Note: Descriptions are shown in the official language in which they were submitted.
~ ~ RD-15,703
THERMOPLASTIC MOLDING OF CERAMIC POWDER
The present invention is directed to thermo-
plastic molding of a ceramic particulate composition.
More particularly, it relates to a thermoplastic
vehicle/binder for thermoplastically forming a ceramic
5 particulate material into a shaped body.
U.S. 4,144,207 and 4,233,256 disclose forming a
mixture Gf a sinterable silicon carbide material, a
thermoplastic resin and an oil or a wax, injection
molding the mixture to produce a molded product, removing
said thermoplastic resin by baking said molded product at
a temperature between about 450C and about lOOO~C to
produce a porous baked product and sintering the baked
product between about 2000C and 2200C to produce a
sintered product.
Injection molding and other thermoplastic
forming techniques of a ceramic powder such as extrusion,
blow molding, compression molding, transfer molding,
drawing, rolling, etc., places stringent requirements on
the vehicle/binder selected. The thermoplastic medium
selected must be suitable as a vehicle for the ceramic
powder. A composite of the thermoplastic medium and
ceramic powder can be formed into various shapes by a
number of techniques. The vehicle must also behave as a
binder maintaining the desired shape and be easily
removed leavinq behind a shaped powder compact. Binder
removal i~ normally done by thermal decomposition.
A major pr~blem with the vehicle/binder for a
ceramic composition has been that thermal decomposition
of the binder from the shaped powder compact generally
introduces defects such as cracks, pits and voids.
~Z~746~ KD-15,703
An object of the present invention is to
provide a binder, i.e. a vehicle/binder system, sui~able
for fabrication of complex ceramic shapes by injection
molding or other thermoplastic forming techniques.
A copolymer of ethylene and vinyl acetate was
found to possess some properties which makes it suitable
as a binder/vehicle or a number of ceramic particulate
compositions. However, its removal was not possible in a
reasonable time without resulting damage to the
remaining powder compact. In the present invention, it
was found that certain additions of stearic acid allowed
binder removal without defects to the compact and with no
serious compromise to the molding and release behavior of
the ethylene-vinyl acetate copolymer.
Briefly stated, one embodiment of the present
invention comprises a thermoplastically moldable ceramic
composition comprising a substantially ho~ogeneous
dispersion comprising from about 40% by volume to about
60% by volume of a ceramic powder having an average
particle size which is less than about 10 microns and
from about 40% by volume to about 60yO by volume of a
binder consistlng essentially of an organic acid
containing from 12 to 26 carbon atoms per molecule and
having a melting point ranging from about 44C to about
88C and a thermoplastic copolymer of ethylene and from
greater than about 12 weight % to about 33 weight % vinyl
acetate, said copolymer having a melt index according to
ASTM Dl238 ranging from about 8 to about 43, said organic
acid ranging from greater than about 18% by weight up to
about 45% by weight of the total weight of said binder,
~aid binder having no significantly deleterious effect on
said ceramic powder.
~7~ R~-15,703
Briefly stated, in another embodiment, the
present invention comprises a process for producing a
~haped baked body for densifying into a polycrystalline
ceramic body having a porosity of less than about 20% ~y
volume of the total volume of said polycrystalline body,
which compri~e6 forming a thermoplastically moldable
ceramic ~omposition comprising a substantially
homogeneous dispersion comprising from about 40% by
volume to about 60% by volume of a ceramic powder having
an average particle size which is less than about 10
microns, and the balance being a binder consisting
essentially of an organic acid containing from 12 to 26
carbon atoms per molecule and having a melting point
ranging from about 44C to about 88C and a thermoplastic
copolymer of ekhylene and from greater than about 12
weight % to about 33 weight % vinyl acetate, said
copolymer having a melt index according to ASTM Dl238
ranging from about 8 to about 43, said organic acid
ranging from greater than about 18% by weight up to about
45% by weight of the total weight of said binder, said
binder having no significantly deleterious effect on said
ceramic powder, thermoplastically molding said ceramic
composition producing a molded body, embedding said
molded body in an embedding powder, said embedding powder
having no significantly deleterious effect on said body,
ba~ing ~aid embedded molded body at a heating rate which
- has no ~ignificant deleterious effect thereon at a
- temperature ranging up to 450C removing said binder
leaving no amount ther2in which would have a
sig~ificantly deleterious effect on said densified body,
~aid baking having no ~ignificant deleterious effect on
said body, and recovering the resulting baked body.
~74~ P~-~5,703
In the present invention, the thermoplastically
moldable ceramic composition is comprised of a uniform or
~ubstantially uniform mixture, e.g. a homogeneous or
eubstantially homogeneous dispersion, of the ceramic
powder and binder. More specifically, the moldable
ceramic composition contains the ceramic powder as a
homogeneous or substantially homogeneous dispersion.
The present binder is a thermoplastic material
which exhibits ~ very high viscosity at room temperature,
and as a practical matter can be considered a solid at
room temperature. Its viscosity decreases with
increasinq temperature. The binder is comprised of the
present organic acid and a thermoplastic copolymer of
ethylene and greater than about 12 weight % to about 33
weight % vinyl acetate.
The present copolymer also exhibits a very high
viscosity at room temperature, and also, as a practical
matter can be considered a solid at room temperature.
The copolymer has a melt index according to ASTM Dl238
ranging from about 8 to about 43, and preferably it
ranges from about 12 to 30 and most preferably it is
about 19. For a given molecular weight, the melt index
of the present copolymer increases with increasing vinyl
acetate concentration. An ethylene-vinyl acetate
copolymer wherein the vinyl acetate content is about or
below 12 weight % or higher than about 33 weight % is not
useful since it will not produce, or it will be
substantially more difficult to produce, the present
densified body free of service-limiting defects.
Specifically, with decreasing vinyl acetate content in
the copolymer, i.e. about or below 12 weight % o vinyl
acetate, it becomes significantly increasingly difficult
to produce the present substantially homogeneous
12 ~4~5 RD-15,703
dispersion as well as to produce a part without
significant defects such as surface cracks. On the other
hand, above about 33 weight % vinyl acetate, the
copolymer is less viscous and it becomes substantially
more difficult to remove the binder without creating
significant defects such as bloating. Preferably, the
presen~ copolymer contains vinyl acetate in an amount of
- at least about 13 weight %, more preferably from about 14
w~ight % to about 30 weiyht %, most preferably from about
18 weight % to about 28 weight %, and particularly
preferred is about 25 weight % vinyl acetate.
The present organic acid contains from 12
carbon atoms to 26 carbon atoms per molecule and has a
melting point ranging from about 44C to about 88C.
Preferably, the organic acid is selected from the group
consisting of lauric acid (melting point ~44C), stearic
acid (melting point ~70C), cerotic acid (melting point
~88C), and mixtures thereof, and most preferably it is
stearic acid.
The present organic acid allows the binder
during bake-out to be thermally decomposed in a
reasonable time without sacrificing the beneficial
molding propert.es of the copolymer. The useful range
for the present organic acid is greater than about 1~3% by
weight up to about 45% by weight of the total binder.
With decreasing amounts of the organic acid, i.e. below
and about 18% by weight of the organic acid, the binder
will behave increasingly like the ethylene-vinyl acetate
copolymer alone, i.e. it was found tha~ the
ethylene-vinyl acetate copolymer by itself would not
thermally decompose during bake-out without leaving
defects in the powder compact. On the other hand, with
increasing amounts of organic acid, i.e. about and above
RD-15,7C3
~L27464~
45% by weight of the organic acid, the binder will beha~e
in an increasingly brit~le manner like wax and is apt to
leave, or will leave, service limiting defects consistent
with low molecular weight binders. Preferably, the
present organic acid is used in an amount ranging from
about 20% by weight to about 40% by weight, most
preferably from about 25% by weight to about 35% by
weight, of the total amount of the binder.
The present ceramic powder is a densifiable
powder, i.e. it can be densified to produce the present
polycrystalline ceramic body. More specifically, the
present ceramic powder is a particulate ceramic material
which, when formed into the present baked body, can be
densified without the application of uniaxial mechanical
pressure to produce a polycrystalline ceramic body having
a porosity of less than about 20% by volume. Examples of
such densification of the present baked body include
sintering or firing the baked body in a vacuum or gaseous
atmosphere, reaction bonding of the baked body and/or hot
isostatic pressing of the baked body with a gas.
The present ceramic powder can be, for example,
a sinterable silicon carbide powder, a sinterable silicon
nitride powder, a sinterable mullite powder, a sinterable
aluminum nitride powder or a sinterable alumina powder.
A sinterable silicon carbide powder is comprised of, for
example, silicon carbide and a suitable sintering
additive such as a combination of boron and free carbon.
A sinterable silicon nitride powder i8 comprised of, for
example, silicon nitride and a suitable sintering
additiv2 such as MgO. A sinterable mullite powder may or
may not contain sintering additive. A sinterable
aluminum nitride powder is comprised of, for example,
aluminum nitride and a suitable sintering additive such
127~S RD 15703
as Y2O3 or CaO. a sinterable alumina powder may or
may not contain a sintering additive but a useful
sintering additive is MgO. Generally, a sinterable
ceramic powder contains sintering additive up to about
5% by weight of the powder.
Firing or sintering of the present baked
body of sinterable ceramic powder is carried out at an
elevated temperature in a vacuum or gas which has no
significant deleterious effect thereon to produce the
present polycrystalline body.
Useful examples of the present ceramic
powder, and the present densification to produce the
present polycrystalline ceramic body are disclosed in
Unites Patent Numbers 4,004,934; 4,041,117; 4,119,475
and 5,017,319, all of which are assigned to the
assignee hereof.
U.S. Patents 4,004,934 and 4,041,117 to
Prochazka disclose a sinterable ceramic, i.e. silicon
carbide, powder comprised of silicon carbide and
additives of boron and carbon, and sintering a body
thereof at about or below atmospheric pressure
producing a polycrystalline silicon carbide body with
a porosity of less than about 20% by volume.
U.S. Patent 4,119,475 to Prochazka et al
discloses a sinterable ceramic, i.e. silicon nitride,
powder comprised of silicon nitride and a combination
of beryllium and magnesium sintering additives, and
sintering a body thereof at from about 1800C to about
2200~C under a superatmospheric pressure of nitrogen
producing a polycrystalline silicon nitride body with
a porosity of less than about 20% by volume.
U.S. 4,225,356 to Prochazka et al discloses a
sinterable ceramic powder comprised of silicon nitride
-- 7
''I 5 ` '~
RD-15,7~3
and beryllium sintering additive, and sintering a body
thereof at from about l900~C to about 2200C under a
superatmospheric pressure of nitrogen producing a
polycrystalline silicon nitride body with a porosity sf
less than about 20% by volume.
US 4,017,319 to Greskovich et al discloses a
~eramic powder or reaction bonding comprised of silicon
containing a boron additive, sintering a body thereof to
a density ranging from 65% to 75%, and nitriding the
sintered body by reacting it in a gaseous nitrogen
atmosphere from 1100C to below the melting point of
silicon producing a polycrystalline body with a porosity
of less than about 20% by volume.
An example of reaction bonding densification
comprises forming a ceramic powder comprised of about
equivalent amounts of silicon carbide and free carbon,
infiltrating a body thereof with silicon liquid or vapor
and reacting the carbon and silicon producing a
polycrystalline body with a porosity of less than 20% by
volume and generally comprised of about 85% silicon
carbide balance free silicon.
Generally, reaction bonding comprises
contacting the present shaped baked body at an elevated
temperature with a liquid or gas with which it reacts
thereby densifying and producing the present
polycrystalline body.
To carry out the hot isostatic pressing, the
present baked body is made gas impermeable and then it is
hot isostatically pressed with a gas at superatmospheric
pressure and at an elevated temperature which has no
significantly deleterious effect on it to produce a
polycrystalline body having a porosity of less than 20%
by volume. The particular gas pressure depends largely
~7~S RD- 15,703
on the density desired in the final product, and the
particular temperature depends largely on the composition
of the body and should have no significant deleterious
effect thereon. Generally, isostatic pressing is
carried out at a pressure ranging from about 5000 p5i to
about 30,000 psi at a temperature ranging from about
1400C to about 2200C. The hot isostatic pressing gas
should have no significant deleterious effect on the body
and examples of useful gasses are argon, helium, nitrogen
and mixtures thereof.
The present baked body can be treated to make
it gas impermeable by a number of techniques depending
largely on its composition. It may, for example, be
sintered only sufficiently to close off its surface
pores, making it gas impermeable. A specific example
comprises firing the present baked body comprised of
mullite in oxygen at from about 1500C to 1675C at
ambient pressure closing off its surface pores, and hot
isostatic pressing the resulting gas impermeable body
with argon at a pressure of about 10,000 psi at a
temperature ranging from 1500C to about 1700C producing
the present polycrystalline body having a porosity of
less than about 20% by volume.
The present baked body may also, for example,
be provided with a coating of a material which makes it
gas impermeable but which has no significantly
deleterious effect on it. For example, the present baked
body i6 coated completely with a slurry of small glass
spheres, heated in a vacuum to a temperature which melts
the glass but which is below the sintering, bonding or
decomposition temperature of the ceramic powder producing
a gas impermeable glass cvating enveloping the body and
hot isostatic pressing the coated body with a gas. A
~ z ~ D-15,703
specific example comprises coating the present baked body
comprised of silicon nitride and suitable sintering
additive such as Y2O3, A12O3 or MgO with a slurry of
glass spheres, heating the coated body in a vacuum below
the decomposition temperature of the silicon nitride
melting the glass enveloping the body with glass coating
and hot isostatic pressing the resulting gas impermeable
coated body with nitrogen gas at a pressure of about 5000
p6i and a temperature ranging from about 1600C to about
2000~C.
The average size of the present ceramic powder
ranges up to about 10 microns and depends largely on the
particular densification techniques, i.e. larger particle
sizes can be used in reaction bonding whereas smaller
particle sizes would be used in sintering a compact
thereof. Preferably, however, the ceramic powder has an
average particle size which is submicron and most
preferably, it has an average particle size ranging from
about 0.05 micron up to about 1 micron.
The binder is intimately mixed with the ceramic
powder in a ratio that maintains the thermoplastic
behavior of the binder but contains enough powder to form
a self-supporting powder compact free of significant
defect once the binder is removed. Specifically, the
thermoplastically moldable ceramic composition is
comprised of from about 40% by volume to about 60% by
volume, and preferably about 50% by volume, of solids,
i.e. th~ ceramic powder composition, and the balance is
the present binder. An amount of solids less than about
49% by volume or higher than about 60% by volume i5 not
operable to produce the present densified polycrystalline
body without significant defect.
RD-15,703
lZ7~
The ceramic powder and the present binder can
be admixed by a number of conventional techniques to
produce the present thermoplastically moldable ceramic
composition. Preferably, the ceramic powder and the
S binder are mixed at temperatures at which the present
binder is molten, preferably at temperatures ranging from
about ~0C to about 180C. Preferably, the resulting
- ceramic mixture is broken up inko pieces to produce a
more useul feed material.
A number of thermoplastic molding techniques
can be used to produce the present molded body.
Representative of such techniques are injection molding,
extrusion, blow molding, compression molding, transfer
molding, drawing and rolling.
To carry out the present thermoplastic molding,
sufficient heat and pressure is applied to the ceramic
composition to force it to flow to the desired degree
depending on the particular thermoplastic molding
process. The ceramic composition is heated to a
temperature at which the binder is soft or molten
depending upon the particular thermoplastic molding
process. For most commercial thermoplastic forming
techniques, the present ceramic composition is heated to
make the binder molten at from about 80C to about 200C,
shaped under a pressure ranging from about 5 psi to about
30,000 psi depending upon the particular thermoplastic
forming technique, and then allowed to cool and harden.
For example, in the case of injection molding, the molten
ceramic composition is forced into a die to produce the
molded product. Specifically, for injection molding, the
molten ceramic mixture, preferably at a temperature from
about 130C to about 180C and under a pressure ranging
from about 1000 psi to about 30,000 psi, is forced into a
RD-15,703
12~6~
die where it is allowed to harden and then removed from
the die. Preferably, the die is preheated to roughly
from about 30~C to about 60C.
The resulting molded body is baked to remove
the binder leaving no significant amount thereof, i.e.
leaving no amount of binder which would have a
significantly deleterious effect during the densification
of the body or on the resulting densified body.
GeneraLly, the present baking of the molded body leave~
the binder in an amount of lcss than about 2% by weight
and preferably less than about 1% by weight, of the baked
body. The molded body is embedded, preferably immersed,
in a supporting powder which prevents significant
distortion of the body during baking to remove the
binder. The embedding powder should be chemically
compatible with the molded body, i.e. it should have no
significant deleterious effect on the body. An example
of an embedding powder is charcoal. Preferably, the
embedding powder is spherical or nominally spherical and
preferably has an average diameter ranging from about lO
microns to about 1000 microns. Representative of
embedding powders useful for molded bodies of silicon
carbide and silicon nitride are charcoal, sintered
polycrystalline silicon carbide having a density greater
than 80% of the theoretical density of silicon carbide,
amorphous and/or crystalline free carbon-coated sintered
polycrystalline silicon carbide wherein said carbon has a
density greater than 80% of the theoretical density of
graphite and wherein said polycrystalline silicon carbide
has a den~ity greater than 80% of the theoretical density
of silicon carbide, amorphous and/or crystalline free
carbon having a density greater than 80% of the
theoretical den~ity of graphite and mixtures thereof.
RD-15,7~3
~2~ 5
For chemical compatibility reasons, silicon nitrid~
powder would be particularly preferred as an embedding
powder for a molded body of silicon nitride. With
rçspect to a molded body of mullite, an embedding po~d~r
of A1203 or SiO2 is useful but an embedding powder of
mullite would be preferred.
Baking of the embedded molded body is carried
out under a vacuum or in an atmosphere which has no
significant deleterious effect thereon.
The molded body is baked at a heating rate or
on a time-temperature schedule which removes the binder
without imparting significant defect to the body up to a
temperature of 450C. During baking, the binder
evaporates and/or thermally decomposes and is removed
predominantly as a vapor. The baking should not
introduce any significant defect, i.e. any
service-limiting defect, such as, for example, cracks,
voids and pits to the resulting baked and/or sintered
body.
The allowable average heating rate or schedule
to remove binder to produce parts without serious defect
is dependent on the size, shape and especially the
maximum cross-sectional thickness of the molded part.
Faster average heating rates are acceptable for thinner
parts, and slower average heating rates are necessary for
thicker cross-sectional thicknesses. More specifically,
the average heating rate to remove binder is inversely
proportional or substantially inversely proportional to
the maximum cross-sectional thickness of the molded
piece. Therefore, for the range o useful products of
commercial interest, the average heating rate to remove
binder can range from about 0.1C/hr to about 400C/hr.
As an example, the following co~ditions of binder removal
~D-1-,753
~Z7~S
are specific for a part with a maximum cross-sectional
thickness of ~.45 inches. The temperature is ramped from
room temperature to 400~C at ~4C/hr, held ~t 400C for
24 hours and then furnace cooled to room temperature.
If desired, the baked body can be additionally
heated to impart additional mechanical strength thereto
Such strength-imparting heating should have no
significant deleterious effect on the body. Such
strength-imparting heating can be carried out at a
temperature higher th~n about 1000DC at a heating rate
which does not cause thermal shock, usually no greater
than about 1000~C per hour.
The baked body is recovered from the embedding
powder and densified to produce the present
polycrystalline ceramic body.
The present polycrystalline ceramic body has a
porosity less than about 20% by volume, preferably less
than about 10% by volume and most preferably less than
about 5% by volume of the total volume of the densified
body. Porosity is the percent by volume of the densified
body occupied by voids, i.e. pores, and can be determined
by liquid displacement and/or metallographic procedures.
The pores are distributed throughout the body.
The present invention makes it possible to
fabricate complex and/or hollow shaped articles of a
polycrystalline ceramic as well as simple shaped
articles. Thus, articles such as gas turbine air foils,
crucibles, thin-walled h~llow tubes, long rods, spherical
bodies and nozzles can be produced directly by the
present invention.
~,,4~ Canadian S.N. 4 ~ ,"THERMOPLASTIC MOLDING OF
SINTERABLE SILICON CARBIDE" filed for G.M. Renlund
and C.-A. Johnson on ~ ~ /O~
14
lZ~45 RD 15,703
and assigned to the assignee hereof and
discloses a thermoplastically moldable ceramic
composition comprised of from about 40% to about
60% by volume of a sinterable silicon carbide
powder and a binder comprised of an organic acid
and a copolymer of ethylene and from greater than
about 12 weight % to about 33 weight % vinyl acetate,
said organic acid having a melting point ranging from
about 44C to about 88C and ranging from greater than
about 18% by weight up to about 45% by weight of the
binder. The ceramic composition is thermoplastically
molded into a body which is baked to remove the
binder and then sintered.
United States Patent Number 4,530,808,
entitled: "BINDER REMOVAL EROM THERMOPLASTICALLY
FORMED SiC ARTICLE", issued July 23, 1985 to
G.M. Renlund and C.A. Johnson and assigned to
the assignee hereof and discloses a method of
producing a sintered silicon carbide body which
comprises forming a thermoplastically moldable ceramic
composition comprised of~sinterable silicon carbide
powder and binder, thermoplastically molding the
ceramic composition into a body, embedding the body
in nominally spherical particles selected from the
group consisting of polycrystalline silicon carbide,
carbon-coated polycrystalline silicon carbide,
dense free carbon and mixtures thereof, baking the
embedded body to remove the binder therefrom,
recovering the baked body and sintering the baked
body.
United States Patent Number 4,551,436,
entitled: "FABRICATION OF SMALL DENSE SILICON CARBIDE
SPHERES" issued November 5, 1985 to C.A. Johnson;
G.M Renlund; C.E. VanBuren and S. Prochazka and assigned
to the assignee hereof and discloses the production of
small dense silicon carbide spheres ranging in average
RD-15,703
3~Z74~
diameter from about lO microns to 5000 microns by spray
drying or tumbling a sinterable silicon carbide powder
producing spherical agglomerates thereof and sintering
the agglomerates.
The embedding particles of spherical or
nominally spherical polycrystalline silicon carbide and
carbon coated polycrystalline silicon carbide are
produced according to the disclosures of U.S. Patents
~ 3 0, ~o ~ and ~ 3 ~ -
The invention is further illustrated by the
following Examples which, unless otherwise noted, were
carried out as follows:
The melt index was according to ASTM D-1238.
The sinterable silicon carbide powder was a
substantially homogeneous dispersion, i.e. mixture, with
an average particle size which was submicron and which
was comprised of ~-silicon carbide, free uncombined
carbon in an amount of about 1.0% by weight of the
silicon carbide and elemental boron in an amount of about
0.5% by weight of the silicon carbide. The powder
contained less than about 0.4% by weight of oxygen.
E~AMPLE l
The binder was comprised of stearic acid and a
thermoplastic copolymer of ethylene and 25 weight % vinyl
acetate. The copolymer was sold under the trademark
"Elvax 350", had a melt index of l9 and softened at about
90C. The ~tearic acid was present in an amount of 30%
by weight of the total amount of binder.
Fifty volume % of the sinterable silicon
carbide powder was admixed with 50 volume % of the binder
to produce a substantially homogeneous mixture.
Specifically, the sinterable silicon carbide powder along
RD-15,703
~LZ7~4~
with the stearic acid and ethylene-vinyl acetate
copslymer were mixed in a Sigma-blade mixer at ambient
pressure for about an hour at a temperature of roughly
about 120C to about 140C and then continued mixing for
about 10 minutes under a vacuum of approximately 29
inches of Hg to remove air bubbles therefrom producing a
substantially homogeneous mixture. The resulting mixture
was placed on a sheet of aluminum foil, chopped into
pieces, and allowed to cool to room temperature.
The chopped mixture, i.e. thermoplas~ically
moldable ceramic composition, was a substantially
homogeneous mixture of the binder and sinterable silicon
carbide powder. It was injection molded in a 100 ton
injection molding press. The press was provided with a
barrel and nozzle for heating the material and a sprue
bushing through which the hot thermoplastic material was
passed into a die shaped to give a molded part in the
form of a rotor of complex shape weighing about 150
grams. The barrel and nozzle were preheated to 130C,
- 20 the sprue bushing was preheated to 70C and the die was
preheated to 50~C.
The mixture was placed in the barrel where it
was heated for about 15 minutes until it reached uniform
temperature. The resulting molten mixture was then
forced under pressure through the sprue bushing into the
die filling the die where its residence time was about 3
minutes allowing it to solidify. The inje~tion molding
pressure ranged up to about 10,000 psi. The resulting
molded body was removed from the die. It had a maximum
cross-sectional thickness of ~0.65 inch.
The molded body appeared free of visual
defects. ~t was totally immersed in an embedding powder
comprised of spherical or nominally spherical particles
~D-15,703
~:74~
of sintered polycrystalline silicon carbide which had an
average diameter of approximately 50 microns and a
density greater than 90% of the theoretical density for
silicon carbide. The spherical particles were comprised
of silicon carbide, about 0.5% by weight boron and about
1.0% by weight free carbon, based on ~ilicon carbide.
The embedding particles were produced according to the
:r ~ ~ disclosure of U~S. Patent ~,55~'~3G issued ~ ~he~ ~s'
for "FABRICATION OF SMALL DENSE SiC SPHERES" by forming
spray dried spherical or nominally spherical agglomerates
containing ~-SiC, about 0.5% by weight boron and about
1.0% by weight free carbon, based on silicon carbide, and
sintering in ~2 atmosphere helium at about 2080C.
The resulting embedded structure was baked
under a vacuum ranging from about 5 millitorr to about
100 millitorr at a heating rate of 1C per hour to about
400C, held at about 400C for 24 hours and then
furnace-cooled to about room temperature. The resulting
baked body was recovered from the embedding powder and
appeared free of defects.
The baXed body was sintered in an atmosphere
comprised of about ~ atmosphere of helium at about 2080C
for 30 minutes and then furnace-cooled to room
temperature. The sintered body had a density greater
than 95% of the theoretical density for silicon carbide,
i.e. it had a porosity of less than about 5% by volume of
the sintered body, and had a substantially uniform small
~rained microstructure. The sintered body appeared free
of defects and would be useful as a rotor.
EXAMPLE 2
This example was carried out in substantially
the same manner as disclosed for Example 1 except that
18
RD-15,7~3
~:79~5
the sinterable silicon carbide powder along with the
stearic acid and ethylene-vinyl acetate copolymer were
placed in a one liter bowl and mixed at ambient pressure
in a Haake mixer with cam rotors at approximately 110C
for approximately 15 minutes. The resulting hot mixture
was transferred to a one quart Sigma-blade mixer and
mixed at roughly 90C for about 10 minutes at ambient
pressure and then continued mixing for about ten minutes
under a vacuum of roughly about 29 inches Hg to remove
air bubbles therefrom producing a substantially
homogeneous mixture.
In this example, the barrel and nozzle of the
injection molding press were preheated to 180C. The
resulting molded body was immersed in the enbedding
powder and was heated at a rate of 1C per hour to about
400C, held at about 400C for 24 hours, then heated at
10C per hour to about 500C and then it was
furnace-cooled to room temperature.
The resulting sintered body had a density
greater than 95% of the theoretical density for silicon
carbide, i.e. it had a porosity of less than about 5% by
volume of the sintered body, and had a substantially
uniform small grained microstructure. The sintered body
appeared free of defects and would be useful as a rotor.
EXAMPLE 3
This example was carried out in substantially
the same manner as disclosed for Example 2, except that
the ethylene-vinyl acetate copolymer contained 12 weight
% vinyl acetate. This copolymer was sold under the
trademark "Elvax 6S0. ?1
Some surface cracks were seen in the baked
body. The resulting sintered body showed the same
19
R~-15,703
~'~7~
surface cracks seen in ~he baked body but no additional
defects were visible.
Based on other experiments and past experience,
it was determined that these surface cracks were due to
the complex shape of the part and its relatively large
size, and that a higher molding pressure and/or
temperature, or an ethylene-vinyl acetate copolymer
containing more than 12 weight % vinyl acetate, would
have produced a baked and sintered part free of these
surface cracks, i.e free of any significant defect.
EXAMPLE 4
The binder was comprised of a copolymer of
ethylene and 25 weight % vinyl acetate and had a melt
index of 19 and stearic acid in an amount of 30% by
weight of the total amount of binder.
55 volume % of the sinterable silicon carbide
powder was admixed with 45 volume % of the binder
producing a substantially homogeneous mixture.
Specifically, the sinterable silicon carbide powder along
with the stearic acid and ethylene-vinyl acetate
copolymer were mixed in a Sigma-blade mixer at roughly
about 120C to about 140C at ambient pressure for about
10 minutes and then continued mixing for about 10 minutes
under a vacuum of approximately 29 inches of Hg to remove
bubbles therefrom producing a substantially homogeneous
mixture.
The resulting mixturs was placed on a sheet of
aluminum foil, chopped into pieces, and allowed to cool
to room temperature.
The chopped mixture, i.e. thermoplastically
moldable ceramic composition, was a substantially
homogeneous mixture of the binder and sinterable silicon
7 ~ RD-15,703
carbide powder. It was injection molded in an injection
molding press which was of substantially the same t~pe a~
disclosed in Example 1, except that it was smaller and
produced a molded part in the form of a turbine blade
S weighing about 10 grams. The barrel and nozzle were
preheated to 160C, and the die was preheated to about
50C.
The mixture was placed in the barrel where it
was heated for about 15 minutes until it reached uniform
temperature. The resulting molten mixture was then
force~ under a pressure through the sprue bushing into
the die filling the die where its residence time was a
few minutes, long enough to allow it to solidify. The
injection molding pressure ranged up to about 6,000 psi.
The resulting molded body was removed from the die. It
had a maximum cross-sectional segment of about 0.35
inches.
The molded body appeared free of visual
defects. It was totally immersed in an embedding powder
comprised of 50-200 mesh coconut charcoal. The resulting
structure was baked under a vacuum ranging from about 5
millitorr to about 100 millitorr at a heating rate of 4C
per hour to about 400C, held 24 hours at about 400C,
and then furnace-cooled to about room temperature. The
resulting baked body was recovered from the embedding
powder and did not show any significant defects.
The baked body was sintered in an atmosphere
somprised o about ~ atmosphere of helium at about 2080C
for 30 minutes and then furnace-cooled to room
temperature. The sintered body had a density of greater
than 95~0 of the theoretical density for silicon carbide,
i.e. it had a porosity of less than about 5% by volume OI
RD-15,703
the sintered body, and had a substantially uniform
small-grained microstructure.
The sintered body appeared free of defect and
would be useful for high temperature structural
applications such as gas turbine blades.
~XAMPLE 5
This example was carried out in substantially
the same manner as disclosed for Example 4 except that
45% by weight stearic acid was used.
The baked body exhibited some small surface
cracks. The resulting sintered body had a density
greater than 95% of theoretical density and showed the
same small surface cracks as in the baked body, but no
additional defects were visible. Based on other
experiments and past experience, it was determined that
an amount of stearic acid less than about 45% by weight
would have produced a sintered body free of the observed
surface cracks, i.e. a sintered body with no significant
defect.
EXAMPLE 6
This example was carried out in substantially
the same manner as disclosed for Example 4 except that
18% by weight stearic acid was used.
No visible defects were seen in the molded body
but the baked body exhibited some small bubbles.
The resulting sintered body showed the same
bubbles but no additional defects were seen. Based on
other experiments and past experience, the small bubbles
seen in the baked and sintered bodies indicated that an
amount of stearic acid greater than about 18% by weight
would have produced a baked body as well as a ~intered
RD-1~,703
~Z~6~5
body which would have been free of such bubbles, i.e.
which would have no significant defects.
Examples 7, 8 and 9 were carried out in
substantially the same manner as disclosed for Example 4
except as indicated herein and in Table I.
In Example 8, the polyisobutene was a
thermoplastic polymer with an average molecular weight of
100,000 and a volatization temperature lower than 450C.
In Example 9, the binder was compris~ of a
wax composition comprised of 30.3~0 by wt1~-21 wax
(melting point 53-57C), 30.3% by wt P-22 wax (melting
point ~63C), 30.3% by wt ceresin (melting point
73-78C~, 5.0% by wt oleic acid and 4.1% by wt aluminum
stearate. In Example 9, mixing was carried out by
mechanically stirring the binder components for a few
hours above their melting point and then mixing the
resulting binder with the sinterable silicon carbide
powder in a Sigma-blade mixer at about 100C under a
vacuum of about 29 inches of Hg for about 10 minutes.
The binders of Examples 7 and 9 caused the
production of sintered bodies which were not useful.
The binder of Example 8 produced a baked body
with internal voids which was useless and which would
have resulted in a useless sintered product.
The examples are illustrated in Table I where
~xamples 1, 2 and 4 of Table I illustrate the present
invention.
~.27~ RD 15,703
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p~-1~,703
~746~5
EXAMPLE 10
This example illustrates extrusion molding.
The chopped thermoplastically moldable cerami-
composition was the same as disclosed in Example 1.
Conventional extrusion molding equipment was
used which was preheated to a temperature of about 90C
to 100C.
The present thermoplastic ceramic composition
was extruded into the form of a solid rod about 0.15 inch
in diameter and about 11 inches long.
To carry out the baking, a supporting graphite
substrate with an open groove therein for supporting the
rod was used. The preform, i.e. molded rod, was
positioned in the groove, covered with 50-200 mesh
coconut charcoal and baked under substantially the same
conditions as disclosed in Example 4.
The charcoal was then removed, and the baked
rod supported in the groove of the graphite substrate was
sintered in substantially the same manner as disclosed in
Example 4.
The resulting sintered rod had a density
greater than 95% of the theoretical density for silicon
carbide, i.e. it had a porosity of less than about 5% by
volume of the sintered body, and appeared free of
defects.
EXAMPLE 11
The procedure and materials used in this
Example were the same as disclosed in Example 10 except
that the thermoplastic molding composition was extruded
into a preform which was a hollow tube about 5 inches
long with about a 1/~ inch inner diameter and about a 5/8
inch outer diameter.
~D-~-,703
~Z7~
The resulting hollow sintered tube had a
density greater than 95% of the theoretical density for
silicon carbide, i.e. it had a porosity of less than
about 5~ by volume of the sintered body, and appeared
free of defects.
EXAMPLE 12
This example illustrates blow molding.
An extruded preform was produced as disclosed
in Example 11, i.e. a hollow tube about 5 inches long
with about a l/2 inch inner diameter and about 5/8 inch
outer diameter.
The central portion of the preform was
positioned within a glass tube which had an inner
diameter of about one inch. A clamp was placed around
the outer diameter of one end of the preform which closed
that end to air flow. A second clamp was placed around
the outer diameter of the opposite end of the preform
which left that end open to air flow. Both clamps
prevented expansion of the end portions of the preform.
The preform was heated in air to about 80C to
100C. Compressed nitrogen, roughly 5 to 1~ psi gauge,
was forced through the end of the hot preform open to air
flow causing the central portion of the preform to expand
and such expansion was limited by the supporting glass
tube.
The compressed nitrogen was then removed, the
preform cooled to ambient temperature, the clamps were
then removed and the preform was removed from ~he glass
tube.
The resulting blow molded preform was baked
under substantially the same conditions set forth in
Example 4. The resulting baked body was sintered in
26
127464~ RD-15,703
substantially the same manner as disclosed in Example 1.
The resulting hollow sintered tube had a density greater
than 95% of the theoretical density for silicon carbide,
i.e. it had a porosity of less than about 5% by volume of
the sintered body, and its central portion had a diameter
significantly larger than its outer open end portions.
EXAMPLE 13
This example illustrates roll-forming of the
present thermoplastically moldable ceramic composition
into the form of a sheet. The roll-forming e~uipment was
conventional equipment used in the plastics industry for
forming sheet.
The thermoplastically moldable ceramic
composition was the same as disclosed in Example 1.
The rolls were made of steel, set with an
approximately .005 to 0.010 inch gap therebetween and
preheated to about 100C.
The present thermoplastic ceramic composition
was rolled through the gap producing a molded sheet
ranging from about .005 to about .010 inches in
thickness.
The molded sheet ~was immersed in 50-200 mesh
charcoal powder and baked in substantially the same
manner as disclosed in Example 4.
The baked sheet was rec~vered and sintered in
substantially the same manner as disclosed in Example 1.
The resulting sheet had a density greater than
95% of the theoretical density for silicon carbide, i.e.
it had a porosit~J of less than about 5% by volume of the
sintered body, and would be useful as substrates.
EXAMPLE _
RD-l~,703
~7~4S
In this example the ceramic powder was a-A1203
with an average particle size which was submicron. The
binder disclosed in Example 1 was used in this example.
A uniform mixture comprised of the a A1203
S powder and binder was produced in substantially the same
manner as disclosed in Example 1 except that the
resulting mixture contained 47% by volume solids, i.e.
the a-A1203 powder.
A conventional single screw extruder was used.
The resulting chopped mi~ture of binder and ~-A1203 was
used as feed material for the extruder and was extruded
at a nozzle temperature of about 100C forming a hollow
tube with an outer diameter of about 0.2a inch and an
inner di~meter of about 0.11 inch. The extruded tube was
formed into a three foot long section and was cut into
lengths 4 inches long.
A 4" length of the extruded hollow tube was
totally immersed in 50-200 mesh charcoal embedding powder
and baked in substantially the same manner as disclosed
in Example 4. The resulting baked tube was recovered
from the embedding powder and did not show any
significant defect.
The resulting baked tube was sintered in air at
ambient pressure at about 1650~C for about 30 minutes and
2S then furnace cooled to ambient temperature.
The resulting sintered hollow tube appeared
free of defects and had a porosity of less than about 1%
by volume o the sintered body. It would be useful as
protective tubing.
EXAMPLE 15
In this example the ceramic powder was
comprised of aluminum nitride with a surface area of
~7~ 5 RD-15,703
about 4m /gram and 3% by weight Y203. The binder
disclosed in Example 1 was used in this example.
The aluminum nitride, Y203 and binder were
mixed in a Brabender mixer with roller blades a~ about
100C in air producing a substantially uniform mixture.
The resulting mixture was placed on a sheet of aluminum
foil, broken into pieces and allowed to cool to room
temperature. The mixture was comprised of 45% by volume
of ceramic powder and the balance was binder.
In this example, thermoplastic molding was
carried out in a die with a 3 inch inner diameter with
the faces of the die punches covered with three mil Mylar
to prevent sticking. The die was preheated from about
80C to 100C.
A portion of the resulting mixture of binder
and ceramic powder was placed in the die and pressed,
i.e. thermoplastically molded, under a pressure of about
lO00 psi to about 5000 psi. The resulting molded body,
i.e. disc, had a diameter of 3 inches and a thickness of
about .010 inches.
The disc was totally immersed in 50-200 mesh
charcoal and baked under a vacuum ranging from about 5
microns to about lO0 microns from 20C to 4209C in a
linear ramp of 8C per hour, held at 420C for 24 hours
and then furnace cooled to ambient temperature. The
baked disc was recovered from the charcoal, and a portion
thereof placed on a tungsten setter and heated in a
flowing nitrogen atmosphere at ambient pressure to
1900C. It was held at 1900C for about 60 minutes and
then furnace cooled to ambient temperature.
The resulting sintered body appeared free of
defects and had a porosity of less than about 1% by
29
~ Z~ RD-15,7~3
volume of the sintered body. It would be useful as a
substrate.
" _
