U.S. patent application number 10/756209 was filed with the patent office on 2005-07-14 for durable thermal barrier coating having low thermal conductivity.
Invention is credited to Lawton, Paul, Liu, Yourong.
Application Number | 20050153160 10/756209 |
Document ID | / |
Family ID | 34739789 |
Filed Date | 2005-07-14 |
United States Patent
Application |
20050153160 |
Kind Code |
A1 |
Liu, Yourong ; et
al. |
July 14, 2005 |
DURABLE THERMAL BARRIER COATING HAVING LOW THERMAL CONDUCTIVITY
Abstract
This invention provides a thermal barrier ceramic coating for
application to a metallic article, with the ceramic coating having
a formula of Re.sub.xZr.sub.1-xO.sub.y with Z dissolved in wherein
Re is a rare earth element selected from the group consisting of
Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu, where
0<x<0.5 and 1.75<y<2 and wherein Z is an oxide of a
metal selected from the group consisting of Y, Mg, Ca, Hf and
mixtures thereof. A preferred embodiment is wherein Re is Nd and Z
is yttrium oxide.
Inventors: |
Liu, Yourong; (Hillsdale,
NJ) ; Lawton, Paul; (Rock Hill, NY) |
Correspondence
Address: |
MITCHELL D. BITTMAN
SEQUA CORPORATION
3 UNIVERSITY PLAZA
HACKENSACK
NJ
07601
US
|
Family ID: |
34739789 |
Appl. No.: |
10/756209 |
Filed: |
January 12, 2004 |
Current U.S.
Class: |
428/633 ;
428/701; 428/702 |
Current CPC
Class: |
C23C 28/321 20130101;
C23C 28/325 20130101; F01D 5/288 20130101; C04B 35/488 20130101;
F05D 2300/15 20130101; Y10T 428/12937 20150115; C23C 14/025
20130101; Y10T 428/12611 20150115; C23C 28/345 20130101; C23C
28/3455 20130101; C23C 14/30 20130101; C23C 28/3215 20130101; Y10T
428/12618 20150115; C23C 14/08 20130101; F01D 5/284 20130101; C23C
14/083 20130101; F05D 2300/2118 20130101 |
Class at
Publication: |
428/633 ;
428/701; 428/702 |
International
Class: |
B32B 015/01 |
Claims
What is claimed:
1. A metallic article comprising a metallic substrate and a thermal
barrier ceramic coating on its surface with the ceramic coating
having a formula of Re.sub.xZr.sub.1-xO.sub.y with Z dissolved in
where Re is a rare earth element selected from the group consisting
of Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu, where
0<x<0.5 and 1.75<y<2 and Z is an oxide of a metal
selected from the group consisting of Y, Mg, Ca, Hf and mixtures
thereof.
2. Article of claim 1 further comprising a metallic bond coat
between the ceramic coating and the metallic substrate.
3. Article of claim 2 wherein the metallic bond coat is a MCrAlY,
wherein M is selected from Ni and/or Co.
4. Article of claim 2 wherein the metallic bond coat is
intermetallic aluminide.
5. Article of claim 1 wherein the ceramic coating has a columnar
microstructure.
6. Article of claim 5 wherein the columnar microstructure is a
straight or a saw tooth structure.
7. Article of claim 1 wherein the ceramic coating has a layered
microstructure.
8. Article of claim 1 further comprising a protective ceramic top
coat overlaying the ceramic coating.
9. Article of claim 8 wherein the protective ceramic top coat is a
dense or a wide columnar microstructure.
10. Article of claim 8 wherein the protective ceramic top coat is 6
to 8 wt % YSZ.
11. Article of claim 1 wherein the ceramic coating has a thermal
conductivity of from about 0.78 to 1.02 W/mK from 600.degree. C. to
1100.degree. C.
12. Article of claim 1 wherein the ceramic coating has a
coefficient of thermal expansion of about 0.67 to
0.95.times.10.sup.-6/.degree. C. from room temperature to
1400.degree. C.
13. Article of claim 1 wherein the ceramic coating has been applied
by EBPVD, air plasma spray or HVOF.
14. Article of claim 2 wherein the metallic article is a turbine
component.
15. Article of claim 14 wherein the turbine component has a
metallic substrate of a nickel or cobalt based superalloy.
16. Article of claim 1 wherein the ceramic coating has a thickness
within the range of about 5 to 500 .mu.m.
17. Article of claim 8 wherein the protective ceramic top coat has
a thickness of about 5 to 50 .mu.m.
18. A metallic article comprising a metallic substrate and a
thermal barrier ceramic coating on its surface, the ceramic coating
having a formula of Nd.sub.xZr.sub.1-xO.sub.y with yttria dissolved
in wherein 0<x<0.5 and 1.75<y<2.
19. Article of claim 18 wherein the ceramic has a cubic crystal
structure.
20. Article of claim 18 further comprising a metallic bond coat
between the ceramic coating and the metallic substrate.
21. Article of claim 20 wherein the metallic bond coat is a MCrAlY,
wherein M is selected from Ni and/or Co.
22. Article of claim 20 wherein the metallic bond coat is
intermetallic aluminide.
23. Article of claim 18 wherein the ceramic coating has a columnar
microstructure.
24. Article of claim 23 wherein the columnar microstructure is a
straight or a saw tooth structure.
25. Article of claim 18 wherein the ceramic coating has a layered
microstructure.
26. Article of claim 18 further comprising a protective ceramic top
coat overlaying the ceramic coating.
27. Article of claim 25 wherein the protective ceramic top coat is
a dense or a wide columnar microstructure.
28. Article of claim 25 wherein the protective ceramic top coat is
6-8 wt % YSZ having a thickness of about 5 to 50 .mu.m.
29. Article of claim 18 wherein the ceramic coating has a thermal
conductivity of from about 0.72 to 1.02 W/mK from 600.degree. C. to
1100.degree. C.
30. Article of claim 18 wherein the ceramic coating has been
applied by EBPVD, air plasma spray or HVOF.
31. Article of claim 18 wherein the metallic article is a turbine
component.
32. Article of claim 31 wherein the turbine component has a
metallic substrate of a nickel or cobalt based superalloy.
33. Article of claim 18 wherein the ceramic coating has a density
of about 4.7 g/cm.sup.3.
34. A method for applying a thermal barrier ceramic coating to a
metallic article comprising: forming a ceramic having a formula
Re.sub.xZr.sub.1-xO.sub.y with Z dissolved where Re is a rare earth
element selected from the group consisting of Ce, Pr, Nd, Pm, Sm,
Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu, wherein 0<x<0.5 and
1.75<y<2 and wherein Z is an oxide of a metal selected from
the group consisting of Y, Mg, Ca, Hf and mixtures thereof, by
doping Z and oxides of Re into a zirconia ceramic; and applying the
ceramic as a coating onto the metallic article.
35. Method of claim 34 wherein the ceramic is applied by electron
beam physical vapor deposition.
36. Method of claim 34 wherein 4 to 15 mole % of Nd.sub.2O.sub.3
and 2 to 14 mole % Y.sub.2O.sub.3 are doped into ZrO.sub.2.
37. Method of claim 36 wherein the ceramic is applied by electron
beam physical vapor deposition.
38. Method of claim 37 wherein 10 mole % of Nd.sub.2O.sub.3 and 2.6
mole % Y.sub.2O.sub.3 is doped into ZrO.sub.2 forming
Nd.sub.0.1Zr.sub.0.9O.sub.- 1.95 with yttria dissolved in.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to the field of
thermal barrier coatings that are used in elevated temperature
applications such as gas turbine engines. In particular, this
invention relates to a thermal insulating ceramic coating, which
has a low thermal conductivity as well as a long service life, and
to the metallic articles such as turbine components, (e.g. blades
and vanes) that the coatings are applied to prevent the components
from overheating during high temperature operation.
BACKGROUND OF THE INVENTION
[0002] Advanced gas turbine engines are continuously pursuing
higher thrust and efficiency by the use of increased operating
temperatures. However, the demand of increasing temperature is
limited by the ability of most advanced nickel and cobalt based
superalloy turbine blades and vanes to maintain their mechanical
strength when exposed to the heat, oxidation, erosion and corrosion
environment. Thus it is desirable to increase turbine engine
operating temperatures, while decreasing the heat transfer to the
metallic parts. One approach is to apply a thermal barrier coating
onto the turbine blades and vanes to insulate the components from
the high temperature operating environment. The ability of the
thermal barrier coating to decrease the temperature to the metallic
substrate depends upon the thermal conductivity of the thermal
barrier coating. It is therefore desirable to develop thermal
barrier coatings having low thermal conductivity to insulate
effectively the thermal transfer to the components used in gas
turbine engines, as well as providing a coated component having a
long service life.
[0003] Efforts have been made to lower the thermal conductivity of
thermal barrier coatings by modifying the chemistry and
microstructure of current 7YSZ thermal barrier coating systems as
disclosed in EP0816526A2, U.S. Pat. No. 6,071,628, U.S. Pat. No.
5,846,605, U.S. Pat. No. 6,183,884B1, U.S. Pat. No. 5,792,521, U.S.
Pat. No. 5,687,679, W01/63008 and U.S. Pat. No. 6,284,323B1. These
approaches decreased thermal conductivity of thermal barrier
coatings to various extents with the minimum reported to be about
half the thermal conductivity of typical 7 wt % yttria stabilized
zirconia (7YSZ).
[0004] It is a main object of this invention to decrease thermal
conductivity of thermal barrier coatings by providing a new ceramic
material with low thermal conductivity. This invention also
provides a method of applying such a thermal barrier coating system
onto the metallic parts providing increased thermal insulation
capability and prolonged durability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 shows the ceramic coating, Re.sub.xZr.sub.1-xO.sub.y
with Z dissolved in, which was applied by EBPVD onto a metallic
bond coat
[0006] FIG. 2 shows the ceramic coating, Re.sub.xZr.sub.1-xO.sub.y
with Z dissolved in, applied in a layered microstructure.
[0007] FIGS. 3(a) and 3(b) shows a protective ceramic top coat on
the ceramic coating, Re.sub.xZr.sub.1-xO.sub.y with Z dissolved
in.
[0008] FIG. 4 shows the specific heat of coatings vs.
temperature.
[0009] FIG. 5 shows the thermal diffusivity of coatings vs.
temperature.
[0010] FIG. 6 shows the thermal conductivity of coatings as
deposited vs. temperature.
SUMMARY OF THE INVENTION
[0011] Briefly, this invention provides a thermal barrier ceramic
coating for application to a metallic article, with the ceramic
coating having a formula of Re.sub.xZr.sub.1-xO.sub.y with Z
dissolved in, wherein Re is a rare earth element selected from the
group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb
and Lu, where O<x<0.5 and 1.75<y<2 and Z is an oxide of
a metal selected from the group consisting of Y, Mg, Ca, Hf and
mixtures thereof. A preferred embodiment is wherein Re is Nd and Z
is yttrium.
DETAILED DESCRIPTION OF THE INVENTION
[0012] This invention provides a thermal barrier ceramic coating
having a formula of Re.sub.xZr.sub.1-xO.sub.y with Z dissolved in,
where 0<x<0.5, 1.75<y<2, Re is a rare earth element
selected from Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu
and Z is an oxide of a metal selected from the group consisting of
Y, Mg, Ca, Hf and mixtures thereof. The ceramic is formed by doping
oxides of the selected rare earth elements and the selected metal
oxides into a host zirconia ceramic. A preferred embodiment is
where Re is Nd with the formula Nd.sub.xZr.sub.1-xO.sub.y, where
0<x<0.5 and 1.75<y<2 and Z is yttria. The
Nd.sub.xZr.sub.1-xO.sub.y ceramic coating with yttria dissolved in
can be prepared by doping 4 to 15 mole % of Nd.sub.2O.sub.3 and 2
to 14 mole %, preferably 2.6 to 5.6 mole %, Y.sub.2O.sub.3 into
ZrO.sub.2. One example of Nd.sub.xZr.sub.1-xO.sub.y is
Nd.sub.0.1Zr.sub.0.9O.sub.1.95 having a non-pyrochlore, cubic
crystal structure with 10 mole % of Nd.sub.2O.sub.3 and 2.6 mole %
Y.sub.2O.sub.3 doped into ZrO.sub.2. The ceramic coating of this
invention is applied to a metallic article providing a thermal
barrier coating with low thermal conductivity and high resistance
to cyclic oxidation.
[0013] The ceramic coating of this invention has a low thermal
conductivity generally within the range of about 0.78 to 1.02 W/mK
from 600.degree. C. to 1100.degree. C. This thermal conductivity is
around 50% of the measured thermal conductivity of a typical 7YSZ
coating (1.65-2.22 W/mK from 600.degree. C. to 1100.degree. C.). In
addition the ceramic coating of this invention has a high
resistance to cyclic oxidation. Cyclic oxidation testing of the
coating Nd.sub.0.1Zr.sub.0.9O.sub.1.95 with 2.6 mole %
Y.sub.2O.sub.3 dissolved in demonstrated a lifetime greater than
twice that of 7YSZ.
[0014] Techniques for applying the ceramic coatings,
Re.sub.xZr.sub.1-xO.sub.y with Z dissolved in include air plasma
thermal spray (APS), low pressure plasma spray (LPPS), high
velocity oxygen fuel (HVOF), sputtering and electron beam physical
vapor deposition (EBPVD), etc. In a preferred embodiment the
ceramic coating is applied by electron beam physical vapor
deposition (EBPVD) due to the columnar microstructure with
inter-column gaps produced. The ceramic coating can be deposited as
a straight columnar microstructure or a saw tooth microstructure or
a layered microstructure or mix of thereof for further reduction in
thermal conductivity. Generally, the ceramic coating is applied to
a thickness within the range of about 5 to 500 .mu.m, preferably
about 25 to 400 .mu.m. In a layered microstructure, the ceramic
coating can have at least 2 layers, preferably from 5 to 100
layers, each at least about 1 .mu.m thick, preferably about 5 to 25
.mu.m thick.
[0015] The process of applying the ceramic coating by EBPVD is
similar to that of applying 7YSZ in production. The evaporating
source in a crucible is a solid ingot of the
Re.sub.xZr.sub.1-xO.sub.y with Z dissolved in, which is sintered
zirconia doped with the selected rare earth oxide and selected
metal oxide. The layered microstructure of the ceramic coating,
Re.sub.xZr.sub.1-xO.sub.y with Z dissolved in, is applied by
evaporating the solid ingots from two crucibles under controlled
gun on/off program of electron beam physical vapor deposition. The
ceramic coating, Re.sub.xZr.sub.1-xO.sub.y with yttria dissolved in
with 6-8 wt % YSZ at the top is deposited by evaporating the solid
ingot of Re.sub.xZr.sub.1-xO.sub.y with Z dissolved in from one
crucible and 6-8 wt % YSZ ingot from another crucible by electron
beam physical vapor deposition.
[0016] For increased adhesion a metallic bond coat is applied onto
the metallic article, such as a nickel or cobalt based superalloys
prior to deposition of the ceramic coating. The metallic bond coat
can be a MCrAlY alloy, wherein M is Ni, Co or mixtures thereof.
Such alloys have a broad composition of 10 to 35% chromium, 5 to
15% aluminum, 0.01 to 1% yttrium, or hafnium, or lanthanum, with M
being the balance. Minor amounts of other elements such as Ta or Si
may also be present. The MCrAlY bond coat can be applied by EBPVD,
though sputtering, low pressure plasma or high velocity oxy fuel
spraying or entrapment plating may also be used.
[0017] Alternatively, the metallic bond coat can be comprised of an
intermetallic aluminide such as nickel aluminide or platinum
aluminide. The aluminide bond coating can be applied by standard
commercially available aluminide processes whereby aluminum is
reacted at the substrate surface to form an aluminum intermetallic
compound, which provides a reservoir for the growth of an alumina
scale oxidation resistant layer. Thus the aluminide coating is
predominately composed of aluminum intermetallic [e.g., NiAl, CoAl
and (Ni, Co) Al phase] formed by reacting aluminum vapor species,
aluminum rich alloy powder or surface layer with the substrate
elements in the outer layer of the superalloy component. This layer
is typically well bonded to the substrate. Aluminizing may be
accomplished by one of several conventional prior art techniques,
such as, the pack cementation process, spraying, chemical vapor
deposition, electrophoresis, sputtering, and appropriate diffusion
heat treatments. Other beneficial elements can also be incorporated
into diffusion aluminide coatings by a variety of processes.
Beneficial elements include Pt, Pd, Si, Hf, Y and oxide particles,
such as alumina, yttria, hafnia, for enhancement of alumina scale
adhesion, Cr and Mn for hot corrosion resistance, Rh, Ta and Cb for
diffusional stability and/or oxidation resistance and Ni, Co for
increasing ductility or incipient melting limits. In the specific
case of platinum modified diffusion aluminide coating layers, the
coating phases adjacent to the alumina scale will be platinum
aluminide and/or nickel-platinum aluminide phases (on a Ni-base
superalloy).
[0018] Through oxidation an alumina (i.e., aluminum oxide) layer is
formed over the metallic bond coat. This alumina layer provides
both oxidation resistance and a bonding surface for a ceramic
coating. The alumina layer may be formed before the ceramic coating
is applied, during application of the coating or subsequently by
heating the coated article in an oxygen containing atmosphere at a
temperature consistent with the temperature capability of the
superalloy, or by exposure to the turbine environment. The
sub-micron thick alumina scale will thicken on the aluminide
surface by heating the material to normal turbine exposure
conditions. The thickness of the alumina scale is preferably
sub-micron (up to about one micron). The alumina layer may also be
deposited by chemical vapor deposition following deposition of the
metallic bond coat.
[0019] Alternatively, the metallic bond coat may be eliminated if
the substrate is capable of forming a highly adherent alumina scale
or layer. Examples of such substrates are very low sulfur (<1
ppm) single crystal superalloys, such as PWA 1487 and Rene N5,
which also contain 0.1% yttrium to enhance adhesion of the
thermally grown alumina scale.
[0020] FIG. 1 shows the ceramic coating, Re.sub.xZr.sub.1-xO.sub.y
with Z dissolved in, 40 which was applied by EBPVD onto a metallic
bond coat 20, such as a MCrAlY and/or platinum modified aluminide.
The bond coat 20 was applied to the metallic article 10, of nickel
or cobalt based superalloys prior to the application of the ceramic
coating 40. The bond coat 20 provides strong adhesion between the
metallic substrate 10 and the ceramic coating 40. The ceramic
coating adheres to the bond coat 20 through a thermally grown
alumina film 30 on the bond coat 20.
[0021] FIG. 2 shows the ceramic coating, Re.sub.xZr.sub.1-xO.sub.y
with Z dissolved in, 40 applied in a layered microstructure. The
interface boundaries between the layers are another potential
source of phonon scattering for thermal conductivity reduction.
[0022] FIGS. 3(a) and 3(b) show a protective ceramic top coat 50
which is coated after the columnar ceramic coating
Re.sub.xZr.sub.1-xO.sub.y with Z dissolved in 40, to provide
increased erosion resistance on the top surface which is subject to
hot gas impact during turbine engine operation. This protective
ceramic top coat can be a dense and/or wide column of ceramic
coating Re.sub.xZr.sub.1-xO.sub.y with Z dissolved in, or
alternatively, layer 50 could also be 6-8 wt % YSZ. This protective
ceramic top coat, 50 for erosion resistance, generally has a
thickness of about 5 to 50 .mu.m preferably about 10 to 25 .mu.m
thick. A protective top coat of 7YSZ with an appropriate thickness
for erosion resistance on the ceramic coating,
Nd.sub.0.1Zr.sub.0.9O.sub.1.95 with 2.6 mole % Y.sub.2O.sub.3
dissolved in, provides a thermal conductivity which is equivalent
to the ceramic coating without the protective ceramic top coat.
[0023] The ceramic coating system of this invention provides many
advantages for use in gas turbine engines. The reduction in thermal
conductivity of 50 percent can reduce the thickness required for
the thermal barrier coating (TBC) by approximately one half for the
same degree of thermal insulation. This will lower the cost of the
TBC due to the time saved in applying the coating, ingot material
savings and energy savings in production. Decreasing in the coating
thickness will also lower the weight of the gas turbine component,
e.g. blades and vanes, which can provide a significant reduction in
the weight of the disk that holds these components. Depositing the
same thickness of the ceramic coating will allow an increased
operating temperature to be achieved without overheating the
metallic parts allowing the engine to operate a higher thrust and
efficiency. The increased insulating capabilities of the ceramic
coating could also reduce the requirements for air cooling the
part.
[0024] This invention is generally applicable to any metallic
article which uses thermal barrier coating system, and includes
various modifications according to the principles of this
invention.
EXAMPLE
[0025] A ceramic coating having a formula
Nd.sub.0.1Zr.sub.0.9O.sub.1.95 with yttria dissolved in was applied
by EBPVD evaporating ZrO.sub.2 ceramic ingots doped with 10 mole %
of Nd.sub.2O.sub.3 and 2.6 mole % Y.sub.2O.sub.3. The coating
displayed a saw tooth columnar structure oriented perpendicularly
to the surface of the substrate. The intercolumnar gaps are visible
and tend to be gradually wider from bottom to top. Phase
identification conducted on as-deposited ceramic coating of
Nd.sub.0.1Zr.sub.0.9O.sub.1.95 with yttria dissolved in by XRD
showed a ceramic coating of Nd.sub.0.1Zr.sub.0.9O.sub.1.95 having a
non-pyrochlore, cubic crystal structure produced on the top layer
of the EBPVD thermal barrier coating system.
[0026] It is known that the thermal conductivity of material K
(W/mK) is determined by its thermal diffusivity a (cm.sup.2/s),
density p (g/cm.sup.3) and specific heat C.sub.p (J/g K), i.e.
K=.alpha..multidot..rho..multidot.C.sub.p. The specific heat of the
ceramic coating Nd.sub.0.1Zr.sub.0.9O.sub.1.95 with yttria
dissolved in was tested using Differential Scanning Calorimetry
(DSC) on an Omnitherm DSC 1500 in Oak Ridge National Lab. The
samples are free standing ceramic coating, i.e. an intact ceramic
coating without substrate. The free standing samples of ceramic
coating are 180 to 230 .mu.m thick and are machined to 6 mm in
diameter to meet the requirements of the testing instrument. The
test was run in the temperature range of 100.degree. C. to
1100.degree. C. using sapphire as the baseline standard. The
results in FIG. 4 show that the specific heat of
Nd.sub.0.1Zr.sub.0.9O.sub.1.95 ceramic coating with yttria
dissolved in is about 5%-7% lower than that of typical 7YSZ.
[0027] The thermal diffusivity (a) was measured by the laser flash
technique at Oak Ridge National Laboratory on a Flashline 5000
Thermal Diffusivity System, see H. Wang, R. B. Dinwiddie and P. S.
GAAL, "Multiple Station Thermal Diffusivity Instrument", THERMAL
CONDUCTIOVITY 23, Proceedings of the Twenty-Third International
Thermal Conductivity Conference, P119-126. Two or three free
standing ceramic samples of each kind were measured at every
100-degree interval from 600.degree. C. to 1100.degree. C. Three
measurements of each sample were conducted at every temperature.
The time-temperature curves were analyzed by the method of Clark
and Taylor, which takes into account radiation losses and uses the
heating part of the curve to calculate thermal diffusivity. The
average readings of two or three samples with three measurements in
each at temperature from 600.degree. C. to 1000.degree. C. are
plotted in FIG. 5. It shows the thermal diffusivity of
Nd.sub.0.1Zr.sub.0.9O.sub.1.95 ceramic coating with yttria
dissolved in is around 40% lower than that of typical 7YSZ coating.
A layered microstructure further decreases the thermal diffusivity
of the Nd.sub.0.1Zr.sub.0.9O.sub.1.95 ceramic coating with yttria
dissolved in. Applying a thin layer of 7YSZ on top of the
Nd.sub.0.1Zr.sub.0.9O.sub.1.95 ceramic coating with yttria
dissolved in did not significantly change the thermal
diffusivity.
[0028] The density of the ceramic coating is about 4.7 g/cm.sup.3,
which is less than that of the typical 7YSZ coating (5.0
g/cm.sup.3). This lower density allows the gas turbine component
coated with the ceramic coating to have less coating weight than
that currently used for typical 7YSZ coated components.
[0029] The thermal conductivity of Nd.sub.0.1Zr.sub.0.9O.sub.1.95
ceramic coatings with yttria dissolved in is calculated according
to their value of thermal diffusivity, density and specific heat,
and then is plotted in FIG. 6, which shows the thermal conductivity
of the ceramic coating as deposited at a temperature 600.degree. C.
to 1100.degree. C. The ceramic coating as deposited shows a low
thermal conductivity of 0.78-1.02 W/mK, which is 46%-47% of the
measured thermal conductivity of typical 7YSZ coating (1.65-2.22
W/mK from 600.degree. C. to 1100.degree. C.). The thermal
insulation capability of the ceramic coating is primarily
attributed to its crystal structure and chemistry. Heat conduction
is a motion of carriers of thermal energy. In dielectric ceramic
materials, the carriers are lattice vibration, i.e. phonon motion.
The high intrinsic point defects of substitutional atoms Nd and
oxygen vacancy leads to the reduction in the mean free path length
of a phonon. The large difference in atomic mass and ionic radius
between Nd (144.2 g in atomic mass, 1.05.ANG. in ionic radius) and
Zr (91.2 g in atomic mass, 0.79.ANG. in ionic radius) in the
compound, results in a high reduction in thermal conductivity.
[0030] Another significant concern is the thermal conductivity of a
thermal barrier coating during operation in a turbine gas engine
where the coatings are subject to high temperature for a long
period of time. There are two factors that will affect the
intrinsic thermal conductivity during engine operation--sintering
and radiation. To investigate the effect of sintering on thermal
conductivity of the ceramic coatings, samples of the
Nd.sub.0.1Zr.sub.0.9O.sub.1.95 ceramic coatings with yttria
dissolved in applied by EBPVD were aged heat treat at 1200.degree.
C. for 50 hours. FIG. 7 shows the thermal conductivity of the
ceramic coatings and typical 7YSZ coating after aging heat treat.
Comparing the thermal conductivity of the coatings as deposited in
FIG. 6, all the coatings as aged have higher thermal conductivity
than those as deposited. This is because that the gaps between the
adjacent columns in EBPVD applied thermal barrier coatings will be
narrowed or even be closed by sintering at high temperature. The
lack of scattering sites of vertical pores will increase thermal
conductivity of the coating. However, it is noticeable that the
thermal conductivity of the Nd.sub.0.1Zr.sub.0.9O.sub.1.95 ceramic
coatings with yttria dissolved in as aged is still quite low at
1.00 W/mK after aging, which is about 50% of the thermal
conductivity of 7YSZ coating as aged.
[0031] During engine operation at high temperature, a thermal
barrier coating is subjected to incident radiation from the hot
combustor. Radiation is then absorbed by the soot that is usually
covered on the exposed coating due to the combustion environment. A
translucent coating, such as typical yttria stabilized zirconia,
permits the energy to be transported internally by radiation,
thereby increasing the total energy transfer and acting to increase
thermal conductivity. However, the Nd.sub.0.1Zr.sub.0.9O.sub.1.95
ceramic coating with yttria dissolved in has a sky-blue color,
which can reduce the internal radiation transport. Therefore, the
effect of radiation on the insulating ability of the new coatings
is expected to be negligible.
[0032] Nd.sub.0.1Zr.sub.0.9O.sub.1.95 with 2.6 mole % of yttria
dissolved in also demonstrates a high resistance to cyclic
oxidation. The cyclic oxidation test was conducted at 2025.degree.
F., one hour cycle, i.e. 7 minutes heat up to 2025.degree. F., soak
at 2025.degree. F. for 50 minutes, followed by 3 minutes air-forced
cooling down to 200.degree. F. 7YSZ coated specimens used as a
reference were loaded in the same furnace chamber. The specimens
were inspected at specified intervals. Six samples of each coating
were tested. The lifetime is measured by the number of cycles to
failure (spallation of 30% of the coating area). The results show
that the lifetime of the new coating Nd.sub.0.1Zr.sub.0.9O.sub.1.95
with yttria dissolved in is more than twice that of 7YSZ, with the
7YSZ coating sample having an average lifetime of 707 cycles, while
the Nd.sub.0.1Zr.sub.0.9O.sub.1.95 with 2.6 mole % of yttria
dissolved in had an average lifetime of 1429 cycles with 4 of the 6
samples tested not failing at 1473 cycles when the test was
stopped.
[0033] The coefficient of thermal expansion (CTE) of the ceramic
coating Nd.sub.0.1Zr.sub.0.9O.sub.1.95 with 2.6 mole % of yttria
dissolved in is about 6.7 to 9.5.times.10.sup.-6/.degree. C. from
room temperature to 1400.degree. C., which is larger than that of
typical 7YSZ. Thus, the thermal stress in the new ceramic coating,
which is caused by the difference of thermal expansion between the
substrate and the ceramic coating, is lower than that of a 7YSZ
thermal barrier coating system.
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