U.S. patent application number 13/815152 was filed with the patent office on 2016-01-14 for coating and coating method for gas turbine engine component.
This patent application is currently assigned to Howment Corporation. The applicant listed for this patent is William C. Basta, Kenneth S. Murphy. Invention is credited to William C. Basta, Kenneth S. Murphy.
Application Number | 20160010472 13/815152 |
Document ID | / |
Family ID | 47747471 |
Filed Date | 2016-01-14 |
United States Patent
Application |
20160010472 |
Kind Code |
A1 |
Murphy; Kenneth S. ; et
al. |
January 14, 2016 |
Coating and coating method for gas turbine engine component
Abstract
The present invention provides a protective coating for a gas
turbine blade or other component wherein the duplex coating
includes an aluminum-bearing coating, such as a diffusion
aluminide, formed on a first, relatively higher temperature region
of the blade/component and a later-applied chromium-bearing
diffusion coating formed on an adjacent relatively lower
temperature region of the blade/component subject to hot corrosion
in service. The chromium-bearing coating is applied after the
aluminum-bearing coating by masking that coating and depositing a
metallic chromium coating on the adjacent region followed by
diffusing the chromium into the blade/component alloy to form a
chromium-enriched diffusion coating thereon.
Inventors: |
Murphy; Kenneth S.; (North
Shores, MI) ; Basta; William C.; (Montague,
MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Murphy; Kenneth S.
Basta; William C. |
North Shores
Montague |
MI
MI |
US
US |
|
|
Assignee: |
Howment Corporation
Independence
OH
|
Family ID: |
47747471 |
Appl. No.: |
13/815152 |
Filed: |
February 4, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61633935 |
Feb 21, 2012 |
|
|
|
Current U.S.
Class: |
416/241R ;
204/484; 204/485; 205/122; 205/156 |
Current CPC
Class: |
C25D 13/12 20130101;
C23C 10/58 20130101; F01D 5/3007 20130101; C23C 10/02 20130101;
F05D 2230/31 20130101; F01D 5/288 20130101; C25D 5/44 20130101;
F05D 2300/121 20130101; C25D 3/04 20130101; F05D 2300/132 20130101;
C25D 5/50 20130101; C25D 13/02 20130101; C25D 13/16 20130101; C25D
5/022 20130101; C25D 5/12 20130101 |
International
Class: |
F01D 5/28 20060101
F01D005/28; C25D 13/16 20060101 C25D013/16; C25D 13/02 20060101
C25D013/02; C25D 5/02 20060101 C25D005/02; C25D 5/12 20060101
C25D005/12 |
Claims
1. A method of forming a coating on a substrate, comprising the
steps of first applying an aluminum-bearing coating on a first
region of the substrate, then depositing a metallic coating
comprising chromium on the substrate, and then diffusing the
chromium into the substrate to form a chromium-enriched diffused
layer thereon.
2. The method of claim 1 including applying masking on said second
region before the aluminum-bearing coating is applied.
3. The method of claim 1 including applying masking on the
aluminum-bearing coating before applying the metallic coating.
4. The method of claim 1 including applying the aluminum-bearing
coating on both said first region and second region followed by
removal of the aluminum-bearing coating from said adjacent region
before the metallic coating is applied.
5. The method of claim 1 wherein the metallic coating is applied
using a liquid deposition medium.
6. The method of claim 5 wherein the liquid deposition medium is a
electroplating bath or electrophoretic bath.
7. The method of claim 5 wherein the liquid deposition medium is a
slurry of chromium-bearing particles.
8. The method of claim 1 wherein the aluminum-bearing coating is
applied as a diffusion aluminide.
9. A method of forming a duplex coating on a nickel or cobalt based
alloy turbine blade, comprising the steps of first applying an
aluminum-bearing coating on an airfoil region of the blade, then
depositing a metallic coating comprising chromium on at least a
portion of a root region of the blade using a liquid deposition
medium, and then diffusing the chromium into the alloy to form a
chromium-enriched layer on said portion of said root region.
10. The method of claim 9 including applying masking on the root
region before the aluminum-bearing coating is applied on the
airfoil region.
11. The method of claim 9 including applying the aluminum-bearing
coating on the airfoil region and on the root region followed by
removal of the aluminum-bearing coating from the root region before
the metallic overlay is applied to the root region.
12. The method of claim 9 including applying masking on the
aluminum-bearing coating before applying the metallic coating.
13. The method of claim 9 wherein the metallic coating is
electroplated at a temperature less than 212 degrees F.
14. The method of claim 9 wherein the aluminum-bearing coating is
applied to also cover a shank portion of said root region such that
the root region includes the shank portion covered by the
aluminum-bearing coating and an adjacent portion covered by the
chromium-enriched coating.
15. The method of claim 9 including leaving an attachment portion
of the root region uncoated.
16. The method of claim 9 wherein the aluminum-bearing coating is
applied as a diffusion aluminide.
17. The method of claim 9 wherein the liquid deposition medium is
an electroplating bath or electrophoretic bath.
18. The method of claim 9 wherein the liquid deposition medium is a
slurry of chromium-bearing particles.
19. A nickel or cobalt based alloy turbine component precursor
having an aluminum-bearing coating applied on an airfoil region of
the precursor and a metallic electroplated or electrophoretic
coating comprising chromium applied on at least a portion of a root
region of the precursor.
20. The precursor of claim 19 wherein the metallic coating
comprises a majority of chromium.
21. The precursor of claim 19 wherein the aluminum-bearing coating
also covers a shank portion of said root region such that the root
region includes a shank portion covered by the aluminum-bearing
coating and an adjacent portion covered by the metallic
coating.
22. The precursor of claim 19 wherein an attachment portion of the
root region is left uncoated.
23. The precursor of claim 19 wherein the aluminum-bearing coating
comprises a diffusion aluminide.
24. The precursor of claim 19 that includes a platform region
between the airfoil region and the root region, wherein a surface
of the platform facing toward the airfoil region includes the
aluminum-bearing coating.
25. The precursor of claim 24 wherein a surface of the platform
region facing away from the airfoil region includes the metallic
coating.
26. The precursor of claim 24 wherein a surface of the platform
region facing away from the airfoil region includes the
aluminum-bearing coating.
27. A nickel or cobalt based alloy turbine component precursor
having an aluminum-bearing coating applied on an airfoil region of
the precursor and a metallic coating comprising chromium-bearing
slurry particles applied on at least a portion of a root region of
the precursor.
28. A nickel or cobalt based alloy turbine component having an
aluminum-bearing coating applied on an airfoil region of the blade
and a chromium-enriched coating formed on at least a portion of a
root region by depositing a metallic electroplated or
electrophoretic coating comprising chromium and diffusing the
chromium into the alloy at said portion of said root region.
29. The component of claim 28 that includes a platform region
between the airfoil region and the root region, wherein a surface
of the platform facing toward the airfoil region includes the
aluminum-bearing coating.
30. The component of claim 29 wherein a surface of the platform
region facing away from the airfoil region includes the
chromium-bearing coating.
31. The component of claim 29 wherein the surface of the platform
region facing toward the airfoil region includes the
aluminum-bearing coating.
32. The component of claim 31 wherein the aluminum-bearing coating
comprises a diffusion aluminide.
33. The component of claim 31 wherein the aluminum-bearing coating
also covers a shank portion of said root region such that the root
region includes a shank portion covered by the aluminum-bearing
coating and an adjacent portion covered by the chromium-enriched
coating.
34. The component of claim 28 wherein an attachment portion of the
root region is uncoated.
35. A nickel or cobalt based alloy turbine component having an
aluminum-bearing coating applied on an airfoil region of the blade
and a chromium-enriched coating formed on at least a portion of a
root region by depositing a metallic coating comprising
chromium-bearing slurry particles and diffusing the chromium into
the alloy at said portion of said root region.
Description
RELATED APPLICATION
[0001] This application claims benefit and priority of U.S.
provisional application Ser. No. 61/633,935 filed Feb. 21, 2012,
the entire disclosure of which is incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a protective coating for a
gas turbine blade or other component wherein the coating includes
an aluminum-bearing coating applied at a relatively high
temperature region of the component and a chromium-bearing coating
applied at another relatively lower temperature region of the
component depending on coating functionality needed.
BACKGROUND OF THE INVENTION
[0003] Current gas turbine designs are requiring that a variety of
coatings be applied to different areas of the turbine part for
different functional reasons. Examples of coating functionality
include wear, oxidation, thermal barrier, and hot corrosion.
Turbine designers choose an appropriate coating for a particular
functionality in the gas turbine environment.
[0004] Hot corrosion is a form of accelerated oxidation when a
liquid salt is present on the surface of a Ni and Co based
superalloy component. The salt is usually sodium sulfate with other
naturally occurring constituents, such as K, Ca, and/or Mg,
present. It is well known that as the Cr content of an alloy
increases, its resistance to hot corrosion attack increases.
Current methods to increase surface Cr content are pack and vapor
phase chromizing, which comprise one-step deposition and reaction
with the Ni substrate alloy, forming a Cr-enriched alloy zone. The
chromizing process is facilitated by halide (Cl or F) activators
that form Cr-halide gases at relatively high temperatures, such as
greater than 1900 degrees F.
[0005] Since pack and vapor phase chromizing require high
temperature application above 1900 F and are difficult to apply to
localized part areas of interest, these processes must be applied
early in the part routing to the entire the part. Masking has not
been effective in these processes as a means for controlling the
localized deposition of the chromium on certain areas of interest
and, as a result, has not been applied in these high temperature
processes.
SUMMARY OF THE INVENTION
[0006] The present invention provides in an embodiment a method of
forming a protective coating on a gas turbine component wherein the
duplex coating includes an aluminum-bearing coating applied at one
region of the gas turbine component where relatively higher
temperatures are encountered in service and a chromium-bearing
coating applied at another region of the turbine blade or other
component where relatively lower temperatures and hot corrosion are
encountered in service, thereby providing coating functionality for
the different temperatures and oxidation/corrosion environments to
be encountered by the gas turbine component.
[0007] In an illustrative embodiment of the present invention, the
method involves forming a duplex coating on a superalloy substrate
by first applying an aluminum-bearing coating on the first
relatively higher temperature region of the substrate, secondly
applying a metallic coating comprising chromium on an adjacent
relatively lower temperature region of the substrate followed by
diffusing chromium into the substrate to form a chromium-enriched
diffused coating thereon at the adjacent relatively lower
temperature region. The aluminum-bearing coating is applied in a
first step by high temperature vapor deposition, while the
chromium-bearing coating is applied in a subsequent second step at
a relatively lower temperature, such as less than 500.degree. F.
The method typically involves applying masking on the relatively
lower temperature region before the aluminum-bearing coating is
applied on the relatively higher temperature region and
subsequently applying masking on the relatively higher temperature
region before the metallic coating of chromium is applied on the
relatively lower temperature region.
[0008] In the event the substrate is a gas turbine component, the
method is practiced by first applying a mask on a root region of
the component, then applying an aluminum-bearing coating, such as a
diffusion aluminide coating, on an airfoil region, de-masking the
root portion, and then masking the already-coated airfoil region.
Then, the method involves depositing a metallic coating comprising
chromium on at least a portion of a relatively lower temperature
root region that will be subject to hot corrosion, de-masking the
airfoil region followed by diffusing the chromium into the alloy at
the coated portion of the root region to form a chromium-enriched
diffused surface coating on the portion of the root region. The
aluminum-bearing coating optionally can be applied to cover the
airfoil region and also an intermediate platform region and root
shank region. An attachment portion, such as a fir tree portion, of
the root region may be left uncoated to enhance fatigue life of the
root region where it is connected to a turbine disk.
[0009] In a particular embodiment of the present invention, a
relatively low temperature deposition process embodying a liquid
deposition medium, such as electroplating bath, electrophoretic
bath, liquid slurry, and others, is used to form a metallic coating
comprising a majority of chromium on at least a portion of the
relatively lower temperature region of a precursor component. The
chromium coating is applied as a very thin layer having a thickness
of 0.00005 to 0.005 inch. Diffusion of the as-deposited chromium
into the substrate typically is effected by elevated temperature
heat treatment after the masking is removed from the
previously-applied aluminum-bearing coating on the airfoil
region.
[0010] The present invention envisions a nickel or cobalt based
alloy turbine component precursor having an aluminum-bearing
coating applied on an airfoil region and metallic coating
comprising substantially pure chromium or a chromium alloy applied
on at least a portion of the root region of the component. The
chromium coating then is diffused into the alloy to form a diffused
chromium-enriched coating on the portion of the root region of the
gas turbine blade. The diffused chromium-enriched coating has an
outermost region that comprises at least about 20%, preferably
about 25%, and more preferably about 30% to about 60% by weight
Cr.
[0011] Advantages, features, and embodiments of the present
invention will become apparent from the following description.
DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic view of a gas turbine blade having an
aluminum-bearing coating AL on the airfoil region and on the
platform region surface facing the hot gas path in order to form a
protective alumina scale or layer in service and having a diffused
chromium-enriched coating CR on the root region and on the platform
region facing away from the hot gas path in order to form a
protective chromia scale or layer in service.
[0013] FIG. 2 is a schematic view of an intermediate gas turbine
blade precursor having an aluminum-bearing coating AL on the
airfoil region and on the platform region surface facing the hot
gas path to form a protective alumina scale or layer in service and
having a metallic chromium coating ECR on the root region and on
the platform region facing away from the hot gas path in order to
be diffused into the alloy to form a protective diffused
chromium-enriched coating.
[0014] FIGS. 3 and 4 are further schematic views of other
embodiments of a gas turbine blade where the aluminum-bearing
coating and the chromium-bearing coating reside on various
illustrative regions of the turbine blade.
[0015] FIG. 5 is a graph of chromium concentration profiles (Cr
concentration versus distance into the CMSX-4 nickel base
superalloy substrate) at a shank portion of the root region after
pack chromizing and after masking the shank region and aluminizing
to form a Pt-modified diffusion aluminide (Pt--Al) coating on the
airfoil. The root region includes the shank portion and the fir
tree attachment portion of the turbine blade.
[0016] FIG. 6 is graph of Cr concentration versus distance into the
CMSX-4 nickel base superalloy substrate showing effects of Cr
plating thickness and diffusion conditions on Cr concentration in
the substrate. The distance of "0" is the surface of the
substrate.
[0017] FIG. 7 is a photomicrograph of the microstructure of a
CMSX-4 specimen electroplated with 8.7 .mu.m of Cr plating followed
by a diffusion treatment by heating at 1975 degrees F. for four
hours.
[0018] FIG. 8 shows hot corrosion test results at 700 degrees C.
plotted as weight change versus exposure hours for the various
CMSX-4 specimens shown, which were tested in duplicate. The test
involved applying 1-2 mg/cm.sup.2 to the specimen surface at each
25 hour specimen inspection.
[0019] FIG. 9 is a photomicrograph of a CMSX-4 specimen
electroplated with 8.7 .mu.m of Cr plating followed by a diffusion
treatment by heating at 1975 degrees F. for four hours and
subjected to hot corrosion with Na.sub.2SO.sub.4 applied to the
specimen surface as in FIG. 8. Through-holes (one shown) through
the specimen were not coated and show aggressive hot corrosion
attack while coated surfaces are protected.
[0020] FIG. 10 is a photomicrograph of the microstructure of a
CMSX-4 specimen electroplated with 8.7 .mu.m of Cr plating followed
by a diffusion treatment by heating at 1975 degrees F. for four
hours and then subjected to hot corrosion at 700 degrees C. with
Na.sub.2SO.sub.4 applied to the specimen surface as in FIG. 8.
[0021] FIG. 11A is photomicrograph of a CMSX-4 specimen
electroplated with 8.7 .mu.m of Cr plating followed by a diffusion
treatment by heating at 1975 degrees F. for four hours. FIG. 11B
shows microprobe results as a table for specimen of FIG. 11A. FIG.
11C is a graphic plot showing variation of Cr content over distance
into the substrate alloy for the As-Plated sample (open triangle
symbols), plated/diffusion-heat treated sample (open square
symbols), and plated/diffusion heat treated/hot corrosion tested
sample (open diamond symbols-Type I hot corrosion test). The
distance of "0" is the surface of the substrate.
DETAILED DESCRIPTION OF THE INVENTION
[0022] In one embodiment of the present invention, a method is
provided for forming a protective coating on a gas turbine
component wherein the coating includes an aluminum-bearing coating
applied at one region of the turbine blade or other component where
relatively higher temperatures are encountered in service and a
chromium-bearing coating formed at another adjacent region of the
component where relatively lower temperatures and hot corrosion are
encountered in service. Such a duplex coating provides coating
functionality for the different temperatures and
oxidation/corrosion environments to be encountered in service.
[0023] The present invention is especially useful for protecting
different regions of a gas turbine blade component from oxidation
and hot corrosion in service in a gas turbine engine, although the
invention is not limited to gas turbine components since it can be
practiced to protect other components against oxidation and hot
corrosion. The present invention can be practiced to protect nickel
based superalloy gas turbine components, nickel-cobalt based
superalloy gas turbine components, or cobalt based superalloy gas
turbine components from hot corrosion, although the invention is
not limited to these alloys. For purposes of illustration and not
limitation, the present invention will be described below with
respect to protection of different regions of a gas turbine engine
blade made of CMSX-4 nickel based superalloy against oxidation and
hot corrosion in service in a gas turbine engine.
[0024] In particular, FIG. 1 shows an exemplary gas turbine blade
10 having an airfoil region 12, a root region 14 and an
intermediate platform region 20 between the airfoil and root
regions. The airfoil 12 includes a tip 12a, a leading edge 12b, and
trailing edge 12c subjected to the gas path of the engine turbine
section. A platform region 20 typically separates the gas-path
surfaces from non-gas path surfaces and includes an upper platform
surface facing the gas path and an lower platform surface facing
away from the gas path. The root region 14 includes non-gas path
surfaces beneath the lower side of the platform region 20 wherein
the root region includes a an attachment portion 16, such as a
conventional fir tree portion, by which the turbine blade is
connected to the turbine disk (not shown) in usual manner and an
adjacent shank portion 18 between the attachment portion 16 and the
platform region 20. The fir tree typically comprises machined
serrations which fit into the turbine disk and which can be
machined before or after coating pursuant to customer
specifications. The shank region 18 includes the region between the
fir tree and lower side of the platform region 20 and may include
as-cast and machined surfaces as well as features to aid sealing
the gas path from the non-gas path regions.
[0025] A first hotter region of the turbine blade 10 is subjected
to relatively higher temperatures and oxidation degradation in
service in the gas turbine engine and comprises the airfoil region
12 and surface 20a of a platform region 20 that faces toward the
airfoil region such that the airfoil region 12 and platform surface
20a operate in or near the hot gas path of the turbine section of
the gas turbine engine. The airfoil region 12 and platform surface
20a are the hottest regions of the turbine blade and usually
operate above 1900 degrees F. for purposes of illustration and not
limitation.
[0026] As a result of the relatively high operating temperatures
encountered, the airfoil region 12 and the platform surface 20a
preferably are provided with a so-called alumina-former coating
thereon that produces an adherent protective scale of alumina in
service in the gas turbine engine.
[0027] A second relatively cooler region of the turbine blade 10 is
subjected to relatively lower temperatures and hot corrosion by
salts, such as sodium sulfates and other constituents such as K,
Ca, and/or Mg, in service in the gas turbine engine. The second
region comprises the under (lower) surface 20b of a platform region
20 that faces away from the airfoil region 12 and the root region
14. The second region thus involves a cooler region that on older
turbine blades may operate uncoated. However, as the combustor
efficiency has improved, the operating temperature of the second
region is generally increasing and spread more uniformly over the
second region. Hence the first region comprised of the airfoil 12
and platform surface surface 20a is also becoming hotter. When
salts of sodium sulfate are deposited on a surface that operates
between 1200 F and 1850 F, hot corrosion attack can occur.
Combining the high stress state of the blade root 14, with hot
corrosion conditions, rapid attack and fracture of the turbine
blade in the root region can occur. For turbine blades with
uncoated root regions heretofore used in the lower temperature
operating conditions, hot corrosion resistance can be increased by
increasing the chromium content of the turbine blade alloy.
[0028] The present invention provides a multiplex coating and a
method for applying the coating to protect the different hotter and
cooler regions of the turbine blade exposed to more aggressive
temperature/hot corrosion conditions associated recent engine
designs. The present invention provides a nickel or cobalt based
alloy turbine blade 10 having an aluminum-bearing coating AL
applied on an airfoil region of the blade and a metallic coating
comprising chromium applied on at least a portion of the root
region of the blade and diffused into the alloy to form the
diffused chromium-enriched coating CR on the portion of the root
region. FIG. 1 illustrates the aluminum-bearing coating AL and the
chromium-bearing diffused coating CR on the airfoil and the root
portion, respectively. The aluminum-bearing coating AL comprises a
so-called alumina-forming coating in that it forms a thin adherent
alumina scale on the coating in service. The chromium-bearing
diffused coating CR comprises a so-called chromia-forming coating
in that it forms a thin adherent chromia scale on the coating in
service. Also, after or before the diffusion of the chromium
coating, a thermal barrier coating (TBC), such as yttria-stablized
zirconia, can be applied as an outermost coating to all or portion
of airfoil region 12 and platform surface 20a to provide thermal
insulation properties of the TBC.
[0029] FIG. 2 illustrates a gas turbine blade precursor
(intermediate component) that includes the aluminum-bearing coating
AL and the as-deposited metallic chromium-bearing coating ECR on
the airfoil and the root portion, respectively, applied using a
method pursuant to another embodiment of the present invention
described below that overcomes problems and difficulties that can
be otherwise associated with providing a duplex coating based on
needed coating functionality as described in the COMPARISON EXAMPLE
set forth below. The chromium coating is applied as a very thin
layer having a thickness of 0.00005 to 0.005 inch.
[0030] The aluminum-bearing coating is applied in a first one step
procedure by high temperature vapor deposition, such as by chemical
vapor deposition at or above 1900 degrees F. pursuant to U.S. Pat.
Nos. 5,264,245; 4,132,816; and 3,486,927, by conventional
above-the-pack processes, or other vapor deposition processes.
[0031] The chromium-bearing coating is applied after the
aluminum-bearing coating using a two step procedure that involves
depositing a metallic coating comprising chromium on the substrate
at a relatively low temperature below 212 degrees F. when a liquid
electrolytic deposition bath or liquid carrier medium is employed
followed by a high temperature heat treatment to diffuse chromium
into the substrate. Exemplary low temperature processes for
depositing the metallic chromium coating include, but are not
limited to, electroplating or electrophoetric deposition using a
liquid bath, and slurry coating with chromium-bearing particles
(e.g. Cr or Cr alloy particles) in a liquid carrier followed by
drying, all of which can be conducted below 212 degrees F. using
liquid baths or liquid slurries. Certain other relatively low
temperature deposition processes can be employed to deposit the
metallic coating comprising chromium including, but not limited to,
electro-spark discharge conducted typically at less than
500.degree. F., cladding conducted typically at less than
100.degree. F., plasma spray conducted at less than 500.degree. F.,
and entrapment plating wherein Cr particles are entrapped in a Ni
electroplated layer.
[0032] When the substrate comprises a gas turbine component having
airfoil, platform and root regions, a method embodiment is
practiced by first applying a mask on a root region of the
component, then applying an aluminum-bearing coating, such as a
diffusion aluminide coating, on an airfoil region, de-masking the
root portion, and then masking on the already-coated airfoil
region. Then, this method embodiment deposits a metallic coating
comprising chromium on at least a portion of a relatively lower
temperature root region that will be subject to hot corrosion,
de-masks the airfoil region, followed by diffusing the chromium
into the alloy at the coated portion of the root region to form a
chromium-enriched surface coating on the portion of the root
region. The aluminum-bearing coating optionally can be applied to
cover the airfoil region and also an intermediate platform region
and root shank region. An attachment portion, such as a fir tree
portion, of the root region may be left uncoated to enhance fatigue
life of the root region where it is connected to a turbine
disk.
[0033] The chromium-enriched diffused coating applied on at least a
portion of the root region of a nickel base superalloy substrate
typically comprises in the diffused condition a Cr-enriched
outermost diffusion zone comprising chromium, nickel, and other
substrate alloy elements in solid solution wherein Cr is present as
a majority of the zone, FIGS. 7 and 11A, 11B and inner diffusion
zone between the outermost diffusion zone and the substrate and
comprising nickel, chromium, and other substrate alloy elements
wherein Cr is a minority of the zone, FIGS. 7, 11A, 11B. Another
diffusion or reaction zone may be present between the inner
diffusion zone and substrate and comprise refractory rich phases.
This diffusion or reaction zone is very thin and is not visible in
FIGS. 7 and 11A.
[0034] Practice of embodiments of the invention allow control of
the Cr content and Cr depth profile into the substrate to tailor
hot corrosion protection as needed for a particular service
application. Typically, more Cr at the outermost coated substrate
surface will be more protective than less. More Cr can be provided
by varying the thickness of the Cr metallic coating and the
diffusion heat treatment conditions.
COMPARISON EXAMPLE
[0035] This Comparison Example is offered to help illustrate the
problems and difficulties of forming such a duplex coating on a gas
turbine blade by processing other than that pursuant to the present
invention.
[0036] For example, available high temperature (above 1900 F) pack
or vapor phase chromizing processes and high temperature (above
1700 degrees F.) pack, vapor phase or CVD aluminizing processes can
be used to produce a turbine blade with two environmental
protection coatings (duplex coating). However problems in
processing and retaining high surface Cr have been observed. The
masking used for aluminizing can remove Cr from the chromized shank
during the high temperature aluminizing process. Namely, in order
to coat a turbine blade with the duplex coating, the turbine blade
must be entirely chromized by a high temperature pack or vapor
phase process and then the resulting Cr-rich layer must be removed
from the gas path surfaces 12, 20a prior to aluminizing or
overcoating the gas path surfaces. To prevent aluminizing of the
root region 14, the root region is masked by placing it in masking
powder (e.g. alumina powder, NiO powder, etc.) residing in a
containment box. However, this procedure has resulted in unwanted
reductions in Cr content of the previously applied Cr-enriched
layer on the root region and a reduction in its hot corrosion
resistance as will now be demonstrated.
[0037] A cast turbine blade having airfoil, platform, and root
features of FIG. 1 and made of CMSX-4 nickel based superalloy
(nominal composition in weight % of about 9.6% Co, about 6.6% Cr,
about 0.60% Mo, about 6.4% W, about 3.0% Re, about 6.5% Ta, about
5.6% Al, about 1.0% Ti, about 0.1% Hf, balance Ni and incidental
impurities) was chromized all over and then grit blasted to remove
the Cr-enriched coating from the first hotter region that included
the airfoil region 12 and platform surface 20a. The first hotter
region was electroplated to deposit a Pt metal layer and then
aluminized by CVD to form a Pt-modified diffusion aluminide coating
on the first region. The second cooler region (that included the
platform surface 20b and root region 14) was then masked with
commercially available powder maskant M1 available from Akron Paint
& Varnish Co., 1390 Firestone Parkway, Akron Ohio.
[0038] The chromizing process was conducted using the following
pack parameters: pure chromium powder with aluminum oxide and
NH.sub.4Cl activator for 5 hours at 1950 degrees F.
[0039] The Pt electroplating was conducted using the following
parameters set forth in U.S. Pat. No. 5,788,823, which is
incorporated herein by reference to this end, to deposit 0.3 mils
of Pt on the substrate. The aluminizing process was conducted using
the following parameters: 1975 degrees F. for 1440 minutes in
H.sub.2/AlCl.sub.3 atmosphere pursuant to U.S. Pat. No. 5,264,245,
which is incorporated herein by reference to this end.
[0040] FIG. 5 shows concentration depth profiles of Cr at the shank
portion 18 of the turbine blade for the as-chromized condition
(Pack Cr) and after masking and aluminizing (Pack Cr+Aluminizing
Cycle) to form the Pt-modified diffusion aluminide coating on the
first region that included the airfoil and platform surface
20a.
[0041] The enriched Cr content of the as-chromized coating on the
second region as shown in FIG. 5 is a desirable chemistry for
resisting hot corrosion attack. However, as the graph shows, the Cr
enrichment formed by the pack chromizing process is depleted
following the aluminizing process even when masking is present on
the root (shank 18 and attachment portion 16 to prevent aluminum
from depositing thereon). The Cr content at the shank portion has
been is lowered to below the CMSX-4 superalloy content (nominal
alloy Cr composition: 6.4% Cr by weight and is directionally
exactly the opposite Cr content desired to improve hot corrosion
resistance. It is apparent that the duplex coating processing of
this Comparison Example failed to produce a turbine blade with a
desired Cr-enriched coating for hot corrosion attack
resistance.
[0042] Pursuant to method embodiments of the present invention, the
duplex coating is applied using a sequence processing steps that
overcomes the above-discussed problems and difficulties
demonstrated in the Comparison Example.
Example 1
[0043] Pursuant to an illustrative embodiment of the present
invention, the following processing steps are employed:
[0044] 1. If a platinum-modified diffusion aluminide coating is to
be formed on the gas path surfaces 12, 20a, then these surfaces are
optionally electroplated with a layer of Pt pursuant to U.S. Pat.
No. 5,788,832 which is already incorporated herein by reference. If
a simple diffusion aluminde coating is to be formed, then this step
is omitted.
[0045] 2. Masking the second region of the turbine blade (i.e. root
region 14 and platform surface 20b) with the M1 maskant powder
mentioned above in a containment box. That is, the root region 14
and platform surface 20b are embedded in the maskant powder in the
containment box.
[0046] 3. Aluminize the first hotter region (i.e. airfoil 12 and
platform surface 20a) to form a diffusion aluminide coating, such
as a Pt-modified diffusion aluminide coating if step 1 is
practiced, with the masking covering the second region.
[0047] 4. Masking the diffusion aluminide coating on the first
region.
[0048] 5. Cr electroplating the second cooler region with the
masking of step 2 covering the diffusion aluminide coating formed
in step 3. The Cr electroplating is conducted at low temperature
such as less than 212 degrees F. using a liquid (e.g. aqueous)
electroplating bath. The Cr electroplate can be locally deposited
by virtue of the masking on the first region being effective under
the low temperature plating bath conditions.
[0049] 6. Diffusing the Cr plating into the CMSX-4 substrate alloy
to form the Cr-enriched hot corrosion resistant coating wherein
diffusing of the Cr plating improves bonding with the superalloy
substrate and makes the resulting Cr-rich layer more ductile.
[0050] The chromium electroplating process is conducted using
plating conditions to deposit a hexavalent hard, dense chromium
electroplate comprising substantially pure Cr that meets AMS
(Aerospace Material Specification) 2438B for hard, dense chromium
coatings for aerospace material applications on steel materials.
AMS 2338B is incorporated herein by reference to this end.
[0051] In this example, the hard, dense substantially pure chromium
electroplate was applied commercially by a commercial electroplater
Armoloy of Illinois, 118 Simonds Ave., DeKalb, Ill., using
proprietary plating conditions. The deposited Cr electroplating was
applied to a thickness of 8.7 micrometers or 3.5 micrometers. The
electroplated layer was substantially pure Cr; e.g. 99.9% by weight
pure Cr and balance plating impurities. The invention envisions
electroplating Cr alloys, rather than pure Cr, and also plating
alternating layers of Cr and Ni.
[0052] The chromium electroplating can be conducted using any
suitable parameters. For purposes of illustration and not
limitation, the following plating conditions can be used:
[0053] 1. Vapor hone surfaces with an alumina slurry to clean
surfaces to be plated.
[0054] 2. Activate the surfaces to be plated by immersion in
plating bath containing 250-400 g/L chromic acid and 2.5-4 g/L of
sulfate catalyst (sulfuric acid) at 52-63.degree. C. and applying a
current (30-54 A/dm.sup.2 at 3 to 12 volts) such that the parts are
anodes (which is opposite of Cr plate deposition) for 30 seconds to
2 minutes.
[0055] 3. Cr plate surfaces to be plated by immersion in plating
bath and applying current (such that the parts are cathodes) for 4
minutes to 30 minutes or as long as needed to meet the thickness
requirement for the Cr plating.
[0056] 4. Rinse in 120.degree. F. de-ionized water to remove
majority of plating bath.
[0057] 5. Rinse in hot de-ionized water to remove remaining plating
bath and dry.
[0058] The CVD aluminizing process is conducted using the following
parameters: 1975 degrees F. for 1440 minutes in H.sub.2/AlCl.sub.3
atmosphere pursuant to U.S. Pat. No. 5,264,245, which is
incorporated herein by reference to this end. Other aluminizing
processes which can be used include, but are not limited to, pack,
vapor phase, sputtering, physical vapor deposition and slurry
followed by diffusion heat treatment, electrophoretic followed by
diffusion heat treatment, and others.
[0059] For this example, the diffusion heat treatment of Cr was
conducted at 1975 degrees F. for 4 hours in an Ar partial pressure
atmosphere or at 2050 degrees F. for 2 hours in an Ar partial
pressure atmosphere to prevent oxidation.
[0060] FIG. 6 is graph of Cr concentration versus distance into the
CMSX-4 nickel base superalloy substrate showing effects of Cr
plating thickness and diffusion conditions on Cr concentration in
the substrate. In FIG. 6, the distance of "0" is the surface of the
substrate. FIG. 6 shows that the surface Cr content can be
controlled to be as low as 15 weight % or as high as 63 weight %.
Also, FIG. 6 shows that the depth of Cr enrichment can be
controlled as well. Typically, the Cr content and enrichment depth
can be balanced to provide acceptable hot corrosion resistance
while minimizing fatigue debit for the strains to be experienced by
the component in use in the turbine engine. FIG. 6 presents two Cr
plating thicknesses and two different diffusion heat treatments
illustrating a range of resultant Cr enrichments.
[0061] FIGS. 7 and 11A contain photomicrographs of the
microstructure of a CMSX-4 specimen electroplated with 8.7 .mu.m of
Cr plating followed by a diffusion treatment by heating at 1975
degrees F. for four hours. FIG. 11B includes microprobe data of the
Cr-enriched diffused coating as a table and FIG. 11C includes a
plot showing variation of Cr content over distance into the
substrate alloy. The distance of "0" is the surface of the
substrate.
[0062] FIGS. 7 and 11A, 11B (microprobe table) and 11C (plot)
results reveal that the chromium-enriched diffused coating
comprised a Cr-enriched outermost (Top) diffusion zone comprising
chromium, nickel, and other substrate alloy elements in solid
solution wherein Cr is present as a majority of the Top zone and
inner diffusion zone (Diffusion) between the outermost diffusion
zone and the substrate and comprising nickel, chromium, and other
substrate alloy elements wherein Cr is a minority of the Diffusion
zone. Another diffusion or reaction zone may be present between the
inner diffusion zone and the substrate and comprise refractory rich
phases. This diffusion or reaction zone is very thin and is not
visible in FIGS. 7 and 11A.
[0063] FIG. 8 illustrates hot corrosion test results at 700 degrees
C. plotted as weight change versus exposure hours for the various
CMSX-4 specimens shown, which were tested in duplicate. The test
applied 1-2 mg/cm.sup.2 of Na.sub.2SO.sub.4 to the specimen surface
at each 20 hour specimen inspection and then exposed the salted
sample to 700.degree. F. in a furnace which had a 1000 ppm
SO.sub.2/O.sub.2 gas passing through a heated Pt catalyst to form
SO.sub.3. The SO.sub.3 reacted with the salt at the test
temperature to provide corrosion attack.
[0064] FIG. 8 shows that the tested specimens coated pursuant to
embodiments of the invention exhibited essentially the same weight
change over time, regardless of the thickness of the Cr
electroplate and Cr diffusion parameters employed. Bare (uncoated)
CMSX-4 specimen lost substantial weight during the test.
[0065] FIG. 9 is a photomicrograph of a CMSX-4 specimen
electroplated with 8.7 .mu.m of Cr plating followed by a diffusion
treatment by heating at 1975 degrees F. for four hours and
subjected to the above 700.degree. F. hot corrosion with
Na.sub.2SO.sub.4 applied to the specimen as in FIG. 8.
Through-holes (one shown) through the specimen were not coated and
show aggressive hot corrosion attack while coated surfaces are
protected.
[0066] FIG. 9 shows the aggressive nature of these test conditions
to the bare (uncoated) CMSX-4 alloy substrate and the resistance of
the coated specimen surfaces to hot corrosion attack.
[0067] FIG. 10 is a photomicrograph of the microstructure of a
CMSX-4 specimen electroplated with 8.7 .mu.m of Cr plating followed
by a diffusion treatment by heating at 1975 degrees F. for four
hours and then subjected to hot corrosion at 700 degrees C. with
Na.sub.2SO.sub.4 applied to the specimen surface as in FIG. 8.
[0068] FIG. 11C is a graphic plot showing variation of Cr content
over distance into the substrate alloy for the As-Plated sample
(open triangle symbols), plated/diffusion-heat treated sample (open
square symbols), and plated/diffusion heat treated/hot corrosion
tested sample (open diamond symbols-Type I hot corrosion test). The
distance of "0" is the surface of the substrate.
[0069] Comparing the later two samples in FIG. 11C, the Cr content
after the hot corrosion test is virtually unchanged.
Example 2
[0070] Pursuant to another illustrative embodiment of the present
invention, the following processing steps are employed:
[0071] 1. If a platinum-modified diffusion aluminide coating is to
be formed on the gas path surfaces 12, 20a, then these surfaces are
optionally electroplated with a layer of Pt pursuant to U.S. Pat.
No. 5,788,832 which is already incorporated herein by reference. If
a simple diffusion aluminide coating is to be formed, then this
step is omitted.
[0072] 2. Aluminize the first hotter region and the second region
to form a diffusion aluminide coating, such as a Pt-modified
diffusion aluminide coating. No masking covering the second
region.
[0073] 3. Removing the diffusion aluminide coating selectively from
the second region by grit blasting, machining or other technique to
expose the substrate alloy, while leaving the diffusion aluminide
coating on the first region.
[0074] 4. Masking the diffusion aluminide coating on the first
region as described in Example 1.
[0075] 5. Cr electroplating the exposed second cooler region with
the masking of step 4 covering the diffusion aluminide coating
formed in step 2. The Cr electroplating is conducted at low
temperature such as less than 212 degrees F. using a liquid
electroplating bath. The Cr electroplate can be locally deposited
by virtue of the masking on the first region being effective under
the low room temperature plating bath conditions.
[0076] 5. Diffusing the Cr plating into the CMSX-4 substrate alloy
to form the Cr-enriched hot corrosion resistant coating wherein
diffusing of the Cr plating improves bonding with the superalloy
substrate and makes the resulting Cr-rich layer more ductile.
[0077] The chromium electroplating process is conducted by the
commercial electroplater of Example 1. The CVD aluminizing process
is conducted using the parameters of Example 1. The diffusion heat
treatment of Cr is conducted using the parameters of Example 1.
Further Examples
[0078] FIGS. 3 and 4 illustrate alternative embodiments of the
invention where, in FIG. 3, an aluminized layer AL' with or without
Pt is applied on the upper region of the shank portion 18 from the
platform surface 20b to a preselected distance or plane below the
platform 20 along with other coatings AL, CR as indicated; and
where, in FIG. 4, an uncoated attachment portion 17 of the root
region is provided at an even cooler region of the turbine blade
closer to the turbine disk, which portion 17 is masked during both
aluminizing and Cr plating. The attachment portion 17 comprises
bare (uncoated) substrate alloy where the blade design cannot
tolerate any coating along with other coatings AL, CR as
indicated.
[0079] The invention allows for many combinations of Al, Al/Cr, Cr,
and bare, uncoated areas on a turbine blade to provide desired
coating functionality as needed to suit different service
conditions in the gas turbine engine.
[0080] Although the invention has been described with respect to
certain detailed embodiments thereof, it will be understood by
those skilled in the art that various changes in form and detail
thereof may be made without departing from the spirit and scope of
the claimed invention.
* * * * *