CA1178425A - Silicon carbide composite and process for production - Google Patents

Abstract

IMPROVED SILICON CARBIDE COMPOSITE
AND PROCESS FOR PRODUCTION
ABSTRACT OF THE DISCLOSURE
Sintered silicon carbide compositions enveloped with diamond or cubic boron nitride (CBN) crystals are described.
They are made through a process comprising;
(a) forming a first and second dispersion of uncoated diamond crystals and carbon black in paraffin;
(b) forming a mixture of carbon fiber, carbon black and filler in paraffin;
(c) compacting said dispersions and mixture together to produce an integral compact wherein said dispersions from an evelope about said mixture;
(d) subjecting said compact to a vacuum for a period of time at a temperature sufficient to vaporize essentially all of said paraffin;
(e) heating silicon to cause liquefaction, direct infiltration and diffusion into said compact in vacuum, and (f) sintering the compact containing silicon under conditions sufficient to produce a .beta.-silicone carbide binder uniting said composite without applying pressure.
In preferred composites, the dispersions contain different properties of diamond or cubic boron nitride crystals.
One may be utilized for a peripheral band about the mixture;
the other connecting the edge as a surface layer of the composition. These composites are particularly useful as cutting material and/or wear components, where they exhibit extreme wear resistance.

Description

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IMPROVED SILICON CARBIDE COMPOSITE
AND PROCESS FOR PRODUCTION
BACKG~OU~D OF THE INVENTION
Articles composed of materials having refractory characteristics, hardness and resistance to erosion haYe myriad important uses. Representative materials are described in U.S. Patent No. 2,938,807 of James C. Anderson, issued May 31, 1960.
Reaction sintering of -silicon carbide and B-silicon carbide has been known for making high temperature components.
For example, B-silicon carbide is described as an excellent binder in the Anderson U.S. Patent No. 2,938,807, however, no diamond is incorporated in this silicon carbide technology.
Another useful component of these mate~ials would be superhard crystals such as diamond or cubic boron nitride.
Their superior properties of, for example, hardness have long been appreciated. A satisfactory means of incorporating superhard crystals into the critical area of such articles would be 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, 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 to Lee et al, issued November 7, 1978; and U.S. Patent No. 4,173,614 to

- 2 - 60MP-2037 Lee et al, issued November 6, 1979, all of which patents are assigned to the assignee of the present invention.
Reference is also made to U.S. Patent No. 4,220,~55 which issued to St. Pierre, et al on September 2, 1980 and which is also assigned to the assignee of the present invention. The latter patent discloses a process for making a homogeneous diamond composite throughout an article, wherein inidividual crystals are coated, and Si is infiltrated into a porous preform indirectly through a wick material. The process of manufacture of U.S. Patent No. 4,220,455 is not readily adaptable to mass production.
Many of the problems associated with the prior art have been overcome by the inventions disclosed in Canadian Patent Application Serial Nos. 381,405, filed July 9, 1981 and 381,415 filed July 9, 1981 by John ~lichio Ohno. In brief, they describe bi-layer diamond composites having a special binder of B-silicon carbide and Si. That binder forms a matrix throughout the composite so as both to hold the diamond crystals and to unite the composite layers.
INTRODUCTION TO THE INVENTION
The present invention employs diamond or cubic boron nitride crystals: ~-SiC, B-SiC or other filler crystals;
carbon black; carbon fiber and paraffin to produce composite bodies having a cap or envelope of sintered diamond or cubic boron nitride as a thin, surface layer.
The process of the present invention is more productive than the prior art of making diamond tools or wear components in that it involves:
(1) Room temperature pressing of diamond incorporated material to a desired shape, using high quality carbons and paraffin, without any use of other additives described in the prior art.
(2) Simple direct infiltration without use of coherent continuous coating and wick material to guide infiltration. No pressure is applied, but a diffusion process is utilized in a vacuum.

s _ 3 _ 60MP-2037 t3) Shaping is greatly simplified because it is largely performed prior to in situ formation of binder by reaction between silicon and carbon.
(4) Reduction of stress between dissimilar composites by use of an intermediate dispersion and also by use of an envelop3-shaped compo9ite.
t5) Elimination of warpage by use of a set of plugers with compensation.
t6) P~einforcement of cutting edge.
(7) Elimination of major grinding operation by making side relief before sintering.
The subject invention has a definite saving of time and material, such as diamond grinding wheel, to produce a final shape. A drastic cost saving is achieved by limiting the use of superabrasive crystals only in the critical area.
The composites of the present invention are prepared by the steps of:
(a) forming a first and second dispersion of diamond or cubic boron nitride crystals and carbon black in paraffin;
(b) forming a mixture of carbon fiber, carbon black and filler in paraffin;
(c) compacting said dispersions and mixture together to produce an integral compact wherein said dispersions form an envelope about said mixture;
(d) subjecting said compact to a vacuum for a period of time at a temperature sufficient to vaporize essentially all of said paraffin;
(e~ heating silicon to cause liquefaction, direct infiltration and diffusion into said compact;
and (f~ sintering the compact containing silicon under conditions sufficient to produce a B-silicon carbide binder uniting said composite.
As a result of this process, a highly stable bonded composite having a super wear resistance surface layer is produced.

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- ~ - 60MP-2037 That diamond or cubic boron nitride crystal containing surface, held t~htly bya strongsilicon carbidebonding matrix,isparticularlysuitable asatooling or cutting edge. Furthermore,a significantcostsaving inmachiningc~abrasive materials is realized due to the excellent per~ormance in sp~ed and wear resistance of the tooling or cutting edge of the subject invention.
DESCRIPTION OF T~E DRAWINGS
In the drawings:
~gure 1 i9 aschematicdiagram oft~eprocessc,ft~epresent invention;
Figures2,3, 4A, 5, 6A, 7A are sequential illustrativedepictions ~ a preferred lQ approach andspecificapparatus usefulint~e process of the present invention;
Figure 4B is a cross-section of the tip of one of the plungers used in carrying out the present invention;
Figure 6B iS a view similar to Figure 4B showing the tip of one form of opposing plunger;
Figure 7B is a cross-section of a compact formed in accordance with the present invention;
Figures 8A and 8B are cross-sectional views of two embodiments of a compact of the present invention;
Figures 9A and 9B are cross-sectional views of sintered inserts respectively made from the compacts of Figures 8A and 8B; and-Figures lOA, lOB and 10C sequentially illustrate a process according to an alternate embodiment of the subject invention for making a neutral insert with the side relief.
DESCRIPTION OF THE INVENTION
The present shaped composites may have any of the overall geometric shapes known for like tools. In use, a round insert is customarily rotated about a central axis for use in cutting, whereas a square-shaped insert is indexable four times without reqrinding of the edges.
The present process for preparing silicon carbide composites is dia-grammed in representative manner in Figure 1. As shown by that diagram, one of the initial steps involves the formation of two dispersions of dia-mond or cubic boron nitride crystals and carbon black in paraffin.
For various reasons, small and less defective crystals are preferably employed in these dispersions. In a preferred embodiment, the diamonds employed include crystals having a size less than 400 mesh. Crystals of this preferred size ,`

~.~ 7~ ~P5 will, when bonded with B-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 superhard crystals such as diamond or cubic boron nitride crystals must be added carbon black to aid in compaction. This carbon serves subsquently by reacting to yield B-silicon carbide for the bonding matrix of the present composites. This carbon 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 the superhard crystals has proven optimum.
The paraffin utilized in the dispersions 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 a small but effective amount to adi in admixture and is removed. It generally constitutes from 3 to 6% by total weight of each dispersion.
The foregoing consituents may simply be mixed together to form each of the two dispersions. Very intimate and homogeneous dispersions are however, preferred. Consequently, a step-wise process such as that outlined in the flow diagram of Figure 1 is desirable.
In accordance with the subject process, superhard crystals such as diamond crystals and carbon black are throughly blended to permit an even distribution. Only after this step is completed is the paraffin mixed into the blend. Thereafter, the dispersions are preferably subjected to a further step of fining.

The dispersions may be identical but preferably are differentiable by their respective diamond or cubic boron nitride crystal contents. One deslrably comprises between 20 to 40% by total weight less diamond than the other.
The higher diamond crystal content dispersion is (as described in greater detail below) ordinarily utilized to form the predominant working surface portion of an eventual composite.
In preferred composites, therefore, one dispersion will contain from about 80 to 95% diamond; another, from about 40 to 80~.
By utilizing two such different dispersions for producing the present superhard envelopes of the composites, several advantages are achieved. Most particularly, many problems of cracking and warpage commonly incident to sintering of dissimilar layers may be obviated.
Returning to the process illustrated in Figure 1, the paraffin and carbon black utilized in the mixture (or core~ material for the present process may be any of those previously described. For convenience, the same ones as are ordinarily utilized in forming both the first and second dispersions are employed. Generally, the mixture also contains from 3 to 6% paraffin and 2 to 4~ carbon black by weight. The amount, quality and type of carbon black are again important. For example, sulfur contamination in carbon black should be avoided.
The carbon fiber employed is desirably of ~ery small size to facilitate homogeneous 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 pro~ided 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 s-silicone carbide is ..

3'1f~5 preferred. Ordinarily, from 40 to 75% of filler by total weight of the mixture is employed.
As it the case in production to the two dispersions, the paraffin, carbon black, carbon fiber and filler should be intimately admixed. They are also desirably screened, for example, by passage through a 20 mesh screen, to insure fineness.
Due to the presence of paraffin, each dispersion and mixture is independently capable of being compacted (or molded) to desired shape(s). Application of pressure provides a compacted composition with sufficient "green strength"
or physical stability to retain its imparted shape during subsequent operations and/or handling.
In the process of the subject invention, one or two of the two dispersions and mixture are desirably compacted to form an envelope of the composite. After the intermediate soft compact has been formed, it may be recompacted with the remaining mixture. For this step, the intermediate compact may be positioned where desired within a mold having the shape of the desired composite. The application of pressure higher than that normally employed for tungsten carbide then yields a physically stable compact.
This type of sequential technique for forming the instant compacts is much preferred. Figures 2-7 illustrate in greater detail a preferred such sequence of steps for this operation of forming a compact from the two dispersions and mixtures. This sequence corresponds to the single compacting step of Figure 1.
Referring to Figure 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. Alternatively, the mold M may be of square configuration, and the various plungers would be of a corresponding square configuration. Mold M contains a tightly fitting, cylindrical plunger Pl which has a symmetrical end tip E. Due to the difference between the diameter "d" of the cylindrical bore of mold M and the diameter of the end tip E, and annular -s - 8 - 60MP-aO37 gap G is created. This gap G is filled or loaded with a dispersion 1 containing diamond crystals (see Figure 3).
The other dispersion 2 is then loaded into the bore G on top of dispersion 1. Plunger P2 is then inserted into bore G for compacting dispersions l and 2, as shown in Figure 4A. Figure 4s illustrates in cross-section the tip portion of plunger P2 which is configured to include a chamfer c2 that contacts the dispersions. $he chamfer tip C2 f the plunger P2 defines the configuration of the formed dispersions 1 and 2, and is designed to compensate for warpage in sintering to produce flat and parallel surfaces of the resulting composite, as more fully described herein-after. If this compensation for warpage was not included in the subject process, the resulting sintered composite (which would include warpage) would require subsequent grinding utilizing a diamond grinding whell to produce flat and parallel surfaces for the composite. These extra operations are obviously time-consuming and expensive.
It is noted that the movement of plunger P2 is without much pressure as it merely corresponds to the weight of the plunger Pl when the assembly is inverted, as illustrated in Figure 5. Preferably, the volume of the dispersions 1 and 2 are just enough to complete the side and top of the intermediate soft envelope of the compact.
As the plunger Pl is withdrawn, there remains a cap-like intermediate compact 3 composed of a bottom layer 2 and a ring-like side 1. $he remaining central cavity within the cap-like compact 3 is then filled or loaded with the mixture 4, as shown in Figure 6A. As also shown therein, a third plunger P3 is then inserted into the mold M and is pressed into the dispersions and mixture at a pressure higher than that utilized for pressing tungsten carbide powder. Plunger P3, similar to plunger P2 includes a chamfer c3, as illustrated in Figure 6B. Chamfer C3 is provided to compensate for warpage of the resulting composite due to sintering. Vpon the application of high pressure by plunger P3, the two dispersions l and 2 as _ 9 - 60MP-2037 as well as the mixture 4 are tightly pressed to form a stable green compact, designated by the numeral 5 in Figures 7A and 7B. As shown, in Figure 7A, the green compact S
is ejected from the mold M by advancement of plunger P3 Figure 7B illustrates the tri-layer compact 5 after ejection from the mold M. The compact 5 is composed of the compressed layers of dispersions 1 and 2, as well as mixture 4 and is in a form suitable for further processing according to the subject process, as depicted in Figurel.
One item 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 is impressed upon a compact during molding. Thus the time consuming and difficult steps of finishing to a desired shapel 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 tri-layer compact is (as may be seen in Figure l) subjected to vacuum and temperature conditions sufficient to vaporize the paraffin from its entire volume. Generally, however, a pressure of less than 200 microns and temperature of about 500C 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 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 compacted mixture is next infiltrated with liquid silicon. There must be sufficient eIemental 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.

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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 ~rom 5 to 12%, excess silicon is even desirable to ensure substantially complete reation in the mixture and Si diffusion into the superhard crystal layer to react with carbon black in the crystal boundary.
An optimum amount of silicon may be readily determined by experimentation prior to undertaking mass production of the subject invention whereby a smooth and clean surface may be produced without any residue on the surfaces of the compacts. This is in sharp contrast and more efficient thab the indirect process of feeding silicon through wick material according to the process described in the afore-mentioned U.S. P~tent No. 4,220,455 to St. Pierre, et al.
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 compacted misture, and reaction of diffused silicon with the carbon black, in the envelope to produce B-silicon carbide throughout 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 rnage for at least 10 minutes at 1490C~ pre~erably at least 30 minutes at 1450-1490~C. This ensures substantially complete reaction of available carbon black and carbon fiber with infiltrated silicon in the mixture and diffusion sintering of the envelope. Consequently, the entire operation may proceed essentially simultaneously under a single set of conditions or in a sequential, step-wise progression, as desired.

~ 8'~5 The process of the present invention does not require application of pressure during silicon infiltration and diffusion. This, of course, means that there i9 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. Patent No. 4,124,401, of Lee et al, which 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 or other conventional article for which its properties are particularly desirable.
During sintering, the different materials of a compact undergo unequal amounts of volume change. This - change is largely a function of the chemical reaction which takes place between carbon and silicon to produce s-silicon carbide in each layer. The differential between the changes of the three layers may unduly stress the compact and resultant composite. The magnitudes of these stresses have been found to be dependent upon the compositions of the materials involved, through the differences in their respective unit volume changes on sintering.
Stress, as revealed by warpage, follows certain general rules. It is oriented directionally so as to be primarily toward larger surfaces and thinner layers of superhard crystals.
Through use of an envelope according to the present invention, however, these stresses may be greatly alleYiated.
The dispersion having a diamond content intermediate to those of the mixture and other dispersion mitigates the stress.
Consequently improved composites are obtained.
Still further improvements may be achieved in accordance with a preferred embodiment of optimizing the sub-configurations of these two dispersions within the present composites.

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~,i7~ 5 - 12 - 60MP~2037 In composites having envelopes derived from such dispersions, varying shapes and thicknesses of the individual layers of these two dispersions may be utilized to counterbalance those stresses normally encountered in sintering of a composite.
Each exterior or overall configuration of a composite leads to different stresses. For example, the presence of a conventional relief angle in the composite ordinarily increases warpage. All however, are capable of being resolYed. The variables of dispersion compositions and individual layer configurations may be coordinated to harmonize the differential changes during sintering of each material present. This is possible because it has been discovered that the unit differential changes of given dispersion compositions may be offset through these variables. Consequently, incident cracking or warpage is substantially eliminated.
These bonded composites generally contain strata (or layers~ which evidence their process of production. In the main, the strata are evidence by the filler of the mixture (or core material) and by the diamond or cubic boron nitride crystals of the dispersions on its surface.
Figures 8A and 8B respectively illustrate cross-sectional views of two embodiments of compacts in the green state made according to the subject invention. Figure 8A
illustrates an embodiment of a positive insert compact 6 in green state, whereas Figure 8B illustrates a neutral insert compact 7 in green state. In Figures 8A and 8B, the designation "t" indicates an inclined or undercut portion of the respectiYe compact which if formed during the pressin~ of the compact and which is provided for warpage compensation. During the sintering operation of the green compacts 6 and 7, the beveled portions, or undercuts "t"
disappear, and this is respectiveIy illustrated in Figures qA and 9B. The dimensional change due to sintering of the respective compacts is also exhibited by the thicknesses of the structures. For example, in Figures 8A and 8B, the .

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thickness T1 of the compacts is equal to 0.068-0.069 inches whereas in the resulting sintered composites of Figures 9A
and 9s, the thickness T2 equals 0.070-0.072 inches. Figures 9A and 9B respectively depict cross-sectional views o~ a positive insert and a neutral insert with side relief, after the step of sintering the compacts of Figures 8A and 8B. In the positive insert 8 of Figure 9A, the angle 9 is generally less than 90, whereas in the neutral insert 10 of Figure 9B, the angle 11 is equal to 90. Numeral 12 designates a relief angle, generally in the range of 5 to 10, in the neutral insert 10.
It is noted that in order to achieve the 90 cutting edges (or 90 circular edges) in the neutral insert 10, the angle "dl" in the top surface of the compact as illustrated in Figure 8B must be greater than the inclined portion "t"
in the upper surface of the compact as illustrated in Figure 8A. The amount and degree of inclination of the various angles o the compactmay be readily determined by experimentation such that the resulting sintered composites have the desired geometric configurations. It is noted that increasing a relief angle 9 in a positive insert 8 as shown in Figure 9A weakens the cutting edge, whereas a 90 cutting edge in the neutral insert 10 (as illustrated in Figure 9B) reinforces the cutting edge.
Figures lOA through lOC illustrate an alternate process of the subject invention for forming a neutral insert of a configuration as illustrated in Figure 9B. In Figure lOA, a green compact 20 is formed according to the subject process as described hereinabove, and is made of three layers including dispersions 21 and 22 and mixture 24. The mold utilizes for forming the green compact 20 is configured to include straight side walls whereby the side walls 25 of the green compact 20 are generally perpendicular to the upper and lower surfaces of the compact. As illustrated in Figure lOB, a relief angle 26 is made in the green compact 20 by machining the side wall or periphery of the green compact, after which the compact is sintered .

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- 14 - 60.~P-2037 to the final configuration as illustrated in Figure 10C.
By employing the special plungers as utilized in the subject process and as depicted in Figures 2 through 7, flat and parallel upper and lower surfaces are produced in the resulting neutral insert 27 as shown in Figure 10C.
Obviously, other modifications and variations of the present invention are possible, in the light of the above teachings. It is therefore, to be understood that changes may be made in the particular embodiment of the invention described which are within the full and intended scope of the invention as defined by the appended claims.

Claims (19)

1. The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
   l. A process for preparing a bonded composite comprising:
   (a) forming a first and second dispersion of uncoated diamond or cubic boron nitride crystals and carbon black in paraffin;
   (b) forming a mixture of carbon fiber, carbon black and filler in paraffin;
   (c) compacting said dispersions and mixture together to produce an integral compact wherein said dispersions form an envelope about said mixture;
   (d) subjecting said compact to a vacuum for a period of time at a temperature sufficient to vaporize essentially all of said paraffin;
   (e) placing said compact in direct contact with elemental silicon;
   (f) heating said silicon to cause liquefaction, direct infiltration and diffusion into said composite in a vacuum; and (g) sintering the compact and infiltrated silicon under conditions sufficient to produce a .beta.-silicon carbide binder uniting said composite in the absence of applied pressure.
2. 2. A process as defined in claim 1, wherein said second dispersion is substantially free of any diamond.
3. 3. The process of claim 1 or 2, wherein the first dispersion comprises from about 40% to 80% diamond or cubic boron nitride crystals by total weight.
4. 4. The process of claim 1 or 2, wherein the second dispersion comprises from about 80% to 95% diamond or cubic boron nitride crystals by total weight.
5. 5. The process of claim 1, wherein the side of the composite is prepared essentially from the first dispersion, while the second dispersion forms on surface of the composite.
6. 6. The process of claim 2, wherein the side of the composite is prepared essentially from the first dispersion, while the second dispersion forms on surface of the composite.
7. 7. The process of claim 5 or 6, wherein the other surface and the core of the composite is prepared essentially from the mixture.
8. 8. The process of claim 1 or 2, wherein step (c) is performed by:
   (i) compacting the dispersions to produce a soft intermediate compact; and (ii) recompacting said soft intermediate compact with mixture to produce said composite.
9. 9. The process of claim 1 or 2, wherein the step of compacting the dispersions and mixture is performed utilizing a set of plungers of a design to compensate for warpage during sintering of the compact.
10. 10. A composite produced in accordance with the process of any one of claim claim 1, 2 or 6.
11. 11. A bonded composite comprising at least one layer and a core united by a .beta.-silicon carbide matrix, said layer containing diamond or cubic boron nitride crystals and forming an envelope about said core.
12. 12. A bonded composite as in claim 11 substantially free of any diamond.
13. 13. A bonded composite as in claim 11 or 12, wherein two layers of diamond or cubic boron nitride crystals are provided.
14. 14. A bonded composite as in claim 11 or 12, wherein said core is formed of a mixture including carbon fiber, carbon black and filler crystal such as SiC.
15. 15. A bonded composite as in claim 11, wherein said core is essentially free of diamond or cubic boron nitride crystals.
16. 16. The composite of claim 11 or 12, wherein the diamond or cubic boron nitride crystal containing layer forming a surface of said composite is internally concave.
17. 17. A process for preparing a bonded composite comprising:
    blending cubic boron nitrdie (CBN) crystals and carbon black to permit an even coating of the surfaces of said crystals;
    forming a CBN dispersion of said blended CBN
    crystals and carbon black in paraffin;
    forming a core mixture of carbon fiber, carbon black and filler in paraffin;
    compacting said dispersions together to produce an integral bi-layer composite having a CBN layer and a core layer produced from said CBN dispersion and core mixture, respectively;
    subjecting said composite to a vacuum for a period of time at a temperature sufficient to vaporize essentially all of said paraffin;
    placing said composite in direct contact with elemental silicon;
    heating said silicon to cause liquefaction and infiltration into both layers of said composite; and sintering the composite and infiltrated silicon under conditions sufficient to produce a binder of silicon and .beta.-silicon carbide uniting said composite..
18. 18. A bonded composite produced in accordance with claim 17.
19. 19. A cutting insert composite comprising:
    a CBN layer containing cubic boron nitride (CBN) crystals;
    a core layer containing fillter material; and a matrix of silicon and .beta.-silicon carbide binder material dispersed substantially throughout said CBN and core layers for bonding said CBN and core layers internally and to each other.