Abstract:
The present invention is directed to an improved cutting element for use with rotating downhole tools. More specifically, the present invention is directed to a compact cutter which includes unique configurations for the interface regions between the substrate the abrasive element to promote superior impact resistance and adhesion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 09/391,033 as filed on Sep. 7, 1999, now U.S. Pat. No. 6,315,067 which in turn was a continuation-in-part from application Ser. No. 09/129,179 filed Apr. 16, 1998 which issued as U.S. Pat. No. 6,026,919 on Feb. 22, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to abrasive cutters to be applied to rotating downhole tools useful in creating subterranean boreholes. More specifically, the present invention is directed to a compact cutter including an interface region between the substrate and the abrasive element to promote superior impact resistance and adhesion. 
     2. Description of the Prior Art 
     Polycrystalline diamond compacts (PDC) are commonly used in oil field drilling and machine tools. A PDC is a synthetic form of diamond that is made by pressing diamond powder and cobalt onto a cemented tungsten carbide substrate. In the press, the cobalt becomes liquid and acts as a catalyst for diamond grain growth. The result is a highly abrasive, e.g. roughly 90% as abrasive as natural diamond, and environmentally resistant component which is very adaptable to drilling systems for resistant rock formations. 
     Although PDC is resistant to abrasion and erosion, a PDC compact cutter demonstrates several disadvantages. The main components of the PDC system, diamond and tungsten carbide, are brittle materials subject to impact fracturing. Moreover, because tungsten carbide and diamond have different coefficients of thermal expansion, there are residual stresses in a PDC system because the tungsten carbide demonstrates greater contraction during the cooling phase than that of the synthetic diamond. 
     As a result of the aforereferenced disadvantages, attempts have been made in the art to limit the affects by modifying the geometry at the interface between the diamond and the tungsten carbide. Such modifications have usually taken the place of an irregular, non planar interface geometry. The most beneficial resultant of the non-planar interface, defined as any design where the interface between the diamond and tungsten carbide is not a circular plane, is the redistribution of residual stresses. Redistributing residual stresses allow the PDC manufacturer to increase the diamond thickness, thereby providing increased wear resistance. An irregular interface is advantageous since brittle materials are more resistant to compressive loads than tensile loads. The existence of a flat interface causes tensile stress plane between the diamond and tungsten carbide. This plane generally defines a main failure locus for delamination of the diamond layer. 
     One cutter which first utilized a non-planar interface geometry was the “Claw” cutter, so named as a result of the wear pattern of a worn cutter which looked like the remnants of claw marks. The interface of the “Claw” cutter, when viewed in cross section, consists of a plurality of parallel ridges and grooves disposed across the diameter. The “Claw” cutter provided advantages in the areas of wear resistance, but demonstrated a number of disadvantages which included the need to orient the cutter in order to position the parallel diamond inserts normal to the cutting surface. This required orientation of the cutter vis-a-vis the drill bit body complicates the manufacture process. 
     The so called “ring claw” cutter adopted a similar design to that of the Claw cutter except that the Ring Claw included a enhanced thickness ring of synthetic diamond which bounded a series of parallel inserts which also includes diamond of an enhanced thickness. The Ring Claw cutter demonstrated improved wear resistance over the Claw cutter, but when the outer diamond ring became worn, demonstrated similar disadvantages as to the need for precise orientation vis-a-vis the work surface. 
     Another prior art cutter is known as the “target cutter”, and is characterized by an alternating grooves and ridges formed on the cutting face in the form of a target. The target cutter, while addressing the issue of orientation presented by the “Ring Claw cutter,” demonstrated vulnerability to hoop stresses. Hoop stresses are created on the bounding ridges of tungsten carbide positioned interior to grooves filled with synthetic diamond. Hoop stresses are caused by uninterrupted concentric grooves and ridges in the PDC. During cooling of the PDC after pressing, the tungsten carbide ridges will contract and compress on the synthetic diamond rings disposed in the internal grooves. Such contraction simultaneously pulls the tungsten carbide substrate away from diamond disposed in external rings. These differential stresses create a tensile load between all of the internal tungsten carbide ridges and synthetic diamond disposed in all external grooves. such stresses can be severe enough to completely delaminate the synthetic diamond layer. A more common failure is the creation of stress zone in the interface, where fractures due to impact can originate. 
     Moreover, both the “Claw” and the “target cutter” suffered from brazing problems associated with attempts to increase the thickness of the diamond layer. Such additional thicknesses also resulted in reduced impact resistance. In all such prior art cutters, the highest level of stress is found at the edge where cutting forces and impact forces are the highest. Thus, even thought the “Claw” and “target cutter” incorporated a substrate to abrasive interface which included one or more grooves, the uninterrupted thickness and width of the abrasive in these grooves still gave rise to stresses which would often result in stress fracturing and ultimately the complete failure of the cutting element. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above and other disadvantages of prior cutter designs by providing a tool insert comprising a generally disc-shaped abrasive compact having a major front surface and a beveled or arcuate back surface, where at least a part of the periphery of the insert defines a cutting edge. The insert itself is comprised of a hard metal substrate backed to an abrasive compact material, e.g. synthetic diamond, where the substrate defines a partially beveled or tapered surface. 
     In a first embodiment, the substrate defines a major planar surface which incorporates a circumferential slot or groove at its outermost radial border such that the thickness of the abrasive layer about the slot or groove is greater than at the planar region. The circumferential groove is defined by an upper and lower inner boundary and the radial border or periphery of the element. In this embodiment, the trace between the upper and lower portions of the inner boundary is characterized by an arcuate or beveled edge. 
     In a second embodiment, the substrate defines a major planar surface which incorporates a circumferential slot or groove at its outermost radial border such that the thickness of the abrasive layer about the slot or groove is greater than at the planar region. The circumferential groove is again defined by an upper and lower inner boundary and the radial border or periphery of the element. In this embodiment, however, the trace between the upper and lower portions of the inner boundary includes one or more steps which may themselves include an arcuate or beveled subtrace. 
     The present invention offers a number of advantages over the prior art. One such advantage is the ability to increase the thickness of the cutting material where it is most needed to resist stresses experienced in the cutting processes. 
     The present invention also serves to minimize failures occasioned as a result of differential expansion coefficients between the abrasive material and the underlying substrate during the cooling phase. The cutter also presents a uniform thickness of abrasive material around the circumference of the cutter with relative radial symmetry. 
     Further, the cutter of the present invention facilitates drill bit manufacture since the cuter can be oriented at any angle on the drill bit body during assembly. 
     Yet other advantages of the invention will become obvious to those skilled in the art in light of the drawings and the description of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side cross-section of a typical cutting element illustrating sheer stresses. 
     FIG. 2 is a side cross-section of a “ring claw” cutter illustrating compressive stresses. 
     FIG. 3 is a side cross-section of a typical cutting element illustrating the combination of stresses which act on a typical cutting element when applied to a downhole cutting tool. 
     FIG. 4 is a top view of a prior art “ring claw” cutting element. 
     FIG. 5 is a cross-section of the cutting element illustrated in FIG.  4 . 
     FIG. 6 is a top view of a prior art “target” cutting element. 
     FIG. 7 is a cross-section view of the cutting element of FIG.  6 . 
     FIG. 8 is a top view of one embodiment of the cutting element of the present invention. 
     FIG. 9 is a side cross-section of the embodiment shown in FIG.  8 . 
     FIG. 10 is a top cross-section of one embodiment of a second embodiment of the invention. 
     FIG. 11 is a side cross-section of the embodiment shown in FIG.  10 . 
     FIG. 12 is a side cross-section of a third embodiment of the invention. 
     FIG. 13 is a side cross-section of a fourth embodiment of the invention. 
     FIG. 14 is a side cross-section of a fifth embodiment of the invention. 
     FIG. 15 is a side cross-section of a sixth embodiment of the invention. 
     FIG. 16 is a side cross-section of a seventh embodiment of the invention. 
     FIG. 17 is a side cross-section of an eighth embodiment of the invention. 
     FIG. 18 is a side cross-section of a ninth embodiment of the invention. 
     FIG. 19 is a side cross-section of a tenth embodiment of the invention. 
     FIG. 20 is a side cross-section of an eleventh embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention achieves a means of mitigating the level of stresses in the geometrical features defining the interface between the substrate and the superabrasive material of a cutting element. 
     By reference to FIG. 1, sheer stresses between the superabrasive compound table and the substrate are caused by the differential expansion rates between the materials most often comprising these features, polycrystalline diamond  2  and carbide  4 , respectfully. The stress caused by this differential expansion varies as the distance increases along the interface  5 . For example, in the original PDC cutter designs sheer stresses were very low near the center axis “A” but increased toward the periphery  7 . The amount of this stress was related to the distance from the cutter center to its edge. 
     In a “step” design utilizing grooves and channels, the stress is lowest at the bottom  12  of the groove or step  10  and greatest at the top  14 , with stresses increasing as a factor of the step height. (See FIG. 2) “Stepped” designs also suffer from problems of compression interface stresses which occur along the inside wall  15  of the step. 
     Cutters which do not incorporate a ring describe the highest stresses at the periphery  20 . (See FIG. 3) These stresses can oftentimes exceed the strength of the diamond  22  or the diamond to carbide interface  24  resulting in a loss of cutting material. Cutters which do not include a compressive ring describe the highest stress at the top of the step where sheer stresses from the face, sheer stress from the ring wall and compressive stresses are all at their highest. When the stresses are combined with cutting forces the strength of the diamond to carbide interface can be exceeded. 
     FIGS. 4-7 illustrate top, cross-sectional views of prior art cutters sold, in the instance of FIGS. 4-5, under the name “ring claw cutter” and in the instance of FIGS. 6-7, under the name “target cutter”. 
     By reference to FIGS. 4-5, the “ring claw” cutter  42  comprises a disc shaped body  44  defining a peripheral cutting edge  45  bounding a top, cutting surface  46  comprised of a superabrasive material, commonly polycrystalline diamond. As illustrated, the polycrystalline cutting surface  46  is bonded to an underlying hard metal substrate  47 , e.g. cemented tungsten carbide, defining a series of axial ridges  48  bounded by grooves  49  about which the superabrasive is formed and subsequently bonded. The “ring claw” cutter is characterized by a radial groove  50  formed at the outer periphery of body  44 , which groove receives the polycrystalline diamond to form cutting edge  45 , as shown. 
     The outer radial groove  50  is defined by an upper  41  and lower  42  inner boundary and the periphery  44 , where an uninterrupted linear trace is formed therebetween of a given length “L.” The presence of this trace gives rise to hoop stresses caused as a result of differential coefficients of expansion during cooling. 
     FIGS. 6-7 illustrate the prior art “target cutter”  60  which also includes a disc shaped body  62  defining a peripheral cutting edge  63  bounding a top cutting surface  65  again comprised of a polycrystalline diamond. In this prior embodiment, the carbide substrate  67  forms a series of concentric ridges  67  defining complementary grooves  69  in which the polycrystalline diamond is formed and subsequently bonded. 
     Similar to the ring claw embodiment of FIGS. 4-5, the grooves  69  formed in the substrate  67  include an upper  64  and lower  68  inner boundary which define an uninterrupted linear trace. As a result, the “target” cutter embodiment also suffers from problems of hoop stresses caused as a result of differential coefficients of expansion exhibited during cooling. These hoop stresses, in some cases, are severe enough as to result in delamination of the polycrystalline diamond layer. 
     A first embodiment of the cutting element  100  of the present invention may be seen by reference to FIGS. 8-9 in which is illustrated a disc-shaped body  102  comprised of a substrate  104  and an abrasive layer  106  which together define an interface  107 . In a preferred embodiment, the substrate  104  is comprised of a tungsten carbide while the abrasive layer  106  is comprised of polycrystalline diamond. The substrate  104  defines a generally planar central region  109  bounded by an outer groove  110 . In such a fashion, the thickness of the abrasive layer is thicker about the groove  110  than about the central region  109 . 
     FIG. 9 illustrates in phantom the longitudinal and radial trace  101  which would be formed if a concentrical groove were incorporated as in the “target” and “ring claw” designs, where said traces would have a length of “L” and “R,” respectively. It has been discovered that the foreshortening of “R” and “L” substantially decreases the hoop and other stresses associated with the cutting element. In this embodiment, therefore, both “L” and “R” are foreshortened by the incorporation of an outwardly beveled trace  112  which is included at an angle θ, by reference to a plane normal to axis “A.” In such a fashion, the abrasive layer attains a maximum thickness at the periphery  111  of body  102 . It is preferred that the angle of bevel θ be less than or equal to 45 degrees, though other angulations are contemplated within the spirit of the invention. 
     A second embodiment of the invention is shown in FIGS. 10-11 which is illustrated a generally disc-shaped body  120  comprised of a substrate  122  and an abrasive layer  124  which together define an interface  123 . Similar to the embodiment of FIGS. 8 and 9, this embodiment also includes a central planar region  126  bounded by an outer groove  128 . The complete trace  121  of outer groove  128  is illustrated in phantom. Outer groove  128  defines an upper  129  and lower  130  inner boundary defining a height “H” and a radius “R.” The lower, inner boundary is located at the lowest level in groove  128  and in some instances may be at the periphery. In this embodiment, the intersection between groove  128  and central planar region  126  defines a step  127  terminating in an outwardly sloping bevel  125 . In such a fasion, the thickness “H” and width “R” of the trace of groove  128  are foreshortened. Additional desired thickness at the cutting edge  131  is therefore rendered possible while still addressing issues of compressive stresses. 
     Still other embodiments are illustrated at FIGS. 12-13. In FIG. 12, the cutting element  140  is comprised of a substrate  142  and an abrasive layer  144 . In this embodiment, the substrate  142  includes an exterior groove  149  which defines an arcuate intersection boundary  147  with an internal planar region  148 . The phantom trace  146  of a conventional groove is illustrated. The use of arcuate or curvilinear intersection  147  serves to foreshorten radius “R” and thickness “H,” again resulting in stress reduction thereby results in a further reduction in compressive and combination stresses while still providing maximum thickness of the abrasive compound at the cutting edge  143 . 
     The cutter  150  of FIG. 13 comprises a disc-shaped body  152  including a substrate  154  and an abrasive layer  155 , where the substrate  154  defines an external groove or channel  156  and a generally planar central area  158 . The phantom trace of the architecture of a concentrical groove is again illustrated. In this embodiment, the radius of the phantom trace is designated “R” and the maximum thickness of the abrasive layer in groove  156  is designated “H.” This embodiment defines a pronounced “step”  159  which would ordinarily result in high compressive stress. The upper  151  and lower inner  154  boundary of step  159 , however, has been modified to foreshorten both “H” and “R” by the inclusion of curvilinear points of intersection. 
     Variations on this principle are seen in FIGS. 18 and 19. In both of these embodiments, the radius “R” and thickness “H” are foreshortened, albeit incorporating differing internal architecture. In both examples, compressive and total stresses are minimized. 
     Yet an additional embodiment is illustrated at FIG.  14 . This embodiment also defines a disc-shaped body  180  which is comprised of a substrate  82  and abrasive layer  84 , the combination defining an interface  85 . The embodiment of FIG. 14 includes a central planar region  88  bounded by an outer groove  89 . Outer groove  89  defines an upper  91  and lower  93  inner boundary, where the trace of the concentrical groove is illustrated in phantom. In this embodiment, the thickness “H” and radius “R” are foreshortened by the inclusion of a convex region, as illustrated. 
     FIGS. 15,  16 , 17  and  20  illustrate yet additional embodiments of the invention. In each of these embodiments, a disc-shaped body includes an internal, planar portion which is bounded by a two-stage channel or groove of varying depths. As illustrated, this depth increases as one progresses radially outwardly from the axis “A.” The interface between the two channels is variably characterized by arcuate or beveled surfaces such that the thickness of the abrasive compound variably increases toward the periphery. In each embodiment, however, the compressive stresses are reduced by architecture which results in a foreshortening of the thickness “H” and the radius “R” of the original channel trace. As illustrated in FIGS. 16 and 20, this foreshortening may be accomplished incrementally through a successive series of stepped, beveled or curvilinear surfaces. 
     Although particular detailed embodiments of the apparatus and method have been described herein, it should be understood that the invention is not restricted to the details of the preferred embodiment. Many changes in design, composition, configuration and dimensions are possible without departing from the spirit and scope of the instant invention.