U.S. patent application number 17/318631 was filed with the patent office on 2021-08-26 for ceramic material for high temperature service.
This patent application is currently assigned to OERLIKON METCO (US) INC.. The applicant listed for this patent is OERLIKON METCO (US) INC.. Invention is credited to Jacobus C. DOESBURG, Mitchell R. DORFMAN, Matthew GOLD, Liangde XIE.
Application Number | 20210261465 17/318631 |
Document ID | / |
Family ID | 1000005582904 |
Filed Date | 2021-08-26 |
United States Patent
Application |
20210261465 |
Kind Code |
A1 |
DOESBURG; Jacobus C. ; et
al. |
August 26, 2021 |
CERAMIC MATERIAL FOR HIGH TEMPERATURE SERVICE
Abstract
A high purity yttria or ytterbia stabilized zirconia powder
wherein a purity of the zirconia is at least 99.5 weight percent
purity and with a maximum amount of specified oxide impurities.
Inventors: |
DOESBURG; Jacobus C.;
(Westbury, NY) ; DORFMAN; Mitchell R.; (Westbury,
NY) ; GOLD; Matthew; (Westbury, NY) ; XIE;
Liangde; (Mason, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OERLIKON METCO (US) INC. |
Westbury |
NY |
US |
|
|
Assignee: |
OERLIKON METCO (US) INC.
Westbury
NY
|
Family ID: |
1000005582904 |
Appl. No.: |
17/318631 |
Filed: |
May 12, 2021 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
15836369 |
Dec 8, 2017 |
11046614 |
|
|
17318631 |
|
|
|
|
13913101 |
Jun 7, 2013 |
9975812 |
|
|
15836369 |
|
|
|
|
11790430 |
Apr 25, 2007 |
8603930 |
|
|
13913101 |
|
|
|
|
11520043 |
Sep 13, 2006 |
7723249 |
|
|
11790430 |
|
|
|
|
60724268 |
Oct 7, 2005 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 2235/3225 20130101;
C04B 2235/3224 20130101; C23C 14/083 20130101; Y10T 428/24471
20150115; C04B 2235/3246 20130101; F01D 5/288 20130101; C04B
35/62665 20130101; C23C 30/00 20130101; C04B 35/486 20130101; C23C
4/11 20160101; C04B 2235/72 20130101; Y10T 428/24273 20150115; C04B
35/482 20130101; C09D 1/00 20130101; Y10T 428/24331 20150115; C04B
2235/3227 20130101 |
International
Class: |
C04B 35/482 20060101
C04B035/482; C04B 35/486 20060101 C04B035/486; C04B 35/626 20060101
C04B035/626; C23C 14/08 20060101 C23C014/08; C23C 30/00 20060101
C23C030/00; F01D 5/28 20060101 F01D005/28; C23C 4/11 20060101
C23C004/11; C09D 1/00 20060101 C09D001/00 |
Claims
1. A ceramic powder comprising: a primary stabilizer oxide
comprising 4-20 weight percent and oxide impurities up to a maximum
of 0.15 weight percent; and a first material comprising a balance
of the ceramic powder, wherein the first material is partially
stabilized by the primary stabilizer oxide into a first partially
stabilized material; wherein the oxide impurities, when combined
with each other or with the first partially stabilized material,
form phases with melting temperatures at least 100.degree. C. lower
than the melting temperature of the first material.
2. The ceramic powder according to claim 1, wherein the first
material comprises at least one of zirconia or hafnia; and the
primary stabilizer oxide comprises at least one of a yttria or a
ytterbia stabilizer.
3. The ceramic power according to claim 2, wherein the oxide
impurities comprise: less than 0.005 weight percent SiO.sub.2; less
than 0.015 weight percent Al.sub.2O.sub.3; less than 0.002 weight
percent TiO.sub.2; and less than 0.002 weight percent MgO.
4. The powder of claim 3, wherein the oxide impurities further
comprise: less than 0.04 weight percent Fe.sub.2O.sub.3; less than
0.02 weight percent CaO; less than 0.001 weight percent
Cr.sub.2O.sub.3; and less than 0.002 weight percent Na.sub.2O.
5. The powder of claim 1, wherein the powder is a thermal sprayable
powder.
6. The powder of claim 1 has a high purity of at least 99.5 weight
percent purity.
7. The powder of claim 1, wherein said powder has a particle size
of from about 5 to 150 microns.
8. A blended ceramic powder comprising: a first material
composition comprising: a first primary stabilizer oxide comprising
4-16 weight percent and first oxide impurities up to a maximum of
0.15 weight percent; and a first material comprising a balance of
the first material composition, wherein the first material is
partially stabilized by the first primary stabilizer oxide into a
first partially stabilized material; wherein the first oxide
impurities, when combined with each other or with the first
partially stabilized material, form phases with melting
temperatures at least 100.degree. C. lower than the melting
temperature of the first material; and a second material
composition comprising: a second primary stabilizer oxide
comprising 4-20 weight percent and second oxide impurities up to a
maximum of 0.15 weight percent; and a second material comprising a
balance of the second material composition, wherein the second
material is partially stabilized by the second primary stabilizer
oxide into a second partially stabilized material; wherein the
second oxide impurities, when combined with each other or with the
second partially stabilized material, form phases with melting
temperatures at least 100.degree. C. lower than the melting
temperature of the second material.
9. The blended ceramic powder according to claim 8, wherein the
first material comprises at least one of zirconia or hafnia, the
first primary stabilizer oxide comprises 4-12 weight percent of a
yttria stabilizer, and a fraction of the first material in the
blended powder is 5-50 weight percent, and wherein the second
material comprises at least one of zirconia or hafnia, the second
primary stabilizer oxide comprises 4-16 weight percent of a
ytterbia stabilizer, and a fraction of the second material in the
blended powder is 50-95 weight percent.
10. The blended ceramic powder according to claim 9, further
comprising a third material composition comprising: a third primary
stabilizer oxide comprising 4-20 weight percent and second oxide
impurities up to a maximum of 0.15 weight percent, a third material
comprising a balance of the third material composition, wherein the
third material is partially stabilized by the third primary
stabilizer oxide into a second partially stabilized material,
wherein the third first oxide impurities, when combined with each
other or with the third partially stabilized material, form phases
with melting temperatures at least 100.degree. C. lower than the
melting temperature of the third material.
11. The blended ceramic powder according to claim 10, wherein the
third material composition comprises at least one of zirconia or
hafnia, the third primary stabilizer oxide comprises at least one
of 4-16 weight percent of a neodymium stabilizer or 4-20 weight
percent Europa stabilizer, wherein the fraction of the first
material in the blended powder is 10-90 weight percent, wherein the
fraction of the second material in the blended powder is 5-45
weight percent and wherein the fraction of the third material in
the blended powder is 5-45 weight percent.
12. The blended ceramic powder according to claim 8, wherein the
first material comprises at least one of zirconia or hafnia, the
first primary stabilizer oxide comprises 4-16 weight percent of a
ytterbia stabilizer, and the first material fraction of the blended
powder is 5-50 weight percent, and wherein the second material
comprises at least one of zirconia or hafnia, the second primary
stabilizer oxide comprises at least one of 4-16 weight percent of a
neodymium stabilizer or 4-20 weight percent Europa stabilizer, and
the second material fraction of the blended powder is 50-95 weight
percent.
13. The blended ceramic powder according to claim 8, wherein the
first material comprises at least one of zirconia or hafnia, the
first primary stabilizer oxide comprises 4-12 weight percent of a
yttria stabilizer, and the first material fraction of the blended
powder is 50-95 weight percent, and wherein the second material
comprises at least one of zirconia or hafnia, the second primary
stabilizer oxide comprises at least one of 4-16 weight percent of a
neodymium stabilizer or 4-20 weight percent Europa stabilizer, and
the second material fraction of the blended powder is 5-50 weight
percent.
14. The blended ceramic powder according to claim 8, wherein at
least one of the first, second and third oxide impurities comprise:
less than 0.005 weight percent SiO.sub.2; less than 0.015 weight
percent Al.sub.2O.sub.3; less than 0.002 weight percent TiO.sub.2;
and less than 0.002 weight percent MgO.
15. The blended ceramic powder of claim 14, wherein the at least
one of the first, second or third oxide impurities further
comprises: less than 0.04 weight percent Fe.sub.2O.sub.3; less than
0.02 weight percent CaO; less than 0.001 weight percent
Cr.sub.2O.sub.3; and less than 0.002 weight percent Na.sub.2O.
16. The blended ceramic powder of claim 8 is a thermal sprayable
powder.
17. The blended ceramic powder of claim 8 has a particle size of
from about 5 to 150 microns.
18. The blended ceramic powder of claim 8 has a high purity of at
least 99.5 weight percent purity.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of parent U.S.
application Ser. No. 15/836,369 filed Dec. 8, 2017, which is a
continuation of U.S. application Ser. No. 13/913,101 filed Jun. 7,
2013 (now U.S. Pat. No. 9,975,812 issued May 22, 2018), which is a
continuation of U.S. application Ser. No. 11/790,430 filed Apr. 25,
2007 (now U.S. Pat. No. 8,603,930 issued Dec. 10, 2013), which is a
continuation-in-part of U.S. patent application Ser. No. 11/520,043
filed Sep. 13, 2006 (now U.S. Pat. No. 7,723,249 issued May 25,
2010), which claims the benefit under 35 U.S.C. .sctn. 119(e) of
U.S. Provisional Patent Application No. 60/724,268 filed Oct. 7,
2005, the disclosures of which are expressly incorporated by
reference herein in their entireties.
STATEMENT REGARDING SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable.
REFERENCE TO SEQUENCE LISTING
[0003] Not Applicable.
BACKGROUND OF THE INVENTION
Field of the Invention
[0004] The invention relates to ceramic materials for thermal
barriers and abradable coating systems in high temperature and high
temperature cycling applications, and more particularly to
ultra-pure zirconia materials for use in thermal barrier and
abradable coating applications.
Description of Related Art
[0005] Superior high-temperature properties are required to improve
the performance of heat resistant and corrosion resistant members.
These members include, for example gas turbine blades, combustor
cans, ducting and nozzle guide vanes in combustion turbines and
combined cycle power plants. Turbine blades are driven by hot
gasses, and the efficiency of the gas turbine increases with the
rise in operational temperature. The demand for continued
improvement in efficiency has driven the system designers to
specify increasingly higher turbine operating temperatures. Thus,
there is a continuing need for materials that can achieve higher
operational temperatures.
[0006] Thermal barrier coatings are used to insulate components,
such as those in a gas turbine, operating at elevated temperatures.
Thermal barriers allow increased operating temperature of gas
turbines by protecting the coated part (or substrate) from direct
exposure to the operating environment. An important consideration
in the design of a thermal barrier is that the coating be a ceramic
material having a crystalline structure containing beneficial
cracks and voids, imparting strain tolerance. If there were no
cracks in the coating, the thermal barrier would not function,
because the differences in thermal expansion between the metal
substrate system and the coating will cause interfacial stresses
upon thermal cycling that are greater than the bond strength
between them. By the creation of a crack network into the coating,
a stress relief mechanism is introduced that allows the coating to
survive numerous thermal cycles. Repeating crack networks are
typically imparted into the coating on varying space scales by
manipulating the thermodynamic and kinetic conditions of the
manufacturing method, and different structures known to perform the
coating task have been optimized likewise. In addition to this,
cracks are also formed during service, so the structure formed upon
coating manufacture changes with time, depending on the starting
material phases in the manufactured coating and thermal conditions
during service.
[0007] Another design factor determining coating lifetime is the
sintering rate of the coating. When the coating is cycled above
half of its absolute melting temperature, the coating begins to
sinter causing volume shrinkage. As the coating shrinks, the stress
difference between the coating and substrate increases. At a
certain amount of shrinkage (which varies depending on the type of
structure and thermal conditions during service), the stress
difference exceeds the bonding strength of the coating and it
becomes detached. Decreasing the sintering rate of the thermal
barrier increases the amount of time before the catastrophic
shrinkage is experienced, so it can become a major design
consideration. For high purity zirconia alloys, the onset of
sintering commences at temperatures above 1000.degree. C.
[0008] Historically, high temperature thermal barrier coatings have
been based on alloys of zirconia. Hafnia may also be employed due
to its chemical similarity to zirconia, but is generally
cost-prohibitive. Hafnia also is typically present in most zirconia
materials in more than trace amounts due to difficulty in
separating the two oxides. Zirconia and/or hafnia have the
following combination of desirable properties that other known
ceramic systems do not possesses for the application. First,
zirconia alloys have some of the highest melting points of all
ceramics, and this means theoretically some of the highest
temperatures for which the onset of sintering occurs. Second,
zirconia alloys have one of the lowest thermal conductivities of
all ceramics. Third, zirconia has one of the highest coefficients
of thermal expansion of all ceramics, so it is most compatible with
transition metal alloys during thermal cycling.
[0009] Zirconia alone cannot fulfill the coating requirements
because it undergoes a phase transformation from tetragonal to
monoclinic during thermal cycling. This transformation is presumed
to cause a detrimental volume change resulting in large strain
differences between the coating and the substrate. When the
resulting stresses exceed the bond strength of the coating to the
substrate, the coating will detach. For this reason a phase
stabilizer is added to the zirconia and/or hafnia, such as yttria,
which suppresses the tetragonal to monoclinic phase
transformation.
[0010] Thermal spray abradable coatings are commonly used in gas
turbine applications. Abradable coatings are designed to
preferentially abrade when contact is made with a mating part.
These coatings have low structural integrity so they are readily
abraded when they come into contact with a moving surface with
higher structural integrity (such as the blade of a turbine). The
coatings are designed so as not to damage the mating surface. In
many applications abradable coatings are subject to the same
thermal cycling conditions as the thermal barriers described above.
Thus, there is a continuing need for materials suitable for
abradable coatings that can achieve higher operational
temperatures.
[0011] Some previous efforts to improve coating life have focused
on the coating material and microstructure upon entry into service.
However, the heat cycle of in service parts also causes cracks
throughout the service life of the part. Thus, the microstructure
formed upon coating manufacture changes with time, depending on the
starting material phases in the manufactured coating and thermal
conditions during service. Because a consistent optimal crack
network is not typically maintainable throughout the service life
of the part, coating lifetime is ultimately determined by the
material selection and its manufacturing process. There remains a
need in the art for a coating material, coating material
manufacturing method, and coating manufacturing method that address
the changes in the coating microstructure during its service
lifetime.
SUMMARY
[0012] Accordingly, the invention is directed to a ceramic material
for use in thermal barriers for high temperature cycling
applications and high temperature abradable coatings. The material
is an alloy formed predominantly from ultra-pure stabilized
zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) alloys that have
uncharacteristically high sintering resistance to achieve a high
service lifetime. The invention provides a desired coating material
so that the changes in the coating microstructure over the
in-service lifetime are retarded.
[0013] The limits for impurities discovered to decrease sintering
rate and therefore increase service lifetime compared with current
impurity concentrations when used as a coating and partially
stabilized with a rare earth oxide, for example, yttria
(Y.sub.2O.sub.3) and/or ytterbia (Yb.sub.2O.sub.5), are disclosed
herein. Oxide impurities are defined as materials which, when
combined with each other or with zirconia and/or hafnia, form
phases with melting points much lower than that of pure zirconia
and/or hafnia.
[0014] In one aspect, the invention provides a ceramic material for
use in high-temperature thermal barriers or abradable seal
coatings. The said material has about 4 to 20 weight percent of a
stabilizer of one or more rare earth oxides; and a balance of at
least one of zirconia (ZrO.sub.2), hafnia (HfO.sub.2) and
combinations thereof, wherein the zirconia (ZrO.sub.2) and/or
hafnia (HfO.sub.2) is partially stabilized by the stabilizer, and
wherein the total amount of impurities is less than or equal to
0.15 weight percent.
[0015] In another aspect of the invention a blended ceramic
material of one or more ceramic materials is provided. Each of the
ceramic materials is for use in high-temperature thermal barriers
or abradable seal coatings and is supplied in the form of one of a
powder or a slurry of partially stabilized powder. Each of the
ceramic materials has about 4 to 20 weight percent of a stabilizer
of one or more rare earth oxides and a balance of at least one of
zirconia (ZrO.sub.2), hafnia (HfO.sub.2) and combinations thereof,
wherein the zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) is
partially stabilized by the stabilizer, and wherein the total
amount of impurities of the blended ceramic material is less than
or equal to 0.15 weight percent. Additional ceramic materials or
placeholder materials may also be included in the blended
material.
[0016] Conventional approaches to improving coating life-cycles
have focused on adding stabilizers to the base ceramic material.
The approach of the present invention provides previously
unexpected results in sintering data by identifying low-impurity
materials. When looking at the sintering data, changing the amount
of impurities slightly has a much greater effect on performance
compared with changing the amount and types of stabilizers.
[0017] In embodiments, there is provided a high purity yttria or
ytterbia stabilized zirconia powder wherein a purity of the
zirconia is at least 99.5 weight percent purity and a maximum
amount of oxide impurities is as follows: less than 0.005 weight
percent SiO.sub.2; less than 0.015 weight percent Al.sub.2O.sub.3;
less than 0.002 weight percent TiO.sub.2; less than 0.04 weight
percent Fe.sub.2O.sub.3; less than 0.02 weight percent CaO; less
than 0.002 weight percent MgO. The impurities may include less than
0.001 weight percent Cr.sub.2O.sub.3 and less than 0.002 weight
percent Na.sub.2O.
[0018] In embodiments, the powder is a thermal sprayable
powder.
[0019] In embodiments, the zirconia is partially stabilized by the
yttria.
[0020] In embodiments, the powder has a particle size of from about
5 to 150 microns.
[0021] In embodiments, there is provided a method of applying a
thermal barrier coating on a substrate, the method comprising
thermally spraying the powder of claim 1 onto the substrate so as
to form the thermal barrier coating having from about 5 to 250
vertical macro cracks per 25.4 mm length measured along a coating
surface and being oriented perpendicular to a surface of the
substrate containing said coating.
[0022] In embodiments, there is provided a high purity yttria or
ytterbia stabilized hafnia powder wherein a purity of the hafnia is
at least 99.5 weight percent purity and a maximum amount of oxide
impurities is as follows: less than 0.002 weight percent Na.sub.2O;
less than 0.005 weight percent SiO.sub.2; less than 0.015 weight
percent Al.sub.2O.sub.3; less than 0.002 weight percent TiO.sub.2;
less than 0.04 weight percent Fe.sub.2O.sub.3; less than 0.02
weight percent CaO; less than 0.002 weight percent MgO; and less
than 0.001 weight percent Cr.sub.2O.sub.3.
[0023] In embodiments, the powder is a thermal sprayable
powder.
[0024] In embodiments, the hafnia is partially stabilized by the
yttria.
[0025] In embodiments, the powder has a particle size of from about
5 to 150 microns.
[0026] In embodiments, there is provided a method of applying a
thermal barrier coating on a substrate, the method comprising
thermally spraying the powder of claim 6 onto the substrate so as
to form the thermal barrier coating having from about 5 to 250
vertical macro cracks per 25.4 mm length measured along a coating
surface and being oriented perpendicular to a surface of the
substrate containing said coating.
[0027] In embodiments, there is provided a high purity yttria
stabilized zirconia powder comprising a purity of the zirconia
being at least 99.5 weight percent purity, less than about 0.005
weight percent silicon dioxide (silica), less than about 0.015
weight percent aluminum oxide (alumina), less than about 0.02
weight percent calcium oxide, less than about 0.04 weight percent
ferric oxide, less than about 0.002 weight percent magnesium oxide,
and less than about 0.002 weight percent titanium dioxide.
[0028] In embodiments, the powder is a thermal sprayable
powder.
[0029] In embodiments, the zirconia is partially stabilized by the
yttria.
[0030] In embodiments, the powder has a particle size of from about
5 to 150 microns.
[0031] In embodiments, there is provided a method of applying a
thermal barrier coating on a substrate, the method comprising
thermally spraying the powder of claim 11 onto the substrate so as
to form the thermal barrier coating having from about 5 to 250
vertical macro cracks per 25.4 mm length measured along a coating
surface and being oriented perpendicular to a surface of the
substrate containing said coating.
[0032] In embodiments, there is provided a method of coating a
substrate with a thermal barrier coating (TBC) on a substrate via
thermal spray, the method comprising spraying a powder coating
material comprising a yttria stabilized material comprising
zirconia and hafnia, wherein a purity of each of said zirconia and
said hafnia being at least 99.5 weight percent and total impurities
of said zirconia and said hafnia comprises less than or equal to:
about 0.005 weight percent silica, about 0.002 weight percent
titania, and about 0.002 weight percent magnesia and forming a TBC
coating by depositing the coating material in the form of a
collection of frozen droplets or splats, wherein the TBC coating
comprises vertical cracks arranged at repeating intervals and
extending in a thickness direction of said thermal barrier
coating.
[0033] Embodiments are directed to a ceramic powder that includes a
primary stabilizer oxide comprising 4-20 weight percent and oxide
impurities up to a maximum of 0.15 weight percent and a first
material comprising a balance of the ceramic powder. The first
material is partially stabilized by the primary stabilizer oxide
into a first partially stabilized material. The oxide impurities,
when combined with each other or with the first partially
stabilized material, form phases with melting temperatures at least
100.degree. C. lower than the melting temperature of the first
material.
[0034] According to embodiments, the first material can include at
least one of zirconia or hafnia; and the primary stabilizer oxide
can include at least one of a yttria or a ytterbia stabilizer. The
oxide impurities may include less than 0.005 weight percent
SiO.sub.2; less than 0.015 weight percent Al.sub.2O.sub.3; less
than 0.002 weight percent TiO.sub.2; and less than 0.002 weight
percent MgO. The oxide impurities may further include less than
0.04 weight percent Fe.sub.2O.sub.3; less than 0.02 weight percent
CaO; less than 0.001 weight percent Cr.sub.2O.sub.3; and less than
0.002 weight percent Na.sub.2O.
[0035] In embodiments, the powder can be a thermal sprayable
powder.
[0036] In accordance with other embodiments, powder may have a high
purity of at least 99.5 weight percent purity.
[0037] According to still other embodiments, the powder may have a
particle size of from about 5 to 150 microns.
[0038] Embodiments are directed to a blended ceramic powder that
includes a first material composition having: a first primary
stabilizer oxide comprising 4-16 weight percent and first oxide
impurities up to a maximum of 0.15 weight percent and a first
material comprising a balance of the first material composition,
wherein the first material is partially stabilized by the first
primary stabilizer oxide into a first partially stabilized
material. The first oxide impurities, when combined with each other
or with the first partially stabilized material, form phases with
melting temperatures at least 100.degree. C. lower than the melting
temperature of the first material. The blended ceramic powder also
includes a second material composition having: a second primary
stabilizer oxide comprising 4-20 weight percent and second oxide
impurities up to a maximum of 0.15 weight percent and a second
material comprising a balance of the second material composition,
wherein the second material is partially stabilized by the second
primary stabilizer oxide into a second partially stabilized
material. The second oxide impurities, when combined with each
other or with the second partially stabilized material, form phases
with melting temperatures at least 100.degree. C. lower than the
melting temperature of the second material.
[0039] In accordance with embodiments, the first material may
include at least one of zirconia or hafnia, the first primary
stabilizer oxide may include 4-12 weight percent of a yttria
stabilizer, and a fraction of the first material in the blended
powder may be 5-50 weight percent, and the second material may
include at least one of zirconia or hafnia, the second primary
stabilizer oxide may include 4-16 weight percent of a ytterbia
stabilizer, and a fraction of the second material in the blended
powder may be 50-95 weight percent.
[0040] According to further embodiments, the blended ceramic powder
may further include a third material composition having a third
primary stabilizer oxide comprising 4-20 weight percent and second
oxide impurities up to a maximum of 0.15 weight percent and a third
material including a balance of the third material composition,
wherein the third material is partially stabilized by the third
primary stabilizer oxide into a second partially stabilized
material. The third first oxide impurities, when combined with each
other or with the third partially stabilized material, may form
phases with melting temperatures at least 100.degree. C. lower than
the melting temperature of the third material. Moreover, the third
material composition can include at least one of zirconia or hafnia
and the third primary stabilizer oxide can include at least one of
4-16 weight percent of a neodymium stabilizer or 4-20 weight
percent Europa stabilizer. The fraction of the first material in
the blended powder can be 10-90 weight percent, the fraction of the
second material in the blended powder can be 5-45 weight percent
and the fraction of the third material in the blended powder can be
5-45 weight percent.
[0041] In other embodiments, the first material can include at
least one of zirconia or hafnia, the first primary stabilizer oxide
can include 4-16 weight percent of a ytterbia stabilizer, and the
first material fraction of the blended powder can be 5-50 weight
percent, and the second material can include at least one of
zirconia or hafnia, the second primary stabilizer oxide can include
at least one of 4-16 weight percent of a neodymium stabilizer or
4-20 weight percent Europa stabilizer, and the second material
fraction of the blended powder can be 50-95 weight percent.
[0042] According to other embodiments, the first material may
include at least one of zirconia or hafnia, the first primary
stabilizer oxide may include 4-12 weight percent of a yttria
stabilizer, and the first material fraction of the blended powder
may be 50-95 weight percent, and the second material may include at
least one of zirconia or hafnia, the second primary stabilizer
oxide may include at least one of 4-16 weight percent of a
neodymium stabilizer or 4-20 weight percent Europa stabilizer, and
the second material fraction of the blended powder may be 5-50
weight percent.
[0043] In accordance with still other embodiments, at least one of
the first, second and third oxide impurities can include less than
0.005 weight percent SiO.sub.2; less than 0.015 weight percent
Al.sub.2O.sub.3; less than 0.002 weight percent TiO.sub.2; and less
than 0.002 weight percent MgO. Moreover, the at least one of the
first, second or third oxide impurities can further include less
than 0.04 weight percent Fe.sub.2O.sub.3; less than 0.02 weight
percent CaO; less than 0.001 weight percent Cr.sub.2O.sub.3; and
less than 0.002 weight percent Na.sub.2O.
[0044] In embodiments, the blended ceramic powder may be a thermal
sprayable powder.
[0045] In other embodiments, the blended ceramic powder can have a
particle size of from about 5 to 150 microns.
[0046] In accordance with still yet other embodiments, the blended
ceramic powder may have a high purity of at least 99.5 weight
percent purity.
[0047] Additional aspects, embodiments and advantages of the
invention will be set forth in the description which follows, and
in part will be obvious from the description, or may be learned by
practice of the invention. The objects and advantages of the
invention may be realized and obtained by means of the
instrumentalities and combinations particularly pointed out
hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] The accompanying drawings are included to provide further
understanding of the invention and are incorporated in and
constitute a part of this specification. The accompanying drawings
illustrate embodiments of the invention and together with the
description serve to explain the principles of the invention. In
the figures:
[0049] FIG. 1 illustrates a perspective view of a turbine blade
coated with a thermal barrier of ceramic material;
[0050] FIG. 2 provides a graph showing the effect of impurities on
the sintering rates;
[0051] FIG. 3 provides a phase diagram for ZrO.sub.2;
[0052] FIG. 4 provides a standard phase diagram for stabilized
ZrO.sub.2 showing the general alloying trends for various
stabilizers [Ceramic Phase Diagram, Volume 4, Fig 05241];
[0053] FIG. 5 provides a phase diagram for ZrO.sub.2 with
stabilizer;
[0054] FIG. 6 provides a diagram showing a lamellar thermal barrier
coating structure containing porosity and microcracks;
[0055] FIG. 7 provides a diagram showing a thermal barrier coating
containing porosity, microcracks and macrocracks;
[0056] FIG. 8 provides a diagram showing a thermal barrier coating
deposited from the vapor phase;
[0057] FIG. 9 provides a diagram showing the Thornton model for
predicting structure of a coating formed from the gas phase;
and
[0058] FIG. 10 provides a diagram showing a thermal barrier coating
deposited from both vapor and liquid phase.
DETAILED DESCRIPTION
[0059] Reference will now be made in detail to the preferred
embodiments of the invention, examples of which are illustrated in
the accompanying drawings.
[0060] In an exemplary use of a material of the invention, FIG. 1
shows one component of a turbine. Turbine blade 100 has a leading
edge 102 and an airfoil section 104, against which hot combustion
gases are directed during operation of the turbine, and which
undergoes severe thermal stresses, oxidation and corrosion. A root
end 106 of the blade anchors the blade 100. Venting passages 108
may be included through the blade 100 to allow cooling air to
transfer heat from the blade 100. The blade 100 can be made from a
high temperature resistant material. The surface of the blade 100
is coated with a thermal barrier coating 110 made of ultra-pure
zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) alloys in accordance
with the invention. The thermal barrier coating 110 may be applied
on, for example, a MCrAlY bonding layer with an alumina scale (not
shown) applied between the blade surface and the coating 110. The
coating 110 may be applied onto the bond coating surface through a
variety of methods known in the art including thermal spray
techniques such as powder flame spray and plasma spray and vapor
deposition methods such as electron beam physical vapor deposition
(EBPVD), high speed physical vapor deposition and low pressure
plasma spraying (LPPS).
[0061] When applied, the coating 110 contains a crack network that
allows it to survive the stress of numerous thermal cycles. As
described in the above background section, the crack network is
altered to a less desirable state by sintering and temperature
cycling during service. Thus the structure formed upon coating
manufacture changes with time, the rate depending on the starting
material phases. Decreasing the sintering rate increases the amount
of time before the closing of microcracks and creation of massive
cracks, increasing coating lifetime.
[0062] A dominant factor affecting sintering is the presence of
specific impurity phases within the structure made up of oxides
which when combined with each other or the zirconia alloy result in
melting points hundreds of degrees lower than that of the zirconia
alloy itself. These impurity oxides increase the sintering rate.
FIG. 2 shows the effect of impurity on the sintering rate.
[0063] In one embodiment of the present invention, the material
contains zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) partially
stabilized by a total of 4 to 20 weight percent of one or more rare
earth oxides having total impurities less than or equal to 0.15
weight percent, and preferably less than or equal to 0.1 weight
percent. For purposes of the invention, rare earth oxides can be
defined as any oxide from group IIIB (column 3) of the periodic
table of elements, which includes scandia (Sc.sub.2O.sub.3), yttria
(Y.sub.2O.sub.3), lanthanide oxides and actinide oxides.
[0064] The material of the present invention contains zirconia
(ZrO.sub.2) and/or hafnia (HfO.sub.2) partially stabilized by a
total of 4 to 20 weight percent of a primary stabilizing oxide such
as ytterbia and/or yttria, (and optionally additional stabilizers
of one or more rare earth oxides) having total impurities less than
or equal to 0.15 weight percent, and preferably less than or equal
to 0.1 weight percent. For purposes of the present invention, oxide
impurities can be defined as materials which when combined with
each other or with zirconia form phases with melting points much
lower than that of pure zirconia, especially--but not limited
to--soda (Na.sub.2O), silica (SiO.sub.2), and alumina
(Al.sub.2O.sub.3). Other specific concentration ranges of
stabilizers are provided herein and in co-pending and commonly
assigned U.S. patent application Ser. No. 11/520,041, entitled
"HIGH PURITY CERAMIC ABRADABLE COATINGS," U.S. patent application
Ser. No. 11/520,044, entitled "OPTIMIZED HIGH TEMPERATURE THERMAL
BARRIER," and U.S. application Ser. No. 11/520,042, entitled
"OPTIMIZED HIGH PURITY COATING FOR HIGH TEMPERATURE THERMAL CYCLING
APPLICATIONS" each filed on Sep. 13, 2006 and each incorporated
herein by reference.
[0065] In accordance with embodiments of the invention, the limits
for known impurities in order to achieve a desirable sintering rate
and therefore increase service lifetime when used as a coating are
about:
TABLE-US-00001 soda (Na.sub.2O) 0.1 weight percent silica
(SiO.sub.2) 0.05 weight percent alumina (Al.sub.2O.sub.3) 0.01
weight percent titania (TiO.sub.2) 0.05 weight percent hematite
(Fe.sub.2O.sub.3) 0.05 weight percent calcia (CaO) 0.05 weight
percent, and magnesia (MgO) 0.05 weight percent.
[0066] In a preferred embodiment, the limits for known impurities
are about:
TABLE-US-00002 Na.sub.2O 0.01 weight percent SiO.sub.2 0.01 weight
percent Al.sub.2O.sub.3 0.01 weight percent TiO.sub.2 0.01 weight
percent Fe.sub.2O.sub.3 0.01 weight percent CaO 0.025 weight
percent, and MgO 0.025 weight percent.
[0067] The impurity limits in the embodiments above are not
indicative that any or all of the impurities listed will be
included in the material in any amount. The embodiment of the
invention may include zero weight percent of one or more of the
above-listed impurities.
[0068] FIG. 3 provides a phase diagram for pure zirconia. (The
diagram can be found, for example, in Ceramic Phase Diagrams vol.
3, figure 04259.) As shown in FIG. 3, pure zirconia exists in three
crystal phases at different temperatures. At very high temperatures
(>2370.degree. C.) the material has a cubic structure. At
intermediate temperatures (1200 to 2372.degree. C.) it has a
tetragonal structure. At relatively lower temperatures (below
1200.degree. C.) the material transforms to the monoclinic
structure. The transformation from tetragonal to monoclinic is
rapid and is accompanied by a 3 to 5 percent volume increase that
causes extensive stress in the material. Thus, pure zirconia cannot
fulfill the coating requirements for high-temperature cycling. The
resulting strain difference between the coating and substrate
caused by the phase transformation results in a stress that is
greater than the bond strength between them, so the coating will
detach.
[0069] In accordance with embodiments of the invention, in order to
overcome the volume change caused by the undesired phase
transformation described above, one or more elements are added to
the zirconia to modify the amount of phase transformation that
occurs. The stabilizing elements, which are suitable for changing
the amount and rate of phase transformation that occurs in the
oxide coating, may include the following: scandium, yttrium and the
rare earths, particularly the lanthanides, since they have
solubility in zirconia. Scandium is not typically used due to its
rarity and resulting prohibitive cost. Use of rare earths metals
from the actinide group such as uranium and thorium may be limited
due to their radioactivity. Thus, yttrium is a preferred
stabilizing element.
[0070] FIG. 4 provides a standard phase diagram for stabilized
zirconia showing the general alloying trends for the zirconia
stabilizers. A specific diagram for zirconia with yttria as a
stabilizer is given in FIG. 5. (The diagram can be found, for
example, in Ceramic Phase Diagram, vol. Zirconia, figure
Zr-157.)
[0071] Phase transformation in partially stabilized zirconia may
possibly cause localized stresses that lead to the formation of
micron-sized micro-cracks in the coating upon thermal cycling that
cancel out some of the massive stress caused by coating volume
shrinkage. Thus, these two phenomena of the coating
structure--shrinking and cracking--work against each other and
finding a balance between them will maximize coating lifetime. This
mechanism implies then that the structure of the crack network of
the coating is changing with time as the phase of the ceramic
material changes. This mechanism is required for a thermal barrier
or high temperature abradable coatings to survive thermal
cycling.
[0072] The addition of a stabilizing element affects two main
properties of the zirconia coating system in a positive manner.
First, the addition of a stabilizer as illustrated in FIG. 4
generally increases the melting temperature of the zirconia (in the
partially stabilized composition ranges). Second, the addition of a
stabilizer generally decreases the thermal conductivity. Once the
critical composition that has the highest thermal cycling values is
found experimentally for a stabilizer, the stabilizers can be
compared by the melting point at the critical composition.
[0073] Rising fuel cost and other factors continue to drive the
need for improved operational efficiency, and thus higher operating
temperatures, of gas turbines. While yttria stabilized zirconia is
the material of choice for stabilization, greater operational
temperatures can be achieved using ytterbia (FIG. 4) for example.
Zirconia partially stabilized by ytterbia provides a better
composition, since it also has one of the lowest thermal
conductivities of the potential stabilizers when alloyed with
zirconia. As the need for higher operating temperatures increases,
a higher coating material cost may be tolerated, so ytterbia
partially stabilized zirconia may become a preferred thermal
barrier or high temperature abradable coating system. Given then
the trade-offs of cost and performance, a combination of both
yttria and ytterbia stabilizers is expected to have optimum
performance to cost ratio.
[0074] A blend of two or more partially stabilized high-purity
material compositions may also be used. For example, in another
embodiment, a blended ceramic material for use in high-temperature
thermal barriers is provided. The blended materials include a first
material with a yttria (Y.sub.2O.sub.3) stabilizer, and a balance
of at least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and
combinations thereof, wherein the zirconia (ZrO.sub.2) and/or
hafnia (HfO.sub.2) is partially stabilized by the yttria
stabilizer, and having total impurities less than or equal to 0.15
weight percent, and preferably less than or equal to 0.1 weight
percent. The range of Y.sub.2O.sub.3 stabilizer is about 4-12
weight percent, and preferably 6-9 weight percent. The second
material of the blended material may contain a ytterbia
(Yb.sub.2O.sub.5) stabilizer and a balance of at least one of
zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and combinations
thereof, wherein the zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2)
is partially stabilized by the ytterbia stabilizer, and having
total impurities less than or equal to 0.15 weight percent, and
preferably less than or equal to 0.1 weight percent. The range of
Yb.sub.2O.sub.5 stabilizer is about 4-16 weight percent, and
preferably 10-16 weight percent. In the blended material, the
ytterbia (Yb.sub.2O.sub.5) stabilized zirconia (ZrO.sub.2) and/or
hafnia (HfO.sub.2) fraction may include about 5-50 weight percent
of the total and preferably about 15-30 weight percent of the
total. The yttria stabilized zirconia (ZrO.sub.2) and/or hafnia
(HfO.sub.2) fraction may include about 50-95 weight percent of the
total and preferably about 70-85 weight percent of the total
blend.
[0075] In another embodiment the blended material includes a first
material with a ytterbia (Yb.sub.2O.sub.5) stabilizer, and a
balance of at least one of zirconia (ZrO.sub.2) and hafnia
(HfO.sub.2) and combinations thereof, wherein the zirconia
(ZrO.sub.2) and/or hafnia (HfO.sub.2) is partially stabilized by
the ytterbia stabilizer, and having total impurities less than or
equal to 0.15 weight percent, and preferably less than or equal to
0.1 weight percent. The range of Yb.sub.2O.sub.5 stabilizer is
about 4-16 weight percent, and preferably 10-16 weight percent. The
second material of the blended material may contain a stabilizer of
at least one of neodymium (Nd.sub.2O.sub.3), europia
(Eu.sub.2O.sub.5), and combinations thereof and a balance of at
least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and
combinations thereof, wherein the balance is partially stabilized
by the stabilizer, and having total impurities less than or equal
to 0.15 weight percent, and preferably less than or equal to 0.1
weight percent. The range of Nd.sub.2O.sub.3 stabilizer is about
4-20 weight percent, and preferably 8-16 weight percent. The range
of Eu.sub.2O.sub.3 stabilizer is about 4-16 weight percent, and
preferably 10-16 weight percent. The range of the combined
Nd.sub.2O.sub.3 and Nd.sub.2O.sub.3 stabilizer is about 4-16 weight
percent. In the blended material, the ytterbia (Y.sub.2O.sub.3)
stabilized zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction
may include about 5-50 weight percent of the total and preferably
about 15-30 weight percent of the total. The yttria stabilized
zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction may include
about 50-95 weight percent of the total and preferably about 70-85
weight percent of the total blend.
[0076] In another embodiment of the invention the blended material
includes a first material with a yttria (Y.sub.2O.sub.3)
stabilizer, and a balance of at least one of zirconia (ZrO.sub.2)
and hafnia (HfO.sub.2) and combinations thereof, wherein the
zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) is partially
stabilized by the yttria stabilizer, and having total impurities
less than or equal to 0.15 weight percent, and preferably less than
or equal to 0.1 weight percent. The range of Y.sub.2O.sub.3
stabilizer is about 4-12 weight percent, and preferably 6-9 weight
percent. The second material of the blended material may contain a
stabilizer of at least one of neodymium (Nd.sub.2O.sub.3), europia
(Eu.sub.2O.sub.5), and combinations thereof and a balance of at
least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and
combinations thereof, wherein the balance is partially stabilized
by the stabilizer, and having total impurities less than or equal
to 0.15 weight percent, and preferably less than or equal to 0.1
weight percent. The range of Nd.sub.2O.sub.3 stabilizer is about
4-20 weight percent, and preferably 8-16 weight percent. The range
of Eu.sub.2O.sub.3 stabilizer is about 4-16 weight percent, and
preferably 10-16 weight percent. The range of the combined
Nd.sub.2O.sub.3 and Eu.sub.2O.sub.3 stabilizer is about 4-16 weight
percent. In the blended material, the neodymium (Nd.sub.2O.sub.3)
and/or europia (Eu.sub.2O.sub.5) stabilized zirconia (ZrO.sub.2)
and/or hafnia (HfO.sub.2) fraction may include about 5-50 weight
percent of the total and preferably about 15-30 weight percent of
the total. The yttria stabilized zirconia (ZrO.sub.2) and/or hafnia
(HfO.sub.2) fraction may include about 50-95 weight percent of the
total and preferably about 70-85 weight percent of the total
blend.
[0077] In a further embodiment of the invention the blended
material includes a blend of at least three materials. The first
material may contain a yttria (Y.sub.2O.sub.3) stabilizer, and a
balance of at least one of zirconia (ZrO.sub.2) and hafnia
(HfO.sub.2) and combinations thereof, wherein the zirconia
(ZrO.sub.2) and/or hafnia (HfO.sub.2) is partially stabilized by
the yttria stabilizer, and having total impurities less than or
equal to 0.15 weight percent, and preferably less than or equal to
0.1 weight percent. The range of Y.sub.2O.sub.3 stabilizer is about
4-12 weight percent, and preferably 6-9 weight percent. The second
material of the blend may contain a ytterbia (Yb.sub.2O.sub.5)
stabilizer, and a balance of at least one of zirconia (ZrO.sub.2)
and hafnia (HfO.sub.2) and combinations thereof, wherein the
zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) is partially
stabilized by the ytterbia stabilizer, and having total impurities
less than or equal to 0.15 weight percent, and preferably less than
or equal to 0.1 weight percent. The range of Yb.sub.2O.sub.5
stabilizer is about 4-16 weight percent, and preferably 10-16
weight percent. The third material of the blend may contain a
stabilizer of at least one of neodymium (Nd.sub.2O.sub.3), europia
(Eu.sub.2O.sub.5), and combinations thereof and a balance of at
least one of zirconia (ZrO.sub.2) and hafnia (HfO.sub.2) and
combinations thereof, wherein the balance is partially stabilized
by the stabilizer, and having total impurities less than or equal
to 0.15 weight percent, and preferably less than or equal to 0.1
weight percent. The range of Nd.sub.2O.sub.3 stabilizer is about
4-20 weight percent, and preferably 8-16 weight percent. The range
of Eu.sub.2O.sub.3 stabilizer is about 4-16 weight percent, and
preferably 10-16 weight percent. The range of the combined
Nd.sub.2O.sub.3 and Eu.sub.2O.sub.3 stabilizer is about 4-16 weight
percent. In the blended material, the ytterbia (Y.sub.2O.sub.3)
stabilized zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction
may include about 5-45 weight percent of the total, and preferably
about 15-30 weight percent of the total. The neodymium
(Nd.sub.2O.sub.3) and/or europia (Eu.sub.2O.sub.5) stabilized
zirconia (ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction may also
include about 5-45 weight percent of the total and preferably about
15-30 weight percent of the total. The yttria stabilized zirconia
(ZrO.sub.2) and/or hafnia (HfO.sub.2) fraction may include about
10-90 weight percent of the total, and preferably about 30-60
weight percent of the total blend.
[0078] Material of embodiments of the present invention can be
provided in a variety of forms for use in thermal spray
applications. For example, the material is supplied in the form of
a powder, a slurry of powder, or a chemical solution of the
constituents. If in powder form, the powder may be in the form of a
spray dried powder of the individual constituents and organic
binder, spray dried powder of the combined individual constituents
and organic binder, fused and crushed powder, agglomerated and
sintered powder, plasma densified material or powder from chemical
solution routes. Typical particle sizes for the powders may vary
but typically range between about 5-150 microns when deposited by
various thermal spray equipment, preferably ranging between about
15-125 microns for air plasma spray and ranging between about 5-30
microns for low pressure plasma spray.
[0079] Typically for thermal spray applications, a polymer or
organic material in powder form can be added to the material blend.
Powder may be in the form of a spray dried powder of the individual
constituents and an organic binder, spray dried powder of the
combined individual constituents and an organic binder, fused and
crushed powder, agglomerated and sintered powder, plasma densified
material or powder from chemical solution routes. The organic
binder may be used to at least partially bond together the
placeholder material and the ceramic material. For high temperature
abradable coatings, the benefit of adding a fugitive phase is that
a higher porosity than is achievable with conventional deposition
methods. The increased porosity aids abradability by introducing
more surfaces to the crack network of the coating, decreasing the
coating elastic modulus and thus promoting controlled material
removal. Thus, according to an embodiment of the invention, a
coating material may have about 70 to 99 weight percent of an
ultra-pure ceramic material as previously described and about 1-30
weight percent (and preferably 2-15 weight percent) of a
placeholder material. The placeholder material may be an organic
powder material or an inorganic powder material that can be burned
out subsequent to deposition of the coating material.
[0080] Porosities and cracks provide strain tolerance to TBCs and
help to reduce thermal conductivities. Using thermal spray process,
such as air plasma spray, flame spray or low pressure plasma spray,
a high purity coating structure 120 (shown in FIG. 6) that comprise
a ceramic matrix, porosity and microcracks can be achieved. The
high purity coating structure is formed by injecting particles of
invention high purity materials into a high temperature and high
velocity flame. These particles are then heated and accelerated in
the flame. Before reaching the substrate, some particles are
molten, while some other particles are semi-molten or not melted.
Referring to FIG. 6, molten and semi-molten particles strike on the
substrate 100 (or optional bond coat 112) and then spread and
solidify rapidly to form disk-like deposits 111, which are referred
to as splats. Although some unmelted particles are entrapped and
incorporated into the coating, most of them bounce off when they
hit the substrate. The accumulation of splats and small amount of
unmelted particles results in the coating formation. Due to
shrinkage occurred during rapid solidification and imperfect
packing of splats and unmelted particles, voids and cracks are
generated in the coating. Herein, porosity refers to a void with an
aspect ratio (length divided by width) of less than about 10.
Typical porosity is in the range of about 5.about.20 volume
percent, preferably in the range of about 7.about.15 volume
percent. Micro cracks refers to a void with an aspect ratio (length
divided by width) of larger than about 10 and the length of the
void is less than about 100 micrometers. Typical volume percentage
of micro cracks is in the range of about 2.about.15 volume percent,
preferably in the range of about 5.about.10 volume percent.
[0081] In order to enhance the strain tolerance of the
aforementioned high purity TBCs, macro cracks that runs normal to
the top coat and substrate interface can be introduced into the
coating. As a result, another high purity coating structure that
comprise a ceramic matrix, porosity, macro cracks and micro cracks
(FIG. 7) can be achieved by thermal spray processes, such as air
plasma spray, flame spray or low pressure plasma spray. The high
purity coating structure 130 of FIG. 7 is formed by injecting
particles of inventive materials into a high temperature and high
velocity flame. These particles are then heated and accelerated in
the flame. Before reaching the substrate, some particles are
molten, while some other particles are semi-molten or not melted.
Molten and semi-molten particles strike on the substrate 100 (or
optional bond coat 112) and then spread and solidify rapidly to
form disk-like deposits, which is referred to as splats. Although
some unmelted particles are entrapped and incorporated into the
coating, most of them bounce off when they hit the substrate. The
accumulation of splats 131 and small amount of unmelted particles
results in the coating formation 130. Due to shrinkage occurred
during rapid solidification and imperfect packing of splats and
unmelted particles, voids and cracks are generated in the coating.
When coating deposition conditions are controlled to generate large
shrinkage stress and improve the packing of splats to reduce voids
and gaps between splats, cracks 132 normal to the coating 130 and
substrate 100 interface are created. Herein, macro cracks refers to
a void with an aspect ratio (length divided by width) of larger
than about 10 and the length of the void is longer than about 100
micrometers. More than about 90% of the macro cracks are arranged
in the direction normal to the top coat and substrate interface.
These macro cracks are referred to as vertical macro cracks, while
the macro cracks parallel to the top coat and substrate interface
are referred to as horizontal vertical cracks. For this coating
structure, typical volume percentage of porosity and micro cracks
is less than about 10% and 5%, preferably less than about 5% and
3%, respectively. The average number of vertical macro cracks in a
length of 25.4 mm along the top coat and substrate interface is in
the range of about 5 to 250, preferably in the range of about 50 to
150.
[0082] When coatings are produced using a vapor deposition process,
such as electron beam assisted physical vapor deposition process
(EB-PVD) or low pressure (lower than ambient) plasma spraying, the
resulting coating has a unique columnar structure. The gaps between
columns impart excellent strain tolerance to the coating.
Accordingly, TBCs produce using vapor deposition process, such as
EB-PVD or low pressure (lower than ambient) plasma spraying,
usually have a higher durability than TBCs produced using thermal
spray processes. As illustrated in FIG. 8, if vapor deposition
process was employed, another high purity coating structure 140
that comprises ceramic columns 143 and gaps 141 between them can be
achieved. An optional bond coat 112 is shown between the substrate
100 and the coating 140. The high purity coating structure 140 is
formed by vaporizing the inventive high purity materials in a form
of powder, ingot, target, solution or suspension. The formed vapor
then deposited atomically on the substrate. By controlling
processing temperature and pressure according to the Thornton's
model (FIG. 9), a coating with columnar structure is formed.
Herein, ceramic columns 143 are basically a cluster of crystals.
More than about 90% of the crystals are at an angle of about 45 to
135 degree to the top coat and substrate interface. Within the
cluster of crystals, voids smaller than about 20 micrometers are
present. The gaps 141 between the columns have an aspect ratio
(length divided by width) of larger than about 10. More than about
90% of the gaps are at an angle of about 45 to 135 degree to the
top coat and substrate interface.
[0083] In low pressure (lower than ambient) plasma spraying
process, if molten droplets are also generated during the
vaporization of the invention high purity materials, then the
entrapment and incorporation of these droplets into the coating
results in the formation of another high purity coating structure.
As illustrated in FIG. 9, the high purity coating structure 150
comprises ceramic columns 143, gaps between the columns 141, and
nodules 142 distributing randomly in the gaps and columns. An
optional bond coat 112 is shown between the substrate 100 and the
coating 150. Herein, ceramic columns 143 are basically a cluster of
crystals. More than 90% of the crystals are oriented at an angle of
45 to 135 degree to the top coat and substrate interface. Within
the cluster of crystals 143, voids smaller than 20 micrometers are
present. The gaps 141 between the columns have an aspect ratio
(length divided by width) of larger than about 10. More than 90% of
the gaps 141 are oriented at an angle of 45 to 135 degree to the
top coat and substrate interface. The nodules 142 distributing
randomly in the gaps and columns are frozen droplets. The size of
these nodules 142 is typically less than about 45 micrometers,
preferably less than about 30 micrometers.
[0084] While exemplary embodiments of the invention have been shown
and described herein, it will be obvious to those skilled in the
art that such embodiments are provided by way of example only.
Numerous insubstantial variations, changes, and substitutions will
now be apparent to those skilled in the art without departing from
the scope of the invention disclosed herein by the Applicants.
Accordingly, it is intended that the invention be limited only by
the spirit and scope of the claims, as they will be allowed.
* * * * *