Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
Field of the Invention The present invention rela~e~ ~o
abrasive materials and their manuacture and, more particularly,
to abrasive particulate and its dispersal in a metal matrix to
provide an article~of use having superior abrasive properties.
Description of the Prior Art - Improved efficiency is an
increasingly important factor in the development of gas turbine
engines. Such engines have rows of rotating blades fixtured
within a generally cylindrical case and leakage of gas between
the ends of the blades and case contributes toward inefficiency.
In current engine design, this leakàge is minimized by blade
and sealing systems in which the blade tips rub against a seal
which is attached to the interior of the engine case. Generally,
it has been found most desirable to construct the system such
that the blade tips are harder and more abrasive than the seal
and thus cut into the seal during certain sta~es Qf engine
operation to establish essentially a zero cLearance therebetween.
Previous abrasive blade tips have been constructed of various
materials including certain nickel base supexalloys, U.S.
2,994,605, and nickel matrices having tungsten carbide partîcles
randomly dispersed therein, U.S. 3,199,836. However, as a
result of the increasing temperatures enco~ntered in gas turbine
engines, especially in the turbine section, and of the presence
of corrosive gases, ~he abrading and cutting effectiveness of
these blades tips has not been entirely satisfactory. The prior
art blade tips have proved especial~y deficient when used in con-
junction with ceramic seal materials which have been developed
for ~2S turbine engine use, for example, graded ceramic seals
based on zirconia.
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SUMMAR~ OF THE INVE~TION
The present invention contemplates an improved abrasive
material particularly useful as a dispersoid in a metal matrix
to provide an article of use having superior abxasive properties,
especially at elevated temperatures. Typically, the invention
utilizes abrasive silicon carbide particulate, the individual
particles of which have a coating substantially of alumina
(A12O3) thereon. m e alumina coating functions to inhibit
diffusion or dissolution of the dispersed silicon carbide
particles into the metal matrix during service at elevated
temperature and thereby increases the useful life of the part-
iculate as an abrasive dispersoid. The invention also envisions
providing an intermediate coating of silica (SiO2) between the
particle and outer alumina coating to improve high temperature
bonding by formation of a mullite phase (3~12O3 . 2SiO2) there-
between. According to the invention, the alumina and silica
coatings are preferably applied by sputtering or vapor deposit-
ion processes.
m e alumina coated silicon carbide particulate can be
dispersed in a wide variety of metal matrices to suit particular
needs. Dispersal of the par~iculate can be affected by powder
metallurgical and casting techniques as well as others.
~ he invention finds special application in abrasive
blade tips for service in gas turbine engines. Such blade tips
typically comprise the alumina coated silicon carbide particles
dispersed in nickel base or cobalt base superalloy matrices.
Abrasive blade tips produced in accordance with the present
invention have been found highly effective in abrading and cut-
ting ceramic seal materials at elevated temperatures.
In accordance wlth a broad aspect of the invention,
there is provided a high temperature particulate, useful as an
improved dispersoid in a metal matrix, comprising silicon
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carbide particulates having a predominantly alumina coating,
the coating being resistive to diffusion or dissolution in
superalloys at temperatures of 2000 F. or more.
In accordance with another broad aspect of the
invention, there is provided a method for making a sili~on
carbide and metal matrix abrasive article useful at elevated
temperatures comprising depositing an alumina coating on in-
dividual silicon carbide particles and dispersing the coated
particles in a metal matrix, the alumina coating inhibiting
diffusion of the particles into the matrix at elevated temper- ;
atures and increasing the useful life of the article.
These and other objects, advantages and uses of the
present invention will become more readily apparent from the
; following detailed description of the preferred embodiments.
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DESCRIPTIOI~ 0~ THE PREFEP~ED EMBODIMENT
As previously discussed, the improved abraslve Ma-te~ia
the present invention finds particular usefulness as a dispersoid
in ~etal matrices to provîde articles of use havlng excellen~
high temperature abrasive properties. One such article of use
is the abrasive blade tip used in the blade and seal systems o
modern gas turbine engines. For the sake of illustration, the
present invention will be described in detail hereinbelow with
re~erence to such blade tips.
In modern gas turbine engines, the abrasive blade tips must
be capable of providing effective abrading and cutting action
into the seal material at very high temperatures. In the turbine
section of the engine, for example, blade tip temperature may
approach or exceed 2000F during normal engine operation. Under
- these severe conditions, prior art blade tips have not been
entirely satisfactory, especially- when used in conjunction with
ceramic seal materials. The present invention provides an im-
proved abrasive material which can be dispersed in a nickel base
or cobalt base superalloy matrix, or other appropriate matrix,
to provide an abrasive blade tip characterized by superior
ability to abrade and cut seal materials, including ceramîc
seal materials, at the very high temperatures encountered in
engine operation. More specifically, the invention involves
the discovery that silicon carbide particulate, when dispersed
in nickel base and cobalt base superalloy matrices, exhibits
superior abrasive ability and that by coating the individual
particles of silicon carbide with alumina, such superior
abraslve characteristics can be maintained at turbine operating
temperatures.
Commercially available silicon carbide particulate appears
satisfactory in sizes from 8 mils to 30 mils average diameter,
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although the particular size utilized can of course be varied
as desired depending on the particular c~brasive application
In order to inhibit the di~usion or dissolutlon o~ the par~icl~s
in the metal matrix at high temperatures, thereby maintaining
abrasive ability, the individual particles are coated with
commercially available alumina (A1203) or, for better bonding,
with a first coating of silica (Si02) followed by a second
coating of alumina. The use of the intermediate silica coating
fosters improved high temperature bonding of the alumina coating
to the particle by the formation of a mullite phase (3A1~03 2Si02)
therebetween during high temperature service. Or, if desired,
the duplex coated particles can be suitably heat treated prior
to dispersal in the matrix to effect formation of the mullite
phase. The alumina and silica coatings can be conveniently
deposited on the individual silicon carbide particles by
sputtering, physical vapor deposition and chemical vapor depo-
sition techniques wherein the particles are rotated slowly during
deposition. Of course, the thickness of the coatings can be
varied as desired. For abrasive blade tip applications> an
alumina coating of about 0.1 mil in thickness has been found
satisfactory.
As those skilled in the art will recognize, dispersal o
the alumina coated silicon carbide particulate in the nickel
base or cobalt base superalloy matrices can be effectPd by a
variety o~ techniques including, but not limited to, powder
metallurgy, casting and others. Powder ~etallurgical techniques
are preferred since random dispersal of the particles is optimized.
For example, a preferred tec.hnique is to mix the coated silicon
carbide particles and superalloy powder in desired amounts and
then hot press the mixture to a final or intermediate consolidat~d
shape. However, in some cases, it may be desirable to provide
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for a nonrandom dispersion~ that is, the particulate being
segregated in certain regions of the tip, such as near the end
contacting the seal material. A significant advan-~age af the
present invention is t'hat the alumina coated silicon carbide
particulate can be dispersed in a wide variety o~ matrix alloys
with no signi~icant deleterious interaction therebetween. This
feature allows the matrix alloy to be selected and tailored to
the specific enviro~ment and conditions of service, resulting in
an overall-improved abrasive blade tip or other article of use.
For abrasive blade tips, various nickel base and cobalt base
alloys can be utilized', examples o~ w~ich are a nickel base
alloy of nominal composition, by weight, 21-25% Cr, 4.5-7% Al,
4-10% W, ~.5-7% Ta, .02-.15V/o Y, .1-.3% C, balance nickel and
a cobalt base alloy of nominal composition, by weight, 18-30% Cr,
10-30% Ni+Fe, 5-15% ~+Mo, 1-5% Ta+Cb, .05-.6% C, 3.5-8.0% Al,
.5-2.0% Hf, .02-.1% Y, balance cobalt. Blade tips having the
alumina coated silicon carbide particulate dispersed in these
alloys can be attached to the end of a nickel basQ- or cobalt base
superalloy turbine blade by solid state diffusion bonding, TLP~
bonding, brazing and other processes.
l'hose skilled in the art will recognize that the amount of
alumina coated silicon carbide particulate dispersed in the
alloy matrix can be varied as desired. In blade tip applications,
the amount has ranged from 45 volume percent for 8 mil diameter
particles to 30 volume percQnt ~or 15 mil diameter particles. In
addition, other abrasive particulate may be simultaneously dis-
persed in the matrix to provide useful properties. For example,
hot pressed silicon nitride (Si3N4) can be dispersed along with
alumina coated silicon carbide. Abrasive blade tips of the in-
3~ vention have been wear tested and found to possess acceptable
abrasive'properties to tip temperatures of about 2100F, the
matrix alXoy being a cobalt base alloy.
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Although the following invention has been shown and described
with respect to a pre~erxed embodiment thereo~ should be
understood by those skilled in the art that various changes
and omissions in the form and detail thereof may be made therein
without dep-rting f-om he ~pirit and the scope of the inve~tion.
