U.S. patent application number 11/627582 was filed with the patent office on 2007-06-21 for smooth outer coating for combustor components and coating method therefor.
This patent application is currently assigned to General Electric Company. Invention is credited to Daniel Peter Ivkovich, Timothy Lance Manning, Tara Easter McGovern, Jane Ann Murphy, Thomas Walter Rentz, Mathew Curtis Roling, Raymond Grant Rowe, Andrew Jay Skoog, Wiliam Randolph Stowell.
Application Number | 20070141269 11/627582 |
Document ID | / |
Family ID | 34941713 |
Filed Date | 2007-06-21 |
United States Patent
Application |
20070141269 |
Kind Code |
A1 |
Stowell; Wiliam Randolph ;
et al. |
June 21, 2007 |
SMOOTH OUTER COATING FOR COMBUSTOR COMPONENTS AND COATING METHOD
THEREFOR
Abstract
A coating and method for overcoating a TBC on a component used
in a high-temperature environment, such as the combustor section of
an industrial gas turbine. The coating defines the outermost
surface of the component and is formed of at least two layers
having different compositions. An inner layer of the coating
contains alumina in a first silica-containing matrix material that
is free of zinc titanate. An outer layer of the coating contains
alumina, a glass material, and zinc titanate in a second
silica-containing matrix material. The outer layer of the coating
has a surface roughness of not greater than three micrometers Ra
and forms the outermost surface of the component. The coating
reduces the component temperature by reducing the convective and
radiant heat transfer thereto.
Inventors: |
Stowell; Wiliam Randolph;
(Rising Sun, OH) ; Ivkovich; Daniel Peter;
(Fairfield, OH) ; Manning; Timothy Lance;
(Cincinnati, OH) ; McGovern; Tara Easter;
(Simpsonville, SC) ; Murphy; Jane Ann; (Franklin,
OH) ; Rentz; Thomas Walter; (Cincinnati, OH) ;
Roling; Mathew Curtis; (Simpsonville, SC) ; Rowe;
Raymond Grant; (Niskayuna, NY) ; Skoog; Andrew
Jay; (West Chester, OH) |
Correspondence
Address: |
HARTMAN AND HARTMAN, P.C.
552 EAST 700 NORTH
VAIPARAISO
IN
46383
US
|
Assignee: |
General Electric Company
1 River Road
Schenectady
NY
12345
|
Family ID: |
34941713 |
Appl. No.: |
11/627582 |
Filed: |
January 26, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10710110 |
Jun 18, 2004 |
|
|
|
11627582 |
Jan 26, 2007 |
|
|
|
Current U.S.
Class: |
427/419.2 ;
427/456 |
Current CPC
Class: |
Y10T 428/256 20150115;
C23C 24/08 20130101; C23C 28/345 20130101; C23C 28/3215 20130101;
C23C 28/324 20130101; Y02T 50/60 20130101; C23C 28/3455 20130101;
Y10T 428/252 20150115 |
Class at
Publication: |
427/419.2 ;
427/456 |
International
Class: |
B05D 1/36 20060101
B05D001/36; C23C 4/08 20060101 C23C004/08 |
Claims
1. A method of reducing convective and radiant heat transfer to a
combustor component of a gas turbine, the method comprising the
steps of: depositing a thermal barrier coating on the component;
preparing first and second slurries, the first slurry being free of
zinc titanate and containing first and second alumina particles in
a first silica-forming binder material, the first alumina particles
having a particle size range of about 3 to about 6 micrometers, the
second alumina particles having a particle size range of about 0.05
to about 0.8 micrometers, the second slurry containing third
alumina particles, a glass material, and zinc titanate in a second
silica-forming binder material, the third alumina particles having
a particle size range of less than the first and second alumina
particles; depositing and firing the first and second slurries to
form first and second layers of a multilayer outer coating
overlying the thermal barrier coating, the first layer being free
of zinc titanate and comprising alumina in a first
silica-containing matrix material, the second layer comprising
alumina, the glass material, and the zinc titanate in a second
silica-containing matrix material; wherein the second layer defines
an outermost surface of the component, the outermost surface having
a surface roughness of not greater than 3 micrometers Ra.
2. A method according to claim 1, wherein the first alumina
particles constitute about 20 to about 40 weight percent of the
first slurry.
3. A method according to claim 1, wherein the second alumina
particles constitute about 20 to about 55 weight percent of the
first slurry.
4. A method according to claim 1, wherein the third alumina
particles constitute about 25 to about 65 weight percent of the
second slurry.
5. A method according to claim 1, wherein the first and second
slurries are deposited to yield the outer coating having a
thickness of less than the thermal barrier coating and the
outermost surface has a surface roughness of not greater than 1
micrometer Ra.
6. A method according to claim 1, wherein the first slurry consists
of the first and second alumina particles, the first silica-forming
binder material, and a carrier liquid that is eliminated during the
firing of the first slurry.
7. A method according to claim 1, wherein the second slurry
consists of the third alumina particles, the glass material, the
zinc titanate, the second silica-forming binder material, and a
carrier liquid that is eliminated during the firing of the second
slurry.
8. A method according to claim 1, further comprising depositing a
bond coat on the component prior to depositing the thermal barrier
coating.
9. A method according to claim 8, wherein the bond coat has a
chemical composition consisting essentially of nickel, chromium,
aluminum, yttrium and incidental impurities, and the bond coat has
an average surface roughness Ra of at least about 8
micrometers.
10. A method according to claim 1, wherein the thermal barrier
coating is deposited by air plasma spraying.
11. A method according to claim 10, wherein the thermal barrier
coating has a chemical composition consisting essentially of
zirconia, yttria and incidental impurities.
Description
[0001] This application is a Division of U.S. patent application
Ser. No. 10/710,110, filed Jun. 18, 2004.
BACKGROUND OF THE INVENTION
[0002] The present invention generally relates to coatings for
components exposed to high temperatures, such as the hostile
thermal environment of a gas turbine. More particularly, this
invention relates to a smooth outer coating for combustor
components of a gas turbine component, in which the coating reduces
the component temperature by reducing the convective and radiant
heat transfer to the component in the combustor section of the
turbine.
[0003] Hot section components of aircraft and industrial (power
generation) gas turbine engines are often protected by a thermal
barrier coating (TBC), which reduces the temperature of the
underlying component substrate and thereby prolongs the service
life of the component. Ceramic materials and particularly
yttria-stabilized zirconia (YSZ) are widely used as TBC materials
because of their high temperature capability, low thermal
conductivity, and relative ease of deposition by plasma spraying,
flame spraying and physical vapor deposition (PVD) techniques. Air
plasma spraying (APS) is often preferred over other deposition
processes due to relatively low equipment costs and ease of
application and masking. TBC's deposited by APS are characterized
by a degree of inhomogeneity and porosity that occurs as a result
of the deposition process, in which "splats" of molten material are
deposited and subsequently solidify. The resulting surface of the
TBC is relatively rough, with a surface roughness of 250 to 350
microinches Ra (about 6 to 9 micrometers Ra) being typical for YSZ
deposited by APS (APSTBC). The inhomogeneity and porosity of a
plasma-sprayed TBC enhances the thermal insulating property of the
TBC, and thus helps to reduce the temperature of the component on
which the TBC is deposited. In regard to infrared (IR)
transmissivity, analysis has shown that APSTBC is about 20% to 70%
transparent to thermal radiation (wavelengths of about 780 nm to
about 1 mm) when deposited at typically thicknesses of about 250 to
500 micrometers. As a result, the thermal protection provided by
APSTBC is compromised in environments that have high thermal
radiation loads, such as within the combustor section of a gas
turbine.
[0004] To be effective, TBC systems must strongly adhere to the
component and remain adherent throughout many heating and cooling
cycles. The latter requirement is particularly demanding due to the
different coefficients of thermal expansion (CTE) between ceramic
materials and the substrates they protect, which are typically
superalloys though ceramic matrix composite (CMC) materials are
also used. To promote adhesion and extend the service life of a TBC
system, an oxidation-resistant bond coat is often employed. Bond
coats are typically in the form of an overlay coating such as
MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or
another rare earth element), or a diffusion aluminide coating.
During the deposition of the ceramic TBC and subsequent exposures
to high temperatures, such as during turbine operation, these bond
coats form a tightly adherent alumina (Al.sub.2O.sub.3) layer or
scale that adheres the TBC to the bond coat.
[0005] The service life of a TBC system is typically limited by a
spallation event brought on by thermal fatigue. In addition to the
CTE mismatch between a ceramic TBC and a metallic substrate,
spallation can be promoted as a result of the TBC being subjected
to substances within the hot gas path of a gas turbine. For
example, spallation of TBC from combustor components such as
liners, heatshields and transition pieces can be accelerated in
industrial gas turbines that burn liquid fuel or utilize water
injection for NOx abatement.
[0006] In view of the above, further improvements would be
desirable for the ability of TBC on combustor components to reject
heat and resist spallation.
BRIEF SUMMARY OF THE INVENTION
[0007] The present invention generally provides a coating and
method for overcoating a TBC on a component used in a
high-temperature environment, such as the combustor section of a
gas turbine. The invention is particularly directed to a coating
that reduces the component temperature by reducing the convective
and radiant heat transfer to the component in the combustor section
of an industrial gas turbine.
[0008] The coating of this invention defines the outermost surface
of the component it protects, and is formed of at least two layers
having different compositions. An inner layer of the coating
contains first and second alumina particles in a first
silica-containing matrix material that is free of zinc titanate and
consists essentially of silica, silicate and/or mullite. The first
alumina particles have a particle size that is coarser than the
second alumina particles. An outer layer of the coating contains
third alumina particles having a particle size distribution finer
than the first and second alumina particles, a glass material, and
zinc titanate in a second silica-containing matrix material
consisting essentially of silica, silicate and/or mullite. The
outer layer of the coating has a surface roughness of not greater
than 120 microinches Ra (about 3 micrometers Ra) and, as the
outermost surface of the component, is subjected to the hot
combustion gases within the combustor section.
[0009] The method of this invention involves preparing first and
second slurries from which the inner and outer layers of the
coating are formed. As such, the first slurry is free of zinc
titanate and contains the first and second alumina particles in a
first silica-forming binder material, while the second slurry
contains the third alumina particles, glass material, and zinc
titanate in a second silica-forming binder material. Following
deposition of the thermal barrier coating on the component, the
first slurry is deposited on the thermal barrier coating after
which the second slurry is deposited on the inner layer. The slurry
layers formed by the first and second slurries are fired to form
the inner and outer layers, respectively, of the coating, with the
outer layer defining the outermost surface of the component.
[0010] As noted above, the coating of this invention reduces the
component temperature by reducing the convective and radiant heat
transfer to the component. In particular, the fine particle size
distribution of the outer layer enables the outermost surface
defined by the outer layer to be sufficiently smooth to
significantly reduce convective heat transfer to the component, and
the zinc titanate contained in the outer layer serves to reduce the
IR transmissivity of the coating. The bimodal particle size
distribution of the inner layer promotes the chemical inertness and
stability of the inner layer. Furthermore, the inner layer is
chemically compatible with the outer layer and the absence of zinc
titanate in the inner layer promotes the adhesion of the outer
layer to the component. In addition to the above benefits, the
coating of this invention improves the spallation and erosion
resistance of the TBC, and is therefore capable of significantly
extending the life of the gas turbine component protected by the
thermal barrier coating.
[0011] Other objects and advantages of this invention will be
better appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a partial cross-sectional view through a single
annular combustor structure.
[0013] FIG. 2 is a cross-sectional view of the combustor structure
of FIG. 1, and shows a multilayer outer coating overlaying a
thermal barrier coating in accordance with this invention.
DETAILED DESCRIPTION OF THE INVENTION
[0014] The present invention will be described in reference to a
combustor 12 of an industrial gas turbine 10, a portion of which is
shown in cross-section in FIG. 1. The combustor 12 is one of
multiple can-annular combustors located about the periphery of the
turbine 10, and has a can-type liner 14 whose interior defines a
combustion chamber of the turbine 10. The liner 14 is inserted into
a transition piece 18 with multiple fuel nozzle assemblies 16
located at the head end of the liner 14. Both fuel and water may be
injected into the combustion chamber through the nozzle assemblies
16, with the injection of water being for the purpose of reducing
combustion temperatures and consequently NOx emissions. The
invention is not limited to combustors having the configuration
shown in FIG. 1, but instead is applicable to other combustor
configurations, such as the well-known annular type.
[0015] A thermal barrier coating (TBC) system 20 of a type suitable
for thermally insulating the interior surfaces of the liner 14 is
represented in cross-section in FIG. 2. As shown, the TBC system 20
includes a bond coat 24 overlying a substrate 22, which is
typically but not necessarily the base material of the liner 14.
Suitable materials for the substrate 22 (and therefore the liner
14) include nickel, iron and cobalt-base superalloys, as well as
nonmetallic structural materials including ceramic matrix composite
(CMC) materials. The TBC system 20 further includes a thermal
barrier coating, hereinafter TBC 26, that provides the thermal
protection for the substrate 22. A preferred material for the TBC
26 is an yttria-stabilized zirconia (YSZ), a preferred composition
being about 3 to about 8 weight percent yttria, though other
ceramic materials could be used, such as alumina, nonstabilized
zirconia, or zirconia partially or fully stabilized by magnesia,
ceria, scandia or other oxides. The bond coat 24 may be an overlay
coating such as MCrAlX (where M is iron, cobalt and/or nickel, and
X is yttrium or another rare earth element), or a diffusion
aluminide coating such as a platinum aluminide.
[0016] The TBC 26 is depicted as having been deposited by air
plasma spraying (APS), by which "splats" of molten material are
deposited on the bond coat 24. As indicated, the TBC 26 has a
degree of inhomogeneity and porosity that typically occurs in
coatings produced by plasma spraying. In addition, the surface of
the TBC 26 is relatively rough, with a surface roughness of about
200 to 500 microinches Ra (about 5 to 13 micrometers Ra) being
typical for YSZ deposited by APS (APSTBC). While depositing the TBC
26 by APS is of particular interest to this invention, other plasma
spraying techniques could also be used, such as low pressure plasma
spraying (LPPS; also known as vacuum plasma spraying (VPS)). The
TBC 26 is deposited to a thickness that is sufficient to provide
the required thermal protection for the underlying substrate 22 and
liner 14.
[0017] The bond coat 24 is preferably an NiCrAlY overlay coating
and the TBC 26 is zirconia stabilized by about eight weight percent
yttria (8% YSZ). The bond coat 24 is preferably deposited by APS to
a thickness of about 0.007 to about 0.010 inch (about 175 to about
250 micrometers) and has an average surface roughness R.sub.a of at
least about 320 microinches (about 8 microinches) to promote
adhesion of the TBC 26. Though not required by the invention, the
TBC 26 is depicted as having a construction disclosed in
commonly-assigned U.S. Pat. No. 6,047,539 to Farmer, whereby the
TBC 26 has vertical microcracks 36 that extend through at least
one-half the thickness of the TBC 26, and the density of the TBC 26
is preferably at least 90% of theoretical, i.e., contains less than
10% porosity by volume and more preferably less than 8% porosity by
volume. A suitable thickness for the TBC is about 760 to about 2500
micrometers.
[0018] While many TBC systems use YSZ deposited by APS as the
outermost layer, drawbacks include the roughness of the TBC
surface, erosion resistance, and transmissivity to infrared (IR)
radiation. Within the operating environment of a gas turbine,
surface roughness increases turbulent heat transfer from the hot
combustion gases to the component and reduces aerodynamic
performance. While surface roughness can be reduced by polishing,
such as tumbling or hand polishing, the final surface finish and
thickness of the TBC cannot be closely controlled and the
additional processing costs are undesirable. Though crystalline YSZ
is very resistant to erosion, the erosion resistance of a YSZ
APSTBC is significantly reduced as a result of its porosity and
microcrack structure, the result of which is that fine particle
bombardment dislodges small pieces of the TBC. In regard to IR
transmissivity, analysis has shown that YSZ is about 20% to 70%
transparent to thermal radiation (wavelengths of about 780 nm to
about 1 mm) when deposited by APS to thicknesses of about 250 to
500 micrometers. As a result, the thermal protection provided by
YSZ APSTBC is compromised in environments that have high thermal
radiation loads, such as within the combustor 10 of FIG. 1.
Finally, another consideration is the susceptibility of YSZ TBC's
to attack by CMAS, which is a relatively low melting eutectic that
when molten is able to infiltrate conventional TBC and promote
spallation during thermal cycling.
[0019] To address the above concerns, the TBC 26 in FIG. 2 is
overcoated by a multilayer outer coating 28. As the outermost
coating on the liner 14, the coating 28 defines the outermost
surface 34 of the liner 14 and therefore also determines the
surface roughness of the liner 14. The outer coating 28 of this
invention is also tailored to serve as a barrier to thermal
radiation, while also having the advantage of being more resistant
to erosion and CMAS infiltration than the TBC 26. The outer coating
28 achieves these features of the invention as a result of its
composition and methods of deposition as described below.
[0020] The outer coating 28 is generally an alumina-base
silica-bound ceramic material. More particularly, the outer coating
28 contains alumina (Al.sub.2O.sub.3) dispersed within a binder
matrix material composed of silica (SiO.sub.2), silicates and/or
mullite (3Al.sub.2O.sub.3.2SiO.sub.2), the relative amounts of
which will vary depending on the firing temperature and subsequent
service temperatures seen by the coating 28, with greater amounts
of mullite forming at higher temperatures. The coating 28 is
depicted as comprising an inner layer 30 contacting the TBC 26 and
an outer layer 32 defining the outermost surface 34, with the
combined thicknesses of the layers 30 and 32 being less than that
of the TBC 26. The compositions of the layers 30 and 32 are
tailored for their particular function. The inner layer 30 is
preferably limited to containing alumina in a silica matrix
material, while the outer layer 32 includes alumina as well as a
glass and zinc titanate (Zn.sub.2TiO.sub.4) in a silica matrix
material. While alumina is the preferred constituent of the coating
28, up to about 65 percent by weight of the alumina could be
replaced by other metal oxides, such as zirconia (ZrO.sub.2),
magnesia (MgO), titania (TiO.sub.2), or mullite.
[0021] A more particular composition for the inner layer 30
contains about 5 to about 85 weight percent alumina, more
preferably 40 to about 60 weight percent alumina, with the balance
being essentially the silica matrix material. The inner layer 30 is
deposited on the TBC 26 in the form of a slurry that is
subsequently dried and fired. The slurry is preferably formulated
to contain alumina particles in two discrete particle size ranges.
In such a bimodal size distribution, a suitable particle size range
for the coarser constituent is about 3.0 to about 6.0 micrometers
in diameter. A preferred alumina powder for the coarser constituent
has a particle size range of about 3.0 to about 5.5 micrometers in
diameter, and is commercially available under the designation A-14
from ALCOA. A suitable particle size range for the finer alumina
particles is about 0.05 to about 0.8 micrometers in diameter. A
preferred alumina powder for the finer constituent has a particle
size range of about 0.10 to about 0.6 micrometers in diameter, and
is commercially available under the designation Baikalox SM8 from
Baikowski International Corporation. The SM8 material has an
agglomerate size distribution (on a cumulative weight basis) of 65%
below 0.3 micrometer, 78% below 0.4 micrometer, 95% below 0.6
micrometer, and 100% below 1.0 micrometer.
[0022] In the preferred size ranges, the finer particles are able
to fill the spaces between the larger particles at the surface of
the inner layer 30 to reduce its surface roughness. Another benefit
of the bimodal size distribution of the alumina particles is that
at very high temperatures, silica within the matrix material of the
inner layer 30 preferentially reacts with the finer alumina
particles to form a mullite phase.
[0023] The slurry is prepared by combining the alumina powders with
a silica precursor and a sufficient amount of carrier liquid to
enable the slurry to be applied by spraying. A suitable precursor
for the slurry is a silicone such as polymethyl siloxane, a
particular example of which is a resin manufactured by GE Silicones
under the name SR350, and classified as a methylsesquisiloxane
mixture of the polysiloxane family. A suitable carrier liquid is an
anhydrous alcohol such as methanol or ethanol, though acetone,
isopropyl alcohol or trichloroethylene could be used. A suitable
slurry contains about 40 to about 65 weight percent of the alumina
powder (preferably having the two particle size ranges discussed
above), about 1 to about 45 weight percent of the silica precursor,
and about 5 to about 90 weight percent of the carrier liquid. The
coarser and finer alumina particles preferably constitute, by
weight, about 20% to about 55% and about 20% to about 40%,
respectively, of the slurry. After being sprayed on the TBC 26
using any suitable sprayer known in the art, the composition can be
dried at room temperature and then fired to burn off the carrier
liquid and yield a substantially homogeneous inner layer 30. A
suitable thickness for inner layer 30 is in a range of about 0.0003
to about 0.007 inch (about 7.5 to about 180 micrometers).
[0024] To achieve the desired surface roughness of not more than
120 microinches Ra (about 3 micrometers Ra) for the outermost
surface 34, the outer layer 32 must have a smoother surface finish
than the underlying TBC 26. As noted above, the outer layer 32 of
the coating 28 preferably contains, in addition to alumina and
silica, a glass material and zinc titanate, the latter of which
promotes the reflectivity of the outer layer 32 by promoting the
Mie-like scattering effect of the coating 28. To achieve this
capability, the zinc titanate content is dispersed in the outer
layer 32 of the coating 28. A particular composition for the outer
layer 32 contains, by weight, about 5 to about 85% alumina, about 0
to about 35% zinc titanate, about 0 to about 35% of the glass
material, and the balance the silica-containing matrix material. A
more preferred composition for the outer layer 32, by weight, is
about 25 to about 65% alumina, about 10 to about 25% zinc titanate,
about 10 to about 25% glass material, and the balance the silica
matrix material.
[0025] As with the inner layer 30, the outer layer 32 is deposited
in the form of a slurry that is subsequently dried and fired.
Contrary to the slurry for the inner layer 30, the slurry for the
outer layer 32 preferably contains alumina particles in a single
particle size range and which are finer than the alumina particles
used to form the inner layer 30. The alumina particles constitute
about 5 to about 80 weight percent of the slurry, more preferably
about 25 to about 65 weight percent of the slurry for the outer
layer 32. A suitable alumina powder for the outer layer 32 is
commercially available under the designation A-16SG from ALCOA, and
has an average particle size of about 0.48 micrometers.
[0026] A glass frit, zinc titanate, a silica precursor, and a
liquid carrier preferably make up the balance of the slurry. Glass
frit particles constitute about 0 to about 35 weight percent of the
slurry, more preferably about 10 to about 25 weight percent of the
slurry for the outer layer 32. A preferred glass frit material is a
proprietary composition commercially available from Vitripak, Inc.
under the name V212, with a particle size of -325 mesh (less than
45 micrometers in diameter). While other glass frit materials could
foreseeably be used, such as V55B and V213 glass frit available
from Vitripak and 7052 glass frit available from Corning, the V212
material has been shown to be suitable for having a melting
temperature and coefficient of thermal expansion that are
compatible with the superalloy substrate 22 and the operating
environment within a gas turbine. Zinc titanate particles
constitute about 0 to about 35 weight percent of the slurry, more
preferably about 10 to about 25 weight percent of the slurry for
the outer layer 32, with a suitable particle size being -325 mesh
(less than 45 micrometers in diameter).
[0027] The above solid components are combined with an appropriate
amount of silica precursor and a sufficient amount of carrier
liquid to yield a slurry. Similar to the inner layer 30, a suitable
precursor for the silica-containing matrix material of the outer
layer 32 is a silicone such as polymethyl siloxane, a particular
example of which is a resin manufactured by GE Silicones under the
name SR355. This silicone is also classified as a
methylsesquisiloxane mixture of the polysiloxane family, but yields
less silica when fired than the SR350 silicone used to form the
inner layer 30. A higher silica content is preferred for the inner
layer 30 to promote the yield strength of the inner layer 30,
thereby increasing the compliance of the inner layer 30 to promote
strain isolation resulting from CTE mismatch between the TBC 26 and
the outer layer 32. Finally, the same liquid carrier used to form
the slurry for the inner layer 30 can be used to form the slurry
for the outer layer 32. A suitable slurry contains about 1 to about
45 weight percent of the silica precursor, and about 5 to about 95
weight percent of the carrier liquid.
[0028] After being sprayed on the inner layer 30, the slurry can be
dried at room temperature and then fired to burn off the carrier
liquid and yield a substantially homogeneous outer layer 32. The
surface roughness of the outer layer 32 is in the range of about 20
to about 120 microinches Ra (about 0.5 to 3 micrometers Ra),
preferably not more than 40 microinches Ra (about 1 micrometer Ra),
which is significantly smoother than that possible for the TBC 26
when deposited by APS. A suitable thickness for outer layer 32 is
about 0.0005 to about 0.005 inch (about 10 to about 130
micrometers).
[0029] As a result of their different compositions, the inner and
outer layers 30 and 32 define distinct inner and outer zones of the
coating 28, respectively, as represented in FIG. 2. The thickness,
structure and properties of the outer coating 28 can be tailored by
the firing temperatures and durations used for each layer 30 and
32. A suitable firing technique is to heat the sprayed composition
at a rate of about 10.degree. F. per minute (about 5.5.degree.
C./minute) to a maximum hold temperature of about 800.degree. F. to
about 2500.degree. F. (about 425.degree. C. to about 1370.degree.
C.). The hold temperature is held for a duration of at least one
hour to convert the precursor to the desired silica-containing
matrix material and at least partially sinter the resulting ceramic
constituents of the layers 30 and 32. The degree to which the
layers 30 and 32 are sintered can be tailored for the service
temperature of the component. In a preferred embodiment, the layers
30 and 32 are not sintered to full density, so that voids (not
shown) are present in the coating 28. The voids serve to reduce the
thermal conductivity of the coating 28 as well as provide stress
relief.
[0030] An important feature of the outer coating 28 of this
invention is that it reduces the temperature of the component it
protects by reducing the convective and radiant heat transfer to
the component. In particular, the outermost surface 34 defined by
the outer layer 32 of the coating 28 is sufficiently smooth to
significantly reduce convective heat transfer to the component, and
the zinc titanate contained in the outer layer 32 serves to reduce
the IR transmissivity of the coating 28. The inner layer 30 is
formulated to be compliant (for strain isolation) and chemically
compatible with the outer layer 32, and the absence of zinc
titanate in the inner layer 30 has been shown to promote the
adhesion of the outer layer 32 to the TBC 26. Voids within the at
least the outer layer 32 also potentially serve as
radiation-scattering centers to significantly reduce heating of the
liner 14 by thermal radiation. The voids are capable of providing
this advantage by having an index of refraction different from that
of the alumina particles, glass frit, zinc titanate, and
silica-containing matrix material. Portions of the radiation
propagated through the coating 28 are forward-scattered and
back-scattered by the voids, similar to Mie-scattering that occurs
when solar radiation is scattered in all directions by water
droplets in the atmosphere. A suitable level of porosity for the
outer coating 28 appears to be on the order of about 10% porosity,
though lesser and greater levels of porosity are foreseeable. The
voids form as a result of spaces between the alumina particles as
well as from the decomposition of the organic portions of the
matrix material precursor and the carrier of the as-deposited
slurry coating.
[0031] In an investigation leading to the invention, testing was
performed with one-inch diameter (about 25 mm) superalloy buttons
on which a bond coat of NiCrAlY was deposited by APS to a thickness
of about 0.006 inch (about 150 micrometers), over which a TBC of
YSZ was deposited by APS to have a thickness of about 0.005 to
about 0.008 inch (about 125 to about 200 micrometers. Some of the
buttons were additionally coated with a two-layer outer coating in
accordance with the invention. The outer coatings were formed by
preparing separate slurries for the inner and outer layers of the
outer coatings, as discussed above. The slurry composition for the
inner layers contained about 316 grams SR350 silicone, about 376
gams of the fine SM8 alumina powder, about 516 gams of the coarser
A-14 alumina powder, and about 500 gams of reagent alcohol as the
liquid carrier. The slurry composition used to form the outer
layers of the coatings contained about 150 grams SR355 silicone,
about 500 gams of the A-16SG alumina powder, about 250 gams of zinc
titanate, about 250 grams of the V212 glass, and about 500 gams of
reagent alcohol as the liquid carrier. When preparing the slurries,
their respective silicone constituents were first dissolved in the
liquid carrier, after which the powder materials were added and
then the mixtures ball milled for about twelve hours.
[0032] The slurry compositions were individually applied to the
buttons and then sintered at a temperature of about 1650.degree. F.
(about 900.degree. C.) for a duration of about one hour to convert
the SR350 and SR355 precursors to the desired silica-containing
matrix materials. The inner layer of the outer coating on each
button had a final thickness of about 0.003 to about 0.008 mils
(about 75 to about 200 micrometers) and a final composition of
about 79 weight percent alumina with the balance essentially the
silica matrix material. The outer layers were approximately about
0.0005 to about 0.005 mils (about 10 to about 125 micrometers)
thick and contained, in weight percent, about 48% alumina, about
24% glass, about 24% zinc titanate, with the balance essentially
the silica matrix material.
[0033] The buttons were then subjected to a test in which a flame
was directed at their coated surfaces, followed by cooling air. The
backsides of the buttons were continuously subjected to cooling
air. During the test, in which a temperature of about 2550.degree.
F. (about 1400.degree. C.) was attained at the coating surfaces,
the buttons protected with the outer coating of this invention
exhibited backside temperatures of about 1730.degree. F. (about
943.degree. C.) compared to about 1830.degree. F. (about
998.degree. C.) fo the buttons coated only with TBC, for a
difference of about 100.degree. F. (about 55.degree. C.).
[0034] While the invention has been described in terms of a
preferred embodiment, it is apparent that other forms could be
adopted by one skilled in the art, such as by substituting other
TBC, bond coat and substrate materials. Accordingly, the scope of
the invention is to be limited only by the following claims.
* * * * *