Process for the production of silicone carbide composite

Sintered silicon carbide composites containing diamond crystals are made through a process wherein a first dispersion of diamond crystals and carbon black in paraffin is formed, along with a second dispersion of carbon fiber, carbon black and filler in paraffin. One of the dispersions is compacted to produce a physically stable intermediate compact which is then recompacted with the remaining dispersion to produce a binary compact. The latter is subjected to a vacuum for a period of time at a temperature sufficient to vaporize essentially all of the paraffin, after which the binary compact is infiltrated with liquid silicon and sintered to produce a .beta.-silicon carbide binder uniting the resulting composite.

BACKGROUND OF THE INVENTION 
Articles composed of materials having refractory characteristics, hardness 
and resistance to erosion have myriad important uses. Representative 
materials are described in U.S. Pat. No. 2,938,807 of Andersen. 
Reaction sintering of .beta.-silicon carbide and .alpha.-silicon carbide 
has been known for making high temperature components. For example, 
.beta.-silicon carbide is described as an excellent binder in the Andersen 
U.S. Pat. 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. Pat. No. 4,063,909 to 
Robert D. Mitchell. 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. Pat. No. 
4,124,401 to Lee et al, U.S. Pat. No. 4,167,399 to Lee et al, and U.S. 
Pat. No. 4,173,614 to Lee et al, all of which patents are assigned to the 
assignee of the present invention. 
Many of these problems have been overcome by the invention disclosed in 
U.S. patent application Ser. No. 167,196, filed July 9, 1980 currently 
herewith by John Michio Ohno. The disclosure of this application is 
incorporated herein by reference. 
In brief, that application describes bi-layer diamond composites having a 
special binder of .beta.-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 infiltration 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 .beta.-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 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 sufficient to produce a .beta.-silicon carbide binder uniting 
said composite. 
As a result of this process, a bonded composite having a superior wear 
resistance surface layer is 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 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 involve 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 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 sides 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 than 400 mesh. Crystals of this preferred size 
will, when bonded with .beta.-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 .beta.-silicon carbide for the bonding 
matrix of the present composites. This carbon black is desirably of high 
purity to reduce the presence of contaminants. 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 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 is 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 the second (or core) dispersion 
of the process may be any of these previously described. 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 black by weight. The amount of carbon black, 
particularly in the first dispersion, the quality and type of carbon 
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 .alpha. or .beta. silicon carbide is 
preferred. Ordinarily, from 40% to 75% of filler by total weight 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, such dispersion is independently capable 
of being compacted (or molded) to desired shape(s). Application of 
pressure provides a compacted dispersion with sufficient "green strength" 
or physical stability to retain its imparted shape during subsequent 
operations and/or handling. The amount of pressure applied may vary 
widely, although at least 2300 kg/cm.sup.2 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 portion--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 
intermediate 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 mold to complete filling. The application of pressure as 
previously described then yields a physically 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. 
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 P.sub.1 which has a symmetrical end tip 4. Due 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 P.sub.2 is placed into the bore of the mold M in abutment 
with plunger P.sub.1 (see FIG. 3). Next, the apparatus is reversed and 
plunger P.sub.2 is forced upwardly against plunger P.sub.1 and moves from 
point C.sub.1 to point C.sub.2 thereby forming a ring-like intermediate 
compact, having a peripheral apex e, and designated by the numeral 1 in 
FIG. 4. 
In the next step of the subject process, plunger P.sub.1 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 P.sub.3, the second dispersion forms 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 P.sub.3. The compact 3 is physically 
stable despite its two strata comprising a peripheral ring 1 formed from 
the first dispersion and a central bore 2 formed from the second. 
One thing of great importance in these 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.mu. and temperature of about 
500.degree. 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 15 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 the conditions of 
sintering, infiltration of silicon to, and reaction with, substantially 
all of the carbon black and carbon fiber 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 .beta.-silicon carbide through the 
resultant composite. 
To induce this last set of reactions between silicon and carbon, a minimum 
temperature of at least about 1450.degree. C. is required. Higher 
temperatures may also be utilized. A maximum of about 1490.degree. C. 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 1490.degree. C., preferably at least 30 
minutes at 1450.degree.-1490.degree. C. This ensures substantially 
complete reaction of available carbon black and carbon fiber with 
infiltrated silicon. Consequently, 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 is no need for a hot press mold at this stage of the present 
process. Such other processes as are, for example, described in U.S. Pat. 
No. 4,124,401 of 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 
.beta.-silicon carbide. Thus, if the filler of the second dispersion is 
.beta.-silicon carbide as preferred, that layer may consist essentially of 
.alpha.- and .beta.-silicon carbide. 
The peripheral side surface portion derived from the first dispersion 
ordinarily consists predominantly of diamond crystals and a small amount 
of .beta.-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 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 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 is defined by the 
appended claims.