Superhard cutting structures for earth boring with enhanced stiffness and heat transfer capabilities

Abstract: A cutting element for a rotary drill bit for subterranean drilling, including a substantially planar table of superhard material (12) having a cutting face (16) and a cutting edge (18). The table (12) may be reinforced against bending with one or more strut portions (20) extending from the rear of the table to the rear of the substrate (14) and at least partially across the cutting element. The strut portions (20) may be integral with the diamond table (12), or separately fabricated and of another material. The strut portions, in addition to or in lieu of providing stiffness to the superhard table, may also serve to enhance heat transfer away from the cutting face and cutting edge of the superhard table. The structure supporting the cutting element on the drill bit and the body of the drill bit itself may be additionally configured to include structures to facilitate heat transfer away from the superhard table.

Description: The invention relates to a cutting element for a rotary drill bit for drilling subterranean formations comprising a substantially planar, substantially circular table of superhard material, presenting a substantially planar cutting surface having a cutting edge, a single strut member extending rearwardly from said table and at least partially across the rear of the table and being of the same material as the table and integral therewith, and a substrate supporting said table from the rear, the strut member extending into said substrate. Superhard materials, normally diamond, have been employed in cutting elements for rotary drill bits for decades. For about the past twenty years there has been widespread use of synthetic diamond cutters, specifically in the form of polycrystalline diamond compacts. Polycrystalline diamond compact cutting elements, commonly known as PDC's, have been commercially available for over 20 years. PDC's may be self-supporting or may comprise a substantially planar diamond table bonded during formation to a supporting substrate. A diamond table/substrate cutting element structure is formed by stacking into a cell layers of fine diamond crystals (100 microns or less) and metal catalyst powder, alternating with wafer-like metal substrates of cemented tungsten carbide or other suitable materials. In some cases, the catalyst material may be incorporated in the substrate in addition to or in lieu of using a powder catalyst intermixed with the diamond crystals. A loaded receptacle is subsequently placed in an ultra-high temperature (typically 1450-1600°C) ultrahigh pressure (typically 50-70 kilobar) diamond press, wherein the diamond crystals, stimulated by the catalytic effect of the metal power, 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 catalysis. A so-called thermally stable PDC product (commonly termed as 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°C) without degradation in comparison to normal PDC's, which experience thermal degradation upon exposure to temperatures of about 750-800°C. While PDC and TSP cutting elements employed in rotary drag bits for earth boring have achieved major advances in obtainable rate of penetration while drilling and in greatly expanding the types of formations suitable for drilling with diamond bits at economically viable cost, the diamond table/substrate configurations of state of the art planar cutting elements leave something to be desired. First, bending attributable to the loading of the cutting element by the formation may cause fracture or even delamination of the diamond table from the substrate. It is believed that such degradation of the cutting element is due at least in part to lack of sufficient stiffness of the cutting element so that, when encountering the formation, the diamond table actually flexes due to lack of sufficient rigidity or stiffness. As diamond has an extremely low strain to failure (diamond cannot tolerate large values of absolute strain), only a small amount of flex can initiate fracture. In addition, fracture may also be initiated in the highly stressed carbide substrate when cutting loads are applied to the cutting element. The carbide is stressed in tension during cooling after the previously-described fabrication process, due to the difference in coefficients of thermal expansion between the diamond and the substrate material. A second limitation of PDC's is due to excessive buildup of heat due to frictional forces generated during the cutting process. While the superhard material of the cutting element table has an extremely high thermal conductivity (on the order of 400 to over 600 watts/meter Kelvin) and the substrate has a relatively high thermal conductivity (on the order of 100 watts/meter Kelvin), the bit body, typically steel or WC matrix, has a far lower thermal conductivity (on the order of 30 watts/meter Kelvin). As the cutting element wears and the point of contact with the formation becomes an ever-wider wear flat, the cutting element is subjected to higher cutting energies and the substrate becomes ever-smaller, limiting and actually reducing the potential rate of heat transfer. The heat buildup causes overheating of the cutting element and accelerated wear of the diamond table and supporting substrate. In "dull" or used bits, such excessive heating is often manifested on the WC substrate behind the diamond table by the phenomenon of "heat checking", which comprises vertically running fractures in a checkerboard pattern. It has been proposed to enhance the stiffness of superhard cutting elements by providing the superhard table with a linearly-extending portion of enhanced thickness. Such a configuration provides additional stiffness for the cutting structure, and also beneficially increases compressive stresses in the superhard material table while lowering tensile stresses in the supporting substrate. It has been proposed to promote heat transfer from a PDC element to the underlying bit structure in U.S. Patent 4,478,297. This patent proposes to use a hollow cylindrical stud with a recess extending into about the middle of the stud from the bottom thereof, the recess being filled with a soft, heat-conducting metal to facilitate heat transfer from the PDC at the upper or outer end of the stud. The cutting element of the generic kind is shown in EP 0 322 214 A1. This print describes a tool insert comprising an abrasive compact bonded to a cemented carbide support. The cutting edge of the tool is provided by the periphery of the compact. A plurality of circular concentric recesses or a single spiral recess are/is each filled with the abrasive compact material that extends to the cemented carbide support from the compact/carbide interface. Because of the method by which the tool insert is made the abrasive compact-forming material is a one-piece body enclosed in the cemented support or substrate. It is the object of the invention to enhance both cutting element stiffness and heat transfer capabilities significantly. This object is achieved with the cutting element of the generic kind in that the strut member is diametrically disposed and elongated, includes a relatively wide base, has a web to which the base tapers, and has a tip of the web that extends to or does not extend completely to the rear of the substrate. Preferably the strut member comprises arcuate strut side surfaces extending from the rear of the table providing a broad smooth surface for the substrate to support. It is convenient that the strut member includes a laterally enlarged tip after narrowing from the enlarged base portion to an intermediate web portion. The strut member may include at least one cavity in the web thereof. At least one cavity may comprise an aperture extending through the strut member. Advantageously the strut member extends laterally from one side to the other of the table. According to the invention, the strut member reinforces the superhard table against cutting loads which would otherwise give rise to bending stresses in the table as it curves or bends under the loads in the manner of a cantilever beam. The superhard table, strut member and supporting substrate, are cooperatively configured to place the superhard material in compression and to minimize the tensile stresses in the substrate. The invention is further described referring to the drawing, in which: FIGS. 1A and 1B are respective top and side elevations of a first variation of a first preferred cutting element embodiment of the invention, FIG. 1B having a portion of the substrate material removed; FIG. 2 is a top elevation of another modification of the variation of FIGS. 1A and 1B; FIG. 3 is a perspective view of the rear of the superhard material table of yet another structural modification of the variation of FIGS. 1A and 1B; FIG. 4 is a top elevation of an intermediate product from which the first variation of the first preferred embodiment may be cut; FIG. 5 is a top elevation of an unbacked version of the first variation of the first preferred embodiment; FIGS. 1A and 1B of the drawings depict cutting element 10 including a substantially planar, circular table 12 of superhard material of, for example, PDC, TSP, diamond film or other suitable superhard material such as cubic boron nitride. Table 12 is backed by a supporting substrate 14 of, for example, cemented WC, although other materials have been known and used in the art. Table 12 presents a substantially planar cutting surface 16 having a cutting edge 18, the term "substantially planar" including and encompassing not only a perfectly flat surface or table but also concave, convex, ridged, waved or other surfaces or tables which define a two-dimensional cutting surface surmounted by a cutting edge. Integral elongated strut portion 20 of superhard material project rearwardly from table 12 to provide enhanced stiffness to table 12 against loads applied at cutting edge 18 substantially normal to the plane of cutting surface 16, the resulting maximum tensile bending stresses lying substantially in the same plane as cutting surface 16. In this variation of the invention, elongated strut portion 20 is configured as a single, diametrically-placed strut. Cutting element 10 is rotationally oriented about its axis 22 on the drill bit on which it is mounted so that elongated strut portion 20 placed directly under the anticipated cutting loads. The strut thus serves to stiffen the superhard table against flexure and thereby reduces the damaging tensile portion of the bending stresses. The orientation of the plane of the strut portion 20 may be substantially perpendicular to the profile of the bit face, or at any other suitable orientation dictated by the location and direction of anticipated loading on the cutting edge 18 of the cutting element 10. As shown in FIG. 1A, strut portion 20 includes a relatively wide base 24 from which it protrudes rearwardly from table 12, tapering to a web 25 terminating at a thin tip 26 at the rear 28 of substrate 14. Optionally, tip 26 may be foreshortened and so not extend completely to the rear 28 of substrate 14. Arcuate strut side surfaces 30 extending from the rear 32 of table 12 reduce the tendency of the diamond table/strut junction to crack under load, and provide a broad, smooth surface for substrate 14 to support. Upon cooling of cutting element 10 after fabrication, the differences in coefficient of thermal expansion between the material of substrate 14 and the superhard material of table 12 and strut portion 20 results in relative shrinkage of the substrate material, placing the superhard material in beneficial compression and lowering potentially harmful tensile stresses in the substrate 14. As shown in FIG. 4, cutting element 10 may be formed with a one-piece substrate blank 14' for the sake of convenience when loading the blanks and polycrystalline material into a cell prior to the high-temperature and high pressure fabrication process. The rear area 34 of bank 14' may then be removed by means known in the art, such as electro-discharge machining (EDM) to achieve the structure of cutting element 10, with elongated strut portion 20 terminating at the rear 28 of substrate 14. Alternatively, as noted above, rear area 34 may remain in place, covering the tip 26 of strut portion 20. FIG. 2 depicts an alternative cutting element configuration 110, wherein the strut portion 120 extending from superhard table 12 includes a laterally-enlarged tip 126 after narrowing from an enlarged base portion 124 to an intermediate web portion 125. This configuration, by providing enlarged tip 126, may be analogized to an I-beam in its resistance to bending stresses. From the side, cutting element 110 would be indistinguishable from cutting element 10. FIG. 3 depicts a cutting element 10 from a rear perspective with substrate 14 stripped away to reveal transverse cavities or even apertures 36 extending through web 25 of strut portion 20. Cavities or apertures 26 enhance bonding between the superhard material and the substrate material, and further enhance the compression of the superhard material as the cutting element 10 cools after fabrication. FIG. 5 depicts a diamond table 12 and strut portion 20 configuration similar to that of FIGS. 1A and 1B, forming cutting element 210. Cutting element 210 may comprise a PDC or preferably a TSP which is furnaced or otherwise directly secured to a bit face or supporting structure thereon, without the use of a substrate 14. It may be preferred to coat cutting element 210, and specifically the rear 32 of diamond table as well as the side surfaces of base 24 and web 25 with a single- or multi-layer metal coating in accordance with the teachings of U.S. Patent 5,030,276 or U.S. Patent 5,049,164, each of which is hereby incorporated herein by this reference, to facilitate a chemical bond between the diamond material and the WC matrix of the drill bit or between the diamond material and a carrier structure secured to the drill bit. At this point, it should be noted that the structures depicted in FIGS. 1-5 in addition to enhancing stiffness of the superhard table, also promote heat transfer away from the table 12 and specifically cutting edge 18. Superhard materials, such as PDC's and TSP's are excellent heat conductors, and far superior to the cemented carbide of substrate 14. Thus, strut portions 20 provide a conduit for heat transfer away from cutting face 16 and cutting edge 18 which avoids the limitations imposed by substrate 14. As previously noted, heat transfer problems become more serious as the table 12 and substrate 14 wear and more frictional heat is generated, while at the same time the cutting element's heat transfer capabilities are reduced. Strut portion 20 may also act as a conduit for excess heat from table 12 to another, separate heat transfer structure such as is later disclosed herein. Further, the presence of strut portion 20 permits heat transfer from the top and rear of the strut portion to the borehole environment with a suitable mounting structure for the cutting element on the bit face. The strut portion also acts as a conduit for heat transfer to the bit body, which acts as a heat sink and which may be more easily cooled with the flow of drilling fluid therethrough.

Claims: Cutting element (10) for a rotary drill bit for drilling subterranean formations comprising a substantially planar, substantially circular table (12) of superhard material, presenting a substantially planar cutting surface (16) having a cutting edge (18), a single strut member (20) extending rearwardly from said table (12) and at least partially across the rear (32) of the table (12) and being of the same material as the table (12) and integral therewith, and a substrate (14) supporting said table (12) from the rear (32), the strut member (20) extending into said substrate (14) characterized in that the strut member (20) is diametrically disposed and elongated includes a relatively wide base (24, 124) has a web (25, 125) to which the base (24, 124) tapers, and has a tip (26, 126) of the web (25, 125) that extends to or does not extend completely to the rear (28) of the substrate (14). Cutting element (10) according to claim 1, characterized in that the strut member (20) comprises arcuate strut side surfaces (30) extending from the rear (32) of the table (12) providing a broad smooth surface for the substrate (14) to support. Cutting element (10) according to claim 1 or 2, characterized in that the strut member (120) includes a laterally enlarged tip (126) after narrowing from the enlarged base portion (124) to an intermediate web portion (125). Cutting element (10) according to one of claims 1 to 3, characterized in that the strut member (20) includes at least one cavity in the web (25) thereof. Cutting element (10) according to claim 4, characterized in that at least one cavity comprises an aperture (36) extending through the strut member (20). Cutting element (10) according to claim 1, characterized in that the strut member (20) extends laterally from one side to the other of the table (12).

Inventors: COOLEY CRAIG H, HANSEN WAYNE R, TIBBITTS GORDON A, COOLEY, CRAIG H., HANSEN, WAYNE R., TIBBITTS, GORDON A.
Applicants: BAKER HUGHES INC, BAKER HUGHES INCORPORATED
Application year: 1995
Main classification: E21B10
IPC codes: E21B10