U.S. patent application number 15/384712 was filed with the patent office on 2017-04-13 for thermal barrier coating systems and methods of making and using the same.
The applicant listed for this patent is General Electric Company. Invention is credited to Larry Steven Rosenzweig, James Anthony Ruud, Shankar Sivaramakrishnan.
Application Number | 20170101875 15/384712 |
Document ID | / |
Family ID | 48949279 |
Filed Date | 2017-04-13 |
United States Patent
Application |
20170101875 |
Kind Code |
A1 |
Rosenzweig; Larry Steven ;
et al. |
April 13, 2017 |
THERMAL BARRIER COATING SYSTEMS AND METHODS OF MAKING AND USING THE
SAME
Abstract
A method for coating a surface of a substrate is provided. The
method includes providing a suspension or a precursor comprising
feedstock material suspended in a liquid medium. Further, the
method includes spraying the suspension or the precursor onto the
surface at a spray angle less than about 75 degrees to a tangent of
the surface.
Inventors: |
Rosenzweig; Larry Steven;
(Clifton Park, NY) ; Ruud; James Anthony; (Delmar,
NY) ; Sivaramakrishnan; Shankar; (Schenectady,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
General Electric Company |
Schenectady |
NY |
US |
|
|
Family ID: |
48949279 |
Appl. No.: |
15/384712 |
Filed: |
December 20, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13600273 |
Aug 31, 2012 |
9556505 |
|
|
15384712 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F05D 2240/35 20130101;
C23C 4/18 20130101; C23C 4/134 20160101; C23C 4/00 20130101; F01D
5/288 20130101; F05D 2230/312 20130101; F05D 2300/502 20130101;
Y02T 50/60 20130101; F05D 2300/21 20130101; F05D 2220/32 20130101;
C23C 4/12 20130101; Y10T 428/24355 20150115; Y10T 428/31 20150115;
C23C 4/10 20130101; Y02T 50/6765 20180501; C23C 4/02 20130101; F01D
9/02 20130101 |
International
Class: |
F01D 5/28 20060101
F01D005/28; F01D 9/02 20060101 F01D009/02; C23C 4/02 20060101
C23C004/02; C23C 4/134 20060101 C23C004/134; C23C 4/10 20060101
C23C004/10 |
Claims
1. A method for coating a surface of a substrate, comprising:
providing a suspension comprising feedstock material suspended in a
liquid medium; and spraying the suspension onto the surface at a
spray angle less than about 75 degrees to a tangent of the
surface.
2. The method of claim 1, wherein the feedstock material comprises
fine particles suspended in the liquid medium or a precursor
material of a thermal barrier coating.
3. The method of claim 2, wherein the fine particles have a median
diameter in a range from about 0.1 micrometers to about 10
micrometers.
4. The method of claim 2, wherein an amount of the fine particles
suspended in the liquid medium is in a range from about 5 weight
percent to about 40 weight percent.
5. The method of claim 2, wherein the fine particles comprise an
oxide.
6. The method of claim 2, wherein the fine particles comprise an
oxide stabilized with a stabilizing agent, stabilized zirconia,
yttria-stabilized zirconia, a zirconate, a hafnate, or a
cerate.
7. The method of claim 1, wherein providing the suspension
comprises: forming first and second intermediate suspensions,
wherein the feedstock material in each intermediate suspension
comprises fine particles having a median diameter that is different
from that of the other intermediate suspension; and mixing together
the two intermediate suspensions to obtain the suspension.
8. The method of claim 1, wherein providing and spraying the
suspension comprises: feeding the suspension to a plasma spray gun;
and injecting the suspension into a plasma plume of the plasma
spray gun for deposition onto the surface of the substrate.
9. The method of claim 1, wherein the substrate comprises a
component of a gas turbine assembly.
10. The method of claim 1, wherein spraying is performed onto the
surface at a spray angle of about 20 degrees to about 60
degrees.
11. The method of claim 1, wherein spraying is performed onto the
surface at a spray angle of about 45 degrees to about 75
degrees.
12. The method of claim 1, further comprising depositing a bondcoat
on the surface of the substrate using vapor phase aluminizing, or
vapor phase deposition.
13. The method of claim 12, comprising roughening a surface of the
bondcoat.
14. A method for providing a thermal barrier on a substrate,
comprising: depositing a bondcoat on a surface of the substrate;
providing a suspension comprising feedstock material suspended in a
liquid medium; and plasma-spraying the suspension onto the surface
at a spray angle in a range from about 20 degrees to about 75
degrees to a tangent of the surface.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This patent application is a divisional of U.S.
Non-Provisional patent application Ser. No. 13/600,273 filed on
Aug. 31, 2012, which is incorporated by reference herein in its
entirety.
BACKGROUND
[0002] The invention relates to thermal barrier coatings and
thermal barrier coating systems for high temperature applications,
such as gas turbine assemblies.
[0003] The design of modern gas turbines is driven by the demand
for higher turbine efficiency. It is widely recognized that turbine
efficiency may be increased by operating the turbine at higher
temperatures. Typically, various techniques are used to apply
bondcoats and thermal barrier coatings to airfoils and combustion
engine components of the turbine, such as transition pieces and
combustion liners, to assure a satisfactory life span at these
higher temperatures.
[0004] Usually, the thermal barrier coatings are configured to
tolerate strain in the underlying component without detaching from
the component. The thermal barrier coatings are usually made of
ceramic materials, which have relatively lower inherent ductility
than their underlying metallic components; hence, various
microstructural features are typically incorporated into the
thermal barrier coating to provide the thermal barrier coating with
improved strain tolerance. For instance, the thermal barrier
coatings deposited by plasma spray processes typically incorporate
significant porosity, vertical microcracks, or both, as a means to
enhance the ability of the thermal barrier coating to tolerate
strain. By way of example, thermal barrier coatings deposited by
vapor processes, such as physical vapor deposition (PVD), typically
are fabricated under conditions that encourage nucleation and
growth of discrete, tightly packed, columnar grains, which provides
a compliant microstructure with a relatively high degree of strain
tolerance.
[0005] Although PVD processes provide coatings with suitable strain
tolerance on relatively small components as compared with plasma
spray processes. However, compared to the plasma spray processes
the PVD processes require expensive set-up including a vacuum
chamber and supporting equipment. On the other hand, conventional
thermal spray processes tend to produce coatings with lower strain
tolerance and substrate adhesion than PVD processes, and generally
require ancillary surface preparation processes, such as grit
blasting and deposition of rough bondcoats, to provide adequate
adhesion to the underlying component.
[0006] The bondcoats are typically used to promote adhesion of the
thermal barrier coating layer to the underlying component and
inhibit oxidation of the underlying component during high
temperature exposure of the component. Typically, bondcoats having
aluminide coatings are used in thermal barrier coating systems to
provide oxidation resistance to the substrate and to enhance
adhesion of the thermal barrier coatings. In order to have adequate
adhesion, plasma sprayed thermal barrier coatings are typically
deposited on bondcoats with rough surfaces such as overlay MCrAlY
bondcoats. Relatively smoother bondcoats, such as those formed by
vapor phase aluminide (VPA), often are not considered suitable
candidates for depositing thermal barrier coatings deposited by
plasma spray methods.
[0007] Therefore, there is a need for thermal barrier coatings that
exhibit high strain tolerance, high adhesion, and reduced need for
surface preparation processes that can be applied via comparatively
inexpensive and scalable processes such as plasma spray
processes.
BRIEF DESCRIPTION
[0008] In one embodiment, a coating comprising a first surface and
a second surface is provided. The coating includes a plurality of
growth domains, wherein an orientation of at least one growth
domain of the plurality of growth domains is non-vertical with
respect to the first surface of the coating. One or more growth
domains of the plurality of growth domains comprise a plurality of
at least partially melted and solidified particles.
[0009] In another embodiment, a thermal barrier coating system may
be provided. The system includes a substrate comprising a first
surface and a second surface, a bondcoat disposed on at least a
portion of the first surface of the substrate, and a coating
disposed on at least a portion of the bondcoat, wherein the coating
comprises a plurality of growth domains, wherein an orientation of
at least one growth domain of the plurality of growth domains is
non-vertical with respect to an interface between the bondcoat and
the coating. One or more growth domains of the plurality of growth
domains comprise a plurality of at least partially melted and
solidified particles.
[0010] In yet another embodiment, a method for coating a surface is
provided. The method includes providing a suspension comprising
feedstock material suspended in a liquid medium, and spraying the
surface at a spray angle less than about 75 degrees to a tangent of
the surface.
DRAWINGS
[0011] These and other features, aspects, and advantages of the
invention will become better understood when the following detailed
description is read with reference to the accompanying drawings in
which like characters represent like parts throughout the drawings,
wherein:
[0012] FIG. 1 is a cross-sectional view of an example thermal
barrier coating system having a component comprising a planar
surface, in accordance with embodiments of the present
technique;
[0013] FIG. 2 is a cross-sectional view of an example thermal
barrier coating system having a component comprising a non-planar
surface, in accordance with embodiments of the present
technique;
[0014] FIGS. 3-5 are micrographs of portions of thermal barrier
coating systems having the thermal barrier coating deposited at
three different spray angles, in accordance with embodiments of the
present technique;
[0015] FIG. 6 is a flow chart for a method of making a thermal
barrier coating system, in accordance with embodiments of the
present technique; and
[0016] FIGS. 7-10 are micrographs of coatings produced using a
feedstock having a suspension consisting of YSZ powder with a d50
of about 1 micron.
DETAILED DESCRIPTION
[0017] Embodiments disclosed herein generally relate to thermal
barrier coating systems. In certain embodiments, the thermal
barrier coating systems may include a thermal barrier coating
disposed on a bondcoat. In these embodiments, the thermal barrier
coatings may have a microstructure that is configured to enhance
adhesion and strain tolerance. In one embodiment, the values of the
enhanced adhesion and strain tolerance of the thermal barrier
coatings may approach that of coatings deposited using expensive
plasma vapor deposition methods, such as, but not limited to,
electron beam plasma vapor deposition methods. In certain
embodiments, the thermal barrier coatings may be deposited using
comparatively inexpensive suspension or precursor plasma spray
techniques, or a combination thereof.
[0018] As will be appreciated, in gas turbine applications higher
efficiencies of the system may be achieved by using higher
operating temperatures. However, as operating temperatures
increase, it is desirable to have enhanced high temperature
durability of the components of the engine.
[0019] Further, to be effective, it is desirable that a thermal
barrier coating in a thermal barrier coating system is configured
to exhibit low thermal conductivity, strong adherence to the
substrate (e.g., an engine component), and continued adherence
throughout many heating and cooling cycles. However, differences in
the coefficients of thermal expansion between materials of the
thermal barrier coating system pose additional challenges. For
example, materials of the thermal barrier coating may have a
substantially low coefficient of thermal expansion compared to the
underlying metallic bondcoat and substrate (e.g., a superalloy
substrate), thereby providing the risk of delamination of the
thermal barrier coating during the thermal cycles.
[0020] In some embodiments, the thermal barrier coatings facilitate
high temperature durability of the components of the engine while
protecting the components from erosion, hot corrosion, etc. In some
of these embodiments, the thermal barrier coatings further serve to
reduce heat transfer to the underlying substrate, such as but not
limited to, an engine component.
[0021] In addition to the thermal barrier coating, the turbine
engine components often employ a bondcoat to protect against high
temperature oxidation. In certain embodiments, the bondcoats may
comprise a diffusion bondcoat. In certain other embodiments, the
bondcoats may comprise an overlay bondcoat. In some embodiments,
the aluminum concentrations within diffusion bondcoats or overlay
bondcoats may be in a range from about 5 to about 50 percent by
weight.
[0022] In some embodiments, a diffusion bondcoat may include an
aluminum based intermetallic compound, such as but not limited to,
a nickel aluminide. Non-limiting examples of the diffusion bondcoat
may include platinum nickel aluminide or simple nickel aluminide
applied by vapor or pack diffusion methods. In one embodiment, the
aluminide based bondcoat may be disposed on a substrate using a
diffusion based process. Non-limiting examples of the diffusion
based processes may include pack cementation, vapor phase
aluminiding (VPA), or chemical vapor deposition (CVD). In some
embodiments, the diffusion process may result in a bondcoat that
includes two distinct zones; an outer zone which contains an
oxidation resistant phase (such as beta-NiAl), and a diffusion zone
which includes the oxidation resistant phase and secondary phases
(such as gamma prime, gamma, carbides and sigma). In one example,
the aluminide based bondcoat may be deposited using vapor phase
aluminizing (VPA). In one embodiment, the diffusion bondcoats may
be modified with platinum or platinum group metals. In this
embodiment, the phase of aluminide may include
.gamma.-Ni+.gamma.'-Ni3Al alloy compositions. In some embodiments,
the diffusion aluminide may be applied using commercially available
aluminide processes. In some of these embodiments, aluminum may be
reacted at the substrate surface to form an aluminum intermetallic
compound which provides a reservoir for the formation of an alumina
oxidation resistant interlayer. The aluminide bondcoat layer may
include aluminum intermetallic phases (e.g. NiAl, CoAl, and
(Ni/Co)Al phases) formed by reacting aluminum vapor or
aluminum-rich alloy powder with the substrate elements in the outer
surface layers of the superalloy component. The bondcoat layer is
typically well bonded to the substrate. Aluminizing may be
accomplished by techniques, such as but not limited to, pack
cementation process, spraying, chemical vapor deposition,
electrophoresis, sputtering, and slurry sintering with an aluminum
rich vapor and appropriate diffusion heat treatments.
[0023] In some embodiments, overlay bondcoats may be used to
provide high temperature oxidation and corrosion protection to the
underlying substrate, such as the turbine components. The overlay
bondcoats may include MCrAlY type, where M may represent nickel,
cobalt, iron, or combinations thereof. Additions of various
quantities of other elements such as titanium, zirconium, hafnium,
silicon, tantalum, tungsten, niobium, rhenium, or combinations
thereof, may be used to enhance the performance of the bondcoat.
Advantageously, the overlay bondcoats may not be significantly
influenced by the composition of the underlying substrate. In one
example, the overlay bondcoats may be applied by a number of
different deposition methods such as, but not limited to, thermal
spray, sputtering, electron beam physical vapor deposition (EBPVD),
cathodic arc deposition, electro-deposition, or combinations
thereof.
[0024] Typically, air plasma sprayed bondcoats are often deposited
with intentionally rough surfaces to enhance mechanical
interlocking with subsequently deposited thermal barrier coating.
In stark contrast to these conventional coating systems, the
thermal barrier coatings disclosed herein may exhibit relatively
high adhesion strength, even to comparatively smooth surfaces, such
as a surface of a VPA bondcoat.
[0025] In some embodiments, the bondcoats are relatively smooth,
with a surface roughness of between about 10-60 micro-inches Ra.
Typically, such smooth bondcoats are not suitable candidates for
less expensive coating techniques, such as spray based coating
techniques. Vertical cracks produced within the thermal barrier
coating layer are not desirable in such coatings as they reduce the
adhesion strength of the coatings. In certain embodiments, coatings
disclosed herein may be disposed on the bondcoats using spray
methods. It has been unexpectedly discovered that the coatings
disclosed herein exhibit enhanced adhesion and thermal cycle
performance when deposited on the aluminide based bondcoats or the
overlay bondcoats.
[0026] Typically, thermal barrier coatings having vertical cracks
are desirable on bondcoats with smooth surfaces, such as aluminide
based bondcoats, as they would allow the coatings to be more
compliant. For example, the vertical cracks may allow the coating
to flex without delaminating. However, it has been demonstrated
that coatings without cracks have better adhesion to the aluminide
based bondcoats and overlay bondcoats. Similarly, coatings without
cracks also have longer thermal cycling lives in case of the
aluminide based bondcoats and overlay bondcoats. For example,
coatings with vertical cracks applied to relatively smooth
aluminide based bondcoats, such as but not limited to VPA bondcoat,
exhibit delamination at the interface between the thermal barrier
coating and the bondcoat. Advantageously, in the absence of such
vertical cracks, interfacial delamination may not be present when
the microstructure of the thermal barrier coating does not contain
vertical cracks. In certain embodiments, the thermal barrier
coating may have non-vertical growth domains, and the delamination
may be minimal or absent in these thermal barrier coatings.
[0027] Advantageously, the substantially vertical crack free and
compliant microstructures in the thermal barrier coating facilitate
enhanced adhesion between the thermal barrier coating and the
underlying bondcoat. Further, the thermal cycling performance of
the thermal barrier coating system may be substantially improved
over more dense and vertically cracked thermal barrier
coatings.
[0028] In some embodiments, the bondcoat may have a surface
roughness of less than about 150 micro-inches Ra. In one
embodiment, the surface roughness of the bondcoat may be less than
or equal to about 100 micro-inches Ra. Even at such reduced
bondcoat roughness values, the adhesion strength of the thermal
barrier coating described herein is unexpectedly high. In some
embodiments, this adhesion strength of the thermal barrier coating
to the aluminide based bondcoat may be greater than about 7
megapascals (MPa). In one example, the adhesion strength of the
thermal barrier coating to the aluminide based bondcoat may be
greater than about 28 MPa. Adhesion strengths as referred to herein
refer to values measured in accordance with the procedure set forth
in ASTM Standard C633.
[0029] In certain embodiments, the thermal barrier coatings may be
configured to tolerate strain in the underlying component without
being detached from the component. The thermal barrier coatings may
be made of ceramic materials, which may have relatively lower
inherent ductility than the underlying metallic components; hence,
in certain embodiments, various microstructural features may be
incorporated into the thermal barrier coating to provide the
thermal barrier coating with improved strain tolerance and
adherence. As described in detail below with regard to FIGS. 1-5,
in some embodiments, the thermal barrier coatings may comprise a
plurality of growth domains in the thermal barrier coating. In
certain embodiments, as the material is deposited to form the
thermal barrier coating, the material accumulates in domains
defined as regions of comparatively high density separated by
domain boundaries of comparatively low (though not necessarily
zero) density. In accordance with embodiments of the present
invention, one or more domains may comprise a plurality of at least
partially melted and solidified particles. Without being bound by
any particular theory, it is believed that domain boundaries are
formed between the growth domains as the particles accumulate
within the growth domains and that the growth domains may have
boundary surfaces comprising partially melted and solidified
particles. Domain boundaries are to be contrasted with vertical
cracks. Vertical cracks are separations that occur in a coating
when a crack grows through previously integral and continuous
material. Crack surfaces comprise the faces of grain boundaries or
grain interiors that have been exposed through crack growth. The
orientation of at least one growth domain of the plurality of
growth domains may be non-vertical with respect to a first surface
of the coating, where the first surface of the coatings is disposed
on the bondcoat.
[0030] FIG. 1 illustrates an example thermal barrier coating system
10. The system 10 comprises a component 12 having a first surface
14 and a second surface 15. A bondcoat 18 may be disposed on the
first surface 14 of the component 12. The bondcoat 18 is disposed
on at least a portion of the first surface 14. The system 10
further comprises a thermal barrier coating 16 having a first
surface 17 and a second surface 19. The first surface 17 of the
thermal barrier coating 16 is disposed on the bondcoat 18 and
defines an interface 24 with the bondcoat 18.
[0031] The first surface 14 of the component 12 may be curved or
planar, or combinations thereof. The thermal barrier coating 16 and
the bondcoat 18 may be conformally disposed on the first surface 14
of the component 12.
[0032] The thermal barrier coating system 10 may be used in
elevated temperature applications. In one example, the thermal
barrier coating system 10 may be employed in a gas turbine
assembly, including, for instance, a gas turbine assembly for power
generation or for propulsion of a ship, aircraft, or other craft.
Non-limiting examples of the component 12 may include turbine
blades, stator vanes, and combustion components. In some
embodiments, the thermal barrier coating 16 comprises a ceramic
material, such as an oxide. Particular examples of the thermal
barrier coating 16 may include stabilized zirconia, such as
yttria-stabilized zirconia; zirconates; and other oxides, such as
hafnates and cerates, and including oxides that may be stabilized
with yttria or other stabilizing agents, such as ceria.
[0033] In certain embodiments, the thermal barrier coating 16 may
be generally characterized by a plurality of growth domains,
generally represented by reference numeral 20. One or more growth
domains of the plurality of growth domains comprise a plurality of
at least partially melted and solidified particles. In some
embodiments, an orientation of at least one growth domain may be
non-vertical with respect to the first surface 17, or an interface
24 between the first surface 17 and the bondcoat 18. As used
herein, the term "non-vertical" refers to an alignment angle 22
formed within the cross-sectional plane, where the angle 22 is
defined as an angle that is 90 degrees minus the angular
displacement between (a) a normal 21 to a tangent 23 of the first
surface 17 of the coating 16, and (b) a tangent 25 to the domain
boundary 26. In one embodiment, the angle 22 may be in a range from
about 30 degrees to about 75 degrees to the interface 24 between
the thermal barrier coating 16 and the bondcoat 18. That is, a
longest axis of a growth domain may be oriented at an angle 22 in a
range from about 20 degrees to about 75 degrees to the first
surface 17 of the coating 16. In one embodiment, all the growth
domains 20 may be oriented at similar angles with respect to each
other. In this embodiment, the angles of the growth domains 20 may
be within 5 degrees of each other. In one embodiment, the growth
domains 20 may comprise an aspect ratio of greater than about
1.
[0034] In some embodiments, the growth domains 20 may contain
randomly oriented, substantially equiaxed grains. As used herein,
the term "substantially equiaxed" means the population of grains in
the coating 16 has a median aspect ratio of less than about 3:1.
Moreover, "randomly oriented" refers to the general lack of a
preferred orientation such that long axes of grains (if such a long
axis is present) are not as a whole oriented with respect to a
spray direction or solidification direction. Further, in some
embodiments, the thermal barrier coating 16 may be generally
characterized by the absence of distinct lamellar features. Note
that this use of the term "orientation" referring to the placement
of a grain in space should not be confused with crystallographic
orientation, also called in the art "texture," of a material.
[0035] In certain embodiments, during deposition of the coating 16,
as the material is deposited to form the coating 16, the material
accumulates in the domains 20. In some embodiments, the domains 20
may have comparatively high density of the coating material. The
growth domains 20 may be defined by domain boundaries 26. The
growth boundaries 26 may have comparatively low (though not
necessarily zero) density than the growth domains 20.
[0036] The density of material contained within the growth domains
20, also referred to herein as "intra-domain density," may be at
least about 75% of the theoretical density. In some embodiments,
this density is even higher, such as greater than 85% and, in
certain embodiments, greater than 95%. A high intra-domain density
may provide desirable resistance to erosion and may enhance
cohesive strength of the coating 16.
[0037] In certain embodiments, at least 50% of the material present
in domains 20 comprises at least partially melted and solidified
particles; in particular embodiments this amount may be at least
about 80%, and in more particular embodiments substantially all of
the material in the domains 20 may be made of at least partially
melted and solidified particles. Moreover, in some embodiments,
domains 20 may generally lack substantial crystallographic texture,
in stark contrast to coatings deposited via a vapor deposition
mechanism. Instead, domains 20 typically have a substantially
isotropic crystallographic orientation. In this context, a
"substantially isotropic crystallographic orientation" means that
the material in question has a texture coefficient in the range
from about 0.75 to about 1.25, as that coefficient is defined in D.
S. Rickerby, A. M. Jones and B. A. Bellamy, Surface and Coatings
Technology, 37, 111-137 (1989).
[0038] In one embodiment, one or more growth domains may extend
through a thickness 25 of the thermal barrier coating 16. In these
embodiments, the growth domains 20 may extend from the first
surface 17 to the second surface 19 of the thermal barrier coating
16. In another embodiment, one or more growth domains may extend
through only a portion of the thickness 25 of the thermal barrier
coating 16. In some embodiments, the growth domains extend at least
about 50% of the coating thickness, and in certain embodiments the
growth domains length is at least about 75% of the coating
thickness. As used herein, the "length" of the growth domains in
this context may be defined as the median length measured for at
least a representative sample of the population of the growth
domains 20 in the coating 16. The extent to which a growth domain
may extend through the thickness 25 of the thermal barrier coating
16 may depend on factors such as but not limited to, orientation of
the growth domains, material of the coating, spray angle during
deposition of the coating 16, or combinations thereof. In one
embodiment, it may be desirable to have one or more growth domains
that extend through the thickness of the thermal barrier coating
16. The domain boundaries 26 may, in some embodiments, extend for a
length equal to at least about 50% of the coating thickness, and
this length may be at least about 75% of the thickness in certain
embodiments. Generally, the strain tolerance of the coating 16 may
be enhanced by the presence of longer, well-defined boundaries
26.
[0039] The presence of the growth domains 20 and the domain
boundaries 26 may provide desirable compliance and strain tolerance
properties for the coating 16. In one embodiment, the coating 16
with narrower domains 20 has a greater density of domain
boundaries. In some embodiments, a median width 28 of the domain 20
may be in the range from about 20 micrometers to about 100
micrometers. In some other embodiments, the width 28 may be in a
range from about 30 micrometers to about 90 micrometers. In one
embodiment, the width 28 may be in a range from about 40
micrometers to about 80 micrometers. In certain embodiments, the
width 28 of the growth domains 20 may be measured based on the
average number of boundaries respectively intercepted by lines of
known length drawn along a cross section of the coating 16 in a
direction parallel to the spray direction at 33% of the coating
thickness 25 and at 67% of the coating thickness 25. In these
embodiments, the mean domain width may be calculated as a length
divided by the number of intercepted boundaries. In certain
embodiments, at least about 50% by volume of the coating contains
domains 20; thus it is not necessary that the entire coating 16
comprise the structure described herein.
[0040] The advantages described herein may enhance the suitability
of thermal barrier coating system 10 for use in elevated
temperature applications. In some embodiments, unlike conventional
dense coatings deposited on smooth surfaces, the coatings 16 may
have minimal or no segmentation cracks within the TBC, a condition
referred to herein as "substantially crack-free." Segmentation
cracks, also known in the industry as vertical cracks, are
typically more prevalent within dense coatings. These types of
cracks may extend from the outermost surface through the entire
thickness or partially through the coating thickness. Such cracks
are distinguishable from domain boundaries in that the space within
a crack is bound by a fracture surface, and is essentially void of
coating particles along its length. In contrast, the space within a
domain boundary contains at least some deposited material, such as
coating particles, along its length. In certain embodiments, the
substantially crack free and compliant coatings 16 may provide
significantly improved adhesion to the bondcoat 18. Further, the
substantially crack free and compliant coatings 16 may provide
coatings with significantly enhanced thermal cycle performance,
including improved reliability, adherence, and longer life. In
certain embodiments, the coatings 16 may not comprise any
delamination cracks.
[0041] Typically, in plasma spray deposition, for surfaces with
curvatures, it is difficult to continuously maintain a desirable
spray angle for the surface to be coated. For example, it may be
difficult for a robotic arm to keep up with a change in a curvature
of the surface. Conventional coatings may exhibit lower adhesion
and mechanical strength when the incident angle of the plasma is
not perpendicular to the surface to be coated. In certain
embodiments, methods and coatings are provided that exhibit
enhanced adhesion at spray angles in a determined range. Further,
the methods are cost effective relative to the existing PVD
techniques.
[0042] FIG. 2 is a cross-sectional view of a thermal barrier
coating system 30 having a non-planar surface 31. In the
illustrated embodiment, the system 30 comprises a non-planar
substrate 32 having concave and convex portions 34 and 36,
respectively, viewing perpendicular to a direction of a spray gun,
generally referred to by arrow 33. The system 30 further comprises
a bondcoat 38 and a thermal barrier coating 40. The bondcoat 38 and
the coating 40 may be conformal to the underlying surface. The
coating 40 may comprise a plurality of growth domains 42 having
domain boundaries 43.
[0043] In one example, the portions 34 and 36 may be such that the
spray angle 44 may be in a range from about 20 degrees to about 75
degrees. As used herein, the term "spray angle," is an angle
measured in a clockwise direction at a point of incidence 37, the
angle is measured between a plane tangent to the surface 31 at the
point of incidence 37 and a line 39 connecting the angle of
incidence and the position of the spray gun, where the position of
the spray gun is represented by a point 33, and the plane tangent
to the surface 31 is represented by a line 35.
[0044] FIGS. 3-5 illustrate thermal barrier coatings deposited at
different spray angles relative to the surface on which the coating
is deposited. In certain embodiments, different spray angles may
provide a plurality of growth domains that are oriented at
different non-vertical angles with respect to the interface between
the first surface of the coating and the bondcoat.
[0045] FIG. 3 illustrates a thermal barrier coating system 60
having a coating 62 disposed on a bondcoat 64, where the bondcoat
in turn is disposed on a substrate (not shown). The coating 62
includes a plurality of non-vertical growth domains 68, which are
oriented at an angle of about 70 degrees with respect to an
interface 70 between the coating 62 and the bondcoat 64. In the
illustrated embodiment, the growth domains 68 are substantially
parallel to each other. The growth domains 68 may extend through
the entire thickness of the coating 62. Alternatively, the growth
domains 68 may be disposed in only a portion of the thickness of
the coating 62.
[0046] FIG. 4 illustrates a thermal barrier coating system 70
having a coating 72 deposited on a bondcoat 74 disposed on a
substrate (not shown). The coating 72 includes a plurality of
non-vertical growth domains 78, which are oriented at an angle of
about 60 degrees with respect to an interface 80 between the
coating 72 and the bondcoat 74. The growth domains 78 are
substantially parallel to each other. The growth domains 78 run
through a portion of the thickness of the coating 72.
[0047] FIG. 5 illustrates a thermal barrier coating system 90
having a coating 92 deposited on a bondcoat 94 disposed on a
substrate (not shown). The coating 92 includes a plurality of
non-vertical growth domains 98, which are oriented at an angle of
about 45 degrees with respect to an interface 100 between the
coating 92 and the bondcoat 94. The growth domains 98 are
substantially parallel to each other. As illustrated, the growth
domains 98 run through the entire thickness 102 of the coating
92.
[0048] In the illustrated embodiments of FIGS. 3-5, the angle of
spray gun and orientation of the growth domains may be same or
different. For example, referring back to FIG. 3, the gun angle may
be about 75 degrees to the surface of the substrate, and the
orientation of the growth domains 68 may be about 70 degrees.
Similarly, referring back to FIG. 4, the gun angle is about 60
degrees, and the orientation of the growth domains 78 is about 52
degrees. Referring back to FIG. 5, the gun angle is about 45
degrees, and the orientation of the growth domains 98 is about 40
degrees.
[0049] In certain embodiments, the thermal barrier coatings may be
comparatively more reliable, adherent, compliant, and may have
longer life relative to conventional coatings.
[0050] As a cost effective alternative to electron beam plasma
vapor deposition (EBPVD) application of thermal barrier coatings to
turbine engine components, a suspension plasma spray method may be
used to deposit the thermal barrier coating system disclosed
herein. The suspension plasma spray method utilizes fine particles
of the intended coating material, or precursor thereof, dispersed
into suspension in a liquid medium and passed through a plasma
spray torch to deposit the material on the surface of the
component. In certain embodiments, the thermal barrier coatings may
be applied on a surface of a diffusion bondcoat or an overlay
bondcoat.
[0051] Coatings of the present invention owe their remarkable
structures and properties at least in part to the processing used
in their fabrication. The process involves air plasma spraying,
which, as discussed above, provides certain economic and
manufacturing advantages over processes that require the use of
vacuum equipment, such as PVD or vacuum plasma spray deposition. In
certain embodiments, the process uses a feedstock comprising fine
particles suspended in a liquid agent that is fed to a plasma spray
torch in a controlled manner and injected into the plasma plume for
deposition onto a substrate. The particles have a median diameter
typically, but not necessarily, in the range from about 0.1
micrometers to about 10 micrometers, or 0.2 micrometers to 10
micrometers.
[0052] FIG. 6 is an example flow chart of a method for depositing
the coatings on an aluminide based bondcoat. At block 110, a
substrate may be provided. The substrate may be an engine
component. The substrate may be a metallic substrate, which may
have planar or non-planar surfaces. In one embodiment, the
substrate may comprise a diffusion bondcoat on one or more
surfaces. In another embodiment, the substrate may comprise an
overlay bondcoat on the one or more surfaces. In one example, the
diffusion bondcoat may be deposited on the surface of the substrate
by employing vapor phase aluminizing.
[0053] Optionally, at block 111, a bondcoat may be deposited using
known techniques, such as but not limited to, vapor phase
deposition. In one example, the aluminide based bondcoat may be
disposed on at least a portion of the substrate.
[0054] Optionally, at block 112, the surface of the bondcoat may be
processed to facilitate adhesion of the thermal barrier coating to
the bondcoat. Non-limiting examples of the processing include
roughening prior to depositing the coating. In one example, the
surface of the bondcoat may be grit blasted prior to depositing the
coating.
[0055] At block 114, a feedstock comprising fine particles of
material or precursor material of the thermal barrier coating may
be suspended in a liquid medium. In one embodiment, the particles
may have a median diameter in the range from about 0.4 micrometers
to about 2 micrometers. In one example, solids content of powders
in suspensions may be in a range from about 5 weight percent to
about 40 weight percent.
[0056] At block 114, the suspension may be fed to a plasma spray
gun in a controlled manner and injected into the plasma plume for
deposition onto a substrate.
[0057] At block 116, the spray gun may scan the surface of the
substrate to deposit the coating. The surface of the substrate may
be scanned in a regular or irregular pattern. In one example, the
plasma spray torch may raster the surface of the substrate.
[0058] The plasma spray torch or gun, the substrate, or both may be
oriented relative to each other such that the plasma spray is
incident on the first surface of the substrate at an angle in a
range from about 20 degrees to about 75 degrees. In one embodiment,
the angle at which the plasma is incident on the substrate may be
varied during the deposition. In another example, the angle at
which the plasma is incident on the substrate may not be varied
during the deposition.
[0059] In one example, where the surface of the substrate has a
changing curve, it may not be easy to continuously adjust the spray
angle as the surface is being scanned. In this example, the angle
at which the plasma is incident on the substrate may vary during
the deposition as the spray gun moves from one position on the
substrate to another.
Example 1
[0060] Yttria-stabilized-zirconia (YSZ) coatings were produced on
25 mm diameter, 3 mm thick button substrates of Rene N5 with vapor
phase diffusion nickel aluminide bondcoat. The YSZ coatings were
deposited on the bondcoat surface using a Northwest Mettech Axial
III DC plasma torch. Prior to YSZ deposition, the surface of the
VPA bondcoat was roughened by abrasive blasting using a 220 mesh
white aluminum oxide media at 60 psi air pressure.
[0061] The feedstock material was YSZ powder (ZrO.sub.2-- (7-8)
weight % Y.sub.2O.sub.3) with a median particle diameter (d.sub.50)
of about 0.75 .mu.m suspended in ethanol at 20 wt % using
polyethyleneimine as a dispersant (at 0.2 wt % of the solids). Two
different suspensions were made using different YSZ powders,
obtained from UCM Ceramics in Laufenburg, Germany. The two
different suspensions were mixed together to obtain the final
particle size distribution. Each powder had a unimodal particle
size distribution with a different mean size. The d.sub.50 of one
of the powders as provided by the manufacturer was approximately
0.6 microns, and the d.sub.50 of other powder was approximately 1.0
micron.
[0062] The suspension was injected into a Northwest Mettech Axial
III torch through the center tube of a tube-in-tube atomizing
injector with a nitrogen atomizing gas sent through the outer tube.
A 3/8'' diameter nozzle was used at the end of the plasma torch.
The torch power was about 105 kW. The suspension feed-rate was
about 25 grams/minute or about 0.6 pounds per hour of YSZ. The
plasma torch was rastered across the substrate at 600 mm/sec while
maintaining a constant spray distance of 100 mm distance between
the torch nozzle and substrate. Coatings were produced at different
spray angles of the plasma torch relative to the sample surfaces.
Angles of 90 degrees, 75 degrees, 60 degrees, and 45 degrees were
utilized to evaluate adhesion and thermal cycling performance.
Coating thicknesses of about 150 to about 220 microns were
obtained. Plasma conditions used for the YSZ deposition were 350
slpm total gas flow with 15% nitrogen, 10% hydrogen, and 75% argon.
A current of 200 A was used for each of the three electrodes,
resulting in a total gun power of approximately 105 kW. Nitrogen
carrier gas of 6 slpm was also used.
[0063] In certain embodiments, unexpectedly, the number of thermal
cycles to cause 20% spallation showed a general increase as the
coating angle was reduced from 90 degrees to 45 degrees. The
average number of thermal cycles to cause 20% spallation for a
coating deposited at a spray angle of 45 degrees was 60% greater
than that of a coating deposited at a spray angle of 90
degrees.
Example 2
[0064] Coatings illustrated in FIGS. 7-10 were produced the same
way as described above, except the feedstock for these samples was
a suspension consisting of the YSZ powder with a d50 of about 1
micron.
[0065] FIG. 7 illustrates a coating 142 deposited at a spray angle
of 90 degrees, FIG. 8 illustrates a coating 144 deposited at a
spray angle of 75 degrees, FIG. 9 illustrates a coating 146
deposited at a spray angle of 60 degrees, and FIG. 10 illustrates a
coating 148 deposited at a spray angle of 45 degrees. With coarser
particles in the feedstock, it was found that the spray angles that
produced adherent and compliant coatings were less than coatings
produced from finer particle containing feedstocks, suggesting an
unexpected interaction between particle size and spray angle.
[0066] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the scope of the
invention.
* * * * *