A polycrystalline diamond compact comprising a diamond table is formed in a high-pressure, high-temperature process using a catalyst, the catalyst being substantially removed from the entirety of the diamond table, and the diamond table attached to a supporting substrate in a subsequent high-pressure, high-temperature process using a binder material differing at least in part from a material of the catalyst. The binder material is permitted to penetrate substantially completely throughout the diamond table from an interface with the substrate to and including a cutting surface, and the binder material is selectively removed from a region or regions of the diamond table by a conventional technique (e.g., acid leaching). Cutting elements so formed and drill bits equipped with such cutting elements are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/145,155, filed Jan. 16, 2009, the disclosure of which application is incorporated herein in its entirety by reference. 
    
    
     TECHNICAL FIELD 
     Embodiments of the invention relate to methods of forming polycrystalline diamond cutting elements having at least a portion of a diamond table substantially free of catalytic material, and to cutting elements so formed. 
     BACKGROUND 
     Superabrasive cutting elements in the form of Polycrystalline Diamond Compact (PDC) structures have been commercially available for almost four decades, and PDC cutting elements having a polycrystalline diamond table formed on the end of a supporting substrate for a period in excess of twenty years. The latter type of PDC cutting elements commonly comprises a thin, substantially circular disc (although other configurations are available), commonly termed a “table,” including a layer of superabrasive material formed of diamond crystals mutually bonded under ultrahigh temperatures and pressures and defining a substantially planar front cutting face, a rear face and a peripheral or circumferential edge, at least a portion of which is employed as a cutting edge to cut the subterranean formation being drilled by a drill bit on which the PDC cutting element is mounted. PDC cutting elements are generally bonded over their rear face during formation of the superabrasive table to a backing layer or substrate formed of tungsten carbide, although self-supporting PDC cutting elements are also known, particularly those stable at higher temperatures, which are known as Thermally Stable Polycrystalline Diamond, or “TSPs.” Such cutting elements are widely used on rotary fixed cutter, or “drag,” bits, as well as on other bits and tools used to drill and ream subterranean formations, such other bits and tools including without limitation core bits, bi-center bits, eccentric bits, hybrid (e.g., rolling components in combination with fixed cutting elements), roller cone bits, reamer wings, expandable reamers, and casing milling tools. As used herein, the term “drill bit” encompasses all of the foregoing, and equivalent structures. 
     In the formation of either type of cutting element, a catalyst is usually employed to stimulate diamond-to-diamond bonding of the diamond crystals. Unfortunately, the presence of a catalyst in the diamond table may lead to thermal degradation commencing at about 400° C. due to differences in the coefficients of thermal expansion (CTEs) of the diamond and the catalyst, and commencing around 700° C.-750° C. due to stimulation of back-graphitization of the diamond to carbon by the catalyst. Such temperatures may be reached by the cutting edge of a PDC cutting element during drilling of a formation, despite the use of drilling fluid as a cooling agent and despite relatively rapid heat transfer into the diamond table, the substrate and the body of the drill bit on which the cutting element is mounted. 
     It has been recognized in the art that removal of the catalyst used in the original synthesis manufacturing of the diamond table from the cutting surface of the diamond table, particularly at the cutting edge thereof and along the side of the diamond table proximate the cutting edge and extending toward the substrate, reduces the tendency of those portions of the diamond table to degrade due to thermal effects. Consequently, provided the depth of removal of the catalyst is sufficient, the life of the diamond table is extended. The recognition of the aforementioned thermal degradation effects and how and from what portion of the diamond table the catalyst may be beneficially removed is disclosed in, among many other documents, Japanese Patent JP59-219500, as well as in U.S. Pat. Nos. 4,224,380, 5,127,923, 6,544,308 and 6,601,662, U.S. Patent Publications Nos. 2006/0060390, 2006/0060391, 2006/0060392, 2006/0086540, 2008/0223623, 2009/0152018 and PCT International Publication Nos. WO 2004/106003, WO 2004/106004 and WO 2005/110648. The disclosure of each of the foregoing documents is hereby incorporated herein in its entirety by this reference. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the present invention relate to methods of forming polycrystalline diamond elements, such as cutting elements suitable for subterranean drilling, exhibiting enhanced thermal stability, and resulting cutting elements. 
     In one embodiment of the invention, a polycrystalline diamond compact comprising a diamond table is formed in a high-pressure, high-temperature process using a catalyst, and the catalyst is then substantially removed from the entirety of the diamond table. The diamond table is then attached to a supporting substrate in a subsequent high-pressure, high-temperature process using a binder material differing at least in part from a material of the catalyst. The subsequent high-temperature, high-pressure process may be conducted at a pressure comparable to that used to form the diamond table, or may conducted at a higher pressure or a lower pressure. Different temperatures may also be employed, respectively, to form the diamond table and during attachment of the diamond table to a supporting substrate. 
     In one specific embodiment, the binder material is permitted to penetrate substantially completely throughout the diamond table from an interface with the substrate to a cutting surface and side of the diamond table, and the binder material is selectively removed from a desired region or regions of the diamond table by a conventional technique. 
     Cutting elements formed and exhibiting structures according to embodiments of the methods of the present invention are also disclosed, and encompassed within the scope of the invention. 
     Drill bits employing cutting elements formed and exhibiting structures according to embodiments of the present invention are also disclosed and encompassed within the scope of the invention. 
     Other features and advantages of the present invention will become apparent to those of ordinary skill in the art through consideration of the ensuing description, the accompanying drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a flowchart of an embodiment of a method to form a polycrystalline diamond compact cutting element according to the present invention; 
         FIGS. 2A-2D  depict the formation of a polycrystalline diamond compact cutting element according to the embodiment of  FIG. 1 ; and 
         FIG. 3  depicts one example of a rotary drag bit having cutting elements according to an embodiment of the present invention mounted thereto. 
     
    
    
     DETAILED DESCRIPTION 
     Process flow of an embodiment of a method of the present invention is illustrated in  FIG. 1 , and the associated structures formed during the process are illustrated in  FIGS. 2A-2D . Referring to the foregoing drawing figures, in act  100 , a polycrystalline diamond compact  200  ( FIG. 2A ) in the form of diamond table  202  is formed from a mass of diamond particles (e.g., grit) in the presence of a catalyst  204  in a high-pressure, high-temperature process. As used herein, the terms diamond “particles” or diamond “grit” each include not only individual particles of diamond, but aggregates of individual diamond particles having diamond-to-diamond bonds therebetween. The diamond table  202  may be formed on a supporting substrate  206  (as shown) of cemented tungsten carbide or other suitable material as known in the art in a conventional process of the type described, by way of non-limiting example, in U.S. Pat. No. 3,745,623 or may be formed as a freestanding polycrystalline diamond compact (e.g., without supporting substrate) in a similar conventional process as described, by way of non-limiting example, in U.S. Pat. No. 5,127,923. The diamond grit may comprise natural diamond, synthetic diamond, or a mixture, and may comprise diamond grit of different sizes, or diamond grit in layers or other specific regions of different grain sizes or different average grain sizes, and the diamond table or one or more regions thereof may comprise a gradient of different grain sizes. The catalyst  204  may be supplied in a supporting substrate  206 , if employed, or may be admixed with the diamond grit. The supporting substrate  206 , which is to be removed as described below, may be thin, on the order of a few millimeters, to permit simultaneous fabrication of relatively more diamond tables  202  in a given diamond press cell volume. In act  102 , the supporting substrate  206  (if present) is removed from diamond table  202  by leaching the material of the supporting substrate  206  from the diamond table  202  while simultaneously substantially removing the catalyst  204  from the diamond table  202 . Specifically, as known in the art and described more fully in the aforementioned U.S. Pat. No. 5,127,923 and in U.S. Pat. No. 4,224,380, aqua regia (a mixture of concentrated nitric acid (HNO 3 ) and concentrated hydrochloric acid (HCl)) may be used to dissolve at least a portion of the supporting substrate  206  (if present), to substantially remove the catalyst  204  from interstitial voids between the diamond crystals of the diamond table  202  and from the crystal surfaces, and to dissolve catalytic binder material at an interface between the supporting substrate  206  and the diamond table  202  resulting in separation therebetween. It is also known to use boiling hydrochloric acid (HCl) and boiling hydrofluoric acid (HF), as well as mixtures of HF and HNO 3  in various ratios. Other techniques for catalyst removal are also known in the art. 
     In additional embodiments, the supporting substrate  206  may be removed from the diamond table  202  prior to removing catalyst  204  from interstitial voids between the diamond crystals of the diamond table  202 , or the supporting substrate  206  may be removed from the diamond table  202  after removing catalyst  204  from interstitial voids between the diamond crystals of the diamond table  202 . Furthermore, methods other than acid leaching may be used to remove the supporting substrate  206  from the diamond table  204 . Such methods may include, for example, one or more of grinding, cutting, and laser ablation. 
     The resulting structure ( FIG. 2B ) is diamond table  202 ′ with substantially no catalyst  204  present. As used herein, a diamond table or polycrystalline diamond compact having “substantially no catalyst” therein, or being “substantially free of catalyst” does not require complete removal of catalyst, as there may be some residual catalyst on the surfaces of diamond grit particles, as well as in some substantially closed voids between particles wherein the leaching agent has not penetrated fully. In act  104 , another supporting substrate  208  is placed adjacent diamond table  202 ′ and secured thereto in another conventional high-temperature, high-pressure process in the presence of a binder material differing at least in part from a material of the catalyst  204 . Supporting substrate  208  may comprise a cemented tungsten carbide or other suitable material as known to those of ordinary skill in the art. As depicted in  FIG. 2C , binder material  210  may be present at the commencement of act  104  in (for example) powder form or in the form of a thin disc  210   a  in a layer disposed between diamond table  202 ′ and supporting substrate  208 , as an integral portion  210   b  of the material of supporting substrate  208 , or both. At the conclusion of act  104 , polycrystalline diamond compact  200 ′ having diamond table  202 ″ including binder material  210  therein results due to migration of the binder material  210  from the source or sources thereof into interstitial voids between the diamond crystals in the polycrystalline diamond compact  200 ′ that were vacated upon removal of the catalyst  204  therefrom in act  102 . 
     As noted above, the another conventional high-temperature, high-pressure process conducted in the presence of a binder material  210  may be at a temperature and pressure comparable to that used to form the diamond table  202  or may be at a lower pressure and temperature. For example, the diamond table  202  may be formed at a pressure of at least about 5 GPa and a temperature of about 1500° C., while the another high-temperature, high-pressure process may be conducted at a substantially different, higher pressure, such as in the range of about 6 GPa to about 7 GPa, or even as much as about 8 GPa or more, and at a temperature in the range of about 1650° C. to about 2200° C. Conversely, the pressure used to form the diamond table  202 ′ may be in the range of about 6 GPa to about 7 GPa, or even about 8 GPa or more, and the temperature may be in the range of about 1650° C. to about 2200° C., and the another high-temperature, high-pressure process conducted in the presence of a binder material may be conducted at a substantially different, lower pressure, for example at least about 5 GPa, and at a temperature of about 1500° C. to stay within the diamond stable region and prevent back-graphitization of the diamond table  202 ′ during act  104 . Such back-graphitization tendencies of the diamond table  202 ′ may be of particular concern in light of catalytic properties of the binder employed. In each of the foregoing examples, only pressure may be varied while temperatures employed to respectively form diamond table  202  and attach diamond table  202 ′ to supporting substrate  208  may be substantially the same. Conversely, temperatures may also be varied in the two respective acts  100  and  104 . Furthermore, the times at temperature and pressure for each of the processes may vary in a range extending from about twenty seconds to about twenty minutes or more. 
     In the first example set forth in the above paragraph, the diamond table  202  may be formed at a relatively lower temperature and pressure to produce a diamond-to-diamond bonded structure of lesser density and greater porosity to facilitate removal of catalyst  204  using an acid leaching or other conventional, invasive process. Subsequently, attachment of diamond table  202 ′ to supporting substrate  208  may be conducted at a significantly higher (e.g., by about an additional ten percent or more) pressure and temperature to enhance the density and strength of the resulting diamond table  202 ″. In the second example set forth in the above paragraph, the relatively higher pressure and temperature used to form diamond table  202  will provide a diamond structure of high density and strength, while the relatively lower pressure and temperature used to attach diamond table  202 ′ to supporting substrate  208  will not compromise the density and strength of the resulting diamond table  202 ″ while reducing cycle time for addition of binder material  210  and attachment of substrate  208 . 
     In a further act  106 , a region or regions  212   a ,  212   b  of the diamond table  202 ″ (being, respectively and by way of non-limiting example, a region adjacent a cutting face and a region adjacent a side surface  214  of diamond table  202 ″) have the binder material  210  substantially and selectively removed therefrom while precluding contact with the supporting substrate  208  and, by way of non-limiting example, a portion of the side surface  214  of diamond table  202 ″ with a leaching agent. Of course, the binder material  210  may be removed from diamond table  202 ″ to any substantial extent, or depth, desired. Suitable depths may range from, by way of non-limiting example, about 0.04 mm to about 0.5 mm. Any of the abovementioned leaching agents may be employed, and one particularly suitable leaching agent is hydrochloric acid (HCl) at a temperature of above 110° C. for a period of about three to about 60 hours, depending upon the depth of desired removal of the binder material  210  from a surface of diamond table  202 ″ exposed to the leaching agent, as depicted in  FIG. 2D . Contact with the leaching agent may be precluded, as known in the art, by encasing substrate  208  and a portion of the diamond table  202 ″ in a plastic resin, by coating substrate  208  and a portion of the diamond table  202 ″ with a masking material, or by the use of an “O” ring-type seal resistant to the leaching agent, compressed against the side surface  214  of diamond table  202 ″ using a plastic fixture. The resulting polycrystalline diamond compact  200 ″ offers enhanced thermal stability and consequently improved wear resistance, during use due to the removal of binder material  210  from at least the region or regions  212   a ,  212   b  of diamond table  202 ″. The presence of binder material in another region or regions of the diamond table  202 ″ may enhance durability and impact strength thereof. The inventor herein has noted, surprisingly and contrary to conventional thought in the industry, that the strength of the resulting diamond table having a binder introduced therein after the initial removal of catalyst therefrom, is substantially the same as that of a diamond table having catalyst therein used to form the diamond table, for diamond tables of equal diamond density. 
     By way of non-limiting example, materials suitable for use as catalysts and binder materials in implementation of embodiments of the invention include Group VIII elements and alloys thereof, such as Co, Ni, Fe and alloys thereof. Thus, in one implementation, Co may be used as a catalyst in formation of a polycrystalline diamond compact, which is then leached of the catalyst and the supporting substrate removed. Ni may then be used as a binder material to attach the resulting leached diamond table to another supporting substrate. In another implementation, an Fe alloy is used as a catalyst in formation of a polycrystalline diamond compact, which is then leached of the catalyst and the supporting substrate removed. Co may then be used as a binder material to attach the resulting leached diamond table to another supporting substrate. In another implementation, Co may be used as a catalyst in formation of a polycrystalline diamond compact, which is then leached of the catalyst and the supporting substrate removed. A Co/Ni alloy may then be used as a binder material to attach the resulting leached diamond table to another supporting substrate. In a variation of the foregoing implementation, Co may be used as a catalyst in formation of a polycrystalline diamond compact, which is then leached of the catalyst and the supporting substrate removed. An Fe/Ni alloy may then be used as a binder material to attach the resulting leached diamond table to another supporting substrate. As noted above, the binder material  210  may be incorporated into a cemented tungsten carbide or other suitable substrate, may be applied to an interface between the leached diamond table and the another supporting substrate, or both. In a further variation, binder material  210  may be placed adjacent a surface or surfaces (for example, a surface of diamond table  202 ′ opposite substrate  210 ) to facilitate introduction of binder material  210  into diamond table  202 ′ in act  104 . 
     Referring to  FIG. 3  of the drawings, drill bit  10  in the form of a rotary drag bit is shown. The drill bit  10  includes bit body  11 . The drill bit  10  includes conventional male threads  12  on a shank thereof configured to API standards and adapted for connection to a component of a drill string, not shown. The face  14  of the bit body  11  has mounted thereon a plurality of cutting elements  16 , at least some of which exhibit structure according to an embodiment of a cutting element of the present invention, each cutting element  16  comprising polycrystalline diamond compact (PDC) table  18  formed on a supporting carbide substrate. The cutting elements  16  are positioned to cut a subterranean formation being drilled while the drill bit  10  is rotated under weight-on-bit (WOB) in a borehole about centerline  20 . The bit body  11  may include gage trimmers  23 , at least some of which may exhibit structure according to an embodiment of a cutting element of the present invention, each gage trimmer  23  including one of the aforementioned PDC tables  18 , such tables  18  being configured with an edge (not shown) to trim and hold the gage diameter of the borehole, and pads  22  on the gage, which contact the walls of the borehole and stabilize the drill bit  10  in the hole. As used herein, the term “drill bit” includes and encompasses drag bits, roller cone bits, hybrid bits, reamers, mills and other subterranean tools for drilling and enlarging well bores. 
     During drilling, drilling fluid is discharged through nozzle assemblies  30  located in nozzle ports  28  in fluid communication with the face  14  of bit body  11  for cooling the PDC tables  18  of cutting elements  16  and removing formation cuttings from the face  14  of drill bit  10  into passages  15  and junk slots  17 . The apertures  24  of nozzle assemblies  30  may be sized for different fluid flow rates depending upon the desired flushing required at each group of cutting elements  16  to which a particular nozzle assembly  30  directs drilling fluid. 
     Although the foregoing description contains many specifics and examples, these are not limiting the scope of the present invention, but merely as providing illustrations of some embodiments. Similarly, other embodiments of the invention may be devised that do not depart from the scope of the present invention. The scope of this invention is, therefore, indicated and limited only by the appended claims and their legal equivalents, rather than by the foregoing description. All additions, deletions and modifications to the invention as disclosed herein and which fall within the meaning of the claims are embraced within their scope.