U.S. patent application number 10/904325 was filed with the patent office on 2005-03-31 for nickel aluminide coating and coating systems formed therewith.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Boutwell, Brett Allen Rohrer, Darolia, Ramgopal, Pfaendtner, Jeffrey Allan, Rigney, Joseph David, Ruud, James Anthony, Spitsberg, Irene, Walston, William Scott.
Application Number | 20050069650 10/904325 |
Document ID | / |
Family ID | 32907512 |
Filed Date | 2005-03-31 |
United States Patent
Application |
20050069650 |
Kind Code |
A1 |
Darolia, Ramgopal ; et
al. |
March 31, 2005 |
NICKEL ALUMINIDE COATING AND COATING SYSTEMS FORMED THEREWITH
Abstract
A beta-phase NiAl overlay coating containing a dispersion of
ceramic particles and a process for depositing the overlay coating.
If the coating is used to adhere a thermal barrier coating (TBC),
the TBC exhibits improved spallation resistance as a result of the
dispersion of ceramic particles having a dispersion-strengthening
effect on the overlay coating. The overlay coating contains at
least one reactive element and is deposited so that the some of the
reactive element deposits as the ceramic particles dispersed in the
overlay coating.
Inventors: |
Darolia, Ramgopal; (West
Chester, OH) ; Rigney, Joseph David; (Loveland,
OH) ; Walston, William Scott; (Mason, OH) ;
Pfaendtner, Jeffrey Allan; (Blue Ash, OH) ; Boutwell,
Brett Allen Rohrer; (Liberty Township, OH) ;
Spitsberg, Irene; (Loveland, OH) ; Ruud, James
Anthony; (Delmar, NY) |
Correspondence
Address: |
HARTMAN AND HARTMAN, P.C.
552 EAST 700 NORTH
VAIPARAISO
IN
46383
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
32907512 |
Appl. No.: |
10/904325 |
Filed: |
November 4, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10904325 |
Nov 4, 2004 |
|
|
|
10249564 |
Apr 18, 2003 |
|
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Current U.S.
Class: |
427/446 |
Current CPC
Class: |
C23C 4/02 20130101; C23C
14/06 20130101; C23C 28/324 20130101; C23C 28/345 20130101; C23C
28/3215 20130101; Y10T 428/12771 20150115; C23C 28/321 20130101;
Y10T 428/12611 20150115; Y10T 428/12618 20150115; Y10T 428/252
20150115; Y10T 428/12736 20150115; Y10T 428/12806 20150115; C23C
4/12 20130101; C23C 28/3455 20130101; Y10T 428/12576 20150115; Y10T
428/12944 20150115 |
Class at
Publication: |
427/446 |
International
Class: |
H05H 001/26 |
Goverment Interests
[0002] This invention was made with Government support under
Agreement No. F33615-98-C-2893 awarded by the U.S. Department of
the Air Force. The Government has certain rights in the invention.
Claims
What is claimed is:
1. A process of depositing a coating system on a superalloy
substrate, the coating system comprising a beta-phase NiAl
intermetallic overlay coating containing at least one reactive
element, the process comprising the step of depositing the overlay
coating by a physical vapor deposition or thermal spray deposition
so that during deposition at least some of the at least one
reactive element is reacted to form ceramic particles that are
dispersed in the overlay coating, the ceramic particles being
present in an amount and size sufficient to be stable and
unreactive at temperatures up to about 1300.degree. C. and to
increase the strength of the overlay coating by a dispersion
strengthening mechanism.
2. A process according to claim 1, further comprising the step of
depositing a thermal-insulating ceramic layer on the overlay
coating.
3. A process according to claim 1, wherein the overlay coating
further contains chromium.
4. A process according to claim 1, wherein the overlay coating
consists of nickel, aluminum, chromium, the at least one reactive
element, and the ceramic particles.
5. A process according to claim 1, wherein the at least one
reactive element is selected from the group consisting of
zirconium, hafnium and yttrium.
6. A process according to claim 1, wherein the ceramic particles
comprise at least one compound selected from the group consisting
of oxides, nitrides, and carbides of the at least one reactive
element.
7. A process according to claim 6, wherein the at least one
reactive element is selected from the group consisting of
zirconium, hafnium and yttrium.
8. A process according to claim 1, wherein the overlay coating
further contains at least one of tantalum and silicon.
9. A process according to claim 8, wherein the ceramic particles
comprise at least one compound selected from the group consisting
of oxides, nitrides, and carbides of tantalum and silicon.
10. A process according to claim 1, wherein at least some of the
ceramic particles are formed by reacting the at least one reactive
element with a gaseous species chosen from the group consisting of
CH.sub.4, CO, CO.sub.2, N.sub.2, and O.sub.2.
11. A process according to claim 1, wherein the ceramic particles
are present in an amount of about 0.5 to about 5.0 volume percent
of the overlay coating.
12. A process according to claim 1, wherein the size of the ceramic
particles is in a range of about 1 to about 2000 nanometers.
13. A process of depositing a coating system on a nickel-base
superalloy substrate of a gas turbine engine component, the coating
system comprising a ceramic layer on a beta-phase NiAlCr
intermetallic overlay bond coat, the overlay bond coat containing
nickel, aluminum, chromium and zirconium, the process comprising
the steps of: depositing the overlay coating by a physical vapor
deposition or thermal spray deposition so that some of the chromium
and zirconium content of the coating is reacted to form ceramic
particles that are dispersed in the overlay coating, the ceramic
particles being at least one compound selected from the group
consisting of oxides, nitrides, and carbides of chromium and
zirconium, the ceramic particles being present in the overlay bond
coat in an amount and size sufficient to be stable and unreactive
at temperatures up to about 1300.degree. C. and to increase the
strength of the overlay bond coat by a dispersion strengthening
mechanism; and then depositing the ceramic layer on the overlay
coating.
14. A process according to claim 13, wherein at least some of the
ceramic particles are formed by reacting chromium and zirconium
with a gaseous species chosen from the group consisting of
CH.sub.4, CO, CO.sub.2, N.sub.2, and O.sub.2.
15. A process according to claim 13, wherein the ceramic particles
are present in an amount of 0.5 to less than 5.0 volume percent of
the overlay coating.
16. A process according to claim 13, wherein the size of the
ceramic particles is in a range of about 10 to about 1000
nanometers.
17. A process according to claim 13, wherein the chromium content
of the overlay bond coat is about 2 to about 15 atomic percent, and
some of the chromium content is in the ceramic particles.
18. A process according to claim 13, wherein the zirconium content
of the overlay bond coat is about 0.05 to about 0.8 atomic percent,
and some of the zirconium content is in the ceramic particles.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Division of U.S. patent application
Ser. No. 10/249,564, filed Apr. 18, 2003.
FIELD OF THE INVENTION
[0003] The present invention generally relates to coatings of the
type used to protect components exposed to high temperature
environments, such as bond coats and environmental coatings for gas
turbine engine components. More particularly, this invention is
directed to a ceramic-containing beta-phase (.beta.) NiAl
(beta-NiAl) overlay coating for use as a bond coat or environmental
coating.
DESCRIPTION OF THE RELATED ART
[0004] Components within the turbine, combustor and augmentor
sections of gas turbine engines are susceptible to oxidation and
hot corrosion attack, in addition to high temperatures that can
decrease their mechanical properties. Consequently, these
components are often protected by an environmental coating alone or
in combination with an outer thermal barrier coating (TBC), which
in the latter case is termed a TBC system. Ceramic materials such
as zirconia (ZrO.sub.2) partially or fully stabilized by yttria
(Y.sub.2O.sub.3), magnesia (MgO) or other oxides, are widely used
as TBC materials.
[0005] Various metallic coating systems have been used as
environmental coatings for gas turbine engine components, the most
widely used being diffusion coatings such as diffusion aluminides
and platinum aluminides (PtAl), and MCrAlX overlay coatings (where
M is iron, cobalt and/or nickel, and X is an active element such as
yttrium or another rare earth or reactive element). Used in
combination with TBC, a diffusion aluminide or MCrAlX overlay
coating serves as a bond coat to adhere the TBC to the underlying
substrate. The aluminum content of these bond coat materials
provides for the slow growth of a strong adherent continuous
aluminum oxide layer (alumina scale) that protects the bond coat
and underlying substrate from oxidation and hot corrosion, and
chemically bonds the TBC to the bond coat.
[0006] Diffusion and MCrAlX bond coats containing ceramic particles
have been evaluated. For example, commonly-assigned U.S. Pat. Nos.
6,168,874 to Gupta et al. and 6,485,845 to Wustman et al.
incorporate oxide particles in diffusion aluminide coatings to slow
oxide scale growth, thereby increasing the spallation resistance of
a TBC. Furthermore, commonly-assigned U.S. Pat. No. 4,101,713 to
Hirsch et al. discloses that an oxide dispersion-strengthened
MCrAlY coating exhibits improved mechanical integrity. Others, such
as U.S. Pat. No. 4,447,503 to Dardi et al., disclose that oxide
particles in an MCrAlY coating promote pinning protective oxide
scales, while still others, such as U.S. Pat. No. 4,451,496 to
Gedwill et al., U.S. Pat. No. 6,306,515 to Goedjen et al. and U.S.
Pat. No. 6,376,015 to Rickerby, disclose the use of oxide particles
in MCrAlY as an inhibitor to interdiffusion between an underlying
substrate and an environmental coating deposited on the MCrAlY
coating. The incorporation of oxide particles in an MCrAlY for the
purpose of modifying its coefficient of thermal expansion has also
been suggested, e.g., U.S. Pat. No. 6,093,454 to Brindley et al.,
EP 0 799 904 to Movchan et al., and EP 0 340 791 to Kojima et al.
Finally, the incorporation of other types of ceramic particles in
bond coat materials has been suggested, as reported in U.S. Pat.
No. 6,291,014 to Warnes et al. (silicides and carbides for high
temperature oxidation resistance).
[0007] More recently, overlay coatings of predominantly beta-nickel
aluminide intermetallic have been proposed as environmental and
bond coat materials. The NiAl beta phase exists for nickel-aluminum
compositions of about 30 to about 60 atomic percent aluminum, the
balance of the nickel-aluminum composition being nickel. Notable
examples of beta-NiAl coating materials include commonly-assigned
U.S. Pat. No. 5,975,852 to Nagaraj et al., which discloses a NiAl
overlay bond coat optionally containing one or more reactive
elements, such as yttrium, cerium, zirconium or hafnium, and
commonly-assigned U.S. Pat. No. 6,291,084 to Darolia et al., which
discloses a NiAl overlay coating material containing chromium and
zirconium. Commonly-assigned U.S. Pat. Nos. 6,153,313 and 6,255,001
to Rigney et al. and Darolia, respectively, and commonly-assigned
U.S. patent application Ser. No. 10/044,618 to Pfaendtner et al.
also disclose beta-phase NiAl bond coat and environmental coating
materials. The alloying additions to these beta-NiAl coating
materials have been shown to improve the adhesion of a ceramic TBC
layer, thereby inhibiting spallation of the TBC and increasing the
service life of the TBC system.
[0008] NiAlCrZr overlay coatings described in the above-noted
commonly-assigned patents derive their performance benefits from
optimum combinations of aluminum and the reactive elements,
chromium and zirconium. At certain levels, zirconium promotes an
adherent slow growing (low values of the parabolic scale growth
parameter, k.sub.p) alumina scale, which helps to extend the TBC
spallation life and improve oxidation performance. While oxidation
performance suffers if the zirconium level is too low (e.g., below
0.05 atomic percent), higher levels of zirconium result in Zr-rich
intermetallic precipitates that can increase internal oxidation. In
spite of this internal oxidation phenomenon, levels of zirconium
above 0.2 atomic percent (about 0.4 weight percent) have shown to
significantly improve TBC spallation resistance as a result of the
potent strengthening effect of zirconium additions to beta-NiAl
alloys. Strengthening in beta-NiAl by zirconium additions has been
attributed to two mechanisms: solid solution strengthening, and the
formation of zirconium-containing intermetallic precipitates, the
most common being a Heusler phase (Ni.sub.2AlZr) that results in
further ordering of the NiAl structure. The increased strength of
beta-phase NiAl-based bond coats has been shown to contribute to
better TBC lives.
[0009] However, and as mentioned above, the higher zirconium levels
required to optimize TBC spallation resistance also promote
internal oxidation (oxidation of Zr-intermetallic precipitates),
which degrades the overall oxidation resistance of the bond coat by
effectively increasing the parabolic scale growth parameter,
k.sub.p. While it is critical that a bond coat provide TBC
spallation resistance, bond coats must also exhibit oxidation
resistance in the event of TBC spallation. Therefore, further
improvements are needed in beta-phase NiAl-based overlay coatings
that can result in both improved oxidation resistance and, if used
as a bond coat, improved spallation resistance.
BRIEF SUMMARY OF THE INVENTION
[0010] The present invention generally provides a beta-NiAl overlay
coating and a process for forming the coating on a component
designed for use in a hostile thermal environment, such as
superalloy turbine, combustor and augmentor components of a gas
turbine engine. According to one aspect of the invention, the
coating system includes a ceramic topcoat that exhibits improved
spallation resistance as a result of the beta-phase NiAl overlay
coating containing a dispersion of ceramic particles, yielding a
coating system capable of exhibiting improved oxidation and
spallation resistance.
[0011] More particularly, the beta-NiAl overlay coating contains at
least one reactive element and is deposited so that at least some
of the reactive element deposits as ceramic particles dispersed in
the overlay coating. The ceramic particles are present in an amount
and size sufficient to be stable and unreactive at temperatures up
to about 1300.degree. C. and to increase the strength of the
overlay coating by a dispersion strengthening mechanism. In one
example, zirconium and chromium are constituents of the overlay
coating, and are reacted during the deposition process to form
oxides, carbides and/or nitrides. Notably, significantly improved
oxidation and spallation resistance can be achieved with a
beta-NiAl overlay coating containing zirconium and chromium by
reacting some of the zirconium and chromium during the coating
process to form a fine and uniform dispersion of submicron ceramic
particles throughout the overlay coating.
[0012] Other objects and advantages of this invention will be
better appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a perspective view of a high pressure turbine
blade.
[0014] FIG. 2 is a cross-sectional view of the blade of FIG. 1
along line 2-2, and shows a thermal barrier coating system in
accordance with an embodiment of this invention.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The present invention provides a composition and process for
achieving a desirable combination of environmental properties in a
predominantly beta-phase NiAl-based coating containing reactive
element additions. In NiAl-based coatings of this invention,
reactive element additions are incorporated to a level necessary
for optimum oxidation resistance (i.e., below levels that would
lead to excessive k.sub.p values) without significantly exceeding
the solid solution limit of the element, while additional bond coat
strength (needed for TBC spallation resistance) is provided by a
stable dispersion of oxides, carbides and/or nitrides of the
reactive element, e.g., zirconia (ZrO.sub.2), hafnia (HfO.sub.2),
chromia (Cr.sub.2O.sub.3), yttria (Y.sub.2O.sub.3), ceria
(CeO.sub.2), zirconium carbide (ZrC), hafnium carbide (HfC), etc.,
and possibly oxides, carbides and/or nitrides of other elements of
the coating, e.g., alumina (Al.sub.2O.sub.3), aluminum nitride
(AlN), etc.
[0016] In view of the above, beta-NiAl-based coatings of this
invention are useful on components that operate within environments
characterized by relatively high temperatures, and therefore
subjected to severe thermal stresses and thermal cycling. Notable
examples of such components include the high and low pressure
turbine nozzles and blades, shrouds, combustor liners and augmentor
hardware of gas turbine engines. One such example is the high
pressure turbine blade 10 shown in FIG. 1. The blade 10 generally
includes an airfoil 12 against which hot combustion gases are
directed during operation of the gas turbine engine, and whose
surface is therefore subjected to severe attack by oxidation,
corrosion and erosion. The airfoil 12 is anchored to a turbine disk
(not shown) with a dovetail 14 formed on a root section 16 of the
blade 10. Cooling holes 18 are present in the airfoil 12 through
which bleed air is forced to transfer heat from the blade 10. While
the advantages of this invention will be described with reference
to the high pressure turbine blade 10 shown in FIG. 1, and
particularly nickel-base superalloy blades of the type shown in
FIG. 1, the teachings of this invention are generally applicable to
any component on which a coating system may be used to protect the
component from its environment.
[0017] FIG. 2 depicts a representative TBC system 20 for discussing
the present invention. As shown, the coating system 20 includes a
ceramic layer 26 bonded to the blade substrate 22 with a beta-phase
NiAl-based overlay coating 24, which therefore serves as a bond
coat to the ceramic layer 26. The substrate 22 (blade 10) is
preferably a high-temperature material, such as an iron, nickel or
cobalt-base superalloy. The ceramic layer 26 is depicted as having
a strain-tolerant columnar grain structure as a result of being
deposited by a physical vapor deposition (PVD) technique, though
other deposition techniques could be used. A preferred material for
the ceramic layer 26 is an yttria-stabilized zirconia (YSZ), with a
suitable composition being about 3 to about 20 weight percent
yttria, though other ceramic materials could be used, such as
yttria, nonstabilized zirconia, or zirconia stabilized by ceria
(CeO.sub.2), scandia (SC.sub.2O.sub.3) or other oxides. The ceramic
layer 26 is deposited to a thickness that is sufficient to provide
the required thermal protection for the underlying substrate 22 and
blade 10, generally on the order of about 100 to about 300
micrometers. As with prior art TBC systems, the surface of the
overlay coating 24 oxidizes to form an oxide surface layer (scale)
28 to which the ceramic layer 26 chemically bonds.
[0018] In accordance with this invention, the overlay coating 24 is
predominantly of the beta NiAl phase, with limited alloying
additions. The overlay coating 24 can be deposited using a PVD
process such as cathodic arc or electron beam PVD (EBPVD), or by
thermal spraying such as plasma spraying (air, vacuum and low
pressure) and high velocity oxy-fuel (HVOF) spraying. A suitable
thickness for the overlay coating 24 is about 50 micrometers to
protect the underlying substrate 22 and provide an adequate supply
of aluminum for oxide formation, though thicknesses of about 10 to
about 125 micrometers are believed to be suitable. Following
deposition, the coating 24 may be heat treated to promote
homogenization and adherence (by interdiffusion with the
substrate). If performed, a suitable heat treatment is about two to
about four hours at about 1800.degree. F. to 2100.degree. F. (about
980.degree. C. to about 1150.degree. C.) in a vacuum or an inert
atmosphere such as argon.
[0019] To attain the beta-NiAl intermetallic phase, the NiAl
overlay coating 24 preferably has an aluminum content of about 30
to 60 atomic percent, more preferably about 30 to 50 atomic
percent, and most preferably at the stoichiometric ratio of 1:1
with nickel. According to this invention, the coating 24 also
contains one or more reactive elements, preferably zirconium,
though additions of hafnium and/or yttrium are also contemplated by
this invention. If zirconium, the reactive element is present in an
amount of at least 0.05 atomic percent (about 0.1 weight percent)
up to as much as about 0.8 atomic percent (about 1.6 weight
percent), with a preferred range being about 0.2 to about 0.5
atomic percent (about 0.4 to about 1.0 weight percent). If hafnium
in stoichiometric beta-NiAl, the reactive element is present in an
amount of at least 0.1 atomic percent (about 0.4 weight percent) up
to as much as about 1.0 atomic percent (about 4 weight percent),
preferably about 0.2 to about 0.7 atomic percent (about 0.8 to
about 2.8 weight percent). If the coating 24 contains more than one
reactive element, the total reactive element content is preferably
not greater than about 1.0 atomic percent (about 4.0 weight
percent).
[0020] In a preferred embodiment of the invention, the coating 24
is further alloyed to contain chromium, such as in an amount of
about 2 to about 15 atomic percent (about 2.2 to about 18 weight
percent), more preferably about 2 to about 10 atomic percent (about
2.2 to about 12 weight percent). According to U.S. Pat. No.
6,291,084 to Darolia et al., the presence of chromium in a
beta-NiAl overlay coating has a significant effect on the
spallation resistance of the ceramic layer 26 adhered to the NiAl
overlay coating 24 as a result of solid solution strengthening by
chromium and precipitation strengthening from fine .alpha.-Cr
phases dispersed within the beta phase of the coating 24. However,
as with the reactive element content of the overlay coating 24, the
chromium content of the coating 24 is also beneficial if present in
a nonmetallic form.
[0021] As depicted in FIG. 2, the beta-NiAl overlay coating 24 of
this invention contains a fine dispersion of ceramic particles 30
(not to scale). These particles, which are oxides, carbides and/or
nitrides of at least the reactive element(s) of the coating 24,
have been shown to increase the spallation resistance of the
ceramic layer 26 deposited on the NiAl overlay coating 24. During
testing of EBPVD-deposited NiAlCrZr bond coats of the type
disclosed in U.S. Pat. Nos. 6,153,313, 6,255,001, and 6,291,084,
certain coatings have at times exhibited exceptional TBC spallation
performance in furnace cycle testing (FCT) and burner rig testing.
However, these results were accompanied by a prohibitively large
amount of scatter in FCT performance that could not be correlated
to the concentration of reactive elements in the coatings. In
analyzing various factors that influence FCT life, it was noticed
that significant shifts in FCT performance tended to occur
following major changes in the EBPVD process, such as a change in
the deposition rate or the type of heating element used to heat the
samples during coating. Although the effects of these process
changes were not fully understood, it was speculated that such
changes could influence the concentration, size, and distribution
of impurity defects within a NiAlCrZr overlay coating.
[0022] The present invention arises from data obtained from FCT and
burner rig testing that suggested that carbide and oxide impurities
may play an important role in the performance of NiAlCrZr bond
coats. These investigations involved NiAlCrZr coatings deposited by
EBPVD and HVOF and chemically analyzed using a time-of-flight
secondary ion mass spectroscopy (TOF-SIMS) technique.
[0023] In the FCT investigation, a NiAlCrZr coating was deposited
by EBPVD to a thickness of about 25 micrometers on a one-inch
(about 25 mm) diameter button coupon formed of a single-crystal
superalloy known as RenN6 (U.S. Pat. No. 5,455,120), with a nominal
composition of, by weight, about 12.5% Co, 4.2% Cr, 7.2% Ta, 5.75%
Al, 5.75% W, 5.4% Re, 1.5% Mo, 0.15% Hf, 0.05% C, the balance
nickel and incidental impurities. The concentrations of aluminum,
chromium and zirconium in the coating were determined by electron
microprobe analysis (EMA) to be, by weight, about 21%, about 3%,
and about 0.9%, respectively. A TBC of 7YSZ (zirconia stabilized by
about seven weight percent yttria) was deposited on the overlay
coating to a thickness of about 125 micrometers. After removing a
small piece that included a portion of the NiCrAlZr coating, the
button underwent FCT testing at about 2125.degree. F. (about
1160.degree. C.) in one-hour cycles to evaluate the spallation life
potential of the TBC coating. Testing was terminated when
approximately twenty percent of the TBC had spalled. The FCT life
of the coating system was about 1380 cycles, approximately 6.times.
the baseline life of 230 cycles for an identical TBC deposited on a
PtAl diffusion coating. A post-test metallographic examination
revealed the NiAlCrZr coating to be very flat with no visible
plastic deformation or rumpling.
[0024] The as-coated piece from this button was then
metallographically mounted, polished and subjected to TOF-SIMS
analysis. Analysis of the integrated signal intensities and
observation of the intensities of oxide and carbide maps
unexpectedly showed that some of zirconium and chromium in the
coating was in the form of a fine and uniform dispersion
(sub-micron scale) of zirconium and chromium carbides and, to a
lesser extent, zirconium oxides. A small amount of zirconium
remained in the elemental state to strengthen the bond coat by a
solid-solution mechanism. Notably, a zirconium-free NiAlCr overlay
bond coat subjected to identical FCT testing exhibited a life of
only about 160 cycles--only about 11% of the FCT life of the
Zr-containing sample. The NiAlCr sample exhibited rumpling
(large-scale surface deformation) after testing that was
characteristic of PtAl bond coats. These results confirmed that
zirconium is a very desirable constituent of an NiAlCr coating for
long TBC spallation life. Of significance, these results also
indicated that longer TBC life can be achieved, perhaps more
consistently, if the coating is deposited so that a portion of the
zirconium is in the form of zirconium carbides and/or oxides, which
appear to further promote spallation life through a mechanical
strengthening mechanism. While not wishing to be held to any
particular theory, it was believed that the pressure and/or
atmosphere within the EBPVD deposition/coating chamber (resulting
from residual gases, especially oxygen, and carbon contamination
from a graphite heater) were the cause of the zirconium oxide and
carbide precipitates found in the coating.
[0025] For the burner rig test, NiAlCrZr coatings were deposited by
HVOF to a thickness of about 60 micrometers on pin samples formed
of RenN5 (U.S. Pat. No. 6,074,602), with a nominal composition of,
by weight, about 7.5% Co, 7.0% Cr, 6.5% Ta, 6.2% Al, 5.0% W, 3.0%
Re, 1.5% Mo, 0.15% Hf, 0.05% C, 0.004% B, 0.01% Y, the balance
nickel and incidental impurities. The approximate chemistry of the
coatings was, in atomic percent, about 45% aluminum, about 10%
chromium, about 0.5% zirconium, and the balance nickel and
incidental impurities. A TBC was not deposited on the NiCrAlZr
coatings. The pins were then subjected to a cyclic hot
corrosion-oxidation cycle in a burner rig with a combined cycle of
about fifteen minutes at about 1700.degree. F. (about 930.degree.
C.) plus about five minutes at about 2075.degree. F. (about
1135.degree. C.). Testing was terminated when an approximately 0.1
inch (about 2.5 mm) diameter region of the superalloy was exposed
due to environmental attack of the coating. One of the coatings
performed exceedingly well, with a life of approximately 800
cycles, as compared to the typically observed life of about 300 to
400 cycles for the other NiAlCrZr coatings. After the test, a piece
was cut from the cold section of the high-performing pin and
metallographically mounted, polished and subjected to TOF-SIMS
analysis, by which it was determined that the high-performing pin
contained chromium oxide precipitates. The lower-performing pins
were also analyzed and found to contain lower levels of chromium
oxide. While not wishing to be held to any particular theory, it
was believed that inadvertent variations in the fuel-to-oxygen
ratio in the HVOF process resulted in the different oxide levels
observed in the coatings.
[0026] From the results of the FCT and burner rig tests, it was
speculated that the presence of reactive elements having a high
affinity for oxygen, e.g. zirconium and chromium, when present in
the form of very fine oxide and/or carbide particles at the coating
interfaces (e.g., grain boundaries), may reduce the driving force
for inward diffusion of oxygen during high temperature exposure,
thereby reducing internal oxidation of a NiAl coating. In order to
increase the strength of a beta-NiAl overlay coating by a
dispersion strengthening mechanism, the particles must be present
in a sufficient amount and size to be stable and unreactive at
temperatures at which the coating will be exposed, e.g., up to
about 1300.degree. C. for gas turbine engine components. On this
basis, it is envisioned that an effective size for the particles is
in the sub-micron range, and that an effective particle content is
not greater than about five volume percent. A particularly suitable
particle size range is believed to be in a range of about 1 to
about 2000 nanometers, more preferably about 10 to about 1000
nanometers, while a suitable particle content is believed to be 0.5
to less than 5 volume percent.
[0027] A fine dispersion of ceramic particles can be more
consistently and controllably obtained by introducing CH.sub.4, CO,
CO.sub.2, N.sub.2, NH.sub.3, and/or O.sub.2 into a PVD or thermal
spray coating chamber so as to react the reactive coating
constituents (e.g., zirconium and chromium) during the coating
process. Alternatively, the dispersions can be introduced with the
source materials from which the coating is deposited. Using such
techniques, it is believed that a suitable beta-NiAl overlay
coating for use as a bond coat or an environmental coating contains
up to about 0.8 atomic percent (about 1.4 weight percent) zirconium
and up to about 15 atomic percent (about 18 weight percent)
chromium, but with some of the zirconium and chromium content being
present as oxides, carbides and/or nitrides.
[0028] It is recognized that the potent strengthening effect of
oxide dispersion-strengthening (ODS) has been demonstrated in
superalloys (e.g. MA754 and MA6000 alloys) with Y.sub.2O.sub.3
dispersions, as well as AlN precipitates in bulk (i.e.,
non-coating) beta-NiAl alloys. It is further noted that the concept
of particle dispersion-strengthened MCrAlY and diffusion bond coats
have been previously investigated. However, such coatings differ in
composition and microstructure from the beta-NiAl overlay coatings
that are the subject of the present invention. Furthermore, while
it was previously recognized that limited quantities of zirconium
can improve beta-NiAl overlay coatings through solid solution
strengthening and intermetallic precipitation strengthening, the
effect of higher zirconium contents in the form of carbide and
oxide precipitates was not.
[0029] While the invention has been described in terms of a
preferred embodiment, it is apparent that modifications could be
adopted by one skilled in the art. For example, based on
commonly-assigned U.S. Pat. Nos. 5,975,852, 6,291,084, 6,153,313
and 6,255,001 and commonly-assigned U.S. patent application Ser.
No. 10/044,618, it is believed that the beta-NiAl overlay coatings
of this invention could be modified to contain a reactive element
other than zirconium, such as yttrium, cerium or hafnium, as well
as other alloying ingredients, such as tantalum and silicon.
Accordingly, the scope of the invention is to be limited only by
the following claims.
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