U.S. patent application number 14/052104 was filed with the patent office on 2014-02-06 for protective coating with high adhesion and articles made therewith.
This patent application is currently assigned to General Electric Company. The applicant listed for this patent is General Electric Company. Invention is credited to Luc Stephane Leblanc, Larry Steven Rosenzweig, James Anthony Ruud, Scott Sheridan Smith, Paul Matthew Thomas.
Application Number | 20140037903 14/052104 |
Document ID | / |
Family ID | 41267092 |
Filed Date | 2014-02-06 |
United States Patent
Application |
20140037903 |
Kind Code |
A1 |
Rosenzweig; Larry Steven ;
et al. |
February 6, 2014 |
PROTECTIVE COATING WITH HIGH ADHESION AND ARTICLES MADE
THEREWITH
Abstract
Coatings and articles suitable for use in high temperature
environments, for example, are presented. One embodiment is a
coating that comprises a plurality of elongate material growth
domains defined between domain boundaries. The domains have an
intra-domain density of at least about 75% of theoretical density,
have a substantially equiaxed grain morphology, and comprise a
plurality of at least partially melted and solidified particles.
Another embodiment is a coating that comprises a matrix comprising
a substantially equiaxed grain morphology and a plurality of
vertically oriented cracks disposed in the matrix. Further
embodiments include articles comprising one or more of the coatings
described above.
Inventors: |
Rosenzweig; Larry Steven;
(Clifton Park, NY) ; Ruud; James Anthony; (Delmar,
NY) ; Leblanc; Luc Stephane; (Schenectady, NY)
; Thomas; Paul Matthew; (Queensbury, NY) ; Smith;
Scott Sheridan; (Scotia, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
General Electric Company |
Schenectady |
NY |
US |
|
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
41267092 |
Appl. No.: |
14/052104 |
Filed: |
October 11, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12115819 |
May 6, 2008 |
8586172 |
|
|
14052104 |
|
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|
Current U.S.
Class: |
428/155 |
Current CPC
Class: |
Y10T 428/26 20150115;
Y02T 50/6765 20180501; Y10T 428/24479 20150115; C23C 30/00
20130101; Y02T 50/60 20130101; Y10T 428/24893 20150115; F01D 25/005
20130101; Y02T 50/67 20130101; C23C 4/11 20160101; Y10T 428/249986
20150401; Y10T 428/24802 20150115; Y10T 428/24926 20150115; Y10T
428/24471 20150115; Y10T 428/31 20150115 |
Class at
Publication: |
428/155 |
International
Class: |
F01D 25/00 20060101
F01D025/00 |
Claims
1. An article for use at elevated temperatures, comprising: a
substrate; and a coating disposed on the substrate, wherein the
coating comprises a matrix comprising a substantially equiaxed
grain morphology, and a plurality of vertically oriented cracks
disposed in the matrix.
2. The article of claim 1, wherein the coating has a density of at
least about 90%.
3. The article of claim 1, wherein the cracks are present at a
level of greater than about 250 cracks per linear inch.
4. The article of claim 1, wherein the coating comprises a ceramic
material.
5. The article of claim 1, wherein the coating comprises an
oxide.
6. The article of claim 1, wherein the coating comprises a material
selected from the group consisting of stabilized zirconia,
zirconates, and stabilized oxides.
7. The article of claim 1, further comprising a bondcoat disposed
between the substrate and the coating.
8. The article of claim 7, wherein the bondcoat has a surface
roughness of less than about 150 microinches R.sub.a.
9. The article of claim 7, wherein the adhesion strength is greater
than about 28 megapascals.
10. The article of claim 1, wherein the coating has a density of at
least about 95%.
11. The article of claim 1, wherein the article comprises a
component of a gas turbine assembly.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/115,819, filed May 6, 2008, which is herein
incorporated by reference.
BACKGROUND
[0002] This invention relates to coatings 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 can be increased by operating the turbine at higher
temperatures. In order to assure a satisfactory life span at these
higher temperatures, thermal barrier coatings (hereinafter referred
to as "TBCs") are applied to airfoils and combustion components of
the turbine, such as transition pieces and combustion liners, using
various techniques.
[0004] One important aspect of TBC's is their ability to tolerate
strain in the underlying component without becoming detached from
the component. Because TBC's are typically made of ceramic
materials having much lower inherent ductility than their
underlying metallic components, various microstructural features
are typically incorporated into the TBC to provide it with improved
strain tolerance. For instance, TBC's deposited by plasma spray
processes typically incorporate significant porosity, vertical
microcracks, or combinations thereof as a means to enhance the
ability of the TBC to tolerate strain. TBC's 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 very attractive
strain tolerance properties, they tend to be relatively expensive
and applicable to relatively small components when compared with
plasma spray processes, because PVD processes require a vacuum
chamber and supporting equipment. On the other hand, traditional
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 bond coats, to provide adequate
adhesion to the underlying component.
[0006] Therefore, there is a need for coatings with 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
[0007] Embodiments of the present invention are provided to meet
this and other needs. One embodiment is a coating that comprises a
plurality of elongate material growth domains defined between
domain boundaries. The domains have an intra-domain density of at
least about 75% of theoretical density, have a substantially
equiaxed grain morphology, and comprise a plurality of at least
partially melted and solidified particles.
[0008] Another embodiment is a coating that comprises a matrix
comprising a substantially equiaxed grain morphology; and a
plurality of vertically oriented cracks disposed in the matrix.
[0009] Further embodiments include articles comprising one or more
of the coatings described above.
DRAWINGS
[0010] These and other features, aspects, and advantages of the
present 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:
[0011] FIGS. 1-3 are schematic cross-sections of exemplary
embodiments of the present invention.
DETAILED DESCRIPTION
[0012] Embodiments of the present invention include a coating
having a unique microstructure that provides adhesion and strain
tolerance approaching that of PVD-deposited coatings, but that is
deposited by comparatively inexpensive air plasma spray
techniques.
[0013] In one embodiment, as depicted schematically in FIG. 1, the
coating 100 comprises a plurality of elongate material growth
domains 102. As used herein, the term "elongate" refers to a
structure having an aspect ratio of greater than 1. The growth
domains 102 are generally vertically oriented, meaning that their
longest axes are substantially normal to a coating interface 104.
In accordance with embodiments of the present invention, as
material is deposited to form coating 100, the material accumulates
in domains 102 defined as regions of comparatively high density
separated by domain boundaries 106 of comparatively low (though not
necessarily zero) density. Boundaries 106 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 coating 100 is enhanced by the presence of longer, well-defined
boundaries 106.
[0014] The density of material contained within domains 102, also
referred to herein as "intra-domain density," is at least about 75%
of 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 coating
100.
[0015] The presence of domain boundaries 106 may provide desirable
compliance and strain tolerance properties for coating 100. A
coating with narrower domains has a greater density of domain
boundaries. The width of the domain can be in the range from about
20 micrometers to about 100 micrometers, in some embodiments; in
certain embodiments this width is in the range from about 30
micrometers to about 90 micrometers, and in particular embodiments
the width is from about 40 micrometers to about 80 micrometers.
Domain width as used herein is measured based on the average number
of boundaries respectively intercepted by lines of known length
drawn along a cross section of the coating at 33% of the coating
thickness and at 67% of the coating thickness; mean domain width is
simply the known length divided by the number of intercepted
boundaries. In certain embodiments, at least about 50% by volume of
the coating contains domains 102; thus it is not necessary that the
entire coating 100 comprise the structure described herein. In some
embodiments, however, the advantages provided by this unique
structure are such that substantially all of the coating 100
comprises the described structure.
[0016] Without being bound by any particular theory, certain
characteristics of domains 102 indicate that the deposition
mechanism for coating 100 is substantially by accumulation of
pre-condensed matter at a growth surface; in this instance
"pre-condensed matter" refers to solid and/or liquid matter that
impinges upon a growth surface, rather than matter that condenses
at the growth surface from the vapor phase. For example, domains
102 comprise a plurality of at least partially melted and
solidified particles (meaning the particles were at least partially
liquid before impinging the surface and finally fully solidifying),
which are often (though not always) discernable as "prior
particles" in the microstructure of coating 100 via microscopy
techniques. In certain embodiments, at least 50% of the material
present in domains 102 comprises at least partially melted and
solidified particles; in particular embodiments this amount is at
least about 80%, and in more particular embodiments substantially
all of the material in domains 102 is made of at least partially
melted and solidified particles. Moreover, in some embodiments,
domains 102 generally lack substantial crystallographic texture, in
stark contrast to coatings deposited via a vapor deposition
mechanism. Instead, domains 102 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).
[0017] Conventional thermal barrier coatings include the so-called
"dense vertically cracked" coatings, which are characterized in
part by a lamellar microstructure that includes elongate grains
readily indicative of directional solidification to those skilled
in the art. See, for example, U.S. Pat. No. 6,306,517. The elongate
grains within the lamellae tend to have a preferred orientation
perpendicular to the lamella boundaries, and they typically have
aspect ratios greater than about 4:1 and often as high as 10:1. In
contrast to dense vertically cracked coatings, coating 100 of the
present invention is generally characterized by randomly oriented,
substantially equiaxed grains contained within domains 102, and by
the absence of distinct lamellar features. In this context,
"substantially equiaxed" means the population of grains in coating
100 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. Note that this use of the
term "orientation" referring to the placement of a grain in space
should not be confused with crystallographic orientation, or
"texture" of a material.
[0018] In certain embodiments, coating 100 is a thermal barrier
coating. In some embodiments, coating 100 comprises a ceramic
material, such as an oxide. Particular examples of thermal barrier
coatings include stabilized zirconia, such as yttria-estabilized
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.
[0019] In another embodiment, referring to FIG. 2, the coating 200
more closely resembles a dense vertically cracked coating, in that
it comprises a plurality of vertically oriented cracks 202 to
provide strain tolerance. However, unlike conventional dense
vertically cracked coatings, coating 200 comprises a matrix in
which cracks 202 are disposed, where the matrix comprises a
substantially equiaxed (as this term is defined above in the
discussion of coating 100) grain morphology. In some embodiments
the grains in the matrix are randomly oriented, as that term has
been used above. This lack of preferred grain orientation, as noted
previously, further distinguishes coating 200 from conventionally
fabricated dense vertically cracked coatings. Cracks 202 are
typically present in the matrix at a level greater than about 250
cracks per inch (about 100 cracks per centimeter); in some
embodiments, this level is greater than about 300 cracks per inch
(about 120 cracks per centimeter). The number and length of cracks
202 generally affect the strain tolerance of the coating. In some
embodiments, the cracks 202 extend at least about 50% of the
coating thickness, and in certain embodiments the crack length is
at least about 75% of the coating thickness. Crack length in this
context is defined as the median length measured for the crack
population of coating 200. Coating 200 is generally characterized
by a relatively high "intracrack" density (that is, the density of
the coating exclusive of cracks), which in some embodiments is at
least about 90% of theoretical density, and in particular
embodiments is at least about 95% of theoretical density. Materials
suitable for use in coating 100 are also suitable for coating
200.
[0020] Further embodiments of the present invention include
articles comprising either of the coatings 100, 200 described
above. Referring to FIG. 3, article 300 comprises a substrate 302
and a coating 304 disposed on substrate 302. Coating 304 comprises
coating 100 or coating 200 as described above. Substrate 302 may be
any suitable support material, but in particular embodiments
comprises a high temperature alloy, such as a nickel-based or
cobalt-based superalloy.
[0021] In some embodiments, article 300 further comprises a
bondcoat 306 disposed between coating 304 and substrate 302.
Bondcoats, such as MCrAlY or aluminide coatings, are commonly used
in thermal barrier coating systems to provide oxidation resistance
to the substrate and to enhance adhesion of ceramic topcoats. In
fact, air plasma sprayed bondcoats as commonly used in the art are
often deposited with intentionally rough surfaces to enhance
mechanical interlocking with subsequently deposited ceramic
topcoats. In stark contrast to these conventional coating systems,
coating 300 of the present invention may exhibit very high adhesion
strength, even to comparatively smooth surfaces. In some
embodiments, bondcoat 306 has a surface roughness on a deposition
surface 308 of less than about 150 microinches R.sub.a. In some
embodiments, this roughness is even lower, such as less than 100
microinches R.sub.a and, in certain embodiments, less than about 75
microinches R.sub.a. Even at such reduced bondcoat roughness
values, the adhesion strength of coating 300 is unexpectedly high.
In some embodiments, this adhesion strength is greater than about 7
megapascals (MPa), and in certain embodiments this strength is
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.
[0022] The advantages described above for coating 100, 200 make
them suitable for use in elevated temperature applications.
Accordingly, in some embodiments, article 300 comprises a component
of a gas turbine assembly, including, for instance, a gas turbine
assembly for power generation or for propulsion of a ship,
aircraft, or other craft. Exemplary components include turbine
blades, stator vanes, and combustion components.
[0023] 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
particular, 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 0.4
micrometers to about 2 micrometers.
[0024] Those skilled in the art will appreciate that many different
processing parameters are available for adjustment in a given air
plasma spray process, and that various combinations of these
parameters may result in coatings with different structures and
properties.
[0025] Kassner et al. (J. Thermal Spray Technol. v 17, 115-123
(2008)) have produced coatings from suspensions of sub-micron
yttria stabilized zirconia (YSZ) powders using plasma spray guns
rated at powers between about 25-60 kW. Using 0.3 micrometer
particles, they have observed lamellar coating microstructures with
anisotropic grains. Using 25 nm particles, they have produced
porous vertically cracked coatings with crack densities up to 180
cracks per inch (7 cracks/mm) They report high vertical crack
counts with lower density than traditional thermal spray processes
for thermal conductivity improvements. This process is distinct
from embodiments of the present invention, in which high density,
highly cracked coatings with equiaxed grain morphologies are
described using significantly higher power thermal spray processes
(see the examples, below).
[0026] Kassner, et al. also describe using 25 nm YSZ particles and
a 60 kW-rated plasma spray gun to produce highly porous columnar
microstructures with up to 70% open porosity. They describe the
potential application of such structures for catalytic processes
utilizing high specific surface area. In stark contrast, the
coatings of the present invention include high-density elongate
growth domains with the required mechanical properties for durable
thermal barrier coatings.
[0027] Gell, et al., in Surf. Coating Technol. v 177-178, 97
(2004), and in U.S. Patent Application Publication 2004/0229031,
have produced stabilized zirconia coatings from a solution
precursor with a 35-45 kW plasma gun. Splat-like microstructures
with columnar grains are observed in the coatings, which have
15-40% porosity and vertical crack densities up to about 250
cracks/in (average crack spacing of 100 micrometers). Those
coatings are distinct from embodiments of the present invention,
which include coatings with highly dense, equiaxed grain
structures.
[0028] The following examples are presented to further describe the
fabrication of coatings of the present invention, but should not be
read as limiting, because variations still within the scope of
embodiments of the present invention will be apparent to those
skilled in the art.
EXAMPLE 1
[0029] An yttria-stabilized-zirconia (YSZ) coating was produced on
an Alloy 718 plate using a DC plasma gun. The feedstock material
was 8 wt % YSZ powder with a median particle diameter (d.sub.50) of
0.4 micrometers suspended in ethanol at 10 percent by weight, using
polyethyleneimine as a dispersant (at 0.2 percent by weight of the
solids). The suspension was injected at a feed rate of about 0.25
pounds (about 113 grams) of YSZ per hour into a Northwest Mettech
Axial III torch through the center tube of a tube-in-tube atomizing
injector with a nitrogen atomizing gas flowing through the outer
tube. The total torch power was about 120 kW, with a current of
about 200 amperes maintained for each of the three torch
electrodes, and a total gas flow of 245 liters per minute that
consisted of a mixture of 30% argon, 55% nitrogen, and 15%
hydrogen. The plasma torch was rastered across the substrate at 600
millimeters (mm) per second while maintaining a constant spray
distance of 76 mm distance between the torch nozzle and substrate.
A coating thickness of approximately 165 micrometers was
obtained.
[0030] The resultant coating comprised the structure illustrated in
FIG. 1, with a plurality of elongate material growth domains. The
intra-domain density and average domain width were measured from
cross-sectional images of polished surfaces. The area fraction of
solid phases was determined at a magnification of 400.times. from
the contrast in light microscopy to be about 95.5%. The average
domain width was determined from the average linear intercept to be
about 41 micrometers.
[0031] The grain morphology was evaluated from scanning electron
microscopy images of coating fracture surfaces. The aspect ratios
were measured for a random sampling of the grains. The median
aspect ratio was 1.7:1, indicating a substantially equiaxed grain
morphology.
EXAMPLE 2
[0032] YSZ coatings were produced on substrates of Rene N5 with a
platinum nickel aluminide diffusion bondcoat. The bondcoat surface
was prepared by grit blasting with 80 grit size aluminum oxide at
40 psi pressure. The average Ra roughness of the substrate surface
was about 50 microinches. Coatings were sprayed with the same
coating parameters as for Example 1. The coatings were about 228
micrometers thick. The tensile adhesion strength was measured using
the ASTM C633 specification. The average adhesion strength was 55.6
+/-11.6 MPa.
EXAMPLE 3
[0033] A YSZ coating was produced on a 304 stainless steel plate
grit blasted with 60 grit aluminum oxide at 50 psi pressure. The
coating was produced using the same parameters as described in
Example 1, except that the torch to substrate spray distance was 50
mm instead of 76 mm (which was used for examples 1 and 2).
[0034] The resultant coating comprised the structure illustrated in
FIG. 2, with a matrix comprising a randomly oriented, substantially
equiaxed grain morphology; and a plurality of vertically oriented
cracks disposed in the matrix. The intracrack density and vertical
crack density were measured from cross-sectional images of polished
surfaces. The area fraction of solid phases was determined at a
magnification of 400.times. from the contrast in light microscopy
to be about 99%. The linear density of vertical cracks that
extended at least 50% through the thickness was 560 cracks per inch
and the density that extended at least 75% through the thickness
was 340 cracks per inch.
[0035] The grain morphology was evaluated from scanning electron
microscopy (SEM) images of coating fracture surfaces. The aspect
ratios were measured for a random sampling of the grains using an
image analysis technique. The median aspect ratio was 2:1,
indicating a substantially equiaxed grain morphology. There was a
lack of distinct lamellar features and very few individual lamellar
regions were evident.
[0036] 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 true spirit of the
invention.
* * * * *