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The present invention relates to a method for making infiltration products
of reaction of molten silicon and blends of particulated carbon and a
particulated inorganic material, such as boron nitride particles. More
particularly, the present invention relates to readily machinable silicon
carbide compositions, and composites of such compositions as a contiguous
layer on a silicon carbide base structure.
As shown in copending application Ser. No. 572,969, filed Apr. 30, 1975, of
William Laskow and Robert Morelock, there is described a method for making
silicon carbide-silicon matrix composites by the infiltration of carbon
fibers by molten silicon. Although the resulting silicon carbide-silicon
matrix materials and method for making such shaped structures represented
a significant advance in manufacture of high performance shaped ceramics,
it has been found that in many instances the impact resistance of the
shaped ceramic structures is not sufficiently high to qualify them for a
variety of uses.
The present invention is based on the discovery that readily machinable
reaction products also based on molten silicon infiltration can be made by
effecting molten silicon infiltration into a substantially uniform blend
of particulated carbon and a finely divided inorganic material
substantially nonreactive to molten silicon, such as boron nitride
particles.
In addition to the above-described machinable reaction products based on
molten silicon infiltration, there is also provided by the present
invention, silicon carbide refractory composite structures having
substantially improved impact strength over shaped silicon carbide
refractories of the prior art. These composites of silicon carbide or
silicon-silicon carbide can have a refractory base structure, and an
exterior contiguous layer of the above-described machinable reaction
product based on molten silicon infiltration. The silicon carbide
composites of the present invention can be made by introducing molten
silicon into a mold whereby the contiguous machinable layer in contact
with the base structure is formed in-situ. The latter silicon carbide base
structure moreover may be formed simultaneously with the contiguous layer
by molten silicon infiltration of the base structure in the form of a
carbon fiber preform.
Further features of the present invention can be seen from the drawings,
where in FIG. 1 there is shown a mold filled with a mixture of carbon
particles and a particulated inorganic material, such as boron nitride,
and above the mold there is shown silicon powder or granules in contact
with carbon fiber wicks constituting a molten silicon source for
infiltration into the mixture of carbon particles and particulated
inorganic material.
An additional aspect of the present invention can be seen by FIG. 2,
showing how a silicon carbide composite can be formed in accordance with
one method of the present invention based on the infiltration by molten
silicon into a mold cavity containing a refractory base structure in
contact with a contiguous preform consisting of a mixture of carbon
particles and a particulated inorganic material. The source of the silicon
is contained in the space above the mold cavity.
Another feature of the present invention can be seen from FIG. 3, where
there is shown silicon in contact with carbon wicks above a mold. The mold
is filled with a carbon fiber preform which is surrounded by a contiguous
preform of a mixture of particulated carbon and a particulated inorganic
material.
An additional aspect of the present invention can be seen from FIG. 4,
showing the use of a multilayer contiguous preform on a base structure to
produce a further modification of the composite of the present invention.
As shown by FIG. 1, there is provided by the present invention, machinable
castings having a density of from 1.6 g/cm.sup.3 to 2.7 g/cm.sup.3 which
are the infiltration products of reaction of molten silicon and a
substantially uniform mixture comprising by volume:
(A) from 45% to 90% of particulated carbon having up to an equal proportion
by volume based on the volume of (A), of silicon carbide particules, and
(B) from 10% to 55% of a particulated inorganic material substantially
inert to molten silicon at temperatures up to 1600.degree. C. having an
average particle size of from 0.1 to 2000 microns and a Mohs hardness
value falling within the range of 1-7.
The machinable castings of the present invention can be initially molded
into any desired configuration and thereafter cut by conventional means.
These relatively light weight silicon carbide containing materials
resulting from the infiltration by molten silicon into the mixture
containing carbon particles can be cut with a steel saw, drilled, sanded,
filed, etc., to any desired shape. If desired to be used as a protective
impact resistant coating or layer, the machinable castings can be welded
at a self supporting thickness onto a silicon or silicon carbide substrate
to improve the substrate impact strength. This procedure can be used as an
optional alternative to the procedure shown by FIG. 2, where the
machinable layer is cast in-situ.
In addition to the above described uses, the machinable castings of the
present invention can be cut to a thickness of from 0.01 to 1 or more
inches and used as thermal gradient barriers on various substrates where
attachment can be achieved by mechanical insertion, bolting, etc.,
combustors in such applications as diffusers, transition pieces, etc.
Other uses are, for example, molds for metal casting, gas burner
components, tooling, lap surface plates, and high temperature fixtures.
There is also provided by the present invention a method for making a
silicon carbide composite, comprising a refractory base structure and a
contiguous machinable or compliant layer structure, which comprises,
(1) introducing molten silicon into a mold substantially filled with a
composite having a base structure, and a contiguous exterior layer, where
the base structure is a member selected from
(i) a shaped mass of silicon carbide,
(ii) a composite of silicon and silicon carbide, and
(iii) a carbon fiber preform and the contiguous exterior layer comprises a
uniform mixture of particulated carbon and a particulated inorganic
material substantially non-reactive to molten silicon and having a Mohs
hardness in the range of from about 1 to about 7,
(2) allowing the molten silicon to fully infiltrate into said mold, while
allowing gases of reaction to vent therefrom and
(3) removing the resulting silicon carbide composite from the mold.
Included by the types of particulated carbon which can be used in making
the machinable castings of the present invention, or the contiguous layers
on the composites of the present invention are carbon fiber or graphite
fiber, carbonized plant fibers, lamp black, fine divided coal, wood,
charcoal, etc. The particulated inorganic material which can be employed
in combination with the carbon particles includes, for example, boron
nitride, aluminum oxide, magnesium oxide, silicon nitride, etc., having an
average aggregate size of from 1 to 2000 microns. It is understood that
individual crystallites or subparticles which may comprise the aggregates
can be substantially smaller.
The machinable castings of the present invention also can include the
reaction product of molten silicon and the mixture of carbon fiber, or
graphite fiber, or mixtures thereof, and the aforedescribed inorganic
material which mixture can also comprise up to 50% by volume of other
fillers, such as silicon carbide whiskers, or other particulated forms of
silicon carbide. In addition to silicon carbide, other filler materials
also can be used which are substantially nonreactive to molten silicon and
include, for example, aluminum oxide, or zirconium oxide filaments. It may
be advantageous to coat such filler materials with carbon, for example, by
pyrolytic deposition from carbonaceous gas or gas mixture. The carbon
coating promotes wetting and provides a chemical barrier between the oxide
and the filament. The mixture of carbon fiber and inorganic material, such
as boron nitride, can be present in the mold either in the form of a free
flowing powder, or as a rigid preform which can be made by mixing together
the inorganic material and optionally any other filler with a bonding
agent for graphite, or carbon fiber, such as the graphite suspensions
available from the Dylon Company. In accordance with the method of the
present invention, shaped parts made from the carbon fiber and inorganic
material containing mixture have improved impact resistance and
abradability.
In FIG. 1, there is more particularly shown apparatus for making the
machinable castings of the present invention. There is shown a side view
of a mold support at 10 and a powdered silicon charge at 11. Infiltration
by molten silicon into the mixture of particulated carbon and particulated
inorganic material at 13 can be effected at temperatures of 1400.degree.
C. to 1700.degree. C. through wicks at 12 which can be carbon fibers, such
as WYK braid, or WYB tow of Union Carbide typical lengths being 3 CM. At
14 and 15 hot gas vents are provided to relieve build-up of mold pressure.
Melting of the powdered silicon can be effected by placing the apparatus of
FIG. 1, into a suitable furnace. If desired, heating coils can be used to
surround the powdered silicon source. A mold release agent, such as boron
nitride, can be sprayed onto the inside walls of the mold, as shown in the
application of William B Hillig, Ser. No. 419,286, filed Nov. 27, 1973,
now abandoned and assigned to the same assignee as the present invention.
There is further shown in FIG. 2, a side view of a mold having a base part
of silicon carbide refractory at 23, which includes silicon-silicon
carbide refractories. There is shown at 22, contiguous self-supporting
preforms of the mixture of carbon particles and particles of the inorganic
material as previously defined. If desired, the contiguous preforms can be
made in a variety of shapes from pastes of the mixture of particulated
ingredients by standard techniques using Dylon as previously indicated.
Again, infiltration of molten silicon from the powdered source at 20
through carbon wicks at 21, can provide the in-situ formation of a
composite of the refractory base and a machinable, compliant contiguous
layer.
At FIG. 3, there is shown an additional way of making a composite by an
in-situ procedure to form a refractory silicon-silicon carbide base and a
compliant machinable exterior layer. A molten silicon source is shown at
30 and carbon fiber wicks at 31. Contiguous layers of the mixture of
carbon particles and particles of inorganic material are shown at 32. At
33, there is shown a carbon fiber preform which can be fabricated by
standard techniques.
As used hereinafter, the term carbon fiber or filaments includes
commercially available carbon fiber as previously defined. The carbon
fiber includes, for example, "high strength" graphite having a tensile psi
of typically 250,000 psi, a modulus of 20.times.10.sup.6 psi and a
carbonized density of 1.6 g/cc, as shown by Johnson et al. U.S. Pat. No.
3,412,062. Preferably, the carbon fiber has a specific gravity of about
1.3 to 1.5 and includes, for example, WYK braid, WYB tow of Union Carbide
Corp., and other carbonized fibers, such as carbon felt. In addition to
carbonized rayon fibers, any carbon fibers having a specific gravity as
defined above derived from polymeric or natural organics, such as
polyacrylonitrile, polyacetylene, such as shown by Krutchen U.S. Pat. No.
3,852,235, assigned to the same assignee as the present invention,
polyvinyl chloride, polyvinyl acetate, etc., can be employed. The term
"preform," as used hereinafter, is preferably a shaped structure of
oriented carbon fibers, such as a pre-preg. To make a preform, a carbon
fiber tow, braid or cloth is treated with molten wax or other binder, such
as cellulose nitrate, colloidal graphite, etc.
More particularly at FIG. 4, there is shown a base structure at 44, in
contact with a multi-layer contiguous compliant exterior structure at
42-44. The base structure can consist of a silicon carbide refractory, as
shown by FIG. 2, or a carbon fiber preform, as shown in FIG. 3. The
contiguous layer structure can consist of a preform mixture of carbon
particles and particles of inorganic material at 42, a middle layer of
carbon fiber or carbon sheet at 43, and another preform similar to 42 and
44. Improved impact strength and toughness can be imparted to the
machinable contiguous layer and the base layer upon infiltration by molten
silicon.
In addition to the above described machinable castings, there is also
provided by the present invention, composites comprising,
(C) a contiguous machinable exterior layer, and
(D) a refractory base structure, where the contiguous machinable exterior
layer is the infiltration product of reaction by molten silicon and a
substantially uniform mixture comprising by volume,
from 45% to 90% of particulated carbon having up to an equal proportion by
volume, based on the total volume of the mixture of silicon carbide
particles, and
from 10% to 55% of a particulated inorganic material, substantially inert
to molten silicon at temperatures up to 1600.degree. C., having an average
particle size of from 0.1 to 2000 microns, and a Mohs hardness value
falling within the range of 1-7, and
the refractory base structure is a member selected from the class of shaped
silicon carbide refractories and the infiltration product of reaction by
molten silicon and a carbon fiber preform.
In accordance with the method of the present invention, the above-described
composites of the present invention can be fabricated to gas turbine
shroud sections, aircraft engine shroud sections, gas turbine transition
pieces, diesel engine pistons and rings, heat exchange pipes, hot
processing dies, combustion liners, fusion reactor hardware, wear
resistant tiles, etc.
As previously indicated, in making the above composites of the present
invention, the contiguous machinable layer can be applied onto the base
structure either directly or by in-situ formation based on molten silicon
infiltration. For example, the contiguous layer at thicknesses of from
0.01" to 1" or more can be fabricated onto an appropriate base structure,
where contiguous layer thickness may correspondingly vary from 0.1 to 100
times the thickness of the base structure.
In order that those skilled in the art will be better able to practice the
invention, the following examples are given by way of illustration and not
by way of limitation. All parts are by weight, unless otherwise specified.
EXAMPLE 1
A mixture of 25% by weight or 20% by volume of boron nitride obtained from
the Carborundum Company, designated SHP-40 and 75% by weight or 80% by
volume of a graphite suspension obtained from the Dylon Company,
designated grade AE was blended to a thick paste consistancy. The paste
was then molded to a flat rectangel having a 1/8" thickness and allowed to
dry in the air.
A mold was fabricated from Armco Speer 580 graphite having an initial
thickness of about 3/4" by machining it to produce a mold cavity of about
1/8".times.1/8".times.3". The inside of the mold cavity was then coated
with a boron nitride aerosol. The above blend of boron nitride and
graphite was then cut to size and inserted into the mold. A carbon
crucible having an inside diameter of 11/4" and about 2" high having a
1/8" diameter hole drilled at the bottom and a carbon fiber wick extending
through the hole was placed on top of the mold. The carbon fiber wick was
WYK braid of Union Carbide and it extended about 0.125 inches above the
top of the hole and also touched the above-described blend in the mold.
The crucible was then charged with solid silicon pieces employing about
15% excess of the amount of silicon required to fill the mold cavity in
the molten state. The assembly was transferred to a resistance furnace in
which a vacuum of 1.times.10.sup.-2 torr would be achieved. The assembly
was heated to a temperature of about 1600.degree. C. It was found that the
mixture in the mold reacted immediately with the molten silicon. The mold
assembly was kept in the furnace for about 15 minutes after reaching
1600.degree. C. The mold assembly was then allowed to cool and the
resulting part was removed from the mold. There was obtained a cast part
having the same dimensions as the original blend of boron nitride and
graphite.
Based on method of preparation, the cast part was the infiltration product
of reaction by molten silicon and a mixture of 25% by weight of boron
nitride and 75% by weight of graphite which expressed in terms of volume
is about 20% by volume of boron nitride and 80% by volume of graphite. The
density of the casting was found to be 2.1 g/cm.sup.3. The casting was
then placed in a vice and cut with a steel saw to produce a specimen
1/8".times.11/2".times.1/8". It was found that the casting was readily
machinable; the remaining portion of the casting could be readily filed
out with a steel file.
A 1".times.2".times.1/8" flat plate of the above machinable casting is
placed as a contiguous layer on a 1".times.2".times.1/2" block of silicon
carbide. The resulting composite is then furnace heated in an inert
atmosphere for 15 minutes at a temperature of 1500.degree. C. There is
obtained a composite upon cooling having the machinable casting integrally
welded onto the silicon carbide refractory base.
An impact test was performed to compare the impact strength of a
1".times.2".times.1/2" block of silicon carbide and the above composite.
Impact resistance is determined by striking the surface of a test sample
with a 4.5 mm ball bearing at a velocity of about 200 meters per second
and at an incident angle of about 80.degree.. It is found that the
composite of the silicon carbide having the machinable casting welded on
as a contiguous layer has an impact resistance superior to the impact
resistance of the silicon carbide base structure free of such contiguous
layer.
EXAMPLE 2
Blends of graphite and boron nitride were fabricated in accordance with the
procedure of Example 1 to produce slabs having a thickness of about 1/8".
A mold was then machined out of a Speer 580 graphite, in accordance with
the procedure of Example 1 having a cavity of 1".times.3".times.0.1". The
mold was coated with boron nitride. A silicon carbide part was then placed
in the mold surrounded by the preformed strips of graphite and boron
nitride cut to size which totally filled the mold. As shown in FIG. 2,
three holes having a 0.125 diameter were cut into the top of the mold and
several vent holes were drilled into the bottom of the mold. Into the
holes at the top of the mold there was placed carbon fiber wicks which
protruded about 1/8" above the top of the mold while contacting the boron
nitride-graphite strips within the mold. Particulated silicon was then
charged to the cavity above the top of the mold using about a 15% excess
over that volume required in molten form to fully infiltrate the mold.
As described in Example 1, the mold and the supporting structure were
placed inside of a furnace and heated to a temperature of about
1600.degree. C. Upon being converted to molten silicon, infiltration was
immediate. After about 15 minutes at 1600.degree. C. the mold was allowed
to cool.
Removal of the part from the mold was readily achieved because the mold
surface had been sprayed with boron nitride prior to being filled with the
carbon-boron nitride strips and silicon carbide part. Based on method of
preparation there was obtained a composite of a silicon carbide refractory
base having approximately a 1/8" contiguous layer of the molten silicon
infiltration product of reaction by molten silicon and a mixture of
graphite and boron nitride. It was found that the contiguous layer was
integrally bonded to the silicon carbide refractory base.
The impact strength of the above described composite was substantially the
same as the impact strength of the composite formed in accordance with
Example 1 by welding the piece of the machinable casting onto the surface
of the silicon carbide structure by a furnace treatment. In addition to
improved impact strength, the contiguous layer was found to be readily
machinable as evidenced by being readily abraded by use of a diamond saw
blade which was capable of cutting into the surface of the layer at a rate
of between 0.05 to 0.5" per second. The diamond wheel had a 4" diameter
having a 3/8" width, a diamond grit size of 150 mesh, rotating at 5500
RPM, at a depth of 0.002" and a cutting speed of 0.2" per second, pulling
forces of 90 to 600 grams were used.
EXAMPLE 3
A carbon fiber preform is prepared from low modulus WCA carbon cloth of
Union Carbide Corporation, using an aqueous colloidal suspension of
graphite as a binder. The carbon fiber preform was machined to a shape
similar to that shown by FIG. 3. Following the procedure of Example 2,
strips of boron nitride-graphite mixture which had been molded to a
thickness of about 1/8" were placed in a 1".times.3".times.0.1" mold
previously coated with boron nitride which had been machined from Speer
580 graphite. The space above the mold was then charged with silicon
powder.
Following the procedure of Example 2, infiltration by molten silicon in the
mold resulted in the production of a composite having a silicon-silicon
carbide base structure and a contiguous exterior layer of the reaction
product of molten silicon and a mixture of graphite and graphite-boron
nitride. The impact strength of the aforementioned composite as tested in
accordance with the procedure of Example 1, showed that it exhibited
improved impact as compared to a silicon-silicon carbide refractory part
which was molded from the same mold without the use of the exterior
graphite-boron nitride contiguous layer allowing for the use of a
proportionately larger carbon fiber preform which was sufficient to
completely fill the mold cavity.
EXAMPLE 4
As shown by FIG. 4, a mold was vertically fabricated to accommodate a
carbon fiber preform in the mold adjacent to a multi-layer contiguous
structure consisting of carbon cloth sandwiched in between two strips of a
graphite-boron nitride blend which was fabricated in accordance with
Example 1. Prior to the incorporation of the carbon fiber preform and the
aforementioned contiguous layer structure, the mold surface had been
coated with boron nitride in a standard manner. Molten silicon
infiltration was effected in accordance with the procedure of Example 2.
Upon cooling a composite part was separated from the mold. It was found
that molten silicon infiltration occurred in the contiguous graphite-boron
nitride layers and the carbon fiber preform. Infiltration also occurred in
the intermediate layer of carbon cloth constituting part of the contiguous
layer. It was further found that the contiguous layer on the
silicon-silicon carbide refractory substrate was somewhat tougher and less
machinable than the contiguous layer of Example 2. However, the impact
resistance of the resulting composite is superior to that of Example 2.
Although the above examples are limited to only a few of the very many
variables and compositions which can be employed in the practice of the
present invention, it should be understood that the present invention is
broadly directed to machinable castings as set forth in the broad
description preceding these examples as well as composites of silicon
carbide having a machinable contiguous layer which can be formed in
accordance with the practice of the present invention as well as a method
for making such composites.
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