CA1178424A - Silicon carbide composite and process for production - Google Patents

Abstract

SILICON CARBIDE COMPOSITE
AND PROCESS FOR PRODUCTION
ABSTRACT OF THE DISCLOSURE
Sintered silicon carbide composites containing diamond crystals are described. They are made through 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 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 at 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 the resultant composite.
These composites may be formed in a variety of specialized shapes and are particularly useful as cutting materials and/or wear components, where they exhibit extreme wear resistance.

Description

~ 78~2~ 60MP-2044 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 ~ 7~'~2~
60Mp-2044 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 ~7~3424 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 1~'7~

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 `` 60MP-2004 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 1178~Z~ 60MP-2004 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 ~ 8'~ 60MP-200~

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 ~ 60MP-2004 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.

~ 4~ (~ 60MP-2004 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 ~ ~ 60MP-200 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 ~1 7~ 60MP-200~

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 11 7~ 60MP-200~

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 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.

Claims (8)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A process for preparing a bonded composite comprising:
(a) blending diamond crystals and carbon black to permit an even coating of said diamond crystal surfaces;
(b) forming a first dispersion of said blended diamond crystals and carbon black in paraffin;
(c) forming a second dispersion of carbon fiber, carbon black and filler in paraffin;
(d) compacting one of said dispersions to produce a physically stable intermediate compact;
(e) recompacting said intermediate compact with the remaining dispersion to produce a binary compact;
(f) subjecting said binary compact to a vacuum for a period of time at a temperature sufficient to vaporize essentially all of said paraffin;
(g) infiltrating said binary compact with liquid silicon; and (h) sintering the binary compact containing infiltrated silicon under conditions sufficient to produce a .beta.-silicon carbide binder uniting said composite, elemental silicon forming approximately 3-6% by weight of said first dispersion after sintering.

2. The process of claim 1 wherein said diamond crystals form approximately 82 to 92% by weight of said first dispersion after sintering.

3. The process of claim 2, wherein the composite has two essentially parallel and planar surfaces a predetermined distance apart, the connecting peripheral sides of said composite forming a circle at each of said surfaces.

4. The process of claim 3 wherein the core of the composite is formed from the second dispersion and the sides comprise a peripheral layer formed from the first dispersion.

5. The process of claim 4, wherein the layer has a radially-measured thickness at one surface which is at least twice that at the other surface.

6. The process of claim 3 wherein the circular sides are parallel to an axis normal to the planar surfaces.

7. The process of claim 2, wherein the first dispersion is compacted to form the intermediate compact and, upon recompacting with the second dispersion, said intermediate compact forms the side periphery portion of the composite.

8. The process of claim 2, wherein the second dispersion is compacted to form the intermediate compact and, upon recompacting, the first dispersion forms side periphery portion of the composite.