U.S. patent application number 11/333088 was filed with the patent office on 2007-03-01 for nickel-base superalloy having an optimized platinum-aluminide coating.
This patent application is currently assigned to General Electric Company. Invention is credited to Jon C. Schaeffer.
Application Number | 20070048538 11/333088 |
Document ID | / |
Family ID | 24307161 |
Filed Date | 2007-03-01 |
United States Patent
Application |
20070048538 |
Kind Code |
A1 |
Schaeffer; Jon C. |
March 1, 2007 |
Nickel-base superalloy having an optimized platinum-aluminide
coating
Abstract
A nickel-base superalloy substrate includes a surface region
having an integrated aluminum content of from about 18 to about 24
percent by weight and an integrated platinum content of from about
18 to about 45 percent by weight, with the balance components of
the substrate. The substrate is preferably a single-crystal
advanced superalloy selected for use at high temperatures. The
substrate may optionally have a ceramic layer deposited over the
platinum-aluminide region, to produce a thermal barrier coating
system. The platinum-aluminide region is produced by diffusing
platinum into the substrate surface, and thereafter diffusing
aluminum into the substrate surface.
Inventors: |
Schaeffer; Jon C.; (Milford,
OH) |
Correspondence
Address: |
HUNTON & WILLIAMS LLP;INTELLECTUAL PROPERTY DEPARTMENT
1900 K STREET, N.W.
SUITE 1200
WASHINGTON
DC
20006-1109
US
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
24307161 |
Appl. No.: |
11/333088 |
Filed: |
January 17, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10375537 |
Feb 27, 2003 |
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11333088 |
Jan 17, 2006 |
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09955390 |
Sep 18, 2001 |
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10375537 |
Feb 27, 2003 |
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09244578 |
Feb 10, 1999 |
7083827 |
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09955390 |
Sep 18, 2001 |
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08577071 |
Dec 22, 1995 |
6066405 |
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09244578 |
Feb 10, 1999 |
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Current U.S.
Class: |
428/471 ;
428/472; 428/472.2 |
Current CPC
Class: |
Y10T 428/12875 20150115;
Y02T 50/67 20130101; C23C 28/322 20130101; C23C 28/325 20130101;
C23C 28/3455 20130101; Y10T 428/12458 20150115; Y02T 50/6765
20180501; Y10T 428/12021 20150115; F01D 5/28 20130101; Y02T 50/671
20130101; C22C 19/03 20130101; C23C 10/02 20130101; F05D 2230/90
20130101; Y10T 428/12944 20150115; F05D 2300/121 20130101; C23C
10/60 20130101; Y10T 428/26 20150115; Y10T 428/12611 20150115; Y10T
428/12618 20150115; Y10T 428/12736 20150115; F05D 2300/143
20130101; Y10T 428/12535 20150115; C23C 10/58 20130101; Y02T 50/60
20130101; C23C 28/321 20130101 |
Class at
Publication: |
428/471 ;
428/472; 428/472.2 |
International
Class: |
B32B 15/04 20060101
B32B015/04; B32B 9/00 20060101 B32B009/00 |
Claims
1-20. (canceled)
21. An article for use in a gas turbine engine, comprising: a
nickel based superalloy substrate, a chemical vapor deposited,
diffusion aluminide layer formed on the substrate, said aluminide
layer having an outer layer region comprising a solid solution
intermediate phase and having an inner diffusion zone region
proximate the substrate, said intermediate phase includes an
average aluminum concentration in the range of about 18 to about
26% by weight, an average platinum concentration in the range of
about 8 to about 45% by weight, and an average nickel concentration
in the range of about 50 to about 60% by weight so as to be
non-stoichiometric relative to intermetallic compounds of aluminum
and nickel, or aluminum and platinum, said outer layer region being
substantially free of phase constituents other than said
intermediate phase, an alumina layer on the aluminide layer, and a
ceramic thermal barrier layer on the alumina layer.
22. The article of claim 21, wherein said intermediate phase
resides in a beta solid solution intermediate phase region of a
binary nickel-aluminum phase diagram.
23. The article of claim 21 wherein said outer layer region is
about 1.5 to about 4.0 mils in thickness.
24. The article of claim 21 wherein said ceramic thermal barrier
layer comprises a columnar microstructure.
25. The article of claim 21 wherein the ceramic thermal barrier
layer comprises yttria stabilized zirconia.
26. An article for use in a gas turbine engine, comprising: a
nickel base superalloy substrate, a chemical vapor deposited,
diffusion aluminide layer formed on the substrate, said aluminide
layer having an outer layer region comprising a nickel-aluminum
solid solution intermediate beta phase and an inner diffusion zone
region proximate the substrate, said intermediate phase including
an average aluminum concentration in the range of about 18 to about
26% by weight, an average platinum concentration in the range of
about 8 to about 45% by weight, and an average nickel concentration
of about 50 to about 60% by weight so as to be non-stoichiometric
relative to intermetallic compounds of aluminum and nickel and of
aluminum and platinum, said outer layer region being free of phase
constituents other than said intermediate beta phase, a thermally
grown alpha alumina layer on the aluminide layer, and a ceramic
thermal barrier layer vapor deposited on the alumina layer to have
a columnar microstructure.
27. The article of claim 26 wherein said outer layer region is
about 1.5 to about 4.0 mils in thickness.
28. The article of claim 26 wherein the ceramic thermal barrier
layer comprises yttria stabilized zirconia.
29. A method of forming a thermal barrier coating on a substrate,
comprising: chemical vapor depositing a diffusion aluminide layer
on the substrate which includes a nickel based superalloy substrate
under deposition conditions effective to provide an outer aluminide
layer region comprising a solid solution intermediate phase and an
inner diffusion zone region proximate the substrate, said
intermediate phase including an average aluminum concentration in
the range of about 18 to about 26% by weight, an average platinum
concentration in the range of about 8 to about 45% by weight, and
an average nickel concentration of about 50 to about 60% by weight
so as to be non-stoichiometric relative to intermetallic compounds
of aluminum and nickel, or aluminum and platinum, said outer layer
region being substantially free of phase constituents other than
said intermediate phase, oxidizing the aluminide layer under
temperature and oxygen partial pressure conditions effective to
form an alpha alumina layer, and depositing a ceramic thermal
barrier layer on the alumina layer.
30. The method of claim 29 wherein said intermediate phase resides
in a beta solid solution intermediate phase region of a binary
nickel-aluminum phase diagram.
31. The method of claim 29 wherein said outer layer region is
formed to a thickness of about 1.5 to about 4.0 mils.
32. The method of claim 29 wherein said ceramic thermal barrier
layer is deposited by vapor condensation on said substrate so as to
have a columnar microstructure.
33. The article of claim 21, wherein said intermediate phase
comprises an average aluminum concentration of about 18 to about
24% by weight, and an average platinum concentration of about 18 to
about 45% by weight.
34. The article of claim 21, wherein said intermediate phase
comprises an average aluminum concentration of about 21 to about
23% by weight and an average platinum concentration of about 30 to
about 45% by weight.
35. The article of claim 21 wherein said outer layer region is
about 2.5 mils in thickness.
36. The article of claim 21, wherein said intermediate phase
comprises a surface, distant from said inner diffusion zone region,
and the intermediate phase includes an average aluminum
concentration and an average platinum concentration which is
relatively high adjacent to the surface and decreases with
increasing depth into the intermediate phase.
37. The article of claim 21, wherein said ceramic thermal barrier
layer is deposited by electron beam physical vapor deposition.
38. The article of claim 26 wherein said intermediate phase
comprises an average aluminum concentration of about 18 to about
24% by weight, and the average platinum concentration of about 18
to about 45% by weight.
39. The article of claim 26 wherein said intermediate phase
comprises an average aluminum concentration of about 21 to about
23% by weight and an average platinum concentration of about 30 to
about 45% by weight.
40. The article of claim 26 wherein said outer layer region is
about 2.5 mils in thickness.
41. The article of claim 26 wherein said intermediate phase
comprises a surface, distant from said inner diffusion zone region,
and the intermediate phase includes the aluminum content and the
platinum content which is relatively high adjacent to the surface
and decreases with increasing depth into the intermediate
phase.
42. The article of claim 26, wherein said ceramic thermal barrier
layer is deposited by electron beam physical vapor deposition.
43. The method of claim 29 wherein said intermediate phase
comprises an average aluminum concentration of about 18 to about
24% by weight and, an average platinum concentration of about 18 to
about 45% by weight.
44. The method of claim 29 wherein said intermediate phase
comprises average aluminum content of about 21 to about 23% by
weight and average platinum content of about 30 to about 45% by
weight.
45. The method of claim 29 wherein said diffusion aluminide layer
is about 2.5 mils in thickness.
46. The method of claim 29 wherein said ceramic thermal barrier
layer is deposited by electron beam physical vapor deposition.
47. An article for use in a gas turbine engine, comprising: a
nickel based superalloy substrate, a chemical vapor deposited,
diffusion aluminide layer formed on the substrate, said diffusion
aluminide layer including an average aluminum concentration in the
range of about 18 to about 24% by weight, an average platinum
concentration in the range of about 8 to about 45% by weight, and a
ceramic thermal barrier layer on the aluminide layer.
48. The article of claim 47 wherein said outer layer region is
about 1.5 to about 4.0 mils in thickness.
49. The article of claim 47 wherein the ceramic thermal barrier
layer comprises yttria stabilized zirconia.
50. An article for use in a gas turbine engine, comprising: a
nickel base superalloy substrate, a chemical vapor deposited,
diffusion aluminide layer formed on the substrate, said diffusion
aluminide layer including an average aluminum concentration in the
range of about 18 to about 26% by weight, an average platinum
concentration in the range of about 8 to about 45% by weight, and a
ceramic thermal barrier layer vapor deposited on the aluminide
layer.
51. The article of claim 50 wherein said diffusion aluminide layer
is about 1.5 to 4.0 mils in thickness.
52. The article of claim 50 wherein the ceramic thermal barrier
layer comprises yttria stabilized zirconia.
53. A method of forming a thermal barrier coating on a substrate,
comprising: chemical vapor depositing a diffusion aluminide layer
on the substrate which includes a nickel based superalloy
substrate, said aluminide layer including an average aluminum
concentration in the range of about 18 to about 24% by weight, an
average platinum concentration in the range of about 8 to about 45%
by weight, and depositing a ceramic thermal barrier layer on the
aluminide layer.
54. The method of claim 53 wherein said aluminide layer is formed
to a thickness of about 1.5 to about 4.0 mils.
55. The method of claim 53 wherein said ceramic thermal barrier
layer is deposited by vapor condensation on said substrate so as to
have a columnar microstructure.
56. The article of claim 47, wherein said diffusion aluminide layer
comprises an average aluminum concentration of about 18 to about
24% by weight, and an average platinum concentration of about 18 to
about 45% by weight.
57. The article of claim 47, wherein said diffusion aluminide layer
comprises average aluminum content of about 21 to about 23% by
weight and average platinum content of about 30 to about 45% by
weight.
58. The article of claim 47 wherein said outer layer region is
about 2.5 mils in thickness.
59. The article of claim 47, wherein said diffusion aluminide layer
comprises a surface, and the aluminum content and the platinum
content is relatively high adjacent to the surface and decreases
with increasing depth into the diffusion aluminide layer and the
substrate.
60. The article of claim 57, wherein said ceramic thermal barrier
layer is deposited by electron beam physical vapor deposition.
61. The article of claim 50 wherein said diffusion aluminide layer
comprises an average aluminum concentration of about 18 to about
24% by weight, and an average platinum concentration of about 18 to
about 45% by weight.
62. The article of claim 50 wherein said diffusion aluminide layer
comprises an average aluminum content of about 21 to about 23% by
weight and an average platinum content of about 30 to about 45% by
weight.
63. The article of claim 50 wherein the diffusion aluminide layer
is about 2.5 mils in thickness.
64. The article of claim 50 wherein said diffusion aluminide layer
comprises a surface, and the aluminum content and the platinum
content is relatively high adjacent to the surface and decreases
with increasing depth into the diffusion aluminide layer and the
substrate.
65. The article of claim 50, wherein said ceramic thermal barrier
layer is deposited by electron beam physical vapor deposition.
66. The method of claim 53 wherein said diffusion aluminide layer
comprises an average aluminum concentration of about 18 to about
24% by weight, and an average platinum concentration of about 18 to
about 45% by weight.
67. The method of claim 53 wherein said diffusion aluminide layer
comprises average aluminum content of about 21 to about 23% by
weight and average platinum content of about 30 to about 45% by
weight.
68. The method of claim 53 wherein said diffusion aluminide layer
is about 2.5 mils in thickness.
69. The method of claim 53 wherein said ceramic thermal barrier
layer is deposited by electron beam physical vapor deposition.
70. An article comprising: a nickel-base superalloy substrate
including a substrate surface; a single phase platinum-aluminide
surface region proximate to the substrate surface, said article
exhibiting an environmental life expressed in hours of exposure per
1 mil of the surface region of more than about 2 relative lives
under high-velocity, 0.5 ppm salt environment at 2150.degree.
F.
71. The article of claim 70, wherein said aluminide surface region
comprises from about 18 to about 24% by weight integrated aluminum
content, from about 6 to about 45% by weight integrated platinum
content and from about 25 to about 76% by weight integrated nickel
content.
72. The article of claim 70 wherein said platinum-aluminide surface
region has a thickness of from about 0.0015 to about 0.004
inches.
73. The article of claim 70, wherein said platinum-aluminide
surface region comprises from about 20 to about 24% by weight
integrated aluminum content and from about 18 to about 45% by
weight integrated platinum content.
74. The article of claim 70, wherein said platinum-aluminide
surface region comprises about 25 to about 62% by weight integrated
nickel content.
75. The article of claim 70, wherein said platinum-aluminide
surface region comprises about 21 to about 23% by weight integrated
aluminum content and about 30 to about 45% by weight of integrated
platinum content.
76. The article of claim 70, wherein said platinum-aluminide
surface region comprises from about 21 to about 23% by weight
integrated aluminum content and about 30 to about 34% by weight
integrated platinum content.
77. The article of claim 70, wherein said platinum-aluminide region
comprises from about 26 to about 49% by weight integrated nickel
content.
78. The article of claim 70, wherein said platinum-aluminide region
comprises from about 37 to about 49% by weight integrated nickel
content.
79. The article of claim 70 further comprising a ceramic layer
adjacent said substrate surface.
80. The article of claim 79 wherein the ceramic layer comprises
yttria-stabilized zirconia.
81. The article of claim 70 wherein said platinum-aluminide surface
region extends from the substrate surface into the substrate to a
distance where the aluminum content is less than about 18% by
weight.
82. The article of claim 70 wherein said nickel-base superalloy
substrate is substantially a single crystal in form.
83. The article of claim 70 wherein said nickel-base superalloy
substrate is RN5 or RN6.
84. An article comprising a single phase platinum-aluminide surface
region proximate the surface of a nickel base superalloy substrate
made by a method comprising: forming a platinum layer at the
substrate surface by a method selected from the group consisting of
electroplating, sputtering and metallo-organic chemical vapor
deposition; heating the substrate to a temperature of from about
1800 to about 2000.degree. F. for a time of about 2 hours, wherein
the heating of the substrate diffuses the platinum into the
substrate; and depositing aluminum onto the nickel-base superalloy
substrate by using an aluminum source and diffusing said aluminum
into the substrate surface at an elevated temperature, at an
aluminum activity of from about 40 to about 50 atomic percent in a
pure nickel foil, and for a time of from about 4 to about 16 hours
to form a substantially single phase platinum-aluminide surface
region proximate the substrate surface, said platinum-aluminide
surface region comprising from about 18 percent to about 24 percent
by weight integrated aluminum content, from about 8 to about 45
percent by weight integrated platinum content and from about 31
percent by weight to about 74 percent by weight integrated nickel
content.
85. An article comprising; a substrate which includes a nickel base
superalloy; a diffusion aluminide layer comprising a substantially
single phase, said single phase comprising an average aluminum
concentration in the range of from about 18 to about 24% by weight,
an average platinum concentration in the range of from about 8 to
about 45% by weight, and an average nickel concentration in the
range of from about 21 to about 74% by weight.
86. The article of claim 85 wherein said diffusion aluminide layer
phase extends from the substrate surface into the substrate to a
distance where the aluminum content is about 18% by weight or
less.
87. The article of claim 85 wherein said nickel superalloy
substrate is substantially a single crystal in form.
88. The article of claim 85 wherein said nickel superalloy
substrate is RN5 or RN6.
89. An article having a platinum-aluminide surface region,
comprising: a substrate having a nickel-base superalloy substrate
bulk composition and a substrate surface; and a surface region at
the substrate surface and extending from the substrate surface into
the substrate to a distance defined by an upper limit of
integration that is the distance where a weight percent of aluminum
has decreased to 18% from a higher value closer to the surface, the
surface region having an integrated aluminum content of from about
18 to about 24% by weight and an integrated platinum content of
from about 18 to about 45% by weight, balance components of the
substrate bulk composition, wherein the sum of the integrated
aluminum content, the integrated platinum content, and the
components of the substrate bulk composition in the surface region
total 100% by weight.
90. An article having a platinum-aluminide surface region,
comprising: a substrate having a nickel-base superalloy substrate
bulk composition and a substrate surface; a surface region at the
substrate surface, the surface region having an integrated aluminum
content of from about 18 to about 24 weight percent and an
integrated platinum content of from about 18 to about 45 percent by
weight, balance components of the substrate bulk composition,
totaling 100 percent by weight; and a ceramic layer overlaying the
surface region, wherein the ceramic layer has a thickness of from
about 0.005 to about 0.015 inches.
91. An article prepared by the method comprising the steps of:
providing a substrate having a nickel-base alloy substrate bulk
composition and a substrate surface; depositing a layer of platinum
upon the substrate surface; diffusing platinum from the layer of
platinum into the substrate surface; providing a source of
aluminum; and diffusing aluminum from the source of aluminum into
the substrate surface for a time sufficient to produce a surface
region at the substrate surface and extending from the substrate
surface to a distance defined by an upper limit of integration that
is the distance where the weight percent of aluminum has decreased
to 18% from a higher value closer to the surface, the surface
region having an integrated aluminum content of from about 18 to
about 24% by weight and an integrated platinum content of from
about 18 to about 45% by weight, balance components of the
substrate bulk composition.
92. An article prepared by a method comprising the steps of
providing a substrate having a nickel-base superalloy substance
bulk composition and a substrate surface; thereafter depositing a
layer of platinum upon the substrate surface; thereafter heating
the substrate and layer of platinum to a temperature of about
1800-2000.degree. F. for a time of about 2 hours; thereafter
providing a source of aluminum in contact with the substrate
surface, the source of aluminum having an activity of about 40 to
about 50 atomic percent as measured in a pure nickel foil; and
simultaneously heating the substrate surface and source of aluminum
to a temperature of about 1925-2050.degree. F. for a time of from
about 4 to about 16 hours.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to nickel-base superalloys used in
high-temperature applications, and, more particularly, to articles
made of such materials and having an optimized platinum-aluminide
protective coating.
[0002] In an aircraft gas turbine (jet) engine, air is drawn into
the front of the engine, compressed by a shaft-mounted compressor,
and mixed with fuel. The mixture is combusted, and the resulting
hot exhaust gases are passed through a turbine mounted on the same
shaft. The flow of gas turns the turbine, which turns the shaft and
provides power to the compressor. The hot exhaust gases flow from
the back of the engine, driving it and the aircraft forwardly.
[0003] The hotter the exhaust gases, the more efficient is the
operation of the jet engine. There is thus an incentive to raise
the exhaust gas temperature. However, the maximum temperature of
the exhaust gases is normally limited by the materials used to
fabricate the turbine vanes and turbine blades of the turbine. In
current engines, the turbine vanes and blades are made of
nickel-based superalloys and can operate at temperatures of up to
1900-2100.degree. F.
[0004] Many approaches have been used to increase the operating
temperature limit of the turbine blades and vanes. The compositions
and processing of the materials themselves have been improved.
Physical cooling techniques are used. In one widely used approach,
internal cooling channels are provided within the components, and
cool air is forced through the channels during engine
operation.
[0005] In another approach, a metallic protective coating or a
ceramic/metal thermal barrier coating system is applied to the
turbine blade or turbine vane component, which acts as a substrate.
The metallic protective coating is useful in
intermediate-temperature applications. One known type of metallic
protective coating is a platinum-aluminide coating that is formed
by depositing platinum and aluminum onto the surface of the
substrate and then diffusing these constituents into the surface of
the substrate.
[0006] The thermal barrier coating system is useful in
high-temperature applications. The thermal barrier coating system
includes a ceramic thermal barrier coating that insulates the
component from the hot exhaust gas, permitting the exhaust gas to
be hotter than would otherwise be possible with the particular
material and fabrication process of the component. Ceramic thermal
barrier coatings usually do not adhere well directly to the
superalloys used in the substrates. Therefore, an additional
metallic layer called a bond coat is placed between the substrate
and the thermal barrier coating. The bond coat is usually made of a
nickel-containing overlay alloy, such as a NiCrAlY or a NiCoCrAlY,
of a composition more resistant to environmental damage than the
substrate. The bond coat may also be made of a diffusional nickel
aluminide or platinum aluminide, whose surface oxidizes to a
protective aluminum oxide scale.
[0007] While superalloys coated with such metallic protective
coatings or ceramic/metal thermal barrier coating systems do
provide substantially improved performance over uncoated materials,
there remains room for improvement in elevated temperature
performance and environmental resistance. There is an ongoing need
for improved metallic protective coatings and bond coats to protect
nickel-base superalloys in elevated-temperature applications. This
need has become more acute with the development of the newest
generation of nickel-base superalloys, inasmuch as the older
protective coatings are often not satisfactory with these
materials. The present invention fulfills this need, and further
provides related advantages.
SUMMARY OF THE INVENTION
[0008] The present invention provides a metallic overcoating for
nickel-base superalloys. The overcoating is a platinum-aluminide
useful as a metallic protective coating or as a bond coat for the
thermal barrier coating system. The overcoating is in the form of a
surface region that is well bonded to the substrate. The
platinum-aluminide coating has good elevated-temperature stability
and resistance to environmental degradation in typical gas-turbine
engine applications.
[0009] In accordance with the invention, an article having a
platinum-aluminide surface region comprises a substrate having a
nickel-base alloy substrate bulk composition and a substrate
surface, and a surface region at the substrate surface. The surface
region has an integrated aluminum content of from about 18 to about
24 percent by weight and an integrated platinum content of from
about 18 to about 45 percent by weight, balance components of the
substrate bulk composition totalling 100 weight percent.
Preferably, the surface region has an integrated aluminum content
of from about 21 to about 23 percent by weight and an integrated
platinum content of from about 30 to about 45 percent by weight.
All compositions stated herein for surface regions are determined
by an integration technique to be discussed subsequently, which
effectively determines an averaged composition throughout the
surface region. Optionally, a ceramic layer overlies the surface
region, to produce a thermal barrier coating system.
[0010] A method for preparing such an article comprises the steps
of providing a substrate having a nickel-base alloy substrate bulk
composition and a substrate surface, depositing a layer of platinum
upon the substrate surface, and diffusing platinum from the layer
of platinum into the substrate surface. The method further includes
providing a source of aluminum and diffusing aluminum from the
source of aluminum into the substrate surface for a time sufficient
to produce a surface region at the substrate surface. The surface
region has an integrated aluminum content of from about 18 to about
24 percent by weight and an integrated platinum content of from
about 18 to about 45 percent by weight, as determined by
integration, balance components of the substrate bulk composition
totalling 100 weight percent. Optionally, the substrate and surface
region may be annealed, and/or a ceramic layer may be deposited
overlying the surface region.
[0011] Platinum-aluminide protective surface regions have been
known previously, but the present approach provides an optimized
platinum-aluminide coating whose elevated-temperature performance
and environmental resistance are improved as compared with prior
platinum-aluminide coatings. Moreover, the platinum-aluminide
coating of the invention can be utilized with advanced nickel-base
superalloys without excessive coating growth during service,
surface roughening, production of undesirable phases during
service, or reduced stress rupture capabilities. Other features and
advantages of the present invention will be apparent from the
following more detailed description of the preferred embodiment,
taken in conjunction with the accompanying drawings, which
illustrate, by way of example, the principles of the invention. The
scope of the invention is not, however, limited to this preferred
embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a perspective view of a gas turbine component;
[0013] FIG. 2A is a schematic sectional view through the component
of FIG. 1 along line 2-2, showing one embodiment of the
invention;
[0014] FIG. 2B is a schematic sectional view through the component
of FIG. 1 along line 2-2, showing a second embodiment of the
invention;
[0015] FIG. 3 is a block flow diagram for a method for applying a
protective coating to a substrate; and
[0016] FIG. 4 is a graph illustrating coating performance as a
function of composition of the coating.
DETAILED DESCRIPTION OF THE INVENTION
[0017] FIG. 1 depicts a component of a gas turbine engine such as a
turbine blade or turbine vane, in this case depicted as a turbine
blade 20. The turbine blade 20 includes an airfoil 22 against which
the flow of hot exhaust gas is directed. The turbine blade 20 is
mounted to a turbine disk (not shown) by a dovetail 24 which
extends downwardly from the airfoil 22 and engages a slot on the
turbine disk. A platform 26 extends longitudinally outwardly from
the area where the airfoil 22 is joined to the dovetail 24. A
number of cooling channels optionally extend through the interior
of the airfoil 22, ending in openings 28 in the surface of the
airfoil 22. A flow of cooling air is directed through the cooling
channels, to reduce the temperature of the airfoil 22.
[0018] The airfoil 22 of the turbine blade 20 is protected by a
protective coating 30, two embodiments of which are illustrated in
FIG. 2A and FIG. 2B. In each case, the protective coating 30 is
present at a surface 31 of the turbine blade 20, which serves as a
substrate 32 for the protective coating 30.
[0019] In the embodiment of FIG. 2A, the protective coating 30
comprises a platinum-aluminide region 34 located at the surface 31
of the substrate 32. In the embodiment of FIG. 2B, the protective
coating 30 comprises a platinum-aluminide region 36 at the surface
31 of the substrate 32 and a ceramic thermal barrier layer 38
overlying the platinum-aluminide region 36. The protective coating
30 shown in FIG. 2B, including the metallic region 36 (in this
context termed a bond coat) and the ceramic layer 38, is sometimes
termed a thermal barrier coating system. The two platinum-aluminide
regions 34 and 36 may be of the same or different structures and
compositions, within the scope of the invention. The
platinum-aluminide regions 34 and 36 are preferably from about
0.0015 inches to about 0.004 inches thick, most preferably about
0.0025 inches thick.
[0020] FIG. 3 is a block flow diagram for a preferred method of
preparing the protective coatings of FIGS. 2A and 2B. The substrate
32 is provided, numeral 50. The substrate is a nickel-base
superalloy, preferably an advanced second or third generation,
nickel-base single-crystal superalloy containing substantial
amounts of both aluminum and rhenium. The substrate is
substantially single crystal in form, although small amounts of
polycrystalline material are tolerated. The aluminum content is
from about 5 to about 16 weight percent, most preferably about 6-7
weight percent, in such advanced superalloys. At least about 5
weight percent aluminum is present in order to produce a
sufficiently high volume fraction of the strengthening .gamma.'
phase. The rhenium content is from about 1 to about 8 weight
percent, most preferably from about 2.5 to about 6 weight percent,
in such advanced superalloys. A most preferred substrate is a
single-crystal substrate made of alloy RN5, having a composition,
in weight percent, of 7.5 percent cobalt, 7 percent chromium, 6.2
percent aluminum, 6.5 percent tantalum, 5 percent tungsten, 1.5
percent molybdenum, 3 percent rhenium, balance nickel. Optionally,
some yttrium and/or hafnium may be present. The approach of use
invention is also operable with other advanced alloy substrates
such as alloy RN6, having a composition, in weight percent, of 12.5
percent cobalt, 4.5 percent chromium, 6 percent aluminum, 7.5
percent tantalum, 5.8: percent tungsten, 1.1 percent molybdenum,
5.4 percent rhenium, 0.15 percent hafnium, balance nickel; and
alloy R142, having a composition, in weight percent, of 12 percent
cobalt, 6.8 percent chromium, 6.2 percent aluminum, 6.4 percent
tantalum, 4.9 percent tungsten, 1.5 percent molybdenum, 2.8 percent
rhenium, 1.5 percent hafnium, balance nickel.
[0021] The optimized platinum-aluminide coating of the invention
exhibits excellent performance on a wide variety of substrate
materials, but this improved performance is particularly important
for these advanced single-crystal nickel-base alloy substrates.
These advanced single crystal alloy substrates have higher aluminum
contents than prior nickel-base superalloys, resulting in a larger
amount of .gamma.' phase, about 60-70 volume percent, than prior
nickel-base superalloys. They are used at higher operating
temperatures, over 200.degree. F., than prior nickel-base
superalloy substrates, and diffusional effects are accordingly more
important. The platinum-aluminide coating of the invention does not
experience excessive coating growth, surface roughening, production
of undesirable phases during service, or reduced stress rupture
capabilities during service at such high temperatures. Accordingly,
the combination of such an advanced single-crystal, nickel-base
alloy substrate and the platinum-aluminide coating described next
is the most preferred embodiment of the invention. The
platinum-aluminide coating is not limited to use on such advanced
single-crystal superalloys, however.
[0022] A layer of platinum is deposited on the surface of the
substrate 32 as it then is presented, numeral 52. The layer of
platinum is preferably deposited by electroplating, but other
operable techniques such as sputtering and metallo-organic chemical
vapor deposition may also be used. The layer of platinum is
desirably about 0.0003 inches thick.
[0023] Platinum from the layer of platinum is diffused into the
surface of the substrate by heating the substrate and the deposited
layer of platinum, numeral 54. The preferred diffusion treatment is
2 hours at 180.degree.-2000.degree. F. The steps 52 and 54 may be
conducted simultaneously or serially.
[0024] A source of aluminum is provided, numeral 56, by any
operable technique. Preferably, a hydrogen and a halide gas is
contacted with aluminum metal or an aluminum alloy to form the
corresponding aluminum halide gas. The aluminum halide gas is
contacted to the previously deposited platinum layer overlying the
substrate, depositing an aluminum layer over the platinum
substrate. The reactions occur at elevated temperature so that
aluminum atoms transferred to the surface diffuse into the surface
of the platinum-enriched region and the substrate, numeral 58. The
steps 56 and 58 are therefore typically conducted
simultaneously.
[0025] The temperature of the treatment, the source composition,
the exposure time, and the quantity of aluminum-source gas
determine the amount of aluminum transferred to the substrate and
diffused into the substrate. The activity of the aluminum is
determined with a pure nickel foil 0.025 millimeters thick that is
placed in the aluminizing reactor at the same locations where
substrates are to be placed. Complications associated with the
measurement of aluminum in multicomponent systems are thereby
avoided. The foil is processed in the reactor so that the foil
saturates with aluminum. The aluminum content of the foil is
measured by acid digestion and analysis with a suitable method such
as inductively coupled plasma emission spectroscopy. From these
measurements, the processing of the aluminizing treatment was
determined. The preferred processing produces an activity of
between 40 and 50 atomic percent in a pure nickel foil. In a
preferred approach, the aluminizing and diffusion treatment is
accomplished at a temperature of 1925-2050.degree. F. for 4-16
hours.
[0026] After the diffusion treatment is complete, the chemical
compositions of the platinum-aluminide region 34, 36 and the
portion of the substrate 32 immediately adjacent to the
platinum-aluminum region 34, 36 vary as a function of depth below
the surface. The aluminum content and the platinum content of the
platinum-aluminum region 34, 36 are relatively high adjacent to the
surface 31, and decrease with increasing depth into the region 34,
36 and the substrate 32. The remainder of the composition,
totalling 100 weight percent, is formed of components of the bulk
composition of the substrate alloy, which is high at a large depth
below the surface 31 and decreases to a lower value immediately
adjacent to the surface 31.
[0027] Because of this variation of composition with, depth, the
compositions of surface regions are measured by an integration
method. The coated substrate is sectioned perpendicular to the
surface. The weight percent of aluminum, platinum, and other
elements of interest as a function of distance from the surface is
determined by any technique that provides local compositions, such
as an electron microprobe with a wavelength dispersive spectrometer
or energy dispersive spectrometer (in conjunction with appropriate
calibration standards). Measurements are taken with an electron
raster that produces at least a 5 micrometer by 5 micrometer
window. Such compositional measurement techniques are known in the
art. Compositional measurements are taken at locations starting
within 2-3 micrometers of the outer exposed surface, and increasing
depth increments of 5 micrometers or less from the prior
measurement. The weight percent content of the element of interest
is plotted as a function of distance from the outer exposed
surface, up to a maximum distance that serves as the upper limit of
integration. The upper limit of the integration is selected as the
distance where the weight percent of aluminum has decreased to 18
percent from the higher values closer to the surface, because below
18 percent aluminum the .beta.-NiAl is not stable. The area under
the curve is determined by any appropriate technique such as a
trapezoidal approximation, and divided by the value of the upper
limit of integration.
[0028] Extensive testing, to be described in greater detail
subsequently, was undertaken to determine the characteristics,
properties, and processing of the optimum platinum-aluminum region
34, 36. The result is that the region 34, 36 has an integrated
composition of from about 18 to about 24 weight percent aluminum
and from about 18 to about 45 weight percent platinum. More
preferably, the integrated composition is from about 21 to about 23
weight percent aluminum and from about 30 to about 45 weight
percent platinum. The balance of the composition is interdiffused
components of the substrate, principally nickel, cobalt, and
chromium, so that the total of aluminum, platinum, and the diffused
components composition is 100 percent.
[0029] This region 34, 36 is a single-phase, relatively ductile
composition of aluminum, platinum, nickel, and the diffused
components of the substrate. In the preferred approach, the region
34, 36 is about 0.0025 inches thick.
[0030] The process of FIG. 3 described to this point may optionally
be followed by either or both of two additional processing steps.
The substrate 32 and interdiffused region 34, 36 may be annealed to
stress relieve the interdiffused region 34, 36, numeral 60. This
annealing procedure, while widely used for some protective
coatings, has not been found necessary with the present approach.
If it is used, a preferred annealing treatment is a temperature of
1800-2000.degree. F. for a time of 1/4 to 2 hours.
[0031] A ceramic layer may optionally be deposited over the surface
31 of the substrate 30, numeral 62, if the final structure is to be
a thermal barrier coating system of the type depicted in FIG. 2B.
The ceramic layer for a thermal barrier coating 38 is preferably
yttria-stabilized zirconia (YSZ) having a composition zirconia and
about 6-8 percent by weight yttria, and about 0.005-0.015 inches
thick. The YSZ is deposited by any operable technique, most
preferably electron beam physical vapor deposition.
[0032] Coatings of a variety of platinum-aluminum region
compositions were prepared by the preferred approach described
above using RN5 substrates. The coated specimens were tested in
burner rigs in a high-velocity 0.5 ppm salt environment at
215.degree. F. The lives of the coated specimens were determined in
hours of exposure per mil (0.001 inch) of coating. FIG. 4 depicts
the results of these tests. There is a distinct region of
significantly improved performance, for platinum-aluminum regions
having an integrated aluminum content of from about 18 to about 24
percent by weight and an integrated platinum content of from about
18 to about 45 percent by weight, balance components of the
substrate bulk composition. Particularly desirable results are
obtained for an optimum compositional range wherein the integrated
aluminum content of the surface region is from about 21 to about 23
percent by weight and the integrated platinum content of the
surface region is from about 30 to about 45 percent by weight.
Outside of these limits, the protection afforded by the surface
region decreases.
[0033] This invention has been described in connection with
specific embodiments and examples. However, those skilled in the
art will recognize various modifications and variations of which
the present invention is capable without departing from its scope
as represented by the appended claims.
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