A metallic coating is plasma sprayed onto a smooth surface of a metal alloy substitute or on a bond coating. An initial thin ceramic layer is low pressure sprayed onto the smooth surface of the substrate or bond coating. Another ceramic layer is atmospheric plasma sprayed onto the initial ceramic layer

Brindley, W. J.

Miller, R. A.

Rouge, C. J.

English

NASA Technical Reports Server

NASA Case No.
PRINT FIG.
NOTICE
/ fI / • _,
LEW-15,164-I
The invention disclosed in this document resulted from research in aero-
nautical and space activities performed under programs of the National
Aeronautics and Space Administration. The invention is owned by NASA and is,
therefore, available for licensing in accordance with the NASA Patent
Licensing Regulation (14 Code of Federal Regulations 1245.2).
To encourage commercial utilization of NASA-owned inventions, it is NASA
policy to grant licenses to commercial concerns. Although NASA encourages
nonexclusive licensing to promote competition and achieve the widest possible
utilization, NASA will consider the granting of a limited exclusive license,
pursuant to the NASA Patent Licensing Regulations, when such a license will
provide the necessary incentive to the licensee to achieve early practical
application of the invention.
Address inquiries and all applications for license for this invention to
NASA Patent Counsel, Lewis Research Center, Mail Stop LE-LAW, 21000 Brookpark
Road, Cleveland, OH 44135. Approved NASA forms for application for
nonexclusive or exclusive license are available from the above address.
Serial No.: 699,130
Filed Date: 5/13/91
LeRC
https://ntrs.nasa.gov/search.jsp?R=19910015984 2020-03-19T18:02:15+00:00Z
NASACASENO. LEW-15,164-1
AWARDS ABSTRACT
CERAMIC COATINGS ON SMOOTH SURFACES
A metallic coating is plasma sprayed onto a smooth surface of a
metal alloy substrate, or on a bond coating. An initial thin ceramic
layer is low pressure plasma sprayed onto the smooth surface of the
substrate or bond coating. Another ceramic layer is atmospheric plasma
sprayed onto the initial ceramic layer.
Inventors: R. A. Miller
W. J. Brindley
C. J. Rouge
Employer: NASA Lewis Res. Ctr.
Evaluator: S. J. Grisaffe
Serial NO.: 699,130
Filing Date: 5/13/91
NASACASENO. LEIT-15,164-1 PATENT
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CERAMICOATINGSONSMOOTHSURFACES
Oriqin of the Invention
The invention described herein was made by employees of the United
States Government and may be manufactured and used by and for the
Government for governmental purposes without the payment of any royal-
ties thereon or therefor.
Technical Field
This invention is concerned with applying durable coatings onto
smooth surfaces. The invention is particularly concerned with plasma
sprayed ceramic coatings, such as zirconia-yttria based thermal barrier
coatings.
Components operating in the hot gas paths of advanced turbine
engines normally must be coated. The coating is conventionally a
metallic "overlay" which may be applied by any of a variety of processes
including pack aluminizing, electron beam-physical vapor deposition (EB-
PVD), and low pressure plasma spraying. Engine designers frequently
specify thermal barrier coating systems instead of overlay coatings.
These coating systems normally comprise an insulating ceramic material
applied over an oxidation resistant metallic bond coat. The ceramic
material is usually applied by atmospheric pressure plasma spraying
(APPS) or by EB-PVD. Plasma sprayed ceramic coatings require rough
metal/ceramic interfaces. The APPS ceramics are generally applied over
a rough low pressure plasma sprayed bond coat which typically has a
roughness of 300 micro-inch or greater. APPS bond coats may be
utilized for less demanding applications. The EB-PVD ceramics are
applied over smooth pack aluminide or EB-PVD bond coats. Alternately,
the smooth bond coats may be obtained by grinding low pressure plasma
sprayed coatings to a roughness of about 100 micro-inch.
While a variety of factors may influence bond coat selection, one
practical approach is to select the original overlay as the bond coat
layer for the thermal barrier system. This enables the proven technolo-
gy of the overlay to be retained while adding the insulation of the
ceramic material. This approach may favor the selection of EB-PVD
ceramics inasmuch as the EB-PVD ceramics can be applied to any type of
NASACASENO. LEi_-15,164-I
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bond coat as long as the bond coat is either smoothor can be ground to
be smooth as pointed out above. The disadvantage of the EB-PVDceramic
coatings is that they do not insulate as well as plasma sprayed ceramic
coatings.
Prior Art
Traditional APPS ceramic coatings are only adherent and durable if
applied over a rough-plasma sprayed metallic bond coat. As a result,
traditional APPS ceramic coatings cannot be used if a designer wishes to
retain a pack aluminide or a EB-PVD overlay coating.
It is, therefore, an object of the present invention to provide a
plasma sprayed ceramic coating on a smooth surface thereby removing the
traditional requirement for a rough metal-ceramic interface.
A further object of the invention is to improve coating durability
by eliminating the stress concentration at roughness asperities.
Another object of the invention is to provide a process for
preparing plasma sprayed ceramic coatings which are well bonded to
smooth bond coats or overlays.
Backqround Art
U.S. patent Nos. 4,382,976, 4,576,874 and 4,861,618 disclose
processes of plasma spraying MCrAIY and ceramics onto base materials.
These coatings improve the thermal or corrosive properties of the end
product.
U.S. patent No. 4,623,555 is directed to the general concept of
plasma spraying various layers on top of one another onto a base
material. U.S. patent No. 4,666,733 is concerned with the broad concept
of low pressure plasma spraying (LPPS) for depositing MCrAIY/Cr3C 2
compositions on a metallic surface.
Disclosure of the Invention
The process of the present invention comprises applying an initial
thin layer of low pressure plasma sprayed ceramic to a smooth bond coat
followed by a layer of conventional, low thermal conductivity atmospher-
ic pressure plasma sprayed ceramic for good insulation. The ceramic
coating is preferably a zirconia-yttria ceramic. It is contemplated
that the yttria component may be replaced by another oxide such as
NASA CASE NO. LE_r-15,164-1
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ytterbium, ceria, magnesia or calcia. Also the zirconia component may
be replaced by hafnia. Other oxides such as alumina, titania, chromia,
calcium silicate or combinations of these materials may replace the
zirconia-or yttria-based oxides.
The smooth bond coat may be of the MCrAIX family of alloys where M
is Ni,Co Fe or combinations thereof and X may be an active element such
as yttrium or ytterbium. Other bond coats such as NiAl, NiCr, pack
aluminide coatings or no bond coat, such as with direct application of
the ceramic to a metallic substrate, may also be employed.
Brief Description of the Drawinq
The foregoing, as well as other objects, features, and advantages
of the invention will become more apparent from the following detailed
description when taken in conjunction with the drawings in which;
FIG. 1 is a graph illustrating increases in the life of smooth
interface thermal barrier coatings produced in accordance with the
present invention, and
FIG. 2 is a bar graph showing the test lives for specimens treated
according to the schedules shown in TABLE I.
Best Mode for Carryinq Out the Invention
Specimens of 0.5 inch thick solid nickel-based superalloy sub-
strates commercially available as Waspaloy were provided with a substan-
tially smooth surface having a roughness less than about 120 microinch
Ra. A NiCrAIYb bond coat was low pressure plasma sprayed onto this
smooth surface to a thickness of approximately 6 mils. Metallographic
analysis of these specimens revealed that the coatings were less than
optimum quality because of excessive porosity.
Conventional thermal barrier coating systems were prepared by
atmospheric pressure plasma spraying 10 mils of Zr02-8% Y203 onto four of
the previously low pressure plasma sprayed specimens. When cycled
between room temperature and 1150 ° C in a burner rig, the test lives of
the four specimens were 306, 331, 397, and 433 cycles, respectively with
a mean value of 367 cycles. While this is about one-third of the
anticipated life, the specimens were judged to be of sufficient quality
for use for exploratory smooth bond coat development.
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NASACASENO. LEW-15,164-1
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Nine of the bond coated specimens were centerless ground to an
average roughness of about 110 micro-inch. The remaining bond coat
thickness was now only 3 mils. Eight of these were low pressure plasma
spray coated with between 0.9 and 1.9 mil of Zr02-8% Y203 followed by
about 8 mil of conventional atmospheric pressure plasma sprayed Zr02-8%
Y203. The last of the ground specimens was coated with 10 mil of the
conventional APPS Zr02-8% Y202 ceramic only.
The arc power levels employed for the low pressure plasma spray
were 54, 63, 72, or 81KW at two levels of preheating resulting from
four or 10 preheat passes. The reverse transfer arc feature of the low
pressure plasma spray unit was used in each case. The roughness of the
low pressure plasma sprayed ceramic, measured on the specimen prepared
at 54 KW after 10 preheat passes, was 230 micro-inch.
The test lives defined by the number of cycles to spalling of the
ceramic specimens were 110, 58, 47, and 43 cycles in order of increasing
arc power for the case of 10 preheat cycles. These are shown by the
line 12 in FIG. I. The thickness of the low pressure plasma sprayed
ceramic layer was 1.7, 1.3, 1.9, and 1.7 mil, respectively for the four
data points shown in FIG. I. For the case of two preheat cycles three
of the specimens failed in the range of 60-106 cycles. However, the 72
KW specimen failed in only four cycles.
These tests illustrated that the life of a low pressure plasma
sprayed-APPS ceramic coating on a smooth bond coat of 110 micro-inch was
up to 10 times greater than the life of a conventional APPS ceramic on a
smooth bond coat. The life of the present invention was over 30% of the
life of a conventional rough interface thermal barrier coating system.
Greater lives were achieved with another set of Waspaloy specimens
which were processed according to the schedule given in table I. All of
these were first coated with about 8 mils of NiCrAIYb bond coat and
centerless ground to a surface roughness of about 18 microinch and a
thickness of about 5 mils. The first ten of these were processed
together as a batch with the first eight of these comprising a standard
24-I fractional factorial design plan. The four variables investigated
in this design were bond coat surface roughness, bond coat pre-oxida-
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tion, specimen preheat temperature (i.e. the temperature of the specimen
immediately before application of the APPSceramic layer), and post-
coating heat treatment.
Roughnesswas achieved by grit blasting four of the specimens to a
roughness of about 46 microinch and another four to about 72 microinch.
The pre-oxidation consisted of heating the bond coated specimen in air
for 70 minutes at 1150% (vs. no oxidation treatment), specimen preheat
temperatures were 150% or 370°C, and the post-coating heat treatment
consisted of 4 hours at I080°C in a vacuumfurnace (vs. no heat treat-
ment).
Each of the first eight specimens had a 0.60 to 0.85 mil layer of
low pressure plasma sprayed zirconia-yttria (sprayed at 54 Kw) and a
layer of air plasma sprayed ceramic to bring the total ceramic layer
thickness to about 10 mils. Specimensnine and ten were deposited on
the as-ground (18 microinch) bond coats using the higher level of both
preheat and heat treatment. Oneof these was preoxidized and the other
was not. As shown in FIG. 2 the I050°C cyclic burner rig test lives of
these 10 specimens varied from 287 to 1086 cycles compared to a standard
thermal barrier coating which survived 1376 cycles. Oneof the longest
lived specimenswas an as-ground specimenwhich survived 1009 cycles.
Statistical analysis of this data indicated that longer test lives were
associated with the 150°CAPPSpreheat and two unintentional
covariates -- namely, the thickness of the low pressure plasma sprayed
ceramic layer and the presence of the gray oxide that is characteristic
of alumina formation.
Specimens11 through 16 were then processed to further test the
effect of pre-oxidation and low pressure plasma sprayed ceramic layer
thickness. As shown in the table, all of the specimens were deposited
on the as-ground bond coats and their lives, in the samerig as above
but with modified cooling air, varied from 231 to 430 cycles. This
comparedwell with the 664 cycles observed for a standard thermal
barrier coating. Statistical analysis supported the observation that
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NASA CASE NO. LEW-1b,164-1
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pre-oxidation had no effect on life and that there was a general upward
trend in life with increasing low pressure plasma sprayed layer thick-
ness for the 5 specimens exhibiting the gray oxide although the effect
was less than expected and may indicate a diminishing effect after about
0.9 mil. The analysis also indicated that the life of the sixth
specimen was diminished by the presence of the green oxide determined by
x-ray analysis to consist primarily of nickel oxide. This was also in
agreement with the previous result.
While a preferred embodiment of the invention is shown and de-
scribed it will be appreciated that various procedural modifications can
be made without departing from the spirit of the invention or the scope
of the subjoined claims. By way of example, it is contemplated that a
smooth uncoated surface of an oxidation resistant heat resistant alloy
may be protected in a hot environment by a durable coating applied in
accordance with the present invention. The surface is substantially
smooth having a roughness less than about 120 microinch Ra. The first
layer of ceramic material is low pressure plasma sprayed onto this
smooth surface. This first layer is then covered by a second layer of
ceramic material.
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NASA CASE NO. LEW-15,]64-1
ABSTRACT OF THE DISCLOSURE
CERAMIC COATINGS ON SMOOTH SURFACES
A metallic coating is plasma sprayed onto a smooth surface of a
metal alloy substrate, or on a bond coating. An initial thin ceramic
layer is low pressure plasma sprayed onto the smooth surface of the
substrate or bond coating. Another ceramic layer is atmospheric plasma
sprayed onto the initial ceramic layer.
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POWER (KW)
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Fig. 1
NASA CASE NO. LEW-15,164-1
STANDARD
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BURNER RIG LIFE (CYCLES)
Fig. 2


Ceramic coatings on smooth surfaces

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