Diamond abrasive matrix

March 5, 1968

3,372,010

M. C. PARSONS

DIAMOND ABRASIVE MATRIX

Filed June 23, 1965

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United States Patent Office 3,372,010

Patented Mar. 5, 1968

3,372,010

DIAMOND ABRASIVE MATRIX

Milton C. Parsons, Redford Township, Wayne County, Mich., assignor to Wall Colmonoy Corporation, Detroit, Mich., a corporation of Delaware ³

Filed June 23,1965, Ser. No. 466,391

Claims. (Ci. 51—309)

ABSTRACT OF THE DISCLOSURE 10

A metallic bonding matrix for diamond abrasive grains having a micro structure containing tungsten carbide and chromium boride particles bonded in a lattice comprising a solid solution of controlled amounts of nickel, iron, copper, chromium, tin, carbon, boron and silicon.

. The present invention broadly relates to abrasive articles, and more particularly to an improved composition for tenaciously bonding hard abrasive particles, such as diamond, or the like, within a bonding matrix which may conveniently be affixed to a supporting substrate. More specifically, the present invention is directed to a novel metallic bonding material which, upon sintering, forms a matrix which is of excellent impact and abrasion resistance, is of high strength and ductility, and is effective for forming tenacious bonds with the diamond abrasive particles embedded therein.

A variety of bonding agents have heretofore been used or proposed for use for mounting diamonds in single point or cluster type cutting tools or for forming diamond abrasive matrices incorporating a plurality of fine sized diamond particles distributed therethrough, providing therewith abrasive tools possessing controlled grinding or polishing action. Bonding agents of the types heretofore known conventionally comprise a variety of metal alloys which are selected so as to provide a resultant matrix which is abrasion and impact resistant, which forms a tenacious bond with diamond particles mounted therein and which requires nominal sintering temperatures to prevent thermal degradation of the diamond particles. Of the various bonding agents heretofore known, copper base alloys, iron base alloys, molybdenum and tungsten base alloys and metallic alloys incorporating cemented carbides are most common. Neither of the foregoing wellknown bonding agents possesses all of the optimum characteristics which are necessary to make diamond abrasive finishing tools of optimum efficiency and durability. For example, copper base alloys are generally characterized by their poor abrasion resistance resulting in an excessive wear of the bonding matrix and a premature release of the diamond abrasive particle or particles bonded therein. Iron base alloys, on the other hand, possess superior abrasion resistance but require higher sintering temperatures such that the differential coefficient of thermal expansion of the bonding matrix and the diamond particles results in a shrinkage of the matrix away from the diamonds during cooling whereupon bonds of less than optimum strength are obtained. Molybdenum and tungsten base alloys require relatively high sintering temperatures which usually exceed about 2350° F. fur° ther aggravating the bonding problem as a result of the differences in the coefficients of thermal expansion in addition to causing possible damage to the diamond abrasive particles themselves by exposure to excessive temperatures during the sintering or heating operation.

Various metallic alloys incorporating carbides have been found to possess excellent abrasion resistance but ire deficient due to the excessively high sintering tempera- ‘ ture required which frequently exceed 2800° F. result20 ing the formation of inferior bonds between the bonding matrix and diamonds and also a deterioration of the diamond abrasive particles due to graphitizing at the elevated temperatures during fabrication. Attempts to overcome the problems associated with carbide containing bonding agents including the formation of cold pressed conglomerates of the carbide mixture and infiltration thereof with copper or silver based alloys have not been wholly satisfactory in producing a bonding matrix possessing optimum properties.

It, is accordingly, a principal object of the present invention to provide a novel metallic bonding agent for abrasive particles which overcomes the disadvantages of bonding agents of the types heretofore known.

Another object of the present invention is to provide a novel bonding material which can be converted into a bonding matrix requiring only nominal temperatures and which bonding material effects a wetting of the surfaces of the diamond abrasive particles obtaining bonds of substantially superior strength than those heretofore obtained.

Still another object of the present invention is to provide an improved metallic bonding agent which possesses self-fluxing characteristics preventing thereby the formation of metallic oxides during the heating and sintering of the bonding agent in powdered form while disposed in compacted relationship and in contact with the diamond abrasive particle or particles, to be bonded.

A further object of the present invention is to provide an improved metallic bonding material which can be heated or sintered at temperatures conventionally at or below 1850° F. thereby concurrently minimizing thermal degradation of the diamond abrasive particles, as well as minimizing the adverse effects of the differential thermal coefficient of expansion between the bonding matrix and the abrasive particles being bonded.

A _ still further object of the present invention is to provide a novel bonding composition which forms a bonding matrix of excellent ductility and toughness, of high abrasion resistance and strength, and which is of a dense structure substantially free of any cracks or fissures therein substantially enhancing the grinding characteristics and durability of the abrasive finishing tool produced therefrom.

Yet a still further object of the present invention is to produce an improved bonding material which is conveniently supplied in the form of a metallic powder and can be hot compacted and sintered in situ forming a bonding matrix possessing optimum characteristics.

Yet still another object of the present invention is to provide an improved metallic bonding agent which is of simple, versatile and economical use and which forms diamond abrasive articles of high grinding and polishing efficiency and of long tool life.

The foregoing and other objects and advantages of the present invention are achieved by the formation of a powder which, upon sintering, at a temperature of from about 1800° F. to about 1850° F. is operative to form a hard, dense abrasion and impact resistant bonding matrix consisting of controlled proportions of tungsten carbide particles and chromium boride particles bonded within a lattice consisting essentially of a ternary soluton containing nickel, iron, copper, chromium, tin, carbon, boron and silicon, as well as a complex eutetic dispersed therethrough containing nickel, chromium boron, silicon and iron. The novel metallic bonding composition is equally applicable for setting one or a plurality of relatively large diamond crystals in appropriate position relative to a tool support or, alternatively, for forming a diamond abrasive matrix containing controlled proportions of diamond particles distributed substantially uniformly therethrough.

3,372,010

Other objects and advantages of the present invention will become apparent upon a reading of the following description, taken in conjunction with the accompanying drawing, wherein:

FIGURE 1 is a magnified fragmentary side elevational view of a typical single point diamond abrasive tool;

FIG. 2 is a perspective view of a typical diamond abrasive polishing wheel incorporating a diamond abrasive matrix on the working face thereof;

FIG. 3 is a photomicrograph of the bonding matrix formed in accordance with the practice of the present invention taken of an unetched sample of a magnification of 100, and

FIG. 4 is a photomicrograph of a marble etched sample of the bonding matrix taken at a magnification of 400.

The bonding composition comprising the present invention and the bonding matrix resulting from the sintering thereof at an elevated temperature, is applicable for the manufacture of metal bonded diamond wheels and tools of the conventional types well known in the art. The use of the bonding agent for making single point diamond cutting tools is exemplified in FIGURE 1, wherein a single point tool indicated at 10, is illustrated, comprising a tool body 12 formed at its end with a cavity in which a relatively large diamond crystal 14 is tenaciously retained by means of a bonding matrix 16. The bonding agent comprising the present invention is equally applicable for the manufacture of diamond abrasive wheels such as the diamond abrasive wheel 18, illustrated in FIGURE 2. As shown, the diamond wheel comprises a hub section 20, adapted to be mounted on a suitable rotating shaft, and a wheel section 22 formed with a conical working face 24 incorporating a layer of a diamond abrasive matrix tenaciously bonded thereto. It will be appreciated by those skilled in the art that a variety of alternative shapes of a single point diamond cutting tool 10, shown in FIGURE 1, including cluster point tools as well as alternative shapes of the diamond wheel 18, illustrated in FIGURE 2, can be satisfactorily fabricated as may be desired to provide an appropriate grinding or polishing action consistent with the intended end use of the tool. It will also be appreciated that while the bonding matrix is particularly applicable for bonding diamond particles forming a diamond abrasive matrix, alternative hard abrasive grains can be satisfactorily bonded in accordance with the practice of the present invention.

Conventionally, single point and cluster point cutting tools are made from relatively large size industrial diamond crystals which are set and tenaciously retained in the bonding matrix, which in turn is tenaciously bonded to a hard metal supporting structure such as steel, for example, forming the tool support. In the formation of a diamond abrasive matrix, either supported or unsupported, in which a plurality of fine sized diamond particles are distributed substantially uniformly throughout the matrix, the quantity of diamonds employed and the specific size and shape thereof can be varied within board limits to provide the requisite grinding or polishing action. Usually, diamond particles of an average size as small as about 1 micron or smaller, up to sizes of a nominal %e inch in diameter or greater and incorporated in amounts broadly ranging from 5% by volume up to about 70% by volume of the abrasive matrix, can be satisfactorily employed. Conventionally, the quantity of the diamond abrasive particles are controlled within a range of from about 25% by volume to about 35% by volume of the resultant abrasive matrix. In either event, it has been found that substantially superior cutting efficiency and tool life is obtained by diamond abrasive finishing tools incorporating the metallic bonding matrix comprising the present invention over tools of similar type, or, alternatively, equal cutting efficiencies are obtained employing a lesser quantity of abrasive particles than other tools of the types heretofore known employing a greater quantity of abrasive particles and a conventional metallic bonding agent. The ίο diamond abrasive particles are of the industrial types conventionally employed and usually consist of bort, which comprises imperfectly crystallized or coarse diamonds or fragments obtained from diamond cutting operations.

The bonding material comprising the present invention is conveniently supplied in the form of a fine sized metallic powder mixture which preferably is blended by ball milling or otherwise tumbling the powder particles so as to effect a simultaneous crushing and/or mechanical smearing or intermingling of the. individual powder particles with adjacent particles. The ball milling of the individual powder particles provides for a substantially permanent homogeneous blend of the several constituents assuring the formation of a sintered bonding matrix possessing consistent high quality and strength.

The powder blend and the resultant sintered’ matrix produced therefrom contains the following essential constituents in the amounts hereinafter specified in terms of percent by weight: from about 13% to about 54% copper; from about 2% to about 33% tungsten; from about 10% to about 50% iron; from about 8% to about 36% nickel; from about 1% to about 25% chromium; from about 1% to about 6% tin; from about .2% to about 2.5% carbon; from about .4% to about 7% boron;: and from about 0.1% to about 2.3% silicon. The foregoing elements can be conveniently introduced in the form of prealloyed powders of the several constituents including, for example, a bronze powder, an iron powder, a nickel based powder incorporating self-fluxing agents, fine sized particles of chromium boride, and particles of tungsten carbide.

The bronze powder incorporates proportions of copper and tin in controlled amounts and is employed in a quantity in consideration of the remaining constituents so as to provide a net percentage of copper and tin in the bonding matrix within the ranges as hereinbefore set forth. Prealloyed bronze powders incorporated about 10% tin, which are of a relatively small particle size, preferably 200 mesh or smaller, have been found to be particularly satisfactory for making a metallic powder blend of the bonding matrix. The iron constituent of the bonding matrix may conveniently be introduced in a prealloyed condition or in essentially pure form to supplement the iron content present in others of the prealloyed powder particles. The iron powder employed is preferably of a particle size of about 325 mesh or smaller and particularly satisfactory results have been obtained by employing an electrolytic flake grade iron powder.

The chromium constituent in the bonding material is predominantly in the form of fine sized particles of chromium boride (CrB), which preferably are of a size of about 325 mesh or smaller. Similarly, the tungsten constituent in the bonding material is principally in the form . of fine sized particles of tungsten carbide which preferably ’ are controlled so as to provide an average particle size of about 140 mesh or smaller. The tungsten carbide constituent may include any of those conventionally known in the art including sintered tungsten carbides, fused tungsten carbides, as well cemented tungsten carbides incorporating nominal amounts of cementing agents such as, for example, cobalt and/or nickel. A commercially available cemented tungsten carbide which has been found to provide satisfactory results comprises a tungsten carbide eutectic of W₂C, WC, including about 6% by weight of a cobalt cementing agent.

The nickel constituent in the bonding agent in addition to supplementary quantities of the other metallic alloying constituents such as chromium, iron, boron, silicon and carbon, for example, is preferably introduced in the form of a prealloyed powder which conventionally contain from about 0.45% to about 0.95% carbon; from about 10% to about 26% chromium; from about 2% to about 4% boron, which acts as a fluxing agent; from about 2% to about 4% silicon; from about 1% to about 5% iron, and the balance thereof nickel. The specific quantity of nickel

8,372,010 alloy powder employed and the quantity and type of the alloying constituents contained therein, can be varied so as to provide a resultant bonding matrix composition incorporating the constituents within the range as hereinbefore set forth. ₅

The blended particulated metallic powder composition can be directly employed for sintering in situ at an elevated temperature, preferably under a reducing atmosphere effecting a partial melting and bonding of the diamond particle or particles embedded therein upon subsequent cooling. The sintering operation is of the so-called compression molding type in which the powder is packed in either a steel or a carbon mold and around the diamonds to be bonded and a carbon plug is usually used in the top of the mold over the diamond or diamond par- j,tides to produce a reducing atmosphere which is found to improve the subsequent sintering action and to minimize surface porosity in the resultant abrasive matrix formed. Conventionally, the bonding powder is sintered under a pressure of about 15 to about 20 p.s.i., which is ₂o effective to obtain a reduction in the volume of the powder of approximately 50%. Sintering temperatures of about 1800° F. to about 1850° F. are satisfactory for effecting a partial melting and sintering of the individual powder particles into a hard integrally bonded matrix. ₂₅Heating of the powder can be achieved by an oxy-acetylene torch and preferably by electric induction heating techniques of the types well known in the art. The heating of the bonding powder is preferably done from the bottom toward the top and the mold containing the powder θθ and the diamond abrasive particles is held at that temperature until sintering takes place after which the molds are cooled to approximately 800° F. after which the pressure on the powder is removed. During the sintering operation, a bonding of the diamond abrasive particle or particles ₃₅occurs, which is simultaneously accompanied by a tenacious bonding of the bonding matrix to a hard metal tool support if employed.

During the sintering operation, a partial melting of the metallic constituents occurs, which upon subsequent 49 cooling, forms a bonding matrix characterized as comprising a solid solution having interspersed therethrough tungsten carbide particles, chromium boride particles, and a complex eutectic of nickel, chromium, boron, silicon and iron. During the solidification of the sintered bonding 45 matrix, the solid solution effects tenacious bonding of the diamond abrasive particles, as well as the particles of the chromium boride and tungsten carbide present in the powder mixture. This bonding action is of a metallurgical nature wherein the individual chromium boride and tung- 50 sten carbide particles are minutely alloyed along the surfaces thereof into the surrounding lattice, which is of a susbtantially lower melting temperature than these particles themselves. During the sintering action, the embedded diamond crystals or uniformly distributed bort 55 blended with the bonding powder, become wetted by the molten lattice and thereby are metallurgically bonded within the matrix.

The presence of the element boron in the matrix powder, provides for a self-fluxing chemical action that takes gq place, during the sintering operation. The boron constituent is present in three portions of the microstructure of the bonding, matrix formed. It is present in the form of hard chromium boride compound particles, as an interstitially dissolved element in the solid solution of the lat- _6gtice. and as finely dispersed borides of chromium or nickel or iron in the.complex eutectic. During the sintering operation wherein the lattice or at least portions thereof become molten, the boron constituent present provides the essential fluxing action required to reduce the oxides on ₇₀the surfaces of the powder particles to attain the requisite ‘ bonding of the resultant bonding matrix and the diamond abrasive particles. The presence of boron as a eutectic boride, is apparent from an examination of the micrographs as illustrated in FIGURES 3 and 4. The eutectic 75 borides are present in a substantially smaller size than the so-called “free” chromium boride compound particles securely bonded in the bonding matrix. The presence of the eutectic borides is essential to the attainment of the improved physical properties of the abrasive bonding matrix comprising the present invention.

The microstructure of the bonding matrix devoid of any diamond abrasive particles is best seen in FIGURES 3 and 4 which are taken in magnifications of 100 and 400, respectively. The comparatively large irregularly shaped particles indicated at 26 correspond to the tungsten carbide particles tenaciously bonded within the matrix. The smaller comparatively lighter shade particles indicated at 28 correspond to the so-called free chromium boride particles which similarly are tenaciously bonded within the lattice. The complex eutectic comprising nickel, chromium, boron, silicon and iron compounds, is represented by the irregularly shaped dark gray areas indicated at 30 in FIGURES 3 and 4. The heterogeneous structure of the complex eutectic 30 is best seen in FIGURE 4 at a magnification of 400. The remaining light portions of the photomicrograph indicated at 32 comprise a solid solution comprising primarily nickel, iron and copper with lesser amounts of chromium, tin, carbon, boron and silicon, and includes small interspersed particles of chromium boride therethrough.

In accordance with the microstructure as illustrated in FIGURES 3 and 4, it will be apparent that the mechanical characteristics and physical properties of the bonding matrix can be varied within the composition ranges as hereinbefore set. forth to provide a resultant matrix possessing the requisite characteristics for a specific diamond abrasive tool. The Rockwell hardness of the bonding matrix will vary depending on the amount, size and distribution of the tungsten carbide and chromium boride particles and can.be controlled within the ranges specified to obtain optimum relationships between abrasion resistance and ductility to provide the desired physical characteristics. of the abrasive finishing tool. The metallic bonding matrix prepared in accordance with the composition limits and procedures as hereinbefore defined, conventionally produces a Rockwell apparent hardness ranging from about 10 C. to about 50 C. The apparent hardness of a diamond abrasive matrix incorporating a plurality of diamond crystals distributed substantially uniformly throughout the metallic bonding matrix, similarly will vary depending on the quantity, size and distribution of the diamond crystals in relationship to the quantity and composition of the metallic bonding matrix.

A particulated ball milled mixture of five particulated constituents was prepared employing a bronze powder containing about 10% tin, which was of 200 mesh size and. finer, an electrolytic flake grade iron powder of a particle size 325 mesh and finer, chromium boride particles of 325 mesh and finer, cemented tungsten carbide particles containing approximately 6% cobalt cementing agent which were of 140 mesh and finer, and prealloyed nickel base self-fluxing powder particles of a composition within the ranges hereinbefore mentioned. The five powder constituents were admixed and ball milled and thereafter employed for forming a diamond abrasive matrix employing from about 25% to 35% fine sized diamond crystals or bort. The nominal composition of the resultant ™^ei^aL^llc.^bondinS ^{matrix was} 26% copper, 22% tungsten, , ^lron&gt; 18% nickel, 4.4% chromium, 2.5% tin 1.7% carbon, 1.5% cobalt (present as a cementing agent for the tungsten carbide), 1% boron and 1.2% silicon. Diamond, abrasive matrices of the foregoing composition were subjected to tests under conditions typical of commerical operation. These tests clearly demonstrated the superior characteristics of the metallic bonding matrix comprising the present invention whereupon the tool possessed the best features of abrasion resistance and hardness corresponding to tungsten carbide bonding agents heretofore employed but without the disadvantages of bond strength and brittleness inherently present in suchprior bonding agents. The abrasive finishing tool was further characterized by its. high impact resistance and toughness as well as the excellent bond developed between the diamond abrasive particles and the metallic bonding matrix. The resultant abrasive matrix formed was a dense compact microstructure substantially devoid of any cracks or fissures therethrough or adjacent to the diamond partides.

While it will be apparent that the preferred embodi- , ments of the present invention disclosed are well calculated to fulfill the objects above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper, scope or fair meaning of the subjoined claims.