Abrasive Particles Encapsulated With A Metal Envelope Of Allotriomorphic Dentrites

Abstract

Abrasive particles are improved in function by encapsulating them with a metallic envelope; preferably the envelope is made of a pure metal in dendritic crystalline form. Desirably the abrasive substrate is placed in contraction by the envelope which is heat shrunk onto the abrasive substrate. The preferred method is to deposit the metal on the substrate at an elevated temperature by contacting a vapor of the metallic compound with the substrate particle under reducing conditions. The preferred primary abrasive is a diamond, and it is preferably etched before coating. Superior formed abraders may be formed by metal bonding such encapsulated abrasives with a metal matrix which forms a continuous phase in which the abrasive particles may be positioned. The encapsulated abrasive particle may form the primary abrasive together with a secondary abrasive which is not as hard as the primary abrasive and which may or may not be encapsulated with a metal. The primary and secondary abrasive may in one form of the abrader be distributed in the continuous phase metal binder forming the matrix.

Description

Abrasive, grinding, cutting and earthboring tools, hereinafter referred to as abraders, have bound abrasive particles into an abrader structure, using a binder such as a resin and, in some cases, metal, which acts as the matrix to hold the abrasive particles in the abrader structure.

The abrasive action places the abrasive particle in tension and resultant excessive fragmentation of the abrasive particles may thereby result in loss of the particle from the matrix.

We have solved the problems referred to above by producing a novel metal encapsulated abrasive particle and novel abrader structures by first encapsulating abrasive particle with a metallic envelope.

When metal is used as a matrix to bind the abrasive particles in the abrader structure, encapsulation of the abrasive particles increases the grip of the metal matrix on the abrasive particle.

The encapsulated particle is more easily wetted by the molten metal than the non-metallic substrate. The improved interfacial tension between the metal envelope and the metal matrix used to bond the particles increases the grip of the metal matrix on the encapsulated particle and thus helps to prevent the loss of the particle in case excessive fragmentation of the particle does occur.

In selecting the metal for the envelope when the encapsulated particle is to be used with the metal matrix acting as a bonding agent, it is desirable that the metal in the envelope have a suitably higher melting point than the metal matrix.

The third advantage of the encapsulated abrasive particle of our invention when used together with a metal matrix resides in the increased rate of heat transfer from the abrasive particle resulting from the more intimate contact surface between the envelope and the substrate particle and the envelope and the metal matrix. Heat generated at the abrading surfaces, if not readily transmitted to and absorbed in the metal matrix, acting as a heat mass, will cause a local rise in temperature which may have a deleterious effect upon the life of the abrasive particle.

In order to obtain the increased bond between the abrasive particles and the metal matrix any convenient method for deposit of the metal envelope on the particle substrate may be employed. Thus electrochemical or electrolytic methods which have been previously employed in coating abrasive particles for use in abrader structure will, when used together with a metal bonding agent in our novel abrader structure, result in an improved bond between the metal matrix and the coated particle due to the improved wetting by the molten metal. In this respect, the use of the coated particle in a composite structure employing a metal matrix is an improvement over the use of an abrasive particle coated by an electrochemical or electrolytic process when used with a resin binder. It is similarly an improvement over the use of uncoated abrasive particles with resin or metal binders acting as a matrix for the abrasive particles.

Abrasive particles coated by such procedures result in deposits which are contaminated by intergranular inclusions of impurities from their aqueous environment. Furthermore, the deposits particularily in the case of electrolytic deposits have intergranular planes of weakness and the coating has a relatively low tensile and bending strength. They do not improve, in any substantial degree, the physical properties of the coated particle as compared with the uncoated particle.

The metallic envelopes which constitute the abrasive particles of our invention employed in the novel abrader structure of our invention differ from the foregoing coatings in composition and crystalline nature.

In contrast to these deposits, the deposits of our invention are substantially pure metal envelopes, substantially free of intergranular inclusions.

The metallic envelope of the abrasive in the abrader structure of our invention is composed of crystal grains which are dendrites starting at and extending from the substrate surface, creating a super position of grain growth interrupted by other grain skeleton deposit on top thereof. The grains thus deposited form a mechanically interlocked grain structure giving to the metal sheath high tensile strength. Such deposits are termed "allotriomorphic."See FIG. 13.

We prefer to produce the aforesaid encapsulated abrasive of our invention by a process of chemical vapor deposition, by subjecting the abrasive particles to contact with a volatile metal compound at an elevated temperature sufficient to maintain the metal compound in vapor form and contact the vapor with a solid substrate under metal deposition conditions.

We have also found it useful, where the chemical nature of the abrasive particle substrate permits, to choose metallic envelopes which form a surface chemical bond with the substrate by reason of a limited chemical reaction between the metal and the substrate surface, thus producing an encapsulated particle in the form of a cermet.

The formation of the intersurface bond between the envelope and the substrate is facilitated by the elevated temperature employed in our preferred method of metal deposition.

Where the metal envelope has a coefficient thermal expansion substantially greater than that of the abrasive particle, by depositing the metal envelope on the substrate particle, at a high temperature the resultant particle on cooling is placed under compression by the metallic envelope. Thus, the tensile force necessary to rupture the abrasive particle must then be greater than in the case of the unencapsulated particle.

This property has an advantage irrespective of the bonding agent employed and may advantageously be used whether resin or metal acts as the bonding agent.

Where the abrader structure is to be used as a cutter or abrader, for example, in oil well drill bits or other boring and shaping tools suitable for example in sawing concrete, masonry, rocks, ceramics, bricks, etc., we prefer to use abrasive materials preferably those having hardness of about 2000 kg/mm.sup.2 (Knoop or Vickers) or more, for example, those shown in Table 1. An additional useful criteria is that the abrasive material should have a melting or softening point in excess of the highest temperature reached in the process by which the abrader structure is formed, such as is described hereinbelow.

We prefer to employ, because of their physical properties, such as hardness, melting point, chemical stability, and other physical properties, one of the following abrasive materials preferring among them diamonds, either natural or synthetic. In addition to diamonds, we may use any one of the following abrasive particles shown in Table 1. The values reported in the table are taken from the available literature.

TABLE 1 __________________________________________________________________________ M.P. .degree.C. Sp.G. Percent Hardness Linear Coeff. kg/mm.sup.2 of Expansion Knoop* .times. 10.sup.4 Vickers** 0-1000.degree. F. __________________________________________________________________________ Diamonds (Synthetic or Natural) 3.5 1.5 8000* Aluminum Oxide (Al.sub.2 O.sub.3) 2060 3.5-4 4.4 3000* Cast Eutectic Tungsten Carbide 4800 15 Tungsten Mono Carbide (WC) 4800 15.8 2.7 Ditungsten Carbide (W.sub.2 C) 4800 17.3 1950-2400 Boron Nitride (Cubic) >1700 3.48 .about.20 4700* Tetrachromium Carbide (Cr.sub.4 C) 1500 6.99 3 Trichromium Dicarbide (Cr.sub.3 C.sub.2) 1910 6.68 2.4 2650 Titanium Diboride (TiB.sub.2) 2870 4.52 4.2 3000-3500* Hafnium Diboride (HfB.sub.2) 3250 11.20 4.2 3800* Zirconium Diboride 10 (ZrB.sub.2) 3100 6.09 4.6 2000* Calcium Hexaboride (CaB.sub.6) 4050 2.46 3.6 2740.+-. 220* Barium Hexaboride (BaB.sub.6) 4100 4.32 3.8 3000.+-. 290** Tantalum Carbide (TaC) 3.7 Silicon Carbide >1000.degree. 3.21 2.4 2200-2900* __________________________________________________________________________

In order to obtain a compressive force on the substrate, we select a metal for the envelope having a substantially greater coefficient expansion than the substrate. In such case, when the metal is deposited on the substrate at an elevated deposition temperature, on cooling, the metal sheath will contract more than the substrate, putting the substrate under compression. Since the linear coefficient of thermal expansion of suitable abrasives are in the range of about 1 to about 5 .times. 10.sup..sup.-6 inches per inch per degree Fahrenheit, we select metal sheaths having a higher coefficient of expansion than the substrate. For example, we select metals having linear coefficients of expansion of about 2 .times. 10.sup..sup.-6 to about 10.sup..sup.-5 inches per inch per degree Fahrenheit. By matching the coefficients of expansion, as described above, a useful encapsulation may be obtained. It is useful to remember that the coefficients of cubic expansion may, for the above purposes, be taken as about three times the linear coefficient of expansion. In such a combination the disruptive force sufficient to fragment the substrate particle must be greater than that which would fracture the unencapsulated particle, since it must overcome initially the compressive force which places the underlying substrate in compression.

The following Table 2 lists metals having coefficients of expansion above the lower limit of the coefficients of the abrasives listed in Table 1. As in Table 1 the values are taken from available literature.

TABLE 2 __________________________________________________________________________ Metal Specific Melting Percent Young's Gravity Point Thermal Modu- .degree.C Coefficient lus of Linear .times.10.sup.6 Expansion .times. 10.sup.4 /.degree.F 0-1000.degree. F __________________________________________________________________________ Tungsten (W) 19.3 3380 4 50 Tantalum (Ta) 16.6 2966 3.9 27 Molybdenum (Mo) 10.2 2610 2.2 50 Niobium (Nb) i.e. Columbium (Cb) 8.5 2500 4 26 Vanadium (V) 5.89 1890 3.2 41 Zirconium (Zr) 6.4 1852 3 11 Titanium (Ti) 4.54 1675 4.7 16.8 Iron (Fe) 7.86 1535 6.5 28.5 Cobalt (Co) 8.9 1492 6.85 30 Nickel (Ni) 8.9 1453 7.2 30 Copper (Cu) 8.9 1083 9.22 16 __________________________________________________________________________

Thus, for example, if diamond be the substrate, we may use for the purpose any one of the metals listed in Table 2 to form the encapsulating envelope. In each of these cases, the coefficient of linear expansion of the metal is substantially greater than that of diamond, and their use would also have the advantage of adding a compressive force upon the diamonds to help in overcoming the tensile forces which would tend to fracture the diamond when used in an abrader structure as the abrasive particle.

In selecting the encapsulating metal with the view of obtaining the advantage of the differential contraction, metals may be selected, depending on the stress desired to be imparted.

For example, for the metals listed in Table 2 and the abrasives of Table 1, metals having a coefficient greater than the substrate coefficient by about 5 to 10 percent or more of the value of the coefficient of the substrate. That is, the coefficient of the metal should be about 1.05 or more, for example, up to about 7 times the coefficient of the substrate.

In selecting the metal encapsulating materials, when employing diamonds as a substrate, when we employ carbide-forming metals, we prefer to employ those which have only a limited reaction rate at the temperatures of deposition, as hereinafter described. For example, we may use molybdenum, tungsten, tantalum, titanium, and niobium, all of which are carbide formers but are unlike iron which under the conditions of deposition or the production of the abrader may result in excessive attack on the diamond forming carbides or graphite.

For all of the foregoing reasons, we prefer to use in combination with the abrasive particles tabulated in Table 1 above, and selected according to their properties as described above, tungsten, tantalum, niobium (columbium) and molybdenum, and, among the primary abrasive particles, we prefer to employ diamonds, either the natural or synthetic forms, and prefer to employ tungsten as the encapsulating material, deposited under conditions to produce pure tungsten of the crystal form as described herein.

Where we employ the metal encapsulated abrasive in abrader structures formed by metal bonding the encapsulated abrasive in a metal continuous phase matrix, we prefer to employ as a bonding agent a metal having a significantly lower melting point than the metal sheath of the abrasive substrate. When employing diamonds as the encapsulated abrasive particle, we prefer to limit the melting point of the metal matrix to a temperature below about 2,800.degree. F. in order not to expose the diamonds to excessive temperature which may impair the mechanical strength of the diamonds.

Another important consideration is the coefficient of thermal expansion of the metal matrix used as bonding agent. Since, in general, the low melting metals and materials have high thermal expansion, in the absence of an encapsulating metal which is wetted by the molten metal, the mass of matrix on cooling would tend to pull away from the abrasive material, thus impairing the bond. It is one advantage of the encapsulating metal that the thermal expansion of the metal sheath matches more closely the thermal expansion of the metal matrix and that the interfacial tensions will tend to prevent the pulling away of the metal matrix from the metal sheath. Such metals having melting points so as to be fluid in the formation of the abrader structure, for example at temperatures below about 2,800.degree. F. when employing diamonds are suitable.

However, we prefer to employ such metals which also have the preferred properties as hereinafter described. The metal chosen should be fluid at the temperature at which it is desired to employ the molten metal in forming the composite abrader structure and desirably should have, when solid, ductility as measured in the terms of microhardness of below about 400 kg/mm.sup.2. Desirably, also, it should have a compressive strength above about 90,000 p.s.i. and an impact strength above about 5 foot pounds.

For this purpose we may use copper-based alloys such as brass and bronze alloys and copper-based alloys containing various amounts of nickel, cobalt, tin, zinc, manganese, iron and silver.

Since when using encapsulated diamonds the diamond is protected from attack by the metal, we may use cobalt-based, nickel-based, and iron-based alloys of suitable properties. These alloys are excluded from use as metal matrixes when using unencapsulated diamonds because under molten conditions they attack the diamond excessively. Thus we may with the encapsulated diamond, for example, employ the nickel-copper-aluminum-silicon alloy having a melting point below 2,000.degree. F.; cast iron, cobalt, chromium, and tungsten alloys having melting points below about 2,800.degree. F. may be used.

Where the abrasive particle is a tungsten carbide or diamond particle which is attacked by nickel, cobalt or iron or alloys of these metals, the encapsulation of the tungsten carbide by a metal envelope of substantially higher melting point according to our invention will prevent the attack which the unencapsulated particle would otherwise suffer under the conditions of fabrication of the abrader structure.

As described above, when using a molten metal to form the matrix, unencapsulated cast tungsten carbide is attacked by iron-based or nickel-based alloys. The W.sub.2 C tungsten carbide is attacked or dissolved in the binder, and on freezing precipitates a new phase called eta. This phase is M.sub.6 C type carbide, and in the case of nickel binders will have the composition Ni.sub.3 W.sub.3 C. Eta phase is more brittle than the original particle. The particle is said to be "haloed." The "haloed" portion of particle will have a hardness only of about 1,500 kilograms per square millimeter, compared for example to 1,950 to 2,100 kilograms per square millimeter (Knoop) for the core of the particle. By employing a tungsten-coated tungsten carbide of the above characterization, the "haloing" effect of these metals is avoided and they then may be used as a binder metal.

In one form of the above abrader structures, a plurality of different abrasive particles are employed. In addition to particles of high hardness values, for example, diamonds which act on the primary abrasives, there is distributed in the continuous phase of the metal matrix binder a secondary abrasive of lower hardness value, for example, those shown in Table 1.

The purpose of this secondary abrasive particle is to wear away preferentially thus exposing new abrasive faces of the primary abrasive particle.

The abrader structures thus formed are deemed self-sharpening. That is, the matrix including the secondary abrasive should wear away preferentially and uniformally exposing new primary abrasive cutting surfaces. This tends to reduce the area of the interfacial surfaces between the bonding metal of the matrix and the primary and secondary abrasive particles. Where the bond is weak, the particles are torn out of the metal matrix, causing excessive wear.

The intermetallic bond between the metal matrix and the encapsulated primary or secondary abrasive increases the retention of the abrasive particle until its cutting life is ended by wearing away of the particle or breaking away of fragments thereof from the portion of the abrasive particle which has become free of the encapsulation at the abrading surface during the abrading action.

In selecting the secondary abrasive, we may, in order to add mass to the abrader select from the abrasive particles having suitable hardness and other desirable physical properties those having a specific gravity to give mass to the abrader; i.e., those with specific gravities substantially in excess of the abrasive substrate.

For example, when employing unencapsulated diamonds as a primary abrasive in an abrader we may use as the secondary abrasive tungsten carbide or hafnium diboride or those of somewhat lower specific gravity, i.e. 6 or more as set forth in Table 1.

However, as described above, we may employ the secondary abrasive having a lower specific gravity by encapsulating the secondary abrasive by metal having sufficiently high specific gravity to produce a particle of substantially higher apparent density.

Thus we may use encapsulated secondary abrasive of suitable hardness chosen for example from the list of Table 1 and encapsulate the secondary abrasive with a metal of suitable specific gravity to increase the apparent density of the particle. This will permit the fabrication of an abrader having the required volume percent of secondary abrasive but impart a greater weight to the abrader structure as compared with one of like composition and volume but employing the unencapsulated secondary abrasive particle.

Thus for example as described above where unencapsulated tungsten carbide has been used we may employ in its place alumina encapsulated with tungsten to give a particle of substantially higher specific gravity than the alumina. Reference to Table 2 will permit the selection of suitable encapsulating metals for the purpose.

The advantages obtained by using an envelope of higher specific gravity than the substrate, in order to impart mass to a given volume of the abrader structure become more evident as the difference between the specific gravity of the substrate and of the envelope becomes the greater.

We prefer to employ for the encapsulation of the abrasive particles the reduction of a vapor of the metal compound.

For such purpose we prefer to select among the metals chosen according to the aforesaid principles of our invention those which form a compound which may be placed in the vapor state in contact with the substrate under conditions to deposit the metal on the substrate surface.

We prefer to employ a compound which may be vaporized at a convenient temperature either because of its relatively low boiling point or by reduction of its partial pressure and be introduced into the contact zone with the abrasive particle for conversion to the metal state deposited on the substrate.

The procedure we prefer, because it produces the superior envelope when applied to produce our novel encapsulated abrasive particle, is the conversion of a volatile compound of the metal into the metal deposited on the substrate and a gaseous or vaporous reaction product which may be removed from contact with the encapsulated metal. This leaves an envelope substantially free of included impurities.

For this purpose we prefer to use the halides or the carbonyls of the metals. Preferably for convenience of operation, we prefer to employ those compounds having a boiling point at atmospheric pressure below the reaction temperature.

While compounds which may be placed in the liquid state and which may be distilled by vacuum distillation or by reduction of their partial pressure by means of a carrier gas are possible, the compounds listed in Table 3, having reasonable boiling points, so that their volatilization may be conveniently allowed, are preferred by us.

TABLE 3 ______________________________________ B.P. .degree.C. at 760 m.m. * ______________________________________ Iron Carbonyl [Fe(CO).sub.6 ] 102.8+ Molybdenum Pentachloride [MoCl.sub.5 ] 268 Molybdenum Hexafluoride [MoF.sub.6 ] 35 Molybdenum Carbonyl [Mo(CO).sub.6 ] 156.4 Tungsten Pentabromide [WBr.sub.5 ] 333 Tungsten Hexabromide [WBr.sub.6 ] 17.5 Tungsten Pentachloride [WCl.sub.5 ] 275.6 Tungsten Hexachloride [WCl.sub.6 ] 346.7 Tungsten Carbonyl [W(CO).sub.6 ] 175 at 766 m.m. Tantalum Pentachloride [TaCl.sub.5 ] 242 Tantalum Pentafluoride [TaF.sub.5 ] 229.5 Titanium Tetraboride [TiB.sub.4 ] 230 Titanium Hexafluoride [TiF.sub.6 ] 35.5 Titanium Tetrafluoride [TiCl.sub.4 ] 136.4 Columbium Pentabromide [CbBr.sub.5 ] 361.6 Columbium Pentafluoride [CbF.sub.5 ] 236 Columbium Pentachloride [CbCl.sub.5 ] 236 Nickel Hexafluoride [NiF.sub.6 ] 4 at 25 m.m. Vanadium Tetrafluoride [VaCl.sub.4 ] 148+ Vanadium Pentafluoride [VaCl.sub.5 ] 111+ ______________________________________ * Unless otherwise indicated

In view of the above consideration, we prefer to employ tungsten as an encapsulating metal because of its high density and high melting point. It gives under the conditions of fabrication according to our invention a coating of exceptional high strength. It is readily wetted by the molten metal matrixes described above and forms a strong metallurgical bond with the metal matrixes employed in our invention. It is particularly useful where the substrate is diamond or other substrates which will react with the tungsten such as those which form cermets with tungsten.

Our preferred primary abrasive is diamond. Where encapsulated with a metal under the preferred conditions as described herein, it will produce a superior abrader structure of longer life. Where encapsulated with tungsten or other suitable metals as described above, it will after the exposed metal sheath in contact with the work has been worn away be exposed to the work but will otherwise be gripped by the encapsulating envelope which is in turn gripped by the metal matrix.

In place of or in addition to the encapsulated diamond, we may use the other abrasives as described above, preferring among them encapsulated alumina but may also use the other abrasives described above, particularly encapsulated tungsten carbide or silicon carbide as is more fully described below.

The invention will be further described by reference to the following figures:

FIG. 1 is a diagrammatic flow sheet of our preferred process of encapsulation.

FIG. 2 is a section through a mold for use in the infiltrant technique of forming abraders according to one form of our invention.

FIG. 3 is a schematic showing of a mold for use in a hot press technique employed in forming an abrader element.

FIG. 4 is a section on line 4--4 of FIG. 3.

FIG. 5 is a section taken on 5--5 of FIG. 2.

FIG. 6 is a schematic view of a saw on which the formed abrader is mounted.

FIG. 7 is a section through a mold for the core bit shown in FIG. 8.

FIGS. 9-14 are photomicrographs of etched sections of metal abrasive particles contained in a metal matrix according to our invention.

FIG. 1 illustrated a flow sheet of our preferred process for producing the novel encapsulated abrasive of our invention. The particles to be coated are placed in the reactor 1, whose cap 2 has been removed. The reactor has a perforated bottom to support the particles of selected mesh size. With cap 2 replaced and the valves 3, 4, 5, and 13 closed, and with valve 7 open, the vacuum pump is started to de-aerate the system. Valve 7 is closed and the system filled with hydrogen from hydrogen storage 11, valve 5 being open.

The reactor is heated by the furnace 9 to the reaction temperature, for example, from about 1,000.degree. to about 1,200.degree. F. while purging slowly with hydrogen. The hydrogen flow rate is increased until a fluidized bed is established. Hydrogen prior to introduction into the reactor passes through a conventional palladium catalyst to remove any impurities, such as oxygen in the hydrogen. Vaporized metallic compound is discharged from the vaporizing chamber 10, which may if necessary be heated by furnace 14, together with an inert gas, for example, argon from argon storage 6 into the reaction chamber.

Preferably we desire to employ the volatile metal halides referred to above, although, in some cases, we may use the carbonyls listed in Table 3. Where the halide is employed, the reaction forms hydrogen halide, which is passed through the bubble traps and is absorbed in the absorber. Where the volatile compound employed is a fluoride, the product formed is a hydrogen fluoride, and we may use sodium fluoride for that absorption. We prefer to employ hydrogen in stoichiometric excess.

The reaction deposits metal on the substrate and the effluent material, being in the vapor state is discharged, leaving no contaminants on or in the metal. The metal is formed in its pure state.

The rate of metal deposition depends on the temperature, and flow rate of the reactants, being the greater the higher the temperature and the greater the flow rate of the hydrogen and volatile metals compound.

After the deposit is formed, the valves 4 and 5 are closed and argon is continued to pass into the reactor and the metal encapsulated abrasive is allowed to cool to room temperature in the non-oxidizing condition of the argon environment.

The conditions in the reactor, both because of the mesh size and particle size distribution of the particles and because of the velocity of the vapors and gases fluidizes the particles. As will be recognized by those skilled in the art, a dense phase is established in the lower part of the reactor in which the particles are more or less uniformally distributed in violent agitation in the dense phase. This results in a substantially uniform deposit per unit of surface of the particles.

The reaction products and the carrier gases and excess hydrogen enter the upper space termed the disengaging space where they are separated from any entrained particles.

Where the diamond particle is smooth as for example in the case of synthetic diamonds, we may improve the bond of the metal envelope to the substrate diamond surface produced in the process described above by first surface etching of the diamond. The etching of the diamonds will also have an advantage where the metal envelope is produced by other processes such as electrochemical or electrolytic deposition methods. However, for the reasons previously described, the product produced by the process of vapor deposition described above is superior and is preferred by us.

EXAMPLE 1

To etch the diamonds, we immerse them in a molten bath of an alkali metal nitrate or alkaline earth nitrate at a temperature below the decomposition temperature; thus in using potassium nitrate, temperature would range from 630.degree.+ F. and under 750.degree. F.; sodium nitrate, about 580.degree. F. and under about 700.degree. F.; barium nitrate, at or above 1,100.degree. F. and below its decomposition temperature. We prefer to employ potassium nitrate at about 630.degree. F. for about an hour. The bath is contained in a nitrogen or other inert gas atmosphere.

At the completion of the heating process, the molten bath is cooled and the cooled bath is then leached with water to dissolve the salt, leaving the etched diamonds which may then be separated and dried.

The degree of etching depends upon the immersion time and a suitable time will be about an hour under which conditions the particles will lose from about 1/2 to 1 percent of their weight. The surface of the diamonds is roughened and pitted and forms a desirable and improved substrate base.

For purposes of illustration, not as limitations of our invention, the following examples are illustrative of the process of depositing a metal sheath upon a substrate.

EXAMPLE 2

Diamonds, either synthetic or natural, preferably etched as above, of mesh size suitable for fluidizing are introduced into the reactor 1. The actual mesh size employed depends upon the service to which the abrader is to be placed. For use in oil well tools, cutters, saws, and grinders, we may use particles of size (Tyler mesh) through a 16 and on a 400 mesh (-16 + 400). Preferably we employ 40 to 100 mesh material, for example -40 + 50 mesh. In depositing tungsten, we may and prefer to employ tungsten hexafluoride, which is contained and vaporized in 10. It is volatile at atmospheric temperatures and need not be heated. In the reactor employed after the system has been deaerated and back filled, hydrogen flow is established at a low flow rate of about 100 ml/min and as described above, the temperatures in the reactor 1 having been adjusted to 1,150.degree. F., as measured by the thermocouples, the hydrogen flow is increased to about 1,250-1,350 ml/min, and the flow of the tungsten fluoride vapor to about 150 ml/min and the argon gas is adjusted to about 285 ml/min, all as measured by the flow meters as indicated in FIG. 1, the hydrogen being in stoichiometric excess over the tungsten hexafluoride.

The thickness of the coat of the tungsten on the diamond depends on the duration of the treatment and suitably for the 40 to 50 mesh diamonds described above, the coat will be 1 mil. thick in about 1 hour. Suitable thickness deposit will run from about 0.1 to about 1.5 mils thick.

EXAMPLE 3

Instead of diamonds, we may use alumina. The mesh size, temperature, and procedure as described in Example 2 may be followed to produce a tungsten coat of the thickness previously described.

EXAMPLE 4

Similarly, a tungsten carbide may be coated with tungsten, following the procedure described in Example 1.

In the above example, the substrate surface is completely coated, indicating that the process of vacuum chemical vapor deposition has great throwing power. The outer surface of the coated particles is topographically congruent to the outer surface of the underlying substrate and reproduces it. The interlocked structure produces a coating of high tensile and bending strength.

Since the coating is produced at high temperature, on cooling the contraction of some 1,100.degree. F. will be substantially in excess of the contraction of the substrate as described and the resultant eventual contraction will produce a compress of the underlying abrasive particle. (See FIGS. 9 through 14).

EXAMPLE 5

The process of Example 2 was employed in coating silicon carbide particles of -80 + 100 mesh.

EXAMPLE 6

The metal coated particles may be employed in producing improved abrader structures by any of the techniques previously used with unencapsulated abrasive particles. These include what have become known as surface set, infiltration, hot pressing, and flame metalizing procedures.

For example, a surface set oil well drill (see FIGS. 7 and 8) (such as described in the Austin Pat. No. 2,838,284) may be formed in a graphite mold which is formed with sockets positioned in the interior surface of the mold adjacent to the boring surface of the drill to be formed in the mold. A steel shank is positioned in the mold spaced from the interior surfaces of the mold. Between the space in the mold between the shank and the diamonds is contained a matrix composed of a mixture of sized particles of cast tungsten carbide as the secondary abrasive and a powdered metal such as nickel or tungsten. This mixture extends in the mold above the surface on which the diamonds are deposited. The grain size of the tungsten carbide is chosen to give the proper compaction and void volume; for example, in the range of 35 to 75 percent of the total volume, e.g., -30 + 60 mesh such as described above. The mold is vibrated to compact the tungsten carbide particles.

Superimposed on the layer of tungsten carbide particles is a layer of powdered binder metal such as is described above. The mold is introduced into a furnace and heated to a temperature sufficient to melt the infiltrant metal, for example, the range of 2,000.degree. to 2,500.degree. F., employing, for example, a copper-nickel-zinc alloy. The metal melts and percolates through the interstices, thus producing a continuous phase metal matrix bonding the tungsten carbide particles and binding the portions of the diamond particles protruding from the mold surface.

In this procedure we prefer to employ the tungsten coated diamonds prepared according to the process previously described, having a coating, for example, of about half a mil or more, e.g. 1 to 1.5 mils. Instead of tungsten carbide, we may employ as the secondary abrasive any of the other abrasives other than diamonds listed in Table 1, or the aforesaid second abrasive particles encapsulated in a metal capsule, as described above, for example, alumina coated with tungsten according to the above procedure.

As previously described, in carrying out this procedure we wish to select a temperature of formation which will be below about 2,800.degree. F., in order not to expose the diamonds to an excessive temperature. Because of the metallic coating which is placed upon the diamond and, if desired, on the secondary abrasive particles, the binder metal will wet the surfaces of the encapsulated particles, thus producing a tight bond to the matrix. The encapsulation of the primary and secondary abrasive materials reduced the attack by the infiltrant metal upon these products, whereas the uncoated abrasives might be readily attacked by infiltrant metals as described above.

The secondary abrasive used in the above construction may be usefully a tungsten carbide ranging from WC having 6.12 wt. percent of carbon to W.sub.2 C having a carbon content about 3.16 wt. percent. A useful material is so-called sintered tungsten carbide and consists of microsized WC crystals and cobalt metal bonded by liquid phase sintering at high temperature. The cobalt content varies from 3 wt. percent to over 25 wt. percent. This material has a harness of about 1,250 to 1,350 kg/mm.sup.2 (Knoop). Another form of eutectic alloy containing about 4 percent by weight of carbon having a hardness in the range of 1900 to 2,000 kg/mm.sup.2 (Knoop) may also be used.

Abrader elements may also be produced by an impregnation technique by mixing the primary and secondary abrasive materials in powder form, vibrating or packing the mixture in a suitable mold, and infiltrating the mixture with a low-melting binder metal alloy as described above.

EXAMPLE 7

FIGS. 2 and 5 show a suitable graphite mold for use in such techniques for producing saw blade segments to be brazed to a saw blade. The mold is composed of a base 101, the mold proper 102, with an anchor 103, carrying a funnel 104, clamped by clamp bolt 105, and covered with a furnace cap 106. The mold proper consists of circumferentially space mold cavities having substantially smaller circumferential extension than their radial length. The primary abrasive, for example, a mix of diamond particles 20 + 45 or - 45 + 60 mesh screen and powdered tungsten carbide is tamped into the mold 102. The funnel contains a bronze-copper-tin-alloy powder through a 200 mesh screen. The diamonds form about 25 percent by volume of the mixture of the metal-diamond finally formed in the mold cavity 103. The mold is heated to about 2,000.degree.-2100.degree. F. to melt the alloy which percolates through the interstices between the diamond particles in the mold cavity, i.e., infiltrates the pores filling them to form the continuous phase binding the coated diamond particles and the tungsten carbide in the continuous metal matrix.

Example 8

The process of Example 7 and the product then produced may also be carried out by replacing the tungsten carbide with a metal coated tungsten carbide for example tungsten carbide coated with tungsten metal or other tungsten coated secondary abrasive such as alumina or silicon carbide.

For example, in forming a 12 inch saw blade on which about 19 of the above sections are brazed at the periphery of the saw blade, we may use sections of about 17/8 inches long, one-eighth inch wide, and about five thirty-seconds inch thick may be formed suitably by introducing about 3,500 stones of mesh size -45 + 60 grit or about 1.1 carats of diamonds grit. (See FIG. 6)

Instead of employing the infiltrant process, we may employ a hot press procedure to formulate the abrader of our invention. In such procedure, the mixture in the mold is a mixture of abrasive particles and powdered metal which is to form the continuous metal matrix to bind the abrasive particles.

Example 9

The mold employed is shown in FIGS. 4 and 3. The mold is similar to that of FIG. 2 except that no funnel is employed and the nut 105 is now a plug 107 and the funnel 104 is replaced by the cap 108 in place of cap 106. The mold is formed for the insertion of the cap as shown. The mold is placed in a press and heated for example in an induction furnace.

To produce the saw blade element according to the hot press method described above, a mixture of tungsten metal power, powdered tungsten carbide -35 + 50 mesh diamond grit which has been coated with a 10 micron tungsten metal envelope as described above is mixed with a -200 mesh bronze-tin alloy and tamped into the mold of FIG. 4. The concentration of diamonds in the mix may be suitably about 25 percent. The mold is heated to about 1,600.degree. F. at about 3,000 p.s.i. pressure to produce the saw blade element.

Instead of tungsten carbide, we may use tungsten coated tungsten carbide or other metal coated secondary abrasive described above, for example, tungsten coated alumina or silicon carbide.

Instead of using the low temperature melting bronze as an example, we may use the higher melting metals as binder matrix such as iron, cobalt, nickel or alloys of these metals and heat the hot press mold to temperatures as high as above 1,535.degree. F. depending on the melting point of the metal selected to form the binder.

In carrying out the procedures of Examples 3 through 9 where we have referred to encapsulated abrasive, we prefer to employ the process of encapsulation described in Example 2 and where diamonds are referred to we prefer, where they are synthetic diamonds having a smooth face, that this be etched, for example, by the procedure of Example 1.

The superior product produced by the encapsulation method of Example 1 and 2 when used in the production of the abraders by the hot press or infiltrant method is illustrated in:

FIG. 9 which shows a 0.025 inch tungsten coat on an aluminum oxide particle in the metal matrix at 140 .times. magnification and having an apparent density of 9.3 grams per cubic centimeter.

FIG. 10 shows a similar tungsten coated alumina particle in a metal matrix at 280 X magnification.

FIG. 11 shows a tungsten coated diamond particle hot pressed into a metal matrix at 210 .times. magnification.

FIG. 12 shows a portion of the particle at 840 .times. magnification.

FIG. 13 shows -80 + 100 mesh silicon carbide particle coated with tungsten hot pressed into a metal matrix at 280 .times. magnification.

FIG. 14 shows tungsten coated Al.sub.2 O.sub.3 hot pressed in a metal matrix at 1700.degree. F., polished and etched at 560 .times. magnification to show the allotriomorphic dendrite crystal structure.

It will be seen the excellent throwing power of the process and the intimate coating produced. The metal sheath is congruent to the substrate surface coproducing it faithfully. The resultant intimate bond produces the advantages of compression and hot transfer referred to above. We claim :

Claims

1. An article of manufacture consisting of abrasive particles having a hardness of above about 2000 kg/mm.sup.2 encapsulated with a metal envelope formed of allotriomorphic dendrites.

2. In the article of claim 1 in which the abrasive particles are chosen from the group consisting of diamonds, tungsten carbide, alumina and silicon carbide.

3. In the article of claim 2 in which the encapsulated metal is chosen from the group consisting of tungsten, tantalum, columbium (niobium) and molybdenum.

4. In the article of claim 1 in which the abrasive is diamond and the encapsulating metal is tungsten.

5. In the article of claim 1 in which the abrasive is tungsten carbide and the encapsulating metal is tungsten.

6. In the article of claim 1 in which the abrasive is alumina and the metal is tungsten carbide.

7. In the article of claim 1 in which the abrasive has a coefficient of linear expansion in the range of about 1 .times. 10.sup..sup.-6 inches per inch per degree Fahrenheit to about 5 .times. 10.sup..sup.-6 inches per inch per degree Fahrenheit and said metal has a linear coefficient of expansion in the range of from about 2 .times. 10.sup..sup.-6 to about 10.sup..sup.-5 inches per inch per degree Fahrenheit and in which the linear coefficient of expansion of said metal is in the range of from about 1.05 to about 7 times the linear coefficient of expansion of said unencapsulated particle.

8. In the article of claim 7 in which the abrasive particles are chosen from the group consisting of diamonds, tungsten carbide, alumina and silicon carbide.

9. In the article of claim 8 in which the encapsulated metal is chosen from the group consisting of tungsten, tantalum, columbium (niobium) and molybdenum.

10. In the article of claim 7 in which the abrasive is diamond and the encapsulating metal is tungsten.

11. In the article of claim 7 in which the abrasive is tungsten carbide and the encapsulating metal is tungsten.

12. In the article of claim 7 in which the abrasive is alumina and the metal is tungsten carbide.

13. A shaped abrader structure comprising a continuous phase of metal matrix and abrasive particles positioned in said matrix said particles having a hardness in excess of about 2,000 kg/mm.sup.2 encapsulated with a metallic envelope formed of allotriomorphic dendrites.

14. In the article of claim 12 in which the abrasive particles are chosen from the group consisting of diamonds, tungsten carbide, alumina and silicon carbide.

15. In the article of claim 14 in which the encapsulated metal is chosen from the group consisting of tungsten, tantalum, columbium (niobium) and molybdenum.

16. In the article of claim 13 in which the abrasive is diamond and the encapsulating metal is tungsten.

17. In the article of claim 13 in which the abrasive is tungsten carbide and the encapsulating metal is tungsten.

18. In the article of claim 13 in which the abrasive is alumina and the metal is tungsten carbide.

19. As an article of manufacture an etched diamond particle encapsulated in a metal envelope in which the metal is composed of allotriomorphic dendrites.

20. In the article of claim 19 in which the metal is tungsten.

21. A method of producing an abrasive particle having a hardness of above about about 1,700 kg/mm.sup.2 encapsulated with a metal envelope formed of allotriomorphic dendrites which comprises contacting said particle with a vaprous metal compound and with hydrogen at an elevated temperature in the range of about 1000.degree. to about 1200.degree. F. and depositing metal as an envelope on said particle and separating the said particles from vapors and gases in contact therewith.

22. In the process of claim 19 in which the abrasive particles are chosen from the group consisting of diamonds, tungsten carbide, alumina and silicon carbide.

23. In the process of claim 19 in which the encapsulated metal compound is chosen from the group consisting of tungsten, tantalum, columbium (niobium) and molybdenum.

24. The process of claim 19 in which the abrasive particles are diamonds etched by contact with a molten alkali metal nitrate or an alkali earth nitrate at temperatures below the decomposition temperature of the nitrate in an inert atmosphere.

25. The process of claim 24 in which the encapsulating metal is tungsten.