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.