Abstract:
The present disclosure generally relates to constructions including a body of polycrystalline diamond (PCD) material attached to a substrate.

Description:
FIELD 
       [0001]    This disclosure relates to a superhard construction comprising, for example, a body of polycrystalline diamond (PCD) material attached to a substrate. 
       BACKGROUND 
       [0002]    Polycrystalline superhard materials, such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) may be used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. In particular, tool inserts in the form of cutting elements comprising PCD material are widely used in drill bits for boring into the earth to extract oil or gas. The working life of superhard tool inserts may be limited by fracture of the superhard material, including by spalling and chipping, or by wear of the tool insert. 
         [0003]    Cutting elements such as those for use in rock drill bits or other cutting tools typically have a body in the form of a substrate which has an interface end/surface and a superhard material which forms a cutting layer bonded to the interface surface of the substrate by, for example, a sintering process. The substrate is generally formed of a tungsten carbide-cobalt alloy, sometimes referred to as cemented tungsten carbide and the superhard material layer is typically polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or a thermally stable product TSP material such as thermally stable polycrystalline diamond. 
         [0004]    Polycrystalline diamond (PCD) is an example of a superhard material (also called a superabrasive material) comprising a mass of substantially inter-grown diamond grains, forming a skeletal mass defining interstices between the diamond grains. PCD material typically comprises at least about 80 volume % of diamond and is conventionally made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, and temperature of at least about 1,200° C., for example. A material wholly or partly filling the interstices may be referred to as filler or binder material. 
         [0005]    PCD is typically formed in the presence of a sintering aid such as cobalt, which promotes the inter-growth of diamond grains. Suitable sintering aids for PCD are also commonly referred to as a solvent-catalyst material for diamond, owing to their function of dissolving, to some extent, the diamond and catalysing its re-precipitation. A solvent-catalyst for diamond is understood be a material that is capable of promoting the growth of diamond or the direct diamond-to-diamond inter-growth between diamond grains at a pressure and temperature condition at which diamond is thermodynamically stable. Consequently the interstices within the sintered PCD product may be wholly or partially filled with residual solvent-catalyst material. Most typically, PCD is often formed on a cobalt-cemented tungsten carbide substrate, which provides a source of cobalt solvent-catalyst for the PCD. Materials that do not promote substantial coherent intergrowth between the diamond grains may themselves form strong bonds with diamond grains, but are not suitable solvent-catalysts for PCD sintering. 
         [0006]    Cemented tungsten carbide which may be used to form a suitable substrate is formed from carbide particles being dispersed in a cobalt matrix by mixing tungsten carbide particles/grains and cobalt together then heating to solidify. To form the cutting element with a superhard material layer such as PCD or PCBN, diamond particles or grains or CBN grains are placed adjacent the cemented tungsten carbide body in a refractory metal enclosure such as a niobium enclosure and are subjected to high pressure and high temperature so that inter-grain bonding between the diamond grains or CBN grains occurs, forming a polycrystalline ultra hard diamond or CBN layer. 
         [0007]    In some instances, the substrate may be fully cured prior to attachment to the superhard material layer whereas in other cases, the substrate may be green, that is, not fully cured. In the latter case, the substrate may fully cure during the HTHP sintering process. The substrate may be in powder form and may solidify during the sintering process used to sinter the superhard material layer. 
         [0008]    Cobalt has a significantly different coefficient of thermal expansion from that of diamond and, as such, upon heating of the polycrystalline diamond material during use, the cobalt in the substrate to which the PCD material is attached expands and may cause cracks to form in the PCD material, resulting in the deterioration of the PCD layer. 
         [0009]    To reduce the residual stresses created at the interface between the substrate and the superhard layer, interface surfaces on substrates are known to have been formed with a plurality concentric annular rings projecting from the planar interface surface. Due to the difference in the coefficients of thermal expansion of the substrate and the superhard material layer, these layers contract at different rates when the cutting element is cooled after HTHP sintering. Tensile stress regions are formed on the upper surfaces of the rings, whereas compressive stress regions are formed on the valleys between such rings. Consequently, when a crack begins to grow in use, it may grow annularly along the entire upper surface of the annular ring where it is exposed to tensile stresses, or may grow along the entire annular valley between the projecting rings where it is exposed to compressive stresses, leading to the early failure of the cutting element. 
         [0010]    It is also known for cutting element substrate interfaces to comprise a plurality of spaced apart projections, the projections having relatively flat upper surfaces projecting from a planar interface surface. 
         [0011]    Common problems that affect cutting elements are chipping, spalling, partial fracturing, and cracking of the superhard material layer. Another problem is cracking along the interface between the superhard material layer and the substrate and the propagation of the crack across the interface surface. These problems may result in the early failure of the superhard material layer and thus in a shorter operating life for the cutting element. 
         [0012]    The working life of PCD material in a cutter used during drilling operations is typically determined, to a large extent, by the initiation and propagation of cracks in the PCD material. In particular, cracks tend to form behind a wear scar and propagate towards the free surface of the PCD material, where they coalesce and may result in spalling and catastrophic failure of the cutter. It is desirable that any cracks that form should be prevented from propagating through the PCD material or at least their propagation be directed to reduce the risk of spalling. One solution is to increase the toughness of the PCD material to minimise crack formation, however, tougher materials tend to have decreased hardness and therefore are less wear resistant so may wear faster and have a shorter lifetime. 
         [0013]    Accordingly, there is a need for a cutting element having an enhanced operating life in high wear or high impact applications, such as boring into rock, with a superhard material layer in which the likelihood of cracking, chipping, and fracturing is reduced or controllable. 
       SUMMARY 
       [0014]    Viewed from a first aspect there is provided a superhard construction comprising:
       a layer of superhard material;   a substrate attached to the layer of superhard material along an interface and formed of a material having a coefficient of thermal expansion; and   a region of material in the substrate having a different coefficient of thermal expansion to the coefficient of thermal expansion of the material forming the substrate;   wherein the region of material comprises a solid of revolution about an axis of symmetry or a partial solid of revolution; and   wherein the region has a point, edge or face in contact with or extending to the layer of superhard material and/or a peripheral surface of the superhard construction.       
 
         [0020]    The superhard material may comprise, for example, polycrystalline diamond material or polycrystalline cubic boron nitride. 
         [0021]    Viewed from a second aspect there is provided a superhard construction comprising:
       a layer of superhard material;   a substrate attached to the layer of superhard material along an interface and formed of a material having a coefficient of thermal expansion;   a region of material in the substrate having a different coefficient of thermal expansion to the coefficient of thermal expansion of the material forming the substrate;   wherein the superhard construction has a longitudinal axis;   wherein the region of material comprises a solid of revolution about an axis of symmetry or a partial solid of revolution about an axis of symmetry, said region of material having a different coefficient of thermal expansion to the coefficient of thermal expansion of the material forming the substrate; and   wherein the axis of symmetry of the solid or partial solid of revolution is offset from the longitudinal axis of the superhard construction and not parallel to said longitudinal axis.       
 
         [0028]    Viewed from a further aspect there is provided a superhard construction comprising:
       a layer of superhard material;   a substrate attached to the layer of superhard material along an interface and formed of a material having a coefficient of thermal expansion;   one or more regions of material in the substrate having different coefficient(s) of thermal expansion to the coefficient of thermal expansion of the material forming the substrate;   wherein one or more of the regions of material comprise one or more annuli.       
 
         [0033]    Viewed from another aspect there is provided a superhard construction comprising:
       a layer of superhard material;   a substrate attached to the layer of superhard material along an interface and formed of a material having a coefficient of thermal expansion;   one or more regions of material in the substrate having different coefficient(s) of thermal expansion to the coefficient of thermal expansion of the material forming the substrate;   wherein one or more of said regions extends into the layer of superhard material and into the substrate, one or more of said regions comprising a solid of revolution about an axis of symmetry or a partial solid of revolution about an axis of symmetry.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    Examples of superhard constructions comprising, for example, polycrystalline diamond bodies will now be described in more detail, with reference to the accompanying figures in which: 
           [0039]      FIGS. 1 to 22  are schematic cross-sections through alternative embodiments of a PCD construction formed of a body of PCD material attached to a substrate; 
           [0040]      FIGS. 23   a ,  23   b ,  24   b ,  25   b ,  26   b  and  27   b  are schematic contour plots through alternative embodiments of a PCD construction showing the regions of stress; and 
           [0041]      FIGS. 24   a ,  25   a ,  26   a  and  27   a  are schematic contour plots through prior art PCD constructions showing the regions of stress. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    The term “solid of revolution” as used herein is to be understood to include solids of revolution about an axis of symmetry and solids derived therefrom, and so includes bodies from which a segment has been removed, that is, a partial solid of revolution such as a cylinder with a segment excised. 
         [0043]    As used herein, the term “bulk of the substrate” refers to the substrate material of largest amount by volume. 
         [0044]    As used herein, the term “stress state” refers to a compressive, unstressed or tensile stress state. Compressive and tensile stress states are understood to be opposite stress states from each other. In a cylindrical geometrical system, the stress states may be axial, radial or circumferential, or a net stress state. 
         [0045]    As used herein, the term “residual stress state” refers to the stress state of a body or part of a body in the absence of an externally-applied loading force. The residual stress state of a PCD structure, including a layer structure may be measured by means of a strain gauge and progressively removing material layer by layer. 
         [0046]      FIGS. 1 to 15  show various embodiments of PCD constructions  1  each comprising a body of PCD material  10  attached to a substrate  12  along an interface  14 . In one embodiment, the PCD construction  1  may be a cutter insert for a drill bit, such as a shear or drag drill bit for boring into the earth, for example to extract oil or gas deposits. Alternatively, the PCD construction  1  may be suitable for machining, boring into or degrading hard or abrasive materials such materials comprising metal, ceramic, wood, composite material, concrete, stone or asphalt. 
         [0047]    In the embodiment shown in  FIGS. 1 to 15 , the PCD construction  1  is formed with at least one solid of revolution  30 ,  40 ,  50 ,  60 ,  70 , as defined above, included in the substrate  12  of the PCD construction  1 . In each of these embodiments, at least one of these solids of revolution  30 ,  40 ,  50 ,  60 ,  70  has at least one point, edge or face  80  of the solid in contact with the body of PCD material  10  or with an exposed surface of the substrate  12  or combinations thereof. 
         [0048]    The same reference numbers refer to the same respective features in all drawings. 
         [0049]    With reference to  FIG. 1 , the PCD construction is formed of a body of PCD material  10 , attached along an interface  14  to a cemented carbide substrate  12 . An annulus  30  with a substantially square cross section is located coaxially on the substrate adjacent the body of PCD material  10  such that the outer diameter of the annulus  30  is substantially equal to the outer diameter of the substrate  12 , the surface of the PCD body  10  opposing the exposed surface thereof abutting a surface  80  of the annulus  30  to form an interface with the substrate and the annulus  30 . The average coefficient of thermal expansion (CTE) of the annulus  30  is lower than the average CTE of the bulk of the cemented carbide substrate  12 . 
         [0050]    The embodiment shown in  FIG. 2  differs from that shown in  FIG. 1  in that an additional annulus  40  is included having an outer radius less than or equal to the inner radius of the first annulus  30  such that the second annulus  40  is positioned within the first annulus  30  and is in contact with the PCD body  10  along a surface  80 . 
         [0051]    The height of the second annulus  40  is not constrained by the height of the first annulus  30  and, as shown in  FIG. 3 , may, for example be greater than that of the first annulus  30 , extending further into the substrate than the first annulus or less (not shown). The average coefficient of thermal expansion (CTE) of the second annulus  40  is lower than the average CTE of the bulk of the substrate  12  and may be formed of the same material or a different material from the first annulus  30 . 
         [0052]    In a further embodiment, as shown in  FIG. 4 , there is a single annulus  30  which has an outer diameter less than that of the substrate  12 , the peripheral edge thereof thereby being enclosed by the substrate  12 . 
         [0053]    The annuli  30  and  40  shown in one or more of  FIGS. 1 to 4 , may be formed of PCD material which may be of a different PCD composition from the body of PCD  10  and of dimensions that are not constrained by the thickness thereof and have the characteristic that the average coefficient of thermal expansion (CTE) is lower than the average CTE of the bulk of the substrate  12 . 
         [0054]    In a further embodiment, as shown in  FIG. 5 , a thin zone of low shear-strength material may be placed along the interface between the body of PCD material  10  and the annulus  30 . 
         [0055]      FIG. 6  shows a further embodiment which differs from that shown in  FIG. 2  in that a third annulus  50  is included in the substrate  12 . The third annulus  50  may be, for example, of similar or larger inner diameter than the first annulus  30  and may be positioned such that it abuts the surface of the first annulus  30  opposing the surface of the first annulus  30  which abuts the interface with the body of PCD material  10 . 
         [0056]    The third annulus  50  may, for example, have a rectangular cross section, as shown in  FIG. 6 , or a trapezoidal cross section as shown in  FIG. 7 . The average coefficient of thermal expansion (CTE) of the third annulus  50  is lower than the average CTE of the bulk of the substrate  12  and may be formed of the same material or a different material from the first and/or second annuli  30 ,  40 . 
         [0057]      FIG. 8  shows a further embodiment which differs from that shown in  FIG. 7  in that an additional cylindrical insert  60  is located in the hole defined by the inner peripheral surface of the second annulus  40  and is recessed into the substrate  12 , with the top surface of the cylindrical insert  40  being in contact with and forming an interface with the body of PCD material  10 . The height of the cylindrical insert  60  is not limited by the height of the second annulus  40 . The average coefficient of thermal expansion (CTE) of the cylindrical insert is different from, for example lower than the average CTE of the bulk of the substrate  12  and may be formed of the same material or a different material from the annuli  30 ,  40 ,  50 . 
         [0058]    With reference to  FIG. 9 , the PCD construction  1  is formed of a body of PCD material  10 , attached along an interface to a substrate  12  such as a cemented carbide substrate. An annulus  30  with a substantially square cross section is located coaxially on the substrate  12  adjacent the body of PCD material  10  such that the outer diameter of the annulus  30  is substantially equal to the outer diameter of the substrate  12 , the surface of the PCD body  10  opposing the exposed surface thereof abutting a surface  80  of the annulus  30  to form an interface with the substrate  12  and the annulus  30 . An additional cylindrical insert  60  is located in the hole defined by the inner peripheral surface of the first annulus  30 , with the top surface of the cylindrical insert  60  being in contact with and forming an interface with the body of PCD material  10 . The height of the cylindrical insert  60  is not limited by the height of the first annulus  30 . 
         [0059]    A further annulus  50  which may be, for example, of similar or larger inner diameter than the first annulus  30  may be positioned such that it abuts the surface of the first annulus  30  opposing that forming the interface with the body of PCD material  10 . The outer diameter of the further annulus  50  may be substantially equal to the outer diameter of the first annulus  30  and the substrate  20 . The inner peripheral surface defining the hole of the further annulus  50  may be curved such that, in cross section as shown in  FIG. 9 , the section includes a curved line defining the inner surface. This line may be convex as shown, straight, concave or otherwise shaped and chosen to best manage residual stresses. 
         [0060]    The average coefficient of thermal expansion (CTE) of the cylindrical insert  60  is different from, for example lower than, the average CTE of the bulk of the substrate  12  and may be formed of the same material or a different material from the annuli  30 ,  40 ,  50  and/or the body of PCD material. 
         [0061]    With regard to the embodiments shown in  FIGS. 1 to 9 , one or more chamfers may be formed on the inner and/or outer faces of the annuli within the substrate. 
         [0062]    A further embodiment is shown in  FIG. 10  in which the PCD construction  1  comprises a body of PCD material  10 , a substrate  12 , and a coaxially located and inverted cone  60  recessed into the substrate  12 , such that the base of the cone  60  is in contact with the body of PCD material  10  at a point  80 . 
         [0063]    The embodiment shown in  FIG. 11  differs from that shown in  FIG. 10  in that the central inverted cone  60  recessed into the substrate bulk  12  is truncated at its apex, the base of the cone being in contact with the body of PCD material  10 . 
         [0064]    The embodiment shown in  FIG. 12  is a combination of those shown in  FIGS. 10 and 11  in that it includes a central cup-shaped solid of revolution  60  where the “cup” is formed by a central inverted cone  62  and a central inverted truncated cone  64  recessed into the substrate bulk  12 , the central inverted cone  62  being embedded in a co-axial with the truncated cone  64 . The bases of the central inverted cone  62  and a central inverted truncated cone  64  are in contact with the body of PCD material  10  along an interface. 
         [0065]    With reference to  FIG. 13 , this differs from the embodiment shown in  FIG. 12  in that the inverted V-section (a hollow cone)  66  in which the inverted central cone  62  is embedded is not truncated. 
         [0066]    In the embodiments of  FIGS. 10 to 13 , the average coefficient(s) of thermal expansion (CTE) of the inserts  60 ,  62 ,  64  is/are different from, for example lower than, the average CTE of the bulk of the substrate  12  and may be formed of the same composition or a different composition from each other and/or the PCD body  10 . 
         [0067]    In the embodiment shown in  FIG. 14 , a central and coaxial solid of revolution is recessed into the substrate  12 . The solid of revolution  72  has a diamond cross-section. An annulus  74  of triangular cross-section is embedded therein such that a side face  76  of the triangular cross-section of the annulus  74  forms part of one surface of the diamond cross-section of the solid of revolution. The annulus  74  is co-axial with the longitudinal axis of the PCD construction  1 . An apex of the diamond cross-section touches the PCD table at one point  80  lying on the longitudinal axis of the PCD construction and on the interface with the substrate  12 . 
         [0068]      FIGS. 15 to 22  show embodiments in which the symmetry axes of the solids of revolution, as defined above, are offset from and are not parallel to the symmetry axis of the PCD construction  1  or the substrate  12 . 
         [0069]    With reference to  FIG. 15 , the embodiment shown therein comprises a PCD construction  1  having a body of PCD material  10 , a substrate  12  attached thereto along an interface  102 , and a first hollow conical solid of revolution  100  embedded in the substrate  12 . The first conical solid of revolution  100  touches body of PCD material  10  at a point  104  on the interface  102  between the body of PCD material and the substrate. The first conical solid of revolution  100  also touches the outer peripheral surface of the substrate at a point  106 . The conical solid  100  is not coaxial with the substrate  12 . A further conical solid of revolution  108  coaxial with and recessed in the hollow of the first conical solid of revolution  100  is positioned such that the bases of the two conical solids  108  and  100  are coplanar. Further conical solids (not shown) may be included.  FIG. 15   a  is a three-dimensional schematic rendering of an embedded conical solid  108 , 100  as described in respect of  FIG. 15 . 
         [0070]    In the embodiment of  FIG. 15 , the average coefficient(s) of thermal expansion (CTE) of the solids of revolution  100 ,  108  are different from, for example lower than, the average CTE of the bulk of the substrate  12 . The CTEs of the solids of revolution  100 ,  108  may be the same or different and these solids may be formed of the same composition as or a different composition from the PCD body  10 . 
         [0071]      FIG. 16  shows a further embodiment in which the solid of revolution  110  is a cylinder having a symmetry axis  111  offset from that  112  of the substrate  12  into which it is embedded. The cylinder  110  contacts the body of PCD material  10  at a point  114  on the interface  116  and also contacts the peripheral surface of the substrate  12  at a point  118  thereon. In other embodiments (not shown), two or more such embedded shallow cylinders with the same or different dimensions, compositions and orientations may be included.  FIG. 16   a  is a three-dimensional schematic rendering of an embedded shallow cylinder as described with respect to  FIG. 16 . In the embodiment of  FIG. 16 , the average coefficient(s) of thermal expansion (CTE) of the solid of revolution  110  is different from, for example lower than, the average CTE of the bulk of the substrate  12 . The solid of revolution  110 , may be formed of the same composition as or a different composition from the PCD body  10 . 
         [0072]    With reference to  FIG. 17 , a further embodiment is shown which includes an embedded solid of revolution  120  which is not coaxial with the PCD construction  1  and is located in the substrate  12 . The solid of revolution  120  is an annulus with a parallelogram cross-section and touches the body of PCD material  121 , to a reasonable approximation at only one point  122  on the interface  124  of the body of PCD body  121  and the substrate  12 . Two or more such embedded shaped solids with the same or different dimensions, compositions and orientations may be included (not shown). The average coefficient of thermal expansion (CTE) of the solid of revolution  120  is different from, for example lower than, the average CTE of the bulk of the substrate  12 . The solid of revolution  120 , may be formed of the same composition as or a different composition from the PCD body  121 . 
         [0073]    With reference to  FIG. 18 , this embodiment differs from that shown in  FIG. 17  in that the embedded solid of revolution  120 , which is not coaxial with the substrate, has an axis of symmetry that is substantially perpendicular to that of the substrate  12  rather than being at an angle of less than 90 degrees thereto as in  FIG. 17 . Furthermore, contact between the solid of revolution  120  and the body of PCD material  121  is made along a portion of the peripheral outer surface of the solid of revolution  120  along the interface  124  of the body of PCD material  121  and the substrate  12 . Two or more such embedded washer-shaped solids of revolution  120  may be included having the same or different dimensions, compositions and orientations. 
         [0074]      FIG. 19  shows a further embodiment in which the PCD construction comprises a body of PCD material  130 , a substrate bulk  132  attached thereto along an interface  134 , and an embedded half cylinder  136  of the same radius as the substrate bulk. The side wall of the half cylinder  136  extends to the side wall of the substrate bulk  132 .  FIG. 19   a  is a three-dimensional schematic rendering of the embedded half cylinder  136  as described above. The average coefficient of thermal expansion (CTE) of the partial solid of revolution, namely the half cylinder  136  is different from, for example lower than, the average CTE of the bulk of the substrate  132 . The partial solid of revolution  136 , may be formed of the same composition as or a different composition from the PCD body  130 . 
         [0075]    With reference to  FIG. 20 , in this embodiment the PCD construction has a body of PCD material  140 , a substrate bulk  142  bonded to the body of PCD material  140  along an interface  143 , and a plurality of half cylinders  144  of various sizes and orientations embedded in the substrate bulk  142 , all being offset from the longitudinal axis  146  of the PCD construction.  FIG. 20   a  is a three-dimensional schematic rendering of a typical embedded half cylinder  144  as described in respect of  FIG. 20 . The half cylinders  144  may be of the same composition or of different compositions. One or more of the half cylinders  144  contacts the body of PCD material  140  at a point/line of contact on the interface  143  and/or at a point/line of contact on/along the peripheral side wall of the substrate  142 . The average coefficient(s) of thermal expansion (CTE) of the partial solids of revolution, namely the half cylinders  144  is/are different from, for example lower than, the average CTE of the bulk of the substrate  142 . The partial solids of revolution  144 , may be formed of the same composition as or a different composition from the PCD body  140 . 
         [0076]    The embodiment shown in  FIG. 21  differs from those of  FIGS. 1 to 19  in that the solid(s) of revolution embedded in the PCD construction do not extend to or touch a peripheral edge or surface of the substrate bulk whether that is external or along the interface with the PCD body. However, in  FIG. 21 , the axes of symmetry of the solid(s) of revolution are offset from the longitudinal axis of the substrate bulk. 
         [0077]    With reference to  FIG. 21 , in this embodiment the solids of revolution  150  embedded in the substrate  152  are substantially ovoid in shape with an end section removed to form a “spinning-top” shaped solid. Two such solids  150  are shown in the embodiment of  FIG. 21  however any number of such solids may be included which may be of different sizes and orientations and the same or different compositions.  FIG. 21   a  is a three-dimensional schematic rendering of a “spinning-top” shaped solid as described with reference to  FIG. 21 . The average coefficient(s) of thermal expansion (CTE) of the partial solids of revolution, namely the ovoids  150  is/are different from, for example lower than, the average CTE of the bulk of the substrate  152 . The partial solids of revolution  150 , may be formed of the same composition as or a different composition from the PCD body  10  and/or each other. 
         [0078]    With reference to  FIG. 22 , this embodiment of a PCD construction has a solid of revolution in the form of an annulus  160  of elliptical cross-section. The solid of revolution  160  is embedded partially in the substrate bulk  162  and partially in the body of PCD material  164  which is attached to the substrate  162  along an interface  166 , the solid of revolution  160  being differentiable from both the substrate bulk  162  and the body of PCD material  164  by its material composition. More generally, a solid of revolution of any geometrical cross-section may be used in this embodiment, provided it is embedded in both the substrate bulk and the body of PCD material, and is differentiable from both the substrate bulk and the PCD table by its material composition. 
         [0079]    Whilst not wishing to be bound by a particular theory, it is believed that the solids of revolution in the PCD constructions described above alter the residual tensile and compressive stresses in the PCD construction. These stresses are illustrated in  FIGS. 23 to 30  for various constructions and compositions. In all of  FIGS. 23 to 27 , only a part corner section of the PCD construction is shown.  FIGS. 23   a ,  23   b ,  24   b ,  25   b  show the residual tensile and compressive stresses in the embodiment of, for example,  FIG. 1  in which the solid of revolution is an annulus. 
         [0080]      FIGS. 24   a ,  25   a ,  26   a  and  27   a  are schematic cross sections of a corner section of a conventional PCD construction with no solid of revolution embedded therein and showing the residual tensile and compressive stresses therein for comparison. 
         [0081]      FIGS. 26   b  and  27   b  show the residual tensile and compressive stresses in an embodiment in which the solids of revolution comprise an outer annulus of rectangular cross-section whose peripheral outer edge is flush with the peripheral outer surface of the substrate, and an inner cylinder spaced from the inner surface of the annulus. Both the annulus and cylinder are co-axial with the substrate and both abut the interface with the PCD body. 
       EXAMPLES 
       [0082]    A number of embodiments are described in more detail with reference to the examples below, which are not intended to be limiting. 
         [0083]    Residual stress analyses were performed using the ABAQUS Finite Element Analysis program to give an indication of stress states of different configurations. For the sake of simplicity, only axisymmetric cases were selected. In the model, each configuration was taken from high pressure and temperature conditions down to room temperature and atmospheric pressure. The residual Maximum Principal Stresses in the cutter were examined and corresponding stress contours in a large area of the cross-section which includes the cutting corner are shown in  FIGS. 23   a  to  27   b . A cutter consisting of a Reference PCD grade, a WC-13 wt % Co cemented carbide bulk substrate and a planar interface was used as a baseline (and is hereinafter referred to as Case 0). 
         [0084]    Most of the examples were variants of the conventional cutter “Case 0” with either Diamond-Enhanced Carbide (DEC) or WC-6 wt % Co cemented carbide used to form the solids of revolution in the substrate  12 . 
         [0085]    A number of variants were considered for manufacture and wear tests. Table 1 below shows the properties of the materials used to form the PCD constructions which were then subjected to linear elastic FEA analysis. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Coefficient of 
               
               
                   
                 Young&#39;s 
                 Poisson&#39;s 
                 Thermal Expansion) 
               
               
                 Material 
                 Modulus (GPa) 
                 ratio (—) 
                 (10 −6  ° C. −1 ) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Reference PCD 
                 927 
                 0.108 
                 4.43 
               
               
                 Bulk substrate 
                 550 
                 0.227 
                 6.5 
               
               
                 WC—13 wt % Co 
               
               
                 DEC 
                 680.3 
                 0.182 
                 4.77 
               
               
                 WC—6 wt %Co 
                 620 
                 0.21 
                 5.40 
               
               
                   
               
             
          
         
       
     
         [0086]    In addition to the reference case (Case 0), the following variants were analysed:
       Case 1: a PCD construction having a solid of revolution comprised of DEC and in the form of an annulus as shown in  FIG. 1 ;   Case 2: a PCD construction having a solid of revolution comprised of WC6 wt % Co and in the form of an annulus as shown in  FIG. 1 ;   Case 3: a PCD construction having a solid of revolution comprised of DEC and in the form of an annulus as shown in  FIG. 1  with an additional solid of revolution in the form of a co-axial cylinder within the annulus, the cylinder also being formed of DEC; and   Case 4: a PCD construction having a solid of revolution comprised of WC6 wt % Co and in the form of an annulus as shown in  FIG. 1  with an additional solid of revolution in the form of a co-axial cylinder within the annulus, the cylinder also being formed of WC6 wt % Co.       
 
         [0091]    For each geometry and material, five sizes of the solid(s) of revolution were considered. 
         [0092]    The constraints on the geometries were as follows:
       1. PCD construction nominally of 16 mm diameter and 12 mm overall height with nominal thickness of the PCD body of 2.2 mm.   2. Annulus of square section and touching the substrate outer diameter and PCD body (as sown, for example, in  FIG. 1 ).   3. Additional solid of revolution in the form of a cylinder, if present, having a height equal to the annulus section side, with the added cylinder coaxial with the PCD construction and its top surface in contact with the PCD body, the radius of the cylinder being equal to the length of the sides of the square cross-section of the annulus.   4. Annulus section having sides of lengths 2.00 mm, 2.25 mm, 2.50 mm, 2.75 mm and 3.00 mm were considered.       
 
         [0097]    A further constraint was that, in all cases, the annulus and cylinder were of the same material. 
         [0098]    Within these constraints the residual stress fields were found to be qualitatively substantially the same for the various annulus section sides (this observation applying to the cases where annuli and cylinders were used and to the cases where only annuli were used). See  FIG. 23  for an example, where the annulus of side length 2.00 mm gives qualitatively substantially the same stress fields as for that of side length 3.00 mm. It was deemed most practical to choose the largest of the explored annulus/cylinder sizes as, in practice, these would facilitate manufacturing, considering the small sizes of the annuli and cylinders. For this reason all results displayed are for annulus section side of 3.00 mm. 
         [0099]      FIGS. 24   b ,  25   b ,  26   b  and  27   b  respectively show these four Cases. 
         [0100]    Selected contours of the residual tensile and compressive stresses are shown in  FIGS. 23 to 27 . In these contour plots, the densely-dotted areas represent regions of compressive Maximum Principal Stress, and the white areas represent regions of tensile Maximum Principal Stress. Hence the heavily-dotted lines around the densely-dotted areas indicate zero stress contours. To provide some resolution in the tensile regions, contours of +100 MPa, +350 Mpa and +600 Mpa are shown. 
         [0101]    In addition, a maximum and a minimum stress location is shown for each Case. The values corresponding to these are shown in Table 2. It will be seen that the maximum stress occurs on the interface of the PCD body and the substrate. Where there is an annulus present, this point is inside the annulus at the junction of the PCD body, the annulus and the substrate, the stress gradient moving from here into the bulk of the PCD as intended. By contrast, in the absence of an annulus, as shown in  FIGS. 23   a  to  27   a , the point of maximum tensile stress is on the outer diameter of the cutter, resulting in an increased risk of delaminations. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Maximum Principal Stress (Mpa) 
                   
               
             
          
           
               
                   
                 Case 
                 Figure 
                 Minimum 
                 Maximum 
               
               
                   
                   
               
             
          
           
               
                   
                 0 
                 — 
                 −54 
                 +754 
               
               
                   
                 1 
                 24 
                 −260 
                 +1,632 
               
               
                   
                 2 
                 25 
                 −142 
                 +1,234 
               
               
                   
                 3 
                 26 
                 −280 
                 +1,609 
               
               
                   
                 4 
                 27 
                 −123 
                 +1,163 
               
               
                   
                   
               
             
          
         
       
     
         [0102]    Of these Cases, Case 1 ( FIG. 24   b ) probably shows the best combination of stress features and design simplicity relative to the benchmark (shown in  FIG. 24   a ). 
         [0103]    For Case 1:
       1. The tensile stress on the interface outer diameter is reduced (which may therefore reduce the probability of delamination of the PCD body from the substrate).   2. The compressive region in the PCD body is removed (which may assist in preventing the deflection of cracks into the PCD body towards the working surface, such deflection being an observed failure mechanism in conventional PCD constructions).   3. The stresses in the bulk substrate material enclosed by the annulus are particularly high with a large gradient away from the PCD body.   4. The stresses in the annulus itself are low tensile to compressive in nature, which may act to keep cracks which may develop in use, substantially away from the substrate outer surface in the region of the PCD body (again reducing the probability of delamination).       
 
         [0108]    Case 2 ( FIG. 25   b ) shows the same design as Case 1 but with a material which is more similar to the bulk of the substrate, namely WC6 wt % Co. The observations are as follows:
       1. The tensile stress on the interface outer diameter is reduced but by less than in the Case 1.   2. The compressive region in the PCD body is not removed.   3. The stresses in the bulk substrate material enclosed by the annulus remain high with a large gradient away from the PCD body.   4. The stresses in the annulus itself are quite low tensile to compressive in nature, keeping cracks substantially away from the substrate outer surface in the region of the PCD body.   5. Again only one feature (the annulus) is introduced into the substrate.       
 
         [0114]    Case 3 ( FIG. 26 ) shows a variation of the design in Case 1, namely a central right cylinder is included in the substrate in addition to the annulus and the cylinder is in contact with the PCD body along the interface with the substrate. The follow observations were made:
       1. The tensile stress on the interface outer diameter is reduced (which may assist in reducing the probability of delamination).   2. The compressive region near the outside diameter of the PCD body is removed (which may assist in preventing the deflection of cracks into the PCD body towards the working surface, such deflection being an observed failure mechanism in conventional PCD constructions).   3. The stresses in the bulk substrate material enclosed by the annulus and cylinder are particularly high with a large gradient away from the PCD body.   4. The stresses in the annulus itself are low tensile to compressive in nature, keeping cracks substantially away from the substrate outer surface in the region of the PCD body (which may also reduce the probability of delamination).   5. The consequent change in stress state is that the volume of the cylinder is almost totally compressive, with a small volume compressive “dome” of material just above it (in the orientation shown) in the core of the PCD body.       
 
         [0120]    Case 4 ( FIG. 27 ) shows the same design as Case 3 but with a material which is more similar to the bulk of the substrate, namely WC6 wt % Co in place of DEC. Alternatively, Case 4 may be viewed as the design and materials of Case 2 with a central right cylinder included in the substrate abutting the PCD body along the interface with the substrate. In this case:
       1. The stress distributions and state in the annulus, adjacent PCD body and substrate are qualitatively much the same as in  FIG. 24   b.      2. The effect of the cylinder seems quite independent of the annulus. It results in a small compressive zone in the cylinder and a quite large compressive zone in the region directly above the cylinder (in the orientation shown), which may be beneficial for arresting cracks should they penetrate that far into the PCD body.   3. The high tensile stress contours are more confined within the annulus than in  FIG. 23   b , which may assist in reducing the risk of delamination of the substrate.       
 
         [0124]    The PCD constructions of Cases 0 to 4 mentioned above may for example, be constructed in the manner described below: 
       Baseline (Case 0). 
       [0125]    About 2 g of a multimodal diamond powder mix with average size of approximately 5 μm admixed with approximately 1 weight percent cobalt was poured into a Niobium inner cup and a cemented carbide substrate was placed on top of the inner cup and a Niobium outer cup placed over this, sealed and the canister pre-treated in an oven (vacuum outgassing at approximately 1050° C.). The canister was sintered at approximately 5.5 GPa and 1450° C. to produce a well-sintered PCD table. The cutter was not subjected to any leaching treatment. The cutter was subjected to a wear test, with the cutter suitably prepared as would be appreciated by the skilled person, to machine a granite block mounted on a vertical turret milling apparatus and counting the number of passes before failure. The wear resistance thus measured serves as a baseline for comparison with other cases. 
       Cases 1 and 2: 
       [0126]    As a first step a green body annulus would be prepared consisting of the powders required to achieve the DEC or WC6 wt % Co material in Table 1 mixed with suitable binders and pressed flat and to the required dimensions (for example a 3 mm annulus side section). A cylinder of the substrate powders with suitable binders, such as a cobalt based alloy would also be pressed flat such that it fitted inside the annulus with the same thickness. About 2 g of a multimodal diamond powder mix with average size of approximately 5 μm admixed with approximately 1 weight percent cobalt would be poured into a Niobium inner cup and compacted to achieve flatness. The green bodies, namely the DEC or WC6 wt % Co annulus and tungsten carbide disc would then be placed flatly on top of the compacted diamond admix. A cemented carbide substrate of approximately 3 mm reduced height would then be placed on top of the inner cup and the open arrangement vacuum outgassed at approximately 1050° C. A Niobium outer cup would be placed over this, sealed and the canister would be sintered at approximately 5.5 GPa and 1450° C. to produce well-sintered PCD table, annulus and carbide disc, with the individual parts well-sintered to one another. The cutter would be subjected to a wear test, with the cutter suitably prepared as would be appreciated by the skilled person, to machine a granite block mounted on a vertical turret milling apparatus and counting the number of passes before failure. The wear resistance thus measured should indicate a significant increase in passes achieved with respect to the baseline. 
       Cases 3 and 4: 
       [0127]    As a first step a green body annulus and cylinder would be prepared consisting of the powders required to achieve the DEC or WC6 wt % Co material in Table 1 mixed with a binder such as a cobalt based alloy and pressed flat and to the required dimensions (for example 3 mm annulus side section; cylinder 3 mm radius and 3 mm thick). An annulus of the substrate powders with suitable binders would also be pressed flat such that it fitted inside the annulus and around the cylinder, with the same thickness. About 2 g of a multimodal diamond powder mix with average size of approximately 5 μm admixed with approximately 1 weight percent cobalt would be poured into a Niobium inner cup and compacted to achieve flatness. The green bodies, namely the annulus, cylinder and tungsten carbide-cobalt ring would then be placed flatly on top of the compacted diamond admix. A cemented carbide substrate of approximately 3 mm reduced height would then be placed on top of the inner cup and the open arrangement vacuum outgassed at approximately 1050° C. A Niobium outer cup would be placed over this, sealed and the canister would be sintered at approximately 5.5 GPa and 1450° C. to produce well-sintered PCD table, annulus and carbide disc, with the individual parts well-sintered to one another. The cutter would be subjected to a wear test, with the cutter suitably prepared as would be appreciated by the skilled person, to machine a granite block mounted on a vertical turret milling apparatus and counting the number of passes before failure. The wear resistance thus measured should indicate a significant increase in passes achieved with respect to the baseline. 
         [0128]    The one or more solid of revolution may have parameters for example geometry, size, position and material properties such that any propagating cracks in the PCD body may tend to be diverted into the substrate, that is, away from the free surfaces of the PCD material. Coefficient of Thermal Expansion (CTE) differences between the bulk of the substrate and the one or more solids of revolution therein may be used to divert the cracks in use. The substrate materials may therefore be considered to be functionally graded either in a monotonic manner or more generally with advantageous maxima and minima. Residual stress fields that are more tensile or less compressive may thereby be formed in certain regions of the substrate, on and away from the interface with the PCD body. 
         [0129]    Whilst various embodiments have been described above with reference to example which are not intended to be limiting, it will be appreciated that many variations may be made. For example, the one or more solids of revolution are not restricted as to position in the substrate and may, for example, touch the body of PCD material or the substrate at one point, or a line segment, or over a circle or a surface or combinations of these. Furthermore, in other embodiments, the one or more solids of revolution may penetrate the body of PCD material and/or the substrate. 
         [0130]    In addition, the materials that are used to form the solids of revolution are not restricted to cemented carbides. Suitable materials may include but are not limited to various grades of PCD, partially or fully leached PCD, diamond-enhanced carbides (DEC&#39;s), PCBN, cBN, cemented tungsten carbides, other carbides, nitrides, borides and carbonitrides of Groups 4, 5 and 6 transition metals, refractory metals; and/or one or more of the following may also be used: Cr3C2, NbN, ZrO2, TiN, Cr2N, Al2O3, VN, Mo2C, TiC, VC, ZrN, NbC, HfN, HfC, ZrC, TaC, WC, AIN, B4C, SiC, TaN, Si3N4, CrN (these latter materials conveniently covering a wide range of Coefficients of Thermal Expansion). 
         [0131]    In some embodiments, if a solid of revolution is of unleached, partially leached or leached PCD, then this PCD may be of different microstructure, for example different binder, binder content, grain size, pool size, from any PCD in the body of PCD material attached to the substrate. 
         [0132]    In some embodiments, if the bulk of the substrate is cemented tungsten carbide, then one or more solids of revolution of cemented tungsten carbide may be included having different microstructure, for example different binder, binder content, grain size, pool size, from those of the cemented tungsten carbide constituting the bulk of the substrate. 
         [0133]    In some embodiments, if the bulk of the substrate is not of cemented tungsten carbide then a solid of revolution may be of the same type of material as the bulk of the substrate, but may have different microstructure such as different grain size, additives and the like from the material constituting the bulk of the bulk of the substrate. 
         [0134]    In some embodiments, a given solid of revolution may be of one continuous material including mixtures and/or compounds of materials whilst other options include laminates of two or more bonded materials, fibres and/or macroscopic inclusions bonded into the bulk of the solid of revolution. 
         [0135]    In some embodiments, in terms of material properties the one or more solid of revolution is primarily identified by different Coefficients of Thermal Expansion, though two or more may have the same Coefficients of Thermal Expansion. 
         [0136]    In some embodiments, for example where a solid of revolution is formed of a PCD material, this may be but is not required to be differentiated from other PCD grades in the PCD construction in terms of contiguity in particular of diamond particles. 
         [0137]    In some embodiments, where PCD materials are used in any solids of revolution then they may have significantly different Coefficients of Thermal Expansion while having statistically the same average particle sizes. This may be due to de facto cases where these conditions hold or by introducing additives such as for example TiC and/or VC which may result in PCD materials with the same statistical average particle sizes but significantly different Coefficients of Thermal Expansion. Another method for achieving this embodiment is to leach or partially leach one or more PCD materials. 
         [0138]    In some embodiments, a solid of revolution positioned close to the body of PCD material may be separated from the PCD body by a thin layer of material with low shear strength compared to the adjoining body of PCD material. 
         [0139]    Extensive use may be made of chamfers or radii or fillets or combinations of these to manage stress concentrations in all embodiments. 
         [0140]    In summary, a number of the embodiments disclosed herein are solids of revolution coaxial with the PCD construction assuming the cutting tool to be axisymmetric: for example but not limited to annuli, a central cylinder, cone, truncated cone or hollow cone, all of any geometrically-allowable cross sectional shape, size and any of the materials described above. More complex embodiments disclosed herein include a right central cone with a truncated right central cone removed axisymmetrically from its core and a truncated right central cone with a right central cone removed axisymmetrically from its core. 
         [0141]    Also disclosed herein are solids of revolution having symmetry axes not coinciding with or parallel to (or not coinciding with and not parallel to) any such symmetry axis in the PCD construction as a whole and/or the substrate as a whole for example annuli, discs, cylinders, cones, hollow cones and cup shapes distributed such that their axes are directed as required by the design parameters for each one selected to provide a desired stress distribution. Further complex embodiments herein disclosed include any of the solids disclosed above cut by surfaces to produce new solids for example wedge shapes, discs with portions sliced off by planes and the like. 
         [0142]    The positioning of materials in the substrate is to assist in managing residual stresses in the PCD construction. In particular, the positioning is to assist in setting up the residual stresses to manage the propagation of cracks away from the surfaces of the body of PCD material, for example into the bulk of the substrate.