1.  Field of the Invention
The present invention relates generally to superhard cutting elements, and more specifically to substantially planar polycrystalline diamond compact cutting elements comprising a polycrystalline diamond table formed and bonded to a supporting substrate during formation of the cutting element. 
2State of the Art
Polycrystalline diamond compact cutting elements, commonly known as PDC's, have been commercially available for over twenty years.  PDC's may be self-supporting, or may comprise a substantially planar diamond table bonded during formation to a supporting substrate.  The diamond table/substrate cutting element structure is formed by stacking in a receptacle layers of fine diamond crystals (100 microns or less) and metal catalyst powder, alternating with wafer-like metal substrates of cemented carbide.  In some cases, the catalyst o material may be incorporated in the substrate in addition to or in lieu of using a powdered catalyst intermixed with the diamond crystals.  The loaded receptacle is subsequently placed in an ultrahigh temperature (typically 1450.degree.-1600.degree.  C.), ultrahigh pressure (typically 50-70 kilobar) diamond press wherein the diamond crystals, stimulated by the catalytic effect of the metal powder, bond to each other and to the substrate material.  The spaces in the diamond table between the diamond to diamond bonds are filled with residual metal catalyst.  A so-called thermally stable PDC product (commonly termed a TSP) may be formed by leaching out the metal in the diamond table.  Alternatively, silicon, which possesses a coefficient of thermal expansion similar to that of diamond, may be used to bond diamond particles to produce a Si-bonded TSP.  TSP's are capable of enduring higher temperatures (on the order of 1200.degree.  C.) without degradation, in comparison to normal PDC's, which experience thermal degradation upon exposure to temperatures of about 750.degree.-800.degree.  C.
While PDC and TSP cutting elements employed in rotary drag bits for earth boring have achieved major advances in obtainable rate of penetration and in greatly expanding the types of formations suitable for drilling with diamond bits at economically viable cost, the diamond table/substrate configuration of state of the art planar cutting elements leave something to be desired. 
First, the interface of the diamond table with the substrate (typically tungsten carbide, or WC) is subject to high residual shear stresses arising from formation of the cutting element, as after cooling the differing coefficients of thermal expansion of the diamond and substrate material result in thermally-induced stresses.  In addition, finite element analysis (FEA) has demonstrated that high tensile stresses exist in a localized region in the outer cylindrical substrate surface and internally in the WC substrate.  Both of these phenomena are deleterious to the life of the cutting element during drilling operations, as the stresses, when augmented by stresses attributable to the loading of the cutting element by the formation, may cause spalling, fracture or even delamination of the diamond table from the substrate. 
In addition to the foregoing shortcomings, state of the art PDC's often lack sufficient diamond volume to cut highly abrasive formations, as the thickness of the diamond table is limited due to the inability of a relatively thick diamond table to adequately bond to the substrate. 
Finally, the benefits of a multi-thickness diamond table, which produces a kerfing action during drilling as the thickness portions, have been recognized.  However, all such prior art PDC configurations (see, for example, U.S. Pat.  Nos.  4,784,023 and 5,120,327) employ parallel linear interleaved ridges of diamond and substrate extending across the cutting element.  Such a configuration, which is purported by the patentee to alleviate the diamond table/substrate interface stresses, may actually by its asymmetrical nature, aggravate rather than alleviate and undesirably concentrate such stresses, as well as substrate outer surface stresses.  In fact, the embodiment of the '327 patent, wherein a circumferential ring of diamond is formed around the substrate ridges to resist crack formation and propagation in the substrate, is a tacit admission of the basic parallel ridge structure's inability to remedy the basic interface stress problem. 
Another PDC cutting element structure which affords a multiple-depth diamond table is disclosed in European Patent Specification Publication No.  0 322 214 B1.  This structure's substrate ridges resemble a "bull's eye" pattern in one embodiment, and a spiral pattern in another.  While allegedly providing curved cutting ridges as the cutting element wears, wear of such ridges causes the primary contact points between the cutting element and the formation to migrate rapidly laterally, so that a deep kerf or cleft in the formation at a substantially constant radial location is never effected.