CA1193451A - Randomly-oriented polycrystalline silicon carbide coatings for abrasive grains - Google Patents
Randomly-oriented polycrystalline silicon carbide coatings for abrasive grainsInfo
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Abstract
RANDOMLY-ORIENTED POLYCRYSTALLINE SILICON CARBIDE COATINGS
FOR ABRASIVE GRAINS
ABSTRACT OF THE DISCLOSURE
Disclosed is a composite abrasive particle com-prising a core abrasive crystal and a silicon carbide coating inherently bonded to substantially all of the exterior surfaces of said core crystal, said silicon carbide coating characterized by silicon carbide crystals having a random orientation substantially independent of the structure of the core crystal, the outer surface of the coating being conformationally irregular. Aggregates of the composite abrasive particles are interconnected by a matrix of silicon carbide which has an open structure. Preferred core abrasive crystals are diamond or cubic boron nitride. The composite abrasive particles preferably are made by infiltrating core crystals bonded together by silicon carbide and elemental silicon, leaching substantially all of the silicon from the bonded mass with a silicon leaching agent, sub-dividing the resulting leached mass, and recovering the composite abrasive particles.
FOR ABRASIVE GRAINS
ABSTRACT OF THE DISCLOSURE
Disclosed is a composite abrasive particle com-prising a core abrasive crystal and a silicon carbide coating inherently bonded to substantially all of the exterior surfaces of said core crystal, said silicon carbide coating characterized by silicon carbide crystals having a random orientation substantially independent of the structure of the core crystal, the outer surface of the coating being conformationally irregular. Aggregates of the composite abrasive particles are interconnected by a matrix of silicon carbide which has an open structure. Preferred core abrasive crystals are diamond or cubic boron nitride. The composite abrasive particles preferably are made by infiltrating core crystals bonded together by silicon carbide and elemental silicon, leaching substantially all of the silicon from the bonded mass with a silicon leaching agent, sub-dividing the resulting leached mass, and recovering the composite abrasive particles.
Description
s~
RANDOMLY-ORIENTED POLYCRYSTALLINE SILICON CAR~IDE COATINGS
FOR AB~ASIVE GRAINS
Background of the Invention The present invention relates to silicon carbide coated abrasive grains, such as diamond and cubic boron nitride, and more particularly to unique randomly-oriented polycrystalline silicon carbide coatings therefor.
Taylor (U.S. Patent No. 3,520,667 dated July l4, 1970) proposes to coat diamond abrasive particles with an adherent surface coating of silicon carbide by suspending the diamond particles in a gaseous atmosphere of a volatile silicon compound which is thermally decomposed to form a silicon carbide coating on the diamond particles by reaction with the carbon present in the volatile silicon compound or added gaseous source of carbon. Kuratomi (U.S. Patent No.3,617,347 dated November 2~ 1971) uses an analogous vapor deposition technique to coat diamond powder with a thin film of silicon carbide formed from the gaseous silicon compound and the diamond particle itself which thin silicon carbide coating then is .recoated with elemental si.licon. Silicon-based alloy coatings for abrasive grains have been proposed by Otopkov (U.S~
Pa~ent No. 4,184,853 dated January 22, 1980) wherein the coating is a three-component system which includes silicon and two other metals selected from a rscited group of metals, and by Ca~eney (U.S. Pa~n-t No.3,929,432 . ~J
~9~
dated December 30, 1975) wherein a titanium alloy coats the diamond grainO U.S. Patent No. 4,174,971 to Schrewelius dated November 20, 1979, describes the in situ formation of silicon carbide by a pyrolysis process.
A prime use of these various coated abrasive particles is in the formation of grinding elements wherein the coated abrasive particles are bonded together with resin, metal, vitreous material, or the like. A prevalent problem in using such bonded coated abrasive particles is that the particles tend to pull out from the bonding matrix, thus resulting in decreased grinding performance and a loss of valuable abrasi~e particles. The coatings on the abrasive particles provide a degree of increased retention of the particles within the bonding matrix.
Still, improvements in this art are needed.
Broad Statement o'f the Ihvention One aspect of the present invention is a composite abrasive particle comprising a core abrasive crystal selected from the group of diamond and cubic boron nitride, and a silicon carbide coating adherently bonded to substantially all of the exterior surfaces of said core crystal. The improved silicon carbide coating is characterized by silicon carbide crystals having a random orientation substantially independent of the structure of the core crystal, the outer surface o~ the coating being conformationally irregular.
Another aspect of the present invention is a composite a~rasive aggregate comprising core crystals selected from diamond, subic boron nitride, and mixtures thereof, disposed within a silicon carbide matrix which matrix is ~dherently bonded to substantially all of the exterior surfaces of each of the core crystals. The silicon carbide matrix is characterized by silicon 60SD~127 -- 3 ~
carbide crystals having a random orientation substantially independent of the structure of the core crystals. The matrix interconnecting said core crystals has an open structure and the outer surface of the matrix is conformationally irregular.
A method for making the co~posite abrasive particle described ahove comprises~ providing core crystals coated with non-diamond carbonaceous material; infiltrating said crystals with fluid silicon under a partial vacuum at a temperature of above 1400C, cooling and recovering the resulting mass of core crystals bonded together by a bonding medium of silicon carbide and elemental silicon, the outer surfaces of said core crystals having silicon carbide adherently bonded to substantially all of the exterior surfaces thereof;
leaching substantially all of said silicon from said bonded mass with a silicon leaching agent; and subdividing (milling) the resulting leached mass and recovering said composite abrasive particles. Fibrous graphit~ can be provided in a uniform mixture with the coated core crystals to promote silicon infiltration.
There are two distinctions between the process as used to make the composite abrasive par-ticles described above and the composite abrasive aggregate. First, in the case of aggregates, the core crystals coated with non-diamond carbonaceous material should be tightly packed ~ince a high degree of contact between the coated crystals is needed. This high packing density may be used in the case of the single particles as well, but is not required. The high crystal concentration also dictates that a minium amount of fibrous graphite be used.
Secondly, and more importantly, to make aggregates, a relatively light or low energy milling (eg. less milling time) is required with sieving done more frequently to remove aggregates of the desired size (more than twice ~ 19 ~5~ 6OSD-127 a, the size o~ the starting core crystals). To obtain just coated diamond or CBN, a more vigorous milling can be used with sieving done at a size only slightly larger than the core crystal. For example, starting with 140/170 mesh (106/90 micron) diamond, silicon carbide coated diamond would be sieved out in the 100/120 mesh (150/125 micron) size while aggregates would be obtained by not milling beyond about 40/60 mesh (425/~50 micron.s).
Another aspect of the present invention is an improved abrasive element of coated abrasive particles embedded in a bonding matrix. The improvement is that said coated abrasive particles comprise a core abrasive crystal selected from the group of diamond and cubic horon nitride, and a silicon carbide coating adherently bonded to substantially all of the exterior surfaces o~ the core crystal, said silicon carbide coating characterized by silicon carbide crystals having a random orientation substantially independent of the structure of said core crystal, the outer surface of said coating being conformationally irregular.
Additionally, the coated abrasive particles in the improved abrasive element can comprise composite abrasive aggregates wherein the silicon carbide coated core abrasive crystals are interconnected by a silicon carbide matrix having an open structure. The preferred bonding matrix can be selected from the group of resin, metal, and vitrous material, and the preferred abrasive element is a grinding element.
Ad~antages of the present invention include a silicon carbide coating which is tightly bonded to the core particle and which provides excellent mechanical bonding in the bonding matrix. Also, the unique silicon carbide coating of the present invention provides a highly wear-resistent region, intermediate between the core abrasi~e crystal and the bonding ~OSD-127 matrix, around each abrasive grain which reduces loss of the coated crystal by chip erosion during use of the abrasive element. FurthQr, the composite abrasive aggregates of the present invention have an open structure between silicon carbide coated grains which, when filled with bonding matrix material during abrasive fabrication, provides for a positive locking of the aggregate in the bonding matrix. ~dditionally, since no metal coating is contained on ~he composite abrasive particles or aggregates of the present invention, the silicon carbide coating can be added at the expense of additional secondary abrasive in the abrasive element without detrimental effects even at higher concentrations of the coated abrasive particles in the abrasive element.
These and other advantages will beeome readily apparent to those skilled in the art based upon the disclosure herein contained.
Brief Description of the Drawings Figs. lA, lB, and lC are photomicrographs at three different magnifications (lOOX, 500X~ and 1500X, respectively) of the randomly-oriented polycrystalline silicon carbide coated diamond grains of the present invention; and Figs. 2A, 2B, and 2C are photomicrographs at three different magnifications (lOOX, 500X, and 1500X, respectively) of comparative silicon carbide coated diamond grains.
Detailed Description of the Drawings - The comparative silicon carbide coated diamond particles shown in Figs. 2A-2C were made by the preferred infiltration, leaching, and subdividing process of the present invention, except that the diamond grains were uncoated, i.e. not coated with non diamond carbonaceous material so that the silicon carbide coating was formed b~ a chemical reaction of diamond carbon with silicon.
The silicon carbide coated diamond particles in Figs.
60SD~127 lA-lC were made hy the pr~ferred method of the present invention wherein the diamond grits were coated with a non-diamond carbonaceous material for infiltration thexeof with fluid silicon. Note the relatively smooth outex surface of the comparative partioles and the apparent related orientation of the silicon carbide crystals of the coatin~. rrhis is to be contrasted with the exterior surface of the no~l particles o the present invention wherein an apparently total random orientation of the silicon carbide crystals can be seen. The silicon carbide crystals on the novel particles also appear to be much smaller in overall exterior dimensions (eg.about 0.1 - 10 microns) than the silicon carbide crys~als of the comparative particles.
Note further that the differen~ faces of the core diamond crystal are distinctly de~ined for the com-parative particles. The comperative particles also display a relationship or ordered pattern between the diamond surace w~ich supplied the carbon and the formed silicon carbide crystals. Note the silicon carhide crystal shape and orientation in Figure 2A on the octahedral (6-sided~ surfaces compared to the cubic (4 sided) surfaces. This illustrates the dependance of the coating sur~ace upon the underlying diamond surface which suppliea the carbon atoms for the formation of silicon carbide. On the oth~r hand~
the exterior surface of the novel particles o~ the - present invention exhibit a completely random orientation of silicon carbide crystals independent of the underlying diamond surface. The outer surface of the novel particles also is conformationally irregular and extremely rough in texture. The random orientation of the silicon carbide cr~tals on the novel coated particles of the present invention apparently s~ems from ~he fac~ that the silicon carbide s~o nucleation arises from the silicon carbide crystals of the coating rather than from the diamond core particle. Apparently, nucleation of the silicon carbide crystals on the comparati~e samples during their for-mation is based on the core diamond particle itself.Also, since nucleation of the silicon carbide in the coating of the novel particles is based upon the silicon carbide crystals in the coating itself, excellent intercrystal silicon carbide crystal bonding is displayed by the novel coated particles of the present invention. ~ased upon the texture and arrangement of the silicon carbide crystals of the coatings of the two different particles shown in the photographs, one would expect that the novel coated particles of the present. invention could be bound in a matrix to a much better degree than the comparative coated crystals. In fact, this has been demonstrated to be true as the example will amply demonstrate.
Detailed Description of the Invention Referring to the pre~erred method for ma]cing the composite abrasive particles and aggregates of the present invention, the preferred core abrasive crystals is selected from the group consisting of diamond, cubic boron nitride, and mixtures thereof when ma~ing the novel abrasive aggregates of the present in~ention. It must be recognized, though, that any abrasive particle (eg~ silicon carbide itself, aluminum oxide, etc.) may serve as the core abrasive - cxystal for coatiny with silicon carbide according to the precepts of the pxesent inventionO In view of the cr~stal arrangement and morphology of the silicon carbide crystals in the coating as shown in the drawing, it is apparent that the source of carbon for foxming the silicon carbide coating should not come from the core ahrasive crystal itself~ but rather an external source of carbon should be used~ Thus, the core abrasive crystals provided with a coating of non-diamond carbonaceous material. The particle size range for the preferred diamond and cubic boron nitride abrasive crystals range from about 325/400 to 60/80 mesh (United States standard sieves series) (45/38 to 250/1~0 microns) with particles of about 100/120 mesh and smaller being preferred. Coating of the core crystals with non-diamond carbonaceous material and the silicon infiltration s~ep of the present process desirably are practiced according to -the general procedure disclosed in U.S. Patent No. 4,220,455 to St. Pierre dated September 2, 1980. Note that somewhat similar processes also are disclosed in U.S.Patent Nos.4,148,894 to Hillig et al dated April 10~ 1979, 4,233,433 dated ~ecember 9, 1980; and 4,242,106 to Morelock dated December 30, 1980.
The carbonaceous material can be elemental non-diamond carbon, an organic material, or mixtures thereof. The organic material decomposes completely at an elevated temperature below 1400C, and ordinarily decomposes completely at a temperature ranging from about 50C to about 1200C, to produce elemental non-diamond carbon and gaseous product of decomposition.
Representative of the organic materials useful in the present process are polymers of aromatic hyd ~carbons such as polyphenylene and polymethylphenylene derivatives of polynuclear aromatic hydrocarbons contained in coal tar such as dibenzanthracene and chrysene. Additional examples of useful organic materials are the epoxy resins such as the reaction product of epichlorohydrin and Bisphenol-A. Still additional examples of useful organic materials are phenolic resins obtained by the condensation of phenol or sub~tituted phenols with aldehydes such as formalde-hyde~ acetaldehyde, and furfural. Specific examplesaxe the condensation products of phenol~formaldehyde, 3~
resorcinol-formaldehyde, aniline-formaldehyde, and cresolformaldehyde.
The non-diamond carbonaceous material coated core crystals nex~ are infiltrated with fluid silicon under partial vacuum at a temperature of above 1400 C, preferably using the apparatus and general procedure shown and described in U.S. Patent No. 4,220,455 to St. Pierre dated September 2, 1980. While the coated core crystals can be admixed with additional carbon for infiltrating with silicon, such is not necessary in forming ~he silicon carbide coated composite abrasive particles of the present invention. As noted in U.S.
Patent No. 4,220,455, the coated core crystals and a source of silicon are heated to an infiltration temperature which is above 1400C. When the carbonaceous material coating the core crystals is an organic material, such organic material decomposes completely at a temperature below 1400C producing elemental non-diamond carbon and gaseous products of decomposition.
Infiltration is carried out at a temperature above 1400C at which silicon becomes fluid and which has no significant deleterious effect on the crystals.
For cubic boron crystals infiltration temperatures significantly higher than about 1450C are not useful ~ince they are likely to cause conversion to hexagonal boron nitride~ On the other hand, for diamond crystals, temperatures higher than 1550C provide no significant advantage. By a temperature at which silicon becomes fluid it is meant herein a temperature at which the silicon is readily 10wable. The fluid silicon is highly moblle and highly reactiwe with elemental non-d~amond carbon, i.e~ it has an affinity for elemental non~diamond carbon, wetting it and reacting with it to foxm silicon carbide. Specifically, when sili~on is at its melting temperature, which has been given in the art to range from about 1412C to about 1430C, it has 5~
a high viscosity, bu-t as its temperature ls raised, it becomes less viscous and at a temperature about ten degress higher than its melting point, it becomes fluid.
The temperature at which the silicon is fluid is the temperature at which it will infuse or infiltrate through the capillary~size passages, interstices or voids of the moldconfined mixture of crystals. With increase in temperature, the flowability of the fluid silicon increases resulting in a faster rate of reaction.
Sufficient silicon is infiltrated throughout the mold-confined mass or mixture infusing or infiltrating through the voids or pores of the mixture by capillary action to react with the total amount of elemental non diamond carbon (including fibrous graphite) present in the confined mixture forming silicon carbide, and also to fill any pores or voids which may remain after formation of the silicon carbide. Any pores remaining after formation of silicon carbide are filled during infiltration with elemental silicon. Also during infiltration, the silicon reacts with the elemental non-diamond carbon coating on the surfaces of the crystals forming a protective adherent coating of silicon carbide on the diamond surfaces and causing no loss or no significant loss of crystals and no change or no significant change in the shape and dimensions of -the crystals. The resulting infiltrated mass is cooled in an atmosphere which has no significant deleterious effect on said infiltrated mass~ preferably it is furnace cooled in ~he partial vacuum to about room temperature, and the resulting polycrystalline body is recovered.
The period of time for full infiltration by the silicon is determinable empirically and depends largerly on the si2e of the shaped mixture, and frequently, infiltration hy the fluid silicon through the mold-confined shaped mixture is completed within about lS
minutes.
The cooled and recovered resulting mass of core crystals bonded togther by a bonding medium of silicon carbide and elemental silicon then is subjected to the action of a silicon leaching agent for leaching substantially all of the silicon from the resulting bonded mass. The silicon leaching agent generally is a suitable acid with a mixture of nitric and hydrofluoric acids in a volume ratio of 9:1, respectively, ~eing preferred. The leaching operation desirably 5s practiced by immersion of the bonded mass in the silicon leaching agent which may be held at room temperature or more preferably at a temperature between abou-t 100 and 120C. Note that sequential leaching of the honded mass or on subdivided portions of the bonded mass may be practiced as is necessary, desirable, or convenient.
The leached bonded mass is quite friable and is composed primarily of silicon carbide coated core abrasive crystals and silicon carbide particles. This leached mass is subdivided by crushing or other conventional technique to provide the composite abrasive particles or aggregates of the present invention in admixture with silicon carbide particles. The sub-divided mass, then is subjected to a separation step by screening or other conventional classification technique for separating and recovering the composite abrasive particle~ or aggregates o~ the present invention.
Several unexpected and beneficial results of this process are achieved. The silicon carbide coating on the core abrasive particles is in a thickness ranging from about 0~1 to 10 microns with a thickness of about 0.5 to 1 micron being preferred. It will be appreciated that due -to the highly rough texture and outer surface conformational irregularity of the particles of the present invention that precise measurements of the coating thickness are difficulk and the figures given here, while believed to be accurate, are not a limitation on the present invention. Still, the coating thickness of the silicon carbide on the core abrasive crystal generally is much greater than can be conveniently achieved by formation of silicon carbide coatings deposited on the core abrasive crystals by conventional vapor deposition techniques. Additionally~
the silicon carbide coating is very tightly bonded to the core crystal. Thicker coatings are possible if the vapor process is carried out for long periods of time. Vapor deposition is inherently a slow process since the layer must be built up atom by atom. Additionally, for this reason vapor deposited coatings tend to be smoother or follow the roughness of the substrate. In the present invention, the coating is rougher because the non-diamond carbonaceous coating is rough. Note the difference in the Figures. Further, the random orientation of the silicon carbide crystals is a noted unique feature. Additionally, the silicon carbide crystals in the coating generally are quite small in particle size, especially when compared to silicon carbide which is formed by reaction of silicon with the ~5 carbon content of diamond, for example.
With respect to the composite aggregates of the present invention, the aggregates are composed of the silicon carbide coated crystals which are interconnected by a silicon carbide matrix in an open structure. This 3Q means that channels and passageways run through the aggregates ~rom the interior thereof to the outside.
The open structure or porosity of -the silicon carbide matrix binding the coated composite abrasive particles is unique and well adapted for bonding of the aggreyates ~5 in the formation of abrasive elements, for example.
The composite abrasive particles and aggregates of 3~
the present invention desirably are used in the formation of abrasive elements wherein the coated ahrasive particles or aggregates are embedded in a bonding matrix for formation of an abrasive element which preferably is a grinding element. The novel composite abrasive particles and aggregates of the present invention in such use provide much better mechanical bonding of the abrasive particles to the matrix as well as much better bondlng of the silicon carbide coating of the core abrasive crystals, especially when the composite abrasive particles of the present invention are compared to conventional metal coated abrasive particles. Further, the silicon carbide coating provides an area of wear resistance about the core abrasive particle which is intermediate between the core abrasive crystal itself and the bonding matrix. This further contributes to the retention of -the composite abrasive particles and aggregate~ in the bonding matrix during use o~ the abrasive elements, since it reduces erosion of the bondîng matrix around the diamond/CBN crys-tals by workpiece chips generated in the grinding action.
A variety of conventional bonding matrices may be used to bond -the composite abrasive particles and aggregates of the present invention. Such conventional bonding matrices include resin, metal, and vitreous materials.
The resin most frequently used in resin-bonded grinding elements or other abrasive elements is a phenol-formaldehyde reaction product. However, other resins or organic polymers may be used such as melamine or urea-~ormaldehyde resins, epoxy resins~ polyesters, po'yamides, and polyimides. A resinoid grinding, wheel with the composite abrasive particles and aggregates of the present invention can be formed, for e~ample, in the manner as taught in U.S. Patent No. 3,6~5,706 to Bovenkerk dated February 29, 1972~ Note that the silicon carbide coating can be added to the abrasive emenet at the expense of secondary abrasive particles customarily in resin grinding elem2nts without detrimentally effecting the performance of the grinding element. The improved resinoid grinding elements of the present invention having diamond as the core crystal are particularly useful in the wet or dry grindlng of various workpieces and especially in grinding non-ferrous materials such as cemented tungsten carbide and cemented tungsten carblde/steel combinations.
It is feasible to modify the composite abrasive particles or aggregates of this invention by coating techniques~ They could be coated with a metal (eg.
nickel or copper) over the silicon carbide coating by any of the well known techniques such as electroless coating (see U.S. Patents 3,556,839 to Roy dated January 19, 1971 and 3,957,461 to Lindstrom dated May 18, 1976, Example 6 and Lowenhein, F.A., Electroplating,, Chapters 17 and 19, McGraw Hill Book Co. (1978). Further re~erences to various metals which are suitable for coating the p~rticles of the pxesent invention can be found in U.S. Patent 3,957,461 to Lindstrom dated May 18, 1276~see especially column 8).
The ~olume percent concentration of coated abrasive particles, when used in a resin bond matrix, is normally between a~out 5 and 35 percen~.
The aggregates as described herein also can be infiltrated with metal (eg. silver or copper) by: (a) mixing them with metal powder; and (b) furnace heating the mixture ~preferably under non-Qxidizing conditions) at su~ficient temperature to melt the metal allowing it to infiltrate into the interstices of the aggregates, the resulting infiltrated masses may be milled and sieved to si~eO Two infiltration process known to the art are described in U.S. Patents 2,216,728 Example I
and 4,024,675 to Naidich dated May 24, 1977.
~1 ~ ~ r~ ~ c- a For further details as to suitable metal, resin, and vitreous matrices for bond.ing the composi~e abrasive particles and aggregates of the present invention, reference is made to U.S~ Patents Nos. 2,137,329,
RANDOMLY-ORIENTED POLYCRYSTALLINE SILICON CAR~IDE COATINGS
FOR AB~ASIVE GRAINS
Background of the Invention The present invention relates to silicon carbide coated abrasive grains, such as diamond and cubic boron nitride, and more particularly to unique randomly-oriented polycrystalline silicon carbide coatings therefor.
Taylor (U.S. Patent No. 3,520,667 dated July l4, 1970) proposes to coat diamond abrasive particles with an adherent surface coating of silicon carbide by suspending the diamond particles in a gaseous atmosphere of a volatile silicon compound which is thermally decomposed to form a silicon carbide coating on the diamond particles by reaction with the carbon present in the volatile silicon compound or added gaseous source of carbon. Kuratomi (U.S. Patent No.3,617,347 dated November 2~ 1971) uses an analogous vapor deposition technique to coat diamond powder with a thin film of silicon carbide formed from the gaseous silicon compound and the diamond particle itself which thin silicon carbide coating then is .recoated with elemental si.licon. Silicon-based alloy coatings for abrasive grains have been proposed by Otopkov (U.S~
Pa~ent No. 4,184,853 dated January 22, 1980) wherein the coating is a three-component system which includes silicon and two other metals selected from a rscited group of metals, and by Ca~eney (U.S. Pa~n-t No.3,929,432 . ~J
~9~
dated December 30, 1975) wherein a titanium alloy coats the diamond grainO U.S. Patent No. 4,174,971 to Schrewelius dated November 20, 1979, describes the in situ formation of silicon carbide by a pyrolysis process.
A prime use of these various coated abrasive particles is in the formation of grinding elements wherein the coated abrasive particles are bonded together with resin, metal, vitreous material, or the like. A prevalent problem in using such bonded coated abrasive particles is that the particles tend to pull out from the bonding matrix, thus resulting in decreased grinding performance and a loss of valuable abrasi~e particles. The coatings on the abrasive particles provide a degree of increased retention of the particles within the bonding matrix.
Still, improvements in this art are needed.
Broad Statement o'f the Ihvention One aspect of the present invention is a composite abrasive particle comprising a core abrasive crystal selected from the group of diamond and cubic boron nitride, and a silicon carbide coating adherently bonded to substantially all of the exterior surfaces of said core crystal. The improved silicon carbide coating is characterized by silicon carbide crystals having a random orientation substantially independent of the structure of the core crystal, the outer surface o~ the coating being conformationally irregular.
Another aspect of the present invention is a composite a~rasive aggregate comprising core crystals selected from diamond, subic boron nitride, and mixtures thereof, disposed within a silicon carbide matrix which matrix is ~dherently bonded to substantially all of the exterior surfaces of each of the core crystals. The silicon carbide matrix is characterized by silicon 60SD~127 -- 3 ~
carbide crystals having a random orientation substantially independent of the structure of the core crystals. The matrix interconnecting said core crystals has an open structure and the outer surface of the matrix is conformationally irregular.
A method for making the co~posite abrasive particle described ahove comprises~ providing core crystals coated with non-diamond carbonaceous material; infiltrating said crystals with fluid silicon under a partial vacuum at a temperature of above 1400C, cooling and recovering the resulting mass of core crystals bonded together by a bonding medium of silicon carbide and elemental silicon, the outer surfaces of said core crystals having silicon carbide adherently bonded to substantially all of the exterior surfaces thereof;
leaching substantially all of said silicon from said bonded mass with a silicon leaching agent; and subdividing (milling) the resulting leached mass and recovering said composite abrasive particles. Fibrous graphit~ can be provided in a uniform mixture with the coated core crystals to promote silicon infiltration.
There are two distinctions between the process as used to make the composite abrasive par-ticles described above and the composite abrasive aggregate. First, in the case of aggregates, the core crystals coated with non-diamond carbonaceous material should be tightly packed ~ince a high degree of contact between the coated crystals is needed. This high packing density may be used in the case of the single particles as well, but is not required. The high crystal concentration also dictates that a minium amount of fibrous graphite be used.
Secondly, and more importantly, to make aggregates, a relatively light or low energy milling (eg. less milling time) is required with sieving done more frequently to remove aggregates of the desired size (more than twice ~ 19 ~5~ 6OSD-127 a, the size o~ the starting core crystals). To obtain just coated diamond or CBN, a more vigorous milling can be used with sieving done at a size only slightly larger than the core crystal. For example, starting with 140/170 mesh (106/90 micron) diamond, silicon carbide coated diamond would be sieved out in the 100/120 mesh (150/125 micron) size while aggregates would be obtained by not milling beyond about 40/60 mesh (425/~50 micron.s).
Another aspect of the present invention is an improved abrasive element of coated abrasive particles embedded in a bonding matrix. The improvement is that said coated abrasive particles comprise a core abrasive crystal selected from the group of diamond and cubic horon nitride, and a silicon carbide coating adherently bonded to substantially all of the exterior surfaces o~ the core crystal, said silicon carbide coating characterized by silicon carbide crystals having a random orientation substantially independent of the structure of said core crystal, the outer surface of said coating being conformationally irregular.
Additionally, the coated abrasive particles in the improved abrasive element can comprise composite abrasive aggregates wherein the silicon carbide coated core abrasive crystals are interconnected by a silicon carbide matrix having an open structure. The preferred bonding matrix can be selected from the group of resin, metal, and vitrous material, and the preferred abrasive element is a grinding element.
Ad~antages of the present invention include a silicon carbide coating which is tightly bonded to the core particle and which provides excellent mechanical bonding in the bonding matrix. Also, the unique silicon carbide coating of the present invention provides a highly wear-resistent region, intermediate between the core abrasi~e crystal and the bonding ~OSD-127 matrix, around each abrasive grain which reduces loss of the coated crystal by chip erosion during use of the abrasive element. FurthQr, the composite abrasive aggregates of the present invention have an open structure between silicon carbide coated grains which, when filled with bonding matrix material during abrasive fabrication, provides for a positive locking of the aggregate in the bonding matrix. ~dditionally, since no metal coating is contained on ~he composite abrasive particles or aggregates of the present invention, the silicon carbide coating can be added at the expense of additional secondary abrasive in the abrasive element without detrimental effects even at higher concentrations of the coated abrasive particles in the abrasive element.
These and other advantages will beeome readily apparent to those skilled in the art based upon the disclosure herein contained.
Brief Description of the Drawings Figs. lA, lB, and lC are photomicrographs at three different magnifications (lOOX, 500X~ and 1500X, respectively) of the randomly-oriented polycrystalline silicon carbide coated diamond grains of the present invention; and Figs. 2A, 2B, and 2C are photomicrographs at three different magnifications (lOOX, 500X, and 1500X, respectively) of comparative silicon carbide coated diamond grains.
Detailed Description of the Drawings - The comparative silicon carbide coated diamond particles shown in Figs. 2A-2C were made by the preferred infiltration, leaching, and subdividing process of the present invention, except that the diamond grains were uncoated, i.e. not coated with non diamond carbonaceous material so that the silicon carbide coating was formed b~ a chemical reaction of diamond carbon with silicon.
The silicon carbide coated diamond particles in Figs.
60SD~127 lA-lC were made hy the pr~ferred method of the present invention wherein the diamond grits were coated with a non-diamond carbonaceous material for infiltration thexeof with fluid silicon. Note the relatively smooth outex surface of the comparative partioles and the apparent related orientation of the silicon carbide crystals of the coatin~. rrhis is to be contrasted with the exterior surface of the no~l particles o the present invention wherein an apparently total random orientation of the silicon carbide crystals can be seen. The silicon carbide crystals on the novel particles also appear to be much smaller in overall exterior dimensions (eg.about 0.1 - 10 microns) than the silicon carbide crys~als of the comparative particles.
Note further that the differen~ faces of the core diamond crystal are distinctly de~ined for the com-parative particles. The comperative particles also display a relationship or ordered pattern between the diamond surace w~ich supplied the carbon and the formed silicon carbide crystals. Note the silicon carhide crystal shape and orientation in Figure 2A on the octahedral (6-sided~ surfaces compared to the cubic (4 sided) surfaces. This illustrates the dependance of the coating sur~ace upon the underlying diamond surface which suppliea the carbon atoms for the formation of silicon carbide. On the oth~r hand~
the exterior surface of the novel particles o~ the - present invention exhibit a completely random orientation of silicon carbide crystals independent of the underlying diamond surface. The outer surface of the novel particles also is conformationally irregular and extremely rough in texture. The random orientation of the silicon carbide cr~tals on the novel coated particles of the present invention apparently s~ems from ~he fac~ that the silicon carbide s~o nucleation arises from the silicon carbide crystals of the coating rather than from the diamond core particle. Apparently, nucleation of the silicon carbide crystals on the comparati~e samples during their for-mation is based on the core diamond particle itself.Also, since nucleation of the silicon carbide in the coating of the novel particles is based upon the silicon carbide crystals in the coating itself, excellent intercrystal silicon carbide crystal bonding is displayed by the novel coated particles of the present invention. ~ased upon the texture and arrangement of the silicon carbide crystals of the coatings of the two different particles shown in the photographs, one would expect that the novel coated particles of the present. invention could be bound in a matrix to a much better degree than the comparative coated crystals. In fact, this has been demonstrated to be true as the example will amply demonstrate.
Detailed Description of the Invention Referring to the pre~erred method for ma]cing the composite abrasive particles and aggregates of the present invention, the preferred core abrasive crystals is selected from the group consisting of diamond, cubic boron nitride, and mixtures thereof when ma~ing the novel abrasive aggregates of the present in~ention. It must be recognized, though, that any abrasive particle (eg~ silicon carbide itself, aluminum oxide, etc.) may serve as the core abrasive - cxystal for coatiny with silicon carbide according to the precepts of the pxesent inventionO In view of the cr~stal arrangement and morphology of the silicon carbide crystals in the coating as shown in the drawing, it is apparent that the source of carbon for foxming the silicon carbide coating should not come from the core ahrasive crystal itself~ but rather an external source of carbon should be used~ Thus, the core abrasive crystals provided with a coating of non-diamond carbonaceous material. The particle size range for the preferred diamond and cubic boron nitride abrasive crystals range from about 325/400 to 60/80 mesh (United States standard sieves series) (45/38 to 250/1~0 microns) with particles of about 100/120 mesh and smaller being preferred. Coating of the core crystals with non-diamond carbonaceous material and the silicon infiltration s~ep of the present process desirably are practiced according to -the general procedure disclosed in U.S. Patent No. 4,220,455 to St. Pierre dated September 2, 1980. Note that somewhat similar processes also are disclosed in U.S.Patent Nos.4,148,894 to Hillig et al dated April 10~ 1979, 4,233,433 dated ~ecember 9, 1980; and 4,242,106 to Morelock dated December 30, 1980.
The carbonaceous material can be elemental non-diamond carbon, an organic material, or mixtures thereof. The organic material decomposes completely at an elevated temperature below 1400C, and ordinarily decomposes completely at a temperature ranging from about 50C to about 1200C, to produce elemental non-diamond carbon and gaseous product of decomposition.
Representative of the organic materials useful in the present process are polymers of aromatic hyd ~carbons such as polyphenylene and polymethylphenylene derivatives of polynuclear aromatic hydrocarbons contained in coal tar such as dibenzanthracene and chrysene. Additional examples of useful organic materials are the epoxy resins such as the reaction product of epichlorohydrin and Bisphenol-A. Still additional examples of useful organic materials are phenolic resins obtained by the condensation of phenol or sub~tituted phenols with aldehydes such as formalde-hyde~ acetaldehyde, and furfural. Specific examplesaxe the condensation products of phenol~formaldehyde, 3~
resorcinol-formaldehyde, aniline-formaldehyde, and cresolformaldehyde.
The non-diamond carbonaceous material coated core crystals nex~ are infiltrated with fluid silicon under partial vacuum at a temperature of above 1400 C, preferably using the apparatus and general procedure shown and described in U.S. Patent No. 4,220,455 to St. Pierre dated September 2, 1980. While the coated core crystals can be admixed with additional carbon for infiltrating with silicon, such is not necessary in forming ~he silicon carbide coated composite abrasive particles of the present invention. As noted in U.S.
Patent No. 4,220,455, the coated core crystals and a source of silicon are heated to an infiltration temperature which is above 1400C. When the carbonaceous material coating the core crystals is an organic material, such organic material decomposes completely at a temperature below 1400C producing elemental non-diamond carbon and gaseous products of decomposition.
Infiltration is carried out at a temperature above 1400C at which silicon becomes fluid and which has no significant deleterious effect on the crystals.
For cubic boron crystals infiltration temperatures significantly higher than about 1450C are not useful ~ince they are likely to cause conversion to hexagonal boron nitride~ On the other hand, for diamond crystals, temperatures higher than 1550C provide no significant advantage. By a temperature at which silicon becomes fluid it is meant herein a temperature at which the silicon is readily 10wable. The fluid silicon is highly moblle and highly reactiwe with elemental non-d~amond carbon, i.e~ it has an affinity for elemental non~diamond carbon, wetting it and reacting with it to foxm silicon carbide. Specifically, when sili~on is at its melting temperature, which has been given in the art to range from about 1412C to about 1430C, it has 5~
a high viscosity, bu-t as its temperature ls raised, it becomes less viscous and at a temperature about ten degress higher than its melting point, it becomes fluid.
The temperature at which the silicon is fluid is the temperature at which it will infuse or infiltrate through the capillary~size passages, interstices or voids of the moldconfined mixture of crystals. With increase in temperature, the flowability of the fluid silicon increases resulting in a faster rate of reaction.
Sufficient silicon is infiltrated throughout the mold-confined mass or mixture infusing or infiltrating through the voids or pores of the mixture by capillary action to react with the total amount of elemental non diamond carbon (including fibrous graphite) present in the confined mixture forming silicon carbide, and also to fill any pores or voids which may remain after formation of the silicon carbide. Any pores remaining after formation of silicon carbide are filled during infiltration with elemental silicon. Also during infiltration, the silicon reacts with the elemental non-diamond carbon coating on the surfaces of the crystals forming a protective adherent coating of silicon carbide on the diamond surfaces and causing no loss or no significant loss of crystals and no change or no significant change in the shape and dimensions of -the crystals. The resulting infiltrated mass is cooled in an atmosphere which has no significant deleterious effect on said infiltrated mass~ preferably it is furnace cooled in ~he partial vacuum to about room temperature, and the resulting polycrystalline body is recovered.
The period of time for full infiltration by the silicon is determinable empirically and depends largerly on the si2e of the shaped mixture, and frequently, infiltration hy the fluid silicon through the mold-confined shaped mixture is completed within about lS
minutes.
The cooled and recovered resulting mass of core crystals bonded togther by a bonding medium of silicon carbide and elemental silicon then is subjected to the action of a silicon leaching agent for leaching substantially all of the silicon from the resulting bonded mass. The silicon leaching agent generally is a suitable acid with a mixture of nitric and hydrofluoric acids in a volume ratio of 9:1, respectively, ~eing preferred. The leaching operation desirably 5s practiced by immersion of the bonded mass in the silicon leaching agent which may be held at room temperature or more preferably at a temperature between abou-t 100 and 120C. Note that sequential leaching of the honded mass or on subdivided portions of the bonded mass may be practiced as is necessary, desirable, or convenient.
The leached bonded mass is quite friable and is composed primarily of silicon carbide coated core abrasive crystals and silicon carbide particles. This leached mass is subdivided by crushing or other conventional technique to provide the composite abrasive particles or aggregates of the present invention in admixture with silicon carbide particles. The sub-divided mass, then is subjected to a separation step by screening or other conventional classification technique for separating and recovering the composite abrasive particle~ or aggregates o~ the present invention.
Several unexpected and beneficial results of this process are achieved. The silicon carbide coating on the core abrasive particles is in a thickness ranging from about 0~1 to 10 microns with a thickness of about 0.5 to 1 micron being preferred. It will be appreciated that due -to the highly rough texture and outer surface conformational irregularity of the particles of the present invention that precise measurements of the coating thickness are difficulk and the figures given here, while believed to be accurate, are not a limitation on the present invention. Still, the coating thickness of the silicon carbide on the core abrasive crystal generally is much greater than can be conveniently achieved by formation of silicon carbide coatings deposited on the core abrasive crystals by conventional vapor deposition techniques. Additionally~
the silicon carbide coating is very tightly bonded to the core crystal. Thicker coatings are possible if the vapor process is carried out for long periods of time. Vapor deposition is inherently a slow process since the layer must be built up atom by atom. Additionally, for this reason vapor deposited coatings tend to be smoother or follow the roughness of the substrate. In the present invention, the coating is rougher because the non-diamond carbonaceous coating is rough. Note the difference in the Figures. Further, the random orientation of the silicon carbide crystals is a noted unique feature. Additionally, the silicon carbide crystals in the coating generally are quite small in particle size, especially when compared to silicon carbide which is formed by reaction of silicon with the ~5 carbon content of diamond, for example.
With respect to the composite aggregates of the present invention, the aggregates are composed of the silicon carbide coated crystals which are interconnected by a silicon carbide matrix in an open structure. This 3Q means that channels and passageways run through the aggregates ~rom the interior thereof to the outside.
The open structure or porosity of -the silicon carbide matrix binding the coated composite abrasive particles is unique and well adapted for bonding of the aggreyates ~5 in the formation of abrasive elements, for example.
The composite abrasive particles and aggregates of 3~
the present invention desirably are used in the formation of abrasive elements wherein the coated ahrasive particles or aggregates are embedded in a bonding matrix for formation of an abrasive element which preferably is a grinding element. The novel composite abrasive particles and aggregates of the present invention in such use provide much better mechanical bonding of the abrasive particles to the matrix as well as much better bondlng of the silicon carbide coating of the core abrasive crystals, especially when the composite abrasive particles of the present invention are compared to conventional metal coated abrasive particles. Further, the silicon carbide coating provides an area of wear resistance about the core abrasive particle which is intermediate between the core abrasive crystal itself and the bonding matrix. This further contributes to the retention of -the composite abrasive particles and aggregate~ in the bonding matrix during use o~ the abrasive elements, since it reduces erosion of the bondîng matrix around the diamond/CBN crys-tals by workpiece chips generated in the grinding action.
A variety of conventional bonding matrices may be used to bond -the composite abrasive particles and aggregates of the present invention. Such conventional bonding matrices include resin, metal, and vitreous materials.
The resin most frequently used in resin-bonded grinding elements or other abrasive elements is a phenol-formaldehyde reaction product. However, other resins or organic polymers may be used such as melamine or urea-~ormaldehyde resins, epoxy resins~ polyesters, po'yamides, and polyimides. A resinoid grinding, wheel with the composite abrasive particles and aggregates of the present invention can be formed, for e~ample, in the manner as taught in U.S. Patent No. 3,6~5,706 to Bovenkerk dated February 29, 1972~ Note that the silicon carbide coating can be added to the abrasive emenet at the expense of secondary abrasive particles customarily in resin grinding elem2nts without detrimentally effecting the performance of the grinding element. The improved resinoid grinding elements of the present invention having diamond as the core crystal are particularly useful in the wet or dry grindlng of various workpieces and especially in grinding non-ferrous materials such as cemented tungsten carbide and cemented tungsten carblde/steel combinations.
It is feasible to modify the composite abrasive particles or aggregates of this invention by coating techniques~ They could be coated with a metal (eg.
nickel or copper) over the silicon carbide coating by any of the well known techniques such as electroless coating (see U.S. Patents 3,556,839 to Roy dated January 19, 1971 and 3,957,461 to Lindstrom dated May 18, 1976, Example 6 and Lowenhein, F.A., Electroplating,, Chapters 17 and 19, McGraw Hill Book Co. (1978). Further re~erences to various metals which are suitable for coating the p~rticles of the pxesent invention can be found in U.S. Patent 3,957,461 to Lindstrom dated May 18, 1276~see especially column 8).
The ~olume percent concentration of coated abrasive particles, when used in a resin bond matrix, is normally between a~out 5 and 35 percen~.
The aggregates as described herein also can be infiltrated with metal (eg. silver or copper) by: (a) mixing them with metal powder; and (b) furnace heating the mixture ~preferably under non-Qxidizing conditions) at su~ficient temperature to melt the metal allowing it to infiltrate into the interstices of the aggregates, the resulting infiltrated masses may be milled and sieved to si~eO Two infiltration process known to the art are described in U.S. Patents 2,216,728 Example I
and 4,024,675 to Naidich dated May 24, 1977.
~1 ~ ~ r~ ~ c- a For further details as to suitable metal, resin, and vitreous matrices for bond.ing the composi~e abrasive particles and aggregates of the present invention, reference is made to U.S~ Patents Nos. 2,137,329,
2,561,709 to Norling dated July 24, 1951, 3,203,775 Cantrell August 31, 1965 and 3,894,673 to Lowder et al dated July 5, 1975 for metals; 3,385,684 to Voter dated May 28, 1968; 3,664,819 Slovi et al dated May 23, 1972, 3,779,727 to Siqui et al dated December 18,1973 10 and 3,957,461 to Lindstrom et al dated May 18, 1976 for resins; and 2,216,728 (page 3) for vitreous bond.
Metal bonded grinding elements can be used in glass edging, lens generating, grinding ceramics and other non-metall.ic materials as well as other articles as is well known in the art. A variety of ferrous and non-ferrous metals and alloys can be suitably ground with vitreous bonded grinding elements also.
The following example shows how the present invention can be practiced, but should not be construed as limitiny. In this application, all units are in the metric system, all mesh sizes are in United States standarded sieves series, and all proportions are by volume, unless otherwise expressly indicated.
EXAMPLE
WET GRINDING TESTS
Abrasive Preparation Two abrasives were silicon carb.ide coated:
140/170 mesh ~106/90 micron) resin bond diamond obtained as RVG from General Electric Company 140,~170 mesh metal bond diamond obtained as MGB
~rom General Electric Company Each was non-diamond carbon coated by pyrolyzing methane at about 1200C. The coating weight was about seve.n percent (7~) by weight for both abrasives.
Each coated abrasive was silicon infiltrated 5~
- 16 ~ 60SD-127 according to U.S. Patent 4,200,455 to St. Pierre dated September 2, 1980 and U.S. Patent No. ~,3~1,271 to Hayden dated April 26, 1983O Each carbon coated abrasive was mixed with fibrous graphite (which had previously been heat treated for 1-5 minutes at 1300 to 1550C. under a vacuum of less than about 1 Torr) in the approximate weight ratio 100:6 coated diamond to fibrous graphite.
This mixture was placed into a hexagonal boron nitride coated graphite mold with a diamond concentration of about 28 percent (23%) by volume. The mixtures then were silicon infiltrated. The infiltrated bodies were placed in a bath of nitric acid and hydrofluoric acid in a volume ratio of ahout ~:1. This bath was heated to about 120C. and held at this temperature until no further reaction was evident, about 4 hours. The resulting friable mass was rinsed in water, dried, and lightly crushed in a mortar and pestle. Because of the low diamond concentration,~only light crushing was required to separate the silicon carbide coated diamond particles. Particles larger than about 140 U.S. mesh were sieved out and acid treated as before until no further reaction was evident, about 30 minutes.
These particles were rinsed in water and dried. The 100/140 mesh particles were sieved out and used for grinding tests. The particles essentially consisted of individual 140/170 mesh diamond grits uniformly coated with silicon carbide.
The silicon carbide coating level was measured by chemically removing the silicon carbide from a representative sample in fused potassium hydroxide.
The result in percent by weight was as follows:
Type SiC Coating Level (%) Resin Bond Diamond 36 Metal ~ond Diamond 23 Prepara-tion of Grinding Wheels lAl type peripheral grinding wheels containing q r-~
~_ ~ 9;;1~ J~
60SD~127 each t~pe of abrasi~e and 140/170 mesh RVG uncoated, resin bond diamond for comparision were made. All wheels contained diamond at 22.5 percent by volume concentration and were 125 mm diameter, 9.5 ~m wide and a 9.5 mm thick abrasive section. The wheels were phenolic resin bonded with fine SiC secondary abrasive in a 1:25 weight ratio to the resin. A minimum of two wheels containing each abrasive type was ~ested.
Grinding Tests Wet grinding tests were run on a Norton surface grinder. A BioCool 500 (a rust inhibitor) and water mixture was used as the coolant. The workpiece ma-terial was Carboloy Grade 370 cemented carbide; a wear resistant grade with a composition by weight of WC 72%, TiC 8%, TaC 11.5%, and Co. 8.5%.
Prior to testing, each wheel was mounted on the grinder and trued using a silicon carbide wheel on a brake-controlled trueing device. The wheel was subjected to a precondikioning grind until energy re~uirements and rate of wheel wear had stabilized.
Testing was done at the following conditions:
Wheel Speed: 24.5 m/sec Table Speed: 15.0 m/min Downfeed: 0.038mm Crossfeed: 1.27 mm Workpiece Size: 145cm2 Grindihg Test ~esults The grindiny test results are expressed in terms of grinding ratio, GR, which is defined as the volume o workpiece material removed divid~d by the volume of wheel consumed. GR is a measure of wheel efficiency.
The grinding test results were as follows. The G~ reported is the average of two wheels with a minimum of 3 tests per wheel~
"
Metal bonded grinding elements can be used in glass edging, lens generating, grinding ceramics and other non-metall.ic materials as well as other articles as is well known in the art. A variety of ferrous and non-ferrous metals and alloys can be suitably ground with vitreous bonded grinding elements also.
The following example shows how the present invention can be practiced, but should not be construed as limitiny. In this application, all units are in the metric system, all mesh sizes are in United States standarded sieves series, and all proportions are by volume, unless otherwise expressly indicated.
EXAMPLE
WET GRINDING TESTS
Abrasive Preparation Two abrasives were silicon carb.ide coated:
140/170 mesh ~106/90 micron) resin bond diamond obtained as RVG from General Electric Company 140,~170 mesh metal bond diamond obtained as MGB
~rom General Electric Company Each was non-diamond carbon coated by pyrolyzing methane at about 1200C. The coating weight was about seve.n percent (7~) by weight for both abrasives.
Each coated abrasive was silicon infiltrated 5~
- 16 ~ 60SD-127 according to U.S. Patent 4,200,455 to St. Pierre dated September 2, 1980 and U.S. Patent No. ~,3~1,271 to Hayden dated April 26, 1983O Each carbon coated abrasive was mixed with fibrous graphite (which had previously been heat treated for 1-5 minutes at 1300 to 1550C. under a vacuum of less than about 1 Torr) in the approximate weight ratio 100:6 coated diamond to fibrous graphite.
This mixture was placed into a hexagonal boron nitride coated graphite mold with a diamond concentration of about 28 percent (23%) by volume. The mixtures then were silicon infiltrated. The infiltrated bodies were placed in a bath of nitric acid and hydrofluoric acid in a volume ratio of ahout ~:1. This bath was heated to about 120C. and held at this temperature until no further reaction was evident, about 4 hours. The resulting friable mass was rinsed in water, dried, and lightly crushed in a mortar and pestle. Because of the low diamond concentration,~only light crushing was required to separate the silicon carbide coated diamond particles. Particles larger than about 140 U.S. mesh were sieved out and acid treated as before until no further reaction was evident, about 30 minutes.
These particles were rinsed in water and dried. The 100/140 mesh particles were sieved out and used for grinding tests. The particles essentially consisted of individual 140/170 mesh diamond grits uniformly coated with silicon carbide.
The silicon carbide coating level was measured by chemically removing the silicon carbide from a representative sample in fused potassium hydroxide.
The result in percent by weight was as follows:
Type SiC Coating Level (%) Resin Bond Diamond 36 Metal ~ond Diamond 23 Prepara-tion of Grinding Wheels lAl type peripheral grinding wheels containing q r-~
~_ ~ 9;;1~ J~
60SD~127 each t~pe of abrasi~e and 140/170 mesh RVG uncoated, resin bond diamond for comparision were made. All wheels contained diamond at 22.5 percent by volume concentration and were 125 mm diameter, 9.5 ~m wide and a 9.5 mm thick abrasive section. The wheels were phenolic resin bonded with fine SiC secondary abrasive in a 1:25 weight ratio to the resin. A minimum of two wheels containing each abrasive type was ~ested.
Grinding Tests Wet grinding tests were run on a Norton surface grinder. A BioCool 500 (a rust inhibitor) and water mixture was used as the coolant. The workpiece ma-terial was Carboloy Grade 370 cemented carbide; a wear resistant grade with a composition by weight of WC 72%, TiC 8%, TaC 11.5%, and Co. 8.5%.
Prior to testing, each wheel was mounted on the grinder and trued using a silicon carbide wheel on a brake-controlled trueing device. The wheel was subjected to a precondikioning grind until energy re~uirements and rate of wheel wear had stabilized.
Testing was done at the following conditions:
Wheel Speed: 24.5 m/sec Table Speed: 15.0 m/min Downfeed: 0.038mm Crossfeed: 1.27 mm Workpiece Size: 145cm2 Grindihg Test ~esults The grindiny test results are expressed in terms of grinding ratio, GR, which is defined as the volume o workpiece material removed divid~d by the volume of wheel consumed. GR is a measure of wheel efficiency.
The grinding test results were as follows. The G~ reported is the average of two wheels with a minimum of 3 tests per wheel~
"
3~5~
Abrasive Type Grinding Ratio Rel. GR*
Uncoated resin bond diamond 50 loO
Resin bond diamond 120 2.4 Metal bond diamond 38 0.76 *Rel. GR is -the relative grindiny ratio, calculated by dividing the grinding ratio for each abrasive type by the GR for the uncoated abrasive.
As can be seen, the GR and SiC coated resin bond diamond is unexpectedly large compared to uncoated resin bond diamond. Metal ~ond diamond is not recommended for grinding cemented carbide since it is less friable and, thus, does not remain sharp. SiC coated metal bond diamond also has thi~ disadvantage. This is evidence that the coating process does not degrade the metal bond crystal to where its grinding performance is effected.
Abrasive Type Grinding Ratio Rel. GR*
Uncoated resin bond diamond 50 loO
Resin bond diamond 120 2.4 Metal bond diamond 38 0.76 *Rel. GR is -the relative grindiny ratio, calculated by dividing the grinding ratio for each abrasive type by the GR for the uncoated abrasive.
As can be seen, the GR and SiC coated resin bond diamond is unexpectedly large compared to uncoated resin bond diamond. Metal ~ond diamond is not recommended for grinding cemented carbide since it is less friable and, thus, does not remain sharp. SiC coated metal bond diamond also has thi~ disadvantage. This is evidence that the coating process does not degrade the metal bond crystal to where its grinding performance is effected.
Claims (11)
1. An improved composite abrasive particle comprising a core abrasive crystal and a silicon carbide coating adherently bonded thereto, said silicon carbide coating being conformationally irregular and comprising silicon carbide crystals having a random orientation substantially independent of the structure of said core abrasive crystal and made by the steps comprising:
(a) providing core crystals coated with non-diamond carbonaceous material;
(b) infiltrating said crystals with fluid silicon under a partial vacuum at a temperature of above 1400°C, cooling and recovering the resulting mass of core crystals bonded together by a bonding medium of silicon carbide and elemental silicon, the outer surfaces of said core crystals having silicon carbide adherently bonded to substantially all of the exterior surfaces thereof;
(c) leaching substantially all of said silicon from said bonded mass with a silicon leaching agent; and (d) sub-dividing the resulting leached mass and recovering said composite abrasive particles.
(a) providing core crystals coated with non-diamond carbonaceous material;
(b) infiltrating said crystals with fluid silicon under a partial vacuum at a temperature of above 1400°C, cooling and recovering the resulting mass of core crystals bonded together by a bonding medium of silicon carbide and elemental silicon, the outer surfaces of said core crystals having silicon carbide adherently bonded to substantially all of the exterior surfaces thereof;
(c) leaching substantially all of said silicon from said bonded mass with a silicon leaching agent; and (d) sub-dividing the resulting leached mass and recovering said composite abrasive particles.
2. The improved composite abrasive particle of claim 1 wherein said core abrasive crystal is selected from the group consisting of diamond and cubic boron nitride.
3. The improved composite abrasive particle of claim 1 which is further coated with a metal.
4. The improved composite abrasive particle of claim 3 wherein said metal is selected from the group consisting of nickel and copper.
5. A porous composite abrasive aggregate com-prising core abrasive crystals disposed within a silicon carbide matrix, said silicon carbide adherently bonded to substantially all of the exterior surfaces of each of said core crystals, said silicon carbide matrix being porous, conformationally irregular, and comprising silicon carbide crystals having a random orientation substantially independent of the structure of the core abrasive crystals and made by the steps comprising:
(a) providing core crystals coated with non-diamond carbonaceous material;
(b) infiltrating said crystals with fluid silicon under a partial vacuum at a temperature of above 1400°C, cooling and recovering the resulting mass of core crystals bonded together by a bonding medium of silicon carbide and elemental silicon, the outer surfaces of said core crystals having silicon carbide adherently bonded to substantially all of the exterior surfaces thereof;
(c) leaching substantially all of said silicon from said bonded mass with a silicon leaching agent; and (d) recovering said composite abrasive aggregates.
(a) providing core crystals coated with non-diamond carbonaceous material;
(b) infiltrating said crystals with fluid silicon under a partial vacuum at a temperature of above 1400°C, cooling and recovering the resulting mass of core crystals bonded together by a bonding medium of silicon carbide and elemental silicon, the outer surfaces of said core crystals having silicon carbide adherently bonded to substantially all of the exterior surfaces thereof;
(c) leaching substantially all of said silicon from said bonded mass with a silicon leaching agent; and (d) recovering said composite abrasive aggregates.
6. The composite abrasive aggregates of claim 5 wherein said core crystals are selected from the group consisting of diamond and cubic boron nitride.
7. The porous composite abrasive aggregate of claim 5 which is coated with a metal or infiltrated with a metal.
8. The porous composite aggregates of claim 7 wherein said metal is selected from the group of nickel and copper.
9. An improved abrasive element of coated abrasive particles embedded in a bonding matrix, wherein the improvement comprises coated abrasive particles comprising core crystals selected from the group consisting of diamond and cubic boron nitride, and an irregular silicon carbide coating adherently bonded to substantially all of the exterior surfaces of said core crystals, said silicon carbide coating characterized by silicon carbide crystals which are conformationally irregular and have a random orientation substantially independent of the structure of said core abrasive crystal and made by the steps comprising:
(a) providing core crystals coated with non-diamond carbonaceous material;
(b) infiltrating said crystals with fluid silicon under a partial vacuum at a temperature of above 1400°C, cooling and recovering the resulting mass of core crystals bonded together by a bonding medium of silicon carbide and elemental silicon, the outer surfaces of said core crystals having silicon carbide adherently bonded to substantially all of the exterior surfaces thereof;
(c) leaching substantially all of said silicon from said bonded mass with a silicon leaching agent; and (d) sub-dividing the resulting leached mass and recovering said composite abrasive particles.
(a) providing core crystals coated with non-diamond carbonaceous material;
(b) infiltrating said crystals with fluid silicon under a partial vacuum at a temperature of above 1400°C, cooling and recovering the resulting mass of core crystals bonded together by a bonding medium of silicon carbide and elemental silicon, the outer surfaces of said core crystals having silicon carbide adherently bonded to substantially all of the exterior surfaces thereof;
(c) leaching substantially all of said silicon from said bonded mass with a silicon leaching agent; and (d) sub-dividing the resulting leached mass and recovering said composite abrasive particles.
10. The improved abrasive element of claim 9 wherein said bonding matrix is selected from the group consisting of a cured resinous material, metal, and a vitreous material.
11. The improved abrasive element of claim 9 wherein said coated abrasive particles comprise porous composite abrasive aggregates of said core abrasive crystals in a matrix of silicon carbide which inter-connects said silicon carbide coated core crystals.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000400768A CA1193451A (en) | 1982-04-08 | 1982-04-08 | Randomly-oriented polycrystalline silicon carbide coatings for abrasive grains |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000400768A CA1193451A (en) | 1982-04-08 | 1982-04-08 | Randomly-oriented polycrystalline silicon carbide coatings for abrasive grains |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1193451A true CA1193451A (en) | 1985-09-17 |
Family
ID=4122547
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000400768A Expired CA1193451A (en) | 1982-04-08 | 1982-04-08 | Randomly-oriented polycrystalline silicon carbide coatings for abrasive grains |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1193451A (en) |
-
1982
- 1982-04-08 CA CA000400768A patent/CA1193451A/en not_active Expired
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