U.S. patent application number 14/386492 was filed with the patent office on 2015-02-12 for method of forming thermal barrier coating, thermal barrier coating formed thereby, and article comprising same.
The applicant listed for this patent is The University of Connecticut. Invention is credited to Maurice Gell, Eric Jordan, Jeffrey Roth.
Application Number | 20150044444 14/386492 |
Document ID | / |
Family ID | 49483796 |
Filed Date | 2015-02-12 |
United States Patent
Application |
20150044444 |
Kind Code |
A1 |
Gell; Maurice ; et
al. |
February 12, 2015 |
METHOD OF FORMING THERMAL BARRIER COATING, THERMAL BARRIER COATING
FORMED THEREBY, AND ARTICLE COMPRISING SAME
Abstract
A thermal barrier coating that includes a YAG-based ceramic is
prepared by a solution precursor plasma spray method that includes
injecting a precursor solution into a thermal jet, evaporating
solvent from the precursor solution droplets, and pyrolyzing the
resulting solid to form a YAG-based ceramic that is melted and
deposited on a substrate. The thermal barrier coating can include
through-coating-thickness cracks that improve the strain tolerance
of the coating.
Inventors: |
Gell; Maurice; (Somerset,
NJ) ; Jordan; Eric; (Storrs, CT) ; Roth;
Jeffrey; (Bolton, CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The University of Connecticut |
Farmington |
CT |
US |
|
|
Family ID: |
49483796 |
Appl. No.: |
14/386492 |
Filed: |
April 22, 2013 |
PCT Filed: |
April 22, 2013 |
PCT NO: |
PCT/US2013/037525 |
371 Date: |
September 19, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61636755 |
Apr 23, 2012 |
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61708226 |
Oct 1, 2012 |
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Current U.S.
Class: |
428/220 ;
415/208.1; 415/230; 416/241R; 427/453; 428/312.8; 501/127;
60/752 |
Current CPC
Class: |
Y10T 428/24997 20150401;
C23C 4/134 20160101; F05D 2300/2112 20130101; Y10T 428/24471
20150115; C04B 35/44 20130101; C23C 28/36 20130101; F01D 5/288
20130101; Y02T 50/6765 20180501; C23C 4/10 20130101; C23C 28/3215
20130101; C23C 4/02 20130101; C23C 28/3455 20130101; Y02T 50/60
20130101; C23C 4/11 20160101; F02C 7/24 20130101 |
Class at
Publication: |
428/220 ;
416/241.R; 415/208.1; 415/230; 60/752; 427/453; 501/127;
428/312.8 |
International
Class: |
C23C 4/10 20060101
C23C004/10; C23C 4/12 20060101 C23C004/12; C23C 4/02 20060101
C23C004/02; F01D 5/28 20060101 F01D005/28 |
Claims
1. A method of forming a thermal barrier coating, comprising:
injecting a precursor solution into a thermal jet; wherein the
precursor solution comprises metal ion precursors to a YAG-based
ceramic; evaporating solvent from the precursor solution in the
thermal jet to form unpyrolyzed solid particles; pyrolyzing at
least a portion of the unpyrolyzed solid particles in the thermal
jet to form pyrolyzed solid particles comprising a YAG-based
ceramic; melting at least a portion of the pyrolyzed solid
particles in the thermal jet to form droplets comprising the
YAG-based ceramic; and depositing the droplets comprising the
YAG-based ceramic on a substrate to form a thermal barrier
coating.
2-4. (canceled)
5. The method of claim 1, wherein the YAG-based ceramic has the
empirical formula Y.sub.3Al.sub.5-xFe.sub.xO.sub.12, where x can
vary continuously from 0 to 5.
6. The method of claim 1, wherein the YAG-based ceramic comprises
yttrium aluminum garnet (Y.sub.3Al.sub.5O.sub.12).
7. The method of claim 1, wherein the precursor solution is a
homogeneous solution.
8. The method of claim 1, wherein the metal ion precursors to the
YAG-based ceramic are provided in the form of a salt selected from
the group consisting of carboxylate salts, alkoxide salts,
carbonate salts, halide salts, nitrate salts, hydrates of the
foregoing salts, and combinations thereof.
9. The method of claim 1, wherein the precursor solution comprises
yttrium nitrate and aluminum nitrate.
10. The method of claim 1, wherein said depositing the droplets
comprising the YAG-based ceramic on the substrate forms splats on
the substrate, the splats comprising the YAG-based ceramic; wherein
the splats have an average diameter less than or equal to 5
micrometers.
11. The method of claim 1, wherein the thermal barrier coating has
a thickness of about 1 to about 5,000 micrometers.
12. The method of claim 1, further comprising incompletely
pyrolyzing at least a portion of the unpyrolyzed solid particles in
the thermal jet to form incompletely pyrolyzed solid particles;
depositing the incompletely pyrolyzed solid particles and/or the
unpyrolyzed solid particles on the substrate; and pyrolyzing the
incompletely pyrolyzed solid particles and/or the unpyrolyzed solid
particles on the substrate.
13. The method of claim 1, wherein said pyrolyzing the unpyrolyzed
solid particles and/or incompletely pyrolyzed solid particles on
the substrate forms through-coating-thickness microcracks in the
thermal barrier coating.
14. The method of claim 1, wherein the thermal barrier coating has
a thickness; and wherein the through-coating-thickness microcracks
are spaced from each other at a distance, on average, of less than
or equal to half the coating thickness.
15. The method of claim 1, wherein the through-coating-thickness
microcracks have a width of about 0.1 to about 5 micrometers.
16. The method of claim 1, wherein the thermal barrier coating has
a porosity of about 10 to about 40 volume percent based on the
total volume of the thermal barrier coating.
17. The method of claim 1, wherein the thermal barrier coating
comprises inter-pass boundaries.
18. The method of claim 1, wherein the YAG-based ceramic has the
empirical formula Y.sub.3Al.sub.5-xFe.sub.xO.sub.12, where x can
vary continuously from 0 to 5; wherein the precursor solution is a
homogeneous solution; wherein the precursor solution comprises
yttrium nitrate and at least one of aluminum nitrate and ferric
nitrate; wherein the thermal barrier coating has a thickness of
about 50 to about 5,000 micrometers; wherein the method further
comprises depositing incompletely pyrolyzed solid particles and/or
the unpyrolyzed solid particles on the substrate and pyrolyzing the
incompletely pyrolyzed solid particles and/or the unpyrolyzed solid
particles on the substrate; wherein the thermal barrier coating
comprises through-coating-thickness microcracks having a width of
about 0.1 to about 5 micrometers; wherein the
through-coating-thickness microcracks are spaced from each other at
a distance, on average, of less than or equal to half the coating
thickness; and wherein the thermal barrier coating has a porosity
of about 10 to about 40 volume percent based on the total volume of
the thermal barrier coating.
19. A thermal barrier coating prepared by the method of claim
1.
20. A thermal barrier coating having a thickness of about 50 to
about 5,000 micrometers and comprising through-coating-thickness
microcracks having a width of about 0.1 to about 5 micrometers and
spaced from each other at a distance, on average, of less than or
equal to half the coating thickness; wherein the thermal barrier
coating comprises a YAG-based ceramic.
21. The thermal barrier coating of claim 20, wherein the YAG-based
ceramic has the empirical formula
Y.sub.3Al.sub.5-xFe.sub.xO.sub.12, where x can vary continuously
from 0 to 5; and wherein the thermal barrier coating comprises
inter-pass boundaries and splats having an average diameter less
than or equal to 5 micrometers.
22. An article comprising the thermal barrier coating of claim
20.
23. The article of claim 22, wherein the article is used in the hot
section of a gas turbine and selected from turbine blades, turbine
vanes, turbine blade outer air seals, and combustor liner segments.
Description
BACKGROUND OF THE INVENTION
[0001] The thermodynamic efficiency of gas turbines can be improved
significantly by maximizing inlet temperature and/or reducing the
volume of air required for cooling airfoils. The most promising
approach to achieve higher operating temperatures while protecting
the superalloy substrates from degradation and failure is the use
of thermal barrier coatings. Yttria-stabilized zirconia with about
7 mole percent yttria (7YSZ) has been recognized as a preferred
material for thermal barrier coatings due to a combination of
outstanding materials properties such as thermodynamic stability in
the operating environment up to 1200.degree. C., very low thermal
conductivity, relatively high thermal expansion coefficient, and
the ability to be processed with relatively high strain-tolerance
through coating techniques such as air plasma spray (APS) and
electron-beam physical vapor deposition (EB-PVD). See, e.g., M.
Gell, E. H. Jordan, M. Teicholz, B. M. Cetegen, N. P. Padture, L.
Xie, D. Chen, X. Ma, and J. Roth, "Thermal Barrier Coatings Made by
the Solution Precursor Plasma Spray Process," Journal of Thermal
Spray Technology, volume 17, no. 1, pages 124-135 (2008). The
practical use of 7YSZ-based thermal barrier coatings is limited to
a maximum operating temperature of approximately 1200.degree. C.
due to its phase instability above 1200.degree. C. APS-processed
7YSZ consists of predominantly metastable t' phase at room
temperature. Upon high-temperature exposure above 1200.degree. C.
for extended periods of time, the t' phase separates into
yttrium-deficient tetragonal (t) and yttrium-rich cubic (c)
fluorite phases. Upon cooling, the tetragonal phase undergoes
martensitic transformation to a monoclinic phase, with a volume
expansion of about four to five percent that has a detrimental
effect on the thermal barrier coating quality leading to spallation
and failure. See V. Luigi and D. R. Clark, "High temperature aging
of YSZ coatings and subsequent transformation at low temperature,"
Surface & Coatings Technology, volume 200, no. 5-6, pages
1287-1291 (2005).
[0002] New materials that can meet the performance standards of
7YSZ but can extend the operating window to higher temperatures,
for example to 1300.degree. C. or above, can facilitate the
development of turbines that can operate at elevated temperature. A
variety of materials have been proposed and evaluated as higher
temperature thermal barrier coatings candidates, but none has yet
emerged as a clear winner due to the difficulty in meeting the
combination of factors required to be a viable high-temperature
thermal barrier coating material. See Z. O. Cao, R. Vassen, and D.
Sroever, "Ceramic material for thermal barrier coatings," Journal
of European Ceramic Society, volume 24, pages 1-10 (2004); D.
Stover, G. Pracht, H. Lehmann, M. Dietrich, J-E Doming, and R.
Vassen, "New material concepts for the next generation of plasma
sprayed thermal barrier coatings," Journal of Thermal Spray
Technology, volume 13, no. 1, pages 76-83 (2004); and R. Vassen, A.
Stuke and D. Stover, "Recent Developments in the Field of Thermal
Barrier Coatings," Journal of Thermal Spray Technology, volume 18,
no. 2, pages 181-186 (2009). In addition to meeting the primary
requirement of a thermal conductivity less than that of 7YSZ, any
candidate thermal barrier coating system must in addition possess
the properties of resistance to fuel-based contaminants, adequate
erosion resistance, higher temperature phase stability to avoid
generally catastrophic volume changes associated with phase change,
and compatibility with the thermally grown oxide (TGO) which forms
on the substrate. An extensive literature exists on alternate
composition thermal barrier coatings. See, e.g., C. G. Levi,
"Emerging materials and processes for thermal barrier systems,"
Current Opinion in Solid State and Materials Science, volume 8,
pages 77-91 (2004); and D. R. Clarke and S. R. Phillpot, "Thermal
barrier coating materials," Materials Today, June 2005 pages 22-29.
The most commonly studied systems include (1) doped zirconia-based
systems with dopants that can prevent the detrimental phase
transformations, (2) zirconates, (3) fluorites, and (4) two-layer
structures containing a YSZ inner layer (in contact with the
substrate) and a zirconate or fluorite outer layer. A key
conclusion from a review of this literature is that most of the
seriously considered alternative systems have some, but not all,
properties superior to the baseline YSZ. For example, phase
stability studies have indicated that all the pyrochlore zirconates
are prone to degrade the TGO by interdiffusion, requiring
incorporation of a compatible "diffusion barrier", typically 7YSZ,
for safe implementation. Zirconates have also shown limited
stability under burner rig conditions. Additional issues arise when
considering environmental effects on alternative thermal barrier
coating materials. It has also been reported that alternate thermal
barrier coating compositions generally exhibit lower erosion
resistance than 7YSZ. For example, the following erosion rates
relative to 7YSZ (rate=1) were measured in EB-PVD coatings: 20YSZ
(129), CeO2 (35), 12YCeO2 (22), and 12YSZ (2.3). See M. J. Maloney,
H. S. Achter, and R. H. Barkalow, "Development of ceria-based low
thermal conductivity thermal barrier coatings," Proceedings of the
1997 Thermal Barrier Coating Workshop (1997). Several multilayer
thermal barrier coating systems have also been proposed, but these
systems face challenges associated with higher processing costs and
complexity unless simple, highly durable, and reliable systems can
be processed in a relatively fast and effective manner.
[0003] Thus, there remains a need for thermal barrier coatings that
are thermally stable above 1200.degree. C. without substantially
compromising other beneficial attributes of 7YSZ coatings.
BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION
[0004] One embodiment is a method of forming a thermal barrier
coating, comprising: injecting a precursor solution into a thermal
jet; wherein the precursor solution comprises metal ion precursors
to a YAG-based ceramic; evaporating solvent from the precursor
solution in the thermal jet to form unpyrolyzed solid particles;
pyrolyzing at least a portion of the unpyrolyzed solid particles in
the thermal jet to form pyrolyzed solid particles comprising a
YAG-based ceramic; melting at least a portion of the pyrolyzed
solid particles in the thermal jet to form droplets comprising the
YAG-based ceramic; and depositing the droplets comprising the
YAG-based ceramic on a substrate to form a thermal barrier
coating.
[0005] Another embodiment is a thermal barrier coating prepared by
the method.
[0006] Another embodiment is a thermal barrier coating having a
thickness of about 50 to about 5,000 micrometers and comprising
through-coating-thickness microcracks having a width of about 0.1
to about 5 micrometers and spaced from each other at a distance, on
average, of less than or equal to half the coating thickness;
wherein the thermal barrier coating comprises a YAG-based
ceramic.
[0007] Another embodiment is an article comprising the thermal
barrier coating.
[0008] These and other embodiments are described in detail
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a schematic illustration of a solution precursor
plasma spray (SPPS) process.
[0010] FIG. 2 is a schematic illustration of the crystal structure
of garnets, showing "A", "C", and "D" sites occupied by metal ions
(oxygen atoms are not shown).
[0011] FIG. 3 is a scanning electron micrograph of a section of an
yttrium aluminum garnet (YAG) coating exhibiting a "feathery"
microstructure.
[0012] FIG. 4 is a scanning electron micrograph of a section of an
SPPS YAG coating exhibiting inter-pass boundaries and vertical
cracks.
[0013] FIG. 5 is a set of scanning electron micrographs of sections
of SPPS YAG coatings from Example 2 and illustrating the effects of
standoff distance and index; the corresponding sample numbers
appear in each micrograph, and the conditions for their formation
are summarized in Table 2; in the first and second rows, the left
micrograph is for a cross-section of the coating, the center
micrograph is a magnified cross section of another coating sprayed
at the same time, and the right micrograph is a magnified cross
section of a third coating sprayed at the same time.
[0014] FIG. 6 is a set of scanning electron micrographs of sections
of SPPS YAG coatings from Example 2 and illustrating the effect of
differences in standoff distance; the corresponding sample numbers
appear in each micrograph, and the conditions for their formation
are summarized in Table 3.
[0015] FIG. 7 is a set of scanning electron micrographs of sections
of SPPS YAG coatings from Example 2 and varying in the presence or
absence of ammonium acetate or urea in the precursor composition;
the corresponding sample numbers appear in each micrograph, and the
conditions for their formation are summarized in Table 4.
[0016] FIG. 8 is an x-ray diffraction pattern of sample 050812-A
from Table 4.
[0017] FIG. 9 is a scanning electron micrograph of the Table 2
sample 041112-A structure after heat treatment at 1100.degree. C.
for one hour.
[0018] FIG. 10 is a set of electron micrographs illustrating the
effect of radial distance on coating morphology; the upper images
correspond to two enlargements of a cross-section of a coating
prepared with a 7.5 millimeter radial distance; the lower images
correspond to a cross section at two enlargements of a coating
prepared with a 5.5 millimeter radial distance.
[0019] FIG. 11 is a set of electron micrographs illustrating the
effect of standoff distance on coating morphology; the upper images
correspond to two enlargements of a cross-section of a coating
prepared with a 1.25 inch standoff distance; the middle images
correspond to two enlargements of a cross section of a coating
prepared with a 1.375 inch standoff distance; the lower images
correspond to two enlargements of a cross section of a coating
prepared with a 1.5 inch standoff distance.
[0020] FIG. 12 is a set of electron micrographs illustrating the
effect of solvent composition on coating morphology; the upper
images correspond to two enlargements of a cross-section of a
coating prepared with a solvent containing 25% water and 75%
ethanol; the middle images correspond to two enlargements of a
cross section of a coating prepared with a solvent containing 50%
water and 50% ethanol; the lower images correspond to two
enlargements of a cross section of a coating prepared with a
solvent containing 75% water and 25% ethanol.
[0021] FIG. 13 is a set of electron micrographs illustrating the
effect of urea addition to the precursor solution; the upper images
correspond to two enlargements of a cross-section of a coating
prepared with no urea added to the precursor solution; the middle
images correspond to two enlargements of a cross section of a
coating prepared with 5 weight percent urea in the precursor
solution; the lower images correspond to two enlargements of a
cross section of a coating prepared with 10 weight percent urea in
the precursor solution.
[0022] FIG. 14 is a set of electron micrographs illustrating two
SPPS coatings; the upper images correspond to two enlargements of a
cross-section of a "dense" coating; and the lower images correspond
to two enlargements of a cross-section of a "feathery" coating;
preparation of both coatings is described in Example 4.
[0023] FIG. 15 is a schematic illustration of a thermal cycling
test conducted at the University of Connecticut (UConn).
[0024] FIG. 16 is a plot of results from the thermal cycling test
conducted at UConn.
[0025] FIG. 17 is a pair of electron micrographs for the dense
coating after 217 hours (before failure; left), and after 636 hours
(after failure; right) in the UConn thermal cycling test.
[0026] FIG. 18 is a temperature profile for a thermal cycling test
conducted at HiFunda; the red line is for the sample holder
temperature, and the blue line is for the furnace temperature.
[0027] FIG. 19 consists of photographic images of SPPS YAG, SPPS
YSZ, and APS YSZ coated coupons before and after 270 hours of
thermal cycling in the HiFunda test.
[0028] FIG. 20 consists of two photographic images and a schematic
diagram of the HiFunda thermal conductivity apparatus.
[0029] FIG. 21 consists of two plots--SPPS YSZ on the left and SPPS
YAG on the right--of effective thermal conductivity versus
temperature as measured on the HiFunda thermal conductivity
apparatus.
[0030] FIG. 22 is a set of electron micrographs illustrating the
"dense" SPPS YAG coating before and after cycling, with the
"before" images magnified 500 times and the "after" images
magnified 1000 times. In each row, the left image is a scanning
electron micrograph, and the right image is a digital enhancement
of the left image emphasizing porosity and vertical cracks.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The present invention generally relates to the use of
solution precursor plasma spray to form thermal barrier coatings
comprising YAG-based ceramics. YAG itself has proven stability at
elevated temperatures and excellent high-temperature mechanical
properties. To date, the limitation in the use of YAG-based
ceramics as thermal barrier coatings has been related to the
difficulty in processing them with a sufficiently compliant
microstructure to achieve the required strain tolerance. The
present method solves that problem by utilizing a solution
precursor plasma spray process (SPPS) to generate thermal barrier
coatings with compliant microstructure and adequate strain
tolerance. The thermal barrier coatings comprising YAG-based
ceramics exhibit markedly improved high temperature properties
relative to 7YSZ.
[0032] Polycrystalline garnet ceramics are used in a variety of
high-temperature applications due to their unique properties. In
particular, YAG (Y.sub.3Al.sub.5O.sub.12) is an excellent choice
due to its robust high temperature properties and phase stability
up to its melting point (1970.degree. C.). Table 1 compares the
properties of YAG with those of YSZ. YAG thermal barrier coatings
have much higher use temperature and erosion resistance, and lower
thermal conductivity and density compared to air plasma spray (APS)
YSZ thermal barrier coatings. Some of the improved properties for
YAG thermal barrier coatings are enabled by use of the Solution
Precursor Plasma Spray (SPPS) process that provides a highly
strain-tolerant microstructure and a microstructure for which
microporosity can be varied over wide ranges.
TABLE-US-00001 TABLE 1 YSZ YAG Melting Point (.degree. C.) 2680
1950 Maximum Operating Temperature 1350 1800 (.degree. C.) Thermal
Conductivity at 1350.degree. C. 2.0-3.0 2.5 (extrapolated)* (W/m-K)
(measured) Thermal Expansion Coefficient 9.5 .times. 10.sup.-6 7.5
.times. 10.sup.-6 (ppm/K) Density (g/cc at 23.degree. C.) 6.10 4.55
Vickers Hardness at 23.degree. C. 1200 1700** Fracture Toughness at
23.degree. C. 3 ~1.8, but with a (MPa m.sup.0.5) modest maximum of
~2.2 at 1000.degree. C. *N. P. Padture and P. G. Klemens, "Low
Thermal Conductivity in Garnets", Journal of the American Ceramics
Society, volume 80, no. 4, pages 1018-20 (1997). **D. B.
Sirdeshmukh et al., "Systematic hardness measurements on some rare
earth garnet crystals," Bulletin of Materials Science, volume 24,
no. 5, pages 469-473 (2001).
[0033] Use Temperature.
[0034] Although YAG has a lower melting temperature than YSZ (Table
1), the maximum use temperature for a thermal barrier coating is
governed by the maximum temperature that the ceramic can withstand
without undergoing a phase change. YAG is phase stable up to its
melting point of 1970.degree. C. (J. S. Abell, I. R. Harris, B.
Cockayne, and B. Lent, "An Investigation of phase stability in the
Y.sub.2O.sub.3--Al.sub.2O.sub.3 system," Journal of Materials
Science, volume 9, pages 527-537 (1974)), whereas YSZ exhibits a
tetragonal to monoclinic transformation when cooled from
temperatures above 1200.degree. C. Thus, YAG has a maximum use
temperature that is over 700.degree. C. higher than YSZ based on
this criterion.
[0035] Density.
[0036] YAG has a very low density of 4.55 gram per cubic centimeter
at 23.degree. C., which represents a 25% reduction compared to YSZ.
The density advantage means that at a standard thermal barrier
coating thickness, YAG would exert less pull (stress) on the blade
root or, for the same blade pull, thicker YAG thermal barrier
coatings could be used for greater thermal insulation.
[0037] Hardness.
[0038] YAG has a much higher hardness than YSZ: 1700 versus 1200
for Vickers Hardness. The higher hardness can be used for improved
thermal barrier coating erosion resistance or can be traded off for
a higher porosity content and reduced thermal conductivity.
[0039] Thermal Conductivity.
[0040] Based on the thermal conductivity measurements of Padture
and Klemens, YAG has equal to or slightly higher thermal
conductivity than YSZ at 1,000.degree. C. YSZ has almost a constant
value of thermal conductivity with temperature; whereas, YAG's
thermal conductivity continually decreases with temperature.
Extrapolation of the YAG thermal conductivity data from the
1000.degree. C. to 1350.degree. C. (a generous estimate of the
maximum use temperature for YSZ), indicates YAG would have lower
thermal conductivity than YSZ at 1350.degree. C. It would be
desirable to achieve a YAG thermal conductivity of 0.5
Wattsmeter.sup.-1Kelvin.sup.-1 by introducing increased
microporosity, while still retaining a YAG hardness greater than
YSZ for improved erosion resistance.
[0041] It is also important to consider that at elevated
temperatures, heat transfer is controlled not only by conduction
but also by radiation. The measured thermal conductivities of both
stabilized zirconia and various zirconates are found to increase
with temperatures above 1200.degree. C. due to these ceramics being
virtually transparent to radiation in the near infrared range. See,
P. C. Patnaik, X. Huang, and J. Singh, "State of the Art and Future
Trends in the Development of Thermal Barrier Coating Systems,"
Innovative Missile Systems (pages 38-1 to 38-20), Meeting
Proceedings RTO-MP-AVT-135 Paper 38. Neuilly-sur-Seine, France
(2006). This radiation effect has been shown to increase the
substrate temperature by as much as 50.degree. C. (R. Siegel and C.
M. Spuckler, "Analysis of Thermal Radiation Effects on Temperatures
in Turbine Engine Thermal Barrier Coatings", Materials Science and
Engineering, volume A245 pages 150-159 (1998)), even at present
turbine operating conditions, and it is expected to have an even
greater effect at higher operating temperatures. Therefore, any
coating designed to operate at elevated temperatures above
1200.degree. C. must consider overall heat transfer through the
thermal barrier coatings, including via radiation effects.
[0042] Thermal Expansion.
[0043] One of the reasons for the long-term success of YSZ thermal
barrier coatings is that they possess a thermal expansion
coefficient that is the highest known for oxide ceramics. This high
thermal expansion coefficient minimizes the thermal expansion
mismatch between the ceramic and the underlying metal. In turn, the
bond line stress is reduced and the spallation life increased. A
number of advanced thermal barrier coating materials with lower
thermal conductivities and higher use temperatures, but with lower
thermal expansion coefficients, exhibited poorer thermal cyclic
durability compared to YSZ and were eliminated from further
consideration. See, X. Q. Cao, R. Vassen, F. Tietz, and D. Stoever,
"New double-ceramic layer thermal barrier coating based on
zirconia-rare earth composite oxides," Journal of the European
Ceramics Society, volume 26, pages 247-251 (2006). As indicated in
Table 1, YAG also has a lower thermal expansion coefficient
compared to YSZ. This limitation can be overcome by the use of the
Solution Precursor Plasma Spray (SPPS) that provides a thermal
barrier coating with a high density of through-coating-thickness
(vertical) cracks that produces a highly strain-tolerant
microstructure. The strain-tolerant microstructure of SPPS YSZ
thermal barrier coatings is so strong a feature that thermal
barrier coatings as thick as 4 millimeters have been made and
thermally cycled with excellent durability. M. Gell, E. H. Jordan,
M. Teicholz, B. M. Cetegen, N. P. Padture, L. Xie, D. Chen, X. Ma,
and J. Roth, "Thermal Barrier Coatings Made by the Solution
Precursor Plasma Spray Process," Journal of Thermal Spray
Technology, volume 17, no. 1, pages 124-135 (2008). The SPPS method
of the present invention provides a superior high temperature/low
thermal conductivity YAG thermal barrier coating, with a lower
thermal expansion coefficient and superior durability compared to
YSZ because of the strain-tolerant microstructure enabled by
SPPS.
[0044] The Solution Precursor Plasma Spray (SPPS) Process.
[0045] FIG. 1 is a schematic illustration of a solution precursor
plasma spray system 10 comprising a plasma discharge unit 20 with
tungsten cathode 30 and tungsten anode 40; a plasma precursor gas
50 is introduced into the plasma discharge unit 20, and a thermal
jet 60 exits; a precursor solution 70 is introduced as a mist to
the thermal jet via atomizing nozzle 80 (in other embodiments, the
precursor solution is introduced to the thermal jet as a solution
stream); the thermal jet 60 produces a coating 90 on substrate 100.
Within the thermal jet, the injected solution droplets are
fragmented (in other embodiments, the injected solution droplets
are not fragmented). Also in the thermal jet, solvent is vaporized
from the precursor solution droplets, at least some of the
resulting salt particles are pyrolyzed to yield YAG-based ceramic
particles, and the YAG-based ceramic particles are deposited on the
substrate. In some embodiments, a portion of the YAG-based ceramic
particles are melted and deposited on the substrate as micron-sized
splats. In some embodiments, at least some of the salt particles
are not pyrolyzed or incompletely pyrolyzed before being deposited
on the substrate, and they are completely pyrolyzed after being
deposited on the substrate. Although not shown in FIG. 1, the
system can utilize more than one precursor solution and a mixer to
apportion the different precursor solutions to the atomizing
nozzle. In addition, multiple atomizing nozzles can be used to
deposit layered compositions or provide graded coatings.
[0046] The SPPS process has been used to fabricate YAG coatings
with a small amount of Dysprosium (Dy) (e.g., 1 mole percent
relative to the total moles of yttrium and dysprosium) of for use
as an in-situ temperature sensor. A strain-tolerant microstructure
was obtained, similar to that obtained with SPPS-coated YSZ. These
YAG:Dy coatings have been exposed to very high temperatures
(1360.degree. C.) at NASA-Lewis and exhibit excellent durability.
The NASA-Lewis method is described in J. T. Eldridge, T. O.
Jenkins, S. W. Allison, G. S. Curzen, J. J. Condevaux, J. R. Senk,
and A. D. Paul "Real time thermographic phosphor-based temperature
Measurement of thermal barrier coatings surfaces subject to a High
velocity combustor burner environment," Proceedings of the 57th
International Instrumentation Symposium, 208 (2011).
[0047] The present method is directed to the formation of thermal
barrier coatings comprising a YAG-based ceramic. Yttrium aluminum
garnet (YAG) has the chemical formula Y.sub.3Al.sub.5O.sub.12. YAG
belongs to the isostructural garnet family of ceramics having the
garnet structure illustrated in FIG. 2, where metal sites labeled
"C", "A", and "D" are shown, and oxygen sites are omitted for
clarity. In YAG, yttrium ions occupy the "C" sites, two-fifths of
the aluminum ions occupy the "A" sites, and three-fifths of the
aluminum ions occupy the "D" sites. "A" sites are octahedrally
coordinated with respect to oxygen, "C" sites are tetrahedrally
coordinated with respect to oxygen, and "D" sites are
dodecahedrally coordinated with respect to oxygen. As used herein,
the term YAG-based ceramics includes ceramics having the FIG. 2
garnet structure in which "C" cites are occupied by one or more
types of trivalent metal ions such as yttrium ions, scandium ions,
lutetium ions, lanthanum ions, cerium ions, praseodymium ions,
neodymium ions, promethium ions, samarium ions, europium ions,
gadolinium ions, terbium ions, dysprosium ions, holmium ions,
erbium ions, thulium ions, ytterbium ions, and the like; and "A"
and "D" sites are occupied by one or more types of trivalent metal
ions such as aluminum ions, gallium ions, iron ions, chromium ions,
scandium ions, or the like.
[0048] In some embodiments, the YAG-based ceramic comprises YAG or
a garnet structure in which part or all of the aluminum ions in the
"A" and/or "D" sites of YAG are substituted with one or a mixture
of iron ions, gallium ions, chromium ions, scandium ions, or the
like.
[0049] In some embodiments, the YAG-based ceramic comprises YAG
(Y.sub.3Al.sub.5O.sub.12).
[0050] In some embodiments, the YAG-based ceramic comprises a
garnet in which part or all of the "C" site yttrium ions of YAG are
substituted with one or a mixture of scandium ions, lutetium ions,
lanthanum ions, cerium ions, praseodymium ions, neodymium ions,
promethium ions, samarium ions, europium ions, gadolinium ions,
terbium ions, dysprosium ions, holmium ions, erbium ions, thulium
ions, ytterbium ions, or the like.
[0051] In some embodiments, the YAG-based ceramic thermal barrier
coating exhibits at least one of a thermal conductivity less than
or equal to about 3 Wattmeter.sup.-1K.sup.-1 at about 1000.degree.
C., an oxygen diffusivity less than or equal to about 10.sup.-15
meter.sup.2second.sup.-1 at about 1000.degree. C., a thermal
coefficient of expansion greater than or equal to about
9.times.10.sup.-6.degree. C..sup.-1, a maximum temperature
capability greater than or equal to about 1400.degree. C., a
hardness greater than or equal to about 14 gigapascals, an elastic
modulus less than or equal to about 280 gigapascals, and a density
less than or equal to about 6.4 gramscentimeter.sup.-3.
[0052] One embodiment is a method of forming a thermal barrier
coating, comprising: injecting a precursor solution into a thermal
jet, wherein the precursor solution comprises metal ion precursors
to a YAG-based ceramic; evaporating solvent from the precursor
solution in the thermal jet to form unpyrolyzed solid particles;
pyrolyzing at least a portion of the unpyrolyzed solid particles in
the thermal jet to form pyrolyzed solid particles comprising a
YAG-based ceramic; melting at least a portion of the pyrolyzed
solid particles in the thermal jet to form droplets comprising the
YAG-based ceramic; and depositing the droplets comprising the
YAG-based ceramic on a substrate to form a thermal barrier
coating.
[0053] The method utilizes a thermal jet that is hot enough to melt
the YAG-based ceramic. Suitable thermal jet coating techniques
include suspension plasma spray coating, air plasma spray coating,
vacuum plasma spray coating, ultra-high vacuum plasma spray
coating, detonation spray coating, high velocity oxy fuel spray
coating, atmospheric fuel spray coating, and combinations thereof.
These techniques are known and need not be described in detail
here.
[0054] The method includes injecting a precursor solution into a
thermal jet. This can be accomplished by injecting a stream of
precursor solution into the thermal jet, where it is broken into
droplets. Alternatively, the precursor solution can be delivered to
a liquid injector, preferably an atomizing injector nozzle or a
piezoelectric crystal induced liquid injector. In some embodiments,
the precursor solution is atomized in the atomizing injector nozzle
into droplets up to hundreds of micrometers in size, specifically
15 to 40 micrometers, and injected into the thermal jet. In other
embodiments, the precursor solution is introduced to the thermal
jet by a piezoelectric crystal induced liquid injector which
produces droplets greater than about 50 micrometers and having
lower velocity. The precursor solution can be injected into the
thermal jet internally or externally, radially or coaxially. When
the precursor solution is injected radially, the injector nozzle
can be oriented at an angle of about 45.degree. to about 90.degree.
relative to the axis of the jet. Preferably the injector nozzle is
oriented at about 90.degree. relative to the jet axis, or somewhat
at an upstream angle. The injection parameters may impact the
porosity of the deposited material and the presence or absence of
through-coating-thickness microcracks. Multiple radial injectors
can be used to increase deposition efficiency.
[0055] The precursor solution comprises solvent and metal ion
precursors to the YAG-based ceramic. The metal ion precursors
include the metal ions described above in the context of the
YAG-based ceramic structure, and they further comprise anions. In
some embodiments, the metal ion precursors to the YAG-based are
provided in the form of a salt selected from the group consisting
of carboxylate salts (including acetate salts, propionate salts,
and citrate salts), alkoxide salts (including methoxide salts,
ethoxide salts, 1-propoxide salts, and 2-propoxide salts),
carbonate salts (including bicarbonate salts), halide salts
(including fluoride salts, chloride salts, bromides salts, and
iodide salts), nitrate salts, hydrates of the foregoing salts, and
combinations thereof.
[0056] In some embodiments, the precursor solution comprises
yttrium nitrate and aluminum nitrate.
[0057] The precursor solution comprises a solvent. Suitable
solvents include, for example, water, methanol, ethanol,
1-propanol, 2-propanol, ethylene glycol, dimethylsulfoxide (DMSO),
acetonitrile, dimethylformamide (DMF), ethyl acetate, formic acid,
acetone, methyl ethyl ketone, and the like, and mixtures thereof.
The solvent can include a flammable organic material, such as urea
or ammonium acetate, that is intentionally included to increase the
temperature of the thermal jet. In some embodiments, the solvent
comprises water, ethanol, or a mixture thereof. The precursor
solution typically contains at least two metal ion salts. The total
concentration of metal ion salts in the precursor solution can vary
according to the identities of the salts and their solubilities in
the solvent. In some embodiments, the total concentration of metal
ion salts in the precursor solution is as less than or equal to
about 80 weight percent, specifically less than or equal to 70
weight percent, based on the total weight of the precursor
solution. There is no particular limitation on the metal ion salt
concentration in the precursor solution, but for coating efficiency
it may be preferred to use a metal ion salt concentration of at
least 1 weight percent, specifically at least 5 weight percent,
more specifically at least 10 weight percent, even more
specifically at least 20 weight percent. The molar ratio of metal
ions in the precursor solution will typically reflect their
respective concentration in the YAG-based ceramic formed by the
process. For example, when the YAG-based ceramic is YAG itself
(that is, yttrium aluminum garnet), the molar ratio of yttrium ions
to aluminum ions will typically approximate the 3:5 molar ratio of
yttrium to aluminum in YAG. Thus, in some embodiments, the mole
ratio of yttrium to aluminum is 2 to 4 moles yttrium:4 to 6 moles
aluminum. In some embodiments the composition of the precursor is
deliberately adjusted to account for material lost in the spray
process by sublimation, evaporation, and so on.
[0058] In some embodiments, the precursor solution is a homogeneous
solution. The precursor solution is therefore distinguished from
sols and suspensions comprising solid materials.
[0059] Once in the thermal jet, the precursor solution droplets
are, optionally, fragmented. Solvent is evaporated from the
precursor solution droplets to form unpyrolyzed solid particles.
Without being bound by theory, it is believed that in some cases
upon entering the thermal jet the droplets form a crust. As the
solvent portion of the droplet inside the crust vaporizes the
droplet can explode resulting in the formation of a large number of
very small droplets and/or particles. Alternatively the droplet may
be fragmented by a dynamic interaction between the velocity of the
droplet and the velocity of the jet. In still other embodiments,
the entering droplets are small enough or robust enough that no
fragmentation occurs in the thermal jet. Independent of whether
droplet fragmentation has occurred, solvent is evaporated from the
precursor solution, and at least some of the resulting unpyrolyzed
solid particles are pyrolyzed in the thermal jet to form pyrolyzed
solid particles comprising a YAG-based ceramic. Pyrolysis is
defined herein as the conversion of the metal ion precursors to the
desired YAG-based ceramic without substantial degradation. For
example, pyrolysis of the particles containing yttrium and aluminum
cations yields yttrium aluminum garnet.
[0060] At least a portion of the pyrolyzed solid particles are
melted in the thermal jet to form droplets comprising the YAG-based
ceramic. The melting can be partial or complete. The droplets are
then deposited on a substrate to form the thermal barrier coating.
There is no particular limitation on the substrate, as long as it
is thermally and structurally stable to the impact of the pyrolyzed
solid particles. Suitable substrates for thermal spray coating
include, for example, metals (including steel, stainless steel,
nickel-based superalloys, aluminum, and titanium), ceramics, and
heat-resistant plastics.
[0061] One advantage of the present solution precursor plasma spray
process over the air plasma spray process is the deposition on the
substrate of smaller units of material than deposited by air plasma
spray. For example, in some embodiments, depositing the droplets
comprising the YAG-based ceramic on the substrate forms splats on
the substrate. The splats can have an average diameter less than or
equal to 10 micrometers, specifically less than or equal to 5
micrometers, more specifically less than or equal to 4 micrometers,
even more specifically less than or equal to 2 micrometers. A splat
is defined as a thin platelet formed when the YAG-based ceramic
droplets impinge on the substrate. Splats can typically be
described as having a length, width, and thickness. For
convenience, the diameter is herein defined as the length or the
width, whichever is greater. In some embodiments, the splats have a
thickness less than or equal to about 800 nanometers, specifically
less than or equal to about 700 nanometers, and more specifically
less than or equal to about 600 nanometers.
[0062] In some embodiments, the method further comprises depositing
pyrolyzed solid particles comprising YAG-based ceramic on the
substrate (that is, co-depositing the droplets and the pyrolyzed
solid particles).
[0063] One important advantage of the present method is its ability
to form thermal barrier coatings with through-coating-thickness
microcracks. The presence of through-coating-thickness microcracks
is associated with significantly improved strain tolerance for the
thermal barrier coating. Without being bound by theory, it is
believed that the deposition of unpyrolyzed and incompletely
pyrolyzed solid particles contributes to the formation of
through-coating-thickness microcracks in the thermal barrier
coating, because pyrolysis of the unpyrolyzed and incompletely
pyrolyzed solid particles on the substrate surface causes a volume
change that promotes formation of the through-coating-thickness
microcracks. Thus, in some embodiments, the method further
comprises depositing incompletely pyrolyzed solid particles and/or
the unpyrolyzed solid particles on the substrate and pyrolyzing the
incompletely pyrolyzed solid particles and/or the unpyrolyzed solid
particles on the substrate. To maximize the stress-relief
advantages associated with the through-coating-thickness
microcracks, these microcracks can be spaced from each other at a
distance, on average, of less than or equal to half the coating
thickness.
[0064] The through-coating-thickness microcracks typically have a
width of about 0.1 to about 5 micrometers, specifically about 0.2
to about 4 micrometers, more specifically about 0.4 to about 3
micrometers, even more specifically about 0.4 to about 2
micrometers. As is implied by their name, the
through-coating-thickness microcracks extend through the entire
thickness of the coating. In some embodiments, the thermal barrier
coating further comprises additional microcracks that penetrate at
least half the coating thickness and less than the complete coating
thickness.
[0065] The droplets comprising the YAG-based ceramic are typically
deposited over multiple passes to form the thermal barrier coating.
Thus, a wide range of coating thicknesses can be produced. For
example, the thermal barrier coating can have a thickness of about
1 micrometer to about 5 millimeters, specifically about 10
micrometers to about 2 millimeters, more specifically about 50
micrometers to about 1 millimeter, still more specifically about
100 micrometers to about 500 micrometers, even more specifically
about 200 micrometers to about 500 micrometers. In some
embodiments, the thermal barrier coating has a thickness of about
50 to about 5,000 micrometers.
[0066] The presence of porosity in the thermal barrier coating is
associated with improved (reduced) thermal conductivity and
improved strain tolerance. In some embodiments, the thermal barrier
coating has a porosity of about 10 to about 40 volume percent based
on the total volume of the thermal barrier coating. Porosity can be
determined by quantitative examination of the microstructure, or by
the Archimedes method.
[0067] In some embodiments, the thermal barrier coating comprises
inter-pass boundaries. Inter-pass boundaries reduce thermal
conductivity. The inter-pass boundaries can have a thickness of
about 0.1 to about 2 micrometers, specifically about 0.5 to about 2
micrometers. The porosity of the inter-pass boundary can affect the
overall thermal conductivity of the deposited material. The
porosity of the inter-pass boundary can be about 20 to about 95
volume percent, specifically about 20 to about 75 volume percent,
more specifically about 20 to about 50 volume percent, based on the
total volume of the inter-pass boundary. The inter-pass boundaries
can exhibit a layered spacing of about 1 to about 10 micrometers.
The inter-pass boundaries can be continuous or discontinuous.
[0068] In a very specific embodiment of the method, the YAG-based
ceramic has the empirical formula
Y.sub.3Al.sub.5-xFe.sub.xO.sub.12, where x can vary continuously
from 0 to 5; the precursor solution is a homogeneous solution; the
precursor solution comprises yttrium nitrate and at least one of
aluminum nitrate and ferric nitrate; the thermal barrier coating
has a thickness of about 50 to about 5,000 micrometers; the method
further comprises depositing incompletely pyrolyzed solid particles
and/or the unpyrolyzed solid particles and/or on the substrate and
pyrolyzing the incompletely pyrolyzed solid particles and/or the
unpyrolyzed solid particles on the substrate; the thermal barrier
coating comprises through-coating-thickness microcracks having a
width of about 0.1 to about 5 micrometers; the
through-coating-thickness microcracks are spaced from each other at
a distance, on average, of less than or equal to half the coating
thickness; and the thermal barrier coating has a porosity of about
10 to about 40 volume percent based on the total volume of the
thermal barrier coating.
[0069] The invention includes thermal barrier coatings prepared by
any of the above-described variations of the method.
[0070] One embodiment is a thermal barrier coating having a
thickness of about 50 to about 5,000 micrometers and comprising
through-coating-thickness microcracks having a width of about 0.1
to about 5 micrometers and spaced from each other at a distance, on
average, of less than or equal to half the coating thickness;
wherein the thermal barrier coating comprises a YAG-based ceramic.
Within the range of about 0.1 to about 5 micrometers, the
through-coating-thickness microcracks can have a width of about 0.2
to about 4 micrometers, specifically about 0.4 to about 3
micrometers, more specifically about 0.4 to about 2
micrometers.
[0071] The invention includes articles comprising the thermal
barrier coating. The thermal barrier coating is particularly
suitable for use on hot-section components in gas turbine engines
for jet aircraft and power generation and the like. Other
applications include use in diesel engines, dielectric coatings,
catalytic films, doped oxide films for use in fuel cells and gas
separation and purification, electronic and ionic conductivity
membranes and sensor devices. In some embodiments, the article is
used in the hot section of a gas turbine, including turbine blades,
turbine vanes, turbine blade outer air seals, and combustor liner
segments.
[0072] The invention includes at least the following
embodiments.
Embodiment 1
[0073] A method of forming a thermal barrier coating, comprising:
injecting a precursor solution into a thermal jet; wherein the
precursor solution comprises metal ion precursors to a YAG-based
ceramic; evaporating solvent from the precursor solution in the
thermal jet to form unpyrolyzed solid particles; pyrolyzing at
least a portion of the unpyrolyzed solid particles in the thermal
jet to form pyrolyzed solid particles comprising a YAG-based
ceramic; melting at least a portion of the pyrolyzed solid
particles in the thermal jet to form droplets comprising the
YAG-based ceramic; and depositing the droplets comprising the
YAG-based ceramic on a substrate to form a thermal barrier
coating.
Embodiment 2
[0074] The method of embodiment 1, wherein the YAG-based ceramic
has the garnet structure of FIG. 1 comprising "C" sites, "A" sites,
and "D" sites; wherein the "C" cites are occupied by one or a
mixture of trivalent metal ions selected from the group consisting
of yttrium ions, scandium ions, lutetium ions, lanthanum ions,
cerium ions, praseodymium ions, neodymium ions, promethium ions,
samarium ions, europium ions, gadolinium ions, terbium ions,
dysprosium ions, holmium ions, erbium ions, thulium ions, ytterbium
ions; and wherein the "A" and "D" sites are independently occupied
by one or a mixture of trivalent metal ions selected from the group
consisting of aluminum ions, gallium ions, iron ions, chromium
ions, and scandium ions.
Embodiment 3
[0075] The method of embodiment 1, wherein the YAG-based ceramic
has the garnet structure of FIG. 1 comprising "C" sites, "A" sites,
and "D" sites; wherein the "C" cites are occupied by yttrium ions;
and wherein the "A" and "D" sites are independently occupied by one
or a mixture of metal ions selected from the group consisting of
aluminum ions, iron ions, gallium ions, and scandium ions.
Embodiment 4
[0076] The method of embodiment 1, wherein the YAG-based ceramic
has the garnet structure of FIG. 1 comprising "C" sites, "A" sites,
and "D" sites; wherein the "C" cites are occupied by one or a
mixture of metal ions selected from the group consisting of yttrium
ions, cerium atoms, neodymium atoms, terbium atoms, and ytterbium
atoms; and wherein the "A" and "D" sites are occupied by aluminum
ions.
Embodiment 5
[0077] The method of embodiment 1, wherein the YAG-based ceramic
has the empirical formula Y.sub.3Al.sub.5-xFe.sub.xO.sub.12, where
x can vary continuously from 0 to 5.
Embodiment 6
[0078] The method of embodiment 1, wherein the YAG-based ceramic
comprises yttrium aluminum garnet (Y.sub.3Al.sub.5O.sub.12).
Embodiment 7
[0079] The method of any of embodiments 1-6, wherein the precursor
solution is a homogeneous solution.
Embodiment 8
[0080] The method of any of embodiments 1-7, wherein the metal ion
precursors to the YAG-based ceramic are provided in the form of a
salt selected from the group consisting of carboxylate salts,
alkoxide salts, carbonate salts, halide salts, nitrate salts,
hydrates of the foregoing salts, and combinations thereof.
Embodiment 9
[0081] The method of any of embodiments 1-8, wherein the precursor
solution comprises yttrium nitrate and aluminum nitrate.
Embodiment 10
[0082] The method of any of embodiments 1-9, wherein said
depositing the droplets comprising the YAG-based ceramic on the
substrate forms splats on the substrate, the splats comprising the
YAG-based ceramic; wherein the splats have an average diameter less
than or equal to 5 micrometers.
Embodiment 11
[0083] The method of any of embodiments 1-10, wherein the thermal
barrier coating has a thickness of about 1 to about 5,000
micrometers.
Embodiment 12
[0084] The method of any of embodiments 1-11, further comprising
incompletely pyrolyzing at least a portion of the unpyrolyzed solid
particles in the thermal jet to form incompletely pyrolyzed solid
particles; depositing the incompletely pyrolyzed solid particles
and/or the unpyrolyzed solid particles on the substrate; and
pyrolyzing the incompletely pyrolyzed solid particles and/or the
unpyrolyzed solid particles on the substrate.
Embodiment 13
[0085] The method of embodiment 12, wherein said pyrolyzing the
unpyrolyzed solid particles and/or incompletely pyrolyzed solid
particles on the substrate forms through-coating-thickness
microcracks in the thermal barrier coating.
Embodiment 14
[0086] The method of embodiment 13, wherein the thermal barrier
coating has a thickness; and wherein the through-coating-thickness
microcracks are spaced from each other at a distance, on average,
of less than or equal to half the coating thickness.
Embodiment 15
[0087] The method of embodiment 13, wherein the
through-coating-thickness microcracks have a width of about 0.1 to
about 5 micrometers.
Embodiment 16
[0088] The method of any of embodiments 1-15 wherein the thermal
barrier coating has a porosity of about 10 to about 40 volume
percent based on the total volume of the thermal barrier
coating.
Embodiment 17
[0089] The method of any of embodiments 1-16, wherein the thermal
barrier coating comprises inter-pass boundaries.
Embodiment 18
[0090] The method of embodiment 1, wherein the YAG-based ceramic
has the empirical formula Y.sub.3Al.sub.5-xFe.sub.xO.sub.12, where
x can vary continuously from 0 to 5; wherein the precursor solution
is a homogeneous solution; wherein the precursor solution comprises
yttrium nitrate and at least one of aluminum nitrate and ferric
nitrate; wherein the thermal barrier coating has a thickness of
about 50 to about 5,000 micrometers; wherein the method further
comprises depositing incompletely pyrolyzed solid particles and/or
the unpyrolyzed solid particles on the substrate and pyrolyzing the
incompletely pyrolyzed solid particles and/or the unpyrolyzed solid
particles on the substrate; wherein the thermal barrier coating
comprises through-coating-thickness microcracks having a width of
about 0.1 to about 5 micrometers; wherein the
through-coating-thickness microcracks are spaced from each other at
a distance, on average, of less than or equal to half the coating
thickness; and wherein the thermal barrier coating has a porosity
of about 10 to about 40 volume percent based on the total volume of
the thermal barrier coating.
Embodiment 19
[0091] A thermal barrier coating prepared by the method of any of
embodiments 1-18.
Embodiment 20
[0092] A thermal barrier coating having a thickness of about 50 to
about 5,000 micrometers and comprising through-coating-thickness
microcracks having a width of about 0.1 to about 5 micrometers and
spaced from each other at a distance, on average, of less than or
equal to half the coating thickness; wherein the thermal barrier
coating comprises a YAG-based ceramic.
Embodiment 21
[0093] The thermal barrier coating of embodiment 20, wherein the
YAG-based ceramic has the empirical formula
Y.sub.3Al.sub.5-xFe.sub.xO.sub.12, where x can vary continuously
from 0 to 5; wherein the YAG-based ceramic has the empirical
formula Y.sub.3Al.sub.5-xFe.sub.xO.sub.12, where x can vary
continuously from 0 to 5; and wherein the thermal barrier coating
comprises inter-pass boundaries and splats having an average
diameter less than or equal to 5 micrometers.
Embodiment 22
[0094] An article comprising the thermal barrier coating of any of
embodiments 19-21.
Embodiment 23
[0095] The article of embodiment 22, wherein the article is used in
the hot section of a gas turbine and selected from turbine blades,
turbine vanes, turbine blade outer air seals, and combustor liner
segments.
[0096] The invention is further illustrated by the following
non-limiting examples.
Example 1
[0097] Thermal barrier coatings were generated with a Sulzer-Metco
Plasma Spray System that included a 9MC controller, a 9MB plasma
torch, and a Bete atomizing nozzle. The precursor solution was a
homogeneous aqueous solution of yttrium nitrate and aluminum
nitrate in an yttrium to aluminum mole ratio of 3:5. The precursor
solution was prepared from 1000 milliliters deionized water, 584.7
grams yttrium nitrate hexahydrate, and 952 grams aluminum nitrate
nonahydrate, yielding a solution with 5.35 weight percent yttrium
and 2.70 weight percent aluminum. Spray coating was conducted using
an argon primary gas flow of 80 to 140 standard cubic feet per hour
(SCFH) at 100 pounds per square inch (689 kilopascals), a hydrogen
secondary gas flow of 8 to 20 SCFH at 50 pounds per square inch
(345 kilopascals), a plasma current of 600 amps, and a plasma
voltage of 65 volts. The precursor solution was atomized using the
Bete atomizing nozzle at a rate of 25 milliliters per minute. The
plasma jet at the substrate was hotter for the dense microstructure
of FIG. 4 because the stand-off distance was 1.375 inches (3.4925
centimeters), compared to 1.75 inches (4.445 centimeters) for the
feathery microstructure of FIG. 3. The inter-pass boundaries were
produced with the dense microstructure of FIG. 4 because the plasma
torch raster scan height was 1 millimeter for the dense
microstructure, compared to 3 millimeters for the feathery
microstructure of FIG. 3. The reduced raster scan height captured
more semi-pyrolyzed and unpyrolyzed material, thus producing the
inter-pass boundaries.
[0098] These experiments illustrate that thermal barrier coatings
comprising YAG-based ceramics can be produced with highly varied
microstructures by varying thermal spray processing parameters.
Example 2
[0099] In this experiment, the following SPPS process parameters
were studied: (1) stand-off distance (gun to substrate distance),
(2) radial distance (atomizing nozzle tip to plasma plume
distance), (3) plasma power, and (4) precursor feed rate. In Table
2, "Index" is the raster scan height of the plasma gun in
successive traverses of the specimen. In initial spray trials,
summarized in Tables 2 and 3, 50/50 deionized water/ethanol was
used as the solvent. The precursor solution was as described in
Example 1. These initial trials demonstrated that uniform
micro-porous YAG coatings (samples 041112-A and 041112-B; FIG. 5)
can be deposited when the standoff distance is less than 1.75''.
Thermal barrier coatings 250 micrometer thick were also achieved
(Samples II-A and II-B) at such standoff distances (FIG. 6).
TABLE-US-00002 TABLE 2 Spray conditions for first set of YAG SPPS
spray trials Precursor solution: YAG mixed with 50/50 Water-Ethanol
(V/V) solution Injection Bete Atomizing Sample # Index Standoff
distance 041112-A 2 mm 1.625'' 041112-B 2 mm 1.75'' 041112-C 2 mm
1.875'' 041112-D 1 mm 1.625'' 041112-E 1 mm 1.75'' 041112-F 1 mm
1.875''
TABLE-US-00003 TABLE 3 Spray conditions for second set of SPPS YAG
spray trials Solution: 50/50 Water/Ethanol (V/V) Solution Bete
Atomizing; Feed rate: 15~18 mL/min Sample # Index Standoff distance
Scan speed 042512-A 2 mm 1.625'' 450 mm/s 042512-B 2 mm 1.750'' 450
mm/s 042512-D 2 mm 1.875'' 450 mm/s 042512-E 2 mm 1.625'' 225 mm/s
042512-F 2 mm 1.750'' 225 mm/s 042512-G 2 mm 1.875'' 225 mm/s
042512-H 2 mm 2'' 225 mm/s
[0100] In order to increase the particle temperature during the
spray, 5 weight percent urea or ammonium acetate was added into the
solution as "fuel" to enhance the melting of the YAG particles and
increase the coating density and mechanical properties. Conditions
are summarized in Table 4, where "SD" is stand-off distance, and
"Delay" is the delay in seconds between passes. The micrographs
shown in FIG. 7 indicate that addition of ammonium acetate or urea
does enhance the melting of YAG particles and thereby the density
of the coatings. A sample sprayed with ammonium acetate (sample
051512-E) with a close standoff distance (1.375'') had a nearly
fully dense top layer. A thermal barrier coating specimen with urea
addition (051512-J, standoff distance 1.375'') had uniform dense
layers and vertical cracks. Another specimen, 051512-A (standoff
distance 1.375''), without the addition of ammonium acetate or
urea, also showed good hardness with uniform columnar
microstructure and micro porosity. Larger standoff distances
(051512-B and 051512-C) tended to generate higher porosity
especially on the coating top surfaces. It was also observed that a
predetermined delay during different spray passes reduced the
melting of YAG particles and increased the porosity (051512-G).
X-ray diffraction patterns (FIG. 8) indicated that the as-sprayed
SPPS YAG sample (III-A) contains mainly the YAG phase with very
small amounts of YAM and YAP, which may due to the insufficient
thermal treatment of some precursor droplets that traveled outside
of the plasma plume. Heat treatment (1100.degree. C., 1 hour) of
SPPS YAG samples with uniform micro-porosity (041112-A and
041112-B) was shown to generate vertical cracks, which are highly
desired for enhanced coating compliance, with a spacing of about
100 micrometers (FIG. 9). It was anticipated that this
microstructure would be highly strain-tolerant.
TABLE-US-00004 TABLE 4 Spray conditions Sample # Precursor Solution
Injection SD Delay Index 051512-A YAG w/Ammonium Bete Atomizing
1.75'' No 3 mm Acetate 051512-B YAG w/Ammonium Bete Atomizing
1.875'' No 3 mm Acetate 051512-C YAG w/Ammonium Bete Atomizing 2''
No 3 mm Acetate 051512-E YAG w/Ammonium Bete Atomizing 1.375'' No 3
mm Acetate 051512-F YAG w/Ammonium Bete Atomizing 1.5'' No 3 mm
Acetate 051512-G YAG w/Ammonium Bete Atomizing 1.375'' 30 s 3 mm
Acetate 051512-H YAG w/Ammonium Bete Atomizing 1.5'' 30 s 3 mm
Acetate 051512-I YAG w/Ammonium Bete Atomizing 1.625'' 30 s 3 mm
Acetate 051512-J YAG w/Urea Bete Atomizing 1.375'' 30 s 3 mm
051512-K YAG w/Urea Bete Atomizing 1.5'' 30 s 3 mm 051512-L YAG
w/Urea Bete Atomizing 1.625'' 30 s 3 mm 051512-M YAG w/Urea Bete
Atomizing 1.75'' No 3 mm 051512-N YAG w/Urea Bete Atomizing 1.875''
No 3 mm 051512-O YAG w/Urea Bete Atomizing 2'' No 3 mm 051812-A YAG
Bete Atomizing 1.375'' 30 s 3 mm 051812-B YAG Bete Atomizing 1.5''
30 s 3 mm 051812-C YAG Bete Atomizing 1.625'' 30 s 3 mm
Example 3
[0101] These examples represent a Taguchi experimental design to
optimize the microstructure of YAG thermal barrier coatings. The
process variables were water/ethanol ratio, standoff distance, and
radial distance. Experimental conditions are summarized in Table
5.
TABLE-US-00005 TABLE 5 Spray conditions Bete Atomizing FC14, Feed
rate: 15 ml/min, scan speed 650 mm/s, radial distance 5.5 mm
(A5/B5/C5), 7.5 mm (A7/B7/C7) Sample # Index Standoff distance
Sample Set 073112-A 50/50 DI water/ethanol A5A 2 mm 1.5'' A5B 2 mm
1.625'' A5C 2 mm 1.75'' A5D 2 mm 1.875'' A7A 2 mm 1.5'' A7B 2 mm
1.625'' A7C 2 mm 1.75'' A7D 2 mm 1.875'' Sample Set 073112-B 75/25
DI water/ethanol B5A 2 mm 1.5'' B5B 2 mm 1.625'' B5C 2 mm 1.75''
B5D 2 mm 1.875'' B7A 2 mm 1.5'' B7B 2 mm 1.625'' B7C 2 mm 1.75''
B7D 2 mm 1.875'' Sample Set 073112-C 25/75 DI water/ethanol C5A 2
mm 1.5'' C5B 2 mm 1.625'' C5C 2 mm 1.75'' C5D 2 mm 1.875'' C7A 2 mm
1.5'' C7B 2 mm 1.625'' C7C 2 mm 1.75'' C7D 2 mm 1.875''
[0102] Analysis of the Taguchi designed experiment showed that the
key factors that affected the microstructure of SPPS YAG are
standoff distance, urea additive, radial distance and water/ethanol
ratio. A closer radial distance (5.5 millimeters) combined with
suitable feed rate and atomizing pressure helps to keep more
precursor droplets in the central area of the plasma plume, which
will enhance the melting of the YAG particles. As a result, more
dense areas are observed in comparison with a radial distance of
7.5 millimeters (FIG. 10; samples correspond to C5A and C7A in
Table 5). A short standoff distance increases the surface
temperature of the coating and therefore also enhances the melting
of the YAG particles. A fully dense YAG coating with vertical
cracks was achieved at a standoff distance of 1.25 inch (FIG. 11).
Increasing the standoff distance resulted in a microstructure with
uniformly distributed dense areas and a columnar structure (FIG.
11). Higher ethanol concentration also provided more thermal energy
during the ethanol combustion and therefore enhanced the melting of
YAG particles (FIG. 12). As demonstrated in previous spray trials,
adding urea enhanced the melting of YAG particles. See FIG. 13,
where the top row images correspond to sample 051512-F in Table 4.
There was no significant difference observed in coating
microstructure and density with 5 weight percent and 10 weight
percent urea addition. The preliminary spray trials and further
optimization show that a wide range of microstructure of SPPS YAG
was achieved, from highly porous microstructure to high density
layered structure with vertical cracks and to nearly fully dense
structure. As a result, the micro-hardness (Vickers hardness) of
the SPPS YAG coatings varies in a wide range also. Table 6 lists
micro-hardness of some samples with typical microstructure.
TABLE-US-00006 TABLE 6 Measured Micro-hardness of some of the SPPS
YAG samples Microstructure Vickers hardness (HV) Highly porous,
"feathery" 100 "Dense" with vertical cracks 334 "Dense" columnar
structure 205 Nearly fully dense 1587 "Dense" columnar structure
305
Example 4
[0103] This example describes the deposition of two specific
microstructures having the potential for very high strain tolerance
(FIG. 14). These included (1) a "dense vertically cracked"
structure; and (2) a "feathery" structure. YAG TBCs with 250 micron
thickness were sprayed by SPPS on HVOF MCrAlY bond coated H230
alloy with a thin SPPS YSZ coating (.about.75 .mu.m) sprayed
between the alloy and SPPS YAG as a diffusion barrier. Preparation
of the "dense vertically cracked" structure utilized water without
ethanol, 5 weight percent urea, and a standoff distance of 1.375
inches. Preparation of the "feathery" structure utilized a 1:1
weight ratio of water to ethanol, no urea, and a standoff distance
of 1.75 inches. Thermal cycling and thermal conductivity tests were
conducted at UConn and HiFunda using as-sprayed SPPS YAG
coupons.
Example 5
[0104] This example describes thermal cycling tests used to
characterize the "dense vertically cracked" and "feathery" coatings
prepared in Example 4. Two types of thermal cycling tests were
carried out. The first type of thermal cycling was conducted at
UConn with an "extreme condition", 12 hour hold-time thermal
cycling at 1180.degree. C. See FIG. 15. APS and EB-PVD YSZ baseline
samples were also tested with the SPPS YAG samples. Prior test
experience with a number of advanced TBCs under these testing
condition yielded lifetimes of 60-200 hrs. The results of this
testing are presented in FIG. 16. During the test, two EB-PVD YSZ
baseline samples failed at 72 and 120 hrs. SPPS YAG samples, for
both "feathery" (failed at 636 hours) and "dense" (failed at 660
hours) microstructures, showed superior high-temperature thermal
cycling life than one of the baseline and APS YSZ and therefore
clearly showed potential to be used as a higher temperature TBC. A
second baseline sample was run which used the same base alloy and
bond coat as the YAG sample. The YAG TBC sample lasted 27% longer
than the baseline sample. In these preliminary tests, SPPS YAG TBCs
outperformed both baseline samples, thus demonstrating that the
thermal expansion mismatch strains of YAG TBCs can be overcome
using the highly strain tolerant SPPS microstructure. This is a key
result enabling SPPS YAG TBCs to be used as thermal barrier
coatings. Thermal barrier coatings can exhibit life factors of two
or more in the same cyclic test. Of note, is the excellent
consistency of the lives for the three dense and the three samples
with "feathery" microstructure.
[0105] Post-failure SEM analysis indicated that spallation occurs
in the SPPS YAG layer, just above the interface with the APS YSZ
inner layer (FIG. 17). This spallation location provides optimum
thermal cycling durability because it benefits from the strain
tolerance of the SPPS microstructure and, being remote from the
bond coat and the TGO, it experiences lower stresses. FIG. 17 also
shows the uniform vertical cracks in SPPS YAG TBCs that provide
strain-tolerant microstructures, thus enhancing thermal cycling
durability.
[0106] The second type of thermal cycling tests carried out at both
HiFunda and UConn using a widely accepted, lower temperature
(1121.degree. C..+-.5.degree. C.) one hour thermal cycling profile
with 10 minute heating and cooling ramps. SPPS YAG, SPPS YSZ, and
baseline YSZ were tested. The test at HiFunda was conducted in an
in-house fabricated, programmable thermal cycling furnace (FIG.
18). The furnace is kept hot and the specimen is inserted in the
hot furnace, comes to temperature, is held at temperature for 50
minutes, then the specimen is removed from the furnace and cooled.
At the time of this report, the test has reached 400 cycles with no
significant spallation for all the samples tested (FIG. 19). The
test at UConn was also completed after 400 cycles with no specimen
distress on the SPPS YAG TBCs, but the baseline APS YSZ failed at
352 cycles. To summarize, testing at two temperatures and at two
testing sources showed SPPS YAG thermal barrier coatings to have a
cyclic life equal to or greater than SPPS and APS YSZ thermal
barrier coatings. This demonstrates the benefit of the SPPS
strain-tolerant microstructure.
[0107] Based on previous test data under these conditions, a TBC
with poor compliance can fail at under 200 cycles, and 400 cycles
are typical for good APS YSZ TBCs. These tests at moderate
temperatures and short duration (1121.degree. C./1 hour) and high
temperature and long duration (1180.degree. C./12 hours) both
confirm the superior durability of SPPS YAG TBCs and the capability
of the strain-tolerant microstructure to overcome the larger
thermal expansion mismatch strains compared to APS YSZ.
Example 6
[0108] This example describes thermal conductivity
measurements.
[0109] UConn's thermal conductivity measurements of as-sprayed SPPS
YAG TBC coatings were made using the laser flash technique. The
measurement showed a thermal conductivity of 0.95 W/m-K at
23.degree. C.
[0110] HiFunda carried out experiments designed to estimate
"effective" heat transfer coefficients resulting from both
conduction and radiation at high-temperature. Initially, the
emphasis was on using this technique to perform comparative
measurements relative to 7YSZ as opposed to measurements with
extreme precision. FIG. 20 shows a schematic of the measurement
system built at HiFunda for this purpose. A radiant heater was used
to drive heat through the TBC-coated superalloy specimen. The back
end of the steel plate was cooled with circulating air. The whole
system was encased in thick high temperature ceramic insulation
such that the heat flow in all the other directions except for the
path through the TBC-coated specimen would be a very small fraction
(less than 3%) of the net heat generated by the radiant heater.
Thermocouples were attached to the top of the TBC and the back of
the superalloy. Knowing the thermal conductivity of the superalloy,
the thickness of the TBC coatings and the top and bottom
temperatures, the heat flux through the TBC system is estimated and
the effective thermal conductivity of the TBCs were calculated.
[0111] The effective thermal conductivity of as-sprayed SPPS YSZ
and the fully dense SPPS YAG coatings as measured by this technique
is shown in FIG. 21. The thermal conductivity of SPPS YSZ was
between 0.9 and 1.2 Watts/meter Kelvin (W/mK) (from 100.degree. C.
to 1200.degree. C.), which is coincident with the measurement for
SPPS YSZ using a similar approach. The thermal conductivity of SPPS
YAG is between 0.6 and 0.8 W/mK (100.degree. C. to 1200.degree.
C.), which is about 30% lower than that of SPPS YSZ. This is
attributed to the higher porosity in SPPS YAG coatings and the
intrinsically lower thermal conductivity of YAG at high
temperature. Both SPPS YSZ and SPPS YAG have lowest thermal
conductivity around 700.degree. C. and an increase of thermal
conductivity at higher temperature due to the contribution of heat
transfer by radiation. The shapes of the effective thermal
conductivity measurements observed here are similar to, but lower
than (presumably due to the greater porosity in our coatings)
previously reported measurements on APS YAG TBCs. N. P. Padture, K.
W. Schlichting, T. Bhatia, A. Ozturk, B. Cetegen, E. H. Jordan, M.
Gell, S. Jiang, T. D. Xiao, P. R. Strutt, E. Garcia, P. Miranzo And
M. I. Osendi, "Towards Durable Thermal Barrier Coatings With Novel
Microstructures Deposited By Solution Precursor Plasma Spray," Acta
Materialia 49 (2001) 2251-2257.
[0112] Both thermal conductivity measurements at UConn and HiFunda
indicate that SPPS YAG has lower thermal conductivity than SPPS YSZ
and conventional APS and EB-PVD YSZ. As shown in FIG. 22, at
optical magnifications, the SPPS YAG TBCs show no microstructural
change with thermal cycling.
[0113] Table 7 summarizes the properties of SPPS YAG in comparison
with bulk YAG and YSZ materials. The key advantage of using SPPS
YAG as a thermal barrier coating is its higher theoretical
operating temperature (1950.degree. C.) in comparison with YSZ
(1200-1300.degree. C.). The highest temperature that SPPS YAG
coatings have been exposed to in these studies is 1250.degree. C.
and SPPS YAG coatings showed superior high-temperature thermal
cycling life than baseline and APS YSZ at elevated temperature.
Experiments are in progress to evaluate the maximum operating
temperature of SPPS YAG. The thermal conductivity of SPPS YAG with
a coating density of 2.87 is 0.95 measured at room temperature. The
effective thermal conductivity measured between 100.degree. C. and
1200.degree. C. is .about.30% lower than that of SPPS YSZ coatings.
SPPS process has the advantage of controlling microstructure via a
wide range of variables, such as precursor formulation and spray
parameters. As demonstrated in Phase I, the ability to vary coating
hardness in a wide range by different microstructure will be
important for optimizing erosion resistance, thermal conductivity
of YAG thermal barrier coatings and also for applications such as
abradable blade outer air seals and graded coatings.
TABLE-US-00007 TABLE 7 Properties of SPPS YAG, YAG and YSZ Material
Property YSZ (bulk) YAG (bulk) SPPS YAG TBCs Melting Point
(.degree. C.) 2680 1950 1950 Maximum Operating 1200-1300 1800 to be
determined Temperature (.degree. C.) (1250.degree. C. in initial
experiments) Thermal Conductivity at 2.0-3.0 (measured 8.7
(measured 23.degree. C.)/2.5 0.95 at 23.degree. C. (UConn)
23/1350.degree. C. (W/m-K) over temp range) (extrapolated
1350.degree. C.) 0.8 at 100.degree. C. (HiFunda) Thermal Expansion
9.5 .times. 10.sup.-6 7.5 .times. 10.sup.-6 7.5 .times. 10.sup.-6
but stresses Coefficient (ppm/K) reduced by microstructure Density
(g/cc) 6.10 4.55 2.87 Vickers Hardness (HV) 1200 1700 300-1550
[0114] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to make and use the invention. The patentable
scope of the invention is defined by the claims, and may include
other examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims if they
have structural elements that do not differ from the literal
language of the claims, or if they include equivalent structural
elements with insubstantial differences from the literal language
of the claims.
[0115] \All cited patents, patent applications, and other
references are incorporated herein by reference in their entirety.
However, if a term in the present application contradicts or
conflicts with a term in the incorporated reference, the term from
the present application takes precedence over the conflicting term
from the incorporated reference.
[0116] All ranges disclosed herein are inclusive of the endpoints,
and the endpoints are independently combinable with each other.
Each range disclosed herein constitutes a disclosure of any point
or sub-range lying within the disclosed range.
[0117] The use of the terms "a" and "an" and "the" and similar
referents in the context of describing the invention (especially in
the context of the following claims) are to be construed to cover
both the singular and the plural, unless otherwise indicated herein
or clearly contradicted by context. Further, it should further be
noted that the terms "first," "second," and the like herein do not
denote any order, quantity, or importance, but rather are used to
distinguish one element from another. The modifier "about" used in
connection with a quantity is inclusive of the stated value and has
the meaning dictated by the context (e.g., it includes the degree
of error associated with measurement of the particular
quantity).
* * * * *