CA1199781A - Nitrided superhard composite material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Sintered silicon carbide cutting tool compos-ites containing diamond crystals are described. They are made through a process comprising separately compacting carbon binder materials, one of which contains superhard crystals, into a cutting tool insert configuration when the crystals are concentrated in the cutting edges, in-filtrating the insert with liquid silicon and sintering and nitriding the insert to convert the silicon to silicon carbide and silicon nitride.

Description

- 1 - 60~P 2073M
NITRIDED SUPERHARD CO~POSI~E MATERIAL
BACKG~OUND OF THE INVENTION
Articles composed of materials having refractory characteristics of hardness and resistance to erosion have myriad important uses. Reaction sintering of ~ -silicon carbide and ~ -silicon carbide has been known for making high temperature components. For example, ~ -silicon carbide is described as an excellent binder in U.S. Patent No. 2,938,807, issued May 31, 1960 to Anderson.
Another useful component of these materials would be superhard crystals such as diamond and cubic biron nitride. Their superior properties, for example hardness, have long been appreciated. A satisfactory means of incorporating diamond for example into such materials would be of a significant advantage and is an object of the process and product of the present invention.
In the area of bonding diamond crystals, certain metals have been employed together ~ith hot-press technology, as for example described in U.S. Patents Nos. 4,124r401, issued November 7, 1978 to Lee et al, 4,167,399, issued September 11, 1979 to Lee et al, and 4,173,614, issued 20 November 6, 1979 to Lee et al, all assigned to the assignee of the present invention.
The Ohno application describes bi-la~er diamond composltes having a special binder of ~silicon carbide.
That binder forms a matrix throu~hout the composite so as both to hold the diamond cr~stals and to unite the composite layers, ~he composites are formed by a process comprising:

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(a) forming a first dispersion of diamond crystals and carbon black in paraffin, ~b) forming a second dispersio~ of carbon fiber, carbon black and filler in S 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 suffi-cient to vaporize essentially all of said paraffin;
(e) li~uefying said silicon to cause infiltra-tion into both layers;
(f) uniting the layers of said composite with 1S liquid silicon; and (g) sintering the composite and infil~rated silicon under conditions sufficient to produce a ~ -sili~on carbide binder unit-ing said composite.

Superhard crystals such as diamond crystals and crystals of cubic boron ni~ride are expensive items com pared to carbide and other materials u~ed in the cutting edge3 of metal cutting tools. Moreover, the use of addi~
tive ma~erials including superhard crystals entailed a rather wide distribution of crystals within the matrix so that a great number of crystals never take part in the cutting actionO The above Ohno process 9 known as the press and treat technique, provides a means o segregat ing the crystals into define~l area~ such a~ discs an~
tria~guloid con~igura~ions with the results ~hat ~he total quan~ity of superhard c!rys~als is reduced.

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It has now been discovered that the compact with smaller volumes of superhard crystals concentrated in predetermined and perhaps spaced relationships is structurally improved by subjecting the compact to a nitriding process to convert the elemental silicon, from the infiltration step, into silicon nitride. The im-proved process permits the manufacture of high strength cuttin~ tool inserts having more discrete concentrations of superhard crystals at predetermined locations, such as a triangular concentration near the edge of an insert.
These inserts use a lesser amount of superhard crystals and are, therefore, more economical. Means are provided for securing these unique inserts in appropriate tool holders.

TH E DRAWI NGS

This invention will be better understood when taken in connection with the following description and the draw-ings, in which:
FIG. 1 is a schematic diagram of the process of the present invention;
FIGS. 2-6 are sequential, illustrative depic-tions of a preferred process and particular apparatus useful in the process of the present invention;
FIG. 7 illu$trates a composite insert having an overlay of superhard crystals ~ith geometrical raised corner portions;
FI5. 8 illustrates a composite insert where superhard corner portions extend in plural directions from the composite;

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FIG. 9 illustrates a composite with a recessed upper surace to contain a clamping plate;
FIG. 10 i.llustrates a circular composite having a triangular cross section edge with a gradation layer;
FIG. 11 illustrates a clamping washer means for a composite;
FIG. 12 illustrates a stress distributing de-vice in a composite clamping means FIG. 13 illustrates a thin composite with an envelope structure of superhard crystals; and DESCRIPTION OF THE INVENTION

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 ~table intermediate compact which, in the case of the diamond dispersion, is in a predetermined configu-ration such as a cutting edge;
(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 suf-ficient to vaportize essentially all of said paraffin;

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(f) infiltrating said binary compact with liquid ~ilicon; and (g) sintering the binary compact containing înfiltrated silicon under conditlons suf-ficient to produce a ~-silicon carbide binder unitlng said composite.
(h) subjecting said binary compact during or a~ter sintering to a nitriding process to ~onvert elemental silicon to silicon nitride.
As a result of this process, a bonded composite having a superior wear resistance surface layer is pro-duced. When diamond crystals are used, ~he ~iamond crystal containing surface, held tightly by a strong silicon carbide bonding matrix, is particularly suitable as a tooling or cutting edge.
The present process for preparing silicon car-bide composi~es i~ diagra~ed in representative manner in PIG. 1~ As shown by that diagram, one of the initial steps involves the formation of a dispersion of diamond crys~als and carbon black in parafEin~ ~or various reasons, small crystals are usually employed in this first dispersion. In a preferred embodiment~ the dia-monds employed include crystals having a si~e less than 400 mesh. Crystals of this preferred size will, when bonded with ~-silicon carbide, exhibit superior resist-ance to chipping. In additlon, they provide sharp edges having desirable relief angles for cutting in~erts and other wear component~.
To the diamond crystals mu~t be added carbon b~ ack. q~his carbon serves subsequently by reac~ing to yi~ld /~-silicon carbide for the bonding matr1x o the present co~posites. Thi~ carbon black i~ desirably of 97~

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 preferably about 2%, by weight of diamond has proven optimumO
The paraffin utilized in the first (or periph-eral) dispersion may be any of the hydrocarbon waxes en-compassed by the common meaning of this term. Again a high purity hydrocarbon should be employed to avoid pos-sible harmful residue. For each 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 crit-ical as it is subsequently removed. It generally consti-tutes from 3% to 6~ by total weight of the first dis-persion.
The foregoing constituents may simply be mixed together to form the first dispersion. A very intimate and homogeneous dispersion is7 however, preferred. Con-sequently, a step-wise technique such as that outlined in the flow diagram of FIG. 1 is desirable~
In accordance with that technique, the diamond crystal 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 con~aining carbon fiber, ~arbon black~
and paraffin may be passed through a screen of~ for ex-ample, about 20 mesh to improve ad~ix~ure and reduce any agglomeration which may have occurred.

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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 hoth the first and second dispersions. Generally, the second dispersion also contains from 3% to 6% paraffin and 2% to 4% carbon black by wei~ht. The amount of carbon black, particular-ly in the first dispersion, the quality and type of car-bon black, are also critical. For example, sulfur con-tamination 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 weight of the second dispersion is employed.
As is the case in production of the first dis persion, the paraffin, carbon black, carbon fiber and filler should be intimately admixed. They are also desirably screened as previously descri~ed to insure . finenes~.
Due to the presence of paraffin, each disper-sion is independently capable of being compacted (or molded) to desired shape~s). Application of pressure _.. ._ ... ~ ._~....................................................... _.. _.. _ : ~ _ ~
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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/cm2 is preferred.
In the process of this invention one or the other of the two dispersion is compacted to form that portion of the composite with which it will ultimately correspondO This compacted dispersion therefore consti-tutes 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 ormed from one dispersion, it may be recompacted with the re-maining dispersion. For this step, the intermediate compact may be positioned where desired within a mold having the shape of the desired compositeO The remaining dispersion may then be added to the mold to complete filling. One dispersion must be compacted in each of the fore~oing steps, but their sequence is not important.
The application of pressure as previously described then yields a physically stable binary compact ~hich has the same shape as the ultimate bonded compositeO
An item of 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 grinding and finishing to a desired shape, common with other refractory materials, may be eliminated in accordance with the pres2nt process~ the mold(s) and/or plunger(s) should thereore have the con-figuration(s) desired for the ultimate portion of the body to which the compact or composite corresponds~

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In FIGS. 2-6 there is shown a molding apparatus and the sequence of events. The apparatus comprises a base B, a mold M defining a cavity C and a plunger P.
Plunger P fits into the mold M and cavity C to closely 5define the final shape of an insert or compact. Insert material 1, FIG. 3, is introduced into the cavity and a block plunger ~ is used to close the cavity and to com pact the material 1 into the annular shape 2 shown in FIG. 4. In FIG. 5 the plunger 3 has a flat face and 10annular inser~ 2 has been filled with the remaining dis-persion 4. Vpon completion of the compacting process, FIG. 6, the composite 5 is removed for sintering. Once molded to the desired shape, the binary compact is (as may be seen in FIG. 1) subjected to vacuum and tempera-15ture conditions sufficient to vaporize the paraffin from its entire volume~ Suitable conditions aret of course, dependent upon the particular paraffin present~ General-ly, however, a pressure of less than 200 and temperature of about 500 are utilized. Alternatively, another tem-20perature and a correspondinyly varied vacuum may be employed.
The vaporization of the paraffin is preferably conducted slowly. This avoids, for exam~le~ violent boiling and/or build-up of gaseous pressure within the 25composi~e. Accordingly, conditions re~uiring at least 10 minutes and preferably from 10 to 15 minu~es for the es-sentially complete removal of the paraffin are preferredO
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, substan-tially all of the carbon black and carbon fiber of the compact. There may also be excess silicon. It is no~

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, ~ ,~, 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.
5T~e operation of bonding a compact to create a composi~e actu~lly involves a series of steps, all of which may occur essentially simultaneouslyO These steps include melting of the silicon, infiltration of mol~en silicon into the compact and reaction of infiltrated 10silicon with both the carbon black and carbon fiber to produce ~-silicon carbide through the resultant composite To induce this last set of reactions between ~ilicon and c~rbon, a minimum temperature of at least about 14S0C is required. ~igher temperature~, may also 15be utilized. A maximum of about 1490C is, however, pre-ferred to avoid graphitization of the diamond crystals.
Normally the compact should be maintained at a tempera-ture within this range for at least 10 minutes at 1490C, preferably at least 30 minutes at 1450-1490 C. This 20ensures substantially complete reaction o~ available car-bon black and carbon fiber with infiltrated silicon.
Consequently, the entire operation may proceed essential-ly simultaneously under a single se~ of conditions or in a sequential~ ~tep-wise prvgression, as desired.
25The process of the present invention does not require application of pressure during silicon inflltra~
tion or f~intering, This, o course, means that there is no need for a hot press mold at this st2ge of the present proces~. 5uch other processes as are, for example~ des~
30cribed in Vnited 5tates Letters Patent NoO 4V124,401 of Lee et al, rely upon a pressure upwards o 20,000 psi for this portion of the process.

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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 shapel it is ready for use. Most commonlyt the~efore, it will be configured as a cutting tool9 wire drawing die or other conventional article for which its properties are particularly desir-able .
These bonded composites generally contain strata which evidence their process of production. In the ~ain, the strata are evidenced by the filler of the second di~persion (or core) and by the diamond crystals on its surface~ Uniting these different strata is the bonding matrix of ~-silicon carbide. Thus, for example, i the fillar of the secon~ dispersion is ~-silicon car-bide, as preferred, that layer may consist esser,tially of ~- and ~ silicon carbideO A residue of unreacted consti-tuents-- generally from about 5~ to 14% silicon and up to about 0.2~ carbon ~y weight -- may ~lso exist. The sili-con residue may be present throughou~ the composite~
However, residual ca~bon in the portion derived from the first dispersion must be less than 0.05% by weight.
The peripheral side surface portion 2, FIG. 5~
derived from the first dispersion ordinarily consists predominantly of diamond crystals and a sm~ll amount of ~ -silicon carbide. Most charac~eristic of this layer is the presence o~ i~s diamond cry~tals~ preferably in the range of from about 75% to 90~ by weight.
The composites of the present invention may be improved by a nitriding ~rea~mentO In this tre3tment, as il ustrated in FIG. 14, a ni~rogen insertion .~tep is ~hown where nitrogen is caused ~o flow into ~he vacuum furnace ~o convert ~he remainin~ elemental silicGn ~o , . .
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silicon nitride. In one example of this step nitrogen is caused to flow into the furnace at a temperature above about 1100C for an hour or less to cause surface nitrid-ing with the production of amorphous 5i3N~. Prior to this step hydrogen and/or nitro~en may be flushed to re-duce the furnace environment. A self clogging process prevents additional nitrogen from penetrating deeply into the insert and providing a surface like treatment only.
The present invention merely requires the in-jection of nitrogen gas into the furna~e and is therefore straightforward, inexpensive, readily lends itself to automated procedures, and is essentially pollution free.
Further, neither hiqh temperature nor high pressure tech-niques are employed as in the prior art, thus producins amorphous Si3N4, rather than conventional ~ or~
Si3N4. Although the hardness of amorphous Si3N4 is lower than that o ~ or ~ Si3N4~ i~ has the advant-age of providing a protective layer which ~oth eliminates buildup of Al-Si alloy on the insert when cutting these materials, a further densification of the surface of the in~ertr a hardening o~ the binder phase, a 20 to 40% in-crease in the bending stren~th of the insert and an in-creased wear-resistance.
The present shaped composites may have any of the geometric shapes known for such cutting tool insertsO
In general, these inserts share an indexable fe2ture that, during use, they are rotated about a central ~xis ~hile their circumferential working sides or edges are oriented ei~her parallel to, or intersecting, that axis.
Cer~ain preferred embodiments of the present invention involve some of these shapes~ For examplet the inser~ 5 ~ of FIG. 6 may have two essentially parallel and planar ~ ~997~

surfaces spaced a predetermined distance apart.
These surfaces would represent the anterior and posterlor surfaces of the insert, their distance of separation, its depth, is ordlnarily from 0.1 to 0.2 cm.
The periphery of the insert 5 is forme~
by peripheral sides joining the upper and lower surfaces. These sides generally form either a cylindrical conical or polygonal shape. The sides of neutral cutting inserts are parallel to an axis normal to the planar surfaces. ~owever, the sides of positive cutting inserts have a relief angle, as shown in composite 5. Therefore, each separate side is trape20idal in configuration.
The insert 5 of FIG. 6 clearly shows a peripheral concentration of hard crystals 2 and a central section 4 with few, if any, hard crystals. The cross section of the peripheral structure 2 is preferably of a triangular configuration. The peripheral structure 2 may also have an upper hard crystal surface which coextensively covers the total top surface of the insert 5 somewhat similar to the surface 6 as shown in FIG. 7. This top surface 6 may be so formed that, in a polygon structure 7 such as illustrated in FIG. 7, there are formed, discrete triangular edge structures 8 which are elevated above the central section 9 -to provide a bilevel effect. By this means most of the hard crystals are concentrated in a streng-thening arrangement in cutting edge structures. The edge structures 8 may also project laterally and forwardly from the insert as shown in FIGS. 7 and 8 where, in FIG. 8 for example, an insert 10 includes hard crystal edge structures 11 having surfaces 12, 13 and 14 which project laterally , ~

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and forwardly as well as upwardly as shown. Moreover, slnce the actual relationship of the cutting edge structure oE a polygon insert to a workpiece is not symmetrical, i.e., that more of one side of the cutting edge is presented to the workpiece than the other side, the edge structure may be formed in a complementary manner. The polygon insert 15 of FIG. 9 shows nonsymmetrical hard crystal cutting edge structures 16 of a generally triangular configuration having one longer side for presentation to the workpiece. Rectangular structures may also be employed in the same manner. These projections minimize subsequent grinding operations.
For mechanical locking means, a pexEormed recess 24 in FIG. 9 contains a plate or washer 25 of a relatively soft ma-terial such as copper which is bonded to the insert 15.
The upper surface of plate 25 extends higher than the upper surface of the hard crystal edge structure 16 and serves to absorb clamping forces on the insert.
As described, the process of this invention may be employed to control, disperse, or concentrate the crystal structures of this invention in predetermined areas. In order to provide a better bond between the edge structures and the matrix, to keep the quantity of hard crystals at a minimum/ and to increase toughness 3Q of the total insert, a supporting crystal layer may be employed as shown in FIG. 10 in the cross section of insert 17A. As illustrated, a cut-ting edge structure 18 is spaced from the matrix 19 by a layer 20 of hard crystals whose concentration is ~ 15 - 60MP-2073M

less than the crystals in -the edge structure 18.
This layer 20 provides for a gradation in stresses in the inser-t and may be stratified as shown or may be a continuing varying concentration either separately or as a part of edge structure 18. Also, as shown in FIG. 10, the edge structure 18 may be formed with a rake angle a-t the sides 21 -to minimize subsequent grinding.
In the compacting process of this invention there is a different force required to compact the hard crystal edge structure 18 and the matrix portion 19. These pressures may be equalized by having the press anvil of a convex section so that the insert has its central and more compressible section 22 compressed further into a thinner section at the time when the crystal edge structure see their maximum compression force.
The process of this invention by permitting the performing of bilevel area, rim structures and recesses provides for unique and better composition.
Where the insert is made in a very thin cross section, such as the insert 26 in FIG. 11 it may be protectively mounted by utilizing an annular hlock member 27 with an upstanding nipple 28. The insert 26 is provided with an aperture 27 and is positioned with a close fit over nipple 28. At the same time a hard metal washer 29 rests on the insert 26 as well as on the front edge of nipple 28, and a pin member~ such as a screw, clamp the 11997~I

washer to the front edge of the nipple, and the block to a tool holder. In this manner excess stress is carried by the nipple and block rather than the insert.
Where stresses may be exessive, the structure o~ FIG. 12 may be employed. In FIG. 12 the insert 30, which may be those of FIGS. 8 and 9 for example, includes a shock absorber block 31 of a hard metal. This block is smaller than the insert and its perimitor rests on all the cutting edge structures. A clamping force on this block distributes the stresses to all the inserts.
Because of the layer concept o this process various additional shapes may be produced such as annular inserts with cutting edges located on any surface, and drawing dies having an inner cutting edge.
With regard to clamping and general dimensional stability of an insert, the insert exemplified by FIG. 13 may be employed. In FIG. 13 t~e insert 32 includes a central matrix 33 which is prepressed to provide struc-tural and dimensional stability. Thereater, a layer or envelope of super hard crystal 34 is compaction of event-ually only the crystal material. The composite is then sintered in the usual manner. Metallic layers 35 serve to bond the insert to a base.

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Claims (10)

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

1. A process for preparing a bonded composite cutting element comprising:
(a) forming a first dispersion of super-hard 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 com-pact 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 contain-ing infiltrated silicon under conditions sufficient to produce a .beta. -silicon carbide binder uniting said com-posite.
(h) subjecting said binary compact to a nitriding treatment to convert elemental silicon to sili-con nitride.

2. The process of Claim 1 wherein nitrogen in gaseous form is introduced into the sintering process.

3. A cutting tool insert made by the pro-cesses of Claim 1.

4. The insert of claim 3 where the insert includes a layer of superhard crystals on an upper surface and a number of concentrated volumes of superhard crystals project from said layer.

5. The insert of claim 3 wherein the superhard crystals are concentrated in a rim portion about a central section where the rim portion has a thickness less than the thickness of the insert.

6. The insert of claim 3 where the superhard crystals are concentrated in a number of cutting edge structures.

7. The insert of claim 3 wherein the cutting edge structures include an underlayer of more dispersed crystals.

8. The insert of claim 6 wherein the edge structures are projecting from the insert.

9. The insert of claim 6 wherein the projection is laterally forwardly and upwardly from the matrix.

10. The invention as recited in claim 6 wherein said insert has a preformed recess in its top or bottom surface or both.