Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to a carbon bearing MCrAlY
coating and to a method of producing the same. More particu-
larly, the invention concerns a wear resisting oxidation
protective coating for a superalloy and to a method of
providing a coating having a wear, corrosion and oxidation
resistance in a gas turbine environment, on a superalloy
substrate. This application is related to Canadian Application
No. 313,565 filed on October 17, 1978. The present invention
also relates to protective coatings and coated components and,
more particularly, to coatings having high temperature oxida-
tion, corrosion, and wear resistance for application to
superalloy parts.
In modern gas turbine engines, certain engine compo-
nents, such as superalloy turbine blades, must be provided with
both oxidation and wear resistance at very high temperatures.
These properties are especially important with respect to the
Z-notch (a "Z" shaped area, when planar viewed, serving to
interlock adjacent blade shrouds) on a turbine blade tip
shroud which rubs against the Z-notches of adjacent turbine
20 blades and is subject to severe wear and oxidation.
In the past, the Z-notch has been protected by
various materials including puddle welded nickel or cobalt
alloy hardface coatings, typical of which is a cobalt base
alloy of nominal composition, by weight, 28% Cr, 5% Ni, 19.5%
W, 1% V, balance cobalt. Although capable of providing pro-
tection to the Z-notch area of the blade tip shroud during
engine operation, such hardface coatings are expensive to
apply by the puddle weld process, can cause base metal cracking,
and, in some cases, service li~e has been less than satisfac-
tory~ Other more economical techniques for applying the alloyhardface coatings, such as conventional plasma spraying, are
J `r
~ " ~ ~2~o~
unsatisfactory due to inadequate adhesi of the coating during X
service. Another type of material which has been used as a
heat, wear, and corrosion resistant coating is that in which
hard particles are embedded in a softer matrix of which, tungs-
ten carbide in a cobalt matrix is a familiar example fox lower
temperatures up to 1000F. Aecording to Wasserman et al, U,SO
Patent 3,023,130, refractory carbide particles are included in
heat resisting iron base welding alloys~ Chromiurn carbide
particles have often been preferred, usually in amounts up to
90 percent by weight. For example, in Pelton et al, U.S.
Patent 3,150,938, 325 mesh and finer sieve size particles have
been included in a nickel chromium (80%-20%) alloy, in Hyde
et al, U.S. Patent 3,556,747, particles have been included in
a molybdenum matrix with minor amounts of niekel chromium,
and, in Fischer, U.S. Patent ~,230,097, they have been included
in a chromium and lower melting point nickel brazing alloy.
The afore-mentioned coatings are applied by various methods,
including welding, but flame or plasma spraying is most
prevalent. There are two characteristics of the alloys and
coatings which are notable. First the matrices do not have
sufficient oxidation-corrosion resistance for gas turbine Z-
notch applications. Second, chromium carbide particles, per
se, are included in the coating in its use condition. That is,
the exact chromium carbide particles in the applied mixture
-~ are the particles intended to be in the adhered coating alloy.
The function of the matrix alloy is simply to be the binder.
Therefore, the chromium carbide particle and metal rnatrix
coatings heretofore ~nown are susceptible to failure due to
undercutting and pullout of the particles due to wear, erosion,
corrosion, or oxidation of the matrix. Consequently, the
performance of composite coatings containing particles, is
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,~ .
limited by the matrix. Therefore there is need for a far
improved coating which tends to be rnore homogeneous or mono-
lithic and have better performance.
It is well known that the family of protective
coatings generally referred to as MCr~lY coatings, where M
is selected from nickel, cobalt and iron and their mixtures,
can provide superior oxidation-corrosion resistance in the
high temperature engine environment compared to other types
of coatings and the matrix materials of the aforementioned
carbide containing coatings. For example, see U.S. Patents
to Evans et al, 3,676,085, Goward et al, 3,754,903, Hecht et
al, 3,928,026 and Talboom, Jr. et al, 3,542,530, all of
common assignee herewith. However, in the past, these MCrAlY
coating alloys have been applied to the airfoil and root
portions of the superalloy blade where there is no rubbing
or like conditions promoting wear nearly as severe as those
to which the Z-notch of the blade tip shroud is subjected.
Heretofore, MCrAlY coatings have not purposefully
contained substantial amounts of carbon, as it was not consi-
dered beneficial. In fact, migration of the carbon fromcertain superalloy base metals has been observed to cause the
undesired formation of chromium carbides at the coating-base
metal interface and suppression was sought, as for example,
is described in Shockley et al, U.S. Patent 3,955,935.
relatively esoteric case to the contrary occurs when some
specialty alloys contain a rather high carbon content, e.g.
Ni-Ta-C eutectic alloys. Here, as described in Jackson et al
4,117,179, a MCrAlY coating containing some ('~0.1 wt.%)
carbon is used to avoid debilitating migration of carbon from
the alloy. But the carbon content in the coating is minimized
to avoid the formation of carbides, and there is no suggestion
--3--
r~
nor likelihood of improved wear resistance.
There is another contemporaneous U.S~ Patent which
has relation to the instant invention. Wolfa et al, 4,124,137
discloses a tantalum carbide containing Co-Cr alloy coating
for resisting wear at high temperature. The coating in its
broadest form consists essentially by weight percent of
17-35 Cr, 5-20 ~a, 0.5-3.5 C, balance Co. Other embodiments
contain rare earth metals, Al, Si, and various metal oxides.
Of course, as is well known and mentioned in Wolfa et al, Ta
is a solid solution strengthener in high temperature alloys.
While preferred for oxidation-corrosion resistance over W and
Mo, as a refractory metal Ta at best does not improve the
oxidation-corrosion resistance of a CoCrAlY alloy, and most
likely degrades it, if only by replacing other elements in
the system.
Of course, as has been well-documented in the litera-
ture, aircraft gas turbines operate at the extreme conditions
of material durability. A material optimized for one condition,
e.g. oxidation at llO0C, may fare poorly at another condition,
20 e.g. hot corrosion at 900C, and vice versa. There are often
necessary compromises as a result. The addition of the requi-
rement for wear resistance adds a further variable to be
addressed. Thus, there is still a need and room for improvement
in coating alloys to achieve the highest performance in a gas
turbine.
An object of the invention is to provide a wear,
oxidation, and corrosion resisting coating alloy and a coated
superalloy article, useful at temperatures up to 1000C or
higher.
According to the invention, the improved coating is
comprised of chromium, aluminum, yttrium, and carbon with the
," .
--4--
balance being selected from the group consisting of nickel,
cobalt, iron, or mixtures thereof. The invention results in
a coating consisting essentially of a carbon rich M~rAlY matrix
containing fine metal carbides of the order of 1-2 microns size
and chromium carbides of the order of 12 microns. More parti-
cularly, the invention relates to a superalloy article having
a wear resisting and oxidation corrosion protective coating
consisting essentially of a carbon rich MCrAlY matrix having
finè transition metal carbides of the order of 1-2 microns size
and coarser chromium carbides of the order of 12 microns size,
produced by plasma spraying MCrAl~ and Cr3C2 powders, the coat-
ing having a carbon content of at least 0.6 weight percent
wherein M is one or more of nickel, cobalt and iron. An embodi-
ment entails a coating composition consisting essentially of,
by weight, 18-80% chromium, 1.2-29% aluminum, up to 4.8% yttrium,
0.6-11% carbon, balance selected from the group consisting of
nickel, cobalt and iron or mixtures thereof. Advantageously, the
coating composition consists essentially of, by weight, 23-68%
chromium, 4-22% aluminum, up to 4.4% yttrium, 1.5-7.8% carbon,
balance selected from the group consisting of nickel~ cobalt,
and iron, or mixtures thereof. In one preferred embodiment, the
coating composition consists essentially of, by weight, 36%
chromium, 10% alumirum, 2.6% carbon, 0.52% yttrium, and balance
cobalt. The improved coating consists of complex compounds of
the deposited elernents compared to simply chromium carbide
particles entrapped in a metal matrix as known in the prior
art. The coating is believed to include within its structure
complex MCrAlY compounds having substantial carbon content
together with non-stoichiometric transition metal carbides, as
well as Cr3C2
The peculiar morphology of the coating, the combina-
tion of fine and coarse carbides, provides a particularly
7 -5-
durable and hard matrix with well-bonded larger wear resisting
chromium carbide complexes.
The aforementioned compositions may be applied by
different methods, but a preferred method is to plasma spray
a mixture of Cr3C2 and MCrAlY of suitable particle sizing and
proportion onto a superalloy substrate. After plasma spraying
the coating is heat treated to prepare the coating for use,
preferably.
The coating of the invention finds special use as
a protective coating on turbiné blade tip shrouds made of
nickel, cobalt, and iron base superalloys to provide signifi-
cantly increased service life in the gas turbine engine en-
vironment.
According to another embodiment of the invention,
an improved M~rAlY coating containing chromium carbides is
applied by plasma spraying a particulate. In one embodiment,
the particulate is comprised of a mixture of MCrAlY and Cr3C2
~- and the deposited coating consists essentially by weight per-
cent of 18-80 Cr, 1.2-29 Al, up to 4.8 Y, 0.6-11 C, balance
selected from the group consisting of Ni, Co, and Fe, or
mixtures thereof. Advantageously the coating consists essen-
tially of 23-68 Cr, 4-22 Al, up to 4.4 Ya 1.5-7.8 C, balance
as set forth before. One particularly preferred composition is
36 Cr, 10 Al, 2.6 C, 0.5 Y, balance Co. The improved coatings
further are characterized by a MCrAlY matrix contai.ning fine
carbides of the order of 1-2 microns and coarse chromium
carbides of the order of 12 microns, the fine carbides being
formed during the spraying process when the mixed partlculate
is used.
According to a broad aspect of the invention, there
is provided a method of providing a coating having wear,
corrosion, and oxidation resistance in a gas turbine environ-
ment, on a superalloy substrate, which comprises the steps of:
(a) mixing MCrAlY powders with Cr3C2 powders, wherein M is a
- metal selected from the group consisting of ~i, Co, Feg or
mixtures thereof, with the Cr3C2 having a first average particle
size and being present in sufficient quantity in the mixture to
- produce the composition and structure after spraying which is
indicated in step (b), (b) plasma spraying the powders so that
they impinge on, adhere to the substrate, and interact during
spraying to produce a coating which consists essentially of a
MCrAlY matrix rich in carbon and containing fine carbides of
the order of 1-2 microns size and coarse chromium carbides of
the order of 12 microns size, the average size of the Cr3C2
carbides being smaller than the said first average particle
size, and the coating having at least 0.6 weight percent
carbon.
Most preferably, the plasma sprayed substrate is
heat treated at about 1080C to diffusion bond the coating
and form further fine transition metal carbides in the matrix
. which is saturated with carbon.
The coating method finds special use for applying
a protective coating on turbine blade tip shrouds made of
nickel, cobalt, and iron base superalloys to provide signifi-
cantly increased service life in the gas turbine engine
environment.
These and other advantages, objects and use of the
invention will appear more fully from the following Figures
and detailed description of the preferred embodiment.
The invention will now be illustrated by means of
the following drawings, in which~
Figures 1 and 2 are conventional light microphoto-
graphs of cross sections through a heat treated coating of
the present invention at 250X and 500X, respectively, after
JJ
a 5% chromic acid electrolytic etch.
Figure 3 is a scanning electron microscope photo~
micrograph of a cross section through a coating of the
present invention at lOOOX.
Figure 4 is similar to Figure 3, but shows the coat-
ing a-fter heat treatment.
The superalloys are generally those alloys character-
ized as nickel, cobalt or iron base alloys which display high
strengths at high tamperatures. There are a number of super-
alloys which are used in gas turbine engines. Of these, thegreatest physical demands are usually placed on those alloys
employed in blades and vanes in such engines since the blades
and vanes face the highest stress at the highest temperature.
- With-respect to blades, the most severe service in terms of
-7a-
oxidation, corrosion, and wear is experienced by the Z-notch
area on the blade tip shrouds which areas rub against each
other during engine operation, Typical nickel base alloys
used for blades are IN 100*, INCONEL 792*, INCO 718*, and
MARM 200*, typical cobalt base alloys are WI-52* and MARM 509*.
Figures 1, 2~3 and 4 show a 0.023 cm thick coating
of the present invention in which the composition comprises, by
weight, 36% Cr, 10% Al, 2.6% C and 0.52% Y, and balance cobalt,
which was applied to a superalloy substrate of Inconel 718.
As can be seen from the Figures, there is a multiphase struc-
ture which on microanalysis appears to include cornplex carbide
particles more or less randomly dispersed through the matrix
which is found by probe to be carbon bearing. The larger
complex carbides are very fine in size, having an average
diameter of about 10 microns and are generally less than 15-20
microns.
To insure high coating density and the desired com-
plex structure, the coating is applied to the substrate by
the advanced plasma spray process and apparatus described in
U. S. Patent ~o. 4,236,059 issued November 25, 1980.
In the advanced process, the powders needed to form the
coating are injected into a cooled plasma gas and then
sprayed onto the substrate. The advanced technique was
used to form the coating shown in the Figures. A physical
mixture of two minus 44 micron particle size powders, one a
MCrAlY type alloy powder comprised by weight of 63% cobalt,
23% chromium, 13% aluminum, and 0.65% yttrium and the other a
chromium carbide ~Cr3C2~ powder comprised by weight of 87%
chromium and 13% carbon, was injected into the plasma ~as
stream. About 50% of the mixture by weight was CoCrAlY
powder. After inert plasma spraying by this technique, the
coated article with the structure shown in Figure 3, was heat
, * Trademark
treated at 1080C (1975F) ~or ~our hours to form a diffusion
bond between the coating and the substrate, and produced a
somewhat different structure, shown in Figures 1, 2 and 4.
0-ther temperature and time combinations will be usable to
achieve the same result as described herein, as the skilled
person will readily ascertain.
The coating described above and others of similar
nature were examined by various metallurgical techniques
including, wet chemistry, light microscopy, x-ray diffraction,
and scanning electron microscopy to identify constituents and
morphology. The chemical composition for the as-deposited
coating shown in Figure 3 was determined using electron micro-
probe x-ray energy analysis, specifically, using an Etec Auto
Probe with a Kevex 5100 x-ray energy analyzer tracing a
number of different locations for Co, Cr, and Al and calculat-
ing Y and C. It was found that the chemical composition by
weight was nominally 51% cobalt, 36% chromium, 10% aluminum,
2.6% yttrium. This indicated that the constituent powder
passed through the plasma spraying device deposited a composi-
` 20 tion which would result from the ratio of 80% MCrAlY and 20%
Cr3C2~ Of course small percentage variations are to be norm-
ally expected in the composition of MCrAlY coating powder
compositions as well as variances in electron microprobe
compositional analysis. Consequently, it will be understood
that the conclusions herein are subject to these limitations
of precision. It is well-known by those skilled in the art
of coating that all the powder passing through a plasma spray-
ing device does not deposit on the substrate, and that differ-
ent powders have different deposition rates, or deposit
efficiencies, for the same spraying condition. Consequently,
we take care herein to distinguish between the material
which is sprayed and that which is deposited.
* Trademark
_ g _
~ ~h~ jy
Such a distinction is not always present in the prior art.
Many compounds were present which were not character-
izable with reference to standard x-ray diffraction patterns
or prior examinations of MCrAlY coatings. Therefore, it is
speculated that the coating is comprised of very fine (1-2
micron) complex metal carbides, non-stoichiometric carbides
and metastable compounds. Phases identifiable as Cr3C2 carbides
were present in the as-deposited coating such as shown in
Figure 3, but in sizes (seldom exceeding 12 microns) consider-
ably smaller than the 15 micron average size carbide particleswhich had been included in the mixture passed through the
spraying device. In addition, the microprobe analysis showed
that only 5 to 10 percent of the as-deposited coating by weight
was the crystallographic compound Cr3C2. The remainder of the
chromium and carbon must therefore be alloyed with or precipita-
ted within the fine compounds of the CoCrAlY matrix. This is
an unexpected result based on the prior art which does not
appear to teach coating systems in which such interactions occur.
It is likely that the regions identified as Cr3C2
may be partially diluted with metals of the matrix, at least
at their periphery, and therefore in reference to a particle,
the term chromium carbide as used herein should be taken to
include these more complex and diluted compounds of Cr3C2.
Examination of the coating after heat treatment showed
the composition to be unchanged, but as seen in Figure 4, the
morphology was significantly different. Particles which may be
fine Co-Cr carbides of the order of 1 micron in diameter are
apparent in the matrix' because of their fineness, the composi-
tion or exact structure was not determinable. However, we
30 characterize these as transition metal carbides inasmuch as
only transition metals are present and capable of forming
--10--
,c,T ~ i
.`
substantial carbides in our coatin~s (excepting the improbable
or insubstantial comhination with Al and Y). We would charac-
terize the fine carbides in the unheat treated coating similarly.
The previously observed Cr3C2 regions are seen to be substanti-
ally altered in appearance and less clearly defined and they
are made substantially smaller-- 10 microns or less. These
results are presumed to be due to dif~usion and alloying.
X-ray fluorescence of the coating as deposited and after heat
treatment indicates that the carbon is dispersed throughout
the coating, rather than all concentrated in the defined
chromium carbide particles.
The amount of carbon and chromium added to basic
MCrAlY type alloys to produce new wear resisting alloys can
be varied to suit the particular service environment to be
encountered. For simplicity we state the chromium and carbon
added to the basic MCrAlY in terms of the amount of Cr3C2
which the additions are represen-tative of, even though as
explained above, the elements are not all chemically combined
as Cr3C2 in the coatingO We find usable coatings to be those
having from 5 to 85 weight percent Cr3C2. This range, when
combined with the MCrAlY composition used in the pre~erred
embodiment, results in a coating with the total weight of
chromium varying from about 26 to 78%, and the carbon from
about 0.65 to 11%. For low temperatures, e.g., below 750C
(1400F), or severe wear applications, the chromium and carbon
contents would be in the high portion of the range as the
carbide phases provide wear resistance. The upper limit is
determined by the need for sufficient matrix to bind the
carbides togetner and to the substrate. Beyond the upper
limit the coating will degenerate due to the physical loss of
carbidesu At higher temperatures, in the 950C (1700F) range,
.
.''
conditions of less severe wear, or those requiring greater
ductility, the lowest portion of the compositional range is
suitable. The lower limit is determined by the need to provide
improved wear resistance over conventional MCrAlY alloys.
Sufficient carbon must be present to cause the pr~sence of
detectable carbides which impart wear resistance. Yttrium is
included in MCrAlY coatings to enhance the oxidation-corrosion
performance at the highest use temperatures, namely, a~ove
950C (1700F). The function of yttrium in MCrAlY alloys has
10 been well set forth in the prior art and the yttrium content
of our inventions are accordingly determined by the same crite-
ria. Since yttrium significantly increases high temperature
properties, we believe at least some yttrium, should be present,
0.01% or more. For applications at lower temperatures it is
possible to omit the yttrium without suffering adverse perfor-
mance effect in carbon bearing MCrAlY coatings of the present
invention.
The hardness of a coating is measured by several
tests with a diamond penetrant hardness (DPH) tester using
20 300 gm loading, producing an impression width of 0.025 mm
(0.001 inches) or larger, thereby giving a nominal hardness
value for the matrix. The average hardness of the invention
coating can be tailored from about 600 DPH to over 1000 DPH
by variation of the carbon-chromium content. The hardness of
the matrix provided by the invention is especially desirable
for wear resistance. Undercutting of the even harder chromium
carbide regions is thus avoided. The measured apparent hard-
ness of the matrix is attributable to the very fine carbides
dispersed therein, provided in the invention. The most suitable
30 thicknesses for the invention coating are determined by the
particular application and the dimension specified is normally
that for a coating which is in its finished condition after
,
-12-
~ ~ ~7~
machining. The preferred coating thickness can range from
0.013-0.09 cm (0.005-0.035 inch~ and typically is in the
0.020-0,038 cm (0.008-0.015 inch) range, though of course for
special applications other than Z-notches thinner coatings of
0.0025 cm (0.001 inch) or less may be usable.
For optimum oxidation, wear resistance, and adhesion
of the coating to the substrate, the density of the coating
should be high, for example, at least 95% of theoretical. The
coating shown has a density of 98%.
The high hardness in combination with the outstanding
oxidation and corrosion resistance of the CoCrAlY alloy provi-
des a versatile invention coating having a unique structure
and combination of properties usable under a wide variety of
harmful serviee conditions. Such properties include a much
better combination of adhesion, oxidation, corrosion and wear
resistance at elevated temperatures than the prior art hard-
facing alloys and composite or cermet coatings such as those
having chromium carbide particles dispersed in a nickel-
chromium or like alloy binder. In addition, the coating of
20 the invention can be economically deposited on substrates by
the advanced plasma spray technique described above as well as
others.
The improved wear resistant coating of novel morpho-
logy can be expected to result from the addition of chromium
` and carbon to the ranges of MCrAlY type coatings disclosed in
the prior art. The ranges have been previously described in
various U.S. patents cited in the background seetion of this
disclosure. (It is also in our contemplation that such impro-
vements or refinements in MCrAlY coating composition as are in
the future revealed will be usable within our invention.~ When
the above-referenced compositions, particularly those in Evans,
-13~
U.S. Patent 3,676,085, are included with from 5 to 85 percent
chromium carbide (Cr3C2), the compositional ranges stated in
the summary of the invention result. While the chromium and
carbon are advantageously added in the form of particulate
Cr3C2,' where chromium and carbon are added in the ratio of
87% chromium and 13% carbon, they might be added in the form
of other compounds such as complex carbides, sub-carbides, or
carbon rich alloys since it is not a requirement that the
carbon containing particles retain entirely intact their
identity as particulate Cr3C2 in the coating to carry out the
invention herein. Also, the coatings of the invention might
be prepared by fabricating a master alloy of the desired compo-
sition, converting same to a powder, and plasma spraying the
powder. Powders ranging in average particle size from 5 to
40 micron can be used, depending on the spraying equipment.
Still other ways of achieving the desired coating composition
on a superalloy article can be utilized by those skilled in the
art of coating.
It may be noted here that compared to other coatings
our coating exhibits unusual effects. First, there is the
interaction of the matrix MCrAlY with the particulate chromium
carbide to form the complex as-deposited structures. With the
less complex alloys of the past such an effect was neither
observed nor thought desirable. Second, the composition of
our coating alloy differs substantially from that of Wolfa et
al in U.S. Patent 4,124,137. We use the transition metal
chromium instead of the refractory metal tantalum, tantalum is
a strengthener whereas chromium is not. Conversely chromium
enhances corrosion resistance whereas tantalum does not.
30 Further in our coating chromium carbides are present whereas
in the coating of Wolfa et al tantalum carbides are present,
and these carbides have differing properties.
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J`7
To further illustrate the invention described herein,
the following examples are given.
Example 1
A mixture of two minus 44 micron particle size
powdersl one, a nichrome alloy comprised of 80% nickel and 20%
chromium by weight, and the other a chromium carbide (Cr3C2),
where the nichrome was 12 percent of the mixture, was applied
with the plasma spray process to a nickel superalloy substrate.
The deposited coating was measured to consist of 25% nichrome
and 75% Cr3C2. Examination of the coating by x-ray diffraction
showed that the constituent nichrome and Cr3C2 were present in
the deposited coating. The Cr3C2 particles were essentially
present in the particle siæe of the original mixture. Since
it is well known to those in the art that nichrome has less
~avorable oxidation and corrosion properties in a gas turbine
environment than MCrAlY coatings and since the chromium carbide
particles are present in a conventional cermet manner, the
matrix can be expected to have the limited properties of
nichrome and the particles can be expected to be susceptible
to pullout. The coating was measured to have a hardness of
400-700 DPH. Examination of a coating after heat treatment
at 1975F for four hours did not sh~w substantial change in
the morphology of the coating from that of the as-deposited
condition. However, when tested on a part, the heat treated
coating was inferior to the unheat treated coating, exhibiting
loss of adhesion from the substrate, spalling, and general
degradation. This served to show the advantage of the inven-
tion compared to a material of the prior art, insofar as the
result produced by heat treatment.
Example 2
A mixture of minus 44 micron particle size powders,
one a MCrAlY alloy comprised of 63% cobalt, 23% chromium,
13% aluminum, and 0.65% yttrium by weight, and the other a
chromium carbide (Cr3C2) powder, where the MCrAlY was 50% of
the mixture, was applied to a I~-718 nickel alloy substrate
using an advanced pl~sma spray process. The coating was heat
treated for four hours at 1975F. The composition of the
coating was found to be nominally 51% Co, 36% Cr, 10% Al,
2.6% C and 0.52% ~. The density was measured at 98% of maximum
possible by metallographic pore counting and calculation.
Examination of the coating by scanning electron microscope and
electron microprobe showed complex unidentifiable carbides
with diffused boundaries, indicating an interaction of the
MCrAlY matrix with the carbides, which would not be expected
in prior art metal matrix-carbide coatings. Smaller carbides
of 1-2 micron diameter were dispersed through the matrix but
could not be identified. The presence of carbides in the metal
matrix of cermets is unexpected, as is the presence of carbides
in a MCrAlY coating. The hardness was measured to be about
600-700 DPH.
Example 3
Eleven blades for the third stage of a high perfor
mance gas turbine were coated at the Z-notch location of the
tip shroud with an 0.008 to 0.010 inch thick layer of the
coating described in Example 2. The parts were installed in
an engine where they were exposed to temperatures at nominally
1700F. After more than 500 hours of engine operation the
coatings showed no indication of degradation or failure.
Example 4
A coating having the composition 64% chromium, 22.8%
cobalt, 5.2% aluminum, 7.8% carbon, and 0.2% yttrium was
applied to turbine blades and tested similarly to that descri-
bed in Example 3. Favorable performance was also observed.
-16-
.
Example 5
A coating having the composition 56.8% cobalt,
29.6% chromium, 11.7% aluminum, 1.3% carbon, and 0.6% yttrium
was applied to turbine blades and tested similarly -to that
described in Example 3. Favo~able performance was also
observed.
It will be appreciated tha~ the invention is not
limited to the specific details shown in the examples and
illustrations and that various modifications may be made
within the ordinary skill in the art without departing from
the spirit and scope of the invention.
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