U.S. patent application number 09/929594 was filed with the patent office on 2002-01-24 for graded reactive element containing aluminide coatings for improved high temperature performance and method for producing.
Invention is credited to Darolia, Ramgopal, Miller, Joshua L., Rigney, Joseph D..
Application Number | 20020009611 09/929594 |
Document ID | / |
Family ID | 22833930 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020009611 |
Kind Code |
A1 |
Darolia, Ramgopal ; et
al. |
January 24, 2002 |
Graded reactive element containing aluminide coatings for improved
high temperature performance and method for producing
Abstract
A diffusion aluminide coating having a graded structure is
applied over a nickel base superalloy substrate. The coating has an
inner region of a diffusion aluminide adjacent to the substrate
rich in a reactive element, typically Hf, Si or combinations of the
two. The near surface region is a diffusion aluminide which is
substantially free of reactive elements. Such coatings when used as
bond coats in thermal barrier coating systems exhibit improved
spallation performance.
Inventors: |
Darolia, Ramgopal; (West
Chester, OH) ; Rigney, Joseph D.; (Milford, OH)
; Miller, Joshua L.; (West Chester, OH) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY
ANDREW C HESS
GE AIRCRAFT ENGINES
ONE NEUMANN WAY M/D H17
CINCINNATI
OH
452156301
|
Family ID: |
22833930 |
Appl. No.: |
09/929594 |
Filed: |
August 14, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09929594 |
Aug 14, 2001 |
|
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09222842 |
Dec 30, 1998 |
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Current U.S.
Class: |
428/680 ;
148/512; 416/241R; 428/610; 428/652 |
Current CPC
Class: |
C23C 28/321 20130101;
B32B 15/018 20130101; Y10S 428/941 20130101; Y10T 428/12458
20150115; B32B 15/01 20130101; Y10T 428/12931 20150115; C23C
28/3455 20130101; Y10T 428/1275 20150115; C23C 10/60 20130101; C23C
10/58 20130101; Y10T 428/12944 20150115; C23C 10/02 20130101; C23C
28/325 20130101 |
Class at
Publication: |
428/680 ;
416/241.00R; 428/610; 428/652; 148/512 |
International
Class: |
B32B 015/01 |
Claims
What is claimed is:
1. A turbine airfoil, comprising: a nickel-base superalloy
substrate; and a diffusion aluminide coating overlying the
substrate, the aluminide coating selected from the group consisting
of PtAl and NiAl, and combinations thereof, the aluminide coating
having two regions, a first region of aluminide including at least
one element selected from the group consisting of Hf, Zr, Y, and Si
and combinations thereof, and a second region adjacent to the first
region substantially free of elements selected from the group
consisting of Hf, Zr, Y and Si.
2. The airfoil of claim 1 wherein the first region is an inner
region positioned between the nickel-base superalloy substrate and
the second region, while the second region is an outer region.
3. The airfoil of claim 2 wherein the inner region has a thickness
of from about 5 to about 50 .mu.m and the outer region has a
thickness of from about 5 to about 50 .mu.m.
4. The airfoil of claim 2 wherein the inner region has a thickness
of from about 15 to about 25 .mu.m and the outer region has a
thickness of from about 15 to about 25 .mu.m.
5. The airfoil of claim 2 wherein the concentration of elements in
the inner region includes from about 0.25%-10% by weight Hf and
from 0 to about 5% by weight Si.
6. A coating for application over a nickel-base superalloy
substrate the coating comprising: a diffusion aluminide coating
selected from the group consisting of PtAl and NiAl and
combinations thereof, the aluminide coating having two regions, a
first region of aluminide including at least one element selected
from the group consisting of Hf, Zr, Y, and Si and combinations
thereof, and a second region adjacent to the first region
substantially free of elements selected from the group consisting
of Hf, Zr, Y and Si.
7. The coating of claim 6 wherein the first region is an inner
region positioned between the substrate and the second region,
while the second region is an outer region.
8. The coating of claim 6 wherein at least one element selected
from the group consisting of Hf, Zr, Y, and Si and combinations
thereof is present in the inner region in concentrations of from
0.25-10% by weight.
9. The coating of claim 8 wherein the concentration of elements in
the inner region includes from about 0.25%-10% by weight Hf and
from 0 to 5% by weight Si.
10. A process for applying over a nickel-base superalloy substrate
a diffusion aluminide coating selected from the group consisting of
PtAl and NiAl and combinations thereof having two regions, a first
region of aluminide including at least one element selected from
the group consisting of Hf, Zr, Y, and Si and combinations thereof,
and a second region adjacent to the first region substantially free
of elements selected from the group consisting of Hf and Si,
comprising the steps of: placing the substrate into an elevated
temperature atmosphere having a high concentration of aluminum and
at least one element selected from the group consisting of Hf, Zr,
Y and Si and combinations thereof for a sufficient period of time
for formation adjacent to the substrate of a first region of
aluminide coating including at least one element selected from the
group consisting of Hf, Zr, Y and Si; and then removing the coated
substrate from the elevated temperature atmosphere and placing the
coated substrate into an elevated temperature atmosphere having a
high concentration of aluminum and substantially free of elements
selected from the group consisting of Hf, Zr, Y and Si for a
sufficient time to allow for formation over the first region of a
second region of aluminide coating substantially free of elements
selected from the group consisting of Hf, Zr, Y and Si.
11. The process of claim 10 wherein the diffusion aluminide coating
is applied by pack cementation by placing the nickel-base
superalloy substrate into a mixture of powders including 0.15-0.50%
by weight of an element selected from the group consisting of Hf,
Zr, Y and Si, 1-5% by weight aluminum source, 0.1 -0.2% by weight
halide activator and the balance inert filler at a temperature for
1850-2050.degree. F. for 2-6 hours to form the first region of
diffusion aluminide coating adjacent to the substrate having at
least one element selected from the group consisting of Hf, Zr, Y
and Si; next removing the substrate from the elevated temperature
atmosphere; then realuminiding the diffusion aluminide-coated
substrate by placing the substrate into the atmosphere having a
high concentration of aluminum substantially free of elements
selected from the group consisting of Hf, Zr, Y and Si at a
temperature of from about 1850-2050.degree. F. for a sufficient
time to allow for formation over the first region of diffusion
aluminide coating of a second region of aluminide coating
substantially free of elements selected from the group consisting
of Hf, Zr, Y and Si having a thickness of at least 5 .mu.m.
12. The process of claim 11 wherein a thin region of platinum is
first electrodeposited on the nickel base superalloy substrate
before the step of placing the substrate in the mixture of
powders.
13. The process of claim 12 wherein the platinum coated substrate
is heat treated at temperature of 1600-2000.degree. F. for 2-20
hours to allow platinum to diffuse into the substrate prior to the
step of placing the substrate in a mixture of powders.
14. The process of claim 10 wherein the nickel-based superalloy
substrate is comprised of a turbine airfoil removed from service
further including removing oxide products and corrosion products
from the surface of the airfoil substrate prior to the step of
placing the airfoil into the elevated temperature atmosphere having
a high concentration of Al and at least one element selected from
the group consisting of Hf, Zr, Y and Si and combinations
thereof.
15. A process for applying over a nickel-base superalloy substrate
a diffusion aluminide coating selected from the group consisting of
PtAl and NiAl and combinations thereof having two regions, a first
region of aluminide substantially free of elements selected from
the group consisting of Hf and Si and a second region of aluminide
adjacent to the first region including at least one element
selected from the group consisting of Hf, Zr, Y, and Si and
combinations thereof, comprising the steps of: placing the
substrate into an elevated temperature atmosphere having a high
concentration of aluminum and substantially free of Hf, Zr, Y and
Si for a sufficient period of time for formation adjacent to the
substrate of a first region of an aluminide coating; then removing
the coated substrate from the elevated temperature atmosphere and
placing the coated substrate into an elevated temperature
atmosphere having a high concentration of aluminum and at least one
element selected from the group consisting of Hf, Zr, Y and Si and
combinations thereof for a sufficient time to allow for formation
over the first region of a second region of aluminide coating; and
holding the coated substrate at an elevated temperature for a
sufficient time so that the elements selected from the group
consisting of Hf, Zr, Y and Si, diffuse from the second region into
the first region.
16. The process of claim 15 further including the step of coating
the nickel-base superalloy substrate with a thin region of Pt, and
wherein the diffusion aluminide coating is formed by: first forming
a single phase PtAl-coated substrate by vapor phase aluminiding the
substrate to form a first region of aluminide coating; then placing
the coated PtAl-coated substrate into a pack powder bed having a
composition of, in weight percent, about 0.25% Hf, about 1%
aluminum-containing powder, about 0.2% halide activator and
optionally about 0.25% Si at a temperature in the range of about
1850.degree. F.-2000.degree. F. for about 2-6 hours to allow for
formation over the first region of a second region of aluminide
coating so that Hf, and optionally Si, diffuse from the second
region into the first region.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to gas turbine engines, and more
particularly, to coatings applied to airfoils in the turbine
portion of the gas turbine engine.
[0003] 2. Discussion of Prior Art
[0004] The current coatings used on airfoils exposed to the hot
gases of combustion in gas turbine engines for both environmental
protection and as bond coats in thermal barrier coating (TBC)
systems include both diffusion aluminides and MCrAlY(X) coatings.
These coatings are applied over substrate materials, typically
nickel-base superalloys, to provide protection against oxidation
and corrosion attack. These coatings are formed on the substrate in
a number of different ways. For example, a nickel aluminide (NiAl)
layer may be grown as an outer coat on a nickel base superalloy by
simply exposing the substrate to an aluminum rich environment at
elevated temperatures. The aluminum diffuses into the substrate and
combines with the nickel to form an outer surface of NiAl. A
platinum modified nickel aluminide coating can be formed by first
electroplating platinum to a predetermined thickness over the
nickel-based substrate. Exposure of the platinum-plated substrate
to an aluminum-rich environment at elevated temperatures causes the
growth of an outer region of NiAl containing platinum in solid
solution. In the presence of excess aluminum PtAl.sub.2 phases may
precipitate in the NiAl matrix as the aluminum diffuses into and
reacts with the nickel and platinum. Of course, an overlay of
MCrAlY where M is an element selected from the group consisting of
Ni and Co and combinations thereof may be applied to the substrate
as a bond coat or as an environmental coating by any known
technique. When applied as bond coats in thermal barrier systems,
an additional thermally resistant coating such as yttria-stabilized
zirconia (YSZ) is applied over top of the coating. However, as the
increased demands for engine performance elevate the engine
operating temperatures and/or the engine life requirements,
improvements in the performance of coatings when used as
environmental coatings or as bond coatings are needed over and
above the capabilities of these existing coatings. Because of these
demands, a coating that can be used for environmental protection or
as a bond coat capable of withstanding higher operating
temperatures or operating for a longer period of time before
requiring removal for repair, or both, is required.
BRIEF SUMMARY OF THE INVENTION
[0005] The present invention is directed to an improved coating for
use on an airfoil in the turbine portion of a gas turbine engine.
In its broadest embodiment, the coating is for application over a
nickel base superalloy substrate requiring environmental protection
and which is subjected to elevated temperatures. The coating is
comprised of a diffusion aluminide coating selected from the group
consisting of NiAl and PtAl. As used herein, unless otherwise
specified, the term platinum aluminide or PtAl refers to a
platinum-modified NiAl in which the NiAl includes platinum in solid
solution and in which PtAl.sub.x phases may be present as
precipitates in the matrix. This platinum aluminide may be
subsequently modified as discussed herein. This coating is applied
by two distinct diffusion aluminiding cycles resulting in two
distinct regions. One region of the coating is a diffusion
aluminide having at least one element selected from the group
consisting of hafnium (Hf), zirconium (Zr), yttrium (Y) and silicon
(Si), and combinations of these elements. The second region of the
diffusion aluminide coating adjacent to the first region is
substantially free of any of the elements selected from the group
consisting of Hf, Zr, Y and Si, except as occurs as a result of
natural diffusion processes during subsequent high temperature
exposures. Airfoils having coatings with such oxygen-active
elements as Hf, Si, Y, and Zr and combinations thereof exhibit
longer life and are capable of withstanding higher temperatures.
When used for environmental protection, these coatings are expected
to exhibit less corrosion and oxidation at higher temperatures than
either the prior art aluminides or MCrAlY coatings. When used as a
bond coat underlying a thermal barrier coating, the bond coat of
the present invention provides an advantage of improved damage
resistance in terms of lower spallation values for comparable times
as compared to a standard aluminide baseline. This improvement
translates into longer life.
[0006] Another advantage of the present invention is that it may be
applied to improve the performance of bond coatings or
environmental coatings for either new airfoils or for airfoils
requiring refurbishment after removal from service. 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is the tip end of an airfoil of the present invention
having the coating of the present invention applied as an
environmental coating;
[0008] FIG. 2 is the tip end of an airfoil of the present invention
applied as a bond coat with thermal barrier coating applied as a
top coat.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0009] FIG. 1 shows the tip 28 of an airfoil having an
environmental coating 30 applied over a substrate 32. The substrate
32 may be any high strength nickel-based material capable of
operation at elevated temperatures over long periods of time.
However, it is preferred that substrate 32 be a nickel based
superalloy substrate having a nominal composition in weight percent
of about 7.0% chromium, 1.5% molybdenum, 5.5% tungsten, 6.2%
aluminum, 7.5% cobalt, 0-0.15% hafnium, 3% rhenium, 6.5% tantalium,
20-300 ppm yttrium, 40 ppm boron, 0.05% carbon and the balance
nickel and incidental impurities. Overlying substrate 32 in the
embodiment shown in FIG. 1 is an aluminide coating 30 applied for
the purposes of environmental protection. The aluminide coating has
two distinct regions: a near surface region 36 and a region 34
immediately adjacent to substrate 32. Regions 34 and 36 comprise
the aluminide coating 30. Coating region 34 immediately overlying
the substrate 32 is a diffusion aluminide coating selected from the
group consisting of platinum aluminide and nickel aluminide, and
contains hafnium (Hf), zirconium (Zr), yttrium (Y), and silicon
(Si) either individually or in combination. Near surface region 36
is substantially the same as the region of the coating 34
immediately adjacent to the substrate 32, except that the near
surface region 36 is substantially free of Hf, Zr, Y, Si, and/or
combinations thereof. The near surface region of the coating can be
very thin, having a thickness of as little as 5 .mu.m in the
examples that follow. However, the thickness of the near surface
region can be increased by lengthening the time the article is
exposed to a coating method free of Hf, Zr, Y, Si, and/or
combinations thereof. As used herein the term "substantially free"
is used to compare the near surface region 36 to the portion of the
coating 34 immediately adjacent to the substrate 32. As the portion
of the coating adjacent to the substrate may contain Hf, Zr, Y, Si,
and/or combinations thereof in concentrations greater than 2%, the
near surface is considered to be substantially free of these
elements when their concentration is well below the concentration
in the coating adjacent to the substrate. While small amounts of
these elements may be present in the near surface region 36 it is
the result of natural diffusion processes that occur during
subsequent high temperature exposures. However, the concentration
of these elements in the near surface region 36 due to the effects
of diffusion is small as compared to the amount in the region
adjacent to the substrate. While this invention is described in
terms of Hf, Zr, Y and Si, it will be understood that other
strength increasing elements such as Ti, Re and Ta can also be
added individually or in combination with any of the above
elements. If any of these elements are added to the coating 30,
they are added in such a way that their concentration in the region
34 immediately adjacent substrate 32 is significantly greater than
their concentration in the coating in the near surface region
36.
[0010] FIG. 2 depicts the tip of a turbine blade 28 having a
coating 30 with the structure of the present invention, the region
nearer the substrate 34 being an aluminide coating including
reactive elements, either singularly or in combination, such as
described previously, and the near surface region 36 opposite the
substrate being substantially free of reactive elements. The
overall coating thickness may vary from about 10 to 100 .mu.m, with
each of regions 36 and 34 varying from 5-30 .mu.m each. However, it
is preferred that each of regions 36 and 34 vary from 15-25 .mu.m
each. The concentration of the reactive elements is greater in the
region 34 of the coating adjacent the substrate than in near
surface region 36 opposite the substrate. A thermal barrier coating
40 may be applied over top of the coating of the present invention
as shown in FIG. 2. There are several methods for accomplishing the
coating structure 30 of the present invention, shown in either FIG.
1 or FIG. 2. Each of these methods produces substantially the same
product.
EXAMPLE 1
[0011] A substrate is electroplated with a thin region of platinum.
After the article has been plated, it is placed in an atmosphere
having a source of aluminum and reactive elements Hf, Y, Zr, and
Si, either individually or in combination. Additional strengthening
elements such as Ti, Ta, or Re may also be present in the
atmosphere. The atmosphere containing a reactive element or
elements and aluminum could be a vapor species of compounds of
these elements such as halides as used in a chemical vapor
deposition process (CVD). Alternatively, the platinum-plated
article may be embedded in a mixture containing each of these
elements as elemental powders or as various compounds plus an
activator, such as is done in a pack cementation process.
Alternatively, the articles can be exposed to vapors containing
these elements such as occurs in above-the-pack (ATP) processes or
in vapor phase aluminiding (VPA) processes. In this situation, the
article is placed above the elements or compounds of the elements
with an activator compound while the temperature is raised. As the
various compounds begin to decompose into their constituent
elements during heating, or as the elemental powders are heated,
the atmosphere experiences increasing concentrations of the
elements. Of course, the driving force for forming a nickel
aluminide containing Pt in solid solution, is very strong. A
diffusion region of nickel aluminide with solid solution Pt begins
to form first and PtAl.sub.x phases may precipitate in the NiAl.
The other elements, Hf, Si, Y and Zr, as their concentration
increases, are then incorporated into the outer nickel platinum
aluminide. The nickel aluminide formation reaction and reactive
element incorporation process are both diffusion-controlled
processes. Because the driving force for reacting with the
aluminum-containing vapors is greater, the aluminide reaction
proceeds more rapidly at first but after a period of time at
elevated temperature, the concentration of reactive elements
increases and these elements are incorporated into the diffusion
aluminide layer. In addition, the platinum aluminide phase
PtAl.sub.x, precipitates, if formed, will also have the reactive
elements incorporated into their structure. The exact composition
of the platinum aluminide precipitate depends upon the
concentration of the platinum, the time that the substrate is held
at temperature, and the specific parameters of the heat treatment.
Because of the lower driving force for the diffusion of reactive
elements, the reactive elements initially diffuse into region 34
slower than the rate that the platinum aluminide coating is formed.
In considering the diffusion rates of reactive elements, Hf
diffusion occurs at a relatively slow rate, while Si diffuses more
rapidly. Y and Zr diffusion occurs at rates intermediate between Hf
and Si. Of course, the longer that the diffusion process is allowed
to continue, the deeper the penetration of the reactive elements
will be into the diffusion aluminide region 34, with the faster
diffusing elements diffusing further than the hafnium.
[0012] Because the reactive elements tend to oxidize in
environments such as the high temperature, corrosive environment of
a turbine portion of a gas turbine engine, it is not desirable to
expose region 34 to this atmosphere. Therefore, after the aluminide
region 34 has been formed by the diffusion process and the reactive
elements have been incorporated into it, region 36 is formed over
region 34 by placing the coated substrate into an aluminum-rich
atmosphere. This atmosphere can be created by any of the processing
methods set forth above for forming the diffusion aluminide
coating, except that the atmosphere is free of any concentrations
of reactive elements. Thus, at an elevated temperature, region 36
is formed by a simple diffusion process. The driving force is the
interdiffusion of Al, Ni and Pt to form an outer region which is
substantially a platinum aluminide free of the slower diffusing
reactive elements. Thus, the substrate is coated with a diffusion
aluminide coating having an inner region including a platinum
aluminide and reactive elements such as Hf, Zr, Y or Si, either
alone or in combination, and an outer region of a platinum
aluminide substantially free of reactive elements. If the substrate
is a nickel-base superalloy, the diffusion aluminide will also
contain a diffusion zone or region having the usual distribution of
TCP phases, .gamma.' and carbides as well as nickel aluminide.
There will be a gradient of these phases from the substrate to the
outer surface.
[0013] Typically, the inner region 34 includes at least one
reactive element in a concentration of 0-10%. In a preferred
embodiment, Hf is present in a concentration, in weight percent, of
about 0.25-10%, while Si is optionally present in the amount of
0-5%.
[0014] While the platinum is deposited by an electroplating method
in the above example, it is understood by those skilled in the art
that the platinum may be deposited by any known method which
deposits a thin region of Pt, including physical vapor deposition
processes, such as sputtering and chemical vapor deposition.
[0015] Samples having the coating of the present invention applied
as set forth in Example 1, were prepared by codepositing Al and Hf
in a pack process for 4 hours followed by vapor phase aluminiding
in an Al atmosphere free of Hf. Unless otherwise noted,
co-deposition (CODEP), was performed at about 1975.degree. F.
Samples having a platinum aluminide coating and samples codeposited
with Hf and Al over the substrate, but having no Pt, followed by
vapor phase aluminiding in an Al atmosphere free of Hf were also
prepared. The substrate for each sample was a 1/4" pin sample of
Rene' N5, a Ni-based superalloy. The samples were then tested to
failure by cycling them in a burner rig. Each cycle comprises
heating to about 1700.degree. F. in 15 seconds, holding for about
10 minutes, heating to about 2075.degree. F. in 15 seconds, holding
for about 5 minutes, followed by a 75-second forced air cool.
Testing was done in a mach 0.5 velocity flame using
salt-contaminated fuel. Table 1 presents the failure results, in
which the number of cycles required to attack the underlying
substrate was recorded. Samples prepared in accordance with Example
1, sample numbers 1, 2, 3, and 4 exhibited longer life than the
PtAl control samples (9, 10 and 11) and samples not including Pt in
the aluminide region (samples 5, 6, 7 and 8).
1TABLE 1 Cycles Sample Coating Type Processing Sequence to Failure
1 PtAl + Hf Pt-plate, Codep (1975.degree. F.), VPA 1360 2 PtAl + Hf
Pt-plate, Codep (1975.degree. F.), VPA 800 3 PtAl + Hf Pt-plate,
Codep (1925.degree. F.), VPA 1120 4 PtAl + Hf Pt-plate, Codep
(1925.degree. F.), VPA 1120 5 NiAl + Hf Codep, (1975.degree. F.),
VPA 400 6 NiAl + Hf Codep, (1975.degree. F.), VPA 400 7 NiAl + Hf
Codep, (1925.degree. F.), VPA 480 8 NiAl + Hf Codep,(1925.degree.
F.), VPA 160 9 PtAl baseline Pt-plate, VPA 300 10 PtAl baseline
Pt-plate, VPA 480 11 PtAl baseline Pt-plate, VPA 560
[0016] It should be noted that the samples of the present invention
without Pt noble metal incorporated (samples 5 through 8) exhibited
equivalent performance to the PtAl baseline samples (samples 9
through 11). The incorporation of Hf using this invented process
sequence counteracts the removal of Pt, an element usually found to
be necessary for erosion resistance.
EXAMPLE 2
[0017] A nickel aluminide coating is formed on the surface of a
nickel-based superalloy substrate by exposing the substrate to an
atmosphere having a source of aluminum and reactive elements Hf, Y,
Zr, and Si, either individually or in combination. Additional
strengthening elements such as titanium or tantalum may also be
present in the atmosphere. The atmosphere containing the reactive
element or elements and aluminum can be a vapor species of
compounds of these elements such as halides as used in a chemical
vapor deposition. Alternatively, the substrate may be surrounded
with a mixture of powders containing each of these elements in
elemental form or as various compounds. Typically, this is
accomplished by placing the substrate directly into the powders,
such as is done in a pack cementation process. Alternatively, the
substrate can be exposed to vapors containing these elements such
as occurs in above-the-pack processes or in vapor aluminide
processes. In this situation, the substrate is placed above the
elements or compounds of these elements while the temperature is
raised. As the various compounds begin to decompose into their
constituent elements during heating or as the elemental powders are
heated, the atmosphere experiences an increasing concentration of
the elements. Of course, the driving force for forming the nickel
aluminide is very strong, therefore a diffusion region of nickel
aluminide begins to form on the surface. The other elements, as
their concentration increases, are also incorporated into the thin
but growing region of nickel aluminide. The nickel aluminide
formation reaction and reactive element incorporation process are
both diffusion-controlled processes. Because the driving force for
reacting with the aluminum containing vapors exceeds that for the
diffusion of the reactive elements, the reactive elements initially
diffuse into nickel aluminide region 34, slower than the nickel
aluminide region is forming. In considering the diffusion rates of
reactive elements, Hf diffusion occurs at a relatively slow rate,
while Si diffuses more rapidly. Y and Zr diffusion occurs at rates
intermediate between Hf and Si. Of course, the longer that the
diffusion process is allowed to continue, the deeper the
penetration of the reactive elements will be into the diffusion
aluminide region 34, with the faster diffusing elements diffusing
to a greater depth than hafnium.
[0018] As in example 1 for the platinum aluminide diffusion region,
it is undesirable for region 34 to be exposed to an oxidizing
atmosphere such as occurs in a gas turbine engine because the
reactive elements such as Hf, Zr, Y and Si tend to rapidly oxidize,
and the oxides are not tightly adherent. When these elements are
present in high concentrations, this oxidation is readily apparent
because large areas of the surface become non-adherent (i.e.
surface oxide spallation). After the reactive elements have been
incorporated into region 34, region 36 is formed over region 34 by
placing the coated substrate into an aluminum rich atmosphere that
is free of any concentrations of reactive elements. Thus, at an
elevated temperature, region 36 is formed by a diffusion process.
The Al and Ni interdiffuse to form an outer region which is
substantially a nickel aluminide free of the slower diffusing
reactive elements. Thus the substrate is coated with a diffusion
aluminide coating having an inner region including a nickel
aluminide and reactive elements such as Hf, Zr, Y or Si either
alone or in combination, and an outer region of a nickel aluminide
substantially free of reactive elements. The diffusion aluminide
region will contain the usual diffusion zone of TCP phases, and
.gamma.' and carbides and there will be a gradient of these phases
from the substrate to the outer surface.
[0019] In each of these examples, region 34 of coating adjacent to
substrate 32 is relatively rich in Hf, Si or any other reactive or
strengthening elements that are added. The coated substrate is now
placed in an atmosphere of aluminum. Because aluminum is a much
faster diffusing element than any of the reactive elements that
that are added in the previous step, the aluminum diffuses into the
matrix and Pt or Ni diffuse outwardly toward the surface faster
than the reactive elements. Thus, the outermost region of the
coating is substantially richer in aluminum than in reactive
elements such as Hf. The gradation of the coating will be such that
near surface region 36 will have a high concentration of aluminum
with little reactive elements such as Hf. With increasing distance
inward from the outer surface towards the substrate, the reactive
element concentration will increase to a peak level and then begin
to decrease until the reactive element (Hf, Y, Zr, and/or Si)
concentration approaches zero near substrate surface 32 or to a
value that is substantially less than their peak level
concentrations. While the inner region typically is formed by a
pack cementation process in which there is a high concentration of
a reactive element included, any process that permits formation of
a diffusion aluminide rich in a reactive element may be used.
Typically, ATP processes, chemical vapor deposition (CVD) processes
or other vapor deposition methods may be used. Temperatures for
forming the diffusion aluminide in the appropriate atmosphere range
from about 1600.degree. F. to about 2000.degree. F. The time the
article is held at the temperature will also vary, from about 2 to
about 20 hours. The time, temperature and atmosphere are all
interdependent, and will vary depending upon the desired results.
All that is applicable to the formation of region 34 is equally
applicable to the formation of region 36, although the atmosphere
for region 36 will be devoid of reactive elements.
[0020] The coating of the present invention has been discussed in
terms of its applications as to a new turbine engine component,
such as a new blade. However, this invention is not so limited. For
example, the coating can be applied to an existing component
removed from service from a gas turbine engine. If the component
has a thermal barrier coating, the thermal barrier coating may be
removed by any suitable technique, and the bond coat can be
inspected. If present as an environmental coating region, the
coating can be inspected. If necessary, corrosion and oxidation
products can be removed from the bond coat or the environmental
coating as set forth above. While it is necessary to remove the
corrosion and oxidation products from the component requiring
refurbishment, it is neither necessary nor desirable to remove any
existing coating because the coating of the present invention can
be incorporated directly into or over the existing coating, whether
the coating is an aluminide or an MCrAlY, where M is an element
selected from the group consisting of Ni and Co and combinations of
Ni and Co. The oxidation and corrosion products may be removed by
techniques well known in the art such as chemical cleaning, caustic
autoclave processing, or grit blasting. Of course, the aluminum
content of the remaining aluminide coating is depleted of aluminum,
as some of the aluminum is incorporated into the oxide by-products
and diffused into the substrate.
EXAMPLE 3
[0021] A Ni-based superalloy turbine blade having a nominal
composition, in weight percent of 7.0% Cr, 7.5% Co, 0.05% C, 1.5%
Mo, 3.0% Re, 0.15 % Hf, 6.2% Al, 5.0% W, 6.5% Ta, 160 ppm Y, 40 ppm
B and the balance Ni and having an environmental coating of
platinum aluminide is removed from service. Oxidation by-products
and corrosion are removed from the surface of the blade by
grit-blasting. After cleaning, the airfoil is embedded in a powder
mixture within a retort. The mixture contains a source of aluminum,
a source of Hf, a halide activator and an inert filler. The airfoil
to be coated is heated in the range of about 1850.degree. F. to
about 2050.degree. F., preferably about 1950.degree.
F.(1070.degree. C.) in an inert atmosphere. The activator vaporizes
and reacts with the aluminum source, such as an aluminum
intermetallic or other aluminum-containing compound to form an
aluminum-rich halide vapor and a vapor of Hf. Aluminum reacts with
the existing platinum aluminide coating to restore aluminum levels,
while Hf also diffuses into the coating. The thickness and the
composition of the coating depends on the time and temperature of
the process, the activity of the powder and the composition of the
workpiece being coated. After the coating has been regenerated by
restoring aluminum, the airfoil is placed into a source of aluminum
that is devoid of Hf, but including a halide activator and an inert
filler. The airfoil to be coated is heated to a temperature in the
range of about 1800-2050.degree. F., preferably about 1950.degree.
F..+-.25.degree. F. (1070.degree. C.) in an inert atmosphere. The
activator vaporizes and reacts with the aluminum source to form an
aluminum-rich halide vapor. Aluminum diffuses into the coating, but
relatively slowly, while Pt and Ni diffuses outward to form a
platinum aluminide coating relatively free of Hf, since the Pt and
Ni will diffuse at a faster rate than the Hf. Thus the structure of
the present invention is formed.
[0022] An airfoil component may then have the graded coating of the
present invention applied to it as set forth in examples 1 or 2. If
a platinum aluminide coating is desired, then the methods set forth
in example 1 are followed. If the component is a nickel-base
superalloy and a nickel aluminide coating is desired, then the
methods set forth in example 2 are followed. If the airfoil
component is one which is undergoing repair, and it is not
desirable to strip the aluminide coating from the airfoil, the
repair procedure of example 3 is followed. It does not matter
whether the prior existing coating is a nickel aluminide or a
platinum aluminide.
[0023] Articles having the graded coating of the present invention
used as a bond coat in a thermal barrier coating system are
expected to exhibit superior spallation performance as compared to
articles having the conventional PtAl bond coat and a ceramic
topcoat of conventional 7% yttria-stabilized zirconia (7YSZ). In
the following examples, button samples of a nickel-base superalloy
substrate having the same nominal composition as the substrate of
example 3, with overlying bond coats and 7YSZ topcoats were
prepared and tested for spallation performance. The spallation
performance was compared to the spallation performance of the same
substrate having a standard PtAl coating as the baseline. The
spallation performance of this baseline substrate with this
standard bond coat and a 7YSZ topcoat, measured when 20% of the
thermal barrier coating spalls from the surface, was 230 cycles at
2125.degree. F. for a one hour furnace cycle test ( FCT), or
alternatively stated, an FCT life of 230 cycles.
EXAMPLE 4
[0024] A button sample of the nickel-base superalloy substrate
having the same nominal composition as the substrate of example 3
was prepared by embedding the substrate in a powder mixture of pure
Hf, an NH.sub.4F activator, an aluminum powder source and alumina
filler powders.
[0025] The Hf was present in the amount of 0.15-0.5% by weight, the
halide activator in the amount of 0.1-0.2% by weight, the aluminum
powder source in the amount of 1-5% by weight of codep-aluminide
and the balance alumina filler. The powders were thoroughly mixed.
The retort was heated to a temperature of about 1950.degree. F. and
held at temperature for about 4 hours to allow formation of a NiAl
aluminide containing Hf. The sample was then removed from the pack
and vapor phase aluminided, by flowing aluminum halide gas over the
surface at a temperature of about 1950.degree. F. for about 4-8
hours. This vapor phase process deposits aluminum on the surface of
the article. However, the driving force for the diffusion process
is such that nickel diffused outwardly to form an outer region of
NiAl relatively free of the slower-diffusing hafnium so that the
duplex coating of the present invention was formed. The button
sample was then coated with a topcoat of 7YSZ and tested for
spallation performance. The sample exhibited an FCT life of 400
cycles which was a significant improvement over the baseline
sample. Microprobe measurements on an as-coated section removed
from samples prior to testing disclosed the presence of, in weight
percent, 34% Al and only 0.11% Hf in the first 5 .mu.m of the bond
coat. The Hf content increased to 0.51 in the region between 5-15
.mu.m, with 32% Al, and to 6.9% Hf in the region between 15-30
.mu.m, with an aluminum content of about 20.7%.
[0026] In this example, the aluminum codep powder consisting
essentially of an aluminum intermetallic was used. However, any
other suitable aluminum compound in powder form may be used.
EXAMPLE 5
[0027] Two button samples of the nickel-base superalloy substrate
having the same nominal composition as the substrate of example 3
were prepared by first electroplating the sample with a thin region
of Pt, about 3-6 .mu.m thick. The Pt-coated specimens were then
subjected to a conventional diffusion heat treatment. The coated
samples were then embedded in a powder mixture of pure Hf, an
NH.sub.4F activator, an aluminum powder source and alumina filler
powders. The Hf was present in the amount of 0.15-0.5% by weight,
the activator in the amount of 0.1-0.2% by weight, the aluminum
powder source in the amount of 1-5% by weight of codep-aluminide
and the balance alumina filler. The powders were thoroughly mixed.
The retort was heated to a temperature of about 1950.degree. F. and
held at temperature for about 5 hours to allow formation of a
platinum-modified nickel aluminide containing Hf. The samples were
then removed from the pack and vapor phase aluminided by flowing
aluminum halide gas over the surfaces at a temperature of about
1950.degree. F. for about 4-8 hours. This vapor phase process
deposited aluminum on the surface of the article. However, the
driving force for the diffusion process is such that Pt and Ni
diffused outwardly to form an outer region of platinum aluminide
relatively free of the slower-diffusing hafnium so that graded
coating of the present invention was formed. The button samples
were then coated with a topcoat of 7YSZ and tested for spallation
performance. The two samples exhibited FCT lives of 320 cycles and
380 cycles respectively, which represent significant improvements
over the baseline sample. Microprobe measurements of the as-coated
samples failed with the lives disclosed indicated that in the first
5 .mu.m of the bond coat of the respective samples, Hf at 0.73% and
1.3%, while Al was at 28.5% and 29.2%, and Pt was at 27.5% and
24.4%, respectively. The Hf content increased to 2.5% and 11% in
the region between 5-15 .mu.m, with 28.7% and 25.1% Al, and 23.6%
and 31.7% Pt, respectively. In the final 15-30 .mu.m of the sample,
Hf was measured at 1.4% and 2%, while Al was 29.0% and 27.7%, and
Pt was 20.5% and 29.2%, respectively. All compositions are given in
weight percent, unless otherwise noted.
EXAMPLE 6
[0028] Ni-based superalloy substrate samples having a nominal
composition of 7.0% Cr, 7.5% Co, 0.05% C, 1.5% Mo, 3.0% Re, 0.15%
Hf, 6.2% Al, 5.0% W, 6.5% Ta, 160 ppm Y, 40 ppm B and the balance
Ni were first plated with a thin coating of Pt by a conventional
electroplating process and then vapor phase aluminided to produce a
single phase PtAl coating on the substrates. The samples were then
inserted into a pack bed having a composition in weight percent of
about 0.25% Hf, about 1% Codep powder as an aluminum source, about
0.25% Si and about 0.2% NH.sub.4F activator in an otherwise typical
pack cementation process. After heating for about four (4) hours at
about 1950.degree. F., three samples were coated with a thermal
barrier topcoat of 7YSZ and subjected to a FCT test at about
2075.degree. F. Samples were also analyzed to determine the
chemistry following the 4-hour temperature exposure at about
1950.degree. F. Although treatment was performed at 1950.degree. F.
for about 4 hours, any suitable combination of time and temperature
may be used to achieve the results set forth below. Typically,
temperatures in the range of about 1850.degree. F. to about
2000.degree. F. for times of about 2-6 hours are used.
[0029] The overall thickness of the resulting coating was about
0.004". The tested chemistry (in weight percent) measured from the
near (or outer surface) was as follows:
2 Region Below Surface Hf Al Si Pt 0-5 .mu.m 0.08 21.34 4.45 26.8
5-15 .mu.m 0.31 20.99 4.80 24.9 15-30 .mu.m 0.36 21.44 4.15
25.02
[0030] The samples subjected to FCT failed at 900 cycles, 1060
cycles and 880 cycles. Spallation at failure for each sample was
about 70% of the TBC, as compared to about 90% of the TBC for the
baseline. Thus, samples prepared in accordance with this procedure
had, on average about twice the cyclic spallation life as baseline
samples.
[0031] Although the present invention has been described in
connection with specific examples and embodiments, those skilled in
the art will recognize that the present invention is capable of
other variations and modifications within its scope. These examples
and embodiments are intended as typical of, rather than in any way
limiting on, the scope of the present invention as presented in the
appended claims.
* * * * *