Note: Descriptions are shown in the official language in which they were submitted.
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SILICON CARBIDE COMPOSIq'E
AND PROCESS FOR PRODUCTION
Background of the Invention
Articles composed of materials having
refractory characteristics, hardness and resistance
to erosion has myriad important uses. Representative
materials are described in U.S. Patent No. 2,938,807,
issued May 31, 1960 to Andersen.
Reaction sintering for B-silicon carbide and
~ -silicon carbide has been known for making high
temperature components. For example, B-silicon carbide
is described as an excellent binder in the Andersen
U.S. patent No. 2,938,807, however, no diamond is
incorporated in this silicon carbide technology.
Another useful component of these materials
would be diamond. Its superior properties of, for
example, hardness have long been appreciated. A
satisfactory means of incorporating diamond into such
articles would be of a significant advantage and such
is an object of the process and product of the present
invention.
A metal is used to bind diamond crystals in
U.S. Patent No. 4,063,909 to Robert D. Mitchell, issued
December 20, 1977. Such metal may be, for example,
Co, Fe, Ni, Pt, Ti, Cr, Ta and alloys containing one
or more of these metals.
The above and other patents in the area of
bonding diamond crystals depend on hot-press technology
as for example described in U.S. Patent No. 4,124,401,
issued November 7, 1978 to Lee et al, U.S. Patent No
4,167,399 issued September 4, 1979 to Lee et al, and
30 U.S. Patent No. 4,173,614 to Lee et al issued
November 6, 1979, all of which patents are assigned
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to the assignee of the present invention.
Many of these problems have been overcome by
the invention disclosed in Canadian Patent Application
Serial No. 381,405, filed July 9, 1981 by John Michio
Ohno.
In brief, that application describes bi-layer
diamond composites having a special binder of B- silicon
carbide. That binder forms a matrix throughout the
composite so as both to hold the diamond crystals and
to unite the composite layers. The composites are
formed by a process comprising:
(a) forming a first dispersion of diamond
crystals and carbon black in paraffin;
(b) forming a second dispersion of carbon
fiber, carbon black and filler in
paraffin;
(c) compacting said dispersions together to
produce an integral bi-layer composite;
(d) subjecting said composite to a vacuum
for a period of time at a temperature
sufficient to vaporize essentially all
of said paraffin;
(e) liquefying said silicon to cause infil-
tration into both layers;
(f) uniting the layers of said composite
with liquid silicon; and
(g) sintering the composite and infiltrated
silicon under conditions sufficient to
produce a B-silicon carbide binder
uniting said composite.
Notwithstanding that invention, however, various
limitations on the construction of shaped diamond
composite useful for these purposes remain. In
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particular, these involve placement of diamond
crystals at desired surface locations.
Introduction to the Invention
The present invention employs diamond crystal,
SiC crystal or other filler crystals, carbon black,
carbon fiber and paraffin to produce bodies with
sintered diamond selectively placed on the lateral
periphery of a composite. Through this preferential
peripheral placement (especially at the cutting edges),
composites having increased wear resistance for reduced
unit costs are obtained.
The composites of the present invention are
prepared by the steps of:
(a) forming a first dispersion of diamond
crystals and carbon black in paraffin;
(b) forming a second dispersion of carbon
fiber, carbon black and filler in
paraffin;
(c) compacting one of said dispersions to
produce a physically stable intermediate
compact;
(d) recompacting said intermediate with the
remaining dispersion to produce a binary
compact;
(e) subjecting said binary compact to a
vacuum for a period of time at a
temperature sufficient to vaporize
essentially all of said paraffin;
(f) infiltrating said binary compact with
liquid silicon; and
(g) sintering the binary compact containing
infiltrated silicon under conditions
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sufficient to produce a B-silicon
carbide binder uniting said composite.
As a result of this process, a bonded composite having
a superior wear re~istance surface layer i9 produced.
That diamond crystal containing surface, held tightly
by a strong silicon carbide bonding matrix, is
particularly suitable as a tooling or cutting edge.
Description of the Drawings
In the drawings,
FIG. 1 is a schematic diagram of the process
of the present invention; and
FIGS. 2-6 are sequential, illustrative
depictions of a preferred approach and particular
apparatus useful in the process of the present
invention.
Description of the Invention
The present shaped composites may have any
of the geometric shapes known for such cutting
utilities. In general, these composites share the
feature that, during use, they are rotated about a
central axis while their circumferential working
sides or edges are oriented either parallel to, or
intersecting, that axis.
Certain preferred embodiments of the present
invention involves some of these shapes. For example,
the composite may have two essentially parallel and
planar surfaces spaced a predetermined distance apart.
These surfaces represent the anterior and posterior
surfaces of the composite; their distance of separation,
its depth. This depth is ordinarily from 0.1 to 0.2 cm.
The periphery of these composites is formed
by sides connecting to edges of the surfaces. These
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sides generally form ~as shown at the edge formed with
a surface) either a circle or a convex regular polygon
(in this last instance, each separate side is
desirably essentially rectangular in appearance).
The sides of neutral cutting inserts are parallel
to an axis normal to the planar surfaces. However,
the saides of positive cutting inserts have a relief
angle, as shown in FIGS. 5 and 6. Therefore, each
separate side is trapezoidal in configuration.
The present process for preparing silicon
carbide composites is diagrammed in representative
manner in FIG. 1. As shown by that diagram, one of
the initial steps involves the formation of a dispersion
of diamond crystals and carbon black in paraffin.
For various reasons, small crystals are
usually employed in this first dispersion. In a
preferred embodiment, the diamonds employed include
crystals having a size less tha~ 400 mesh. Crystals
of this preferred size will, when bonded with ~-silicon
carbide, exhibit superior resistance to chipping. In
addition, they provide sharp edges having desirable
relief angles for cutting inserts and other wear
components.
To the diamond crystals must be added
carbon black. This carbon serves subsequently by
reacting to yield ~-silicon carbide for the bonding
matrix of the present composites. This carbon black
is desirably of high purity to reduce the presence
of contaminents. In particular, its sulfur content
should be low to avoid possible side reactions during
subsequent processing. Although varying amounts of
carbon black are permissible, from 1~ to 3~, most
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preferably about 2~, by weight of diamond has proven
optimum.
The paraffin utilized in the first (or
peripheral) dispersion may be any of the hydrocarbon
waxes encompassed by the common meaning of this term.
Again a high purity hydrocarbon should be employed
to avoid possible harmful residue. For ease of
admixture, a liquid paraffin in employed. This may,
however, be accomplished by operating under a
temperature sufficiently high to melt a paraffin which
is ordinarily solid under ambient conditions. The
amount of paraffin employed is not critical as it is
subsequently removed. It generally constitutes from
3% to 6% by total weight of the first dispersion.
The foregoing constituents may simply be
mixed together to form the first dispersion. A very
intimate and homogeneous dispersion is, however,
preferred. Consequently, a step-wise technique such
as that outlined in the flow diagram of FIG. 1 is
desirable~
In accordance with that technique, the
diamond crystals and carbon black are blended to
permit an even coating of the crystal surfaces.
Only after this step is the paraffin mixed into the
blend. Thereafter, the first dispersion is preferably
subjected to a further step of fining, as by
grinding. However, the admixture of the second
dispersion containing carbon fiber, carbon black,
and paraffin may be passed through a screen of, for
example, about 20 mesh to improve admixture and
reduce any agglomeration which may have occurred.
The paraffin and carbon black utilized in
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the seeond (or eore) disper~ion of the proeess may be
any of these previously deseribed. For convenience,
the same ones are ordinarily utilized in forming both
the first and second dispersions. Generally, the
second dispersion also contains from 3~ to 6% paraffin
and 2% to 4% carbon blaek by weight. The amount of
carbon black, particularly in the first dispersion,
the quality and type of earbon black, are also critical.
For example, sulfur contamination in carbon black
must be avoided.
The carbon fiber employed is desirably of
very small size to facilitate homogenous admixture
and, in particular, the fining operation. The sizes
of fiber are preferably of from 6 to 30 microns in
diameter, and from 250 to 500 microns in length.
The filler is provided to increase bulk
and also to improve the compressibility of the powder
mix containing fiber. It is highly desirable for a
number of applications. Although such a filler may
comprise any material which is stable under the
conditions to which it is subjected during sintering
and use, fine~ or ~silicon carbide is preferred.
Ordinarily, from 40% to 75% of filler by total wei-ght
of the second dispersion is employed.
As is the case in production of the first
dispersion, the paraffin, carbon black, carbon fiber
and filler should be intimately admixed. They are
also desirably screened as previously described to
insure fineness.
Due to the presence of paraffin, each
dispersion is independently carpable of being compacted
(or molded) to desired shape(s). Application of
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pressure provides a compacted dispersion with
sufficient "green strength" or physical stability
to retain its imparted shape during subsequent
operations and/or handling. ~he amount of pressure
applied may vary widely, although at least 2300
kg/cm is preferred.
In the process of this invention one or the
other of the two dispersions is compacted to form that
portion of the composite with which it will ultimately
correspond. This compacted dispersion therefore
constitutes an intermediate compact identical in
shape and volume (but not composition) with a portlon--
such as a core, cutting edge or the like--of the final
composite.
After the intermediate compact has been
formed from one dispersion, it may be recompacted with
the remaining dispersion. For this step, the inter-
mediate compact may be positioned where desired within
a mold having the shape of the desired composite. The
remaining dispersion may then be added to the mode to
complete filling. The application of pressure as
previously described then yields a physcially stable
binary compact which has the same shape as the ultimate
bonded composite.
These alternative routes for the dispersions
are depicted in FIG. 1 by the two sets of dashed
lines. One dispersion must be compacted in each of
the foregoing steps, but their sequence is not
important.
FIGS. 2-6 illustrate in greater detail
a preferred sequence of steps for this operation
of forming a binary compact from the two dispersions.
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Referring to FIG. 2, the apparatus which
may be employed in the subject process includes a
circular mold M which is shown in cross-section
and is mounted on a base ring B. Mold M contains a
tightly fitting, cylindrical plunger Pl which has a
symmetrical end tip 4. ~ue to the difference between
the diameter "d" of the cylindrical bore of mold M
and the diameter of the end tip 4, an annular gap 5
is created. This gap 5 is filled or loaded with a
dispersion containing diamond crystals, and a second
plunger P2 is placed into the bore of the mold M in
abutment with plunger Pl (see FIG. 3). Next, the
apparatus is reversed and plunger P2 is forced
upwardly against plunger Pl and moves from point
Cl to point C2 thereby forming a ring-like inter-
mediate compact, having a peripheral apexe, and
designated by the numeral 1 in FIG. 4.
In the next step of the subject process,
plunger Pl is removed thereby resulting in a central
cavity within the ring-like compact 1, and this
cavity is filled or loaded with the second dispersion.
As shown in FIG. 5, under pressure of a third plunger
P3, the second dispersion froms a core 2 which is
united with the intermediate compact 1 obtained from
the first dispersion.
FIG. 6 illustrates the binary compact 3
after ejection from mold M by advancing the remaining
plunger P3. The compact 3 is physically stable
despite its two strata comprising a peripheral ring 1
formed from the first dispersion and a central core
2 formed from the second.
One thing of great importance in these
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operations is the shape(s) of the mold(s). A
significant advantage of the present invention lies
in the fact that a shape impressed upon a compact
during molding ordinarily need not subsequently be
altered. Thus the time consuming and difficult
steps of finishing to a desired shape, common with
other refractory materials, may be eliminated in
accordance with the present process. The mold(s)
and/or plunger(s) should therefore have the
configuration(s) desired for the ultimate portion
of the body to which the compact or composite corresponds.
Once molded to the desired shape, the binary
compact is (as may be seen in FIG. 1) subjected to
vacuum and temperature conditions sufficient to vaporize
the paraffin from its entire volume. Suitable
conditions are, of course, dependent upon the particular
paraffin present. Generally, however, a pressure of
less than 200~ and temperature of about 500 C are
utilized. Alternatively, another temperature and a
correspondingly varied vacuum may be employed.
The vaporization of the paraffin is
preferably conducted slowly. This avoids, for
example, violent boiling and/or build-up of gaseous
pressure within the composite. Accordingly, conditions
requiring at least 10 minutes and preferably from 10
to 25 minutes for the essentially complete removal
of the paraffin are preferred.
The compact is next infiltrated with liquid
silicon. There must be sufficient elemental silicon
present to permit, under conditions of sintering,
infiltration of silicon to, and reaction with,
substantially all of the carbon black and carbon fiber
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of the compact. There may also be excess silicon.
It is not detrimental if, after sintering, a small
amount of free silicon remains within the resultant
composite. Up to about 14%, preferably from 5% to
12%, excess silicon is even desirable to ensure
substantially complete reaction.
The operation of bonding a compact to create
a composite actually involves a series of steps, all of
which may occur essentially simultaneously. These
steps include melting of the silicon, infiltration of
molten silicon into the compact and reaction of
infiltrated silicon with both the carbon black and
carbon fiber to produce a-silicon carbide through the
resultant composite.
To induce this last set of reactions between
silicon and carbon, a minimum temperature of at least
about 1450C is required. Higher temperatures may
also be utilized. A maximum of about 1490C is,
however, preferred to avoid graphitization of the
diamond crystals. Normally the compact should be
maintained at a temperature within this range for at
least 10 minutes at 1490C, preferably at least 30
minutes at 1450-1490C. This engures substantially
complete reaction of available carbon black and
carbon fiber-with infiltrated silicon.- Conse~uently,
the entire operation may proceed essentially
simultaneously under a single set of conditions or
in a sequential, step-wise progression, as desired.
The process of the present invention does
not require application of pressure during silicon
infiltration or sintering. This, of course, means
that there i9 no need for a hot press mold at this
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stage of the present process. Such other processes
as are, for example, described in United States Letters
Patent No. 4,124,401, issued November 7, 1978 to Lee
et al, rely upon a pressure upwards of 20,000 psi for
this portion of the process.
Once reaction between carbon black and carbon
fiber with silicon has essentially ceased, the bonded
product composite may be cooled. If, as desired, the
composite was formed in the desired shape, it is ready
for use. Most commonly, therefore, it will be configured
as a cutting tool, wire drawing die or other conventional
article for which its properties are particularly desirable.
These bonded composites generally contain
strata which evidence their process of production.
In the main, the strata are evidenced by the filler of
the second dispersion (or core) and by the diamond
crystals on its surface. Uniting these different
strata is the bonding matrix of ~-silicon carbide.
Thus, if the filler of the second dispersion is ~-silicon
carbide as preferred, that layer may consist essentially
of ~- and 4-silicon carbide.
The peripheral side surface portion derived
from the first dispersion ordinarily consists
perdominantly of diamond crystals and a small amount
of ~-silicon carbide. Most characteristic of this
layer is the presence of its diamond crystals,
preferably in the range of from about 82% to 92% by
weight (81% to 91~ by volume).
A residue of unreacted constituents -- generally
30 from about 4% to 14% silicon and up to about 0.2%
carbon by weight -- may also exist in the main body.
The silicon residue may be present throughout
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the composite. However, residual carbon in the portion
derived from the first dispersion must be less than
0.05% by weight, and the optimum Si in the critical
area should be about 3-6%. The precise control of
Si and C is an important feature of the direct
infiltration technique of this invention.
It is to be understood that changes may be
made in the particular embodiment of the invention in
light of the above teachings, but that these will be
within the full scope of the invention as defined
by the appended claims.
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