U.S. patent application number 10/748518 was filed with the patent office on 2005-06-30 for thermal barrier coatings with lower porosity for improved impact and erosion resistance.
Invention is credited to Boutwell, Brett Allen, Spitsberg, Irene.
Application Number | 20050142394 10/748518 |
Document ID | / |
Family ID | 34574772 |
Filed Date | 2005-06-30 |
United States Patent
Application |
20050142394 |
Kind Code |
A1 |
Spitsberg, Irene ; et
al. |
June 30, 2005 |
THERMAL BARRIER COATINGS WITH LOWER POROSITY FOR IMPROVED IMPACT
AND EROSION RESISTANCE
Abstract
A reduced thermal conductivity thermal barrier coating having
improved impact and erosion resistance for an underlying metal
substrate of articles that operate at, or are exposed to, high
temperatures. This coating comprises a zirconia-containing ceramic
composition having a c/a ratio in the range of from about 1.0057 to
about 1.0123 and stabilized in the tetragonal phase by a
stabilizing amount of a stabilizing metal oxide. The coating has a
fraction of porosity of from about 0.15 to about 0.25, and an
impact and erosion resistance property defined by at least one of
the following formulas: (a) I=exp. [5.85-(144.times.s)-(3.68.times-
.p)]; and/or; (b) E=[187-(261.times.p)-(9989.times.s)], wherein
s=1.0117-c/a ratio; p is the fraction of porosity; I is least about
70 g/mil; and E is least about 80 g/mil. This coating can be used
to provide a thermally protected article having a metal substrate
and optionally a bond coat layer adjacent to and overlaying the
metal substrate. The thermal barrier coating can be prepared by
depositing the zirconia-containing ceramic composition on the bond
coat layer, or the metal substrate in the absence of the bond coat
layer.
Inventors: |
Spitsberg, Irene; (Loveland,
OH) ; Boutwell, Brett Allen; (Liberty Township,
OH) |
Correspondence
Address: |
JAGTIANI + GUTTAG
10363-A DEMOCRACY LANE
FAIRFAX
VA
22030
US
|
Family ID: |
34574772 |
Appl. No.: |
10/748518 |
Filed: |
December 30, 2003 |
Current U.S.
Class: |
428/701 ;
427/250; 427/585; 428/332; 428/632; 428/702 |
Current CPC
Class: |
F01D 5/288 20130101;
C23C 28/3215 20130101; Y10T 428/265 20150115; C23C 28/321 20130101;
C23C 14/083 20130101; C23C 30/00 20130101; F05D 2300/2118 20130101;
C23C 14/08 20130101; F05D 2230/90 20130101; Y10T 428/12611
20150115; C23C 28/3455 20130101; Y10T 428/249953 20150401; F05D
2300/611 20130101; Y10T 428/26 20150115 |
Class at
Publication: |
428/701 ;
428/702; 428/632; 428/332; 427/585; 427/250 |
International
Class: |
B32B 009/00; C23C
016/00 |
Goverment Interests
[0001] This invention was made with Government support under
Contract No. N00019-96-C-0176 awarded by the JSF Program Office.
The Government has certain rights to the invention.
Claims
1. A thermal barrier coating for an underlying metal substrate
which comprises a zirconia-containing ceramic composition having a
c/a ratio of the zirconia lattice in the range of from about 1.0057
to about 1.0110 and stabilized in the tetragonal phase by a
stabilizing amount of a stabilizing metal oxide, the thermal
barrier coating having: 1. a fraction of porosity of from about
0.15 to about 0.25; and 2. an impact and erosion resistance
property defined by at least one of the following formulas: I=exp.
[5.85-(144.times.s)-(3.68.times.p)]; (a)
E=[187-(261.times.p)-(9989.times.s)]; (b) wherein s=1.0117-c/a
ratio; p is the fraction of porosity; I is at least about 70 g/mil;
and E is at least about 80 g/mil.
2. The coating of claim 1 which has a strain-tolerant columnar
structure.
3. The coating of claim 2 wherein the c/a ratio is in the range of
from about 1.0069 to about 1.0096.
4. The coating of claim 2 wherein the fraction of porosity is from
about 0.18 to about 0.20.
5. The coating of claim 2 which has an impact and erosion
resistance property defined by both of formulas (a) and (b).
6. The coating of claim 5 wherein I is at least about 90 g/mil and
E is at least about 100 g/mil.
7. The coating of claim 2 wherein the stabilizing metal oxide is
selected from the group consisting of yttria, calcia, ceria,
scandia, magnesia, india, lanthana, gadolinia, neodymia, samaria,
dysprosia, erbia, ytterbia, europia, praseodymia, and mixtures
thereof.
8. The coating of claim 7 wherein the stabilizing metal oxide is
selected from the group consisting of yttria, lanthana, and
mixtures thereof.
9. The coating of claim 8 wherein the stabilizing metal oxide is
yttria.
10. A thermally protected article, which comprises: A. a metal
substrate; and B. a thermal barrier coating which comprises a
zirconia-containing ceramic composition having a c/a ratio of the
zirconia lattice in the range of from about 1.0057 to about 1.0110
and stabilized in the tetragonal phase by a stabilizing amount of a
stabilizing metal oxide, the thermal barrier coating having: 1. a
fraction of porosity of from about 0.15 to about 0.25; and 2. an
impact and erosion resistance property defined by at least one of
the following formulas: I=exp. [5.85-(144.times.s)-(3.68.times.p)];
(a) E=[187-(261.times.p)-(9989.time- s.s)]; (b) wherein
s=1.0117-c/a ratio; p is the fraction of porosity; I is least about
70 g/mil; and E is least about 80 g/mil.
11. The article of claim 10 which further comprises a bond coat
layer adjacent to and overlaying the metal substrate and wherein
the inner layer is adjacent to and overlies the bond coat
layer.
12. The article of claim 11 wherein the thermal barrier coating has
a thickness of from about 1 to about 100 mils.
13. The article of claim 12 wherein the thermal barrier coating has
a strain-tolerant columnar structure.
14. The article of claim 13 wherein the c/a ratio is in the range
of from about 1.0069 to about 1.0096.
15. The article of claim 13 wherein the fraction of porosity is
from about 0.18 to about 0.20.
16. The article of claim 13 wherein the thermal barrier coating has
an impact and erosion resistance property defined by both of
formulas (a) and (b).
17. The article of claim 16 wherein I is at least about 90 g/mil
and E is at least about 100 g/mil.
18. The article of claim 13 wherein the stabilizing metal oxide is
selected from the group consisting of yttria, calcia, ceria,
scandia, magnesia, india, lanthana, gadolinia, neodymia, samaria,
dysprosia, erbia, ytterbia, europia, praseodymia, and mixtures
thereof.
19. The article of claim 18 wherein the stabilizing metal oxide is
selected from the group consisting of yttria, lanthana, and
mixtures thereof.
20. The coating of claim 19 wherein the stabilizing metal oxide is
yttria.
21. The article of claim 13 which is a turbine engine
component.
22. The article of claim 21 which is a turbine shroud and wherein
the thermal barrier coating has a thickness of from about 30 to
about 70 mils.
23. The article of claim 21 which is a turbine airfoil and wherein
the thermal barrier coating has a thickness of from about 3 to
about 20 mils.
24. A method for preparing a thermal barrier coating for an
underlying metal substrate, the method comprising the step of: a.
depositing over the metal substrate a zirconia-containing ceramic
composition having a c/a ratio of the zirconia lattice in the range
of from about 1.0057 to about 1.0110 and stabilized in the
tetragonal phase by a stabilizing amount of a stabilizing metal
oxide selected from the group consisting of yttria, calcia, ceria,
scandia, magnesia, india, lanthana, gadolinia, neodymia, samaria,
dysprosia, erbia, ytterbia, europia, praseodymia, and mixtures
thereof to form a thermal barrier coating having: 1. a fraction of
porosity of from about 0.15 to about 0.25; and 2. an impact and
erosion resistance property defined by at least one of the
following formulas: I=exp. [5.85-(144.times.s)-(3.68.times.p)]; (a)
E=[187-(261.times.p)-(9989.times.s)]; (b) wherein s=1.0117-c/a
ratio; p is the fraction of porosity; I is least about 70 g/mil;
and E is least about 80 g/mil.
25. The method of claim 24 wherein a bond coat layer is adjacent to
and overlies the metal substrate and wherein the thermal barrier
coating is formed on the bond coat layer.
26. The method of claim 25 wherein the zirconia-containing ceramic
composition is deposited by physical vapor deposition to form a
thermal barrier coating having a strain-tolerant columnar
structure.
27. The method of claim 26 wherein the thermal barrier coating is
formed so as to have an impact and erosion resistance property
defined by both of formulas (a) and (b).
28. The method of claim 27 wherein the thermal barrier coating is
formed to have an impact and erosion resistance property defined by
formulas (a) and (b) such that I is at least about 90 g/mil and E
is at least about 100 g/mil.
29. The method of claim 26 wherein the thermal barrier coating is
formed from a zirconia-containing ceramic composition stabilized
with a stabilizing metal oxide selected from the group consisting
of yttria, lanthana, and mixtures thereof.
30. The method of claim 27 wherein the thermal barrier coating is
formed to have a fraction of porosity of from about 0.18 to about
0.20 and is formed from a zirconia-containing ceramic composition
stabilized with yttria and having a c/a ratio in the range of from
about 1.0069 to about 1.0096.
Description
BACKGROUND OF THE INVENTION
[0002] This invention relates to improving the impact and erosion
resistance of thermal barrier coatings having reduced thermal
conductivity. This invention further relates to articles having
such coatings and methods for preparing such coatings for the
article.
[0003] Components operating in the gas path environment of gas
turbine engines are typically subjected to significant temperature
extremes and degradation by oxidizing and corrosive environments.
Environmental coatings and especially thermal barrier coating are
an important element in current and future gas turbine engine
designs, as well as other articles that are expected to operate at
or be exposed to high temperatures, and thus cause the thermal
barrier coating to be subjected to high surface temperatures.
Examples of turbine engine parts and components for which such
thermal barrier coatings are desirable include turbine blades and
vanes, turbine shrouds, buckets, nozzles, combustion liners and
deflectors, and the like. These thermal barrier coatings typically
comprise the external portion or surface of these components are
usually deposited onto a metal substrate (or more typically onto a
bond coat layer on the metal substrate for better adherence) from
which the part or component is formed to reduce heat flow (i.e.,
provide thermal insulation) and to limit (reduce) the operating
temperature the underlying metal substrate of these parts and
components is subjected to. This metal substrate typically
comprises a metal alloy such as a nickel, cobalt, and/or iron based
alloy (e.g., a high temperature superalloy).
[0004] The thermal barrier coating is usually prepared from a
ceramic material, such as a chemically (metal oxide) stabilized
zirconia. Examples of such chemically phase-stabilized zirconias
include yttria-stabilized zirconia, scandia-stabilized zirconia,
calcia-stabilized zirconia, and magnesia-stabilized zirconia. The
thermal barrier coating of choice is typically a yttria-stabilized
zirconia ceramic coating. A representative yttria-stabilized
zirconia thermal barrier coating usually comprises about 7 weight %
yttria and about 93 weight % zirconia. The thickness of the thermal
barrier coating depends upon the metal part or component it is
deposited on, but is usually in the range of from about 3 to about
70 mils (from about 76 to about 1778 microns) thick for high
temperature gas turbine engine parts.
[0005] Although significant advances have been made in improving
the durability of thermal barrier coatings for turbine engine
components, such coatings are still susceptible to various types of
damage, including objects ingested by the engine, erosion,
oxidation, and attack from environmental contaminants. In addition,
in trying to achieve reduced thermal conductivity, other properties
of the thermal barrier coating can be adversely impacted. For
example, the composition and crystalline microstructure of a
thermal barrier coating, such as those prepared from
yttria-stabilized zirconia, can be modified to impart to the
coating an improved reduction in thermal conductivity, especially
as the coating ages over time. However, such modifications can also
unintentionally interfere with desired spallation resistance, as
well as resistance to particle erosion, especially at the higher
temperatures that most turbine components are subjected to. As a
result, the thermal barrier coating can become more susceptible to
damage due to the impact of, for example, objects ingested by the
engine, as well as erosion.
[0006] Accordingly, it would be desirable to be able to improve the
impact and erosion resistance of thermal barrier coatings having
reduced thermal conductivity. It would be further desirable to be
able to modify the chemical composition of yttria-stabilized
zirconia-based thermal barrier coating systems to provide such
reduced thermal conductivity, yet still retain at least acceptable
impact and erosion resistance in such coatings.
BRIEF DESCRIPTION OF THE INVENTION
[0007] An embodiment of this invention relates to improving the
impact and erosion resistance of a thermal barrier coating having
reduced thermal conductivity that is used with an underlying metal
substrate of articles that operate at, or are exposed, to high
temperatures. This thermal barrier coating comprises a
zirconia-containing ceramic composition having a c/a ratio of the
zirconia lattice in the range of from about 1.0057 to about 1.0110
and stabilized in the tetragonal phase by a stabilizing amount of a
stabilizing metal oxide, the thermal barrier coating having:
[0008] 1. a fraction of porosity of from about 0.15 to about 0.25;
and
[0009] 2. an impact and erosion resistance property defined by at
least one of the following formulas:
I=exp. [5.85-(144.times.s)-(3.68.times.p)]; (a)
E=[187-(261.times.p)-(9989.times.s)]; (b)
[0010] wherein s=1.0117-c/a ratio; p is the fraction of porosity; I
is at least about 70 g/mil; and E is at least about 80 g/mil.
[0011] Another embodiment of this invention relates to a thermally
protected article. This protected article comprises:
[0012] A. a metal substrate;
[0013] B. optionally a bond coat layer adjacent to and overlaying
the metal substrate; and
[0014] C. a thermal barrier coating (as previously described)
adjacent to and overlaying the bond coat layer (or overlaying the
metal substrate if the bond coat layer is absent).
[0015] Another embodiment of this invention relates to a method for
preparing the thermal barrier coating on a metal substrate to
provide a thermally protected article. This method comprises the
steps of:
[0016] A. optionally forming a bond coat layer on the metal
substrate;
[0017] B. depositing on the bond coat layer (or on the metal
substrate in the absence of the bond coat layer) the
zirconia-containing ceramic composition previously described to
form the thermal barrier coating having the previously described
porosity and impact/erosion resistance properties.
[0018] The thermal barrier coatings of this invention provide
several significant benefits when used with metal substrates of
articles exposed to high temperatures, such as turbine components.
The thermal barrier coatings of this invention provide a desirable
balance of reduced thermal conductivity for the thermally protected
article with improved impact and erosion resistance. This
improvement in impact and erosion resistance for the thermal
barrier coating can be achieved while allowing flexibility in using
a variety of zirconia-containing ceramic compositions that can
impart to the thermal barrier coating desirable reduced thermal
conductivity properties.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 represents a graphical plot of the calculated c/a
ratios of the zirconia lattice as a function of yttria content.
[0020] FIG. 2 represents graphical plots of the calculated
stability level s of the zirconia lattice as a function of yttria,
lanthana or ytterbia content.
[0021] FIG. 3 represents graphical plots of the predicted
normalized impact resistance values of thermal barrier coatings at
various porosities as a function of yttria equivalent.
[0022] FIG. 4 represents graphical plots of predicted normalized
erosion resistance values of thermal barrier coatings at various
porosities as a function of yttria equivalent.
[0023] FIG. 5 is a side sectional view of an embodiment of the
thermal barrier coating and coated article of this invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] As used herein, the term "zirconia-containing ceramic
compositions" refers to ceramic compositions where zirconia is the
primary component that are useful as thermal barrier coatings that
are capable of reducing heat flow to the underlying metal substrate
of the article, i.e., forming a thermal barrier, and which have a
melting point that is typically at least about 2600.degree. F.
(1426.degree. C.), and more typically in the range of from about
from about 3450.degree. to about 4980.degree. F. (from about
1900.degree. to about 2750.degree. C.).
[0025] As used herein, the term "fraction of porosity" refers to
the volume fraction of porosity defined by unity (i.e., 1), minus
the ratio of the actual density of the thermal barrier coating to
its theoretical density.
[0026] As used herein, the term "comprising" means various
compositions, compounds, components, layers, steps and the like can
be conjointly employed in the present invention. Accordingly, the
term "comprising" encompasses the more restrictive terms
"consisting essentially of" and "consisting of."
[0027] All amounts, parts, ratios and percentages used herein are
by mole unless otherwise specified.
[0028] Zirconia-containing ceramic compositions useful in this
invention impart improved thermal conductivity properties to the
resulting thermal barrier coatings, and in particular lower thermal
conductivity. Thermal conductivity K is defined by the following
equation (1):
K=.alpha..times.(1-p).times.C.sub.p.times.D.sub.t (1)
[0029] where .alpha. is the thermal diffusivity, p is the fraction
of porosity, C.sub.p is the specific heat (in J*K/g), and D.sub.t
is the theoretical density. As be seen from equation (1) above, the
thermal conductivity depends on thermal diffusivity and
porosity.
[0030] Suitable zirconia-containing compositions useful herein
include those which comprise at least about 91 mole % zirconia, and
typically from about 91 to about 97 mole % zirconia, more typically
from about 93.5 to about 95 mole % zirconia. These
zirconia-containing compositions further comprise a stabilizing
amount of stabilizing metal oxide. This stabilizing metal oxide can
be selected from the group consisting of yttria, calcia, ceria,
scandia, magnesia, india, lanthana, gadolinia, neodymia, samaria,
dysprosia, erbia, ytterbia, europia, praseodymia, and mixtures
thereof. The particular amount of this metal oxide that is
"stabilizing" will depend on a variety of factors, including the
metal oxide used and the erosion and impact resistance. Typically,
the stabilizing metal oxide comprises from about 3 about 9 mole %,
more typically from about 5 to about 6.5 mole %, of the
composition. The zirconia-containing ceramic compositions used
herein typically comprise yttria, lanthana, or mixtures thereof as
the stabilizing metal oxide, and more typically yttria. The
zirconia-containing ceramic compositions used herein can also
optionally comprise small amounts of hafnia, titania, tantala,
niobia and mixtures thereof.
[0031] Particularly suitable zirconia-containing ceramic
compositions for use herein are disclosed in copending U.S.
nonprovisional applications entitled "CERAMIC COMPOSITIONS USEFUL
FOR THERMAL BARRIER COATINGS HAVING REDUCED THERMAL CONDUCTIVITY"
(Spitsberg et al), Ser. No., ______, filed ______, 2003, Attorney
Docket No. 129967 and "CERAMIC COMPOSITIONS USEFUL IN THERMAL
BARRIER COATINGS HAVING REDUCED THERMAL CONDUCTIVITY" (Spitsberg et
al), Ser. No., ______ filed ______, 2003, Attorney Docket No.
129968, both of which are incorporated by reference. The
zirconia-containing ceramic compositions disclosed in the first of
these copending applications comprise at least about 91 mole %
zirconia and up to about 9 mole % of a stabilizer component
comprising a first metal oxide selected from the group consisting
of yttria, calcia, ceria, scandia, magnesia, india and mixtures
thereof; a second metal oxide of a trivalent metal atom selected
from the group consisting of lanthana, gadolinia, neodymia,
samaria, dysprosia, and mixtures thereof; and a third metal oxide
of a trivalent metal atom selected from the group consisting of
erbia, ytterbia and mixtures thereof. Typically, these ceramic
compositions comprise from about 91 to about 97 mole % zirconia,
more typically from about 92 to about 95 mole % zirconia and from
about 3 to about 9 mole %, more typically from about from about 5
to about 8 mole %, of the composition of the stabilizing component;
the first metal oxide (typically yttria) can comprise from about 3
to about 6 mole %, more typically from about 3 to about 5 mole %,
of the ceramic composition; the second metal oxide (typically
lanthana or gadolinia) can comprise from about 0.25 to about 2 mole
%, more typically from about 0.5 to about 1.5 mole %, of the
ceramic composition; and the third metal oxide (typically ytterbia)
can comprise from about 0.5 to about 2 mole %, more typically from
about 0.5 to about 1.5 mole %, of the ceramic composition, with the
ratio of the second metal oxide to the third metal oxide typically
being in the range of from about 0.5 to about 2, more typically
from about 0.75 to about 1.33.
[0032] The zirconia-containing ceramic compositions disclosed in
the second of these copending applications comprise at least about
91 mole % zirconia and a stabilizing amount up to about 9 mole % of
a stabilizer component comprising a first metal oxide having
selected from the group consisting of yttria, calcia, ceria,
scandia, magnesia, india and mixtures thereof and a second metal
oxide of a trivalent metal atom selected from the group consisting
of lanthana, gadolinia, neodymia, samaria, dysprosia, erbia,
ytterbia, and mixtures thereof. Typically, these ceramic
compositions comprise from about 91 to about 97 mole % zirconia,
more typically from about 92 to about 95 mole % zirconia and from
about 3 to about 9 mole %, more typically from about 5 to about 8
mole %, of the composition of the stabilizing component; the first
metal oxide (typically yttria) can comprise from about 3 to about 6
mole %, more typically from about 4 to about 5 mole %, of the
ceramic composition; the second metal oxide (typically lanthana,
gadolinia or ytterbia, and more typically lanthana) can comprise
from about 0.5 to about 4 mole %, more typically from about 0.8 to
about 2 mole %, of the ceramic composition, and wherein the mole %
ratio of second metal oxide (e.g., lanthana/gadolinia/ytterbia) to
first metal oxide (e.g., yttria) is in the range of from about 0.1
to about 0.5, typically from about 0.15 to about 0.35, more
typically from about 0.2 to about 0.3.
[0033] Thermal barrier coatings of this invention comprise a
zirconia-containing ceramic composition that is stabilized in a
certain region of the tetragonal phase. The impact and erosion
resistance properties of these thermal barrier coatings can be
predicted on the basis of the effect of the zirconia lattice
stability equivalent of the respective zirconia-containing ceramic
compositions. Impact and erosion resistance performance have been
found to be related to the zirconia lattice stability equivalent.
This stability equivalent can be calculated based on the ratio of
the zirconia lattice parameters c and a using equation (2) below: 1
c a = k 1 i ( r i - r Zr ) .times. m i + k 2 i ( V i - V Zr )
.times. m i ( 2 )
[0034] wherein c,a are the zirconia tetragonal lattice parameters,
r.sub.i is the ionic radius of the first metal oxide, V.sub.i is
the valence of the metal ion of the metal oxide added, m.sub.i is
the mole fraction of the metal oxide added and k.sub.1 and k.sub.2
are constants. See Kim, "Effect of Ta.sub.2O.sub.5,
Nb.sub.2O.sub.5, and HfO.sub.2 Alloying on the Transformability of
Y.sub.2O.sub.3-Stabilized Tetragonal ZrO.sub.2," J. Am. Ceram.
Soc., (1990) 73 (1) pp. 115-120.
[0035] Using equation (2) above, the lattice stability of these
zirconia-containing ceramic compositions in the tetragonal phase
can be calculated, including the effect of incremental additions of
the stabilizing metal oxide, such as yttria. This is illustrated by
FIG. 1 which represents a graphical plot of calculated c/a ratios
for the zirconia lattice as a function of yttria content. The
dotted line (base line) in FIG. 1 represents a zirconia-containing
ceramic composition stabilized with the equivalent of about 4 mole
% (7YSZ) that has a c/a ratio of about 1.0117. Similar lattice
stability values can also be calculated for the incremental
addition of other stabilizing metal oxides such as lanthana and
ytterbia. This is illustrated by FIG. 2 which represents graphical
plots of the calculated stability level s (s=1.0117-c/a ratio) of
the zirconia lattice as a function of yttria (base line), lanthana
or ytterbia content. As can be seen in FIG. 2, lanthana addition
has the greatest effect on lattice stability.
[0036] Referring again to FIG. 1, as the level of yttria decreases
in the zirconia-containing ceramic composition, the c/a ratio
conversely increases. It has been further found that, as the c/a
ratio increases, impact and erosion resistance improves, i.e.,
lowering the yttria level improves the impact and erosion
resistance performance of the zirconia-containing ceramic
composition.
[0037] While increasing the c/a ratio of the zirconia-containing
ceramic composition generally improves the impact and erosion
resistance performance of the resulting thermal barrier coating,
these higher c/a ratios also have a significant and undesirable
effect in increasing thermal conductivity (i.e., the K value is
higher) of such coatings. Indeed, the zirconia-containing ceramic
compositions of this invention that provide thermal barrier
coatings with-reduced thermal conductivity have lower c/a ratios of
about 1.0110 or less, and typically in the range of from about
1.0057 to about 1.0110, more typically in the range of from about
1.0069 to about 1.0096. Because the zirconia-containing ceramic
compositions of this invention have lower c/a ratios, the thermal
barrier coatings resulting from these compositions will also tend
to have poorer impact and erosion resistance performance,
especially relative to coatings prepared from 7YSZ compositions.
Accordingly, another mechanism is needed in order to achieve
satisfactory erosion and impact resistance performance for thermal
barrier coatings prepared from these zirconia-containing ceramic
compositions having lower c/a ratios.
[0038] While the c/a ratio of the zirconia-containing composition
has a very strong, exponential effect on impact resistance
performance, and a less strong, linear impact on erosion resistance
performance, it has been found that the porosity level of the
resultant thermal barrier coating also has a very significant
effect on impact and erosion resistance performance. Moreover,
while decreasing porosity has an exponential effect in improving
impact performance, it has been further found to have only a more
limited, linear effect in increasing thermal conductivity (i.e.,
increasing the K value) of thermal barrier coating. Accordingly, by
controlling the porosity level of the thermal barrier coating
formed from a zirconia-containing ceramic composition having lower
c/a ratios within the range providing desired reduced thermal
conductivity, an appropriate balance of reduced thermal
conductivity and satisfactory erosion and impact resistance
performance can be obtained. The porosity level of the thermal
barrier coatings of this invention is defined herein by the
fraction of porosity of the coating. The thermal barrier coatings
useful in this invention that provide an appropriate balance of
reduced thermal conductivity with satisfactory impact and erosion
resistance have a fraction of porosity of from about 0.15 to about
0.25, more typically from about 0.18 to about 0.20.
[0039] In order to be able to select zirconia-containing ceramic
compositions having a balance of satisfactory impact and erosion
resistance performance by controlling the porosity within the c/a
ratio range having reduced thermal conductivity, experimental data
is obtained on thermal conductivity, impact resistance and erosion
resistance properties of a variety coatings and compositions.
Thermal conductivity data is obtained by the laser flash method.
See ASTM standard E1461-01. Impact and erosion resistance data (in
g/mil) is obtained by testing thermal barrier coatings by the
method described in Bruce, "Development of 1232C (2250 F) Erosion
and Impact Tests for Thermal Barrier Coatings," Tribology Trans.,
(1998), 41(4); 399-410, which is incorporated by reference. This
data obtained by the Bruce method is then normalized by coating
thickness to provide an impact and erosion index that represents
the impact and erosion resistance of the coating at a thickness of
1 mil. Generally, the higher the index is, the better will be the
impact and erosion resistance of the coating.
[0040] The (normalized) data obtained for thermal conductivity,
impact resistance, and erosion resistance are used to develop
appropriate transfer functions. These transfer functions are
represented as follows:
K=f(c/a, p) (3)
I=f(c/a, p) (4)
E=f(c/a, p) (5)
[0041] wherein K is thermal conductivity, I is the normalized
impact resistance (i.e., impact index), E is the normalized erosion
resistance (i.e., erosion index), c/a is defined as before, and p
is the fraction of porosity. Using a standard Excel "solver"
procedure, equations (3), (4) and (5) can be solved simultaneously
to optimize zirconia-containing ceramic compositions that can be
processed into thermal barrier coatings that have the desired
balance of reduced thermal conductivity with satisfactory impact
and erosion resistance performance. From statistical analysis of a
large population of experimental data (about 50 data points), the
transfer function (4) for impact resistance (I) and the transfer
function (5) for erosion resistance (E) were found to be
represented by formulas (a) and (b) below:
I=exp. [5.85-(144.times.s)-(3.68.times.p)] (a)
E=[187-(261.times.p)-(9989.times.s)] (b)
[0042] wherein s=1.0117-c/a ratio; and p is the fraction of
porosity. The regression fit is about 85% for formulas (a) and (b)
above.
[0043] Using formulas (a) and (b) above, FIGS. 3 and 4 graphically
represent plots of predicted normalized impact and erosion
resistance values (in g/mil), respectively, of thermal barrier
coatings at various porosities (i.e., defined by the fraction of
porosity) as a function of yttria equivalent (in mole %), the
yttria equivalent also corresponding to particular c/a ratios. FIG.
3 shows impact resistance to be an exponential function of c/a
ratio and porosity, while FIG. 4 shows erosion resistance to be a
linear function of c/a ratio and porosity. These normalized impact
and erosion resistance values are also presented in the following
Tables 1 and 2:
1TABLE 1 Predicted Normalized Impact Resistance (g/mil) Yttria
Equivalent Porosity c/a Ratio (Mole %) 0.1 0.15 0.18 0.2 0.25 0.27
0.3 0.33 1.0167 1.6 494 411 368 342 284 264 237 212 1.0148 2.5 370
308 276 256 213 198 177 159 1.0138 3 321 267 239 222 185 171 154
137 1.0117 4 240 200 179 166 138 129 115 103 1.0110 4.1 218 182 163
151 126 117 105 94 1.0096 5 180 150 134 125 104 96 86 77 1.0077 6
135 112 101 94 78 72 65 58 1.0069 6.4 122 102 91 85 70 65 59 52
1.0057 7 101 84 75 70 58 54 49 43
[0044]
2TABLE 2 Predicted Normalized Erosion Resistance (g/mil) Yttria
Equivalent Porosity c/a Ratio (Mole %) 0.1 0.15 0.18 0.2 0.25 0.27
0.3 0.33 1.0167 1.6 211 198 190 185 172 166 159 151 1.0148 2.5 191
178 170 165 152 146 139 131 1.0138 3 181 168 160 155 142 137 129
121 1.0117 4 161 148 140 135 122 117 109 101 1.0110 4.1 154 141 133
128 115 110 102 94 1.0096 5 141 128 120 115 102 97 89 81 1.0077 6
121 108 100 95 82 77 69 61 1.0069 6.4 114 101 93 88 75 70 62 54
1.0057 7 101 88 80 75 62 57 49 41
[0045] The thermal barrier coatings of this invention are defined
in terms of the c/a ratio of the zirconia-containing ceramic
composition, the fraction of porosity p, and an impact and erosion
resistance property defined by at least one of the above formulas
(a) or (b), and more typically defined by both of formulas (a) and
(b). For thermal barrier coatings of this invention having the
previously specified c/a ratios (for the zirconia-containing
ceramic composition) and fraction of porosity, I is typically at
least about 70 g/mil, more typically at least about 90 g/mil, while
E is typically at least about 80 g/mil, more typically at least
about 100 g/mil. Some illustrative zirconia-containing compositions
having the previously specified c/a ratios that can be formulated
into thermal barrier coatings having the previously specified
fraction of porosity, to satisfy the indicated impact and erosion
resistance properties I and E are as follow:
3 TABLE 3 Metal Oxide (Mole %) Composition 1 Composition 2 Zirconia
94.0 95.0 Total Stabilizer 6.0 5.0 Yttria 4.8 3.6 Lanthana 1.2 1.4
Porosity 0.18-0.20 0.23-0.25
[0046] Thermal barrier coatings of this invention are useful with a
wide variety of turbine engine (e.g., gas turbine engine) parts and
components that are formed from metal substrates comprising a
variety of metals and metal alloys, including superalloys, and are
operated at, or exposed to, high temperatures, especially higher
temperatures that occur during normal engine operation. These
turbine engine parts and components can include turbine airfoils
such as blades and vanes, turbine shrouds, turbine nozzles,
combustor components such as liners and deflectors, augmentor
hardware of gas turbine engines and the like. The thermal barrier
coatings of this invention can also cover a portion or all of the
metal substrate. For example, with regard to airfoils such as
blades, the thermal barrier coatings of this invention are
typically used to protect, cover or overlay portions of the metal
substrate of the airfoil rather than the entire component, e.g.,
the thermal barrier coatings could cover the leading edge, possibly
part of the trailing edge, but not the attachment area. While the
following discussion of the thermal barrier coatings of this
invention will be with reference to metal substrates of turbine
engine parts and components, it should also be understood that the
thermal barrier coatings of this invention are useful with metal
substrates of other articles that operate at, or are exposed to,
high temperatures.
[0047] The various embodiments of the thermal barrier coatings of
this invention are further illustrated by reference to the drawings
as described hereafter. Referring to the drawings, FIG. 5 shows a
side sectional view of an embodiment of the thermally barrier
coating used with the metal substrate of an article indicated
generally as 10. As shown in FIG. 5, article 10 has a metal
substrate indicated generally as 14. Substrate 14 can comprise any
of a variety of metals, or more typically metal alloys, that are
typically protected by thermal barrier coatings, including those
based on nickel, cobalt and/or iron alloys. For example, substrate
14 can comprise a high temperature, heat-resistant alloy, e.g., a
superalloy. Such high temperature alloys are disclosed in various
references, such as U.S. Pat. No. 5,399,313 (Ross et al), issued
Mar. 21, 1995 and U.S. Pat. No. 4,116,723 (Gell et al), issued Sep.
26, 1978, both of which are incorporated by reference. High
temperature alloys are also generally described in Kirk-Othmer's
Encyclopedia of Chemical Technology, 3rd Ed., Vol. 12, pp. 417-479
(1980), and Vol. 15, pp. 787-800 (1981). Illustrative high
temperature nickel-based alloys are designated by the trade names
Inconel.RTM., Nimonic.RTM., Ren.RTM. (e.g., Ren.RTM. 80-, Ren.RTM.
(95 alloys), and Udimet.RTM.. As described above, the type of
substrate 14 can vary widely, but it is representatively in the
form of a turbine part or component, such as an airfoil (e.g.,
blade) or turbine shroud.
[0048] As shown in FIG. 5, article 10 can also include a bond coat
layer indicated generally as 18 that is adjacent to and overlies
substrate 14. Bond coat layer 18 is typically formed from a
metallic oxidation-resistant material that protects the underlying
substrate 14 and enables the thermal barrier coating indicated
generally as 22 to more tenaciously adhere to substrate 14.
Suitable materials for bond coat layer 18 include MCrAlY alloy
powders, where M represents a metal such as iron, nickel, platinum
or cobalt, or NiAl(Zr) compositions, as well as various noble metal
diffusion aluminides such as nickel aluminide and platinum
aluminide, as well as simple aluminides (i.e., those formed without
noble metals). This bond coat layer 18 can be applied, deposited or
otherwise formed on substrate 10 by any of a variety of
conventional techniques, such as physical vapor deposition (PVD),
including electron beam physical vapor deposition (EB-PVD), plasma
spray, including air plasma spray (APS) and vacuum plasma spray
(VPS), or other thermal spray deposition methods such as high
velocity oxy-fuel (HVOF) spray, detonation, or wire spray, chemical
vapor deposition (CVD), pack cementation and vapor phase
aluminiding in the case of metal diffusion aluminides (see, for
example, U.S. Pat. No. 4,148,275 (Benden et al), issued Apr. 10,
1979; U.S. Pat. No. 5,928,725 (Howard et al), issued Jul. 27, 1999;
and See U.S. Pat. No. 6,039,810 (Mantkowski et al), issued Mar. 21,
2000, all of which are incorporated by reference and which disclose
various apparatus and methods for applying diffusion aluminide
coatings, or combinations of such techniques, such as, for example,
a combination of plasma spray and diffusion aluminide techniques.
Typically, plasma spray or diffusion techniques are employed to
deposit bond coat layer 18. Usually, the deposited bond coat layer
18 has a thickness in the range of from about 1 to about 20 mils
(from about 25 to about 500 microns). For bond coat layers 18
deposited by PVD techniques such as EB-PVD or diffusion aluminide
processes, the thickness is more typically in the range of from
about 1 about 3 mils (from about 25 to about 76 microns). For bond
coat layers deposited by plasma spray techniques such as APS, the
thickness is more typically in the range of from about 3 to about
15 mils (from about 76 to about 381 microns).
[0049] As shown in FIG. 5, the thermal barrier coating (TBC) 22 is
adjacent to and overlies bond coat layer 18. The thickness of TBC
22 is typically in the range of from about 1 to about 100 mils
(from about 25 to about 2540 microns) and will depend upon a
variety of factors, including the design parameters for article 10
that are involved. For example, for turbine shrouds, TBC 22 is
typically thicker and is usually in the range of from about 30 to
about 70 mils (from about 762 to about 1778 microns), more
typically from about 40 to about 60 mils (from about 1016 to about
1524 microns). By contrast, in the case of turbine blades, TBC 22
is typically thinner and is usually in the range of from about 1 to
about 30 mils (from about 25 to about 762 microns), more typically
from about 3 to about 20 mils (from about 76 to about 508
microns).
[0050] The composition and thickness of the bond coat layer 18, and
TBC 22 are typically adjusted to provide appropriate CTEs to
minimize thermal stresses between the various layers and the
substrate 14 so that the various layers are less prone to separate
from substrate 14 or each other. In general, the CTEs of the
respective layers typically increase in the direction of TBC 22 to
bond coat layer 18, i.e., TBC 22 has the lowest CTE, while bond
coat layer 18 has the highest CTE.
[0051] Referring to the FIG. 5, TBC 22 can be applied, deposited or
otherwise formed on bond coat layer 18 by physical vapor deposition
(PVD), and in particular electron beam physical vapor deposition
(EB-PVD) techniques. The particular technique used for applying,
depositing or otherwise forming TBC 22 will typically depend on the
composition of TBC 22, its thickness and especially the physical
structure desired for TBC 22. PVD techniques tend to be useful in
forming TBCs having a strain-tolerant columnar structure. TBC 22 is
typically formed from zirconia-containing ceramic compositions of
this invention by EB-PVD techniques to provide a strain-tolerant
columnar structure.
[0052] Various types of PVD and especially EB-PVD techniques well
known to those skilled in the art can also be utilized to form TBCs
22 from the zirconia-containing ceramic composition. See, for
example, U.S. Pat. No. 5,645,893 (Rickerby et al), issued Jul. 8,
1997 (especially col. 3, lines 36-63) and U.S. Pat. No. 5,716,720
(Murphy), issued Feb. 10, 1998) (especially col. 5, lines 24-61)
and U.S. Pat. No. 6,447,854 (Rigney et al), issued Sep. 10, 2002,
which are all incorporated by reference. Suitable EB-PVD techniques
for use herein typically involve a coating chamber with a gas (or
gas mixture) that preferably includes oxygen and an inert gas,
though an oxygen-free coating atmosphere can also be employed. The
zirconia-containing ceramic composition is then evaporated with
electron beams focused on, for example, ingots of the ceramic
composition so as to produce a vapor of metal ions, oxygen ions and
one or more metal oxides. The metal and oxygen ions and metal
oxides recombine to form TBC 22 on the surface of metal substrate
14, or more typically on bond coat layer 18. The porosity of TBC 22
can be controlled within the desired range previously described by
adjusting the conditions under which the PVD/EB-PVD technique is
carried out during deposition of the zirconia-containing ceramic
composition on the bond coat layer 18, in particular the pressure
and/or temperature conditions (e.g., by reducing the pressure).
[0053] While specific embodiments of the method of the present
invention have been described, it will be apparent to those skilled
in the art that various modifications thereto can be made without
departing from the spirit and scope of the present invention as
defined in the appended claims.
* * * * *