U.S. patent application number 12/016589 was filed with the patent office on 2009-07-23 for low thermal conductivity, cmas-resistant thermal barrier coatings.
This patent application is currently assigned to Rolls-Royce Corp.. Invention is credited to Kang N. Lee.
Application Number | 20090184280 12/016589 |
Document ID | / |
Family ID | 40513795 |
Filed Date | 2009-07-23 |
United States Patent
Application |
20090184280 |
Kind Code |
A1 |
Lee; Kang N. |
July 23, 2009 |
Low Thermal Conductivity, CMAS-Resistant Thermal Barrier
Coatings
Abstract
A thermal barrier coating composition including a base oxide; a
primary dopant including ytterbia; a first co-dopant including
samaria; and a second co-dopant including at least one of lutetia,
scandia, ceria, gadolinia, neodymia, or europia.
Inventors: |
Lee; Kang N.; (Zionsville,
IN) |
Correspondence
Address: |
SHUMAKER & SIEFFERT, P. A.
1625 RADIO DRIVE, SUITE 300
WOODBURY
MN
55125
US
|
Assignee: |
Rolls-Royce Corp.
|
Family ID: |
40513795 |
Appl. No.: |
12/016589 |
Filed: |
January 18, 2008 |
Current U.S.
Class: |
252/62 |
Current CPC
Class: |
C04B 41/009 20130101;
C23C 30/00 20130101; C23C 4/11 20160101; C23C 14/083 20130101; C23C
4/073 20160101; C23C 14/08 20130101; C09D 1/00 20130101; C23C
28/3215 20130101; C04B 41/52 20130101; C04B 41/89 20130101; C23C
28/3455 20130101; C23C 28/345 20130101; C23C 28/321 20130101; C23C
28/042 20130101; C04B 41/009 20130101; C04B 35/565 20130101; C04B
35/806 20130101; C04B 41/52 20130101; C04B 41/5024 20130101; C04B
41/5037 20130101; C04B 41/5096 20130101; C04B 41/52 20130101; C04B
41/4527 20130101; C04B 41/5044 20130101; C04B 41/52 20130101; C04B
41/4529 20130101; C04B 41/5044 20130101; C04B 41/522 20130101 |
Class at
Publication: |
252/62 |
International
Class: |
E04B 1/74 20060101
E04B001/74 |
Claims
1. A thermal barrier coating composition comprising: a base oxide;
a primary dopant comprising ytterbia; a first co-dopant comprising
samaria; and a second co-dopant comprising at least one of lutetia,
scandia, ceria, gadolinia, neodymia, or europia.
2. The thermal barrier coating composition of claim 1, wherein the
base oxide is selected from the group consisting of zirconia,
hafnia, and combinations thereof.
3. The thermal barrier coating composition of claim 1, wherein the
primary dopant is present in a larger amount than either of the
first co-dopant and the second co-dopant.
4. The thermal barrier coating composition of claim 1, wherein the
thermal barrier coating comprises about 2 mol. % to about 40 mol. %
of the primary dopant, about 0.1 mol. % to about 20 mol. % of the
first co-dopant, about 0.1 mol. % to about 20 mol. % of the second
co-dopant, and the balance base oxide and impurities.
5. The thermal barrier coating composition of claim 1, wherein the
thermal barrier coating comprises about 2 mol. % to about 20 mol. %
of the primary dopant, about 0.5 mol. % to about 10 mol. % of the
first co-dopant, about 0.5 mol. % to about 10 mol. % of the second
co-dopant, and the balance base oxide and impurities.
6. The thermal barrier coating composition of claim 1, wherein the
thermal barrier coating comprises about 2 mol. % to about 10 mol. %
of the primary dopant, about 0.5 mol. % to about 5 mol. % of the
first co-dopant, about 0.5 mol. % to about 5 mol. % of the second
co-dopant, and the balance base oxide and impurities.
7. The thermal barrier coating composition of claim 1, wherein the
thermal barrier coating comprises a cubic phase constitution.
8. The thermal barrier coating composition of claim 1, wherein the
thermal barrier coating comprises a tetragonal t' phase
constitution.
9. The thermal barrier coating composition of claim 1, wherein the
thermal barrier coating comprises a tetragonal t' phase
constitution and a cubic phase constitution.
10. The thermal barrier coating composition of claim 1, wherein the
thermal barrier coating comprises RE.sub.2O.sub.3ZrO.sub.2
compounds, RE.sub.2O.sub.3-HfO.sub.2 compounds and combinations
thereof.
11. The thermal barrier coating composition of claim 1, wherein the
thermal barrier coating comprises a mixture of a cubic phase
constitution and a RE.sub.2O.sub.3-ZrO.sub.2 compound,
RE.sub.2O.sub.3-HfO.sub.2 compounds and combinations thereof.
12. The thermal barrier coating composition of claim 1, wherein the
coating is essentially free of yttria.
13. A thermal barrier coating composition comprising: a base oxide
consisting essentially of zirconia, hafnia, or combinations
thereof; a primary dopant consisting essentially of ytterbia; a
first co-dopant consisting essentially of samaria; and a second
co-dopant selected from the group consisting of lutetia, scandia,
ceria, gadolinia, neodymia, europia, and combinations thereof.
14. The thermal barrier coating composition of claim 13, wherein
the primary dopant is present in a larger amount than either of the
first co-dopant and the second co-dopant.
15. The thermal barrier coating composition of claim 13, wherein
the coating consists of about 2 mol. % to about 40 mol. % of the
primary dopant, about 0.1 mol. % to about 20 mol. % of the first
co-dopant, about 0.1 mol. % to about 20 mol. % of the second
co-dopant, and the balance base oxide and impurities.
16. The thermal barrier coating composition of claim 13, wherein
the coating consists of about 2 mol. % to about 20 mol. % of the
primary dopant, about 0.5 mol. % to about 10 mol. % of the first
co-dopant, about 0.5 mol. % to about 10 mol. % of the second
co-dopant, and the balance base oxide and impurities.
17. The thermal barrier coating composition of claim 13, wherein
the coating consists of about 2 mol. % to about 10 mol. % of the
primary dopant, about 0.5 mol. % to about 5 mol. % of the first
co-dopant, about 0.5 mol. % to about 5 mol. % of the second
co-dopant, and the balance base oxide and impurities.
18. The thermal barrier coating composition of claim 13, wherein
the thermal barrier coating comprises a cubic phase
constitution.
19. The thermal barrier coating composition of claim 13, wherein
the thermal barrier coating comprises a tetragonal t' phase
constitution.
20. The thermal barrier coating composition of claim 13, wherein
the thermal barrier coating comprises a tetragonal t' phase
constitution and a cubic phase constitution.
21. The thermal barrier coating composition of claim 13, wherein
the thermal barrier coating comprises RE.sub.2O.sub.3--ZrO.sub.2
compounds, RE.sub.2O.sub.3-HfO.sub.2 compounds, and combinations
thereof.
22. The thermal barrier coating composition of claim 13, wherein
the thermal barrier coating comprises a mixture of a cubic phase
constitution and a RE.sub.2O.sub.3--ZrO.sub.2 compound,
RE.sub.2O.sub.3-HfO.sub.2 compounds, and combinations thereof.
23. The thermal barrier coating composition of claim 13, wherein
the coating is essentially free of yttria.
24. An article comprising: a thermal barrier coating applied to at
least a portion of a substrate, wherein the thermal barrier coating
comprises: a base oxide; a primary dopant comprising ytterbia; a
first co-dopant comprising samaria; and a second co-dopant
comprising at least one of lutetia, scandia, ceria, gadolinia,
neodymia, or europia.
25. The article of claim 24, further comprising a bond coat applied
to at least a portion of the substrate, wherein the thermal barrier
coating is applied to the bond coat.
26. The article of claim 24, further comprising an environmental
barrier coating applied to at least a portion of the substrate,
wherein the thermal barrier coating is applied to the environmental
barrier coating.
27. The article of claim 24, further comprising a bond coat applied
to at least a portion of the substrate and an environmental barrier
coating applied to the bond coat, wherein the thermal barrier
coating is applied to the environmental barrier coating.
28. The article of claim 24, wherein the thermal barrier coating
consists essentially of: a base oxide selected from the group
consisting of zirconia, hafnia, or combinations thereof; a primary
dopant consisting essentially of ytterbia; a first co-dopant
consisting essentially of samaria; and a second co-dopant selected
from the group consisting of lutetia, scandia, ceria, gadolinia,
neodymia, europia, and combinations thereof.
Description
TECHNICAL FIELD
[0001] The present disclosure generally relates to thermal barrier
coatings for high-temperature mechanical systems, such as gas
turbine engines.
BACKGROUND
[0002] The components of high-temperature mechanical systems, such
as, for example, gas-turbine engines, must operate in severe
environments. For example, the high-pressure turbine blades and
vanes exposed to hot gases in commercial aeronautical engines
typically experience metal surface temperatures of about
1000.degree. C., with short-term peaks as high as 1100.degree.
C.
[0003] Typical components of high-temperature mechanical systems
include a Ni or Co-based superalloy substrate. The substrate can be
coated with a thermal barrier coating (TBC) to reduce surface
temperatures. The thermal barrier coating may include a thermally
insulative ceramic topcoat, and is bonded to the substrate by an
underlying metallic bond coat.
[0004] The TBC, usually applied either by air plasma spraying or
electron beam physical vapor deposition, is most often a layer of
yttria-stabilized zirconia (YSZ) with a thickness of about 100-500
.mu.m. The properties of YSZ include low thermal conductivity, high
oxygen permeability, and a relatively high coefficient of thermal
expansion. The YSZ TBC is also typically made "strain tolerant" and
the thermal conductivity further lowered by depositing a structure
that contains numerous pores and/or pathways.
[0005] Economic and environmental concerns, i.e. the desire for
improved efficiency and reduced emissions, continue to drive the
development of advanced gas turbine engines with higher inlet
temperatures. As the turbine inlet temperature continues to
increase, there is a demand for a TBC with lower thermal
conductivity and higher temperature stability to minimize the
increase in, maintain, or even lower the temperatures experienced
by the substrate.
SUMMARY
[0006] In general, the present disclosure is directed to a TBC with
at least one of lower thermal conductivity or enhanced
CMAS-resistance compared to conventional YSZ TBCs. CMAS is a
calcia-magnesia-alumina-silicate deposit resulting from the
ingestion of siliceous minerals (dust, sand, volcanic ashes, runway
debris, and the like) with the intake of air in gas turbine
engines.
[0007] In one aspect, the present disclosure is directed to a
thermal barrier coating composition including a base oxide; a
primary dopant including ytterbia; a first co-dopant including
samaria; and a second co-dopant including at least one of lutetia,
scandia, ceria, gadolinia, neodymia, or europia.
[0008] In another aspect, the present disclosure is directed to a
thermal barrier coating composition including a base oxide
consisting essentially of zirconia, hafnia, or combinations
thereof, a primary dopant consisting essentially of ytterbia; a
first co-dopant consisting essentially of samaria; and a second
co-dopant selected from the group consisting of lutetia, scandia,
ceria, gadolinia, neodymia, europia, and combinations thereof.
[0009] In yet another aspect, the present disclosure is directed to
an article including a thermal barrier coating applied to at least
a portion of a substrate, wherein the thermal barrier coating
includes: a base oxide; a primary dopant including ytterbia; a
first co-dopant including samaria; and a second co-dopant including
at least one of lutetia, scandia, ceria, gadolinia, neodymia, or
europia.
[0010] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0011] FIG. 1 is a cross-sectional diagram of a substrate coated
with a thermal barrier coating including a bond coat and a thermal
barrier coating.
[0012] FIG. 2 is a cross-sectional diagram of an alternative
embodiment of a substrate coated with only a thermal barrier
coating.
[0013] FIG. 3 is a cross-sectional diagram of a substrate coated
with a bond coat, environmental barrier coating, and a thermal
barrier coating.
[0014] FIG. 4 is a cross-sectional photograph of a CMC substrate
coated with a thermal barrier coating bonded to the substrate by an
ytterbia-stabilized hafnia on a mullite and silicon bond coat.
[0015] FIG. 5 is a cross-sectional photograph of a CMC substrate
coated with a thermal barrier coating bonded to the substrate by an
ytterbia-stabilized hafnia on an ytterbium silicate, mullite and
silicon bond coat.
[0016] FIG. 6 is a cross-sectional photograph of a CMC substrate
coated with a thermal barrier coating including yttrium silicate on
a mullite bond coat.
DETAILED DESCRIPTION
[0017] In general, this disclosure is directed to thermal barrier
coating (TBC) compositions that possess at least one of reduced
thermal conductivity or increased CMAS
(calcia-magnesia-alumina-silicate) degradation resistance compared
to conventional yttria-stabilized zirconia (YSZ) TBCs, and articles
coated with such TBCs. More specifically, the disclosure is
directed to a thermal barrier coating including a base oxide, a
primary dopant and two co-dopants that possesses at least one of
reduced thermal conductivity or increased CMAS degradation
resistance compared to conventional YSZ TBCs.
[0018] As turbine inlet temperatures continue to increase, new
thermal barrier coatings are required with better high temperature
performance. Higher turbine inlet temperatures are detrimental to
conventional TBC performance as YSZ may sinter significantly at
temperatures above about 1200.degree. C. Sintering reduces the
porosity of the YSZ TBC, which may lead to higher thermal
conductivity and reduced strain tolerance. Higher thermal
conductivity results in the substrate being exposed to higher
temperatures, which increases the strain on the substrate and
lowers the useful life of the component.
[0019] The reduced strain tolerance of the TBC may also reduce the
useful life of the component. For example, the thermal cycles
experienced by the component (e.g., ambient temperature when gas
turbine is off, high temperature spike when gas turbine is turned
on, continuous high temperatures during operation) exert large
stresses on the component due to thermal expansion and contraction.
The porosity of the TBC provides strain tolerance, and reduced
porosity due to sintering reduces the strain tolerance of the TBC
and may compromise the mechanical stability of the TBC and/or
TBC-bond coat interface.
[0020] Higher turbine inlet temperatures may also lead to
degradation of the TBC through a reaction with CMAS, a
calcia-magnesia-alumina-silicate deposit resulting from the
ingestion of siliceous minerals (dust, sand, volcanic ashes, runway
debris, and the like) with the intake of air in gas turbine
engines. Typical CMAS deposits have a melting point of about
1200.degree. C. to about 1250.degree. C. (about 2200.degree. F. to
about 2300.degree. F.). As advanced engines run at TBC surface
temperatures above the CMAS melting point, the molten CMAS can
penetrate the microstructural features in the TBC that induce
strain tolerance, leading to a loss of strain tolerance and an
increase in the thermal conductivity. Another mode of degradation
by CMAS is chemical: a reaction between CMAS and yttria in the TBC
may occur, preferentially along grain boundaries, leading to the
destabilization of YSZ by precipitating monoclinic zirconia
(ZrO.sub.2) with a lower yttria content. Both CMAS penetration and
YSZ destabilization may result in a shorter life before failure of
the component due to higher thermal stresses.
[0021] FIG. 1 shows a cross-sectional view of an exemplary article
10 used in a high-temperature mechanical system. The article 10
includes a coating 14 applied to a substrate 12. The coating
includes a bond coat 11 applied to the surface 15 of substrate 12,
and a TBC 13 applied to the bond coat 11.
[0022] The substrate 12 may be a component of a high temperature
mechanical system, such as, for example, a gas turbine engine or
the like. Typical superalloy substrates 12 are alloys based on Ni,
Co, Ni/Fe, and the like. The superalloy substrate 12 may include
other additive elements to alter its mechanical properties, such as
toughness, hardness, temperature stability, corrosion resistance,
oxidation resistance, and the like, as is well known in the art.
Any useful superalloy substrate 12 may be utilized, including, for
example, those available from Martin-Marietta Corp., Bethesda, Md.,
under the trade designation MAR-M247; those available from
Cannon-Muskegon Corp., Muskegon, Mich., under the trade designation
CMSX-4, CMXS-10; and the like.
[0023] The substrate 12 may also include a ceramic matrix composite
(CMC). The CMC may include any useful ceramic matrix material,
including, for example, silicon carbide, silicon nitride, alumina,
silica, and the like. The CMC may further include any desired
filler material, and the filler material may include a continuous
reinforcement or a discontinuous reinforcement. For example, the
filler material may comprise discontinuous whiskers, platelets, or
particulates. As another example, the filler material may include a
continuous monofilament or multifilament weave.
[0024] The filler composition, shape, size, and the like may be
selected to provide the desired properties to the CMC. For example,
in some embodiments, the filler material may be chosen to increase
the toughness of a brittle ceramic matrix. In other embodiments,
the filler may be chosen to provide a desired property to the CMC,
such as thermal conductivity, electrical conductivity, thermal
expansion, hardness, or the like.
[0025] The filler composition may be the same as the ceramic matrix
material. For example, a silicon carbide matrix may surround
silicon carbide whiskers. In other embodiments, the filler material
may comprise a different composition than the ceramic matrix, such
as aluminum silicate fibers in an alumina matrix, or the like. One
preferred CMC includes silicon carbide continuous fibers embedded
in a silicon carbide matrix.
[0026] The article 10 may include a bond coat 11. The bond coat 11
may improve adhesion between the TBC 13 and the substrate 12. The
bond coat 11 may include any useful alloy, such as a conventional
MCrAlY alloy (where M is Ni, Co, or NiCo), a .beta.-NiAl nickel
aluminide alloy (either unmodified or modified by Pt, Cr, Hf, Zr,
Y, Si, and combinations thereof), a .gamma.-Ni+.gamma.'-Ni.sub.3Al
nickel aluminide alloy (either unmodified or modified by Pt, Cr,
Hf, Zr, Y, Si, and combinations thereof), or the like.
[0027] The bond coat 11 may also include ceramics or other
materials that are compatible with a CMC substrate 12. For example,
the bond coat 11 may include mullite (aluminum silicate,
Al.sub.6Si.sub.2O.sub.13), silica, silicides, silicon, or the like.
The bond coat 11 may further include other ceramics, such as rare
earth silicates including lutetium silicates (Lu; Lutetium),
ytterbium silicates (Yb; Ytterbium), erbium silicates (Er: Erbium),
dysprosium silicates (Dy: Dysprosium), gadolinium silicates (Gd:
Gadolinium), europium silicates (Eu: Europium), samarium silicate
(Sm: Samarium), neodymium silicates (Nd: Neodymium), yttrium
silicates (Y: Yttrium), scandium silicates (Sc: Scandium), or the
like. Some preferred bond coat 11 compositions for overlaying a CMC
substrate 12 include silicon, mullite and ytterbium silicate.
[0028] The bond coat 11 may be selected based on a number of
considerations, including the chemical composition and phase
constitution of the TBC 13 and the substrate 12. For example, when
the substrate 12 includes a superalloy with
.gamma.-Ni+.gamma.'-Ni.sub.3Al phase constitution, the bond coat 11
preferably includes a .gamma.-Ni+.gamma.'-Ni.sub.3Al phase
constitution to better match the coefficient of thermal expansion
of the superalloy substrate 12, and therefore increase the
mechanical stability (adhesion) of the bond coat 11 to the
substrate 12. Alternatively, when the substrate 12 comprises a CMC,
the bond coat 11 is preferably silicon and/or a ceramic, such as,
for example, mullite or rare earth silicates.
[0029] In some embodiments, the bond coat 11 may include multiple
layers. For example, in some embodiments where the substrate 12 is
a CMC including silicon carbide, a first bond coat of silicon is
deposited on the CMC substrate 12, followed by the deposition of a
second layer including mullite (aluminum silicate,
Al.sub.6Si.sub.2O.sub.13) or a rare earth silicate. The multilayer
bond coat is desirable because the silicon bond coat provides the
bonding while the ceramic bond coat provides a gradual transition
of thermal expansion and prevents water vapor from reaching the
silicon bond coat.
[0030] In other embodiments, such as the article 20 illustrated in
FIG. 2, the article 20 may not include a bond coat 11. For example,
in some embodiments, the TBC 23 may be applied directly to the
substrate 22. A bond coat 11 may not be required or desired when
the TBC 23 and the substrate 22 are chemically and/or mechanically
compatible. For example, in embodiments where the TBC 23 and
substrate 22 adhere sufficiently strongly to each other, a bond
coat 11 may not be necessary. Additionally, in embodiments where
the coefficients of thermal expansion of the substrate 22 and TBC
23 are sufficiently similar, a bond coat 11 may not be
necessary.
[0031] As shown in FIG. 3, an article 30 may also include an
environmental barrier coating (EBC) 36 applied to the bond coat 31.
In this embodiment, the TBC 33 is applied to the EBC 36.
Environmental barrier coatings may provide water vapor stability,
chemical stability, and environmental durability to substrates,
such as, for example, CMC. Environmental barrier coatings may
include, for example, barium strontium alumina silicate
(BaO--SrO--Al.sub.2O.sub.3-2SiO.sub.2; BSAS), rare earth silicates,
combinations thereof, and the like.
[0032] In alternative embodiments (not pictured) the EBC 36 may be
directly applied to a CMC substrate 32. In these embodiments, the
EBC 36 may also perform the function of the bond coat 31 and
increase the adhesion between the TCB 33 and the CMC substrate 32.
A preferred combination EBC 36 and bond coat 31 layer includes
mullite and rare earth silicates.
[0033] As was described briefly above, one purpose of the TBC 13,
23, 33 (collectively "TBC 13") is to provide thermal insulation for
the substrate from high temperatures of the turbine gas. Because of
this, it is desirable that the TBC 13 has a low thermal
conductivity (both an intrinsic thermal conductivity of the
material(s) that forms TBC 13 and an effective thermal conductivity
of the TBC 13 as constructed). Heat is transferred through the TBC
13 through conduction and radiation. The inclusion of rare earth
elements such as ytterbia, samaria, lutetia, scandia, ceria,
gadolinia, neodymia, europia, and the like as dopants may help
decrease the thermal conductivity (by conduction) of the TBC 13.
While not wishing to be bound by any specific theory, the inclusion
of at least one of these dopant ions in the TBC 13 may reduce
thermal conductivity through one or more mechanisms, as
follows.
[0034] A first proposed mechanism of reducing thermal conductivity
includes introducing lattice imperfections into the crystal
structure of the TBC 13. Lattice imperfections include defects in
the crystalline lattice of the TBC 13. The defects may be caused by
the incorporation of dopants with differing ionic radii or
different crystalline lattice types. Lattice imperfections may be
broadly divided into two categories, point defects and larger
defects, for the purposes of this discussion. The point defects,
such as substitutional defects, interstitial defects, void defects,
and the like, may scatter high frequency phonons (lattice waves),
while larger defects, such as grain boundaries of crystals that are
smaller than about 100 nm, may scatter lower frequency phonons. In
either case, phonon scattering decreases the thermal conductivity
of the TBC 13 by reducing the mean free path of a phonon (i.e., the
average distance the phonon travels between scattering sites).
[0035] Heavier rare earth oxide dopants are expected to lower the
thermal conductivity more than lighter rare earth oxide dopants.
For example, rare earth oxides including ytterbia, lutetia,
gadolinia, samaria, neodymia, europia, and the like are expected to
more effectively lower the thermal conductivity of a TBC 13 than
yttria.
[0036] Inclusion of certain rare earth elements in the TBC 13 may
also decrease the extent to which the TBC 13 sinters at a given
temperature. For example, incorporating rare earth elements with a
larger ionic radius than yttrium can decrease the amount of
sintering at a given temperature. While not wishing to be bound by
any theory, a larger ionic radius can lead to a lower diffusion
coefficient at a given temperature. As sintering is primarily a
diffusion-related process, a lower diffusion coefficient lowers the
amount of sintering at a given temperature.
[0037] Minimizing or eliminating sintering may significantly
improve the stability of the thermal conductivity of the TBC 13
over the service life of the TBC 13. Thermal conductivity of the
TBC 13 is typically lowered by depositing the TBC 13 as a porous
structure. The porosity of the TBC 13 reduces the thermal
conductivity by reducing the area through which heat is conducted
and by providing a large refractive index difference between the
pores and the material from which the TBC 13 is made, which can
reduce heat transfer by radiation. Sintering reduces the porosity
of the structure, and thus increases the thermal conductivity (via
both radiation and conduction) of the TBC 13. Thus, preserving the
porosity (i.e., reducing sintering) of the TBC 13 over repeated
thermal cycles may help maintain the thermal conductivity of the
TBC 13 at or near the level of the originally-applied TBC 13.
[0038] The TBC 13 compositions of the present disclosure may also
improve another significant shortcoming of conventional YSZ TBCs.
Specifically, CMAS is highly reactive with yttria in the TBC 13,
preferentially along grain boundaries, leading to the
destabilization of YSZ by precipitating monoclinic zirconia
(ZrO.sub.2) with a lower yttria content, which may result in a
shorter life before failure of the component due to higher thermal
stresses. Thus, it may be particularly desirable to form a TBC 13
that is substantially free of yttria. Substantially free of yttria
means the exclusion of any amount of yttria greater than that
present in the commercially-available forms of the other rare earth
oxides or base oxides used in the TBC 13.
[0039] Other rare earth elements, including ytterbia, lutetia, and
scandia, for example, are more chemically stable in the presence of
CMAS. Thus, these elements, either alone or in combination, may
desirably replace yttria as the primary dopant in a TBC 13, which
may reduce or slow the destabilization of TBC 13 in the presence of
CMAS.
[0040] The TBC 13 of the current disclosure generally includes a
base oxide, a primary dopant, a first co-dopant, and a second
co-dopant. Including multiple dopants, preferably of different
ionic radii, may decrease the thermal conductivity of the TBC 13
more than a single dopant when present at the same total
concentration. For example, the differing ionic radii may increase
the elastic strain field caused by the lattice imperfections.
[0041] The base oxide of the TBC 13 may include or may consist
essentially of zirconia, hafnia, and combinations thereof. In the
current disclosure, to "consist essentially of," means to consist
of the listed element(s) or compound(s), while allowing the
inclusion of impurities present in small amounts such that the
impurities do not substantially affect the properties of the listed
element or compound. For example, the purification of many rare
earth elements is difficult, and thus the nominal rare earth
element may include small amounts of other rare earth elements.
This mixture is intended to be covered by the language "consist
essentially of." Many conventional TBCs 13 are based on zirconia,
so the processing and production of TBCs 13 including zirconia are
well-understood, but hafnia-based TBCs are expected to have a lower
thermal conductivity due to a higher mean atomic weight, which is
one factor in thermal conductivity.
[0042] The composition of the TBC 13 may be selected to provide a
metastable tetragonal (t'), cubic (c), or a mixture of tetragonal
and cubic (t'+c) phase constitution. A metastable tetragonal phase
constitution is generally preferred.
[0043] The primary dopant is generally selected to provide
increased resistance to CMAS attack. The primary dopant may include
ytterbia, or may consist essentially of ytterbia. The TBC 13
composition may include the primary dopant in concentrations from
about 2 mol. % to about 40 mol. %. Preferred primary dopant
concentrations range from about 2 mol. % to about 20 mol. %, and
about 2 mol. % to about 10 mol. % primary dopant is most preferred.
The primary dopant is preferably present in a greater amount than
either the first or second co-dopants, and may be present in an
amount less than, equal to, or greater than the total amount of the
first and second co-dopants.
[0044] The first and second co-dopants are generally selected to
provide decreased thermal conductivity compared to YSZ and
increased resistance to sintering. The first co-dopant may comprise
samaria, or may consist essentially of samaria. The TBC 13
composition may include the first co-dopant in concentrations from
about 0.1 mol. % to about 20 mol. %, preferably about 0.5 mol. % to
about 10 mol. %, most preferably about 0.5 mol. % to about 5 mol.
%.
[0045] The second co-dopant may include lutetia (Lu.sub.2O.sub.3),
scandia (Sc.sub.2O.sub.3), ceria (CeO.sub.2), gadolinia
(Gd.sub.2O.sub.3), neodymia (Nd.sub.2O.sub.3), europia
(Eu.sub.2O.sub.3), and combinations thereof. In other embodiments,
the second co-dopant may be selected from the group consisting of
lutetia (Lu.sub.2O.sub.3), scandia (Sc.sub.2O.sub.3), ceria
(CeO.sub.2), gadolinia (Gd.sub.2O.sub.3), neodymia
(Nd.sub.2O.sub.3), europia (Eu.sub.2O.sub.3), and combinations
thereof. The second co-dopant may be present in the TBC 13 in an
amount ranging from about 0.1 mol. % to about 20 mol. %, preferably
about 0.5 mol. % to about 10 mol. %, most preferably about 0.5 mol.
% to about 5 mol. %.
[0046] The composition of the TBC 13 may be chosen to provide a
desired phase constitution. Accessible phase constitutions include
metastable tetragonal t', cubic (c), and RE.sub.2O.sub.3--ZrO.sub.2
(and/or HfO.sub.2) compounds, such as RE.sub.2Zr.sub.2O.sub.7 and
RE.sub.2Hf.sub.2O.sub.7 (where RE is a rare earth element). To
achieve a RE.sub.2O.sub.3--ZrO.sub.2 (and/or HfO.sub.2) compound
phase constitution, the TBC 13 includes about 20 mol. % to about 40
mol. % primary dopant, about 10 mol. % to about 20 mol. % first
co-dopant, about 10 mol. % to about 20 mol. % second co-dopant, and
the balance base oxide and any impurities present. To achieve a
cubic phase constitution, the TBC 13 includes about 5 mol. % to
about 20 mol. % primary dopant, about 2 mol. % to about 10 mol. %
first co-dopant, about 2 mol. % to about 10 mol. % second
co-dopant, and the balance base oxide and any impurities present.
In some embodiments, the TBC 13 composition is preferably selected
to provide a metastable tetragonal t' phase constitution.
[0047] The TBC 13 may be applied by any useful coating method,
however, preferred methods include plasma spraying and electron
beam physical vapor deposition (EB-PVD). Plasma spraying is
generally preferred when a lower thermal conductivity is desired
due to the greater prevalence of disk-shaped pores oriented with a
major axis perpendicular to the temperature gradient, which
decreases the thermal conductivity of the TBC 13. EB-PVD is
preferred when higher strain tolerance is desired due to the
columnar structure in the TBC 13 that impart greater strain
tolerance than plasma-sprayed microstructure.
[0048] TBCs 13 of the present disclosure may provide at least one
of a decreased thermal conductivity and an increased resistance to
CMAS attack. The TBC 13 of the current disclosure may also enable
higher gas turbine operating temperatures, up to about 1650.degree.
C.
EXAMPLES
Example 1
Ytterbia-Stabilized Hafnia (YbSH) TBC on CMC
[0049] FIG. 4 shows a cross-sectional view of an article 40
including an ytterbia-stabilized hafnia (YbSH) TBC 42 on a silicon
49/mullite (Al.sub.6Si.sub.2O.sub.13) 48 bond coat 44 applied on
CMC substrate 46. The article 40 was exposed to 300 one hour
thermal cycles at 1300.degree. C. and 90% water vapor prior to the
photograph. Despite the relatively high coefficient of thermal
expansion (CTE) of YbSH (about 10.times.10.sup.-6/.degree. C. for
YbSH versus about 4.times.10.sup.-6/.degree. C. to about
5.times.10.sup.-6/.degree. C. for CMC), the multilayer coating
having the YbSH TBC 42 maintained excellent adherence, protecting
the CMC substrate 46 from water vapor attack. The propensity of
through-thickness cracking is believed to have relieved the CTE
mismatch stress. Adding the YbSH TBC 42 reduced the thermal
conductivity by about 34% from the baseline environmental barrier
coating, from about 2 W/m-K to about 1.3-1.4 W/m-K.
Example 2
YbSH/Ytterbium Silicate TBC on CMC
[0050] FIG. 5 shows a cross-sectional view of an article 50
including an ytterbia-stabilized hafnia (YbSH) TBC 52 on a silicon
59/mullite 58/ytterbium silicate 57 bond coat 54. The bond coat is
applied on a CMC substrate 56. The article 50 was exposed to 100
one hour thermal cycles at 1300.degree. C. and 90% water vapor
prior to the photograph. The TBC 52 again maintained excellent
adherence to the substrate 56.
Example 3
Yttrium Silicate on Mullite Bond-Coated CMC
[0051] FIG. 6 shows a cross-sectional view of an article 60
including an yttrium silicate TBC 62 on a mullite bond coat. The
mullite bond coat was applied to a silicon carbide substrate 66.
The article 60 was exposed to 46 one hour thermal cycles at
1400.degree. C. and 90% water vapor prior to the photograph. The
mullite bond coat and yttrium silicate reacted to form the glassy
bubbles 67, severely damaging the TBC 62. A similar glass formation
reaction due to yttria-mullite reaction was observed when the
yttrium silicate was replaced with an yttria-stabilized zirconia
TBC. Mullite-ytterbium silicate does not show glass formation in
the same condition, suggesting a superior chemical stability of
ytterbia compared to yttria in the presence of CMAS.
[0052] Various embodiments of the invention have been described.
These and other embodiments are within the scope of the following
claims.
* * * * *