The present invention relates to polycrystalline diamond and cubic boron nitride (CBN) compacts made under high pressure/high temperature (HP/HT) processing conditions, and more particularly to HP/HT polycrystalline composite compacts having reduced stresses inherent in a supported compact structure.
A compact may be characterized generally as an integrally-bonded structure formed of a sintered, polycrystalline mass of abrasive particles, such as diamond or CBN. For many applications, however, it is preferred that the compact is supported by its bonding to a substrate material to form a laminate or composite compact arrangement. Typically, the substrate material is provided as a cemented metal carbide which comprises, for example, tungsten, titanium, or tantalum carbide particles, or a mixture thereof, which are bonded together with a binder of about 6% to about 25% by weight of a metal such as cobalt, nickel, or iron, or a mixture or alloy thereof. As is shown, for example, in U.S. Pat. Nos. 3,381,428; 3,852,078; and 3,876,751, compacts and composite compacts have found acceptance in a variety of applications as parts or blanks for cutting and dressing tools, as drill bits, and as wear parts or surfaces.
The basic method for manufacturing the polycrystalline compacts and composite compacts of the type described herein involves the placing of an unsintered mass of abrasive, crystalline particles, such as diamond or CBN, or a mixture thereof, within a protectively shielded metal enclosure which is disposed within the reaction cell of a HP/HT apparatus of a type described further in U.S. Pat. Nos. 2,947,611; 2,941,241; 2,941,248; 3,609,818; 3,767,371; 4,289,503; 4,673,414; and 4,954,139, the disclosures of which are expressly incorporated herein by reference. Additionally placed in the enclosure with the abrasive particles may be a metal catalyst if the sintering of diamond particles is contemplated, as well as a pre-formed mass of a cemented metal carbide for supporting the abrasive particles and thereby form a composite compact therewith. The contents of the cell then are subjected to processing conditions selected as sufficient to effect intercrystalline bonding between adjacent grains of the abrasive particles and, optimally, the joining of the sintered particles to the cemented metal carbide support. Such processing conditions generally involve the imposition for about 3 to 120 minutes of a temperature of at least 1300.degree. C. and a pressure of at least 50 kbar.
As to the sintering of polycrystalline diamond compacts or composite compacts, the catalyst metal may be provided in a pre-consolidated form disposed adjacent the crystal particles. For example, the metal catalyst may be configured as an annulus into which is received a cylinder of abrasive crystal particles, or as a disc which is disposed above or below the crystalline mass. Alternatively, the metal catalyst, or solvent as it is also known, may be provided in a powdered form and intermixed with the abrasive crystalline particles, or as a cemented metal carbide or carbide molding powder which may be cold pressed in to shape and wherein the cementing agent is provided as a catalyst or solvent for diamond recrystallization or growth. Typically, the metal catalyst or solvent is selected from cobalt, iron, or nickel, or an alloy or mixture thereof, but other metals such as ruthenium, rhodium, palladium, chromium, manganese, tantalum, and alloys and mixtures thereof also may be employed.
Under the specified HP/HT conditions, the metal catalyst, in whatever form provided, is caused to diffusely advance or "sweep" through the dense diamond crystalline mass, and thereby is made available as a catalyst or solvent for recrystallization or crystal intergrowth. The HP/HT conditions, which operate in the diamond stable thermodynamic region above the equilibrium between diamond and graphite phases, effect a compaction of the abrasive crystal particles which is characterized by intercrystalline diamond-to-diamond bonding wherein parts of each crystalline lattice are shared between adjacent crystal grains. Preferably, the diamond concentration in the compact or in the abrasive table of the composite compact is at least about 70% by volume. Methods for making diamond compacts and composite compacts are more fully described in U.S. Pat. Nos. 3,141,746; 3,745,623; 3,609,818; 3,850,591; 4,394,170; 4,403,015; 4,797,326; and 4,954,139, the disclosures of which are expressly incorporated herein by reference.
As to polycrystalline CBN compacts and composite compacts, such compacts and composite compacts are manufactured in general accordance with the methods suitable for diamond compacts. However, in the formation of a CBN compacts via the previously described "sweep-through" method, the metal which is swept through the crystalline mass need not necessarily be a catalyst or solvent for CBN recrystallization. Accordingly, a polycrystalline mass of CBN may be joined to a cobalt-cemented tungsten carbide substrate by the sweep through of the cobalt from the substrate and into the interstices of the crystalline mass notwithstanding that cobalt is not a catalyst or solvent for the recrystallization of CBN. Rather, the interstitial cobalt functions as a binder between the polycrystalline CBN compact and the cemented tungsten carbide substrate.
As it was for diamond, the HP/HT sintering process for CBN is effected under conditions in which CBN is the thermodynamically stable phase. It is speculated that under these conditions, intercrystalline bonding between adjacent crystal grains also is effected. The CBN concentration in the compact or in the abrasive table of the composite compact, again, is preferably at least about 70% by volume. Methods for making CBN compacts and composite compacts are more fully described in U.S. Pat. Nos. 2,947,617; 3,136,615; 3,233,988; 3,743,489; 3,745,623; 3,767,371; 3,831,428; 3,918,219; 4,188,194; 4,289,503; 4,334,928; 4,673,414; 4,797,326; and 4,954,139, the disclosures of which are expressly incorporated herein by reference.
Yet another form of a polycrystalline compact, which form need not necessarily exhibit intercrystalline bonding, involves a polycrystalline mass of diamond or CBN particles having a second phase of a metal or alloy, a ceramic, or a mixture thereof. The second material phase is seen to function as a bonding agent for the abrasive crystal particles. Polycrystalline diamond and polycrystalline CBN compacts containing a second phase of a cemented carbide material are exemplary of these so-called "conjoint" polycrystalline abrasive compacts.
With respect to composite compacts, it is speculated, as is detailed in U.S. Pat. No. 4,797,326, that the bonding of the support to the polycrystalline abrasive mass involves a physical component in addition to a chemical component which develops at the bond line if the materials forming the respective layers are interactive. The physical component of bonding is seen to develop from the relatively lower CTE (coefficient of thermal expansion) of the polycrystalline abrasive layer as compared to the cemented metal support layer. That is, upon the cooling of the composite compact blank from the HP/HT processing conditions to ambient conditions, it has been observed that the support layer retains residual tensile stresses which, in turn, exert a radial compressive loading on the polycrystalline compact supported thereon. This loading maintains the polycrystalline compact in compression, which compression assists in securing the compact to the support and improves fracture toughness, impact, and shear strength properties of the composite.
In the commercial production of supported compacts, however, it is common for the product or blank which is recovered from the reaction cell of the HP/HT apparatus to be subjected to a variety or finishing operations which include cutting, such as by electrode discharge machining or with lasers, milling, and especially grinding to remove any adherent shield metal from the outer surfaces of the compact. Such finishing operations additionally are employed to machine the compact into a cylindrical shape or the like which meets product specifications as to diamond or CBN abrasive table thickness and/or carbide support thickness. Especially with respect to diamond and CBN composite compacts, a substantially uniform abrasive layer thickness is desirable since the abrasive tables on the blanks are often machined by the user into final products having somewhat elaborate configurations, e.g., saw-toothed wedges, which are tailored to fit particular applications. It will be appreciated, however, that during such finishing operations, the temperature of the blank, which previously has been exposed to a serve thermal cycle during its HP/HT processing and cooling to room temperature, can be elevated due to the thermal effects of grinding or cutting operations. Moreover, the blank or product finished therefrom may be mounted onto a variety of cutting or drilling tools using braze techniques which again subjects the compacts and supports to thermal gradients and stresses. During each of the thermal cycles of the composite blank, the carbide support, owing to its relatively higher coefficient of thermal expansion (CTE), will have expanded to a greater extent than the abrasive compact supported thereon. Upon cooling, the residual stresses generated during heating are relieved principally through the deformation of the abrasive material which results in stress cracking and nonuniformities in the thickness of the abrasive table layer.
As the composite compacts heretofore known in the art have garnered wide acceptance for use in cutting and dressing tools, drill bits, and the like, it will be appreciated that any improvements in the strength and machining properties of such materials would be well-received by industry. Especially desired would be diamond and CBN composite compacts having improved fracture toughness, impact, and shear strengths which would expand the applications for such material by enhancing their machinability, performance, and wear properties. Thus, there has been and heretofore has remained a need for diamond and CBN composite compacts having improved physical properties.