminum
rich superalloy or a superalloy that has had its surface enriched
in aluminum to promote the formation of aluminum oxide. The
oxidation step may be performed by heating the substrate to a
temperature in the range of from about 900.degree. C. to about
1150.degree. C. for about two hours in air or in a controlled
atmosphere, especially with a partial pressure of oxygen.
[0046] Protective coating 24 comprises at least one platinum group
metal selected from the group consisting of platinum, palladium,
rhodium, ruthenium, and iridium. The platinum group metal or metals
can be deposited on the substrate 22 by a variety of processes,
including electroplating, EB-PVD, sputtering, ion plasma and
thermal spray processes. In one embodiment, the coating 24
comprises at least two, and often at least three, of the above
platinum group metals. In most applications, coating 24 comprises
at least about 40%, and typically at least about 50%, by weight, of
platinum or rhodium, or mixtures thereof. The particular platinum
group metals used, their relative proportions, and the thickness of
the coating can be selected to obtain the desired properties, such
as strength, oxidation resistance, durability, hardness, thermal
expansion, and elastic modulus for the coating application at hand.
Minor amounts of additional elements, such as aluminum, zirconium,
hafnium and chromium, and mixtures thereof, can be added to improve
the mechanical and/or physical properties of the coating 24. Such
elements can be added at levels up to about 25%, typically up to
about 20%, by weight of the coating. Coating 24 typically has a
thickness of from about 5 to about 120 microns, more typically from
about 10 to about 60 microns. When the above metals are deposited
sequentially as individual layers on the substrate, the thickness
of each layer of metal in the protective coating 24 is usually from
about 2 to about 50 microns, more typically from about 5 to about
25 microns.
[0047] Prior to depositing the diffusion barrier layer 28 and/or
the protective coating 24, the surface of the turbine engine
component may be cleaned or conditioned, for example, by using a
caustic solution or grit blasting operation, immersing the
component in a heated liquid solution comprising a weak acid,
and/or agitating the surface of the component while it remains
immersed in the solution. In this manner, any dirt or corrosion
products on the surface can be removed without damaging the
component.
[0048] The protective coating 24, and each of its layers, may be
deposited by a variety of methods as mentioned above. One suitable
approach is electroplating, including the various electroplating
and entrapment plating processes known in the art. Electroplating
processes have relatively high deposition efficiencies that make
them particularly useful for depositing the expensive platinum
group metals herein. The platinum group metals may be deposited
sequentially; two or more metals may be co-plated; the metals may
be deposited using entrapment plating; or any combination of these
processes may be used. Electroplating of individual layers is
typically used, however, to more easily the control bath chemistry
and process parameters. For example, a platinum layer may be
deposited by placing a platinum-comprising solution into a
deposition tank and depositing platinum from the solution onto the
component in an electroplating process. An operable
platinum-comprising aqueous solution is Pt(NH.sub.3).sub.4HPO.sub.4
having a concentration of about 4-20 grams per liter of platinum.
The voltage/current source can be operated at about 0.5-20 amperes
per square foot (about 0.05-0.93 amperes per square meter) of
facing article surface. The platinum layer can be deposited in from
about 1 hour to about 4 hours at a temperature of about
190-200.degree. F. (about 88-93.degree. C.). Other platinum source
chemicals and plating parameters known in the art may also be used.
Similar processes can be used to deposit palladium, rhodium,
ruthenium and iridium, and combinations thereof. A thin flash
coating of a conductive material may be deposited to promote
electroplating over the non-electrically conductive, non-metallic
diffusion barrier layer. Examples include electroless nickel plate
and spluttered platinum.
[0049] In one embodiment, an entrapment plating process is used to
deposit the platinum group metals herein. In this process, standard
electroplating is conducted with a fine dispersion of solid
particulate material suspended in the plating solution. Some of the
particles become entrapped and retained in the plated coating. A
diffusion treatment can then be used to obtain a substantially
uniform composition of the coating. The ratio of plated to
entrapped material and the composition of each material is
controlled to arrive at the desired overall coating composition.
For example, platinum plating can be used to trap rhodium-palladium
particulates and obtain a Pt--Rh--Pd coating. An entrapment plating
process is particularly useful for adding minor amounts (e.g., up
to about 25%, typically up to about 20%, by weight) of non-platinum
group metals such as aluminum, zirconium, hafnium and chromium, and
mixtures thereof.
[0050] After depositing the protective coating 24, or each layer
thereof, the article is often heat treated, typically at about
900.degree. C. to about 1200.degree. C., more typically from about
1000.degree. C. to about 1100.degree. C., for a period of time,
e.g., up to about 24 hours, but generally from about 1 to about 8
hours, typically from about 2 to about 4 hours. This causes the
metals of the protective coating to interdiffuse, increasing the
homogeneity of the coating. Heat treating also improves the
adherence or bond between the coating and the diffusion barrier
layer, and the barrier layer and the substrate. The heat treatment
may be conducted after deposition of single or multiple layers of
the platinum group metals, or after electroplating is complete, to
enhance microstructure and composition uniformity, improve
adherence of the protective coating 24, and reduce residual
stresses within the coating and the adjoining surfaces.
[0051] A ceramic layer 26 may then be deposited on the protective
coating 24. Ceramic layer 26 is formed of a ceramic material that
serves to insulate the substrate 22 from the temperature of the hot
exhaust gas passing over the surface of the airfoil 12 when the
engine is in service. The ceramic layer 26 may be any acceptable
material, but typically is yttria-stabilized zirconia (YSZ) having
a composition of from about 3 to about 20 weight percent yttrium
oxide (e.g., about 7 percent yttrium oxide), with the balance
zirconium oxide. Other thermal barrier materials can also be used,
such as zirconia stabilized by ceria (CeO.sub.2), scandia
(Sc.sub.2O.sub.3), or other oxides. The ceramic layer 26 usually
has a thickness of from about 50 to about 1000 microns, typically
about 75 to about 400 microns. The ceramic layer 26 is typically
applied by air plasma spray, low-pressure plasma spray or physical
vapor deposition techniques. To attain a strain-tolerant columnar
grain structure, the ceramic layer 26 is usually deposited by
physical vapor deposition (PVD), though other deposition techniques
can be used. In contrast with conventional environmental coatings
and bond coats, the surface of the protective coating 24 typically
does not oxidize to any significant degree to form an oxide surface
layer. However, since ceramic thermal barrier coatings are
sufficiently permeable to gas that oxygen from the operating
environment may diffuse through such a coating and react with
non-platinum group metals in the bond coat, a small amount of oxide
may be formed. Any such oxide layer formed adheres well to the
protective coating and chemically bonds with the ceramic layer 26
so that satisfactory performance of a thermal barrier coating
system is provided.
[0052] The following example is intended to illustrate aspects of
the invention, and should not be taken as limiting the invention in
any respect.
EXAMPLE
[0053] A protective coating system of the invention comprising a
diffusion barrier layer and a protective coating is formed on
button specimens of a nickel-base superalloy known as Rene N5
having a nominal composition, in weight percent, of 7.5% Co, 7.0%
Cr, 6.2% Al, 6.5% Ta, 5.0% W, 3.0% Re, 1.5% Mo, 0.15% Hf, 0.05% C,
0.004% B, 0.01% Y, with the balance nickel and incidental
impurities. First, an aluminum oxide diffusion barrier layer about
1 micron thick is formed on the substrate by oxidizing the surface
of the aluminum rich superalloy. The oxidation step is performed by
heating the substrate to a temperature in the range of from about
900.degree. C. to about 1150.degree. C. for about two hours in air
or in a controlled atmosphere, especially with a partial pressure
of oxygen.
[0054] A thin flash layer (about 2 microns thick) of platinum is
applied over the diffusion barrier layer by sputtering. Layers of
platinum (about 76.2 microns thick), rhodium (about 12.7 microns
thick), and palladium (about 12.7 microns thick) are then
sequentially deposited over the diffusion barrier layer by
electroplating the button specimens. The samples are then heat
treated at a temperature of about 1050.degree. C. for 2 hours. All
samples are then coated with a ceramic layer (about 125 microns
thick) of zirconia with about 7% yttria by electron beam physical
vapor deposition. The thermal barrier coating system formed
comprises the ceramic layer and the protective coating comprising
platinum, rhodium and palladium.
[0055] Various embodiments of this invention have been described.
However, this disclosure should not be deemed to be a limitation on
the scope of the invention. Accordingly, various modifications,
adaptations, and alternatives may occur to one skilled in the art
without departing from the spirit and scope of the claimed
invention.
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