U.S. patent application number 16/200938 was filed with the patent office on 2019-05-30 for method for preparing a ceramic thermal barrier coating layer having excellent adhesion and thermal durability.
The applicant listed for this patent is Korea Institute of Ceramic Engineering and Technology. Invention is credited to Hyung Tae KIM, Seong Won KIM, Sung Min LEE, Yoon Suk OH.
Application Number | 20190161843 16/200938 |
Document ID | / |
Family ID | 66634903 |
Filed Date | 2019-05-30 |
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United States Patent
Application |
20190161843 |
Kind Code |
A1 |
KIM; Seong Won ; et
al. |
May 30, 2019 |
METHOD FOR PREPARING A CERAMIC THERMAL BARRIER COATING LAYER HAVING
EXCELLENT ADHESION AND THERMAL DURABILITY
Abstract
The present disclosure relates to a method for preparing a
ceramic thermal barrier coating layer including (a) preparing a
first suspension in which first oxide particles for thermal barrier
coating are dispersed and a second suspension in which second oxide
particles for thermal barrier coating are dispersed, respectively;
(b) forming a first coating layer on a base material by suspension
plasma spraying (SPS) using the first suspension; (c) forming a
buffer layer on the first coating layer by the suspension plasma
spraying using a mixed suspension of the first suspension and the
second suspension; and (d) forming a second coating layer on the
buffer layer by the suspension plasma spraying using the second
suspension.
Inventors: |
KIM; Seong Won; (Seoul,
KR) ; LEE; Sung Min; (Seoul, KR) ; OH; Yoon
Suk; (Seoul, KR) ; KIM; Hyung Tae; (Seoul,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Korea Institute of Ceramic Engineering and Technology |
Jinju-si |
|
KR |
|
|
Family ID: |
66634903 |
Appl. No.: |
16/200938 |
Filed: |
November 27, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 4/11 20160101; C23C
4/134 20160101 |
International
Class: |
C23C 4/11 20060101
C23C004/11; C23C 4/134 20060101 C23C004/134 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 28, 2017 |
KR |
KR10-2017-0161077 |
Claims
1. A method for preparing a ceramic thermal barrier coating layer
comprising: (a) preparing a first suspension in which first oxide
particles for thermal barrier coating are dispersed and a second
suspension in which second oxide particles for thermal barrier
coating are dispersed, respectively; (b) forming a first coating
layer on a base material by suspension plasma spraying (SPS) using
the first suspension; (c) forming a buffer layer on the first
coating layer by the suspension plasma spraying using a mixed
suspension of the first suspension and the second suspension; and
(d) forming a second coating layer on the buffer layer by the
suspension plasma spraying using the second suspension.
2. The method of claim 1, wherein a first oxide for thermal barrier
coating is YSZ (Yttria-stabilized zirconia) and a second oxide for
thermal barrier coating is rare-earth zirconates.
3. The method of claim 2, wherein the rare-earth zirconates are
La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06 (x is 0.24 to 1.65).
4. The method of claim 1, wherein the step (a) comprises: (a-1)
mechanically pulverizing an oxide powder for thermal barrier
coating; and (a-2) forming a slurry containing the pulverized oxide
powder.
5. The method of claim 1, wherein the step (a) comprises: (a-1)
mixing two or more oxide powders to perform mechanical alloying;
and (a-2) forming a slurry containing an oxide powder for thermal
spraying coating obtained in the step (a-1).
6. The method of claim 5, wherein the two or more oxide powders
comprise (i) one or more selected from Y.sub.2O.sub.3,
Gd.sub.2O.sub.3, and La.sub.2O.sub.3, and (ii) one or more selected
from ZrO.sub.2 and CeO.sub.2.
7. The method of claim 4, wherein the mechanical pulverizing or
mechanical alloying of the oxide powder in the step (a-1) is
performed by planetary ball milling, attrition milling, or shaker
milling.
8. The method of claim 4, further comprising, prior to the step
(a-2), calcining the oxide powder obtained in the step (a-1).
9. The method of claim 1, wherein, in the step (c), the buffer
layer is formed so that a content ratio of the first oxide and the
second oxide continuously changes in a thickness direction of a
coating layer.
10. The method of claim 9, wherein the step (c) is carried out
using a suspension plasma spraying apparatus with a suspension
supplier, wherein the suspension supplier comprises: a first
suspension storage tank in which the first suspension is stored; a
second suspension storage tank in which the second suspension is
stored; a first transfer pipe for supplying the first suspension
from the first suspension storage tank to a plasma spraying gun; a
first opening/closing valve provided in the first transfer pipe for
controlling a supply amount of the first suspension to the plasma
spraying gun; a second transfer pipe for connecting between the
first suspension storage tank and the second suspension storage
tank and for transferring the second suspension to the first
suspension storage tank; a second opening/closing valve provided in
the second transfer pipe for controlling a flow rate of the second
suspension to the first suspension storage tank; a third transfer
pipe for supplying the second suspension from the second suspension
storage tank to the plasma spraying gun; and a third
opening/closing valve provided in the third transfer pipe for
controlling a supply amount of the second suspension to the plasma
spraying gun.
11. The method of claim 5, wherein the mechanical pulverizing or
mechanical alloying of the oxide powder in the step (a-1) is
performed by planetary ball milling, attrition milling, or shaker
milling.
12. The method of claim 5, further comprising, prior to the step
(a-2), calcining the oxide powder obtained in the step (a-1).
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of
Korean Patent Application No. 10-2017-0161077 filed in the Korean
Intellectual Property Office on Nov. 28, 2017, the entire contents
of which are incorporated herein by reference.
TECHNICAL FIELD
[0002] The present disclosure relates to a method for preparing a
ceramic thermal barrier coating layer deposited on a surface of a
high-temperature superalloy component of a gas turbine engine.
BACKGROUND
[0003] Thermal barrier coatings (TBCs) are heat-resistant ceramic
coatings deposited on a surface of a high-temperature superalloy
component of a gas turbine engine for power or aviation. It
enhances the thermal efficiency of the gas turbine by increasing a
turbine inlet temperature (TIT).
[0004] Yttria-stabilized zirconia (YSZ), widely used in industry,
is formed in a metastable tetragonal prime phase (t'-phase) with no
phase change depending on a temperature when it is prepared by a
plasma spraying technique or an electron beam physical deposition
technique as a thermal barrier coating. This t'-phase YSZ is
separated into a thermodynamically stable tetragonal prime phase
and a cubic phase when exposed to a temperature equal to or above
1200.degree. C. Also, in this t'-phase YSZ, during a cooling
process, the tetragonal prime phase is transformed into a
monoclinic phase having a large unit volume, and deterioration of a
coating layer occurs. Therefore, this t'-phase YSZ has a limited
application temperature.
[0005] Lanthanide rare-earth zirconates are the next generation
thermal barrier coating material that has recently been studied for
application to high-efficiency gas turbine engines operating at or
above an application temperature of conventional YSZ thermal
barrier coatings. Conventional yttria stabilized zirconia has a low
thermal conductivity and a relatively high coefficient of thermal
expansion as a ceramic, and has high fracture toughness and
high-temperature durability of a thermal barrier coating. However,
an application temperature thereof is limited to 1200.degree. C.
Whereas, the lanthanide rare-earth zirconates have low thermal
conductivity compared to the YSZ and has phase stability existing
in the cubic phase up to a melting point. However, it is known that
the lanthanide rare-earth zirconates have poor adhesion and
high-temperature durability when it is prepared into the thermal
barrier coating due to its relatively low thermal expansion
properties and fracture toughness.
[0006] Therefore, the need is increasing that solve the respective
problems caused by the formation of the thermal barrier coating
with the conventional YSZ or lanthanide rare-earth zirconates and
improve the adhesion of the thermal barrier coating layer as well
as the high-temperature durability during a thermal cycle.
RELATED ART DOCUMENT
[0007] (Non-Patent Document 1) D. R. Clarke, Surf. Coat. Technol.,
163-164 (2003) 67. [0008] (Non-Patent Document 2) C. G. Levi, Curr.
Opi. in Sol. St. Mater. Sci., 8 (2004) 77. [0009] (Non-Patent
Document 3) R. Va en, M. O. Jarligo, T. Steinke, D. E. Mack, D.
Stover, Surf. Coat. Technol., 205 (2010) 938. [0010] (Non-Patent
Document 4) D. R. Clarke, M. Oechsner, N. P. Padture, MRS Bull., 37
(2012) 891. [0011] (Non-Patent Document 5) W. Pan, S. R. Phillpot,
C. Wan, A. Chernatynskiy, Z. Qu, MRS Bull., 37 (2012) 917. [0012]
(Non-Patent Document 6) X. Ren, W. Pan, Acta Mater., 69 (2014) 397.
[0013] (Non-Patent Document 7) N. P. Padture, M. Gell, E. H.
Jordan, Science, 296 (2002) 280. [0014] (Non-Patent Document 8) J.
Wu, X. Z. Wei, N. P. Padture, P. G. Klemens, M. Gell, E. Garcia, P.
Miranzo and M. I. Osendi, J. Am. Ceram. Soc., 85 (2002) 3031.
[0015] (Non-Patent Document 9) H. Lehmann, D. Pitzer, G. Pracht, R.
Vassen, D. Stover, J. Am. Ceram. Soc., 86 (2003) 1338. [0016]
(Non-Patent Document 10) J. W. Fergus, Metall. Mater. Trans. E,
(2014) 1.
SUMMARY
[0017] It is an object of the present disclosure to provide a
method for preparing a two-layer ceramic thermal barrier coating
layer. The method may improve adhesion properties of the thermal
barrier coating layer by forming the two-layer ceramic thermal
barrier coating layer by suspension plasma spraying with a
connected suspension supplier suitable for forming an inclined
functional coating layer using a suspension in which pre-treated
oxide particles are dispersed. The method may also improve
high-temperature durability during a thermal cycle by adjusting a
thermal expansion coefficient in a vertical direction of
coating.
[0018] To achieve the foregoing object, the present disclosure
proposes a method for preparing a ceramic thermal barrier coating
layer including (a) preparing a first suspension in which first
oxide particles for thermal barrier coating are dispersed and a
second suspension in which second oxide particles for thermal
barrier coating are dispersed, respectively; (b) forming a first
coating layer on a base material by suspension plasma spraying
(SPS) using the first suspension; (c) forming a buffer layer on the
first coating layer by the suspension plasma spraying using a mixed
suspension of the first suspension and the second suspension; and
(d) forming a second coating layer on the buffer layer by the
suspension plasma spraying using the second suspension.
[0019] Further, the method for preparing a ceramic thermal barrier
coating layer proposes that a first oxide for thermal barrier
coating is YSZ (Yttria-stabilized zirconia) and a second oxide for
thermal barrier coating is rare-earth zirconates.
[0020] Further, the method for preparing a ceramic thermal barrier
coating layer proposes that the rare-earth zirconates are
La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06 (x is 0.24 to 1.65).
[0021] Further, the method for preparing a ceramic thermal barrier
coating layer proposes that the step (a) includes (a-1)
mechanically pulverizing an oxide powder for thermal barrier
coating; and (a-2) forming a slurry containing the pulverized oxide
powder.
[0022] Further, the method for preparing a ceramic thermal barrier
coating layer proposes that the step (a) includes (a-1) mixing two
or more oxide powders to perform mechanical alloying; and (a-2)
forming a slurry containing an oxide powder for thermal spraying
coating obtained in the step (a-1).
[0023] Further, the method for preparing a ceramic thermal barrier
coating layer proposes that the two or more oxide powders include
(i) one or more selected from Y.sub.2O.sub.3, Gd.sub.2O.sup.3, and
La.sub.2O.sub.3, and (ii) one or more selected from ZrO.sub.2 and
CeO.sub.2.
[0024] Further, the method for preparing a ceramic thermal barrier
coating layer proposes that the mechanical pulverizing or
mechanical alloying of the oxide powder in the step (a-1) is
performed by planetary ball milling, attrition milling, or shaker
milling.
[0025] Also, the method for preparing a ceramic thermal barrier
coating layer further proposes calcining the oxide powder obtained
in the step (a-1) prior to the step (a-2).
[0026] Further, the method for preparing a ceramic thermal barrier
coating layer proposes that in the step (c), the buffer layer is
formed so that a content ratio of the first oxide and the second
oxide continuously changes in a thickness direction of a coating
layer.
[0027] Further, the method for preparing a ceramic thermal barrier
coating layer proposes that the step (c) is carried out using a
suspension plasma spraying apparatus with a suspension supplier, in
which the suspension supplier includes a first suspension storage
tank in which the first suspension is stored; a second suspension
storage tank in which the second suspension is stored; a first
transfer pipe for supplying the first suspension from the first
suspension storage tank to a plasma spraying gun; a first
opening/closing valve provided in the first transfer pipe for
controlling a supply amount of the first suspension to the plasma
spraying gun; a second transfer pipe for connecting between the
first suspension storage tank and the second suspension storage
tank and for transferring the second suspension to the first
suspension storage tank; a second opening/closing valve provided in
the second transfer pipe for controlling a flow rate of the second
suspension to the first suspension storage tank; a third transfer
pipe for supplying the second suspension from the second suspension
storage tank to the plasma spraying gun; and a third
opening/closing valve provided in the third transfer pipe for
controlling a supply amount of the second suspension to the plasma
spraying gun.
[0028] According to the method for preparing a two-layer ceramic
thermal barrier coating layer using suspension plasma spraying with
a connected suspension supplier suitable for forming an inclined
functional coating layer according to the present disclosure, a
two-layer ceramic thermal barrier coating layer is formed by
suspension plasma spraying with a suspension supplier connected
using two types of suspensions of YSZ and rare-earth zirconates
containing pulverized oxide particles through high energy milling
or alloyed composite oxide particles. Here, the rare-earth
zirconates are deposited on an upper layer which is a
high-temperature portion of a corresponding coating layer, in which
the rare-earth zirconates have a pyrochlore or fluorite or a mixed
phase of two phases with low thermal conductivity and excellent
phase stability at a high temperature, and there is no m-ZrO.sub.2
causing deterioration of the thermal barrier coating. Here, YSZ is
deposited on a lower layer which is a low-temperature portion of
the corresponding coating layer, in which the YSZ has a thermal
expansion coefficient close to that of a substrate and has
excellent high-temperature mechanical properties. Accordingly, it
is possible to form a two-layer ceramic thermal barrier coating
layer which can be used at higher temperatures as compared with
conventional YSZ monolayer coatings, and has improved adhesion and
high-temperature durability over rare-earth zirconates monolayer
coatings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a view showing a suspension supplier for a
suspension plasma spraying apparatus according to an embodiment of
the present disclosure.
[0030] FIG. 2A shows the result of X-ray diffraction (XRD) pattern
analysis of La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06 (x=0.24,
0.71, 1.18, 1.65) powder after the synthesis used in a suspension
of a second suspension storage tank in Examples 1 to 4 of the
present application.
[0031] FIG. 2B is the result of X-ray diffraction (XRD) pattern
analysis of a YSZ powder used in a suspension of a first suspension
storage tank in Examples 1 to 4 and Comparative Example of the
present application.
[0032] FIG. 3 is the result of X-ray diffraction (XRD) pattern
analysis of a specimen of a two-layer ceramic thermal barrier
coating prepared in Examples 1 to 4 and a specimen of a monolayer
YSZ thermal barrier coating prepared in Comparative Example of the
present application.
[0033] FIG. 4A is an image of a scanning electron microscope (SEM)
showing a cross-sectional microstructure and coating thickness of a
specimen of a two-layer ceramic thermal barrier coating prepared in
Examples 1 to 4 of the present application.
[0034] FIG. 4B is an image of a scanning electron microscope (SEM)
showing a cross-sectional microstructure and coating thickness of a
specimen of a monolayer YSZ thermal barrier coating prepared in
Comparative Example of the present application.
[0035] FIG. 5 is an image of a scanning electron microscope (SEM)
showing a cross-sectional microstructure of a specimen of a
two-layer ceramic thermal barrier coating prepared in Examples 1 to
4 of the present application and the result of EDS line scanning
showing the distribution of La, Gd, Zr, and Y elements.
[0036] FIG. 6A is images of a coating side and a substrate side
after a adhesion test for a specimen of a two-layer ceramic thermal
barrier coating prepared in Examples 1 to 4 and a specimen of a
monolayer YSZ thermal barrier coating prepared in Comparative
Example, in which the white color indicates that dropouts have
occurred on the coating side and the black color indicates that
dropouts have occurred on the substrate side.
[0037] FIG. 6B is the result of an adhesion test of a specimen of a
two-layer ceramic thermal barrier coating prepared in Examples 1 to
4 and a specimen of a monolayer YSZ thermal barrier coating
prepared in Comparative Example.
[0038] FIG. 7 is the result of an isothermal deterioration test at
1275.degree. C. for a specimen of a two-layer ceramic thermal
barrier coating prepared in Examples 1 to 4 of the present
application and a specimen of a monolayer YSZ thermal barrier
coating prepared in Comparative Example, and it shows the number of
cycles until a coating is soundly attached to a substrate.
DETAILED DESCRIPTION OF THE INVENTION
[0039] In the following description of the present disclosure, a
detailed description of related known functions and configurations
will be omitted if it is determined that the gist of the present
disclosure may be unnecessarily blurred.
[0040] Embodiments according to the concept of the present
disclosure may be variously modified and may take various forms.
Therefore, it is intended to illustrate specific embodiments in the
drawings and describe them in detail in this specification or
application. However, it is not intended to limit the embodiments
according to the concepts of the present disclosure to particular
forms of disclosure. It is to be understood that the present
disclosure covers all modifications, equivalents, and alternatives
falling within the spirit and scope of the present disclosure.
[0041] Terms used herein are only used to describe particular
embodiments, but are not intended to limit the present disclosure.
Unless the context clearly means otherwise, singular expressions
include plural expressions. It should be understood that, herein,
the terms "comprises" or "have," etc. are intended to specify that
there is a stated feature, number, step, operation, component,
part, or a combination thereof, but it does not exclude in advance
the possibility of presence or addition of one or more other
features, numbers, steps, operations, components, parts, or
combinations thereof.
[0042] Hereinafter, the present disclosure will be described in
detail.
[0043] A method for preparing a ceramic thermal barrier coating
layer having excellent adhesion and thermal durability according to
an embodiment of the present disclosure includes (a) preparing a
first suspension in which first oxide particles for thermal barrier
coating are dispersed and a second suspension in which second oxide
particles for thermal barrier coating are dispersed, respectively;
(b) forming a first coating layer on a base material by suspension
plasma spraying (SPS) using the first suspension; (c) forming a
buffer layer on the first coating layer by the suspension plasma
spraying using a mixed suspension of the first suspension and the
second suspension; and (d) forming a second coating layer on the
buffer layer by the suspension plasma spraying using the second
suspension. Hereinafter, each step will be described in detail.
[0044] A first oxide for the thermal barrier coating may be an
oxide which has a low thermal conductivity, a relatively high
coefficient of thermal expansion as a ceramic, and excellent
high-temperature durability of a thermal barrier coating due to
high fracture toughness, such as YSZ (Yttria-stabilized zirconia).
A second oxide for the thermal barrier coating may be an oxide
which has low thermal conductivity and excellent phase stability to
a melting point, such as rare-earth zirconates. An example of the
rare-earth zirconates may include
La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06 (x is 0.24 to 1.65), but
are not limited thereto.
[0045] In the step (a), a suspension in which oxide particles are
dispersed is formed as a feedstock used for the suspension plasma
spraying. In the step (a), an oxide fine powder for the thermal
barrier coating is obtained through high-energy milling from one or
two or more oxide powders as a raw material for forming the thermal
barrier coating layer, and then the oxide fine powder is dispersed
in a solvent to prepare the first suspension and the second
suspension separately.
[0046] For example, when one oxide powder such as commercial YSZ
(Yttria-stabilized zirconia) is used as a raw material, the
suspension is formed through (a-1) mechanically pulverizing an
oxide powder for the thermal barrier coating; and (a-2) forming a
slurry containing the pulverized oxide powder.
[0047] A mechanical milling method capable of applying high energy
may be used in the step (a-1). Specific means for this purpose may
include planetary ball milling, attrition milling, or shaker
milling.
[0048] In the step (a-2), the oxide powder in a pulverized size is
homogeneously dispersed in a solvent such as ethanol through a
means such as ball milling.
[0049] In carrying out the step (a), when the oxide for the thermal
barrier coating such as rare-earth zirconates is prepared by using
two or more commercially available oxide powders as a raw material,
the suspension is formed through (a-1) mixing two or more oxide
powders to perform mechanical pulverization or mechanical alloying;
and (a-2) forming a slurry containing an oxide powder obtained in
the step (a-1).
[0050] In the step (a-1), the two or more oxides may include (i)
one or more selected from Y.sub.2O.sub.3, Gd.sub.2O.sub.3, and
La.sub.2O.sub.3, and (ii) one or more selected from ZrO.sub.2 and
CeO.sub.2. The two or more oxide powders are mixed and then
subjected to mechanical pulverization or mechanical alloying
through high energy milling such as planetary ball milling,
attrition milling, or shaker milling.
[0051] In the step (a-2), a composite oxide powder through the
previous step is homogeneously dispersed in a solvent such as
ethanol through a means such as ball milling.
[0052] In performing the step (a), after performing the
pulverization or mechanical alloying of the oxide particles in the
step (a-1) and before carrying out (a-2), calcining may be
additionally performed in an oxidizing atmosphere if necessary in
order to degrade impurities such as organic substances or to
synthesize a final oxide. Here, the calcination may be performed at
a temperature and for a time sufficient for a compound to be
synthesized while the impurities are completely removed.
[0053] Next, in the step (b), a first coating layer is formed on
the base material by the suspension plasma spraying (SPS) using the
first suspension.
[0054] Here, the suspension plasma spraying is a spraying method in
which liquid suspensions are fed directly into a plasma jet instead
of a powder material, in which the suspension injected into the
plasma jet is atomized in the plasma jet, and a coating layer is
formed through a series of processes such as evaporation of a
solvent by heating, dissolution of a material, and collision in a
base material.
[0055] Next, in the step (c), a buffer layer is formed on the first
coating layer by the suspension plasma spraying using a mixed
suspension of the first suspension and the second suspension. The
buffer layer may be formed such that a content ratio of the first
oxide and the second oxide in a thickness direction continuously
changes. More specifically, a buffer layer may be formed such that
a fraction of the first oxide continuously decreases while a
fraction of the second oxide continuously increases from a lower
end toward an upper end of the buffer layer.
[0056] Finally, in the step (d), a second coating layer is formed
on the buffer layer by the suspension plasma spraying using the
second suspension.
[0057] FIG. 1 is a conceptual diagram for a suspension supplier 1
provided in a suspension plasma spraying apparatus for implementing
a method for preparing a two-layer ceramic thermal barrier coating
layer in which a buffer layer according to an embodiment of the
present disclosure is interposed, in particular, a connected
suspension supplier suitable for forming an inclined functional
coating layer, such as a two-layer ceramic thermal barrier coating
layer in which a buffer layer according to an embodiment of the
present disclosure is interposed.
[0058] The suspension supplier 1 for the suspension plasma spraying
apparatus includes basically a plurality of storage tanks for
storing suspensions in which components constituting a raw material
of a material constituting a coating layer are dispersed; a
transfer pipe for connecting the storage tanks each other and for
moving the suspensions or supplying the suspensions to a plasma
spraying gun; and an opening/closing valve provided in the transfer
pipe for opening and closing a flow of the suspensions.
[0059] More specifically, the suspension supplier 1 for the
suspension plasma spraying apparatus may include a first suspension
storage tank 11 in which the first suspension is stored; a second
suspension storage tank 12 in which the second suspension is
stored; a first transfer pipe 21 for supplying the first suspension
from the first suspension storage tank to a plasma spraying gun; a
first opening/closing valve 22 provided in the first transfer pipe
for controlling a supply amount of the first suspension to the
plasma spraying gun; a second transfer pipe 31 for connecting
between the first suspension storage tank and the second suspension
storage tank and for transferring the second suspension to the
first suspension storage tank; a second opening/closing valve 32
provided in the second transfer pipe for controlling a flow rate of
the second suspension to the first suspension storage tank; a third
transfer pipe 41 for supplying the second suspension from the
second suspension storage tank to a plasma spraying gun; and a
third opening/closing valve 42 provided in a third transfer pipe
for controlling a supply amount of the second suspension to the
plasma spraying gun.
[0060] When the method for preparing a ceramic thermal barrier
coating layer according to an embodiment of the present disclosure
is performed using the suspension supplier 1 for the suspension
plasma spraying apparatus as described above, it is possible to
more easily realize an inclined functional thermal barrier coating
layer, i.e., a two-layer ceramic thermal barrier coating layer
having a buffer layer interposed therebetween, and particularly, it
is possible to easily obtain a buffer layer in which a content
fraction of each component continuously changes from the lowest
layer portion to the uppermost layer portion.
[0061] According to the method for preparing a two-layer ceramic
thermal barrier coating layer having a buffer layer interposed
therebetween, using suspension plasma spraying with a connected
suspension supplier suitable for forming an inclined functional
coating layer as described above, a two-layer ceramic thermal
barrier coating layer is formed by suspension plasma spraying with
a suspension supplier connected using two types of suspensions of
YSZ and rare-earth zirconates containing pulverized oxide particles
through high energy milling or alloyed composite oxide particles.
Here, the rare-earth zirconates are deposited on an upper layer
which is a high-temperature portion of a corresponding coating
layer, in which the rare-earth zirconates have a pyrochlore or
fluorite or a mixed phase of two phases with low thermal
conductivity and excellent phase stability at a high temperature,
and there is no m-ZrO.sub.2 causing deterioration of the thermal
barrier coating. Here, YSZ is deposited on a lower layer which is a
low-temperature portion of the corresponding coating layer, in
which the YSZ has a thermal expansion coefficient close to that of
a substrate and has excellent high-temperature mechanical
properties. Accordingly, it is possible to form a two-layer ceramic
thermal barrier coating layer which can be used at higher
temperatures as compared with conventional YSZ monolayer coatings,
and has improved adhesion and high-temperature durability over
rare-earth zirconates monolayer coatings.
[0062] Hereinafter, the present disclosure will be described in
detail by way of examples to illustrate it. However, the
embodiments according to the present disclosure may be modified in
various other forms, and the scope of the present specification is
not construed as being limited to the above-described embodiments.
The embodiments of the present disclosure are provided to more
fully describe the present disclosure to a person skilled in the
art.
Example 1 to 4
[0063] 1. Preparation of a Suspension for Suspension Plasma
Spraying
[0064] Commercial YSZ powder (7.5 wt % Y2O3-ZrO2, PRAXAIR,
ZR0271-5, USA, <125 .mu.m) was ball milled for 20 hours using
YSZ ball (.phi.1 mm) and IPA as a mixing medium. Then, it was
produced as a suspension by ball milling for 1 hour by dispersing
with YSZ ball and ethanol as a mixing medium at a ratio of 1:9
relative to the powder. FIG. 2B is the result of an XRD phase
analysis of the commercial YSZ powder used in the suspension of the
first suspension storage tank in Examples 1 to 4 of the present
disclosure. It was seen that the YSZ powder was composed of a
tetragonal prime phase and was pulverized into several .mu.m size
powders in the form of several tens .mu.m granules after ball
milling for 20 hours.
[0065] Further, the powder of
La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06 (x=0.24, 0.71, 1.18,
1.65) were synthesized by using raw material powders of
La.sub.2O.sub.3 (High purity chemicals, Japan, 99.99%, 10 .mu.m),
Gd.sub.2O.sub.3 (High purity chemicals, Japan, 99.99%, 2-3 .mu.m),
and ZrO.sub.2 (High purity chemicals, Japan, 98%, 5 .mu.m) as shown
in the composition of Table 1 and produced as a suspension.
TABLE-US-00001 TABLE 1 wt % Specimen Composition ZrO.sub.2
La.sub.2O.sub.3 Gd.sub.2O.sub.3 Example 1
La.sub.1.89Gd.sub.0.24Zr.sub.2.12O.sub.7.06 45.63 46.92 7.46
Example 2 La.sub.1.18Gd.sub.0.75Zr.sub.2.12O.sub.7.06 44.95 33.01
22.04 Example 3 La.sub.0.71Gd.sub.1.38Zr.sub.2.12O.sub.7.06 44.29
19.52 36.19 Example 4 La.sub.0.24Gd.sub.1.53Zr.sub.2.12O.sub.7.06
43.65 6.41 49.94
[0066] Each oxide composition was mixed with a zirconia ball, IPA
(Isopropyl alcohol), and 0.5 wt. % dispersant (Dibutyl phosphate,
Sigma-Aldrich, USA, 96%) for 24 hours by ball milling. The mixture
was heated with stirring using an agitator to evaporate a solvent
and then dried in a dryer at 80.degree. C. The dried powder was
calcined at 1550.degree. C. for 2 hours and then powder particles
were assembled and sieved using induction to prepare a
La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06 (x=0.24, 0.71, 1.18,
1.65) synthetic powder. The prepared
La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06 (x=0.24, 0.71, 1.18,
1.65) powder was dispersed in a ratio of 1:9 with respect to the
powder using YSZ ball and ethanol as a mixing medium, and it was
produced as a suspension by ball milling for 1 hour. FIG. 2A is the
result of an XRD phase analysis of
La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06 (x=0.24, 0.71, 1.18,
1.65) powder after the synthesis used in the suspension of the
second suspension storage tank in Examples 1 to 4 of the present
disclosure. It was seen that the synthesized
La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06=0.24, 0.71, 1.18, 1.65)
powder has a pyrochlore crystal phase clearly showing superlattice
peaks of (331) and (511), and also has a fluorite phase.
[0067] 2. Formation of a Thermal Barrier Coating Layer Using
Suspension Plasma Spraying
[0068] The prepared slurry was deposited using suspension plasma
spraying (Axial III plasma spraying system, Northwest Mettech
Corp., Canada) on a substrate that Amdry 386-2 (Sulzer metco,
Switzerland) Bond coat composition was deposited on a nickel-based
superalloy substrate to a thickness of about 200 .mu.m by
high-velocity oxy-fuel spraying (HVOF). Coating was carried out
with the following coating conditions: a mixing ratio of Ar,
H.sub.2, and N.sub.2 was controlled to 7.5:1.5:1, a distance
between a coated substrate and a plasma torch was 75 mm, a rotation
speed of the coated substrate was 1500 rpm, a suspension supply
rate was 45 mL/min, and coating pressurized voltage and current
were 150V and 220 A. FIG. 1 shows a schematic view of a slurry
supply apparatus for preparing a two-layer ceramic thermal spray
coating by a suspension plasma spraying technique. First, an amount
of slurry consumption over time for a slurry fed at a feeding rate
of 45 ml per minute was calculated. Then, a two-layer ceramic
thermal barrier coating was prepared in such a manner that a slurry
was additionally supplied to the first suspension storage tank from
the second suspension storage tank immediately before the
completion of slurry supply from the first suspension storage
tank.
[0069] In the case of a coating specimen of
La.sub.1.66Gd.sub.0.24Zr.sub.2.12O.sub.7.06/YSZ in Example 1,
first, a suspension of a YSZ composition in the first suspension
storage tank was supplied to plasma flame for deposition for 40
minutes. When the supply of the suspension of the YSZ composition
was carried out for 30 minutes, a slurry having a composition of
La.sub.1.66Gd.sub.0.24Zr.sub.2.12O.sub.7.06 was poured into the
first suspension storage tank from the second suspension storage
tank, and a YSZ+ La.sub.1.66Gd.sub.0.24Zr.sub.2.12O.sub.7.06 layer
was coated for 10 minutes. Also, coating was performed so that a
composition of La.sub.2Zr.sub.2O.sub.7 in the second suspension
storage tank was further supplied for 30 minutes, and deposition
was performed for 1 hour and 10 minutes in total.
Comparative Example
[0070] Formation of a Thermal Barrier Coating Layer Using
Atmospheric Plasma Spraying
[0071] Commercial YSZ powder (7.5 wt % Y.sub.2O.sub.3--ZrO.sub.2,
PRAXAIR, ZR0271-5, USA, <125 .mu.m) was deposited using
atmospheric plasma spraying (TripleX Pro, Oerikon Metco,
Switzerland) on a substrate that Amdry 386-2 (Sulzer metco,
Switzerland) Bond coat composition was deposited on a nickel-based
superalloy substrate to a thickness of about 200 .mu.m by
high-velocity oxy-fuel spraying (HVOF).
[0072] The coating was carried out with the following coating
conditions: a gun speed was controlled at 1000 mm/sec, a distance
between a coated substrate and a plasma torch was 125 mm, a powder
feed rate was 10 g/min, and coating pressure voltage and current
were 102 V and 500 A.
<Experimental Example 1> Crystal Structure Analysis and
Cross-Sectional Microstructure Observation for Specimens Prepared
in Examples and Comparative Example
[0073] FIG. 3 is the result of X-ray diffraction (XRD) analysis for
the specimen of the two-layer ceramic thermal barrier coating
prepared in Examples 1 to 4 and the specimen of the monolayer
thermal barrier coating prepared in Comparative Example.
[0074] FIGS. 4A and 4B are images of a scanning electron microscope
(SEM) showing a cross-sectional microstructure for the specimen of
the two-layer ceramic thermal barrier coating prepared in Examples
1 to 4 and the specimen of the monolayer thermal barrier coating
prepared in Comparative Example.
[0075] In accordance with FIG. 3, it could be seen that it consists
of a single phase of fluorite which appears from the upper layer,
i.e., the La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06 (x=0.24, 0.71,
1.18, 1.65) layer. In addition to a peak from the fluorite phase, a
peak of the t'-YSZ located in the lower was also weakly observed.
In accordance with the cross-sectional microstructure in FIG. 4A, a
ceramic coating layer was deposited to a thickness of approximately
480.about.550 .mu.m. A thickness ratio of the YSZ layer as the
lower layer and the La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06
layer as the upper layer was almost the same as 1:1, but the total
thickness was different. The
La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06 layer and the YSZ layer
each had a dense microstructure and showed a vertical separation
microstructure such as a slight vertical crack.
[0076] On the other hand, in accordance with FIGS. 3 and 4B, in the
case of the specimen of the monolayer coating layer having the
commercial YSZ composition and prepared using atmospheric plasma
spraying in Comparative Example, it could be seen that a coating
having a pore similar in size to a granule size composed of a
single phase of the t'-YSZ was formed. Microstructural observation
revealed a vertical separation microstructure such as a slight
vertical crack.
[0077] FIG. 5 is an image of a scanning electron microscope (SEM)
showing a cross-sectional microstructure for a specimen of a
two-layer ceramic thermal barrier coating prepared in Examples 1 to
4 and the result of EDS line scanning showing the distribution of
La, Gd, Zr, and Y elements. It could be seen that a La/Gd layer, a
Zr layer, and a Y layer are clearly distinguished by the difference
in the composition of the lower layer, i.e., the YSZ layer and the
upper layer, i.e., the La.sub.1.89-xGd.sub.xZr.sub.2.12O.sub.7.06
layer. From this, a thickness of the upper layer, an intermediate
layer, and the lower layer, which were not clearly distinguished
from the SEM cross-sectional microstructure, were more clearly
known. It was also seen that a thickness of the intermediate layer
was 150.about.200 .mu.m.
<Experimental Example 2> Measurement for Adhesion and Thermal
Durability on Specimens Prepared in Examples and Comparative
Example
TABLE-US-00002 [0078] TABLE 2 Specimen Adhesion (MPa) Example 1
>29.9 Example 2 26.8 Example 3 23.7 Example 4 >24.1
Comparative Example >28.1
[0079] FIG. 7 is the result of an isothermal deterioration test at
1275.degree. C. for the specimen of the two-layer ceramic thermal
barrier coating prepared in Examples 1 to 4 and the specimen of the
monolayer YSZ thermal barrier coating prepared in Comparative
Example, and it shows the number of cycles until a coating is
soundly attached to a substrate. It was seen that the specimen of
the two-layer ceramic thermal barrier coating prepared in Examples
1 to 4 has higher thermal durability compared to the specimen of
the monolayer YSZ thermal barrier coating prepared in Comparative
Example through the 1275.degree. C. isothermal degradation test.
Considering that a temperature range where the t'-YSZ stably exists
is from room temperature to 1200.degree. C., it is believed that
the two-layer ceramic thermal barrier coating having stable
rare-earth zirconates as an upper layer in experimental conditions
of a high-temperature of 1275.degree. C. provides more improved
high-temperature durability.
* * * * *