[1] This invention is related in general to the field of polycrystalline diamond elements, which are used in drill bits and other ground engaging products.

BACKGROUND OF THE INVENTION

[2] To improve performance of cutting elements on earth boring tools, such as drill bits, one or more wear or working surfaces of the cutting elements are made from a layer of polycrystalline diamond (PCD) in the form of a polycrystalline diamond compact (PDC) that is attached to a substrate. This layer of PCD is often also called a "diamond table" or a "diamond crown." A common substrate for the PCD layer is cemented tungsten carbide. PDC, though very hard with high abrasion or wear resistance, tends to be relatively brittle. The substrate, typically cobalt bonded tungsten carbide, while not as hard, is tougher than the PDC, and thus has higher impact resistance. The composite structure of a hard table bonded to the tough substrate is suitable for drilling and other downhole applications.

[3] Cubic boron nitride (CBN) is, for many wear applications, a suitable substitute for PCD, and references to polycrystalline diamond, or PCD, and polycrystalline diamond compacts, or PDC, are intended to refer also to CBN and compacts made from CBN unless otherwise indicated.

[4] A polycrystalline diamond compact is made by mixing polycrystalline diamond grains, in powder form, which is referred to as "diamond grit," with one or more powdered metal catalysts and other materials, forming the mixture into a compact, and then sintering it using high heat and pressure. A metal catalyst promotes formation of diamond-to-diamond bonds between adjacent grains of diamond. Although cobalt or an alloy of cobalt is the most common catalyst, other Group VIII metals, such as nickel, iron and alloys thereof can be used as a catalyst. The sintering process produces a table or body of bonded diamond crystals, which has been described as a continuous, contiguous or integral matrix or lattice of diamond having interstitial voids or spaces between the diamond grains. The interstitial voids are at least partly filled with the metal catalyst.

15] For a cutter or other wear element, particularly those used in downhoie tools, with a cemented metal carbide substrate, PDC is typically formed by packing diamond grit adjacent a substrate material of metal carbide such as tungsten carbide and metal catalyst in a mold. The PDC materials are then sintered by applying high temperature and high pressure to the moid. During sintering the metal catalyst in the substrate material typically cobalt or a cobalt alloy sweeps into and infiltrates the diamond grit. The metal catalyst promotes bonds between the diamond grains while also cementing the resulting PDC to the substrate in a single step.

[6] The composite of the PDC and the substrate can be fabricated in a number of different ways. The composite can include transitional layers in which the metal carbide and diamond are mixed with other elements for improving bonding and reducing stress between the PDC and substrate. A reference to a substrate of metal carbide is intended, unless otherwise specifically stated, to include substrates with transitional layers.

[7] Because of the presence of catalyst metal within the diamond matrix, sintered PCD exhibits thermal instability. The metal catalyst will have a larger coefficient of expansion than the diamond. During use of the cutter, the thermal expansion of the catalyst contained in the interstitial spaces of the rigid diamond structure can initiate cracking in the structure compromising the structural integrity of the table. The residual metal catalyst can also cause diamond crystals within the PDC to begin to graphitize, which can weaken the PCD structure.

[8] To make the PDC more thermally stable, a substantial percentage, usually more than 50%, though often 70% to 85% and possibly more of the catalyst, is removed from at least a region of the diamond structure next to one or more working surfaces that experience the highest temperatures due to friction.

[9] The metal catalyst is removed by a leaching process. The leaching process involves placing the PDC in a strong acid, examples of which include nitric acid, hydrofluoric acid, hydrochloric acid, or perchloric acid, and combinations of them. In some cases, the acid mix may be heated and/or agitated to accelerate the leaching process. In any event, the leaching process converts the catalyst metal, such as cobalt, into a soluble salt that can then be removed from the diamond table via an aqueous media, i.e., the acid.

[10] Removal of the cobalt can reduce toughness of the PDC, thus decreasing its impact resistance. Leaching can also result in removal of some of the cobalt near the table/substrate interface that cements or binds the table to the substrate, thus affecting the strength or integrity of the substrate and/or the interface of the substrate and diamond interface. To optimize the structural integrity of the cutter, catalyst is removed only from the PDC to a certain depth or distance measured from a working surface or working surfaces of the PDC, The working surfaces of a cutting element of a drill bit, for example, are the surfaces of the cutter designed or intended for engaging the rock formation, in the case of a PDC cutter, they are typically the top surface of the diamond crown or table, at least part of its side surface, and, if present, a beveled edge, radiused or shaped transition between the top and side surfaces. Generally, only the top, working surface of the PDC is exposed to the leach bath. A mask and seal can be used to protect the substrate from the acid. The depth of the leaching depends on the microstructure of the diamond material, the leaching solution being used, and the leaching time.

[11] The acids used for leaching can be highly toxic and the leaching process can require many hours to as long as weeks to leach the cutter. The leaching process requires careful handling of material by trained operators and complex equipment that adds to the cost of producing the cutter significantly. SUMMARY

[12] Sintered diamond compacts produced by high pressure, high temperature processing generally incorporate residual catalytic materials between grains that can compromise the structural integrity of the compact under operational conditions. Removing or replacing the catalyst materials with more compatible materials can extend the operational temperature range of the diamond compacts and significantly reduce the processing costs. The present invention uses a metal solvent to dissolve solid catalyst in the diamond compact. A solvent metal can be used that is environmentally benign with low toxicity.

[13] In one aspect of the present invention, a consolidated diamond compact with interstitial spaces comprises a first portion of the compact with a catalyst metal in the interstitial spaces and a second portion of the compact with a solvent metal in the interstitial spaces.

[14] In an alternative aspect of the invention, a cutter for use in a ground engaging tool comprises a substrate bonded to a diamond table with interstitial spaces where at least a portion of the interstitial spaces are occupied by a solvent metal.

[ 5] In an alternative aspect of the invention, a method for treating a sintered diamond comprises heating a solvent metal and infusing at least a portion of the sintered diamond with the solvent metal,

[16] In some embodiments of the invention, the residual catalyst in the diamond is cobalt and the cobalt is dissolved by solvent metal such as gallium, tin and/or other metals in the interstitial spaces. In some embodiments the solvent metal is heated to between 600°C and 750°C, and preferably over 650°C. The solvent metal may be heated in a vacuum or an inert gas atmosphere. Where heating is completed in a vacuum or in an inert gas atmosphere, the solvent metal can be heated to a temperature as high as 1500°C before exposing the diamond compact in or to the solvent metal. In some embodiments the diamond compact and/or the cutter are treated with a material such as indium, gallium, tin, galinstan or other element or alloy to enhance the solvation process.

[17] In some embodiments the cobalt is removed or the volume is reduced in the interstitial spaces. In some embodiments the cobalt is at least partially replaced with solvent metal so that in a processed portion of the diamond there is less catalyst by weight than solvent. In some embodiments the cobalt is displaced or replaced at least in part with solvent metal and in a processed portion of the diamond the catalyst occupies less volume than solvent. In some embodiments the cobalt is replaced with solvent metal and in a processed portion of the diamond the catalyst occupies less than 25% of the volume of catalyst and solvent.

[18] Metal catalyst remaining within a sintered polycrystalline diamond (PCD) structure after sintering is removed from at least a portion of the structure, for example near one or more of its working surfaces, by removing and replacing it with a solvent metal that melts at a lower temperature than the metal catalyst and preferably lower than the operating temperature of the end product. In some embodiments the solvent is one or more metals selected from the group of gallium, indium and tin.

[19] Such a PCD structure, with metal catalyst near working surfaces of the PCD structure at least partially replaced with a solvent metai, has improved thermal stability. In its liquid phase the solvent metal will not impart an expansive force or pressure to the matrix of sintered PCD, even though the coefficient of thermal expansion of the solvent metal is different than the sintered PCD, thereby avoiding stress within the PCD structure than can lead to fracturing. Gallium, tin and indium are immiscible with carbon and do not contribute to graphitization of the diamond.

[20] A representative example of a process for removing the metal catalyst within a PCD structure and replacing it with a solvent metal having a lower melting temperature comprises soivating or dissolving the metal catalyst with a solvent metal having a Sower melting temperature than the metal catalyst, and allowing the metal catalyst, once dissolved into the metal solvent, to diffuse into the bath and the solvent metal to diffuse into the diamond table, thereby resulting in the removal of the catalyst from the PCD structure and its replacement with the solvent metal. Examples of metals with lower melting temperatures for dissolving or soivating metal catalysts such as cobalt and cobalt alloys include gallium, indium, tin, rubidium, sodium, thallium, lead, cadmium, bismuth, polonium, potassium, mercury, and alloys of them.

[21] Representative examples of a wear structures comprising sintered PCD comprising at least a region adjacent a surface of the PCD containing a metal with a lower melting temperature than the catalyst used to sinter the PCD include a cutter for drill bit or reamer comprising a tungsten carbide substrate.

[22] Acids used for leaching catalyst are not metal based and typically incorporate fluorine, chlorine, sulfur or nitrogen. These acids have no residual metallic component that remains in the interstitial spaces on dissolving catalyst. For the purposes of this disclosure displacing catalyst will mean removal of catalyst and diffusion away from the interstitial spaces. For the purposes of this disclosure replacing catalyst will mean removal of catalyst and diffusion away W from the interstitial spaces and in-migration of solvent molecules in place of the catalyst in the interstitial spaces.

BRIEF DESCRIPTION OF THE DRAWINGS

[23] Figure 1 is a perspective view of a PDC drag bit.

[24] Figure 2A is a perspective view of a PDC cutter.

[25] Figure 2B is a side view of a PDC cutter.

[26] Figure 2C is an end view of a PDC cutter.

[27] Figure 3 is a cross-section view of a portion of a PCD with catalyst in the interstitial spaces.

[28] Figure 4 is a representation of the dissolution of catalyst in the PCD of

Fig. 3.

[29] Figure 5 is a cross section through a portion of the PCD of Fig. 3 with catalyst and solvent in the interstitial spaces.

[30] Figure 6 is a cross-section of a cutter with catalyst and solvent.

[31] Figure 6A is a detail of the cutter of Fig. 6.

[32] Figure 7 is a Cobalt-Tin phase diagram.

[33] Figure 8 is a chart showing depth of penetration of solvent metal in a cutter table.

[34] Figure 9 is a chart showing depth of penetration of solvent metal in a cutter table.

[35] Figure 10 is a chart showing depth of penetration of solvent metal in a cutter table. [36] Figure 11 is a chart showing depth of penetration of solvent metal in a cutter table.

[37] Figure 12 is a cross-section of a cutter in a solvent bath.

[38] Figure 13 is a flow chart of process for removing the metal catalyst from a region of a PCD structure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[39] Diamond is a common industrial material used in many applications such as abrasives and for hard surfacing tools. Diamond consolidated into cutters are used in many different applications such as downhole bits for advancing boreholes, percussive bits, wear members and other cutting and wear applications found, for example, in mining. These aggressive applications in abrasive environments provide a short service life for the cutters and millions of cutters have been manufactured using processes that include removing catalyst from the diamond. The terms catalyst, catalyst metal or metal catalyst in this application refer to a metal that functions as a catalyst in the formation of a consolidated diamond compact, such as a diamond table in a cutter for a drill bit, unless otherwise specified.

[40] Fig. 1 illustrates an example of a PDC drag bit. The bit is intended to be rotated around its central axis 102, it is comprised of a bit body 104 connected to a shank 106 having a tapered threaded coupling 08 for connecting the bit to a drill string and a "bit breaker" surface 1 1 for cooperating with a tool to tighten and loosen the coupling 108 to the drill string. The exterior surface of the body intended to face generally in the direction of boring is referred to as the face of the bit. The face generally lies in a plane perpendicuiar to the centra! axis 102 of the bit. The body is not limited to any particular materia!. It can, for example, be made of steel or a matrix material such as powdered tungsten carbide cemented by metal binder. Disposed on the body along the bit face and sides are a plurality of raised blades, each designated 110, that rise from the body of the bit. On each blade is mounted a plurality of discrete cutting elements, or cutters 112. Each discrete cutting element is disposed within a recess or pocket. Similar reference numbering is used in the following figures for like elements.

[41] Figs 2A, 2B and 2C illustrate a representative example of a PDC cutter suitable for drill bit, such as the PDC bit of Fig 1. Representative cutter 200 is comprised of a substrate 202, to which is attached a layer of sintered polycrystalltne diamond (PCD) 204, also called a diamond table. Cutter 200 is not drawn to scale and is intended to be representative of cutters generally that have a polycrystaliine diamond structure attached to a substrate, and in particular the one or more of the PDC cutters 112 on the drill bit 00 of Fig. 1. In this example, an edge between top surface 206 and side surface 208 of the layer of PCD 204 is beveled to form a beveled edge 210. The top surface and the beveled surface are, in this example, each a working surface for contacting and cutting through formation. A portion of the side surface, particularly nearer the top surface 206, may also contact the formation or debris.

[42] A wear insert comprising a sintered PCD structure is fabricated by subjecting raw materials in a mold to extremely high temperatures and pressures. Residual metal catalyst occupying the interstices between bonded diamond grains is then displaced or at least partially replaced with a metal or alloy to make it more thermally stable. A typical catalyst such as cobalt has a linear coefficient of expansion of 13X10 ^{"6 m} / _m -K and diamond has a coefficient of expansion on the order of 1x10 ^~6 "Vm-K- Cobalt has a melting temperature of about 1500°C and remains solid through typical operating conditions that can reach 700°C or more in a drilling operation. The solid cobalt in the interstitial spaces of the table expands as the table reaches operational temperatures. The cobalt is confined in the interstitial spaces of the diamond matrix. These interstitial spaces do not expand in a corresponding manner to the cobalt The expanding cobalt can initiate fracture of the diamond and damage to the matrix of the diamond table.

[43] A liquid displacing the catalyst however can flow out of the interstitial spaces when it expands rather than applying significant stress to the diamond matrix. A material with a low melting point can be effective in replacing the catalyst and reducing stress in the diamond during operation. The terms "low melting point material," "lower melting temperature metal" or "metal with lower melting temperature" will refer to both metals and metal alloys, capable of acting as a solvent to the catalyst and having a substantially lower melting temperature as compared to the catalyst, and preferably has a lower melting temperature than the anticipated operating temperature.

[44] Examples of such metals with lower melting temperatures that can act as a solvent for the metal catalyst in a sintered PCD structure include gallium, titanium, molybdenum, indium, iron, tin, zirconium, alloys of each, and/or other transition metal with strong affinity to carbon in order to facilitate wetting of the diamond, Other examples of solvent metals include sodium, potassium, mercury, and alloys of each of them. Alloying any of these metals with titanium, molybdenum, iron, zirconium, and/or other transition metai with strong affinity to carbon will tend to reduce capillary resistance and enhance capillary action. Reduced capillary resistance allows the solvent to infuse into the interstitial areas more freely and increases contact with the catalyst materials when infusing the diamond.

[45J Some of these solvents can also be considered catalysts. For example, gallium (and other metals) can act as a catalyst in the diamond table. In one sense, then, the present inventive process involves replacing a first metal used as a catalyst to form a diamond table with a second metal in the diamond table, which itself may function as a catalyst within the diamond table, fn this application, the terms solvent, solvent metal or metal solvent refers to a nonaqueous metai media that dissolves the catalyst used in the formation of the consolidated diamond compact unless otherwise specified.

[46] Dissolution of solid cobalt by liquid solvent metal is governed by Ftck's Law. Fick's first law determines the rate of dissolution and migration or flux of a first material into a second solvent material. The flux moves material from regions of high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient, or that a solute will move from a region of high concentration to a region of low concentration across a concentration gradient. In one (spatial) dimension, the law is: where J is the "diffusion flux" or the (amount of substance) per unit area per unit time. J measures the amount of the first material that will flow through a small area during a small time interval. D is the diffusion coefficient or diffusivity in dimensions of length ² time ^-1 , Φ (for ideal mixtures) is the concentration in dimensions of amount of first material per unit volume and x is the position [length]. D is proportional to the squared velocity of the diffusing particles, which depends on the temperature, viscosity of the fluid and the size of the particles according to the Stokes-Einstetn relation. In dilute aqueous solutions the diffusion coefficients of most ions are similar and have values that at room temperature are in the range of 0.6x10 ^{" 9} to 2x10 ^" m ² /s.

[47] S. P. Yatsenko et ai in a 2008 paper (Journal of Physics: Conference Series 98 062032 doi:10.1088/1742-6596/98/6/062032) examined the corrosion or dissolution rates of several metals in gallium as a function of temperature. They determined that cobalt dissolves in gallium at a rate of 2.3 miiligrams/cm ² - hr at 673K. The dissolution rate is largely governed by temperature. The dissolution rate can be limited by the rate of migration of dissolved cobalt away from the cobalt gallium interface, and the actual area of the gallium catalyst interface with the solvent metal in the interstitial spaces.

[48] Fig. 3 is a representation of a portion of a table 300 of consolidated diamond grains 302 with catalyst 304 in the interstitial spaces between the grains before contact with the solvent 306. During the high temperature, high pressure consolidation process that forms the diamond table, the liquid catalyst promotes the formation of bonds at diamond grain interfaces. Interstitial areas remain where grains do not make contact and catalyst remains in these spaces between grains as a solid.

[49] Fig. 4 is a representation of a portion of the consolidated diamond table 300 being at least partially immersed in solvent 306 with the catalyst 304 present between the diamond grains as a solid. Solvent 306 is shown infusing between the diamond grains to contact the solid catalyst. The catalyst dissolves in the heated solvent at a rate determined by Fick's law and the catalyst molecules 304' migrate away from the solvent catalyst interface toward the bulk solvent with a lower catalyst concentration.

[50] Fig. 5 is a representation of the portion of the consolidated diamond table of Fig. 4 immersed in solvent 306 where the process has progressed to produce differentiated regions of the diamond table. In a first region 300A, the catalyst largely remains in the interstitial regions between diamond grains. In a second region 300B, the catalyst has dissolved and been replaced at least in part in the interstitial spaces by solvent. In a preferred embodiment the catalyst is cobalt and the solvent is gallium, tin or other solvent metal. Alternatively, other catalysts and solvents can be used that function in a similar manner and will still fail within the scope of this disclosure. Although Fig. 5 shows the upper portion of second region 300B at the level of the solvent metal bath, the catalyst can be replaced above the level of the bath as well.

[51] While the regions are shown as rectangular, the regions can be any shape. For example, rather than the front face as illustrated, the solvent region may be restricted to the work area at the circumference of the front face where the cutter would contact a borehole during operation in advancing a borehole. The boundaries between regions that contain solvent metal and catalyst metal may also not be as sharply or precisely delineated as indicated. The boundaries of the region in which the metal catalyst is at least partially replaced by the lower melting temperature solvent metal, as indicated in the drawing, is schematic in nature and intended to be representative. Multiple regions are possible, each with the same or a different geometry, and different patterns or arrangements of such regions are possible.

[52] The region of displaced cobalt may extend from the surface to a predetermined depth. Fig. 6 shows a cutter 200 with a substrate 202 and a table 204 that has been exposed to a solvent. Fig. 6A is a detail of the po!ycrystaliine diamond table 204 with a region of undepleted catalyst 300A and a region of depleted catalyst 300B. The region of depleted catalyst can extend to virtually any depth. In one example, the depth of the depleted catalyst region may be 100 microns. The region of depleted catalyst can extend from the surface to a depth of about 200 microns, in a preferred embodiment the region of depleted catalyst extends to a depth of 200-250 microns. A deeper catalyst depleted region is also possible. As discussed above, the catalyst depleted region can include complete or partial removal of the catalyst.

[53] This solvation process is different than acid leaching. For example, cobalt metal is consumed slowly in dilute sulphuric acid to form solutions containing the aquated Co(ll) ion together with hydrogen gas, H ₂ . In practice, the Co(ll) is present as the complex ion [Co(OH ₂ )e] ^2÷

Co(s) + H ₂ S0 ₄ (aq)→ Co ²⁺ (aq) + S0 ₄ ^2" (aq) + H ₂ (g) [54] Solvents can also be classified as polar or nonpolar. Generally, the dielectric constant of the solvent provides a rough measure of a solvent's polarity. The strong polarity of water is indicated, at 0°C, by a dielectric constant of 88. Sulfuric acid commonly used as a leaching agent for cobalt in diamond has a dielectric constant of 100 and nitric acid has a value of 50. Solvents with a dielectric constant of less than 15 are generally considered to be nonpolar. Polarity is a function of the bonding of elements to form a molecule so that they have electronegative portions of the molecule. Elements such as tin, indium or gallium are nonpolar and may be assigned a dielectric constant of 1.

[55] When a liquid contacts a solid at an interface, some portion of the solid may dissolve into the liquid. Depending on the chemistry of the solid and the liquid a certain portion of the solid will dissolve into the liquid until it reaches a saturation point described in terms of mole percentage or other terms. The saturation point is dependent on temperature. Typically a higher temperature raises the saturation point or the solubility limit. Fig. 7 shows a phase diagram for the solubility of cobalt and tin. At 250°C tin is a liquid, but the solubility of cobalt in the tin is very low. At 700°C the solubility of cobalt in tin is about 4% and for every 96 tin atoms, 4 cobalt atoms will dissolve in the tin.

[56] The Cobalt-Tin phase diagram of Fig. 7 is used as an example. Other metal solvent and catalyst combinations will have similar phase diagrams that can be used to determine solubility. In a preferred embodiment the catalyst metal has at least 2% solubility in the solvent metal when heated above 600°C, but materials with higher or lower solubility levels at various temperatures may be used to displace catalyst in the diamond. Solvents disclosed here may also be considered non-aqueous solvents. Solvents disclosed here may also be considered inorganic solvents. In some embodiments the solvent chemically combines with the catalyst to form an intermediate alloy and/or intermetaliic compound.

(57] Several solvation metais were applied to the tables of cutters to remove cobalt. In each case after immersion and processing in the solvent bath the diamond table was fractured to provide a section face for analysis. Energy- Dispersive X-ray Spectroscopy (EDS) was used to determine the composition of the material along a line 310 extending from the face of the working section of the diamond table inward to a portion of the table unaffected by the solvent metal (see Fig. 6A). The EDS uses a stream of electrons accelerated toward the target material to impact the constituent atoms. The impacting electrons cause the electrons of the atoms to jump to a higher level of the atomic structure. When the electron decays back to the original level it emits an xray with an energy characteristic of the element. The data presented here is a count of xrays with the energy characteristic of the targeted element. The count then correlates to the number of atoms of the constituent material. Variations of the size of the elemental atoms and characteristics of the atomic shells require a normalizing factor to provide a specific concentration. The count data without normalization presented here indicates the depths of depletion of the catalyst and in-migration of the solvent metal.

[58] The surface of the section face along the analyzed line consists of diamond grains alternating with interstitial spaces. This causes the data for a specific constituent material to be noisy or 'jumpy.' In order to see the general trends of the composition a smoothing function has been applied to the data.

[593 ^{m one} example, a bath of gallium was heated to a temperature of 600°C. The diamond table of a cutter with cobalt catalyst in interstitial spaces was placed in the bath for a period of 72 hours. The results of the EDS line analysis are displayed in Fig. 8. The gallium has penetrated to a depth of approximately 40 microns and the concentration of cobalt is reduced at similar depths.

[60] In another example, a bath of indium was heated to a temperature of 700°C. The diamond table of a cutter with cobalt catalyst in interstitial spaces was placed in the bath for a period of 24 hours. The results of the EDS line analysis are displayed in Fig. 9. The indium has penetrated to a depth of approximately 50 microns and the concentration of cobalt is reduced at similar depths.

[61] In another example, a bath of tin was heated to a temperature of 700°C. The diamond table of a cutter with cobalt catalyst in interstitial spaces was placed in the bath for a period of 24 hours. The results of the EDS line analysis are displayed in Fig. 10. The tin has penetrated to a depth of approximately 80 microns and the concentration of cobalt is suppressed to a similar depth.

[62] In another example, a bath of an alloy of gallium, indium and tin (galinstan) was heated to a temperature of 700°C. The diamond table of a cutter with cobalt catalyst in interstitial spaces was placed in the bath for a period of 24 hours. The results of the EDS line analysis are displayed in Fig. 11. The gaiinstan has penetrated to a depth of approximately 60 microns and the concentration of cobalt is suppressed to about the same depth.

[63J Alternatively, the solvation process can be performed in a vacuum or an inert gas environment that suppresses the conversion of diamond to graphite. The solvent can be heated to a higher temperature in such an environment with limited graphitization of the diamond table. At the higher temperature the dissolution of the catalyst by the solvent will be accelerated according to Fick's law and a larger portion of the table can be treated in the same amount of time. Alternatively, a similar portion of the table can be treated in a shorter period of time. In one example, the gallium is heated to a temperature between 1000°C and 1500°C in a vacuum and the face of the cutter table is infused with the liquid gallium.

[64] The materials of the substrate can experience damage by contact with the solvent metal. The solvent metal can dissolve constituents of the substrate which can result in a loss of structural integrity. To protect the substrate, a protective coating may be applied to the substrate that limits contact of the solvent metal with the substrate material. Fig. 12 illustrates a cutter 200 with a substrate 202 and a diamond table 204 similar to the cutter of Fig. 2. The cutter includes a protective layer 352 that separates the substrate from any contact with the solvent metal. The protective layer can be a metal sputter coated on the substrate or an electroplated metal, Other methods can be used to deposit a protective layer. Alternatively, the protective layer can be a high temperature plastic, ceramic or other material compatible with the solvation temperature and materials. [65] infusing solvent metal into the diamond structure may not require immersion of the diamond. Fig. 12 shows a porous body or sponge material 350 supporting the cutter. The porous body 350 is immersed in the solvent metal 306 and the table 204 of the cutter is on the top surface of the porous body above the level of the solvent metal. The conductive body is porous and easily wet by the solvent metal so that the metal is wicked up through the porous body by capillary action to contact the table. This serves to further limit contact between the substrate and the solvent metal. The porous body 350 and the protective layer 352 are not necessarily used together. Either of the porous body or the applied layer may provide sufficient protection by themselves for the substrate and one or the other can be used.

[66] Referring again to Figure 1 , each blade of the bit extends generally in a radial direction, outwardly to the periphery of the cutting face. In this example, there are six blades substantially equally spaced around the central axis and each blade, in this embodiment, sweeps or curves backwardly in relation to the direction of rotation indicated by arrow 115.

[67] tn a drag bit, the cutters are placed along the forward (in the direction of intended rotation) side of the blades, with their working surfaces facing generally in the forward direction for shearing the earth formation when the bit is rotated about its central axis. The cutters are shown arrayed along blades to form a structure cutting or gouging the formation and the resulting debris is flushed by the drilling fluid which exits the drill bit through the nozzles 117. The drilling fluid transports the debris or cuttings uphole to the surface. [68] In this example of a drag bit, ail of the cutters 112 are PDC cutters. However, in other embodiments, not ail of the cutters need to be PDC cutters. The PDC cutters in this example have a working surface made primarily of super hard, poiycrystalline diamond, or the like, supported by a substrate that forms a mounting stud for placement in a pocket formed in the blade. Each of the PDC cutters is fabricated discretely and then mounted by brazing, press fitting, or otherwise into pockets formed on the bit. However, the PDC layer and substrate are typically used in the cylindrical form in which they are made. This example of a drill bit includes gauge pads 1 14. In some applications, the gauge pads of drill bits such as bit 100 can include an insert of thermally stable, sintered poiycrystalline diamond (TSP). These TSP elements can also be treated by processes in accordance with the present invention.

[69] Although frequently cylindrical in shape, PDC cutters are not limited to a particular shape, size, or geometry, or to a single layer of PCD. Not all of the cutters on a bit must be of the same size, configuration, or shape. In addition to being sintered with different sizes and shapes, PDC cutters can be cut, ground, or milled to change their shapes. Furthermore, a cutter could be formed of multiple discrete PCD structures. Other examples of possible cutter shapes might include pre-flatted gauge cutters, pointed or scribe cutters, chisel-shaped cutters, and dome inserts.

[70] Figure 13 illustrates the basic steps of a representative example of a process 400 for fabricating a wear insert comprising a sintered PCD structure by replacing metal catalyst occupying the interstices between the bonded grains diamond grains with a metal or alloy to make it more thermally stable. The process 400 includes two basic steps or processes: first, the process of forming the sintered PCD structure; and, second, a separate process for replacing metal catalyst remaining in at least a portion of the PCD structure after sintering with a low melting temperature metal. The two processes can be performed separately. The second process comprises dissolving and/or diffusing the metal catalyst from at least one region of the PCD structure using a solvent comprising a metal, in a preferred embodiment the solvent has a lower melting temperature than the catalyst. The second process, through solvation and diffusion, removes at least a majority of the metal catalyst from the at least one region of the PCD and replaces it with the lower melting temperature metal. This second process takes place by immersing or placing in contact at least part of the PCD structure in or with a bath containing liquid metal solvent or otherwise exposing the PCD structure to the liquid metal solvent to allow for solvation and diffusion of the metai catalyst into the solvent metal, and diffusion of the solvent metal into the PCD structure. The bath is preferably heated to a temperature above 625°C, but the temperature could be varied depending on several factors including the particular solvent used, the desired depth of replacing the catalyst, etc.

[71] Step 402 represents a process of sintering polycrystaliine diamond particles using a metal catalyst. One example of a sintering process comprises first forming a compact of small or fine grains of synthetic or natural diamond. These grains are referred to within the industry as diamond grit or powder. The compact may include other materials and structures. The grains of PCD in alternative embodiments can be layered within the compact according to grain size. For example, a layer next to a working layer may be comprised of finer grains (i.e. grains smaller than a predetermined grain size) and a layer further away, perhaps a base layer next to the substrate, with grains larger than the predetermined size. However, any number of arrangements and geometries of diamond grit according to particle size are possible. Such geometries may be formed, for example, to create regions with predetermined geometries that may, relatively, speed up or slow down the metal catalyst being dissolved and/or diffused out of the sintered PCD structure due to different densities of diamond. By altering the rate of dissolution and/or diffusion within a selected parts of the sintered diamond structure, it may be possible to engineer regions, in which metal catalyst has been at least partially replaced with lower melting temperature metal, to have more complex geometries.

72J Unless otherwise specifically indicated, the processes described herein are not limited to any particularly geometry or arrangements of diamond grit, to any particular size of diamond grit, or to a particular percentage of diamond grit within the compact. The formed compact can be sintered under high pressure and high temperature (HPHT) in the presence of a catalyst, such as cobalt, a cobalt alloy, or any group VIII metal or alloy. The process of subjecting the compact to HPHT is sometimes referred to as a "pressing." The catalyst may, instead of being mixed with the diamond grit, be infiltrated into the compact by forming the compact on a substrate of tungsten carbide that is cemented with the catalyst, and then heating and pressing the two together. The catalyst may also be mixed with the diamond grit. A compact made from grains of CBN can be formed in a similar manner. [73] If a PCD structure previously sintered using a metal catalyst is available, and has metal catalyst remaining in at least one region that is adjacent a surface of the PCD structure, process 400 can start at any one of steps 404, 406 or 408. Steps 404 and 406 are optional. A representative example of such a PCD structure includes, without limitation, a PDC cutter comprising a sintered PDC wear surface attached to a substrate, such as by the process performed during step 402 or another process. However, the structure need not have a substrate.

[74] Step 404 includes coating the sides of at least the substrate with a meta! such as tungsten, tantalum, gallium oxide, a mixture of tungsten and tantalum, or another material that resists going into solution with the metal that will be used as a solvent. The step contemplates a PCD structure attached to a cemented carbide substrate. An example of such a substrate is cobalt cemented tungsten carbide. The step may reduce the risk of the solvent metal accidentally dissolving some of the metal in the substrate. Gallium, for example, can quickly damage cemented metal carbide by dissolving the metal matrix.

[75] Tungsten offers an advantage as a coating in applications in which the wear part will be brazed to a structure, such as drill bit, as tungsten is generally wet by most braze alloys. The metal coating can extend at least part way down the sides of the PCD structure. Furthermore, all but one surface of the PCD structure that are exposed to the solvent could, if desired, be coated.

[76] Step 406 involves coating one or more of the surfaces of the PCD structure that will be exposed to the solvent with a pre-wetting material. A coating with a solvent metal such as indium, tin, or gaiinstan can be applied on to the surface or surfaces of the PCD structure that will be exposed to the solvent upon immersion in the solvent bath to facilitate wetting of the diamond table.

[77] At step 408 at least a portion of the PCD structure is submerged into a bath containing a solvent comprised of a solvent metal. Alternatively as discussed above the PCD structure can be put in contact with a structure immersed in the solvent, in either case, the PCD structure is exposed to the solvent. The PCD structure is, in one embodiment, suspended or held so that only a predetermined portion of the PCD is exposed to the bath. For example, this portion could be one or more selected surfaces, or portions of those one or more surfaces, of the PCD structure. The bath containing the solvent metai is heated to, and maintained for a period of time at a temperature above the liquidus of the solvent metal and below a temperature at which there is a substantial risk of graphitization occurring within the PCD structure. The temperature at which graphitization within the PCD begins to occur is generally 750°C. The graphitization temperature can be higher than this in a vacuum or in inert gas environments. Diamond heated in a vacuum can reach 1600°C with limited graphitization.

[78] Depending on the solvent metal, heating beyond the point at which it melts may improve wetting of the diamond structure helping to ensure better capillary action improving dissolution and diffusion. Capillary action helps to maintain the contact between the solvent metai and the catalyst metal. Higher temperatures can also increase rates of dissolution and diffusion.

[79] In one representative embodiment, the temperature of the bath is between the melting temperature of the solvent metal and 750°C. In another representative embodiment, the temperature of the bath is between 600°C and 723°C. An example of a solvent metal for this embodiment is an alloy of gallium, indium and tin sometimes referred to as gaiinstan. The bath may be heated to temperature before placing of the PCD structure in the bath or brought up to temperature after the solvent metal has melted and the PCD structure placed in the bath. During step 410 the solvent flows into the exposed PCD structure and contacts the solid catalyst. The catalyst dissolves in the metal solvent then diffuses out of the structure. Once the PCD structure is removed from the bath at step 412, the bath can be cooled to cause the metal catalyst dissolved in the bath to solidify. The solid phase of the catalyst can be separated mechanically or filtered from the solvent (gaiinstan, for example, with a melting temperature just above room temperature), while still in its liquid phase, thus allowing the solvent (such as gaiinstan) to be reused. Other methods to separate the solvent and catalyst are known to those skilled in the art.

[80] The solvent metal dissolves the solid cobalt and the dissolved cobalt diffuses into the metal solvent and out of the diamond structure. The solvent metai can at least partially replace the cobalt in the interstitial spaces. Gallium and other metal solvents wet the PCD structure more readily than a leaching acid. A positive capillary action pulls the solvent into the PCD structure.

[81] Though the invention is described in terms of cutters used in drag bits, this is for the purpose of illustration, and the PCD structures can be used in a range of other applications such as wear members for excavation, picks for underground wall mining and material processing operations. PDC drag bit 100 is intended to be a representative example of a downhole tool in genera!, and more specifically of earth boring tools, drill bits for drilling oil and gas wells, and PDC drag bits.

[82] The foregoing description is of exemplary and preferred embodiments. The invention, as defined by the appended claims, is not limited to the described embodiments. Alterations and modifications to the disclosed embodiments may be made without departing from the invention. The meaning of the terms used in this specification are, unless expressly stated otherwise, intended to have ordinary and customary meaning and are not intended to be limited to the details of the illustrated or described structures or embodiments.