U.S. patent application number 12/026097 was filed with the patent office on 2008-05-29 for processes for preparing corrosion resistant coating systems for silicon-containing substrates.
Invention is credited to Brett Allen Boutwell, Brian Thomas Hazel, Irene Spitsberg.
Application Number | 20080124479 12/026097 |
Document ID | / |
Family ID | 36775550 |
Filed Date | 2008-05-29 |
United States Patent
Application |
20080124479 |
Kind Code |
A1 |
Hazel; Brian Thomas ; et
al. |
May 29, 2008 |
PROCESSES FOR PREPARING CORROSION RESISTANT COATING SYSTEMS FOR
SILICON-CONTAINING SUBSTRATES
Abstract
Process includes providing a silicide-containing bond coat layer
overlying a silicon-containing substrate, optionally forming a
silica scale layer adjacent to and overlying the bond coat layer by
preoxidizing the bond coat layer, and providing an environmental
barrier coating overlying the bond coat layer. The environmental
barrier coating includes a corrosion resistant outer layer formed
by reacting a metal source with the silica scale layer, if present,
or with the silicide-containing substrate in the absence of the
silica scale layer. The process may include providing a thermal
barrier coating overlying the corrosion resistant outer layer. The
silicon-containing substrate may include at least a portion of a
gas turbine engine component selected from a turbine blade, vane,
and blisk.
Inventors: |
Hazel; Brian Thomas; (West
Chester, OH) ; Spitsberg; Irene; (Loveland, OH)
; Boutwell; Brett Allen; (Liberty Township, OH) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY
GE AVIATION, ONE NEUMANN WAY MD H17
CINCINNATI
OH
45215
US
|
Family ID: |
36775550 |
Appl. No.: |
12/026097 |
Filed: |
February 5, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11150097 |
Jun 13, 2005 |
7354651 |
|
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12026097 |
|
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Current U.S.
Class: |
427/452 ;
427/255.18; 427/299; 427/397.7; 427/419.7; 427/578 |
Current CPC
Class: |
C04B 41/52 20130101;
Y10T 428/12611 20150115; C04B 41/52 20130101; Y10T 428/12674
20150115; Y10T 428/24967 20150115; C04B 41/009 20130101; Y10T
428/263 20150115; C04B 41/52 20130101; C04B 41/52 20130101; C23C
4/02 20130101; C04B 41/009 20130101; C04B 41/5035 20130101; C04B
41/5031 20130101; C04B 41/5071 20130101; C04B 41/4558 20130101;
C04B 41/4527 20130101; C04B 35/584 20130101; C04B 41/5024 20130101;
C04B 41/5042 20130101; C04B 41/4558 20130101; C04B 35/806 20130101;
C04B 41/0072 20130101; C04B 41/4529 20130101; C04B 41/522 20130101;
C04B 35/565 20130101; C23C 28/325 20130101; C04B 41/009 20130101;
C23C 28/3455 20130101; C04B 41/52 20130101; C04B 41/89 20130101;
C04B 41/009 20130101; C23C 28/321 20130101; C04B 41/52 20130101;
C23C 28/345 20130101 |
Class at
Publication: |
427/452 ;
427/419.7; 427/397.7; 427/578; 427/299; 427/255.18 |
International
Class: |
C23C 4/12 20060101
C23C004/12; B05D 3/02 20060101 B05D003/02; C23C 16/06 20060101
C23C016/06 |
Claims
1. A process comprising the following steps: (a) providing a
silicon-containing substrate; (b) providing a silicide-containing
bond coat layer overlaying the substrate; and (c) forming over the
bond coat layer a corrosion resistant layer comprising corrosion
resistant metal silicate
2. The process according to claim 1 wherein the corrosion resistant
layer formed in (c) has a thickness up to about 5 mils.
3. The process according to claim 2 wherein step (c) is carried out
by reacting a metal source and a silica source to form the
corrosion resistant layer comprising a reaction-generated corrosion
resistant metal silicate.
4. The process according to claim 3 further comprising: (d) prior
to step (c), forming a silica scale layer on the surface of the
bond coat layer.
5. The process according to claim 4 wherein in step (d), the silica
scale layer is formed by preoxidizing a portion of the bond coat
layer.
6. The process according to claim 3 wherein step (c) is carried out
by reacting the metal source with a silica powder.
7. The process according to claim 3 wherein the metal source is a
metal oxide.
8. The process according to claim 7 wherein the metal oxide is
yttria.
9. The process of claim 1 wherein step (b) is carried out by: (1)
depositing a corrosion resistant metal silicate powder from a
slurry coating composition; and (2) heating the deposited powder to
form the corrosion resistant layer.
10. The process of claim 1 wherein step (b) is carried out by
plasma spraying the corrosion resistant layer.
11. The process of claim 10 wherein the corrosion resistant layer
formed has a thickness up to about 30 mils.
12. The process according to claim 1 wherein step (a) includes
providing a substrate forming at least a portion of a gas turbine
engine component selected from a turbine blade, vane, and
blisk.
13. A process comprising: (a) providing a silicon-containing
substrate; (b) providing a silicide-containing bond coat layer
overlaying the substrate, wherein the bond coat layer comprises a
metal silicide selected from the group consisting of a silicide of
chromium, tantalum, titanium, tungsten, zirconium, hafnium, a rare
earth, and a compatible combination thereof; and (c) forming over
the bond coat layer an environmental barrier coating, wherein the
environmental barrier coating includes a reaction-generated
corrosion resistant metal silicate layer wherein the metal silicate
is at least one member of the group consisting of a yttrium
silicate, a scandium silicate, a zirconium silicate, a hafnium
silicate, a rare earth silicate, and combinations thereof, and
wherein a silicon source for the reaction-generated metal silicate
includes an optional silica scale layer overlaying the bond coat
layer, if present, or the silicide-containing bond coat layer in
the absence of the silica scale layer.
14. The process according to claim 13 wherein in (a), providing the
silicon-containing substrate includes pretreating the substrate
prior to providing the bond coat layer in (b).
15. The process according to claim 13 wherein in (b), providing the
bond coat layer includes depositing or forming the bond coat on a
surface of the substrate by a technique selected from the group
consisting of vapor phase deposition, pack cementation, high
velocity oxy-fuel spray, plasma spray, physical vapor deposition,
thermal spray, and chemical vapor deposition.
16. The process according to claim 13 including the silica scale
layer, wherein the silica scale layer is formed by subjecting the
silicide-containing bond coat layer to a temperature of from about
800.degree. to about 1300.degree. C. for from about 15 minutes to
about 100 hours.
17. The process according to claim 13 further comprising: (d)
providing a thermal barrier coating overlying the corrosion
resistant outer layer.
18. The process according to claim 13 wherein in (a), providing a
substrate includes providing a substrate forming at least a portion
of a gas turbine engine component selected from a turbine blade,
vane, and blisk.
19. A process comprising: (a) providing a silicon-containing
substrate; (b) providing a silicide-containing bond coat layer
overlying the substrate; (c) forming a silica scale layer adjacent
to and overlying the bond coat layer by preoxidizing a portion of
the bond coat layer under conditions sufficient to provide the
silica scale layer with a thickness of from about 0.5 to about 50
microns; (d) providing an environmental barrier coating overlying
the bond coat layer, wherein providing the environmental barrier
coating includes forming a corrosion resistant outer layer adjacent
to and overlying the silica scale layer by reacting a metal source
with the silica scale layer; and (e) providing a thermal barrier
coating overlying the corrosion resistant outer layer.
20. The process according to claim 19 wherein in (a), providing a
substrate includes providing a substrate forming at least a portion
of a gas turbine engine component selected from a turbine blade,
vane, and blisk.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 11/150,097 filed Jun. 13, 2005, the entirety
of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] Embodiments disclosed herein relate broadly to processes for
preparing corrosion resistant coating systems for
silicon-containing substrates.
[0003] Higher operating temperatures for gas turbine engines are
continuously sought in order to increase their efficiency. However,
as operating temperatures increase, the high temperature durability
of the components of the engine must correspondingly increase.
Significant advances in high temperature capabilities have been
achieved through formulation of iron, nickel and cobalt-base
superalloys. While superalloys have found wide use for gas turbine
components used throughout gas turbine engines, and especially the
higher temperature sections, alternative lighter weight substrate
materials have been proposed and sought.
[0004] Ceramic materials containing silicon, such as those
comprising silicon carbide (SiC) as a matrix material and/or as a
reinforcing material (e.g., as fibers) are currently being used as
substrate materials for higher temperature applications, such as
gas turbine engines, heat exchangers, internal combustion engines,
etc. These silicon-containing matrix/reinforcing materials are
commonly referred to as ceramic matrix composites (CMCs). These
silicon-containing materials used as matrix materials and/or as
reinforcing materials can decrease the weight yet maintain the
strength and durability of turbine components comprising such
substrates, and are currently being considered for many gas turbine
components used in higher temperature sections of gas turbine
engines, such as turbine components comprising airfoils (e.g.,
compressors, turbines, vanes, etc.), combustors, and other turbine
components for which reduced weight is desirable.
[0005] As operating temperatures increase, the high temperature
durability of such CMC materials must also correspondingly
increase. In many applications, a protective coating is beneficial
or required for such silicon-containing substrates. Such coatings
should provide environmental protection by inhibiting the major
mechanism for degradation of silicon-containing materials in a
corrosive water-containing environment, namely, the formation of
volatile silicon monoxide (SiO) and silicon hydroxide
(Si(OH).sub.4) products. Consequently, a necessary requirement of
an environmental barrier coating (EBC) system for a
silicon-containing substrate is stability in high temperature
environments containing water vapors. Other important properties
for these coating systems can include a coefficient of thermal
expansion (CTE) compatible with the silicon-containing substrate,
low permeability for oxidants, low thermal conductivity, and
chemical compatibility with the silicon-containing substrate and
overlaying silica scale formed typically by oxidation.
[0006] Various single-layer and multilayer EBC systems have been
investigated, but each has exhibited shortcomings relating to
environmental protection and compatibility with silicon-containing
substrates. For example, EBC systems have been suggested for
protecting silicon-containing CMC substrates from oxidation at high
temperatures and degradation in the presence of aqueous
environments (e.g., steam). These steam-resistant EBC systems
include those comprising mullites (3Al.sub.2O.sub.3.2SiO.sub.2)
disclosed in, for example, commonly-assigned U.S. Pat. No.
6,129,954 (Spitsberg et al.), issued Oct. 10, 2000, and U.S. Pat.
No. 5,869,146 (McCluskey et al.), issued Feb. 9, 1999. Other
steam-resistant EBC systems comprising barium strontium
aluminosilicate (BSAS), with or without mullite, and with or
without additional thermal barrier coatings are disclosed in, for
example, commonly-assigned U.S. Pat. No. 5,985,470 (Spitsberg et
al.), issued Nov. 16, 1999; U.S. Pat. No. 6,444,335 (Wang et al.),
issued Sep. 3, 2002; U.S. Pat. No. 6,607,852 (Spitsberg et al.),
issued Aug. 19, 2003; and U.S. Pat. No. 6,410,148 (Eaton et al.),
issued Jun. 25, 2002.
[0007] One version of these steam-resistant EBCs comprise an
essentially three-layer system of: (1) a silicon bond coat layer
adjacent the silicon-containing substrate; (2) a combination
mullite-BSAS (e.g., 80% mullite-20% BSAS) transition layer
overlaying and adjacent the bond coat layer; and (3) an outer
barrier layer comprising BSAS. See, e.g., commonly assigned U.S.
Pat. No. 6,410,148 (Eaton et al.), issued Jun. 25, 2002. The
silicon bond coat layer provides good adhesion to the
silicon-containing substrate (e.g., a SiC/SiC CMC substrate) and
can also function as a sacrificial oxidation layer. The
mullite-BSAS transition layer prevents rapid reaction between the
outer barrier layer comprising BSAS and the underlying silica scale
that typically forms on the silicon bond coat layer. The outer
barrier layer comprising BSAS is relatively resistant to steam and
other high temperature aqueous environments.
[0008] These steam-resistant EBCs comprising BSAS are typically
deposited on the silicon-containing CMC substrates by thermal spray
techniques such as plasma spraying. Plasma spraying tends to form
relatively thick coatings or layers that may not be suitable for
certain applications. In addition, these steam-resistant EBCs
comprising BSAS may also not be sufficiently resistant to other
forms of environmental attack.
[0009] These steam-resistant three-layer EBC systems were
originally developed for gas turbine component applications where
the EBC surface temperature of the silicon-containing CMC substrate
did not exceed about 2200.degree. F. (1204.degree. C.). Future gas
turbine component applications are expected to increase the EBC
surface temperature of the silicon-containing CMC substrate well
above about 2200.degree. F. (1204.degree. C.).
[0010] Some thermal insulation from these expected higher surface
temperatures can be addressed by including one or more thermal
barrier coating (TBC) layers on top of the three-layer EBC system.
See commonly assigned U.S. Pat. No. 6,444,335 (Wang et al.), issued
Sep. 3, 2002 (T/EBC system that comprises a thermal insulating YSZ
top coat layer overlying an intermediate layer containing YSZ and
BSAS, mullite and/or alumina that overlies a mullite-containing
layer that can be adhered to the silicon-containing substrate by an
optional silicon layer.) Even with these additional TBC layers, the
silicon-containing CMC substrate, as well as the silicon bond coat
layer, is still expected to experience effective temperatures well
above about 2200.degree. F. (1204.degree. C.).
[0011] Accordingly, it would be desirable to be able to provide an
environmental barrier coating (EBC) for silicon-containing (e.g.,
CMC) substrates that can be formed to: (1) provide coating
thicknesses that are thinner than those provided by thermal spray
techniques such as plasma spray; and/or (2) are resistant to
environmental attack by other corrosive agents besides high
temperature aqueous environments (e.g., steam). It would further be
desirable to be able to provide a bond coat layer that can adhere
such an EBC to the silicon-containing (e.g., CMC) substrate, even
when experiencing effective interface surface temperatures between
the EBC and the substrate that are well above about 2200.degree. F.
(1204.degree. C.).
BRIEF DESCRIPTION OF THE INVENTION
[0012] An exemplary embodiment is broadly directed at a process
comprising the following steps: [0013] (a) providing a
silicon-containing substrate having a silicide-containing bond coat
layer over the substrate; and [0014] (b) forming over the bond coat
layer a corrosion resistant layer comprising corrosion resistant
metal silicate.
[0015] An exemplary embodiment includes a process comprising:
[0016] (a) providing a silicon-containing substrate; [0017] (b)
providing a silicide-containing bond coat layer overlaying the
substrate, wherein the bond coat layer comprises a metal silicide
selected from the group consisting of a silicide of chromium,
tantalum, titanium, tungsten, zirconium, hafnium, a rare earth, and
a compatible combination thereof; and [0018] (c) forming over the
bond coat layer an environmental barrier coating, wherein the
environmental barrier coating includes a reaction-generated
corrosion resistant metal silicate layer wherein the metal silicate
is at least one member of the group consisting of a yttrium
silicate, a scandium silicate, a zirconium silicate, a hafnium
silicate, a rare earth silicate, and combinations thereof, and
wherein a silicon source for the reaction-generated metal silicate
includes an optional silica scale layer overlaying the bond coat
layer, if present, or the silicide-containing bond coat layer in
the absence of the silica scale layer.
[0019] An exemplary embodiment includes a process comprising:
[0020] (a) providing a silicon-containing substrate; [0021] (b)
providing a silicide-containing bond coat layer overlying the
substrate; [0022] (c) forming a silica scale layer adjacent to and
overlying the bond coat layer by preoxidizing a portion of the bond
coat layer under conditions sufficient to provide the silica scale
layer with a thickness of from about 0.5 to about 50 microns;
[0023] (d) providing an environmental barrier coating overlying the
bond coat layer, wherein providing the environmental barrier
coating includes forming a corrosion resistant outer layer adjacent
to and overlying the silica scale layer by reacting a metal source
with the silica scale layer; and [0024] (e) providing a thermal
barrier coating overlying the corrosion resistant outer layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a perspective view of a turbine blade for which
embodiments of this invention comprising the environmental barrier
coating, silicide-containing bond coat layer and silicon-containing
substrate are useful.
[0026] FIG. 2 is an enlarged sectional view through the airfoil
portion of the turbine blade of FIG. 1, taken along line 2-2,
showing an embodiment of the environmental barrier coating,
silicide-containing bond coat layer and silicon-containing
substrate of this invention, including an optional overlaying
thermal barrier coating (TBC).
DETAILED DESCRIPTION OF THE INVENTION
[0027] As used herein, the term "environmental barrier coating"
(hereafter "EBC") refers to those coating systems that can provide
environmental protection for the underlying silicide-containing
bond coat layer and silicon-containing substrate against various
types of environmental attack, and are chemically compatible (e.g.,
relatively inert, etc.) with regard to the underlying
silicide-containing bond coat layer. The various types of
environmental attack that the environmental barrier coating
protects against include those caused by high temperature, aqueous
environments (e.g., steam), other environmental contaminant
compositions and corrosive agents, for example those that are
formed from oxides of calcium, magnesium, etc., or mixtures
thereof, as well as sulfates and/or chlorides of calcium,
magnesium, sodium, etc., or mixtures thereof, etc. These oxides,
sulfates, and/or chlorides of calcium, magnesium, sodium, etc., or
mixtures thereof can come from ingested sea salt or a contaminant
composition comprising mixed
calcium-magnesium-aluminum-silicon-oxide systems (Ca--Mg--Al--SiO),
that are commonly referred to as "CMAS." See, for example, U.S.
Pat. No. 5,660,885 (Hasz et al.), issued Aug. 26, 1997, which
describes these CMAS environmental contaminant compositions. The
EBC comprises an outer corrosion resistant layer, plus one or more
optional layers.
[0028] As used herein, the term "corrosion resistant layer" refers
to one or more layers comprising a sufficient amount or level of
corrosion resistant metal silicate to protect against various types
of environmental attack, including those caused by high
temperature, aqueous environments (e.g., steam), other
environmental contaminant compositions and corrosive agents, for
example those that are formed from oxides of calcium, magnesium,
etc., or mixtures thereof, as well as sulfates and/or chlorides of
calcium, magnesium, sodium, etc., or mixtures thereof, mixed
calcium-magnesium-aluminum-silicon-oxide systems, such as CMAS,
etc. The corrosion resistant layer can comprise at least about 90%
corrosion resistant metal silicate, typically at least about 95%
corrosion resistant metal silicate, and more typically 99%
corrosion resistant metal silicate.
[0029] As used herein, the term "corrosion resistant metal
silicate" refers to a metal silicate that is at least resistant to
environmental attack caused by sulfates and/or chlorides of
calcium, magnesium, sodium, etc., or mixtures thereof (e.g., from
sea salt), oxides of calcium, magnesium, etc., or mixtures thereof
(e.g., such as CMAS), etc. These metal silicates can also be
resistant to environmental attack caused by other environmental
contaminant compositions and corrosive agents, including high
temperature, aqueous environments (e.g., steam), etc. Suitable
corrosion resistant metal silicates for use herein can include
yttrium silicates, scandium silicates, zirconium silicates, hafnium
silicates, rare earth metal silicates such as lanthanum silicates,
cerium silicates, praseodymium silicates, neodymium silicates,
promethium silicates, samarium silicates, europium silicates,
gadolinium silicates, terbium silicates, dysprosium silicates,
holmium silicates, erbium silicates, thulium silicates, ytterbium
silicates, lutetium silicates, etc., as well as various
combinations of these metal silicates. The metal silicates can be
in the form of a monosilicate, a disilicate, an orthosilicate, a
metasilicate, a polysilicate, etc., or combinations thereof.
Typically, the corrosion resistant metal silicate is a yttrium
silicate, a scandium silicate, a lutetium silicate, a ytterbium
silicate, a zirconium silicate, a hafnium silicate, or a
combination thereof, and more typically a yttrium silicate,
ytterbium silicate, or a lutetium silicate.
[0030] As used herein, the term "reaction-generated corrosion
resistant metal silicate" refers to any corrosion resistant metal
silicate that is formed by the reaction of a metal source and a
silica source. The reaction-generated corrosion resistant metal
silicate can be formed as a reaction product between a metal source
(e.g., a metal oxide, metal nitrate, metal chloride, etc.) and a
silica source that can come from, for example, from silica powder
that is simply mixed, blended or otherwise combined with the metal
source (e.g., metal oxide) and then reaction-bonded to the surface
of the silicide-containing bond coat layer. Alternatively, the
source of silica can come from the silicide-containing bond coat
layer, from a silica layer overlaying and typically adjacent to the
silicide-containing bond coat layer, for example, a silica scale
layer that forms naturally from the silicide-containing bond coat
layer or that is formed intentionally or deliberately from the
silicide-containing bond coat layer, e.g., by preoxidizing a
portion of the silicide-containing bond coat layer to form a silica
scale layer thereon, by depositing silicon on the
silicide-containing bond coat layer and then preoxidizing the
deposited silicon to form a silica scale layer; by depositing
silica on the silicide-containing bond coat layer to form a silica
scale layer, etc.
[0031] As used herein, the term "silicide-containing bond coat
layer" refers to any bond coat layer that promotes, improves, etc.,
adhesion of the overlaying EBC system to the silicon-containing
substrate, and which comprises a silicon metal alloy (also referred
to herein as a "metal silicide"). Typically, the
silicide-containing bond coat layer comprises a metal silicide
having a melting point of at least about 2800.degree. F.
(1537.degree. C.), more typically at least about 3000.degree. F.
(1648.degree. C.). These metal silicides can be monosilicides,
disilicides, trisilicides, etc., and can be silicides of chromium,
molybdenum, niobium, tantalum, titanium, tungsten, zirconium, rare
earths (lanthanum, cerium, praseodymium, neodymium, promethium,
samarium, europium, gadolinium, terbium, dysprosium, holmium,
erbium, thulium, ytterbium, lutetium, etc.), etc., or compatible
combinations thereof. Suitable metal silicides for use herein
include titanium trisilicide, titanium disilicide, chromium
trisilicide, molybdenum trisilicide, molybdenum disilicide, niobium
trisilicide, niobium disilicide, hafnium disilicide, tantalum
trisilicide, tantalum disilicide, tungsten disilicide, zirconium
disilicide, rare earth disilicides and trisilicides such as
gadolinium disilicide, lanthanum disilicide, neodymium silicide,
ytterbium trisilicide, etc., or compatible combinations
thereof.
[0032] As used herein, the term "silicon-containing substrate"
refers to any silicon-containing-substrate, including those
comprising silicon-containing ceramic materials, metal silicides
(if compositionally different from those comprising the
silicide-containing bond coat layer), or combinations of such
silicon-containing ceramic materials and silicon metal alloys. The
silicon-containing substrate can comprise a substantially
continuous matrix of silicon-containing materials, can be a
composite comprising a continuous matrix of silicon-containing
materials reinforced with discrete elements such as fibers,
particles, etc. dispersed, embedded, etc., in the continuous
matrix, etc. The discrete elements such as fibers, particles, etc.,
can be formed from silicon-containing ceramic materials, or can be
formed from other materials, e.g., carbon fibers. Such combinations
of dispersed, embedded, etc., fibers, particles, etc., in a
continuous matrix of silicon-containing ceramics are typically
referred to as ceramic matrix composites or CMCs. Typical CMCs
comprise a continuous silicon-containing ceramic matrix that is
fiber reinforced, usually with silicon-based fibers. These
reinforcing fibers typically include a coating material that fully
covers the fiber surfaces to impart and maintain structural
integrity of the composite material systems. Typical fiber coating
materials include boron nitride, silicon nitride, silicon carbide,
carbon, etc. Suitable silicon-containing ceramic materials include
silicon carbide, silicon nitride, silicon carbide nitride, silicon
oxynitride, silicon aluminum oxynitride, etc., or combinations
thereof. Suitable metal silicides useful as silicon-containing
substrates include molybdenum silicides, niobium silicides, iron
silicides, etc, or combinations thereof. Illustrative
silicon-containing substrates suitable for use herein include
silicon carbide coated silicon carbide fiber-reinforced silicon
carbide particles and a silicon matrix, a carbon fiber-reinforced
silicon carbide matrix, a silicon carbide fiber-reinforced silicon
nitride matrix, etc.
[0033] As used herein, the term "thermal barrier coating"
(hereafter "TBC") refers to those coatings that reduce heat flow to
the corrosion resistant metal silicate protective layer,
steam-resistant barrier coating, silicon-containing substrate,
etc., of the article, i.e., form a thermal barrier, and which
comprise ceramic materials have a melting point that is typically
at least about 2600.degree. F. (1426.degree. C.), and more
typically in the range of from about 3450.degree. to about
4980.degree. F. (from about 1900.degree. to about 2750.degree. C.).
Suitable ceramic materials for thermal barrier coatings include,
aluminum oxide (alumina), i.e., those compounds and compositions
comprising Al.sub.2O.sub.3, including unhydrated and hydrated
forms, various zirconias, in particular phase-stabilized zirconias
(e.g., zirconia blended with various stabilizer metal oxides such
as yttrium oxides), such as yttria-stabilized zirconias,
ceria-stabilized zirconias, calcia-stabilized zirconias,
scandia-stabilized zirconias, magnesia-stabilized zirconias,
india-stabilized zirconias, ytterbia-stabilized zirconias, etc., as
well as mixtures of such stabilized zirconias. See, for example,
Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd Ed., Vol.
24, pp. 882-883 (1984) for a description of suitable zirconias.
Suitable yttria-stabilized zirconias can comprise from about 1 to
about 20% yttria (based on the combined weight of yttria and
zirconia), and more typically from about 3 to about 10% yttria.
These phase-stabilized zirconias can further include one or more of
a second metal (e.g., a lanthanide or actinide) oxide such as
dysprosia, erbia, europia, gadolinia, neodymia, praseodymia,
urania, and hafnia to further reduce thermal conductivity of the
thermal barrier coating. See U.S. Pat. No. 6,025,078 (Rickerby et
al.), issued Feb. 15, 2000 and U.S. Pat. No. 6,333,118 (Alperine et
al.), issued Dec. 21, 2001, both of which are incorporated by
reference. Suitable ceramic materials for thermal barrier coatings
also include pyrochlores of general formula A.sub.2B.sub.2O.sub.7
where A is a metal having a valence of 3+ or 2+ (e.g., gadolinium,
aluminum, cerium, lanthanum or yttrium) and B is a metal having a
valence of 4+ or 5+ (e.g., hafnium, titanium, cerium or zirconium)
where the sum of the A and B valences is 7. Representative
materials of this type include gadolinium-zirconate, lanthanum
titanate, lanthanum zirconate, yttrium zirconate, lanthanum
hafnate, cerium zirconate, aluminum cerate, cerium hafnate,
aluminum hafnate and lanthanum cerate. See U.S. Pat. No. 6,117,560
(Maloney), issued Sep. 12, 2000; U.S. Pat. No. 6,177,200 (Maloney),
issued Jan. 23, 2001; U.S. Pat. No. 6,284,323 (Maloney), issued
Sep. 4, 2001; U.S. Pat. No. 6,319,614 (Beele), issued Nov. 20,
2001; and U.S. Pat. No. 6,387,526 (Beele), issued May 14, 2002, all
of which are incorporated by reference.
[0034] As used herein, the term "CTE" refers to the coefficient of
thermal expansion of a material, and is typically defined in units
of 10.sup.-6/.degree. F. or 10.sup.-6/.degree. C.
[0035] As used herein, the term "comprising" means various
compositions, compounds, components, coatings, substrates, layers,
steps, etc., can be conjointly employed in this invention.
Accordingly, the term "comprising" encompasses the more restrictive
terms "consisting essentially of" and "consisting of"
[0036] All amounts, parts, ratios and percentages used herein are
by weight unless otherwise specified.
[0037] Previously, bond coat layers for adhering EBC systems
(including those having an outer corrosion resistant layer
comprising a corrosion resistant metal silicate) to the underlying
silicon-containing substrate have comprised silicon. Typically,
this relatively thin silicon bond coat layer has a thickness of
from about 3 to about 6 mils (from about 76 to about 152 microns).
See, for example, commonly assigned U.S. Pat. No. 6,410,148 (Eaton
et al.), Jun. 25, 2002.
[0038] Bond coat layers of EBCs for silicon-containing substrates
used in higher temperature applications can experience effective
temperatures above about 2200.degree. F. (3992.degree. C.), and
especially well above about 2200.degree. F. (1204.degree. C.),
e.g., upwards of about 2400.degree. F. (1315.degree. C.). The
embodiments of article and processes of this invention are based on
the discovery that the adherence and other mechanical properties of
silicon bond coat layers experiencing such higher temperatures can
be adversely affected, even though silicon has a melting point of
about 2570.degree. F. (1410.degree. C.). For example, such higher
temperatures can adversely affect the ability of the silicon bond
coat layer to adhere the EBC system to the underlying
silicon-containing substrate, as well as cause other mechanical
stresses in the EBC system.
[0039] The silicide-containing bond coat layers used in the
embodiments of the article and processes of this invention solve
these problems caused by the bond coat layer experiencing higher
effective temperatures above about 2200.degree. F. (1204.degree.
C.), and especially well above about 2200.degree. F. (1204.degree.
C.), e.g., upwards of about 2400.degree. F. (1315.degree. C.). The
metal silicides that these bond coat layers comprise are much
higher melting than silicon, e.g., have melting points typically of
at least about 2800.degree. F. (1537.degree. C.), more typically at
least about 3000.degree. F. (1648.degree. C.). As a result, the
silicide-containing bond coat layers of the embodiments of the
article and processes of this invention are less susceptible to
adverse effects on the mechanical properties (e.g., adherence and
stress properties) because of higher temperature exposure,
especially compared to silicon bond coat layers.
[0040] While providing the ability to effectively handle higher
temperatures, the silicide-containing bond coat layers used in the
embodiments of the article and processes of this invention retain
the other desirable physical and chemical properties of the
replaced silicon bond coat layer. For example, the
silicide-containing bond coat layer improves the adherence of the
corrosion resistant layer-containing EBC system to the underlying
silicon-containing substrate. In addition, the silicide-containing
bond coat layer can provide a source of silicon for forming an
optional protective silica scale layer thereon, or for forming the
corrosion resistant layer comprising a reaction-generated corrosion
resistant metal silicate of the EBC in the absence of the silica
scale layer.
[0041] These silicide-containing bond coat layers are useful with a
variety of articles for adhering overlaying corrosion resistant
layer-containing EBC systems to silicon-containing substrates where
the article is operated at, or exposed to, high temperature,
corrosive environments, especially higher temperature, corrosive
environments that occur during normal gas turbine engine operation.
These articles can be in the form of turbine engine (e.g., gas
turbine engine) parts and components, including those comprising
turbine airfoils such as turbine blades, vanes and blisks, turbine
shrouds, turbine nozzles, combustor components such as liners,
deflectors and their respective dome assemblies, augmentor hardware
of gas turbine engines, etc. The silicide-containing bond coat
layers used in the embodiments of the articles of this invention
are particularly useful for articles comprising silicon-containing
substrates in the form of turbine blades and vanes, and especially
the airfoil portions of such blades and vanes. However, while the
following discussion of the embodiments of articles of this
invention will be with reference to turbine blades and vanes, and
especially the airfoil portions thereof, that comprise these blades
and vanes, it should also be understood that these
silicide-containing bond coat layers can be useful for adhering
overlaying corrosion resistant layer-containing EBC systems in
other articles comprising silicon-containing substrates.
[0042] The various embodiments of this invention are further
illustrated by reference to the drawings as described hereafter.
Referring to the drawings, FIG. 1 depicts a component article of a
gas turbine engine such as a turbine blade or turbine vane, and in
particular a turbine blade identified generally as 10. (Turbine
vanes have a similar appearance with respect to the pertinent
portions.) Blade 10 generally includes an airfoil 12 against which
hot combustion gases are directed during operation of the gas
turbine engine, and whose surfaces are therefore subjected to
potential environmental attack by high temperature aqueous
environments (e.g., steam), as well as other environmental
contaminants such as CMAS or sea salt. Airfoil 12 has a
"high-pressure side" indicated as 14 that is concavely shaped; and
a suction side indicated as 16 that is convexly shaped and is
sometimes known as the "low-pressure side" or "back side." In
operation the hot combustion gas is directed against the
high-pressure side 14. Blade 10 is anchored to a turbine disk (not
shown) with a dovetail 18 formed on the root section 20 of blade
10. In some embodiments of blade 10, a number of internal passages
extend through the interior of airfoil 12, ending in openings
indicated as 22 in the surface of airfoil 12. During operation, a
flow of cooling air is directed through the internal passages (not
shown) to cool or reduce the temperature of airfoil 12.
[0043] Referring to FIG. 2, the base material of airfoil 12 of
blade 10 comprising the silicon-containing substrate is indicated
generally as 30. Surface 34 of substrate 30 can be pretreated prior
to forming the bond coat layer thereon to remove substrate
fabrication contamination (e.g., cleaning surface 34) to improve
adherence thereto, etc. For example, substrate 30 can be pretreated
by subjecting surface 34 to a grit blasting step. This grit
blasting step is typically carried out carefully in order to avoid
damage to surface 34 of substrate 30 such as silicon carbide fiber
reinforced CMC substrate. The particles used for the grit blasting
should also be hard enough to remove the undesired contamination
but not so hard as to cause significant erosive removal of
substrate 30. The abrasive particles typically used in grit
blasting are sufficiently small to prevent significant impact
damage to surface 34 of substrate 30. When processing a substrate
30, for example, a silicon carbide CMC substrate, grit blasting is
typically carried out with alumina particles, typically having a
particle size of about 30 microns or less, and typically at a
velocity of from about 150 to about 200 m/sec.
[0044] As shown in FIG. 2, adjacent to and overlaying surface 34 of
substrate 30 is a silicide-containing bond coat layer indicated
generally as 42. Bond coat layer 42 typically has a thickness of
from about 0.5 to about 10 mils (from about 13 to about 254
microns), more typically from about 1 to about 6 mils (from about
25 to about 152 microns). This bond coat layer 42 can be applied
to, deposited or otherwise formed on surface 34 by any process
suitable for forming layers from metal silicides, including vapor
phase deposition techniques, pack cementation techniques, high
velocity oxy-fuel (HVOF) techniques, plasma spray techniques,
physical vapor deposition (PVD) techniques such as electron beam
physical vapor deposition (EB-PVD), ion plasma, etc., thermal spray
techniques such as plasma spray (e.g., air plasma spray), etc.,
chemical vapor deposition (CVD) techniques, etc., as described
hereafter for forming thermal barrier coatings, or as well known to
those skilled in the art.
[0045] As also shown in FIG. 2, adjacent to and overlaying bond
coat layer 42 is a corrosion resistant layer-containing
environmental barrier coating (EBC) indicated generally as 50. As
also shown in FIG. 2, EBC 50 can optionally comprise a protective
inner silica scale layer 58. For example, it can be useful to
preoxidize a small portion or fraction of the silicide-containing
bond coat layer 42 to form a protective inner silica scale layer
58. This preoxidized silica scale layer 58 can be formed, for
example, by subjecting the silicide-containing bond coat layer 42
to a temperature of from about 800.degree. to about 1300.degree. C.
for from about 15 minutes to about 100 hours.
[0046] As shown in FIG. 2, adjacent to and overlaying silica scale
layer 50 (or bond coat layer 42 in the absence of silica scale
layer 58) is the outer corrosion resistant layer of EBC 50 that is
indicated generally as 66. Typically, corrosion resistant outer
layer 66 can be formed by simply applying or otherwise depositing a
corrosion resistant metal silicate on either the silica scale layer
58, or the bond coat layer 42 in the absence of silica scale layer
58, for example, by the use of conventional coating methods such as
physical vapor deposition (PVD) techniques (e.g., electron beam
physical vapor deposition (EB-PVD), ion plasma, etc.), thermal
spray techniques (e.g., plasma spray such as air plasma spray,
etc.), chemical vapor deposition (CVD) techniques, etc., as
described hereafter for forming thermal barrier coatings, by
deposition from a slurry or gel coating composition of the
corrosion resistant metal silicate (e.g., as a powder dispersed in
the slurry), followed by heating or firing the deposited powder to
fuse or sinter the corrosion resistant layer 66 on silica scale
layer 58 or bond coat layer 42, as described hereafter.
Alternatively, corrosion resistant layer 66 can be prepared by
reacting a metal source (e.g., a metal oxide such as yttria, a
metal nitrate, a metal halide, such as a metal chloride, metal
fluoride, metal bromide, etc.) with a silica source that can come
from, for example, silica powder that is mixed, blended or
otherwise combined with the metal source, or alternatively from the
silicide-containing bond coat layer 42 (in the absence of silica
scale layer 58), from silica scale layer 58, etc.
[0047] The corrosion resistant layer 66 can be formed to any
desired thickness, the particular thickness typically being
dependent on the technique used for forming layer 66. For example,
for increased thickness, corrosion resistant layer 66 can be formed
by thermal spray techniques such plasma spray (e.g., air plasma
spray) to have thicknesses up to about 30 mils (762 microns), and
typically in the range from about 1 to about 30 mils (from about 13
to about 762 microns), more typically from about 2 to about 10 mils
(from about 25 to about 254 microns). Corrosion resistant layer 66
can also be formed to have a relatively thin thickness, e.g.,
thicknesses up to about 5 mils (127 microns). When formed as a
relatively thin layer, corrosion resistant outer layer 66 can be
formed to typically have a thickness of from about 0.5 to about 5
mils (from about 13 to about 127 microns), more typically from
about 1 to about 2.5 mils (from about 25 to about 64 microns).
[0048] The embodiments of the processes of this invention for
forming relatively thin corrosion resistant layers 66 include
slurry-gel coating deposition techniques, etc. See commonly
assigned U.S. Pat. No. 5,759,032 (Sangeeta et al.), issued Jun. 2,
1998; U.S. Pat. No. 5,985,368 (Sangeeta et al.), issued Nov. 16,
1999; and U.S. Pat. No. 6,294,261 (Sangeeta et al.), issued Sep.
25, 2001 (the relevant portions of which are herein incorporated by
reference) for suitable slurry-gel coating deposition techniques.
Slurry-gel coating deposition to form relatively thin corrosion
resistant layers 66 typically involves depositing particulates
(e.g., powders) of the corrosion resistant metal silicates from a
slurry or gel coating composition, followed by heating or firing
the deposited particulates to a sufficiently high temperature to
fuse or sinter the particulates into a cohesive corrosion resistant
layer 66.
[0049] In addition to the particulates of the corrosion resistant
metal silicate, the slurry or gel composition also includes a
liquid carrier. Non-limiting examples of liquid carriers include
water, lower alcohols (i.e., 1-4 carbon atoms in the main chain)
such as ethanol, halogenated hydrocarbon solvents such as
tetrachloromethane; and compatible mixtures of any of these
substances. Selection of the liquid carrier depends on various
factors such as: the evaporation rate required during subsequent
processing; the effect of the carrier on the adhesion of the slurry
or gel to the underlying layer (e.g., silica scale layer 58 or bond
coat layer 42); the solubility of additives and other components in
the carrier; the "dispersability" of the particulates (e.g.,
powders) in the carrier, as well as handling requirements; cost;
availability; environmental/safety concerns, etc. The amount of
liquid carrier is usually minimized while keeping the particulates
of the slurry or gel in suspension. Amounts greater than that level
may be used to adjust the viscosity of the slurry or gel
composition, depending on the technique used to deposit the
particulates from the slurry or gel.
[0050] The slurry or gel composition can be deposited by a variety
of techniques well known in the art, including slip-casting,
brush-painting, dipping, spraying, or spin-coating. Spray-coating
is often the easiest way to deposit the particulates from the
slurry or gel onto turbine components such as airfoils 12. The
viscosity of the slurry or gel coating for spraying can be
frequently adjusted by varying the amount of liquid carrier used.
After deposition of the particulates from the slurry or gel, the
deposited particulates are then heated or fired to a sufficient
temperature to fuse or sinter the particulates into a cohesive
corrosion resistant sealant layer. The appropriate time/temperature
for heating/firing the deposited particulates will of course depend
on various factors, including the particular metal silicate
particulates in the slurry-gel.
[0051] Alternatively, relatively thin corrosion resistant layers 66
can be formed on silica scale layer 58 (or on bond coat layer 42 in
the absence of silica scale layer 58) by reaction between the metal
source and the silica source (i.e., a reaction-generated corrosion
resistant metal silicate), by processes or techniques similar to
those used to prepare diffusion coatings (e.g., aluminide diffusion
coatings), including chemical vapor deposition (CVD) techniques,
pack cementation techniques, etc., well known those skilled in the
art. Typically, relatively thin corrosion resistant layers 66
layers are formed by the reaction-bonding of a metal oxide (e.g.,
yttria) with silica powders, by diffusion sintering of finely
divided powders comprising the corrosion resistant metal silicate,
etc.
[0052] As further shown in FIG. 2, an optional thermal barrier
coating (TBC) indicated generally as 74 can be formed on or over
corrosion resistant outer layer 66, but can also be provided with
additional transition layers therebetween (i.e., between TBC 74 and
corrosion resistant outer layer 66) for CTE compatibility. See
commonly assigned U.S. Pat. No. 6,444,335 (Wang et al.), issued
Sep. 3, 2002 (the relevant portions of which incorporated by
reference), for the use of such transition layers comprising BSAS,
mullite and/or alumina with TBCs for CTE compatibility.
[0053] TBC 74 can have any suitable thickness that provides thermal
insulating properties. TBC 74 typically has a thickness of from
about 1 to about 30 mils (from about 25 to about 769 microns), more
typically from about 3 to about 20 mils (from about 75 to about 513
microns). TBC 74 can be formed (with or without transitional
layers) on corrosion resistant outer layer 74 by variety of
conventional thermal barrier coating methods. For example, TBC 74
can be formed on corrosion resistant outer layer 74 by physical
vapor deposition (PVD), such as electron beam PVD (EB-PVD),
filtered arc deposition, or by sputtering. Suitable sputtering
techniques for use herein include but are not limited to direct
current diode sputtering, radio frequency sputtering, ion beam
sputtering, reactive sputtering, magnetron sputtering and steered
arc sputtering. PVD techniques can form TBCs 74 having strain
resistant or tolerant microstructures such as vertical microcracked
structures. EB-PVD techniques can form columnar structures that are
highly strain resistant to further increase the coating adherence.
See, for example, U.S. Pat. No. 5,645,893 (Rickerby et al.), issued
Jul. 8, 1997 (especially col. 3, lines 36-63) and U.S. Pat. No.
5,716,720 (Murphy), issued Feb. 10, 1998) (especially col. 5, lines
24-61) (all of which are incorporated by reference), which disclose
various apparatus and methods for applying TBCs by PVD techniques,
including EB-PVD techniques.
[0054] An alternative technique for forming TBCs 74 is by thermal
spray. As used herein, the term "thermal spray" refers to any
method for spraying, applying or otherwise depositing TBC 74 that
involves heating and typically at least partial or complete thermal
melting of the ceramic material and depositing of the heated/melted
ceramic material, typically by entrainment in a heated gas stream,
onto corrosion resistant outer layer 66. Suitable thermal spray
deposition techniques include plasma spray, such as air plasma
spray (APS) and vacuum plasma spray (VPS), high velocity oxy-fuel
(HVOF) spray, detonation spray, wire spray, etc., as well as
combinations of these techniques. A particularly suitable thermal
spray deposition technique for use herein is plasma spray. Suitable
plasma spray techniques are well known to those skilled in the art.
See, for example, Kirk-Othmer Encyclopedia of Chemical Technology,
3rd Ed., Vol. 15, page 255, and references noted therein, as well
as U.S. Pat. No. 5,332,598 (Kawasaki et al.), issued Jul. 26, 1994;
U.S. Pat. No. 5,047,612 (Savkar et al.) issued Sep. 10, 1991; and
U.S. Pat. No. 4,741,286 (Itoh et al.), issued May 3, 1998 (herein
incorporated by reference) which describe various aspects of plasma
spraying suitable for use herein, including apparatus for carrying
out plasma spraying.
[0055] While specific embodiments of the this invention have been
described, it will be apparent to those skilled in the art that
various modifications thereto can be made without departing from
the spirit and scope of this invention as defined in the appended
claims.
* * * * *