Cemented and Cemented/Sintered Superabrasive Polycrystalline Bodies and Methods of Manufacture Thereof

The present invention relates to superabrasive cutting elements for tools and, more particularly, to cemented and cemented/sintered cutting elements and backed compacts of metal coated diamond and cubic boron nitride crystals and processes for the manufacture thereof.

Composite polycrystalline abrasive masses or compacts composed of diamond or cubic boron nitride ("CBN") crystals are widely used in industry as cutting elements in drill bits, dressing or cutting tools and wire drawing dies. Such compacts have demonstrated advantages over single crystal cutting elements, such as better wear and impact resistance.

Compacts have been formed by sintering individual diamond particles together under the high pressure and high temperature ("HPHT") conditions referred to as the "diamond stable region," typically above 40 kbars and between 1,200.degree. C.-2,000.degree. C., in the presence of a catalyst/solvent which promotes diamond-diamond bonding.

Commercially available sintered diamond compacts have a variety of microstructures. An unleached polycrystalline diamond ("PCD") compact having a temperature stability of up to about 700.degree. C.-750.degree. C., can be formed, for example, in accordance with U.S. Pat. No. 3,609,818. In this patent, diamond powder is mixed with graphite powder and a metal catalyst/solvent such as cobalt, and subjected to HPHT conditions to form a polycrystalline compact having sintered diamond-diamond bonds. Catalyst/solvents for sintered diamond compacts include cobalt, nickel, iron, other Group VIII metals, and alloys containing these metals. Diamond compacts usually have a diamond content greater than 70% by volume, with 80-95% being typical. An unbacked compact can be mechanically bonded to a tool body.

Sintered compacts of polycrystalline CBN can be made, for example, in accordance with the teachings of U.S. Pat. Nos. 3,767,371, 3,742,489 and 4,403,015. Catalyst/solvents for CBN include aluminum, or an aluminum alloy including nickel, cobalt, iron, manganese or chromium.

The compact can be bonded to a substrate such as cemented tungsten carbide by subjecting a layer of diamond powder and a mixture of tungsten carbide and cobalt powders to HPHT conditions. The cobalt diffuses into the diamond powder during processing and therefore acts as both a solvent/catalyst for the sintering of the diamond powder to form diamond-diamond bonds and as a binder for the tungsten carbide. By this method, strong bonds are formed at the interface between the diamond layer and the cemented tungsten carbide. See, for example, U.S. Pat. No. 3,745,623 and 4,403,015.

The strength and wear resistance of sintered compacts are comparable or superior to single-crystal diamonds, but they become thermally degraded and their cutting efficiency deteriorates significantly above 750.degree. C. It is believed that this is due to thermal stress resulting from uneven thermal expansion between the diamond and the catalyst/solvent phase and a graphitization of the diamond by the catalyst at high temperature. Differential expansion rates between the compact and the backing can also cause delamination or cracking at their interface.

A compact with improved thermal stability is described in U.S. Pat. No. 4,224,380, which teaches a method of leaching most of the catalyst/solvent from the compact, yielding a primarily metal free, unbacked coherent diamond-diamond bonded abrasive compact. The diamond compact is 70 to 90 percent by volume diamond, 0.05 to 3 percent by volume catalyst/binder, and 5 to 30 percent porous. Unbacked or free-standing leached compacts are thermally stable up to 1,200.degree. C. without significant structural degradation. The compact is commercially available under the tradename "Geoset.TM.". Such compacts are somewhat brittle.

The porosity of the Geoset can also be a source of weakness, decreasing the compact's strength, wear and impact resistance. In U.S. Pat. No. 4,636,253, a compact's porosity is decreased by adding a metal or carbide selected from Groups IVA, VA or VIA of the Periodic Table and an iron group metal to the diamond powder before HPHT treatment. Most of the iron group metal is then leached out. By this process, the porosity of the compact is decreased to 7% or less.

A further example of an improved thermally stable sintered diamond compact is disclosed in British Patent Application No. 2,158,086. There, the compact comprises a mass of polycrystalline diamond of about 80 to 90 percent and a second phase of silicon and/or silicon carbide of about 10 to 20 percent of the volume of the compact. The silicon is infiltrated into the compact during HPHT treatment. This compact is said to be capable of withstanding temperatures of 1,200.degree. C. under vacuum, inert or reducing atmospheric conditions, but the silicon or silicon carbide increases the brittleness of the compact.

Another characteristic affecting the properties of sintered polycrystalline compacts is the grain size of the crystals. The finer the grain size, the higher the wear resistance of the compact because, in part, only small particles chip off under a stress. Small grained compacts provide a fine finish and are preferred in machining applications.

A compact with too fine a structure and with a high degree of diamond-diamond bonding, however, can act more like a large single crystal than an aggregate of smaller crystals. The resulting hardness and high modulus of elasticity associated with the dense diamond-diamond bonding makes such compacts prone to crack propagation. Sufficient stress could therefore cause a catastrophic fracture, breaking the entire compact.

The fine diamond powder starting material required to manufacture fine grained compacts also cause material handling and processing problems which lower process yields and cause inconsistent product quality. The dense powder impedes infiltration of a catalyst/solvent, therefore sintering fine grain crystals is difficult. The increased surface area of the fine grain crystals increases their sensitivity to surface impurities while cleaning and packing small grain crystals is also difficult. The problems are particularly acute when a larger compact is desired or where the diamond feedstock particle size required is less than 6 microns. Fine grained diamond compacts also suffer the same thermal limitations as the compacts discussed above.

In addition to their use in compacts, superabrasive particles have been used to impregnate abrasive cutting elements such as abrasive grinding wheels and saw blades. For example, U.S. Pat. No. 2,818,850 discloses a cutting element formed of tungsten carbide powder, cobalt and diamond dust. See also U.S. Pat. No. 2,796,706. The diamond concentration in such mixtures is typically less than 40%. The presence of the diamond improves the cutting efficiency of the carbide matrix, but superabrasive compacts with higher diamond concentrations are superior. Furthermore, since the diamonds appear to be held in the tungsten carbide matrix by weak mechanical bonds, the small diamond grit is easily lost as the supporting matrix is eroded away.

To improve the bonding of the diamond grit to the carbide matrix, diamond particles have been coated with metals such as tungsten, tantalum, chromium, niobium or molybdenum. U.S. Pat. Nos. 3,871,840 and 3,841,852 apply chemical vapor deposition to metal coat diamond grit which is dispersed in a metal matrix. There, the coated diamond comprises approximately 25% of the abrasive products. PCD's, however, are still superior in terms of hardness, wear resistance and range of application.

More recently, U.S. Pat. No. 4,378,975 disclosed an abrasive product with superabrasive particles occupying up to 40% of the volume of the product. The particles are coated preferably by chromium and a wear resistant outer layer such as a nickel/iron based alloy. The particles are bonded together in a nickel/chromium alloy matrix, which has a melting point below 1,100.degree. C. The abrasive product is formed by mixing the coated superabrasive particles with a powder of the alloy, applying pressure at ambient temperature to form a green state product and sintering the green state product at approximately 950.degree. C.-1,000.degree. C., well below the diamond stable region. The chromium coating can comprise up to 10% of the weight of the particles while the diameter of the coated particle, including the wear resistant coating, can be 2-3 times the diameter of an uncoated particle. The low diamond concentration of the product limits its properties compared to PCD's and its thermal stability is low. Mixing and packing problems with the powders of the different components may prevent a uniform distribution of diamond particles in such a matrix, which can result in non-uniform mechanical and thermal characteristics.

Metal coatings have also been applied to products of higher diamond concentration. For example, U.S. Pat. No. 3,879,901 describes a compact of 65% diamond, formed of diamond grit coated with titanium or molybdenum, held in a matrix of an iron alloy which can include silicon. The matrix can also comprise cemented tungsten carbide with 10% cobalt. The diamond grit is mixed with powders of the matrix, and processed at HPHT conditions to form the compact. The compact is not backed. While the presence of cobalt, iron or nickel in the matrix could cause thermal instability due to thermal mismatch and back conversion, it might not be a problem at this relatively low diamond concentration. If the concentration of diamond were to be increased, however, the resulting product would probably have low thermal stability.

U.S. Pat. No. 3,650,714 describes a method of coating natural diamond products with a thin layer of titanium or zirconium to improve their bonding to metal, resin or ceramic matrices. The metal coating can comprise up to 5% of the volume of the coated diamond. An additional layer of nickel or copper can be added to prevent oxidation of the inner titanium or zirconium layer or subsequent processing can be performed in a non-oxidizing atmosphere. It has been found however, that the thin layer can be penetrated by liquid binders used in later processing, degrading the metal coating. European Patent Application No. 0,211,642, also states that metal coating polycrystalline diamond cutting elements with a thin layer of titanium or chromium improves their adhesion to a tungsten carbide matrix of the bit body of a cutting tool.

In accordance with the present invention, there is provided a thermally stable superabrasive cutting element and a method for making the same. The cutting element comprises a plurality of individually metal coated superabrasive particles wherein the metal coatings between adjacent particles are bonded to each other, forming a cemented matrix. In the case of diamond superabrasives, the metal coating should be a carbide-former and can be chosen from the groups IVA, VA, VIA, or alloys thereof. Tungsten, tantalum and molybdenum are preferred metals for the coating, with tungsten the most preferred.

In a preferred embodiment, the matrix can further comprise a binding aid improving the bonding between the metal coatings. A binding aid with thermal expansion characteristics close to that of diamond, such as silicon, is preferred in order to maintain the thermal stability of the compact. Less preferably, a catalyst/solvent such as cobalt or a non-catalyst metal such as copper can also be used, although with decreased mechanical and thermal properties.

Optionally, uncoated superabrasive particles of a size smaller than the coated superabrasives may be dispersed in the intersticies between the coated particles. Such uncoated particles can be sintered together in the presence of the binding aid, yielding a cemented/sintered cutting element.

In accordance with a further significant aspect of the invention, the metal coated superabrasive particles may comprise thermally stable polycrystalline fine grained sintered aggregates for improved wear resistance.

A cutting element in accordance with the invention can be free-standing or backed. The free-standing superabrasive cutting element can have a brazeable layer on one side for improved bonding to a supporting substrate of a tool. Cutting elements according to the present invention have superabrasive particle concentrations of at least approximately 70% and preferably at least 80% by a volume of the compact.

In accordance with the method of the present invention, a plurality of metal coated superabrasive particles are placed in a mold and are subjected to high pressure and high temperature, bonding the metal coatings of adjacent particles together. In a preferred embodiment, the coated particles can be exposed to a binding aid during HPHT, by mixing a powder of the binding aid with the particles or placing a layer of the binding aid adjacent to the coated particles in the mold for infiltration at HPHT. In another embodiment, uncoated superabrasive particles can be mixed with the coated particles before HPHT. A powder or disc of a backing or brazeable layer can be inserted into the mold prior to HPHT as well.

By coating the superabrasive before forming the compact, the formation of self bonds between the superabrasive particles are essentially avoided. It is believed that diamond-diamond bonds are a cause of some of the problems with prior art compacts, such as brittleness and low impact resistance. The metal coating also, to a substantial degree, protects the diamond from exposure to catalyst metals which can be present in the matrix or can diffuse from a backing. This minimizes graphitization or back conversion of the diamond to graphite during later exposure to elevated temperature, one source of the thermal instability of sintered polycrystalline compacts.

While in the method of the present invention the bonds between the metal coatings of adjacent particles are formed by sintering, the resulting structure is termed "cemented" with respect to the superabrasive particles because they are bonded to the matrix support structure. "Sintering" as used herein refers to the formation of diamond-diamond or CBN-CBN bonds under HPHT conditions.

FIG. 1 is a scanning electron photomicrograph of the polished surface of a cemented diamond cutting element in accordance with the invention;

FIG. 2 is a photomicrograph of the polished surface of a cemented/sintered diamond cutting element in accordance with the invention;

FIG. 3 is a photomicrograph of a cemented/sintered diamond cutting element utilizing coated fine grain sintered diamond aggregates in accordance with the present invention;

FIG. 4a illustrates various shapes of the free-standing cutting elements of the present invention, with and without a brazeable layer; and

FIG. 4b illustrates a backed compact according to the present invention.

The superabrasive cutting element according to the present invention comprises superabrasive particles coated with and bonded to a carbide-former selected from the Groups IVA, VA or VIA or alloys thereof. The coatings of adjacent particles are bonded together at the high pressures and temperatures of the diamond forming region in the process of the invention, forming a cemented matrix comprised of the coating metal and/or its carbide. The process of the present invention is described more fully below. The photomicrograph of FIG. 1 shows an example of a cutting element of the present invention, magnified 160 times.

Tungsten, tantalum and molybdenum are the preferred metals for use in coating diamond particles. Tungsten is most preferred because it is oxidation resistant, its thermal expansion characteristics are the closest to that of diamond among the carbide forming transition metals, and it has a high melting point. Tantalum and molybdenum share these characteristics to a lesser degree. Tantalum or titanium are preferred for coating CBN because they are good boride and nitride formers and have similar thermal expansion characteristics as CBN.

During exposure to HPHT conditions or prior heat treatment, the inner surface of the metal coating forms a carbide (boride or nitride) layer as a result of reacting with the carbon of the diamond (boron or nitrogen of CBN). Depending on the thickness of the coating and the processing conditions, the rest of the coating may react to form a carbide, boride or nitride, or, if the coating is thick enough, may remain metal. Bonds holding diamond superabrasive particles together may therefore be from diamond to carbide to metal to carbide to diamond, or from diamond to carbide to diamond. In either case, strong chemical bonds are formed. Although not thoroughly understood, it is believed that the bond between the diamond and the metal carbide layer is an atom to atom bond between the carbon of the diamond and the carbide.

The volume of the coating can be between 4-30% of the volume of the coated diamond. The diamond concentration of the compact is greater than about 70% by volume of the cutting element, with concentrations over 80% being preferred. The preferred coating volume is between about 8-12% of the volume of the coated diamond. A 10% volume coating is approximately equivalent to a 1.5 micron coating on a diamond with an average diameter of 100 microns. A thicker metal coating would cause a lower diamond concentration in the compact, with a corresponding decrease in desired properties, such as strength and wear resistance. If the coating is too thin, on the other hand, increased diamond-diamond bonds may form during HPHT treatment in the process of the present invention, resulting in potential instabilities such as brittleness and back conversion. The compacting and handling of the coated particles in the process of the present invention, discussed below, could cause some chipping of the metal coatings, which could expose the diamond to sintering to adjacent diamonds. There is not enough diamond to diamond bonding to have a deleterious effect on the properties of the compact, however.

Preferably, the matrix of the cutting element further comprises a binding aid which improves the strength of the cemented structure and decreases its porosity. The binding aid wets the metal coatings, assisting in the formation of bonds between the coatings. Depending on the binder, there may be chemical and/or mechanical bonding with the metal coatings, as well. The binder can comprise up to 18% by volume of the compact.

The preferred binding aids are silicon or silicon or boron containing compounds such as silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SiC) and boron carbide (B.sub.4 C). These preferred binding aids have thermal expansion characteristics close to those of diamond, decreasing potential thermal stresses. In addition, these binding aids have only a small catalytic effect on diamonds and back conversion of any exposed diamond is minimal.

Silicon is the most preferred binding aid because its thermal expansion most closely matches that of diamond. In fact, as illustrated in the examples below, compacts incorporating a binding aid of silicon have thermal stability comparable with conventional free-standing leached PCD cutters. Silicon reacts with the tungsten and tungsten carbide of the matrix, forming a silicon carbide (SiC) and tungsten silicilide (Si.sub.3 W), which increases the overall strength of the compact. The silicon can also form a carbide with the diamond exposed by chipped metal coatings. The brittleness problem experienced with prior art thermally stable compacts containing silicon is alleviated by the presence of the metal/metal carbide portion of the matrix, which is ductile.

Since the diamonds are coated and there is only minimal diamond exposure, it is believed that catalyst/solvent binding aids such as cobalt, nickel or iron or other Group VIII metals, can also be used with reduced concern over temperature instability due to back conversion during use at temperatures over 750.degree. C. A cutting element utilizing cobalt can be stronger than one of silicon, up to about 700.degree. C. At higher temperatures, the element is weakened due to thermal instabilities. A free-standing cutting element utilizing cobalt as a binding aid would be a good alternative to conventional free standing PCD's in applications involving temperatures less than 800.degree. C. A volume concentration in the cutting element of less than 4% catalyst/solvent is sufficient to achieve the strong bonding between the coated particles and this low concentration mitigates somewhat the thermal instability caused by the catalyst binder. Cross-sectional analysis of the cutting element with cobalt as a binding aid shows some contact between the diamond and the cobalt, in addition to that caused by diamond exposure due to chipped coatings. This suggests that some cobalt diffuses through the metal/metal carbide coating. The tests discussed below show that backed cutting elements utilizing cobalt according to the present invention, are thermally stable up to at least 950.degree. C. This is better than the thermal stability of prior art backed PCD's and the present backed cutting elements offer a good alternative. Other embodiments of the present invention which utilize cobalt as a binding aid, can exhibit thermal stability over 1,100.degree. C. These are discussed below.

Metals such as copper, tin or other Group IB, IIB, IIIB or IVB metals and alloys thereof, can also be used as binding aids. These metals have lower melting points than the other binding aids discussed and therefore the resulting compact has lower thermal stability than other embodiments discussed. Furthermore, copper and related metals and alloys do not react chemically with the coatings on the particles and so only provide mechanical bonds with the metal coatings. This causes somewhat lower wear resistance.

In another preferred embodiment of the present invention, the cutting element additionally includes one or more powders of uncoated, smaller sized superabrasive particles which lie in the intersticies between the coated particles. These smaller particles can comprise up to 20% by volume of the cutting element, decreasing its porosity and increasing its diamond content. The uncoated particles sinter to each other in the presence of the binding aid, and bond to the first and second phase matrices. A cutting element utilizing two different sized particles can be referred to as bimodal. If an additional, smaller superabrasive powder is added, the cutting element can be termed trimodal. A photomicrograph of a trimodal cutting element with a cemented/sintered structure magnified 160 times, is shown in FIG. 2.

The cemented metal/metal carbide structure in multi-modal compacts can be formed completely of the metal coating on the superabrasive particles, which carbides with a small amount of the carbon of the uncoated particles. To improve bonding with the uncoated particles, a fine powder of the metal, such as tungsten, can also be added to the mixture before HPHT. For adequate dispersion, the nominal diameter of the added metal powder should be any size smaller than that of the smallest uncoated particle for dispersion.

The coated superabrasive particles have nearly spherical diameters of less than 1 micron to approximately 1,000 microns, with typical compacts being composed of coated particles of between 10 to 200 microns. The use of uniformly sized coated particles is preferred for better packing, but is not required. In a sample of coated particles with an average diameter of 90 microns, for example, approximately 80% of the particles will be between 85-95 microns, with the remaining particles above and below that range.

The smaller, uncoated diamonds can be of any size smaller than the coated particles. It has been found, however, that utilizing vibrational packing methods, better mixing and packing results when the smaller particles are approximately one-sixth to one-ninth the size of the next larger particle. Such a relation between the sizes of the particles have been found to improve the packing density of the cutting element as the smaller particles fit in the intersticies among the larger particles, and are therefore preferred. As shown in FIG. 2, the presence of the smaller, uncoated particles can increase the distance between the larger, coated particles somewhat.

In accordance with another aspect of the invention, the metal coated diamond particles may themselves be formed of sintered fine grained polycrystalline aggregates for improved wear resistance. Otherwise, the compact is identical to the other embodiments discussed. The aggregates are roughly uniformly sized, with a nominal diameter between approximately 20-1,000 microns. The preferred range is about 50-250 microns. The use of fine grain aggregates improves the wear resistance relative to conventional compacts while the cemented matrix decreases the risk of crack propagation common in fine grained sintered compacts. FIG. 3 is a photomicrograph of a compact in accordance with the present invention, comprised of coated fine grain sintered diamond aggregates, magnified 80 times.

A free-standing cutting element can have a thin brazeable layer on one side for bonding to the supporting tungsten carbide substrate of a tool in ambient conditions. The layer can comprise a 0.020 inch layer of an oxidation resistant carbide former such as tungsten, titanium or molybdenum, or a carbide thereof, such as tungsten carbide. Carbide formers and their carbides are preferred because they are erosion resistant. They also form good metallic bonds with the cutting element and the substrate. The use of the brazeable layer enables bonding to the tool substrate at non-HPHT conditions. FIG. 4a shows free-standing cutting elements in accordance with the present invention, with and without a brazeable layer.

A cutting element according to the present invention can also be bonded to a backing, forming a backed compact. Cemented tungsten carbide is one preferred backing. A ceramic substrate comprising silicon compounds such as Si.sub.3 N.sub.4, or silicon carbide, or other silicon or boride containing compounds can also be used. FIG. 4b shows a backed compact in accordance with the present invention.

Backed compacts according to the present invention exhibit thermal stability up to at least 950.degree. C., which is superior to conventional backed compacts, thermally stable up to 800.degree. C. See Examples 1-7, and Table I below. The different embodiments disclosed improve thermal stability in different ways. For example, as shown in Example 1, a backed trimodal compact can be thermally stable up to 1,100.degree. C. While the reason for this is not fully understood, it is believed that the mixture of fine grains of uncoated superabrasive particles and the tungsten or tungsten carbide of the matrix might form a transition layer of intermediate thermal characteristics between the matrix of cobalt which diffuses into the compact from the backing, and the diamond, relieving some thermal stress. This improvement in thermal stability is exhibited in multimodal compacts including fine uncoated superabrasive powder having a maximum diameter less than approximately 4 microns.

The thermal stability of a backed compact can also be improved in accordance with the present invention by the use of a thin compliant layer 500 microns thick between the cutting element and the backing, which acts as a thermal transition between the two sections. This layer comprises a fine superabrasive powder between 0.5-10 microns, mixed with a Group IVA, VA or VIA metal, such as tantalum. The compliant layer helps to avoid delamination between the cutting element and the backing, and also retards the diffusion of cobalt during HPHT. Testing of Example 3, below, indicated that a compact of this type is thermally stable up to 1,000.degree. C.

A compact utilizing silicon as a binding aid, formed by placing a silicon disc or powder layer between the cemented tungsten carbide backing and the compact, as discussed in the process below, showed thermal stability over 1,100.degree. C. in Example 5. It is believed that the silicon layer retards the diffusion of cobalt into the compact during HPHT.

Boron doped diamonds can also be used as the superabrasive particles and are potentially superior in terms of thermal stability to non-boron doped diamonds. Quality inconsistencies, however, make their use uncertain at this time.

Superabrasive particles can be coated with metal by conventional techniques known in the art, such as chemical vapor deposition or sputtering, for use in the process of the present invention. Chemical vapor deposition of tungsten from tungsten hexaflouride can be conducted for 3 hours at 700.degree. C., for example.

After coating, the particles are cleaned by high temperature firing between 950.degree. C.-1,150.degree. C. for 2 hours in a vacuum or in reducing gas atmosphere, such as hydrogen. This high temperature treatment also causes chemical bonding to occur between the superabrasive particle and the metal coating, forming the carbide, boride or nitride layer.

Cutting elements are formed in accordance with the present invention by placing coated superabrasive particles into a mold and subjecting the mold to the High Pressure and High Temperature conditions of the diamond forming region. An appropriate mold is a tantalum cup, which can be tapped or shaken to settle the particles. Pressures and temperatures are typically between 40-80 kbars and 1,200.degree. C.-2,000.degree. C., respectively, for between 5-60 minutes. 15 minutes is generally preferred. The high pressure compresses and consolidates the particle layer, decreasing voids which cause porosity in the compact. Under HPHT conditions, the metal coatings bond to each other and to the diamonds to form the cemented matrix.

In a preferred process, a binding aid is inserted into the mold prior to the HPHT treatment. The binding aid is preferably added in the form of a disc placed adjacent to the superabrasive particles in the mold for infiltration during the HPHT treatment. It is preferred to place the binding aid disc on one side of the superabrasive particles because as it infiltrates, the binding aid can pick up and carry impurities which are deposited at the end of the compact and can be ground off. If infiltrated from both ends, the impurities tend to concentrate in the middle of the compact, providing a potential source of instability. Alternatively, the binding aid can be coated on the metal coated superabrasive or mixed with the superabrasive particles in powdered form by, for example, vibrational packing techniques known in the art and discussed in more detail below. It is difficult to get an uniform distribution of powder among the coated particles, however, this is alleviated somewhat by the high pressures and high temperatures of the process. Wetting by the binding aid and the high pressures of the HPHT treatment cause a slight rearrangement of the particles, allowing further settling and packing.

When silicon is used as the binder in a free-standing cutting element, infiltration should occur prior to significant bonding between the metal coatings. This can be done by conducting the HPHT treatment at least 1,300.degree. C. for 15 minutes at 55 kbars. The melting point of silicon is between 1,200.degree. C.-1,300.degree. C. at this pressure and the silicon will melt and infiltrate before much bonding between the metal coatings has taken place. The silicon can improve their bonding by first wetting the metal coatings.

If a catalyst solvent such as cobalt is used as a binding aid, it is preferred in the process to mix a powder of the binding aid with the coated particles before HPHT. Infiltration is not recommended, since it is difficult to control the amount of cobalt infiltrating into the compact.

In another embodiment of the process, uncoated superabrasive particles smaller than the coated particles are added to the mold before HPHT. As stated above, if one size of uncoated particles is used, it is preferred that the smaller particles be 1/6-1/9 the size of the larger coated particles. If a trimodal cutting element is being made, it is preferred that the smallest uncoated particles be 1/6-1/9 the size of the intermediate sized particles. The combination of particles can be uniformly mixed and densely packed by, for example, the "True-Vibrational Packing Technique," which is known in the art. A Dual Mode.TM. Power Amplifier, Model Number APS 114, a Perma-Dyne Shaker, Model Number APS 120S, controlled by a 2 MH.sub.z Function WaveTek Generator, Model 20, manufactured by Acoustic Power Systems Corporation, can be used to provide adequate vibration for mixing and packing. The operating parameters for this machine are given in the examples below. The smallest particles may be so fine that they float, preventing their dispersion between the other particles during vibration. These fine particles can be suspended in methanol and poured into the mixture of larger particles. The entire mixture is then vibrated and the methanol will assist in interstitially dispersing the fine particles among the larger particles. It will then evaporate.

In a further embodiment of the process of the present invention, the coated superabrasive particles can be fine grained sintered aggregates. Sintered abrasive aggregates can be formed, for example, by crushing and screening commercially available thermally stable sintered polycrystalline diamond compacts such as the Geoset or Syndax, or sintered polycrystalline CBN ("PCBN") such as BZN or amborite. Used or fractured PCD's from other applications can be used as starting materials as well. By utilizing commercial PCD's or fragments thereof, the manufacturing problems inherent in the use of fine grain powders can be avoided.

To prepare polycrystalline aggregate particles for use in compacts in accordance with the present invention, the sintered PCD compacts are crushed to the desired size and then thoroughly cleaned by boiling in acid, such as aqua regia, a concentrated solution of 3 parts nitric acid and 1 part hydrochloric acid. These grits are then rinsed with deionized water, and dried at high temperature. High vacuum firing at about 1,300.degree. C. and 10-.sup.5 torrs for 1 hour removes oxygen, water and impurities. The PCD grits can then be optionally treated with chemical, ceramic, or metallic additives to decrease their porosity. After metal coating by conventional coating techniques, the coated grits are dealt with in the same manner as described herein in connection with monocrystalline coated particles.

A powder or disc comprising an oxidation resistant carbide former or a carbide thereof can be added to the mold on one side of the diamond particle layer prior to the HPHT treatment to form a brazeable layer on a free-standing compact.

A backing layer material may be placed adjacent the cutting element before HPHT to form a backed compact. The backing can comprise a preformed cemented tungsten carbide disc or a powder of tungsten carbide mixed with cobalt, or a disc or powder of a different ceramic backing. When manufacturing a cemented tungsten carbide backed compact, it is difficult to control the amount of cobalt which infiltrates into the compact during HPHT treatment, therefore volume percentages above 2 must be tolerated.

Cobalt infiltration from the backing can be impeded to some degree by placing a silicon disc between the backing and compact particles prior to the HPHT treatment. The silicon should infiltrate prior to the diffusion of the cobalt which has a melting point of 1,400.degree. C. at 55 kbars. In one process, the HPHT cell is heated to 1,300.degree. C. for 10 minutes, allowing the silicon to melt and substantially infiltrate. Then the temperature is increased to 1,450.degree. C. for 5 minutes to adequately bind the tungsten carbide backing and bond the backing to the particles. Alternatively, the HPHT can be conducted at 1450.degree. C. for 15 minutes.

A compliant layer comprising a mixture of fine superabrasive powder and metal powder, can also be placed between the backing and the mixture of particles before HPHT treatment for improved thermal stability. Layers of silicon or boron containing compounds can also be used.

The following examples illustrate the processes of the present invention and the resulting products. The cutting elements and compacts formed in each of the examples are comprised of approximately 82% diamond, 5% metal coating and 12% binder material. Examples 1-7 involve backed compacts. The wear resistance test results are listed in Table I.

0.6 grams of diamond particles having a nominal diameter of 150 microns and a tungsten coating of 3 microns were cleaned by high temperature firing at between 950.degree. C.-1,150.degree. C. for 2 hours in a vacuum or reducing gas atmosphere in a Lindberg Box Furnace. They were then loaded into a protective tantalum cup. The cup was placed into the holding fixture of an APS 120S permanent magnet and electrodynamic vibrational packing table controlled by a Wavetek 2 MH.sub.z Function Generator Model 20. 0.16 grams of uncoated diamonds having a nominal 22 micron diameter were cleaned by boiling in aqua regia and then rinsed in deionized water. The uncoated diamond particles were then dried and fired at 1,050.degree. C. for 2 hours in a hydrogen gas atmosphere. They were added to the cup and dispersed in the intersticies of the larger coated diamonds by vibration. The powder packing was induced by vibration of 300-500 Hz, 1-2 g acceleration, 1-5 micron amplitude, with 12-24 psi compacting pressure, for 3-5 minutes. 0.1 grams of a third, submicron uncoated diamond powder of between 0.5-1 microns, was similarly cleaned and suspended in methanol. The suspension was added to the cup and vibrationally dispersed in the remaining interstitial voids. The methanol assists in the diffusion of the small particles into the mixture. The resulting mixture was 80 volume percent diamond.

A cobalt cemented tungsten carbide backing disc, approximately 13-16 weight percent of cobalt, was fit snugly on top of the diamond layer inside the cup. The reaction cup was loaded into a HPHT cell assembly and subjected to 1,450.degree. C. at 55 kbs for 15 minutes. The resulting trimodal tungsten carbide supported cemented/sintered body was taken out of the assembly and finished to a desired shaped product by conventional techniques such as surface grinding, lapping and outer diameter grinding.

The wear resistance and thermal stability of the compact was determined by testing the wear ratio (ratio of the volume of rock removed to the volume of the compact removed during cutting) of the compact at various temperatures. The finished compact was attached to a tool holder by a clamp. Barre granite was wet cut with the lathe at a cutting speed of 300 sfm (surface feed per minute), a depth of cut of 0.005 inches per revolution, and a feed of 0.012 inches, to determine the wear ratio at room temperature. The compact was then removed and heat treated for 20 minutes in a closed graphite container at 800.degree. C. It was then reattached to the tool holder for additional wear resistance testing. This process was repeated at 50.degree. intervals between 850.degree. C.-1,000.degree. C., and at 1,100.degree. C. and 1,200.degree. C. The existence of thermal instability was determined by examining the product after each heat treatment for structural degradation such as cracking or delamination between the cutting element and the backing. Thermal degradation cannot always be visually determined, particularly when the thermal threshold temperature is approached. In such a case, thermal instability is indicated by decreased wear resistance. Conventional PCD cutters such as GE 2541 and DeBeers Improved Syndrill were similarly tested for comparison.

The results indicated that this trimodal compact was thermally stable up to 1,100.degree. C., despite the presence of cobalt from the backing. The conventional backed PCD cutters failed due to interface delamination at about 825.degree. C. As discussed above, the superior thermal stability exhibited by this compact was possibly due to the fine superabrasive crystals acting as a thermal expansion buffer.

0.9 grams of 150 micron diamond particles coated with a 3 micron tungsten layer were cleaned as above and loaded into the tantalum cup with a cobalt cemented tungsten carbide disc. The diamonds were not vibration packed. The assembled cup was subjected to HPHT as in Example 1. The resulting cemented diamond structure comprised a cemented metal/metal carbide support structure of tungsten and tungsten carbide, with a small amount of cobalt.

Testing as in Example 1 showed temperature stability up to 900.degree. C., which is better than the conventional cemented tungsten carbide backed PCD's but not as good as the compact of Example 1.

0.4 grams of 150 micron boron doped diamonds with a 2 micron tungsten coating were cleaned as above and placed in a tantalum cup with a mixture of 0.65 grams of 50% by volume fine diamond powder of 4 microns and 50% fine tantalum powder of 1 micron placed on top. The cobalt cemented tungsten carbide disc was fit snugly on top of the powder layer in the cup and subjected to HPHT.

The resulting compact contained a cemented PCD layer with a compliant layer of the fine grain sintered PCD at the tungsten carbide interface with the backing. Testing in accordance with the method described in Example 1 indicated that this sample was temperature stable up to 1,050.degree. C.

Tungsten coated diamond feed of the following dimensions were cleaned and loaded in the tantalum cup for each of the examples indicated below. A layer of pure silicon, 0.010 inches thick, was placed on top of the diamond layer, followed by a cobalt cemented tungsten carbide disc. HPHT treatment was conducted for 10 minutes at 1,300.degree. C. and 5 minutes at 1,450.degree. C.

Testing as described in Example 1 indicated improved thermal stability and wear resistance as compared to ordinary PCD compacts. The greater improvement was shown in Examples 5-7, which had thicker tungsten coatings than Example 4. The thermal stability of the compact was preserved by the infiltration of the silicon, which retarded the diffusion of cobalt. Delamination between the backing and the compact occurred at about 1,050.degree. C., compared to 850.degree. C. for the conventional compacts. The additional mass of the silicon binder decreased the porosity of the compact and improved its overall bulk strength. These results are listed in Table I, below.

The following examples of free-standing compacts were prepared and tested as discussed below. The results of the testing of these samples by the method described in Example 1, conducted at 100.degree. C. intervals between 900.degree.-1,200.degree. C., are listed in Table II.

0.6 grams of 175 micron boron doped diamond particles with 2 micron tungsten coatings, and 0.25 grams of 22 micron uncoated diamonds were cleaned as above and mixed with 0.15 grams of Si.sub.3 N.sub.4 powder with nominal diameters between 0.1-0.9 microns, as a binding aid. The mixture was vibrated in the tantalum cup and subjected to HPHT as in Example 1.

A comparison of the resulting free-standing binded cemented compact with commercially available free-standing diamond sintered compacts such as GE 2102 and 2164 showed comparable thermal stability up to 1,200.degree. C., but the present compact showed a lower wear ratio. The Syndax-3 compact fractured during testing due to its high brittleness.

1.7 grams of 70 micron particle size boron doped synthetic diamond particles with a tungsten coating of 0.25 microns were cleaned and loaded into the tantalum cup. A cobalt disc (5.7 percent by weight iron) was placed on top of the diamond layer. A free-standing cutting element was formed having a mixed cemented and sintered structure. The thin metal coating and the cobalt caused some diamond-diamond bonding.

In Example 10, 3.0 grams of boron doped synthetic diamond particles of 175 microns diameter and a 5 micron tungsten coating were similarly prepared, yielding a cemented body with cobalt uniformly dispersed in the tungsten/tungsten carbide matrix. The thicker tungsten coating prevented the formation of diamond-diamond bonds.

Test results indicated wear ratios comparable to ordinary compacts at room temperature but deterioration began at approximately 800.degree. C. Massive structural degradation occurred at around 1,000.degree. C., which was probably due to the thermal expansion mismatch between the cobalt and the diamond, and back conversion. Analysis of cross-sectional samples indicated that cobalt was in contact with the surface of the diamond, suggesting diffusion through the metal coating layer, as well as chipping of the coating by compaction during processing. Free-standing cutting elements utilizing cobalt according to the present invention are good alternatives to conventional PCD's in low temperature applications.

Tungsten coated diamond particles of the following dimensions were cleaned and loaded in the tantalum cup, with a silicon powder or disc placed either on top, or on top and underneath the diamonds, as indicated. In examples 12-14, a brazeable layer of the indicated materials was included as well.

The wear ratio data is shown in Table II to be comparable to those of the commercially available compacts. Sample 14 was successfully brazed into a tungsten carbide disc using a standard silver solder braze alloy, such as Easy-Flo 45 from the Handy and Harman Co. The sample was shear tested by placing the substrate into the fixture of an Ingstron machine. A gradually increasing force was applied to the side of the cutting element, until it separated from the substrate. The shear strength, or the force which caused separation, was 18,000 psi.

The following examples illustrate the use of fine grained sintered polycrystalline diamond compacts as the starting material for the compact. The aggregates were cleaned by boiling in aqua regia, and rinsed with deionized water. They were dried, high vacuum fired at about 1,300.degree. C. and 10.sup.-5 torrs for 1 hour, and then coated with tungsten to the indicated thicknesses. The coated particles were cleaned as above. The results of the testing of these samples by the method of Example 1, are listed in Table III.

0.8 grams of 200 micron PCD crushed aggregate feed with 4 micron thick tungsten coatings were loaded in the Ta-cup. A cobalt cemented tungsten carbide (6 percent by weight cobalt) backing disc was fit snugly on top of the diamond layer.

HPHT treatment for 10 minutes at 1450.degree. C. and 55 kbs yielded a tungsten carbide supported cemented/sintered compact which was finished as described in Example 1.

Comparison to the conventional backed cutters tested in Table I showed somewhat higher thermal stability and comparable wear resistance. Structural degradation began at about 800.degree. C. in each case. An interface layer crack in the compact at the tungsten carbide/diamond interface (delamination) occurred at about 825.degree. C.

0.8 grams of crushed PCD aggregate feed of 250 microns were coated with 5 microns of tungsten and were loaded in the tantalum cup. A pure silicon disc 0.010 inches thick was placed on top of the diamond layer, followed by a cobalt cemented tungsten carbide disc. HPHT treatment as in Example 18 was applied.

Test results showed that the cemented/sintered compact was thermally stable up to approximately 1,100.degree. C., at which point the diamond layer began to show cracks, but there was no obvious interface layer delamination.

2.4 grams of 200 micron PCD aggregate starting material coated with 4 microns of tungsten was loaded in the tantalum cup. A silicon disc 0.010 inches thick, was placed on top of the diamond layer and processed as above. A free-standing cemented compact was obtained. A comparison with Table II indicates that this cutting element has higher wear resistance and thermal stability than existing commercial products such as GE's Geoset or DeBeers, Syndax-3.

The cutting elements and backed compacts according to the present invention are either superior or comparable to their prior art counterparts. Free-standing cutting elements of the present invention utilizing silicon as a binding aid are thermally stable up to 1,200.degree. C. with no signs of major structural degradation or noticeable graphitization. See examples 9 and 11-17. This is comparable to the thermal stability of conventional PCD's such as Geoset and Syndax-3. Backed compacts according to the present invention with a cemented tungsten carbide backing and a silicon infiltrant are thermally stable up to 1,000.degree. C. A trimodal backed compact utilizing fine diamond powder showed thermal stability up to 1,100.degree. C. The use of a compliant layer without silicon infiltrant demonstrated thermal stability up to 1,000.degree. C. See Examples 1-7. These compacts show superior thermal stability compared to the prior art backed PCD's such as GE 2541 and Syndrill, which become thermally degraded between 750.degree. C.-850.degree. C., as shown in Table I. Cutting elements and backed compacts comprising coated fine grained sintered aggregates show higher wear resistance and comparable or superior thermal stability as compared to conventional PCD's. See Table III. In addition, due to the presence of the matrix, the present invention is more resistant to crack propagation than conventional PCD's. The cemented metal/metal carbide support structure also decreased the hardness and modulus of elasticity of the compact, as compared to conventional PCD's, improving its impact resistance. The brittleness of prior art compacts utilizing silicon is also decreased by the metal matrix.