IE42894B1 - Improvements in coated diamond and cubic boron mitride particles and processes thereof - Google Patents

Abstract

The abrasive particle essentially comprises a coated substrate particle of diamond and/or cubic boron nitride; the coating adhering to the substrate particle is rough and granular and comprises a surface layer and an intermediate layer between this and the surface of the substrate particle; the surface layer comprises (A) metal, (B) a compound of this metal and the substrate particle material or (C) a mixture of the metal (A) with the compound (B) of the metal with the substrate particle material; the intermediate layer comprises a compound of the metal and the substrate particle material; the surface layer is chemically bonded to the substrate particle; the coating has a structure extending in the range from nonuniform to essentially uniform and from discontinuous to continuous, and the coating covers 50-100 % of the surface of the substrate particle.

Description

This invention relates to novel coated particles of diamond, cubic loron nitride (hereinafter often referred to as CBN) and mixtures thereof md io a process for the preparation of the same. More particularly it •elates fe modifying Hie surface of diamond and cubic boron nitride articles to produce the coated particles useful as abrasive.

The superhard materials, diamond and CBN exhibit outstanding ability for machining both metallic and non-metallie materials. In particular, hey perform well in the grinding mode. However, the maximum grinding performance is never attained because of the difficulty in retaining the abrasive particles in the grinding tool matrix or bond system. Premature gross pull-out of only partially used grit is still a major factor in grinding wheel wear, and this is particularly true for diamond particle abrasives, either natural or synthetic, in resin, vitreous or metal bonds. 43894 Thus far a number of methods for modifying diamond particle surfaces in order to improve bond strength in tool matrices have been disclosed. For example, adherent films of molybdenum or chromium have been formed on diamond by sputtering or vapor deposition followed, by appropriate heat treatment to form metal . carbides. Molybdenum films have also been deposited on diamond by chemical and electrodeposition methods. However, as a practical matter, these techniques have certain specific drawbacks which prevent their usage. These processes are expensive and not suitable for coating large lots, lh addition, the metal film thicknesses obtainable are small, and the surfaces of the resulting coatings are usually very smooth. Where the abrasive particles are to be used in resin bond systems, it is desirable to have rough coatings to provide a mechanical bond vith the resin, because not much chemical bonding can be expected. In addition, a heavy coating of reasonable heat capacity is desirable, in order to act as a heat sink and reduce the maximum temperature at the abrasiveresin interface and retard thermal degradation of the polymeric material, A heavy coating, if firmly attached to the abrasive, may also increase its fracture toughness, providing the coating material thermal expansion matches that of the grit closely enough to avoid high interfacial stresses.

Because of the inherent difficulties in obtaining adherent coatings on diamond particles that meet the functional requirements of an abrasive, present practice is to effect a compromise by using thick coatings of metals that can easily be deposited by either chemical or electrodeposition methods. The most common -3'.42S0 4 are nickel or nickel-phosphorous or copper. These are reasonably satisfactory for the heat sink function,and if deposited under carefully controlled conditions, can be made rough enough to give a good mechanical bond to a resin wheel matrix. However, there is no bond between the surface of the diamond particle and these metal coatings thereby limiting abrasive life. In addition, these matal coatings cannot be used in either inetal or vitreous bond systems because of their high thermal expansions compared to diamond, their relatively low melting points, and in the case of Ni-P, its metallurigical instability.

According to the present invention, there is provided an abrasive particle consisting essentially of a substrate particle of diamond in the size range 10 to 2000 microns, cubic boron nitride in the size range 10 to 500 microns or a mixture thereof, having a rough adherent flaky granular covering, said covering being composed of an outside surface coating being composed of metal, a compound of said metal and the material of said substrate particle or a mixture thereof, with all mixtures of said metal and/or said metal/substrate particle material compound capable of being present, and a layer of a compound or compounds of said macal Intermediate said surface1 coating and said substrate particle chemically bonding said surface coating to said substrate particle, said metal being selected from molybdenum, tungsten, titanium, niobium, tantalum, chromium, sircetiium and alloys thereof, said covering being uniform or substantially uniform, and covering at least 50 percent of the surface area of said substrate particle.

The present invention also provides a process for producing abrasive particles consisting essentially of substrate particles of diamond, cubic boron nitride or a mixture thereof which comprises: - 4 42894 1) milling (a) balls made of plastics or elastomer having a diameter ranging from 1/16 inch to 1/4 inch, with (b) particles of a metal compound which is decomposible or reducible at atmospheric pressure at a temperature ranging from 800°C to 1400°C to produce metal and a gaseous product of decomposition, said metal compound being molybdenum, sulphide, tungsten sulphide, titanium sulphide, niobium sulphide, tantalum sulphide, chromium chloride, zirconium sulphide or mixtures thereof, and (o) substrate particles of diamond, cubic boron nitride or a mixture thereof, said particles ranging in size from 10 microns to 2000 microns for diamond, or in the case of cubic boron nitride 10 to 500 microns, to mechanically smear a coating of said metal compound onto the surface of said substrate particles, said compound being coated on at least 50 percent of the surface area of said substrate particles; 2) recovering the metal compound-coated substrate particles; and 3) firing said metal compound-coated substrate particles in a reducing or inert atmosphere at a temperature ranging from 800°C to 1400°C to decompose or reduce the metal compound and produce said abrasive particles.

The process of the present invention produces coatings on diamond and CBN particles that overcome the shortcomings of the prior art. Adherent coatings, are applied, which are chemically bonded to diamond by virtue of an intermediate metal carbide Zone or layer and to CBN by virtue of an intermediate metal boride and/or nitride zone or layer. The coating metals include molybdenum and tungsten, or alloys thereof which closely match diamond and CBN in thermal expansion so as to produce low stress bonds.

Metal/particle material compound(s) is defined as a compound(s) of the coating metal(s) and the material of the substrate particle (i.e., for diamond, metal carbide and for CBN, metal boride and/or metal nitride).

In carrying out the process, milling balls are used which are non-metallic and non-ceramic. Specifically, the milling balls of the present invention are made of plastics or elastomer. Examples of useful plastics are polyethylene, polypropylene and polystyrene. The elastomer can be natural or synthetic rubber. The balls should be sufficiently resilient so that during the milling step, the substrate particles do not cut into the balls and become lodged therein, or alternatively, crush the balls preventing proper mechanical smearing of the metal compound onto the substrate particles. Also, the milling balls should have sufficient elasticity so that they do not crush or chip the substrate particles.

The milling balls can vary in shape as long as such shape is effective ln mechanically smearing the metal compound onto the surface of the substrate particles in accordance with the present process. For best results, the surface of the balls should be round. Preferably, the balls are spherical but they can, for example, be cylindrical. They should have a diameter ranging from 1/16 Inch to 1/4 Inch. Milling balls having a diameter significantly greater than 1/4 inch are not suitable because they would result in an insufficient number of balls per unit volume to provide adequate surface area necessary to effectively coat the compound on the diamond particles whereas milling balls having a diameter less than 1/16 inch are too difficult to separate from the diamond particles. Where cylindrically shaped balls are used, they should not have a length greater than twice their diameter or less than about one-half their diameter.

The metal compound is used in the form of a particulate solid which can range ln size from less than one micron to the size of the diamond particles being coated. Metal compound particles having a size significantly larger than that of the substrate particles are not effective because they do not provide sufficient surface area for proper mechanical smearing of the compound onto the surface of the substrate particles. However, compound particles larger than that of the substrate particles are useful if they crush during the ball milling step. Preferably, the particle size of the metal compound is about one-tenth the size of the substrate particles being coated.

-JThe metal compound used in the present process is substantially completely decomposible or reducible at atmospheric pressure at a temperature ranging from 800°C to 1400°C to metal and gaseous product or products of decomposition.

Also, the metal compound is preferably a layer lattice compound, i.e. a compound with a low shear strength between its lattice layers which allows layers of the compound particles to be rubbed off mechanically in the present ball milling step. Representative of the metal compounds useful in the present process are molybdenum sulfide (MoS2), tungsten sulfide (WS2), titanium sulfide (TiS2), niobium sulfide (Nbs2), tantalum sulfide (TaS2), chromium chloride (CrCl3) and zirconium sulfide (ZrSj).

The diamond crystals, i.e. the first type of substrate particles, for use in the present invention include natural or man-produced diamonds. The present diamond particles range in size from 10 microns to 2000 microns.

The CBN crystals, i.e. second type substrate particles, for use in the present invention range in size from 10 microhs to 500 microns.

In carrying out the present process, the milling or ball milling step should preferably be carried out in a container or mill made of a non-metallic, non-ceramic material which does not crush or chip the substrate particles. Specifically, the container or mill is made of plastics such as polyethylene or of an elastomer such as natural or synthetic rubber. The extent to which the container is filled with milling balls, the amount of substrate particles and the amount of metal compound used is determinable empirically depending largely on the size of the balls and particles as well as the extent of the coating of metal compound on the substrate particles desired. Generally, the metal compound coating on the substrate particle - 8 42894 ranges from 2% to 20% by weight of the substrate particle. For best results, about two-thirds of the container is filled with milling balls, and the substrate particles and metal compound particles are then added to the container. The milling step can be carried out in a conventional manner. Specifically, the mill or container can be rolled, preferably at a moderate rate of speed to prevent chipping of the substrate particles, using conventional ball milling apparatus. When the desired extent of compound is coated on the surface of the substrate particles, the coated substrate particles can be separated from the milling balls and any excess metal compound powder using suitable wire mesh sieves.

Xn a preferred embodiment of the present process which provides good control of the final product, the milling balls and metal compound particles are placed in the container and milled sufficiently to produce a coating of the metal compound on the milling ballB. The compound-coated milling balls can then be separated from excess compound powder by a conventional technique such as by using suitable wire mesh sieves. The coated balls are then placed in the mill along with the substrate particles and milled whereby the compound-coating on the balls is mechanically smeared onto the substrate particles. In this way better control of the metal compound-coating on the substrate particles is achieved and separation of the resulting compound-coated substrate particles from the milling balls is more easily carried out by a conventional technique such as by using suitable wire mesh sieves.

The present process is controllable to produce the present coated substrate particle, i, e, abrasive particle, in a number of embodiments, all of which have metal/particlo material compound(s) chemically bonded to the surface of the substrate particle which forms during firing by solid state diffusion between the atoms of -9tlio materials of the substrate particle and the metal atoms of the initially deposited metal compound. Specifically, when using diamond, tlie abrasive particle consists essentially of diamond particle having an adherent covering composed of an outside surface coating having a composition which id a matal, a carbide of the same metal or a mixture of said, iasfial'sixd cstal carbide, and a layer of carbide of said inatal intermediate said surface coating and diamond chemically bonding the surface coating to the diamond surface. When using CBN, the abrasive particle consists essentially of a CBN particle having an adherent covering composed of an outside surface coating having a composition which is a metal, a boride and/or nitride of the same metal, or a mixture of said mstal and metal boride and/or nitride, and a layer of boride and/or nitride of said metal intermediate said surface coating and CBN chemically bonding the surface coating to the cubic boron nitride surface. the present invention will be further described by way of exanple only, with reference to the accompanying drawings in which: FIGURE 1 is a photomicrograph (magnified 1000 X) showing tungsten metal coated diamonds made by firing WSg coated diamonds in hydrogen at 800°G in accordance with the present process, EIGURE 2 is a photomicrograph (magnified about 1000 X) of molybdenum metal coated diamonds after reducing MoO^ coating by firing in hydrogen at 800°C prepared in accordance with the present process.

FIGURE 3 is a photomicrograph (magnified 20,000 X) of the surface of a diamond particle obtained by firing molybdenum sulfide coated diamond in hydrogen at 1200°C prepared in accordance with the present process. •iO 2884 FIGURE 4 is a photomicrograph (magnified 200 X) showing typical crystals of uncoated cubic boron nitride.

FIGURE 5 is a photomicrograph (magnified about 100 X) showing cubic boron nitride crystals coated with tungsten sulfide in accordance with the present process.

FIGURE 6 is a photomicrograph (magnified 100 X) showing the tungsten coating obtained by firing the coated crystals of FIGURE 5 in accordance with the present process.

The present covering is rough and granular. It can be non-uniform or substantially uniform, or be discontinuous Or continuous. The more interconnecting or continuous the covering, the more contact it has with the substrate particle surface and the less likely it is to be chipped off. Also, ,the more uniform the granular structure, the less likely it is to be chipped off. In contrast to grains which are round, the structure of the grains in the present covering is flaky, or planar and it is the planar surface of the grains that is usually bonded, i. e. at which bonding is effected. The covering covers from at least 50 percent of the surface area of the substrate particle.

The covering on the substrate particle, i. e. outside surface coating and intermediate layer of the metal/partlcle material eompound(s) can vary in thickness and generally ranges in thickness from about 1 micron to 100 microns. Specifically, the intermediate layer of metal/particle material compound(s) can be as thin as 2 Angstroms and detectable by transmission microscopy with thicknesses of 3 Angstroms or greater detectable by x-ray diffraction analysis.

In the present process, the firing temperature ranges from 30 800°C to 1400°C and firing is preferably carried out for a period of time sufficient to substantially or completely decompose or reduce th^ metal compound coating lo produce tbe present -ilcovering of metal and/or mctal/parlicle material compound(s).

Firing temperatures significantly higher than 1400°C are not useful since they tend to graphitize the surface of the substrate particle (for CBN thio means reconversion of the surface to hexagonal boron nitride) which would be deleterious to grinding performance and would prevent adequate retention in metal or vitreous wheel bond systems.

The particular firing temperature used depends largely on the specific metal compound used, the firing atmosphere and thd ' particular type of coated, i, e. covered, abrasive particle o o desired. Generally, firing temperatures of S00 C to 900 C produce a substrate particle having a covering composed of an outer surface coating of metal with a layer of metal/particle material compound(s) intermediate the metal surface coating and the substrate particle. At temperatures about 900°C, the outer surface coating is comprised of metal and the metal/particle material compound(s), with the amount of metal decreasing with increasing firing temperature, or the outer surface coating can be entirely of the metal/particle material compound(s).

* If desired, the uniformity and continuousness of the covering can be:significantly increased at relatively low firing temperatures ranging from 800 C to 1000°C by initially heating the compound-coated substrate particles preferably in air at a temperature ranging from 400°C to 700°C to convert the metal compound to metal oxide, and then firing the resulting oxide-coated particle in a reducing atmosphere such as hydrogen to reduce the oxide and produce the present covering. This resulting covering usually has a substantially webbed or feathery interconnecting structure which is highly adherent to the surface of the substrate particle.

The process of this invention also provides the capability of producing a stable etched surface for substrate particle i.e. diambnd. or.-cubic, boron nitride which is - 12 42894 essentially unique. Specifically, at firing temperatures ranging from about 1100°C to 1400°C, significant diffusion of the metal/ particle material compound(s) or metal and mctal/particle material compound(s) over the surface of the substrate particle occurs accompanied by numerous localized diffusion reactions produced by metal atoms penetrating, i. e. etching, the substrate particle surface forming tracks of metal/particle material compound(s) therein resulting in a change in the contour of the substrate particle. The resulting abrasive particle consists essentially of particle having a highly adherent significantly continuous covering of metal/particle material compound(s). However, where an initially relatively thick deposit or coating is made, the covering may have a minor amount of metal in its outside surface coating so that such surface coating will consist essentially of a minor amount of metal and a major amount of metal/particle material compound(s). Much of the covering, i. e. at least about 50%, is of a substantially uniform fine granular form, i. e. finer and more uniform than that normally obtained at firing temperatures below 1100°C, with a multitude of furrows usually substantially uniformly distributed therein and running significantly parallel to each other. These furrows are indicative of bonding of the covering below the surface of the substrate particle by the metal/particle material compound(s) tracks formed therein. Since this covering is adhered both to the surface and below the surface of the substrate particle, it cannot be broken away from the substrate particle in any significant amount without breaking part of the substrate particle. The metal/particle material compound(s) tracks etching, i.e. penetrating, the surface of the substrate particle are detectable by techniques such as x-ray diffraction analysis and by microprobe analysis.

The firing atmosphere used in the present process can be varied, i. e. it can be a reducing atmosphere such as hydrogen or -l3it can be an inert atmosphere such as argon or a vacuum, but it should be an atmosphere which has no significant deleterious effect on the metal compound-coating or substrate particle or the resulting covering of metal and metal/particle material compound(s).

Generally, it is a reducing atmosphere such as hydrogen which reduces or reacts with the metal compound to produce the present adherent covering composed of metal and/or metal/particle compound(s) coating and layer.

The firing step may be performed in a number of ways.. For example, it can be carried out batchwise, or in a continuous manner using a fluidized bad, or a moving belt, in any suitable furnace using quarts, Vycor ί Trade Mark)' or alumina ceramic crucibles, depending upon maximum temperature used. No significant sintering of the fired diamond particles occurs in the present process, and the final fired coated substrate particles are easily broken up into free flowing powder.

The present method provides a number of advantages. One advantage is that it automatically produces a very rough covering, which can be varied in both roughness and thickness through control of the milling step parameters and relative amount of input feed materials. For example, with longer milling periods, larger amounts of input feed materials of finer size and smaller sized milling balls, thicker coatings of the metal compound on the diamond particles are produced resulting in thicker fired coverings of metal and/or metal carbide. Also, very rough fired coverings of metal and/or metal/particle material compound(s) can be produced by initially milling ths substrate particles with a mixture of large and small sized particles oi the matal compound thereby mechanically smearing a substantially uneven undulating deposit or coating on fche substrate particle which, when fired, results in a correspondingly rough covering. Additional advantages arc that the process equipment is inexpensive, and the technical process control requirements a’rb minimal. ·· ' -3s42894 The coaled, i.e, covered, substrate particles, i.e. abrasive particles, of this invention are suitable for use in all types of abrasive and cutting tools, for example, resin bond or metal bond -15The invention is further illustrated by the following ..Examples 1 to 3, 5, 7, and 9 where conditions were as follows unless otherwise noted: The milling balls in each of the examples were sufficiently resilient so that during the milling of rolling step, the diamond particles did not cut into the balls and become lodged therein or crushed or chipped by the balls.

All milling or rolling was carried out dry in air at room temperature on a conventional laboratory mill.

All firing was carried out in a ceramic tube furnace.

EXAMPLE 1 A clean polyethylene bottle, about 2 3/8 inches high by 7/8 inch inside diameter was charged two-thirds full with polytetrafluoroethylene (Teflon) (Registered Trade Mark) sgSsres, ile. balls, which ware a mixture of 1/4 inch diameter balls and 1/8 inch diameter balls with the number of 1/8 inch diameter balls being used being about double that of the 1/4 inch diameter balls. Three grams of powdered molybdenum sulfide, (M0S2), having a particle size of less than 40 microns were also added to the bottle which was then closed and rolled, i.e. milled, on a laboratory mill, at 60 RPM for 24 hours.

Examination of the resulting balls showed them to be uniformly coated with about 0.73 gram of MoS£, the -164 3 8 9 4 remainder being coated on the inside walls of the bottle.

The coated balls, along with 2 grams of 100/120 mesh, i.e. about 125 microns to 149 microns in particle size, synthetic diamond, MBG (Metal Bond Grinding) Man-liade diamonds, were placed in another identical clean bottle, together with an additional 0.120 gram of MoSg powder less than 40 microns In size, and rolled for 24 hours at 60 RPM on the laboratory mill. The balls and coated diamonds were then separated by suitable wire mesh sieves. The compound10 coated diamonds weighed 2,130 grams, indicating a MoS^ coating weight of 0.130 gram. About 807. of the surface area of the diamond particles was coated with MoSg.

EXAMPLE 2 This example was carried out in the same manner as set forth in Example 1 except that all the Teflon balls had a diameter of 1/8 inch. The resulting MoS^ coated diamond particles appeared similar to those produced in Example 1 with at least about 807. of the surface area of the diamond being coated except that the M0S2 coating weight for 2 grams of diamonds was less, i.e. it was 0.082 gram.

EXAMPLE 3 The already M0S2 coated Teflon balls and bottle which were used in Example 1 to coat diamonds were reused in this example without adding any additional M0S2 powder to coat 2 grams of 100/120 mesh MBG diamond particles -17which were added to the bottle. The milling time was 40 hours at 60 RPM. The Teflon balls and coated diamonds were then separated by suitable wire mesh sieves. The resulting M0S2 Coated diamonds had a smoother coating than that obtained in Example 1 which covered at least 80S • of the surface area of the diamond particles. The total MoSg coating weight on the diamond particles was 0.120 gram,. · EXAMPLE 4 A clean polyethylene bottle identical to that used in Example 1 was charged two-thirds full with 1/8 inch long soft rubber cylinders and 1.5 grams of powdered M0S2 having a particle sise less than 40 microns. The bottle was then closed and rolled for 24 hours at 60 RPM, At the end of this time it appeared that substantially all of the MoSg powder coated the rubber cylinders and no significant amount of powder was left on the inside walls of the bottle. EXAMPLE 5 A clean polyethylene bottle identical to that used in Example 1 was charged two-thirds full with 1/8 inch diameter Teflon balls and 2,5 grams tungsten sulfide (WS£) powder having a particle size of less than 40 microns. The bottle was closed and rolled at 60 RPM for 24 hours. At the end of this time, examination showed essentially all the WSg was smeared onto the Teflon balls, with only 0.18 -1842894 gram on the inside bottle walls. 2 grams of 100/120 mash, i.e, about 125 microns to 149 microns In particle size, synthetic diamond particles, MBG (Metal Bond Grinding) Man-Made'' diamonds, were then added to the bottle which was then closed and rolled for 48 hours at 60 RPM. A substantially smooth WS^ coating was obtained which covered at least about 80% of the surface area of the diamond particles, Samples of the compound coated diamond .particles 10 from Examples 1, 2, 3 and 5 were fired as indicated ln Tabla I at various temperatures, and the resulting fired -19s to •fl Gf •rf ¢3 ti N ti S3 3 ti C Μ p 0) M efi tO ri* O eo « & .ti &M 04 *«»✓ erf •rf J3 · fl 64 fi OH «rl O£g irf 3 u CO η uo 03 . fl ,β k H Ίί ti 44 oj r-a ω a ή * a «ο 64 03 fl $4 63 <13 60 & Η O 03 GJ M .ti fl W ♦α m a < 04 04 •rf § I »4 CO ί fl GJ M G) «a g S ti 0e ti ti fi o ti ti ti (rf *O •ti 64 U3 •Ω Wa >» Pl to o »rf «rf ti J2 O 0 s ti 7*4 $4 ?rf A. ti ti ti & ti I-i •rf ! M to ft 1“ I eo β I ! u to If to •rf in I—i cn ti » ti »0 ti H rrf rf •rf rf eo ti ti ti ua ti •rf /3 64 us b ifi &6 3 8 S ti u 3 C Irf ti ti M *J Cl o jfi 0 3 © W fi •rf a pm 64 •η ω trf ti u £ » «rf Sn .s vo o «rf 64 £^ 64 erf jf q S es ti m T fc ω «rf O P trf ti 03 y £ a a e « • B 2 ι ΐ pf1 73 o cn § ,§ •ti fi O pr ti «rf a o cn &6 a o us » 0) υ u CMti Ο H S w ♦fl •s fi © ’S σ g R •a •d © 0 ti fi fl CM H cu •rf o o ai w •rf •o § §B •rf r© •d «Λ •rf •rf ti o , U 8 i •0 Ό ti * CM 04 » ft ti CM £c O * 1 ft ft fc CO «* & & & 64 ί£ Σ5 ti •rf CM w ti o CM CM CM W W PS © cn I e © cn trf fl o US pf4 pT trf fl © US ti ‘ ti fl. ti g O 3o pH OA 0° o in m o °o © co o % © rrf trf £ © 05 £ o CM erf o % o co O % o o ♦rf U % o o •rf O °O o 00 0° in erf M5 o o tn rrf (rf •rf erf erf rrf erf •rf cn cn in m m in ti trf 6« ti •rf A. trf ti r-i S* ti •rf fl. ti •rf fl. ο •rf A· ti erf CU ri ti »rf £ ti •rf a. G ti •rf λ G ¢3 trf ί 9 g W B 3 ϊ M td | td § W td δ κ td δ κ td δ κ td δ κ td κ w ro M td ti erf! d CO «4 m -SO42834 All of the Sample Nos. in Table I, except Sample No. 5C, illustrate the present invention. Specifically, Sample Nos, 1Λ, ID, 3Λ, 3B, 5A and 5B, illustrating the present invention, show that the morphologies of the coatings do not change significantly when fired in H2 alone. FIGURE 1, which shows tungsten metal covered diamond particles produced in Sample No. 5B, shows the flaky-flat-type granular structure produced by the present process. Sample Nos. IB-1 and IB-2 illustrate that the structure of the Mo coating or covering can be· modified to a more interconnecting, feathery structure shown in FIGURE 2 by firGt converting to MoOj by firing in air, followed by reduction in H2, Sample Nos, 1C, IE and 5D illustrate the present etched diamonds. Specifically, as illustrated by Sample Nos, 1C and IE, reducing the MoS2 coating at 1100°C or 1200°C in results in etching of the diamond surface in accordance with the present process as well as complete conversion of Mo to Mo2C, FIGURE 3, which shows the fired covering produced in Sample No, IE, illustrates the fine granular continuous structure attained with significantly parallel furrows substantially uniformly distributed therein. In Sample 5D the fired covering was composed of W metal and W2C, In all of the Sample Nos, of Table I illustrating the present fired coverings, the fired coverings covered -21• from at least about 707» to about 95% of the surface area of the diamond particles and ranged in thickness from about one micron to 100 microns. Also, in Sample Nos. IB-2, 5Λ and 5B, the intermediate metal carbide bonding layer, although not show by X-ray diffraction analysis, would have been detectable by transmission microscopy. Sample No. 5C showed no conversion because the firing temperature was too low, EXAMPLE 6 In order to determine adhesive strength and abrasion resistance of the fired coverings of the present invention, fired specimens from Sample Nos. IA, 1C, 5A and . 5D were placed in small glass vials and shaken vigorously for 5 minutes, using a dental wiggle-Bug, a device for mixing dental amalgam» After Such shaking, although portions of the outside surface material broke off, microscopic examination of the specimens tested showed that the layer or covering bonded to the diamond surface remained adherent.

EXAMPLE 7 Synthetic diamond particles, 140/170 mesh, i.e, 105 microns to 88 microns in particle size, were used.

A portion of the diamond particles was coated with a coating of tungsten sulfide (l^) substantially as set forth in Example 5. The WS2 coated diamond particles were fired at 1100°C for 1 hour in an atmosphere of hydrogen resulting in a fired covering of tungsten carbide (N^C). -2210 42894.

Another portion of the diamonds was coated with a coating of WSg substantially as set forth in Example 5, The resulting WSg coated diamond particles were fired at 1020°C for 1 hour in an atmosphere of hydrogen resulting in a fired coating of tungsten.

A third portion of tlie uncoated diamonds was used as a control.

Nickel in an amount of 56 weight % was deposited on all of these diamond particles.

Dry Grinding Conditions were as follows: M/min. 1.5 M/rain. ,064 mm Cemented tungsten carbide cobalt Wheel speed Table speed Infeed Material Grinding results are shown in Table II: Table II Test results at 0,064 mm infeed: W2C coated diamonds W coated diamonds Control Test results at 0.076 mm infeed: Grinding Ratio 33.8 33.1 24.8 Grinding Ratio 22.4 22.3 18.4 WgC coated diamonds W coated diamonds Control The Grinding-Ratio Is the ratio of the volume of material removed from the workpiece to the volume of grinding -23tool used during the grinding operation. Obviously, the higher the Grinding-Ratio the better the grinding properties of the particular grinding wheel. Table XI shows higher grinding ratios for the coated diamond particles prepared according to the present invention EXAMPLE 8 A clean polyethylene bottle, about 2 3/8 inches high by 7/8 inch inside diameter was charged two-thirds full with polytetrafluoroethylene (Teflon) 1/8 inch diameter speres, i. e. balls. 1, 239 grams of powdered tungsten sulfide, (117¾). having a particle size of less than 40 microns were also added to the bottle which was then closed and rolled, i. e. milled, on a laboratory mill, at 60 RPM for 3 hours. Examination of the resulting balls showed them to be uniformly coated with about 0. 892 gram of WSg, the remainder being coated on the inside walls of the bottle. The coated balls, along with 1.116 grams of 100/120 mesh, i.e. about 125 microns to 149 microns in particle size, CBN were placed in another identical clean bottle and rolled for 19 hours at 60 RPM on the laboratory mill. The balls and coated CBN particles were then separated by suitable wire mesh sieves. The compound-coated CBN particles weighed 1.415 grams, indicating a WSg coating weight of 0.299 gram. About 100% of the surface area of CBN particles was coated with WSg and is shown in FIGURE 5.

EXAMPLE 9 The WSg coated CBN particles of Example 8 were fired in an atmosphere of hydrogen at a temperature of 1040°C for one hour. The resulting fired particles had a very adherent substantially continuous rough granular coating or covering as shown by FIGURE 6. The granular structure of the covering was flaky in character of substantially uniform fine size, X-ray diffraction analysis of the fired .coated particles determined that the covering ' · ' - 24 - · 4289 4 or coating on the CBN particles was tungsten metal. Also, the intermediate metal boride and/or nitride bonding layer, although not shown by X-ray diffraction analysis, would have been detectable by transmission microscopy.

EXAMPLE 10 In this example, the tungsten metal coated CBN particles produced in Example 9 were tested to determine the adhesive strength and abrasion resistance of the tungsten metal coating or covering. Specifically, the fired coated particles were placed in a small glass vial and shaken vigorously for 5 minutes, using a dental ”wiggle-Bug, a device for mixing dental amalgam. After such shaking, microscopic examination showed that some of the CBN particles had cleaved along cleavage crystal planes but that the tungsten metal covering remained adherent and that there was nob significant chipping or loss of the tungsten covering or coating.

EXAMPLE 11 A clean polyethylene bottle identical to that used in Example 8 was charged two-thirde full with 1/8 inch long soft rubber cylinders and 1.5 grams of powdered MoS^ having a particle size less than 40 microns. The bottle was then closed and rolled for 24 hours at 60 RPM. At the end of this time it appeared that substantially all of the M0S2 powder coated the rubber cylinders and no significant amount of powder was left on the inside walls of the bottle.

The present coated CBN particles are highly useful as 25 abrasives. Specifically, the rough granular adherent covering on the CBN particles produced by the present process results in significantly improved grinding performance and retention in all types of wheel bond systems, i.e. resin, vitreous or metal. Also, the improved bond strength in tool matrices significantly extends the life of the abrasive.

Claims (17)

1. ) providing milling balls made of plastics or elastomer having a diameter ranging from 1/16 inch to 1/4 inch; 1) milling (a) balls made of plastics or elastomer

1. An abrasive particle consisting essentially of a substrate particle of diamond in the size range 10 to 2000 microns, cubic boron nitride in the size range 10 to 500 microns or a mixture thereof, having a rough adherent flaky granular covering, said covering being composed of an outside surface coating being composed of metal a compound of said metal and the material of said substrate particle or a mixture thereof, with all mixtures of said metal and/or said metal/substrate particle material compound capable of being present, and a layer of a compound or compounds of said metal intermediate said surface coating and said substrate particle chemically bonding said surface coating to said substrate particle, said metal being selected from molybdenum, tungsten, titanium, niobium, tantalum, chromium, zidonium and alloys thereof, said covering being non-uniform or substantially uniform, and covering at least 50 percent of the surface area of said substrate particle. 2. ) providing particles of a metal compound which is decomposible or reducible at atmospheric pressure at a temperature ranging from 800°C to 1400°C to produce metal and a gaseous product of decomposition, said metal compound being molybdenum sulphide, tungsten sulphide, titanium sulphide, niobium sulphide, tantalum sulphide, chromium chloride, zirconium sulphide or a mixture thereof. 28 42894 2) recovering the metal compound-coated substrate particles; and

2. An abrasive particle according to Claim 1 wherein said covering is composed of said metal/particle material compound or said covering is composed of an outside surface coating composed of a minor amount of metal and a major amount of said metal/particle material compound and a layer of a compound of said metal and the material of said substrate particle intermediate said surface coating and substrate particle, said covering being significantly continuous with a significantly uniform fine granular structure with a multitude of furrows distributed therein running significantly parrallel to each other, said covering also being bonded to the substrate particle by tracks of a compound of said metal and the material of said substrate particle penetrating the substrate particle surface. 3. ) providing substrate particles ranging in size from 3) firing said metal compound-coated substrate particles in a reducing or inert atmosphere at a temperature ranging from 800°C to 1400°C to decompose or reduce the metal compound and produce said abrasive particles.

3. An abrasive particle according to any one of Claims 1 to 3, wherein said substrate particle is diamond and said com5 pound of said metal and the material of said substrate particle is metal carbide. 4. ) milling said milling balls and said particles of

4. An abrasive particle according to claim 1 or claim 2, wherein said substrate particle is cubic boron nitride and said compound of said metal and the material of said substrate 5. ) recovering the metal compound-coated substrate 5 metal compound and substrate particles to mechanically smear a coating of said metal compound onto the surface of said substrate particles, said compound being coated on at least 50 percent of the surface area of said substrate particles;

5. A process for producing abrasive particles consisting essentially of substrate particles of diamond, cubic boron nitride or a mixture thereof which comprises: 6. ) heating said metal compound-coated substrate particles in air to oxidize said metal compound to metal oxide; and

6. A process according to Claim 5 wherein said firing temperature ranges from 1100°C to 1400°C producing penetration of the substrate particle surface by tracks of a compound of said metal and the material of said substrate particle. 7. ) firing the resulting metal oxide-coated substrate 15 particles in a reducing atmosphere at a temperature ranging from at least 800°C to 1000°C to reduce the metal oxide and produce said abrasive particles.

7. A process according to claim 5 or claim 6 wherein said milling balls are initially milled with said particles of metal compound to produce metal compound-coated balls and the compound-coated balls are then milled with said substrate particles to mechanically smear the metal compound coating thereon.

8. A process for coating substrate particles consisting essentially of substrate particles of diamond, cubic boron nitride or a mixture thereof comprising:

9. A process according to any one of Claims 5 to 8, wherein said substrate particle is diamond and said compound 20 of said metal and the material of said substrate material is metal carbide.

10. A process according to any one of Claims 5 to 8, wherein said substrate particle is CBN and said compound of said metal and the material of said substrate particle is metal boride 25 and/or metal mitride. 10 particles; 10 microns to 2000 microns in the case of diamond and 10 to 500 microns in the case of CBN; 10 particle is metal boride and/or metal nitride.

11. Abrasive particles consisting essentially of a substrate of diamond as claimed in claim 1 substantially as hereinbefore described in any one of Examples 1 to 3,5 and 7.

12. Abrasive particles consisting essentially of a substrate of cubic boron nitride as claimed in claim 1 substantially as hereinbefore described in Example 9.

13. A process· for producing abrasive particles consisting essentially of substrate particles of diamond,- as claimed in claim 5 substantially as t hereinbefore described in any one of Examples 1 to 3, 5 and 7.

14. A process for producing abrasive particles consisting essentially of substrate particles of cubic boron nitride as claimed in claim 5 substantially as hereinbefore described in Example 9.

15. An abrasive particle when produced by a process as claimed in any one of claims 5 to 8. 15 having a diameter ranging from 1/16 inch to 1/4 inch, with (b) particles of a metal compound which is decomposible or reducible at atmospheric pressure at a temperature ranging from 800°C to 1400°C to produce metal and a gaseous product of decomposition, said metal compound being molybdenum sulphide, 20 tungsten sulphide, titanium sulphide, niobium sulphide, tantalum sulphide, chromium chloride, zirconium sulphide or mixtures thereof,and (c) substrate particles of diamond, cubic boron nitride or a mixture thereof, said particles ranging in size from 10 microns to 2000 mocrons for diamond, or in the case of 25 cubic boron nitride 10 to 500 microns, to mechanically smear a coating of said metal compound onto the surface of said substrate particles, said compound being coated on at least 50 percent of the surface area of said substrate particles?

16. An abrasive particle consisting essentially of substrate particles of diamond when produced by a process as claimed in claim 9 or claim 13.

17. An abrasive particle consisting essentially of substrate particles of cubic boron nitride when produced by a process as claimed in claim 10 or claim 14.