U.S. patent application number 14/098864 was filed with the patent office on 2015-06-11 for article for high temperature service.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. The applicant listed for this patent is GENERAL ELECTRIC COMPANY. Invention is credited to Lauraine Denault, Larry Steven Rosenzweig, Shankar Sivaramakrishnan.
Application Number | 20150159507 14/098864 |
Document ID | / |
Family ID | 53270651 |
Filed Date | 2015-06-11 |
United States Patent
Application |
20150159507 |
Kind Code |
A1 |
Sivaramakrishnan; Shankar ;
et al. |
June 11, 2015 |
ARTICLE FOR HIGH TEMPERATURE SERVICE
Abstract
Articles for high temperature service, especially where enhanced
strain tolerance coupled with resistance to ingested dust and
debris (CMAS) is desirable, are provided herein. The article
comprises a substrate and a multi-layered coating system disposed
over the substrate. The coating system comprises a first layer
comprising a first material and a second layer comprising a second
material, with the first layer disposed between the second layer
and the substrate. The second material is more resistant to
infiltration by a nominal CMAS composition relative to 8 weight
percent yttria-stabilized zirconia at a temperature of 1300 degrees
Celsius. The second layer comprises a plurality of
through-thickness cracks, wherein at least 90 percent of the cracks
have a mean crack opening displacement, measured in a distal
surface region, of up to about 5 micrometers.
Inventors: |
Sivaramakrishnan; Shankar;
(Schenectady, NY) ; Denault; Lauraine; (Nassau,
NY) ; Rosenzweig; Larry Steven; (Clifton Park,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GENERAL ELECTRIC COMPANY |
SCHENECTADY |
NY |
US |
|
|
Assignee: |
GENERAL ELECTRIC COMPANY
SCHENECTADY
NY
|
Family ID: |
53270651 |
Appl. No.: |
14/098864 |
Filed: |
December 6, 2013 |
Current U.S.
Class: |
428/596 ;
428/136 |
Current CPC
Class: |
C23C 4/134 20160101;
Y02T 50/6765 20180501; Y10T 428/24314 20150115; C23C 28/3215
20130101; C23C 28/3455 20130101; C23C 4/11 20160101; C04B 41/89
20130101; C23C 4/02 20130101; Y10T 428/12361 20150115; F01D 5/288
20130101; Y02T 50/672 20130101; C04B 41/009 20130101; Y02T 50/60
20130101; C04B 41/52 20130101; C04B 41/009 20130101; C04B 35/565
20130101; C04B 35/806 20130101; C04B 41/52 20130101; C04B 41/5071
20130101; C04B 41/52 20130101; C04B 41/5096 20130101; C04B 41/522
20130101; C04B 41/52 20130101; C04B 41/5024 20130101; C04B 41/52
20130101; C04B 41/4582 20130101; C04B 41/5024 20130101; C04B
2103/0021 20130101 |
International
Class: |
F01D 25/00 20060101
F01D025/00 |
Claims
1. An article comprising: A substrate; and a coating system
disposed over the substrate, wherein the coating system comprises a
first layer comprising a first material and a second layer
comprising a second material, the first layer disposed between the
second layer and the substrate, the second layer comprising
proximal and distal surfaces relative to the substrate and having a
thickness defined between the proximal and distal surfaces, and the
second layer having a distal surface region extending from the
distal surface to a depth below the distal surface of 20% of the
thickness of the second layer; wherein the first material has a
higher fracture toughness than the second material; wherein the
second material is more resistant to infiltration by nominal CMAS
relative to 8 weight percent yttria-stabilized zirconia at a
temperature of 1300 degrees Celsius; and wherein the second layer
comprises a plurality of through-thickness cracks, wherein at least
90 percent of the cracks have a mean crack opening displacement,
measured in the distal surface region, of up to about 5
micrometers.
2. The article of claim 1, wherein the first material comprises
yttria-stabilized zirconia.
3. The article of claim 2, wherein the yttria-stabilized zirconia
has a yttria content in a range from about 7 weight percent to
about 9 weight percent.
4. The article of claim 1, wherein the first material comprises
stabilized hafnia, stabilized titania, or combinations thereof.
5. The article of claim 1, wherein the first layer comprises a
silicate.
6. The article of claim 1, wherein the second material comprises
yttria-stabilized zirconia, with a yttria content greater than 38
weight percent.
7. The article of claim 6, wherein the yttria content is at least
about 55 weight percent.
8. The article of claim 1, wherein the second material comprises an
oxide.
9. The article of claim 8, wherein the oxide comprises at least one
transition metal element, at least one rare-earth element, silicon,
indium, or combinations thereof.
10. The article of claim 8, wherein the oxide comprises zirconium,
hafnium, titanium, or combinations thereof.
11. The article of claim 8, wherein the oxide comprises an
oxyapatite, a garnet, or a pyrochlore.
12. The article of claim 8, wherein the oxide comprises
gadolinium-stabilized zirconia.
13. The article of claim 1, wherein the second material comprises a
carbide, a nitride, or a silicide.
14. The article of claim 1, wherein the first layer has a porosity
in a range from about 2% to about 30%.
15. The article of claim 1, wherein the first layer has a porosity
in a range from about 2% to about 10%.
16. The article of claim 1, wherein the first layer has a porosity
in a range from about 10% to about 30%.
17. The article of claim 1, wherein the second layer has a porosity
in the range from about 2% to about 30%.
18. The article of claim 1, wherein the plurality of
through-thickness cracks has a linear crack frequency of at least
about 5 cracks per inch.
19. The article of claim 1, wherein the plurality of
through-thickness cracks has a linear crack frequency of at least
about 25 cracks per inch.
20. The article of claim 1, wherein the substrate comprises a
metal.
21. The article of claim 20, wherein the substrate comprises a
superalloy.
22. The article of claim 1, wherein the substrate comprises a
ceramic.
23. The article of claim 22, wherein the substrate comprises
silicon nitride or silicon carbide.
24. The article of claim 1, wherein the coating system further
comprises a bondcoat disposed between the first layer and the
substrate.
25. The article of claim 24, wherein the bondcoat comprises an
aluminide or an MCrAlY material.
26. The article of claim 24, wherein the bondcoat comprises a
silicide or elemental silicon.
27. The article of claim 1, wherein the second layer is at least
10% of the combined thickness of the first layer and second
layer.
28. The article of claim 1, wherein the second layer is at least
50% of the combined thickness of the first layer and second
layer.
29. The article of claim 1, wherein the article is a component of a
gas turbine assembly.
30. The article of claim 1, wherein at least 50 percent of the
cracks have a mean crack opening displacement, measured in the
distal surface region, of up to about 1.5 micrometers.
31. The article of claim 1, wherein the second layer comprises a
plurality of through-thickness cracks, wherein at least 95 percent
of the cracks have a mean crack opening displacement, measured in
the distal surface region, of up to about 5 micrometers.
32. An article comprising a substrate comprising a superalloy; and
a coating system disposed over the substrate, wherein the coating
system comprises a bondcoat comprising an aluminide or an MCrAlY
material and disposed over the substrate; a first layer, comprising
yttria-stabilized zirconia, disposed over the bondcoat; and a
second layer, comprising yttria-stabilized zirconia having at least
about 55 weight percent yttria, disposed over the first layer, the
second layer comprising proximal and distal surfaces relative to
the substrate, and the second layer having a distal surface region
extending from the distal surface to a depth below the distal
surface of 20% of the thickness of the second layer; wherein the
second layer comprises a plurality of through-thickness cracks,
wherein at least 90 percent of the cracks have a mean crack opening
displacement, measured in the distal surface region, of up to about
5 micrometers.
33. An article comprising a substrate comprising a ceramic-matrix
composite; and a coating system disposed over the substrate,
wherein the coating system comprises a bondcoat comprising a
silicide or elemental silicon and disposed over the substrate; a
first layer, comprising a silicate, disposed over the bondcoat; and
a second layer, comprising a rare earth silicate disposed over the
first layer, the second layer comprising proximal and distal
surfaces relative to the substrate, and the second layer having a
distal surface region extending from the distal surface to a depth
below the distal surface of 20% of the thickness of the second
layer; wherein the second layer comprises a plurality of
through-thickness cracks, wherein at least 80 percent of the cracks
have a mean crack opening displacement, measured in the distal
surface region, of up to about 5 micrometers.
Description
BACKGROUND
[0001] This invention relates to coatings and components for high
temperature applications, such as gas turbine assemblies.
[0002] The design of modern gas turbines is driven by the demand
for higher turbine efficiency. It is widely recognized that turbine
efficiency can be increased by operating the turbine at higher
temperatures. In order to assure a satisfactory life span at these
higher temperatures, thermal barrier coatings (hereinafter referred
to as "TBCs") are applied to airfoils and combustion components of
the turbine, such as transition pieces and combustion liners, using
various techniques.
[0003] A key concern for turbines used in both power generation and
propulsion applications is with harmful effects of ingested dust,
sand, volcanic ash, and other species entrained in turbine intake
air. These species can adhere to TBCs and damage them through the
formation of various comparatively low-melting point phases
collectively referred to as "CMAS" due to their typical inclusion
of such oxide components as calcia, magnesia, alumina, and silica.
CMAS material generally melts around 1200.degree. C. (about
2250.degree. F.), which is below the surface temperature expected
for TBC's in high-performance turbine components; once molten, the
liquid CMAS infiltrates the cracks, pores, columnar grain
boundaries, and open defects of TBCs and solidifies to form a glass
when the TBCs cool to room temperature. As a result, the TBCs lose
compliance and spall prematurely.
[0004] The industry standard 8YSZ material (zirconia stabilized
with approximately 8 weight percent yttria) used for TBCs is
particularly susceptible to degradation via CMAS. One technique to
combat spallation resulting from CMAS ingestion involves TBC
compositions with higher rare earth contents as compared to
conventional TBCs. These high-rare-earth TBCs are designed to react
with ingested CMAS and thereby limit its penetration. These high
rare earth TBCs, however, have lower fracture toughness than
conventional YSZ-based thermal barrier coatings, and thus, while
attractive for some turbine applications, simply changing the
chemistry of the coating may not be an ideal solution for all
turbine designs.
[0005] As a result of the above, a need persists in the industry
for thermal barrier coatings and related methods for fabricating
coated components, where the coatings are resistant to CMAS
ingestion (i.e., spallation resistant), include high strain
tolerance, are scalable (i.e., compatible with large components),
and are relatively inexpensive as compared with conventional
thermal bather coatings.
BRIEF DESCRIPTION
[0006] Embodiments of the present invention are provided to meet
this and other needs. One embodiment is an article. The article
comprises a substrate; and a coating system disposed over the
substrate. The coating system comprises a first layer comprising a
first material and a second layer comprising a second material,
with the first layer disposed between the second layer and the
substrate. The second layer comprises proximal and distal surfaces
relative to the substrate, and has a thickness defined between the
proximal and distal surfaces; this second layer also has a distal
surface region extending from the distal surface to a depth below
the distal surface of 20% of the thickness of the second layer. The
first material has a higher fracture toughness than the second
material, and the second material is more resistant to infiltration
by a nominal CMAS composition relative to 8 weight percent
yttria-stabilized zirconia at a temperature of 1300 degrees
Celsius. The second layer comprises a plurality of
through-thickness cracks, wherein at least 90 percent of the cracks
have a mean crack opening displacement, measured in the distal
surface region, of up to about 5 micrometers.
[0007] Another embodiment is an article comprising a substrate
comprising a superalloy; and a coating system disposed over the
substrate. The coating system comprises a bondcoat comprising an
aluminide or an MCrAlY material and disposed over the substrate; a
first layer, comprising yttria-stabilized zirconia, disposed over
the bondcoat; and a second layer, comprising yttria-stabilized
zirconia having at least about 55 weight percent yttria, disposed
over the first layer, the second layer comprising proximal and
distal surfaces relative to the substrate, and the second layer
having a distal surface region extending from the distal surface to
a depth below the distal surface of 20% of the thickness of the
second layer. The second layer comprises a plurality of
through-thickness cracks, wherein at least 90 percent of the cracks
have a mean crack opening displacement, measured in the distal
surface region, of up to about 5 micrometers.
[0008] Another embodiment is an article comprising a substrate
comprising a ceramic-matrix composite; and a coating system
disposed over the substrate. The coating system comprises a
bondcoat comprising a silicide or elemental silicon and disposed
over the substrate; a first layer, comprising a silicate, disposed
over the bondcoat; and a second layer, comprising a rare earth
silicate disposed over the first layer. The second layer comprises
proximal and distal surfaces relative to the substrate, and has a
distal surface region extending from the distal surface to a depth
below the distal surface of 20% of the thickness of the second
layer. The second layer comprises a plurality of through-thickness
cracks, wherein at least 80 percent of the cracks have a mean crack
opening displacement, measured in the distal surface region, of up
to about 5 micrometers.
DRAWINGS
[0009] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0010] FIG. 1 is a schematic cross-section view of an article in
accordance with some embodiments described herein; and
[0011] FIG. 2 is a graph showing crack opening displacement data
for specimens fabricated under different processing conditions.
DETAILED DESCRIPTION
[0012] Embodiments of the present invention include a coating
having a unique microstructure that provides desirable resistance
to CMAS infiltration while maintaining desirable levels of
adhesion, strain tolerance, and other mechanical properties. The
coating may include a wide range of materials, including stabilized
zirconia systems, and may be deposited via scalable processes such
as plasma spray techniques.
[0013] In one embodiment, as depicted schematically in FIG. 1, an
article 100 comprises a substrate 102 and a coating system 104
disposed over the substrate. Substrate 102 may be a
high-temperature material, such as a metal or a ceramic. Examples
of high-temperature metals include superalloys, such as
cobalt-based superalloys and nickel-based superalloys. Ceramic
material examples include silicon-bearing materials such as silicon
nitride and silicon carbide. One particular example is a ceramic
matrix composite made from a silicon-carbide-bearing matrix and
reinforced with silicon carbide fibers, whiskers, or particles.
Article 100 may be a component of a gas turbine assembly; examples
include, without limitation, stationary and rotating components
exposed to the hot gas path through a gas turbine assembly, such as
vanes, blades, shrouds, combustors, or transition pieces.
[0014] Coating system 104 comprises a first layer 106 comprising a
first material, and a second layer 108 comprising a second
material. First layer 106 is disposed between the second layer and
the substrate. The first material has a higher fracture toughness
than the second material to provide desirable levels of strain and
damage tolerance to coating system 104. The second material is a
CMAS-resistant material, meaning that the second material is more
resistant to infiltration at 1300.degree. C. by molten "nominal
CMAS" than is 8YSZ. Some of these materials owe their effective
resistance to CMAS infiltration due to their tendency to react very
slowly with CMAS, while others react very quickly but rapidly form
a reaction product that effectively seals off the material from
further exposure to the CMAS. In either case, the material
demonstrates overall degradation rates at 1300.degree. C. that are
less than what is known in the art to be the case for 8YSZ. For the
purposes of this description, the term "nominal CMAS" refers to the
following composition, with all percentages in mole percent: 41.7%
silica (SiO.sub.2), 29.3% calcia (CaO), 12.5% alumina
(AlO.sub.1.5), 9.1% magnesia (MgO), 6.0% iron oxide (FeO.sub.1.5),
and 1.5% nickel oxide (NiO). It will be appreciated that the
1300.degree. C. temperature and the nominal CMAS composition given
in this definition represent a reference temperature and a
reference composition to define a benchmark for the material's CMAS
resistance in a way that can be compared to the CMAS resistance of
8YSZ; use of these reference values does not limit in any way the
actual temperature at which article 100 may operate or the actual
composition of ingested material that becomes deposited on the
coating during operation, both of which, of course, will vary
widely in service.
[0015] Second layer 108, as shown in cross section in FIG. 1, has a
proximal surface 110 and a distal surface 112 relative to substrate
102, with a thickness 114 defined between these two surfaces. In
some embodiments, thickness 114 of second layer 108 is at least 10%
of the combined thickness of first layer 106 and second layer 108.
In particular embodiments, thickness 114 is at least 50% of the
combined thickness of first layer 106 and second layer 108. A
comparatively thicker second layer may provide greater resistance
to CMAS infiltration due to, for instance, higher tortuosity and
chemical reaction surface area. The overall thickness of coating
system 104 is expected to vary depending on the particular
application, much as conventional coating thickness varies in the
industry; typical total thickness is up to about 1000
micrometers.
[0016] A plurality of cracks 116 runs through the thickness 114 of
second layer 108. Typically these cracks run through at least 75%
of thickness 114 and are generally oriented substantially
vertically, meaning within about 45 degrees to either side of
perpendicular relative to proximal surface 110. These cracks 116
are thus referred to herein as "through-thickness cracks," and
provide strain tolerance to second layer 108.
[0017] The presence of through-thickness cracks in second layer 108
is contrary to conventional wisdom in the art, which typically
envisions a continuous sealing layer to be applied to the task of
isolating the CMAS from the underlying TBC. See, for example, U.S.
Pat. No. 7,875,370. Such structures generally lack the degree of
strain tolerance attributable to vertically cracked coatings.
However, the present inventors have made the surprising discovery
that even cracked coatings may sufficiently mitigate CMAS
infiltration to allow the use of more strain-tolerant
architectures, so long as the population of through-thickness
cracks are engineered appropriately, in accordance with the
descriptions herein.
[0018] A distal surface region 118 is herein defined within second
layer 108 as extending from the distal surface 112 to a depth of
20% of the thickness 114 below the distal surface 112. In
embodiments of the present invention, at least 90 percent of the
cracks have a mean crack opening displacement, measured in the
distal surface region, of up to about 5 micrometers. As described
in more detail below, coatings with cracks meeting this criterion
remarkably resisted CMAS infiltration, while coatings having cracks
outside of the stated criterion showed detrimental levels of CMAS
infiltration and resultant degradation. In particular embodiments,
at least 95% of the cracks have a mean crack opening displacement,
measured in the distal surface region, of up to about 5
micrometers. In certain embodiments, in addition to the 90% or 95%
criteria discussed above, at least 50 percent of the cracks 116
have a mean crack opening displacement, measured in the distal
surface region 118, of up to about 1.5 micrometers. In some
embodiments, 50% of the cracks measured in the distal surface
region 118 have a mean crack opening displacement of up to about
1.25 micrometers.
[0019] "Mean crack opening displacement" of a through-thickness
crack, as used herein, is defined to be the arithmetic mean of at
least 14 measurements of a given crack's width taken per 10
micrometers of thickness within the defined distal surface region
118. A method for measuring crack opening displacement, based on an
image analysis technique, may be applied to obtain the mean crack
opening displacement, and a more detailed account of one such
technique is provided in the Examples section of this description,
below.
[0020] The results noted herein are consistent with a mechanism
whereby the infiltration of CMAS into the cracks of a coating is
modeled as a competition between capillary forces drawing liquid
into a crack, on the one hand, and a repellant back-pressure
generated by gas trapped within the crack on the other hand. Thus
it can be theorized that, if this mechanism is indeed operating,
sufficiently thin cracks may trap enough gas to balance the
capillary force drawing the fluid into the crack, thereby limiting
or eliminating the degree of CMAS penetration that is possible.
This phenomenon allows the use of remarkably strain tolerant,
vertically cracked coatings in second layer 108 that maintain
desired levels of CMAS infiltration resistance.
[0021] The number of through-thickness cracks 116 present in second
layer 108 may directly affect the strain tolerance of second layer
108, in that more cracks (that is, a higher linear frequency, also
referred to in the art as "linear density," of cracks intersecting
a theoretical horizontal line drawn at the 75% thickness point
relative to distal surface 112) typically results in higher strain
tolerance for the layer. Thus, in one embodiment, the plurality of
through-thickness cracks 116 in second layer 108 has a linear crack
frequency of at least about 5 cracks per inch (about 2 cracks per
centimeter). In some embodiments, this crack frequency is at least
about 25 cracks per inch (about 10 cracks per cm), and in
particular embodiments, the frequency is at least about 100 cracks
per inch (about 40 cracks per centimeter). Those skilled in the art
typically use a measurement length of at least about 25 mm to
obtain a reasonably representative measurement of linear crack
frequency.
[0022] In addition to vertical cracks, coating porosity is also
known in the art to contribute some measure of strain tolerance to
ceramic coatings of the type described for coating system 104. In
one embodiment, second layer 108 has a porosity in the range from
about 2% by volume to about 30% by volume. Thus it is not necessary
for second layer to have a true "dense vertically cracked"
microstructure as that term is used in the art, though such an
embodiment is not precluded. Regarding porosity in first layer 106,
experimental results suggest both conventional "porous" layers
(porosity generally approximately 10%-30%) and "dense vertically
micro-cracked" layers (porosity generally approximately 2%-10%)
have shown desirable results in coating system 104. Thus in some
embodiments, the porosity of first layer 106 is in the range from
about 2% to about 30% by volume, and in two alternative
embodiments, the porosity is in the range from about 2% by volume
to about 10% by volume, or from about 10% to about 30%. In
particular embodiments, first layer 106 has either a columnar
structure with elongated coating growth domains, or a dense,
equiaxed matrix with vertical micro-cracks, both of which coating
types are associated with liquid-injection plasma spray processes
in accordance with U.S. Pat. No. 8,586,172. These particular
coating types are noted for their high adhesion and strain
tolerance.
[0023] As noted above, the first material, which is present in
first layer 106, has higher fracture toughness than the second
material, which is present in second layer 108. Fracture toughness
of the layers may be characterized relative to one another, for
example, according to one of several standard techniques, such as
indentation techniques, known and widely used in the art. The first
material may be a ceramic material, and in some embodiments is a
material used in thermal barrier coatings or other high temperature
applications. Yttria-stabilized zirconia, including YSZ having a
yttria content in the range from about 7 to about 9 weight percent,
is a well-known example of such a material, as are hafnia and
titania (including stabilized compositions that include these
oxides). In some embodiments, the first layer is a material, such
as a silicate, commonly used in recession-resistant environmental
barrier coating applications and often associated with
silicon-bearing substrates exposed to high temperatures; examples
of such silicate coating materials include barium strontium
aluminosilicate and rare-earth disilicates and monosilicates. As
used herein, the term "rare-earth" will be understood to include
not only the lanthanide series elements, but scandium and yttrium
as well. Specific examples include yttrium disilicate and yttrium
monosilicate.
[0024] The second material, used in second layer 108, is applied
for CMAS resistance, as noted above. Many different materials have
been described in the art as providing enhanced CMAS protection
relative to yttria-stabilized zirconia and other standard TBC
materials, and any of these materials may be considered for use in
the coating system described herein.
[0025] In one embodiment, the second material includes an oxide.
Oxides that include one or more transition metal elements,
rare-earth elements, silicon, and/or indium have been described in
the art as being resistant to CMAS. In one embodiment, the oxide
includes zirconium, hafnium, titanium, or combinations thereof.
Zirconia, hafnia, and/or titania materials stabilized with one or
more rare-earth elements have been described in the art of
CMAS-resistant coatings. Examples of such materials include
coatings containing gadolinia and zirconia, such as
gadolinia-stabilized zirconia; and coatings containing mixtures of
gadolinia and hafnia. Examples of other potentially suitable oxide
materials include pyrochlores, such as lanthanum zirconate;
garnets, such as those described in U.S. Pat. No. 7,722,959; and
oxyapatites, such as those described in U.S. Pat. No. 7,722,959.
Sodium-containing oxides, such as sodium oxide, sodium silicate,
and sodium titanate, are other examples of CMAS resistant oxide
materials.
[0026] In one particular example, the second material includes
yttria-stabilized zirconia having higher yttria content (relative
to the overall YSZ content) than typical 8YSZ. Generally, the
yttria content in this example is greater than 38 weight percent,
and in specific embodiments the yttria content is at least about 55
weight percent. Coatings as described herein using YSZ with yttria
content greater than 38 weight percent were superior to coating
made with lower-yttria YSZ materials.
[0027] Other materials besides oxides have been described for use
in resisting CMAS, and are also considered as potentially useful as
second materials in coating system 104. Examples of such
alternative materials include carbides (such as silicon carbide,
tantalum carbide, titanium carbide, and others), nitrides (such as
silicon nitride, zirconium nitride, tantalum nitride, boron
nitride, and others), and silicides (such as chromium silicide,
molybdenum silicide, tantalum silicide, titanium silicide, and
others).
[0028] The materials described herein for both layers 106, 108 of
coating system 104 may be applied to substrate 102 with the
requisite microstructure using any of various deposition techniques
commonly used in industry for the application of ceramic coatings
for use in high-temperature components. Plasma spray techniques,
including techniques applying liquid feedstock injection, are
particularly attractive due to their scalability and relatively
well understood relationships among processing parameters, coating
structure, and resultant properties. For example, crack opening
displacement and linear frequency are often controlled by
parameters, such as (but not limited to) feedstock feed rate and
particle size distribution, particle velocity, gun-to-substrate
distance, substrate temperature, and plasma temperature, that
affect particle temperature and deposition rate during deposition.
Typically, hotter deposition temperatures tend to promote higher
crack frequency, but may also promote more highly developed, that
is, wider, cracks. Thus a plasma spray method for fabricating any
particular coating system 104 will require some development of the
proper combination of process parameters to develop a consistently
acceptable microstructure as described herein.
[0029] A bondcoat 120 is disposed between first layer 106 and
substrate 102 in some embodiments. Bondcoat 120 provides
functionality--adhesion promotion and oxidation resistance, for
example--in coating system 104 similar to what such coatings
generally provide in conventional applications. In some
embodiments, bondcoat 120 comprises an aluminide, such as nickel
aluminide, or a MCrAlY-type coating well known in the art. These
bondcoats may be especially useful when applied to a metallic
substrate 102, such as a superalloy. In other embodiments, bondcoat
comprises a silicide compound or elemental silicon, which are often
associated with ceramic-based substrates, such as silicon carbide
reinforced silicon carbide ceramic matrix composites (CMC's). These
coatings may be applied using any of various coating techniques
known in the art, such as plasma spray, thermal spray, chemical
vapor deposition, or physical vapor deposition.
[0030] In a particular embodiment, associated with a metallic-based
component--for example, a component of a gas turbine assembly--an
article 100 includes a substrate 102 that includes a superalloy,
and a multi-layered coating system 104 disposed over substrate 102.
Coating system 104 includes a bondcoat 120 that comprises an
aluminide or an MCrAlY-type material; a first layer 106, disposed
over the bondcoat 120, that includes yttria-stabilized zirconia
(such as YSZ with nominally 7-9 weight percent yttria content); and
a second layer 108 disposed over first layer 106 and including
yttria-stabilized zirconia having at least about 55 weight percent
yttria content. Second layer 108 includes a plurality of
through-thickness microcracks 116 having the characteristics set
forth previously for such cracks 116. Such an embodiment may
provide, among other things, durable thermal protection for
metallic components used in high-temperature environments.
[0031] In another particular embodiment, associated with a
ceramic-based component, which, like the above example, may include
a component for a gas turbine assembly, an article 100 includes a
substrate comprising a ceramic-matrix composite, such as a silicon
carbide reinforced composite having a silicon carbide matrix; and a
multi-layered coating system 104 disposed over substrate 102.
Coating system 104 includes a bondcoat 120 comprising a silicide or
elemental silicon; a first layer 106, comprising a silicate,
disposed over bondcoat 120; and a second layer 108 comprising a
rare earth monosilicate disposed over the first layer. Second layer
108 includes a plurality of through-thickness microcracks 116
having the characteristics set forth previously for such cracks
116. Such an embodiment may provide, among other things, durable
environmental protection for silicon-bearing ceramic components,
such as ceramic-matrix composites, used in moisture-containing,
high-temperature environments.
EXAMPLES
[0032] The following examples are presented to further describe the
fabrication of coatings of the present invention, but should not be
read as limiting, because variations still within the scope of
embodiments of the present invention will be apparent to those
skilled in the art.
Example 1
A Well-Performing Coating
[0033] Thermal bather coatings were deposited onto 25 mm diameter,
3 mm thick Rene N5 alloy substrates that had approximately 150
microns of NiCrAlY bondcoat applied utilizing an air plasma spray
process, which provided a surface roughness of about 400 microinch
Ra. The first thermal barrier layer was deposited to a thickness of
430 microns onto the bondcoat surface. An 8 weight percent
yttria-92 weight percent zirconia composition with a particle size
median diameter (d50) of about 2.2 microns was suspended in ethanol
at 20 wt % solids using polyethyleneimine as a dispersant (at 0.2
wt % of the solids). The suspension was injected into a Northwest
Mettech Axial III torch through the center tube of a tube-in-tube
atomizing injector with a nitrogen atomizing gas sent through the
outer tube. A 3/8'' diameter nozzle was used at the end of the
plasma torch. The suspension feed rate was 24 ml/min. The plasma
torch was rastered across the substrate at 600 mm/sec, with stripes
spaced at 4 mm intervals. Spray distance between the torch nozzle
and the substrate samples was 75 mm. Plasma conditions used were
300 standard liter per minute (slpm) total gas flow with 10%
nitrogen, 15% hydrogen, and 75% argon volumetric flow fractions.
Plasma conditions used were 300 slpm total gas flow with 75%
nitrogen, 15% hydrogen, and 10% argon volumetric flow ratios. A
current of 200 A was used for each of the three electrodes,
resulting in a total gun power of approximately 98 kW.
[0034] On top of the first 8YSZ layer, a second layer with a
composition of 55 weight percent yttria-45 weight percent zirconia
and a particle size d50 of about 1.5 microns was deposited to a
thickness of about 370 microns. This layer was deposited using the
same plasma conditions as used for the previous 8YSZ layer.
Example 2
A Second Well-Performing Coating
[0035] As in Example 1, similar substrates and bondcoats were used.
In this example, the first TBC 8YSZ layer deposited was
approximately 300 microns thick and was applied using a
conventional air plasma spray process with Sulzer Metco 204NS
powder. In this example, the second layer of 55 weight percent
yttria-45 weight percent zirconia ("55YSZ"), applied over the first
8YSZ layer, was deposited using the same process as described in
Example 1, but was deposited to a thickness of about 200
microns.
Example 3
A Poor-Performing Coating
[0036] A coating was produced using the same substrate and NiCrAlY
bondcoat as described in Example 1. As in Example 1, the first TBC
layer was also an 8YSZ composition, but with a d50 particle size of
about 0.7 microns. It was deposited to a thickness of about 180
microns using similar conditions to those described in Example 1. A
second layer of 55YSZ was applied over the 8YSZ layer to a
thickness of about 660 microns. In this example the particle size
d50 was about 0.5 microns and the coating deposited with the plasma
torch angled 20 degrees off of perpendicular to the surface.
Example 4
Crack Opening Displacement Measurement
[0037] The samples generated in the previous examples were imaged
with a Nikon Epiphot 200 inverted microscope using reflected light
illumination. A 50.times. objective was used to maximize resolution
of the crack morphology. A Lumenera digital camera with a
resolution of 1600 by 1200 pixels was used to capture images and
was interfaced to a computer using Clemex Vision image analysis
software.
[0038] The 8-bit 256 grey level image was captured and stored into
the image analysis system. The image was segmented into a binary
image (black and white) by selecting the characteristic grey level
range of the cracks. In this way crack features could be
distinguished from the background or matrix microstructure. The
field of view was typically about 100 .mu.m below the surface.
[0039] There were several processing steps to eliminate small pores
and extraneous cracks so that one is left with the extended cracks
for analysis. Detected features less than 50 .mu.m and with an
aspect ratio less than 5 were eliminated. Once this size and shape
filtering was implemented, the remaining feature images were
dilated and then eroded in order to close off small internal gaps
within the detected crack feature and to connect discontinuities.
At this stage the remaining long cracks were processed using an
algorithm to eliminate most branching (horizontal) crack structures
from the longer main spine of the major detected cracks. This
processing removed over 90% of the branches. Any remaining branches
were manually removed to ensure that only the central spines of the
major cracks were being measured.
[0040] The detected major cracks were then measured with test lines
that were automatically generated using the built-in algorithm
designated Thickness Grid Per Object. However, to ensure that the
test lines were in the correct orientation relative to the crack, a
processing step was added which smoothed the edges of the crack
while artificially expanding the crack thickness.
[0041] The generated test lines spanned across the crack width, and
were spaced at user-defined measurement intervals of approximately
0.5 microns. The test lines were combined with the processed binary
image of the main cracks using a Boolean operation so that only the
segment of the test line that coincided with the processed crack
image would remain.
[0042] The image analysis software measured the length of the test
lines which corresponded with the actual crack thickness dimensions
in a sequential manner along with the X and Y centroid of each test
line. The measurements were compiled with thickness variation
versus distance from edge generated.
Example 5
CMAS Resistance Testing
[0043] Specimens described above were tested in a high-temperature
thermal gradient test facility where combustion flame conditions
and bare metal backside cooling conditions were adjusted to produce
an outer coating surface temperature of about 1400 degrees Celsius
(about 2550 degrees Fahrenheit) and bare metal backside temperature
of about 870 degrees Celsius (about 1600 degrees Fahrenheit). The
outer coating surfaces were coated with 24 mg/cm.sup.2 of a CMAS
composition and then exposed directly to the flame, whereupon the
CMAS melted and infiltrated into the outer coating layer. Following
infiltration, specimens were exposed for an additional 10 thermal
gradient exposures consisting of 10 minutes flame exposures each.
The amount of coating spalled from each of the specimen surfaces
was measured. Specimens deemed to be good performers as in examples
1 and 2 exhibited approximately 4 times lower volume of coating
spalled compared to example 3.
[0044] FIG. 2 shows cumulative distributions of the measured crack
opening displacement for the two coatings with good performance
(solid lines) and the coating with poor performance (dashed line).
Good performing coatings were characterized by the absence of wide
cracks, as noted above.
[0045] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
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