WO2014020021A2

WO2014020021A2 - Superhard constructions & methods of making same

Info

Publication number: WO2014020021A2
Application number: PCT/EP2013/065997
Authority: WO
Inventors: Nedret Can
Original assignee: Element Six Abrasives S.A.
Priority date: 2012-07-31
Filing date: 2013-07-30
Publication date: 2014-02-06
Also published as: US20170067294A1; CN104661776A; WO2014020021A3; GB201213596D0; GB2506496B; GB201313571D0; GB2506496A; US20150246427A1

Abstract

A superhard polycrystalline construction comprises a body of polycrystalline superhard material formed of a mass of superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, the superhard grains having an associated mean free path; and a non-superhard phase at least partially filling a plurality of the interstitial regions and having an associated mean free path. The average grain size of the superhard grains is less than or equal to 25 microns; and the ratio of the standard deviation in the mean free path associated with the non-superhard phase to the mean of the mean free path associated with the non-superhard phase is greater than or equal to 80% when measured using image analysis techniques at a magnification of 1000. There is also disclosed a method of forming such a superhard polycrystalline construction.

Description

SUPERHARD CONSTRUCTIONS & METHODS OF MAKING SAME

Field

This disclosure relates to superhard constructions and methods of making such constructions, particularly but not exclusively to constructions comprising polycrystalline diamond (PCD) structures attached to a substrate and for use as cutter inserts or elements for drill bits for boring into the earth.

Background

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.

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.

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.

1 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 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.

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 diamond or polycrystalline CBN layer.

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.

Ever increasing drives for improved productivity in the earth boring field create ever increasing demands on the materials used for cutting rock. Specifically, PCD materials with improved abrasion and impact resistance are required to achieve faster cut rates and longer tool life.

Cutting elements for use in rock drilling and other operations require high abrasion resistance and impact resistance. One of the factors limiting the success

2 of the polycrystalline diamond (PCD) abrasive cutters is the generation of heat due to friction between the PCD and the work material. This heat causes the thermal degradation of the diamond layer. The thermal degradation increases the wear rate of the cutter through increased cracking and spalling of the PCD layer as well as back conversion of the diamond to graphite causing increased abrasive wear.

Methods used to improve the abrasion resistance of a PCD composite often result in a decrease in impact resistance of the composite. There is a need for a PCS composite that has improved abrasion resistance and impact resistance and a method of forming such composites.

Summary

Viewed from a first aspect there is provided a superhard polycrystalline construction comprising a body of polycrystalline superhard material formed of: a mass of superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, the superhard grains having an associated mean free path;

a non-superhard phase at least partially filling a plurality of the interstitial regions and having an associated mean free path;

wherein:

the average grain size of the superhard grains is less than or equal to 25 microns; and

the ratio of the standard deviation in the mean free path associated with the non-superhard phase to the mean of the mean free path associated with the non- superhard phase is greater than or equal to 80% when measured using image analysis techniques at a magnification of 1000.

Viewed from a second aspect there is provided a method of forming a superhard polycrystalline construction, comprising:

providing a mass of grains of superhard material comprising a first fraction having a first average size and a second fraction having a second average size, arranging the mass of superhard grains to form a pre-sinter assembly; and treating the pre-sinter assembly in the presence of a catalyst/solvent material for the superhard grains at an ultra-high pressure of around 6 GPa or greater and a temperature at which the superhard material is more

3 thermodynamically stable than graphite to sinter together the grains of superhard material to form a polycrystalline superhard construction, the superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, a non-superhard phase at least partially filling a plurality of the interstitial regions; wherein the non-superhard phase has an associated mean free path; and

wherein:

Viewed from a further aspect there is provided a tool comprising the superhard polycrystalline construction defined above, the tool being for cutting, milling, grinding, drilling, earth boring, rock drilling or other abrasive applications.

The tool may comprise, for example, a drill bit for earth boring or rock drilling, a rotary fixed-cutter bit for use in the oil and gas drilling industry, or a rolling cone drill bit, a hole opening tool, an expandable tool, a reamer or other earth boring tools.

Viewed from another aspect there is provided a drill bit or a cutter or a component therefor comprising the superhard polycrystalline construction defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 is a polycrystalline diamond (PCD) structure attached to a substrate;

Figure 2 shows a schematic cross-section view of an example of a portion of a PCD structure;

Figure 3 shows a schematic longitudinal cross-section view of an example of a PCD element;

4 Figure 4 shows a schematic longitudinal cross-section view of an example of a PCD element;

Figure 5 shows a schematic perspective view of part of an example of a drill bit for boring into the earth;

Figure 6A shows a schematic longitudinal cross-section view of an example of a pre-sinter assembly for a PCD element;

Figure 6B shows a schematic longitudinal cross-section view of an example of a PCD element;

Figure 7A, 7B, 7C and 7D show schematic cross-section views of parts of examples of PCD structures;

Figure 8 is a perspective view of a cutting element having a non-planar interface according to one example;

Figure 9a is a perspective view of the plurality of projections of Figure 8 in free space;

Figure 9b is a schematic plan view of the substrate of the cutting element of Figure 8;

Figure 9c is a schematic cross-sectional view of the substrate along the axis A-A shown in Figure 9b;

Figure 9d is a schematic perspective view of the substrate of the cutting element of Figure 8;

Figure 10 is a perspective view of a cutting element according to an example;

Figure 1 1 is a perspective view of a substrate according to a further example;

Figure 12a is a perspective view of a substrate of a cutting element according to a further embodiment;

Figure 12b is a schematic plan view of the substrate of the cutting element of Figure 12a;

Figure 12c is a schematic cross-sectional view of the substrate along the axis A-A shown in Figure 12b;

Figure 13 is an interval plot of chipping height for an example and two conventional reference cutters;

5 Figure 14 is a plot from a high energy drop test showing pass rate against drop energy for an example and two conventional reference cutters; and

Figure 15 is a plot of depth of penetration against rate of penetration for an example and five conventional reference cutters.

DESCRIPTION

As used herein, a "superhard material" is a material having a Vickers hardness of at least about 28 GPa. Diamond and cubic boron nitride (cBN) material are examples of superhard materials.

As used herein, a "superhard construction" means a construction comprising a body of polycrystalline superhard material. In such a construction, a substrate may be attached thereto or alternatively the body of polycrystalline material may be free-standing and unbacked.

As used herein, polycrystalline diamond (PCD) is a type of polycrystalline superhard (PCS) material comprising a mass of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume percent of the material. In one embodiment of PCD material, interstices between the diamond grains may be at least partly filled with a binder material comprising a catalyst for diamond. As used herein, "interstices" or "interstitial regions" are regions between the diamond grains of PCD material. In embodiments of PCD material, interstices or interstitial regions may be substantially or partially filled with a material other than diamond, or they may be substantially empty. PCD material may comprise at least a region from which catalyst material has been removed from the interstices, leaving interstitial voids between the diamond grains.

As used herein, PCBN (polycrystalline cubic boron nitride) material refers to a type of superhard material comprising grains of cubic boron nitride (cBN) dispersed within a matrix comprising metal or ceramic. PCBN is an example of a superhard material.

A "catalyst material" for a superhard material is capable of promoting the growth or sintering of the superhard material.

The term "substrate" as used herein means any substrate over which the superhard material layer is formed. For example, a "substrate" as used herein may

6 be a transition layer formed over another substrate. Additionally, as used herein, the terms "radial" and "circumferential" and like terms are not meant to limit the feature being described to a perfect circle.

The superhard construction 1 shown in the Figure 1 may be suitable, for example, for use as a cutter insert for a drill bit for boring into the earth.

Like reference numbers are used to identify like features in all drawings.

In an embodiment as shown in Figure 1 , a cutting element 1 includes a substrate 10 with a body of superhard material 12 formed on the substrate 10. The substrate may be formed of a hard material such as cemented tungsten carbide. The superhard material may be, for example, polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN), or a thermally stable product such as thermally stable PCD (TSP). The cutting element 1 may be mounted into a bit body such as a drag bit body (not shown). The exposed top surface of the superhard material opposite the substrate forms the cutting face 14, which is the surface which, along with its edge 16, performs the cutting in use.

At one end of the substrate 10 is an interface surface 17 that interfaces with the body of superhard material 12 which is attached thereto at this interface surface. The substrate 10 is generally cylindrical and has a peripheral surface 18 and a peripheral top edge 19.

The grains of superhard material, such as diamond grains or particles in the starting mixture prior to sintering may be, for example, bimodal, that is, the feed comprises a mixture of a coarse fraction of diamond grains and a fine fraction of diamond grains. In some embodiments, the coarse fraction may have, for example, an average particle/grain size ranging from about 10 to 60 microns. By "average particle or grain size" it is meant that the individual particles/grains have a range of sizes with the mean particle/grain size representing the "average". The average particle/grain size of the fine fraction is less than the size of the coarse fraction, for example between around 1/10 to 6/10 of the size of the coarse fraction, and may, in some embodiments, range for example between about 0.1 to 20 microns.

In some embodiments, the weight ratio of the coarse diamond fraction to the fine diamond fraction ranges from about 50% to about 97% coarse diamond and the weight ratio of the fine diamond fraction may be from about 3% to about

7 50%. In other embodiments, the weight ratio of the coarse fraction to the fine fraction will range from about 70:30 to about 90:10.

In further embodiments, the weight ratio of the coarse fraction to the fine fraction may range for example from about 60:40 to about 80:20.

In some embodiments, the particle size distributions of the coarse and fine fractions do not overlap and in some embodiments the different size components of the compact are separated by an order of magnitude between the separate size fractions making up the multimodal distribution.

The embodiments may consist of at least a wide bi-modal size distribution between the coarse and fine fractions of superhard material, but some embodiments may include three or even four or more size modes which may, for example, be separated in size by an order of magnitude, for example, a blend of particle sizes whose average particle size is 20 microns, 2 microns, 200nm and 20nm.

In some embodiments, the average grain size of the aggregated mass of superhard grains is less than or equal to 25 microns. In some embodiments, the average grain size is between around 8 to 20 microns.

Sizing of diamond particles/grains into fine fraction, coarse fraction, or other sizes in between, may be through known processes such as jet-milling of larger diamond grains and the like.

In embodiments where the superhard material is polycrystalline diamond material, the diamond grains used to form the polycrystalline diamond material may be natural or synthetic.

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.

The body of superhard material 12 shown in Figure 1 may, in some embodiments, be a layered construction or have multiple regions, as described below and illustrated in Figures 2 to 5c. A first further embodiment is illustrated with reference to Figure 2, which shows an example of a PCD structure 12 comprising at least two spaced-apart compressed regions 21 in compressive

8 residual stress states and at least one tensioned region 22 in a tensile residual stress state. The tensioned region 22 is located between the compressed regions 21 and is joined to them.

Variations in mechanical properties of the PCD material such as density, elastic modulus, hardness and coefficient of thermal expansion (CTE) may be selected to achieve the configuration of a tensioned region between two compressed regions. Such variations may be achieved by means of variations in content of diamond grains, content and type of filler material, size distribution or mean size of the PCD grains, and using different PCD grades either on their own or in diamond mixes comprising a mixture of PCD grades.

With reference to Figure 3, a further example of a PCD structure 12 integrally joined to a cemented carbide support body 10. The PCD structure 12 comprises several compressed regions 21 and several tensioned regions 22 in the form of alternating (or inter-leaved) strata or layers. The PCD structure 12 may be substantially cylindrical in shape and located at a working end and defining a working surface 14. The PCD structure 12 may be joined to the support body 10 at a non-planar interface 17. The compressed and tensioned regions 21 , 22 have a thickness in the range from about 5 microns to about 200 or, in some embodiments, 300 microns and may be arranged substantially parallel to the working surface 14 of the PCD structure 12. A substantially annular region 26 may be located around a non-planar feature 31 projecting from the support body 10.

With reference to Figure 4, an example of a PCD element 1 comprises a PCD structure 12 integrally joined to a cemented carbide support body 10 at a non- planar interface 25 opposite a working surface 14 of the PCD structure 12. The PCD structure 12 may comprise about 10 to 20 alternating compressed and tensioned regions 21 , 22 in the form of extended strata or layers. A region 26 that, in this embodiment, does not contain strata may be located adjacent the interface 25. The strata 21 , 22 may be curved or bowed and yet generally aligned with the interface 25, and may intersect a side surface 27 of the PCD structure. Some of the strata may intersect the working surface 14.

In some embodiments, the region 26 may be of a substantially greater thickness than the individual strata or layers 21 , 22 and, in some embodiments, the thickness of the region comprising the alternating layers 21 , 22 may be of a greater

9 thickness than the thickness of the region 26 adjacent the cemented carbide support body 10 which forms a substrate for the PCD material.

In some embodiments, the region 26 adjacent the support body 10 may include multiple layers or strata (not shown) that are of substantially greater thickness than the individual layers or strata 21 , 22, for example, the layers 21 , 22 may have a thickness in the range from about 5 to 200 microns, and the layers in the region 26 adjacent the support body 10 may have a thickness of greater than about 200 microns.

In some embodiments, such as those shown in Figures 2 to 4, the alternating strata, 21 , 22 may have a thickness or thicknesses in the range of from about 5 to 300 microns with the diamond material being formed of PCD with two or more different average diamond grain sizes, for example a mixture of two or more grades of PCD. For example, strata 21 may be formed of an aggregated diamond mix having average diamond grain sizes A and B and strata 22 may also be formed of a diamond mix having average diamond grain sizes A and B but in a different ratio to that of strata 21. In an alternative embodiment, the strata 21 may be formed of a diamond mix having average diamond grain sizes A and B and the strata 22 may be formed of a diamond mix having an average diamond grain size C. It will be appreciated that any other sequence/mixture of two or more diamond grain sizes may be used to form the alternating layers 21 , 22. In these embodiments, the region 26 adjacent the support body 10 may be formed of a single layer substantially thicker than the individual strata 21 , 22, for example, greater than around 200 microns. Alternatively, the region 26 may be formed of multiple layers, individual layers or strata comprising diamond grains of average grain size A and B, and/or C as used to form the diamond mixes of the strata 21 , 22 or another material or diamond grain size may be used to form the layers in this region 26 adjacent the support body 10.

In some embodiments, the diamond layers or strata 21 , 22 and/or strata formed in region 26 adjacent the support body 10 (not shown), may include, for example, one or more of nanodiamond additions in the form of nanodiamond powder up to 20 wt%, salt systems, borides, metal carbides of Ti, V, Nb or any of the metals Pd or Ni.

In some embodiments, the strata 21 , 22 and/or strata formed in region 26 adjacent the support body 10 may lie in a plane substantially perpendicular to the

10 plane through which the longitudinal axis of the diamond construction 1 extends. The strata may be planar, curved, bowed, domed or distorted, for example, as a result of being subjected to ultra-high pressure during sintering. Alternatively, the alternating strata 21 , 22 may be aligned at a predetermined angle to the plane through which the longitudinal axis of the diamond construction 1 extends to influence performance through crack propagation control.

Figure 5 is a schematic representation of an example of a drill bit 39 for boring into the earth into which are inserted a plurality of cutter elements 1 of the type shown in Figure 1. The cutter elements 1 may comprise any of the variations shown in the remaining figures.

With reference to FIG 6A, an example of a pre-sinter assembly 40 for making a PCD element may comprise a support body 30, a region 46 comprising diamond grains packed against a non-planar end of the support body 30, and a plurality of alternating diamond-containing aggregate masses in the general form of discs or wafers 41 , 42 stacked on the region 46. In some versions, the aggregate masses may be in the form of loose diamond grains or granules. The pre-sinter assembly may be heated to remove the binder material comprised in the stacked discs.

With reference to FIG 6B, an example of a PCD element 100 comprises a PCD structure 200 comprising a plurality of alternating strata 210, 220 formed of different respective multimodal grades of PCD material, and a portion 260 that does not comprise strata. The portion 260 may be cooperatively formed according to the shape of the non-planar end of the support body 300 to which it has integrally bonded during the treatment at the ultra-high pressure. The alternating strata 210, 220 of different grades of PCD or mixes of diamond grain sizes or grades are bonded together by direct diamond-to-diamond intergrowth to form an integral, solid and stratified PCD structure 200. The shapes of the PCD strata 210, 220 may be curved, bowed or distorted in some way as a result of being subjected to the ultra-high pressure. In some versions of the method, the aggregate masses may be arranged in the pre-sinter assembly to achieve various other configurations of strata within the PCD structure, taking into account possible distortion of the arrangement during the ultra-high pressure and high temperature treatment.

The strata 21 , 22, 210, 220 may comprise different respective PCD grades as a result of the different mean diamond grain sizes of the strata. Different

11 amounts of catalyst material may infiltrate into the different types of discs 410, 420 comprised in the pre-sinter assembly since they comprise diamond grains having different mean sizes, and consequently different sizes of spaces between the diamond grains. The corresponding alternating PCD strata 21 , 22, 210, 220 may thus comprise different, alternating amounts of catalyst material for diamond. The content of the filler material in terms of volume percent within the tensioned region may be greater than that within each of the compressed regions.

In one example, the compressed strata may comprise diamond grains having mean size greater than the mean size of the diamond grains of the tensioned strata

Whilst not wishing to be bound by a particular theory, when the stratified PCD structure is allowed to cool from the high temperature at which it was formed, the alternating strata containing different amounts of metal catalyst material may contract at different rates. This may be because metal contracts much more substantially than diamond does as it cools from a high temperature. This differential rate of contraction may cause adjacent strata to pull against each other, thus inducing opposing stresses in them.

The PCD element 100 described with reference to Figure 6B may be processed by grinding to modify its shape to form a PCD element substantially as described with reference to Figure 4. This may involve removing part of some of the curved strata to form a substantially planar working surface and a substantially cylindrical side surface. Catalyst material may be removed from a region of the PCD structure adjacent the working surface or the side surface or both the working surface and the side surface. This may be done by treating the PCD structure with acid to leach out catalyst material from between the diamond grains, or by other methods such as electrochemical methods. A thermally stable region, which may be substantially porous, extending a depth of at least about 50 microns or at least about 100 microns from a surface of the PCD structure, may thus be provided. Some embodiments with 50 to 80 micron thick layers in which this leach depth is around 250 microns have been shown to exhibit substantially improved performance, for example a doubling in performance after leaching over an unleached PCD product. In one example, the substantially porous region may comprise at most 2 weight percent of catalyst material.

12 The use of alternating layers or strata with different grain sizes through, for example, differences in binder content, may controllably give a different structure when acid leaching is applied to the PCD construction 1 , 100, especially for the embodiments in which the binder does not contain V and/or Ti. Such a structure may be created as a result of different residual tungsten in each layer during HCI acid leaching. In essence, the rate of leaching is likely to be different in each layer (unless HF-containing acid is used) and this may enable preferential leaching especially at the edges of the PCD material. This may be more pronounced for layers thicker than 120 microns. This is unlikely to occur if HF acid leaching were applied to the PCD material. The reason for this is that, in such a process, the HCI acid removes Co and leaves behind tungsten, whilst HF acid leaching would remove everything in the binder composition.

With reference to Figure 7A, an example variant of a PCD structure 200 comprises at least three substantially planar strata 210, 220 strata arranged in an alternating configuration substantially parallel to a working surface 240 of the PCD structure 200 and intersecting a side surface 270 of the PCD structure.

With reference to Figure 7B, an example variant of a PCD structure 200 comprises at least three strata 210, 220 strata arranged in an alternating configuration, the strata having a curved or bowed shape, with at least part of the strata inclined away from a working surface 240 and cutting edge 280 of the PCD structure.

With reference to Figure 7C, an example variant of a PCD structure 200 comprises at least three strata 210, 220 strata arranged in an alternating configuration, at least part of the strata inclined away from a working surface 240 of the PCD structure and extending generally towards a cutting edge 280 of the PCD structure.

With reference to Figure 7D, an example variant of a PCD structure 200 comprises at least three strata 210, 220 strata arranged in an alternating configuration, at least part of some of the strata being substantially aligned with a working surface 240 of the PCD structure and at least part of some of the strata generally aligned with a side surface 270 of the PCD structure. Strata may be generally annular of part annular and substantially concentric with a substantially cylindrical side surface 270 of the PCD structure 200.

13 The PCD structure may have a surface region proximate a working surface, the region comprising PCD material having a Young's modulus of at most about 1 ,050 MPa, or at most about 1 ,000 MPa. The surface region may comprise thermally stable PCD material.

Some examples of PCD structures may have at least 3, at least 5, at least 7, at least 10 or even at least 15 compressed regions, with tensioned regions located between them.

In some embodiments, each stratum or layer may have a thickness of at least about 5 microns, in others at least about 30 microns, in others at least about 100 microns, or in others at least about 200 microns. In some embodiments, each stratum or layer may have a thickness of at most about 300 microns or at most about 500 microns. In some example embodiments, each stratum or layer may have a thickness of at least about 0.05 percent, at least about 0.5 percent, at least about 1 percent or at least about 2 percent of a thickness of the PCD structure measured from a point on a working surface at one end to a point on an opposing surface. In some embodiments, each stratum or layer may have a thickness of at most about 5 percent of the thickness of the PCD structure.

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. In some examples of PCD elements, at least one compressed region may have a compressive residual stress of at least about 50 MPa, at least about 100 MPa, at least about 200 MPa, at least about 400 MPa or even at least about 600 MPa. The difference between the magnitude of the residual stress of adjacent strata may be at least about 50 MPa, at least about 100 MPa, at least about 200 MPa, at least about 400 MPa, at least about 600 MPa, at least about 800 MPa or even at least about 1 ,000 MPa. In one example, at least two successive compressed regions or tensioned regions may have different residual stresses. The PCD structure may comprise at least three compressed or tensioned regions each having a different residual compressive stress, the regions arranged in increasing or decreasing order of compressive or tensile stress magnitude, respectively.

14 In one example, each of the regions may have a mean toughness of at most 16 MPa.m1/2. In some embodiments, each of the regions may have a mean hardness of at least about 50 GPa, or at least about 60 GPa. Each of the regions may have a mean Young's modulus of at least about 900 MPa, at least about 950 MPa, at least about 1 ,000 or even at least about 1 ,050 MPa.

As used herein, "transverse rupture strength" (TRS) is measured by subjecting a specimen in the form of a bar having width W and thickness T to a load applied at three positions, two on one side of the specimen and one on the opposite side, and increasing the load at a loading rate until the specimen fractures at a load P. The TRS is then calculated based on the load P, dimensions of the specimen and the span L, which is the distance between the two load positions on one side. Such a measurement may also be referred to as a three-point bending test and is described by D. Munz and T. Fett in "Ceramics, mechanical properties, failure behaviour, materials selection" (1999, Springer, Berlin). The TRS corresponding to a particular grade of PCD material is measured measuring the TRS of a specimen of PCD consisting of that grade.

While the provision of a PCD structure with PCD strata having alternating compression and tensile stress states tends to increase the overall effective toughness of the PCD structure, this may have the effect of increasing the potential incidence of de-lamination, in which the strata may tend to come apart. While wishing not to be bound by a particular theory, de-lamination may tend to arise if the PCD strata are not sufficiently strong to sustain the residual stress between them. This effect may be ameliorated by selecting the PCD grades, and the PCD grade or grades of which the tensioned region in particular is formed, to have sufficiently high TRS. The TRS of the PCD grade or grades of which the tensioned region is formed should be greater than the residual tension that it may experience. One way of influencing the magnitude of the stress that a region may experience is by selecting the relative thicknesses of adjacent regions. For example, by selecting the thickness of a tensioned region to be greater than that of the adjacent compressive regions is likely to reduce the magnitude of tensile stress within the tensioned region.

The residual stress states of the regions may vary with temperature. In use, the temperature of the PCD structure may differ substantially between points proximate a cutting edge and points remote from the cutting edge. In some uses,

15 the temperature proximate the cutting edge may reach several hundred degrees centigrade. If the temperature exceeds about 750 degrees centigrade, diamond material in the presence of catalyst material such as cobalt is likely to convert to graphite material, which is not desired. Therefore, in some uses, the alternating stress states in adjacent regions as described herein should be considered at a temperature of up to about 750 degrees centigrade.

The K1 C toughness of a PCD disc is measured by means of a diametral compression test, which is described by Lammer ("Mechanical properties of polycrystalline diamonds", Materials Science and Technology, volume 4, 1988, p. 23.) and Miess (Miess, D. and Rai, G., "Fracture toughness and thermal resistances of polycrystalline diamond compacts", Materials Science and Engineering, 1996, volume A209, number 1 to 2, pp. 270-276).

Young's modulus is a type of elastic modulus and is a measure of the uniaxial strain in response to a uni-axial stress, within the range of stress for which the material behaves elastically. A preferred method of measuring the Young's modulus E is by means of measuring the transverse and longitudinal components of the speed of sound through the material, according to the equation £ = 2p.C_T ²(1 + υ), where : υ = (1 - 2 (Cj I Cif) / (2 - 2 (Cj l C_L)²), C_L and C_T are respectively the measured longitudinal and transverse speeds of sound through it and p is the density of the material. The longitudinal and transverse speeds of sound may be measured using ultrasonic waves, as is well known in the art. Where a material is a composite of different materials, the mean Young's modulus may be estimated by means of one of three formulas, namely the harmonic, geometric and rule of mixtures formulas as follows:

E = M { f₁ /E₁ + f₂ /E₂) ); E = E₁ ^f1 + E, ^a; and £ = j E* + f₂ E₂,

in which the different materials are divided into two portions with respective volume fractions of fi and f₂, which sum to one.

As described herein, the interface between the body of superhard material and the substrate may be substantially planar or non-planar. Examples of non- planar interface designs are described and illustrated with reference to Figures 8 to 12c.

16 In the embodiments described herein, when projections or depressions are described as being formed on the substrate surface, it should be understood that they could be formed instead on the surface of the super-hard material layer that interfaces with the substrate interface surface, with the inverse features formed on the substrate. Additionally, it should be understood that a negative or reversal of the interface surface is formed on the superhard material layer interfacing with the substrate such that the two interfaces form a matching fit.

As shown in the embodiment illustrated in Figure 8, the cutting element 400 includes the substrate 410 with the layer of super-hard material 412 formed on the substrate 410. At one end of the substrate 410 is the interface surface 418 that interfaces with the superhard material layer 412 which is attached thereto at this interface surface. The substrate 410 is generally cylindrical and has a peripheral surface 420 and a peripheral top edge 422. In the embodiment shown in Figure 8, the interface surface 418 includes a plurality of spaced-apart projections 424 that are arranged in a substantially annular first array and are spaced from the peripheral edge 422, and a second or inner substantially annular array of projections 426 that are radially within the first array 424.

As shown in Figures 8 and 9a to 9d, in this embodiment the spaced-apart projections 424, 426 are arranged in two arrays which are disposed in two substantially circular paths around a central longitudinal axis of the substrate 410. However, the invention is not limited to this geometry, as, for example, the placement of the projections 424, 426 may be in an ordered non-annular array on the interface surface 418 or the projections may be randomly distributed thereon rather than in a substantially circular or other ordered array. Furthermore, in the embodiments where the projections are arranged in annular arrays, these may be elliptical or asymmetrical, or may be offset from the central longitudinal axis of the substrate 410. Also, whilst the projections 426 of the inner array are shown to be closer to the outer array 424 than to the longitudinal central axis of the substrate, in other embodiments the projections 426 of the inner array may be closer to the longitudinal central axis.

The projections 426 in the second array may be positioned to radially align with the spaces between the projections 424 in the first array. The projections 424, 426 and spaces may be staggered, with projections in one array overlapping spaces in the next array. This staggered or mis-aligned distribution of three-

17 dimensional features on the interface surface may assist in distributing compressive and tensile stresses and/or reducing the magnitude of the stress fields and/or arresting crack growth by preventing an uninterrupted path for crack growth.

As shown in Figures 8 and 9a to 9d, in these embodiments, all or a majority of the projections 424, 426 are shaped such that all or a majority of the surfaces of the projections are not substantially parallel to the cutting face 414 of the superhard material 412 or to the plane through which the longitudinal axis of the substrate extends. Also, in the embodiments shown in Figures 8 to 10 and 12a to 12c, the interface surface 418 in the spaces between projections is uneven. This may be interpreted as, but not limited to, covering one or more of these spaces being nonuniform, varying, irregular, rugged, not level, and/or not smooth, with peaks and troughs. This arrangement is thought to act to inhibit uninterrupted crack propagation along the interface surface 418 and to increase the contact surface area between the interface of the substrate 410 and the interface of the super hard material layer 412. Furthermore, it is believed that such a configuration acts to disturb 'elastic' wave formation in the material and deflect cracks at the interface. These spaces or uneven valleys separating each projection 424, 426 from the adjacent projections may be uniform in some embodiments and non-uniform in other embodiments.

The projections 424, 426 may have a smoothly curving upper surface or may have a sloping upper surface. In some embodiments, the projections 424, 426 may be slightly trapezoidal or tapered in shape, being widest nearer the interface surface from which they project.

In Figures 8 and 9a to 9d, the projections 424, 426 are spaced substantially equally in/round the respective substantially annular array, with each projection 424, 426 within a given array having the same dimension. However, the projections 424, 426 may be formed in any desired shape, as described above, and spaced apart from each other in a uniform or non-uniform manner to alter the stress fields over the interface surface 418. The projections 424 in the outer array are, as shown in the embodiment of Figures 8 and 9, larger in size than those in the inner array. However, these relative sizes may be reversed, or the projections 424, 426 in both arrays could be approximately of uniform size, or a mixture of sizes.

18 In the embodiment shown in Figures 8 and 9a to 9d, the outer array includes double the number of projections 424 than the inner layer, for example ten and five projections respectively. This permits the cutter element 400 to have pseudo axi-symmetry thereby providing freedom in positioning the cutter in the tool or drill bit in which it is to be used as it would not require specific orientation. The projections 424, 426 are positioned and shaped in such a way that they inhibit one or more continuous paths along which cracks could propagate across the interface surface 418. Also, in some embodiments, all or the majority of the projections and/or spaces therebetween do not have any surfaces which are substantially normal or parallel to any loads expected to be applied to the cutter element 400 in use, and nor which are substantially normal or parallel to any exterior surfaces thereof.

The arrangement and shape of the projections 424, 426 and spaces therebetween may affect the stress distributions in the cutting element 400 and may act to improve the cutting element's resistance to crack growth, in particular crack growth along the interface surface 418, for example by arresting or diverting crack growth across the stress zones in, around and above the projections 424, 426.

As shown in the embodiment of Figure 10, the depth of super hard material in the region around the central longitudinal axis of the substrate 410 may be substantially the same depth as the depth of the super hard material at the periphery of the super hard material layer 412. This may enable the volume and area of super hard material exposed to the work surface in use not to decrease significantly with wear progression thereby improving the lifespan of the cutter element 400. It may also assist in stiffening the cutter element 400 when loaded in the axial direction. Furthermore, it may assist in decreasing or substantially eliminating the possibility of grooving wear formation during use.

Another embodiment of a substrate 450 and interface surface 451 is shown in Figure 1 1. The interface surface 451 of the substrate 450 includes a plurality of adjacent rows of projections 454, each being substantially pyramidal in shape and abutting one or more adjacent projections along one side of its base along the surface 450 from which the projections 454 project. In this embodiment, all or a majority of the projections 454 do not have any surface substantially parallel to either the cutting face of the super hard layer (not shown) which will be attached

19 thereto, or the plane through which the longitudinal axis of the substrate 450 extends. The projections 454 may be all the same height or some may be of a greater height than others.

In a further embodiment (not shown), rather than the entire interface surface 418, 451 being covered by the projections 454 shown in Figure 1 1 , only a majority of the interface surface 418, 451 may be covered by the abutting projections 454 and any interface surface 418, 451 between any projections 454 or not covered by the projections 454 may be uneven, as described above with respect to Figures 8 to 10.

A further embodiment of a substrate is shown in Figures 12a to 12c. This embodiment differs from that shown in Figures 8 to 9d in that the shape of the projections 424, 426 extending from the interface surface 418 are of a differing shape and the number of projections 424 in the outer array are fewer than that shown in Figure 8. In the embodiment of Figures 12a to 12c, these projections 424, 426 have a peripheral shape having one or more non-planar faces.

In one or more of the above-described embodiments, the features of the interface surfaces 418, 451 may be formed integrally whilst the substrate is being formed through use of an appropriately shaped mold into which the particles of material to form the substrate are placed. Alternatively, the projections and uneven surfaces of the interface surface 418, 451 may be created after the substrate has been created or part way through the creation process, for example by a conventional machining process. Similar procedures may be applied to the superhard material layer 12 to create the corresponding shaped interface surface for forming a matching fit with that of the substrate.

The superhard material layer 12 may be attached to the substrate by, for example, conventional brazing techniques or by sintering using a conventional high pressure and high temperature technique.

The durability of the cutter product including the substrate and superhard material layer with the aforementioned interface features and/or the mitigation of elastic stress waves therein may be further enhanced if the superhard material layer 12 is leached of catalyst material, either partially or fully, in subsequent processing, or subjected to a further high pressure high temperature sintering process. The leaching may be performed whilst the super hard material layer 12 is

20 attached to the substrate or, for example, by detaching the super hard material layer 12 from the substrate, and leaching the detached super hard material layer 12. In the latter case, after leaching has taken place, the superhard material layer 12 may be reattached to the substrate using, for example, brazing techniques or by resintering using a high pressure and high temperature technique.

Although particular embodiments have been described and illustrated, it is to be understood that various changes and modifications may be made. For example, the substrate described herein has been identified by way of example. It should be understood that the super-hard material may be attached to other carbide substrates besides tungsten carbide substrates, such as substrates made of carbides of W, Ti, Mo, Nb, V, Hf, Ta, and Cr. Furthermore, although the embodiments shown in FIGS 1 to 12c are depicted in these drawings as comprising PCD structures having sharp edges and corners, embodiments may comprise PCD structures having rounded, bevelled or chamfered edges or corners. Such embodiments may reduce internal stress and consequently extend working life through improving the resistance to cracking, chipping, and fracturing of cutting elements through the interface of the substrate or the super hard material layer having unique geometries.

In some embodiments, the binder catalyst/solvent may comprise cobalt or some other iron group elements, such as iron or nickel, or an alloy thereof. Carbides, nitrides, borides, and oxides of the metals of Groups IV-VI in the periodic table are other examples of non-diamond material that might be added to the sinter mix. In some embodiments, the binder/catalyst/sintering aid may be Co.

The cemented metal carbide substrate may be conventional in composition and, thus, may be include any of the Group IVB, VB, or VIB metals, which are pressed and sintered in the presence of a binder of cobalt, nickel or iron, or alloys thereof. In some embodiments, the metal carbide is tungsten carbide.

In some embodiments, both the bodies of, for example, diamond and carbide material plus sintering aid/binder/catalyst are applied as powders and sintered simultaneously in a single UHP/HT process. The mixture of diamond grains and mass of carbide are placed in an HP/HT reaction cell assembly and subjected to HP/HT processing. The HP/HT processing conditions selected are sufficient to effect intercrystalline bonding between adjacent grains of abrasive particles and, optionally, the joining of sintered particles to the cemented metal

21 carbide support. In one embodiment, the processing conditions generally involve the imposition for about 3 to 120 minutes of a temperature of at least about 1200 degrees C and an ultra-high pressure of greater than about 5 GPa.

In another embodiment, the substrate may be pre-sintered in a separate process before being bonded together in the HP/HT press during sintering of the superhard polycrystalline material.

In a further embodiment, both the substrate and a body of polycrystalline superhard material are pre-formed. For example, the bimodal feed of superhard grains/particles with optional carbonate binder-catalyst also in powdered form are mixed together, and the mixture is packed into an appropriately shaped canister and is then subjected to extremely high pressure and temperature in a press. Typically, the pressure is at least 5 GPa and the temperature is at least around 1200 degrees C. The preformed body of polycrystalline superhard material is then placed in the appropriate position on the upper surface of the preform carbide substrate (incorporating a binder catalyst), and the assembly is located in a suitably shaped canister. The assembly is then subjected to high temperature and pressure in a press, the order of temperature and pressure being again, at least around 1200 degrees C and 5 GPa respectively. During this process the solvent/catalyst migrates from the substrate into the body of superhard material and acts as a binder-catalyst to effect intergrowth in the layer and also serves to bond the layer of polycrystalline superhard material to the substrate. The sintering process also serves to bond the body of superhard polycrystalline material to the substrate.

The practical use of cemented carbide grades with substantially lower cobalt content as substrates for PCD inserts is limited by the fact that some of the Co is required to migrate from the substrate into the PCD layer during the sintering process in order to catalyse the formation of the PCD. For this reason, it is more difficult to make PCD on substrate materials comprising lower Co contents, even though this may be desirable.

An embodiment of a superhard construction may be made by a method including providing a cemented carbide substrate, contacting an aggregated, substantially unbonded mass of diamond particles against a surface of the substrate to form an pre-sinter assembly, encapsulating the pre-sinter assembly in a capsule for an ultra-high pressure furnace and subjecting the pre-sinter assembly to a pressure of at least about 5.5 GPa and a temperature of at least about 1 ,250

22 degrees centigrade, and sintering the diamond particles to form a PCD composite compact element comprising a PCD structure integrally formed on and joined to the cemented carbide substrate. In some embodiments of the invention, the pre-sinter assembly may be subjected to a pressure of at least about 6 GPa, at least about 6.5 GPa, at least about 7 GPa or even at least about 7.5 GPa or greater.

In a further embodiment, both the substrate and a body of polycrystalline superhard material are pre-formed. For example, the bimodal or multimodal feed of superhard grains/particles with optional carbonate binder-catalyst also in powdered form are mixed together, and the mixture is packed in alternating layers or strata into an appropriately shaped canister and is then subjected to extremely high pressure and temperature in a press. Typically, the pressure is at least 5 GPa and the temperature is at least around 1200 degrees C. The preformed body of polycrystalline superhard material is then placed in the appropriate position on the upper surface of the preform carbide substrate (incorporating a binder catalyst), and the assembly is located in a suitably shaped canister. The assembly is then subjected to high temperature and pressure in a press, the order of temperature and pressure being again, at least around 1200 degrees C and 5 GPa respectively. During this process the solvent/catalyst migrates from the substrate into the body of superhard material and acts as a binder-catalyst to effect intergrowth in the layer and also serves to bond the layer of polycrystalline superhard material to the substrate. The sintering process also serves to bond the body of superhard polycrystalline material to the substrate.

A further example method for making a PCD element is now described. Aggregate masses in the form of sheets containing diamond grains held together by a binder material may be provided. The sheets may be made by a method known in the art, such as by extrusion or tape casting methods, in which slurries comprising diamond grains having respective size distributions suitable for making the desired respective bimodal or multimodal PCD grades, and a binder material is spread onto a surface and allowed to dry. Other methods for making diamond- containing sheets may also be used, such as described in United States patents numbers 5,766,394 and 6,446,740. Alternative methods for depositing diamond- bearing layers include spraying methods, such as thermal spraying. The binder material may comprise a water-based organic binder such as methyl cellulose or polyethylene glycol (PEG) and different sheets comprising diamond grains having

23 different size distributions, diamond content or additives may be provided. For example, at least two sheets comprising diamond having different mean sizes may be provided and first and second sets of discs may be cut from the respective first and second sheets. The sheets may also contain catalyst material for diamond, such as cobalt, and / or additives for inhibiting abnormal growth of the diamond grains or enhancing the properties of the PCD material. For example, the sheets may contain about 0.5 weight percent to about 5 weight percent of vanadium carbide, chromium carbide or tungsten carbide. In one example, each of the sets may comprise about 10 to 20 discs.

A support body comprising cemented carbide in which the cement or binder material comprises a catalyst material for diamond, such as cobalt, may be provided. The support body may have a non-planar end or a substantially planar proximate end on which the PCD structure is to be formed and which forms the interface. A non-planar shape of the end may be configured to reduce undesirable residual stress between the PCD structure and the support body. A cup may be provided for use in assembling the diamond-containing sheets onto the support body. The first and second sets of discs may be stacked into the bottom of the cup in alternating order. In one version of the method, a layer of substantially loose diamond grains may be packed onto the uppermost of the discs. The support body may then be inserted into the cup with the proximate end going in first and pushed against the substantially loose diamond grains, causing them to move slightly and position themselves according to the shape of the non-planar end of the support body to form a pre-sinter assembly.

The pre-sinter assembly may be placed into a capsule for an ultra-high pressure press and subjected to an ultra-high pressure of at least about 5.5 GPa and a high temperature of at least about 1 ,300 degrees centigrade to sinter the diamond grains and form a PCD element comprising a PCD structure integrally joined to the support body. In one version of the method, when the pre-sinter assembly is treated at the ultra-high pressure and high temperature, the binder material within the support body melts and infiltrates the strata of diamond grains. The presence of the molten catalyst material from the support body is likely to promote the sintering of the diamond grains by intergrowth with each other to form an integral, stratified PCD structure.

24 In some versions of the method, the aggregate masses may comprise substantially loose diamond grains, or diamond grains held together by a binder material. The aggregate masses of multimodal grains may be in the form of granules, discs, wafers or sheets, and may contain catalyst material for diamond and / or additives for reducing abnormal diamond grain growth, for example, or the aggregated mass may be substantially free of catalyst material or additives. In some embodiments, the aggregate masses may be assembled onto a cemented carbide support body.

In some embodiments, the pre-sinter assembly may be subjected to a pressure of at least about 6 GPa, at least about 6.5 GPa, at least about 7 GPa or even at least about 7.5 GPa or even greater.

The hardness of cemented tungsten carbide substrate may be enhanced by subjecting the substrate to an ultra-high pressure and high temperature, particularly at a pressure and temperature at which diamond is thermodynamically stable. The magnitude of the enhancement of the hardness may depend on the pressure and temperature conditions. In particular, the hardness enhancement may increase the higher the pressure. Whilst not wishing to be bound by a particular theory, this is considered to be related to the Co drift from the substrate into the PCD during press sintering, as the extent of the hardness increase is directly dependent on the decrease of Co content in the substrate.

In embodiments where the cemented carbide substrate does not contain sufficient solvent / catalyst for diamond, and where the PCD structure is integrally formed onto the substrate during sintering at an ultra-high pressure, solvent / catalyst material may be included or introduced into the aggregated mass of diamond grains from a source of the material other than the cemented carbide substrate. The solvent / catalyst material may comprise cobalt that infiltrates from the substrate in to the aggregated mass of diamond grains just prior to and during the sintering step at an ultra-high pressure. However, in embodiments where the content of cobalt or other solvent / catalyst material in the substrate is low, particularly when it is less than about 1 1 weight percent of the cemented carbide material, then an alternative source may need to be provided in order to ensure good sintering of the aggregated mass to form PCD.

Solvent / catalyst for diamond may be introduced into the aggregated mass of diamond grains by various methods, including blending solvent / catalyst

25 material in powder form with the diamond grains, depositing solvent / catalyst material onto surfaces of the diamond grains, or infiltrating solvent / catalyst material into the aggregated mass from a source of the material other than the substrate, either prior to the sintering step or as part of the sintering step. Methods of depositing solvent / catalyst for diamond, such as cobalt, onto surfaces of diamond grains are well known in the art, and include chemical vapour deposition (CVD), physical vapour deposition (PVD), sputter coating, electrochemical methods, electroless coating methods and atomic layer deposition (ALD). It will be appreciated that the advantages and disadvantages of each depend on the nature of the sintering aid material and coating structure to be deposited, and on characteristics of the grain.

In one embodiment of a method of the invention, cobalt may be deposited onto surfaces of the diamond grains by first depositing a pre-cursor material and then converting the precursor material to a material that comprises elemental metallic cobalt. For example, in the first step cobalt carbonate may be deposited on the diamond grain surfaces using the following reaction:

Co(N0₃)₂ + Na₂C0₃ -> C0CO3 + 2NaN0₃

The deposition of the carbonate or other precursor for cobalt or other solvent / catalyst for diamond may be achieved by means of a method described in PCT patent publication number WO/2006/032982. The cobalt carbonate may then be converted into cobalt and water, for example, by means of pyrolysis reactions such as the following:

CoC0₃ -> CoO + C0₂

CoO + H₂ -> Co + H₂0

In another embodiment of the method of the invention, cobalt powder or precursor to cobalt, such as cobalt carbonate, may be blended with the diamond grains. Where a precursor to a solvent / catalyst such as cobalt is used, it may be necessary to heat treat the material in order to effect a reaction to produce the solvent / catalyst material in elemental form before sintering the aggregated mass.

In some embodiments, the cemented carbide substrate may be formed of tungsten carbide particles bonded together by the binder material, the binder material comprising an alloy of Co, Ni and Cr. The tungsten carbide particles may form at least 70 weight percent and at most 95 weight percent of the substrate;.

26 The binder material may comprise between about 10 to 50 wt.% Ni, between about 0.1 to 10 wt.% Cr, and the remainder weight percent comprises Co. The size distribution of the tungsten carbide particles in the cemented carbide substrate ion some embodiments has the following characteristics:

• fewer than 17 percent of the carbide particles have a grain size of equal to or less than about 0.3 microns;

• between about 20 to 28 percent of the tungsten carbide particles have a grain size of between about 0.3 to 0.5 microns;

• between about 42 to 56 percent of the tungsten carbide particles have a grain size of between about 0.5 to 1 microns;

• less than about 12 percent of the tungsten carbide particles are greater than 1 micron; and

• the mean grain size of the tungsten carbide particles is about 0.6+0.2 microns.

In some embodiments, the binder additionally comprises between about 2 to 20 wt.% tungsten and between about 0.1 to 2 wt.% carbon

A layer of the substrate adjacent to the interface with the body of polycrystalline diamond material may have a thickness of, for example, around 100 microns and may comprise tungsten carbide grains, and a binder phase. This layer may be characterised by the following elemental composition measured by means of Energy-Dispersive X-Ray Microanalysis (EDX):

• between about 0.5 to 2.0 wt% cobalt;

• between about 0.05 to 0.5 wt.% nickel;

• between about 0.05 to 0.2 wt.% chromium; and

• tungsten and carbon.

In a further embodiment, in the layer described above in which the elemental composition includes between about 0.5 to 2.0 wt% cobalt, between about 0.05 to 0.5 wt.% nickel and between about 0.05 to 0.2 wt.% chromium, the remainder is tungsten and carbon.

The layer of substrate may further comprise free carbon.

27 The magnetic properties of the cemented carbide material may be related to important structural and compositional characteristics. The most common technique for measuring the carbon content in cemented carbides is indirectly, by measuring the concentration of tungsten dissolved in the binder to which it is indirectly proportional: the higher the content of carbon dissolved in the binder the lower the concentration of tungsten dissolved in the binder. The tungsten content within the binder may be determined from a measurement of the magnetic moment, σ, or magnetic saturation, M_s = 4πσ, these values having an inverse relationship with the tungsten content (Roebuck (1996), "Magnetic moment (saturation) measurements on cemented carbide materials", Int. J. Refractory Met, Vol. 14, pp. 419-424.). The following formula may be used to relate magnetic saturation, Ms, to the concentrations of W and C in the binder:

M_s oc [C]/[W] x wt.% Co x 201 .9 in units of μΤ.ηι³/Ι¾

The binder cobalt content within a cemented carbide material may be measured by various methods well known in the art, including indirect methods such as such as the magnetic properties of the cemented carbide material or more directly by means of energy-dispersive X-ray spectroscopy (EDX), or a method based on chemical leaching of Co.

The mean grain size of carbide grains, such as WC grains, may be determined by examination of micrographs obtained using a scanning electron microscope (SEM) or light microscopy images of metallurgically prepared cross- sections of a cemented carbide material body, applying the mean linear intercept technique, for example. Alternatively, the mean size of the WC grains may be estimated indirectly by measuring the magnetic coercivity of the cemented carbide material, which indicates the mean free path of Co intermediate the grains, from which the WC grain size may be calculated using a simple formula well known in the art. This formula quantifies the inverse relationship between magnetic coercivity of a Co-cemented WC cemented carbide material and the Co mean free path, and consequently the mean WC grain size. Magnetic coercivity has an inverse relationship with MFP.

As used herein, the "mean free path" (MFP) of a composite material such as cemented carbide is a measure of the mean distance between the aggregate carbide grains cemented within the binder material. The mean free path characteristic of a cemented carbide material may be measured using a

28 micrograph of a polished section of the material. For example, the micrograph may have a magnification of about 1000x. The MFP may be determined by measuring the distance between each intersection of a line and a grain boundary on a uniform grid. The matrix line segments, Lm, are summed and the grain line segments, Lg, are summed. The mean matrix segment length using both axes is the "mean free path". Mixtures of multiple distributions of tungsten carbide particle sizes may result in a wide distribution of MFP values for the same matrix content. This is explained in more detail below.

The concentration of W in the Co binder depends on the C content. For example, the W concentration at low C contents is significantly higher. The W concentration and the C content within the Co binder of a Co-cemented WC (WC- Co) material may be determined from the value of the magnetic saturation. The magnetic saturation 4πσ or magnetic moment σ of a hard metal, of which cemented tungsten carbide is an example, is defined as the magnetic moment or magnetic saturation per unit weight. The magnetic moment, σ, of pure Co is 16.1 micro-Tesla times cubic metre per kilogram ^T.m³/kg), and the induction of saturation, also referred to as the magnetic saturation, 4πσ, of pure Co is 201 .9 μΤ.ιτι³^.

In some embodiments, the cemented carbide substrate may have a mean magnetic coercivity of at least about 100 Oe and at most about 145 Oe, and a magnetic moment of specific magnetic saturation with respect to that of pure Co of at least about 89 percent to at most about 97 percent.

A desired MFP characteristic in the substrate may be accomplished several ways known in the art. For example, a lower MFP value may be achieved by using a lower metal binder content. A practical lower limit of about 3 weight percent cobalt applies for cemented carbide and conventional liquid phase sintering. In an embodiment where the cemented carbide substrate is subjected to an ultra-high pressure, for example a pressure greater than about 5 GPa and a high temperature (greater than about 1 ,400°C for example), lower contents of metal binder, such as cobalt, may be achieved. For example, where the cobalt content is about 3 weight percent and the mean size of the WC grains is about 0.5 micron, the MFP would be about 0.1 micron, and where the mean size of the WC grains is about 2 microns, the MFP would be about 0.35 microns, and where the mean size of the WC grains is about 3 microns, the MFP would be about 0.7 microns. These mean grain sizes correspond to a single powder class obtained by natural comminution processes

29 that generate a log normal distribution of particles. Higher matrix (binder) contents would result in higher MFP values.

Changing grain size by mixing different powder classes and altering the distributions may achieve a whole spectrum of MFP values for the substrate depending on the particulars of powder processing and mixing. The exact values would have to be determined empirically.

In some embodiments, the substrate comprises Co, Ni and Cr.

The binder material for the substrate may include at least about 0.1 weight percent to at most about 5 weight percent one or more of V, Ta, Ti, Mo, Zr, Nb and Hf in solid solution.

In further embodiments, the polycrystalline diamond (PCD) composite compact element may include at least about 0.01 weight percent and at most about 2 weight percent of one or more of Re, Ru, Rh, Pd, Re, Os, Ir and Pt.

A polycrystalline construction according to some embodiments may have a specific weight loss in an erosion test in a recirculating rig generating an impinging jet of liquid-solid slurry below 2x10^"3 g/cm³ at the following testing conditions: a temperature of 50°C, an impingement angle of 45°, a slurry velocity of 20 m/s, a pH of 8.02, a duration of 3 hours, and a slurry composition in 1 cubic meter water of: 40 kg Bentonite; 2 kg Na2C03; 3 kg carboxymethyl cellulose, 5 litres.

Some embodiments of a cemented carbide body may be formed by providing tungsten carbide powder having a mean equivalent circle diameter (ECD) size in the range from about 0.2 microns to about 0.6 microns, the ECD size distribution having the further characteristic that fewer than 45 percent of the carbide particles have a mean size of less than 0.3 microns; 30 to 40 percent of the carbide particles have a mean size of at least 0.3 microns and at most 0.5 microns; 18 to 25 percent of the carbide particles have a mean size of greater than 0.5 microns and at most 1 micron; fewer than 3 percent of the carbide particles have a mean size of greater than 1 micron. The tungsten carbide powder is milled with binder material comprising Co, Ni and Cr or chromium carbides, the equivalent total carbon comprised in the blended powder being, for example, about 6 percent with respect to the tungsten carbide. The blended powder is then compacted to form a green body and the green body is sintered to produce the cemented carbide body.

30 The sintering the green body may take place at a temperature of, for example, at least 1 ,400 degrees centigrade and at most 1 ,440 degrees centigrade for a period of at least 65 minutes and at most 85 minutes.

In some embodiments, the equivalent total carbon (ETC) comprised in the cemented carbide material is about 6.12 percent with respect to the tungsten carbide.

The size distribution of the tungsten carbide powder may, in some embodiments, have the characteristic of a mean ECD of 0.4 microns and a standard deviation of 0.1 microns.

To assist in improving thermal stability of the sintered structure, the catalysing material may be removed from a region of the polycrystalline layer adjacent an exposed surface thereof. Generally, that surface will be on a side of the polycrystalline layer opposite to the substrate and will provide a working surface for the polycrystalline diamond layer. Removal of the catalysing material may be carried out using methods known in the art such as electrolytic etching, and acid leaching and evaporation techniques.

Embodiments are described in more detail below with reference to the following example which is provided herein by way of illustration only and is not intended to be limiting.

Example 1

A diamond powder mixture was prepared in the proportions given in Table 1 below.

Table 1

The diamond powder mixture was then placed into a suitable HpHT vessel, adjacent to a tungsten carbide substrate with a binder composition as given in Table 2.

31 9.5 - 2.75-3.15 0.25 -0.35

10

Table 2

and sintered at a pressure of around 6.8 GPa and a temperature of about 1500 degrees C.

PDC cutters made with the attributes stated above were made and tested in a chipping test. The results are shown in Figure 13. This test targets edge chipping resistance of sharp cutters by impacting at energy levels of 5 Joules and was repeated for up to 8 impacts. As shown in Figure 13, the results of this test confirmed at a statistically significant level that the embodiment tested had a higher edge chipping resistance compared to the tested conventional PDC materials shown in Figure 13 as Refs 1 and 2.

In order to test the integrity under impact loading of the finished PCD compacts formed according to the above example, a high energy drop test was performed on the cutter and two reference conventional cutters. The results are shown in the plot of Figure 14. It will be seen from Figure 14 that the cutter according to an embodiment showed a significantly greater resistance to impact loading at high energy than the conventional PDC cutters subjected to the same test, shown as Refs 1 and 2.

In connection with the above-performed tests, the reference conventional PCD compacts/cutters tested comprised Refs 1 and 2 which were sintered at a pressure of 5.5 GPa and were formed of a multi-modal mix of diamond grains with an average grain size of about 10 microns. The multi modal mix is as set out in Table 3 below:

Table 3

32 For both Refs 1 and 2, the substrate was a tungsten carbide substrate with 13 wt% Co binder and a tungsten carbide grain size of predominantly 1 to 4 microns. The magnetic properties of the substrate prior to sintering were:

Magnetic saturation (%): 1 1 .5 to 12.5

Magnetic coercivity (kA/m): 9.0 to 10.5

The differences between Refs 1 and 2 lay in the non-planar interface designs.

While laboratory testing showed the PDC cutter an embodiment to have superior chipping resistance and impact resistance, as illustrated in Figures 13 and 14, the cutter according to an embodiment is also determined to have significantly longer durability and good penetration rates, as shown from the test results of Figure 15 which is a plot of depth of penetration against rate of penetration for an embodiment and five conventional reference cutters. Observation of the cutters once the drill bit had reached a "dull condition" showed that most of the conventional cutters had failed due to large wear flats and not due to catastrophic failure. As the PDC cutter according to an embodiment had not failed due to catastrophic failure the repair costs to reuse the drill bit are significantly reduced. For example, such a cutter could be rotated and re-used in the drill bit for a second time.

Example cutters Ex1 to Ex 5 which were subjected to the test whose results are shown in Figure 15 are identical to Ref 1 , referred to above in connection with Figures 13 and 14 however, the cutter Ex 5 was mounted in a different drill bit design to those of Ex 1 to Ex 4 for the purposes of this test.

In polycrystalline diamond material, individual diamond particles/grains are, to a large extent, bonded to adjacent particles/grains through diamond bridges or necks. The individual diamond particles/grains retain their identity, or generally have different orientations. The average grain/particle size of these individual diamond grains/particles may be determined using image analysis techniques. Images are collected on a scanning electron microscope and are analysed using standard image analysis techniques. From these images, it is possible to extract a representative diamond particle/grain size distribution.

Generally, the body of polycrystalline diamond material will be produced and bonded to the cemented carbide substrate in a HPHT process. In so doing, it is

33 advantageous for the binder phase and diamond particles to be arranged such that the binder phase is distributed homogeneously and is of a fine scale.

The homogeneity or uniformity of the sintered structure is defined by conducting a statistical evaluation of a large number of collected images. The distribution of the binder phase, which is easily distinguishable from that of the diamond phase using electron microscopy, can then be measured in a method similar to that disclosed in EP 0974566. This method allows a statistical evaluation of the average thicknesses of the binder phase along several arbitrarily drawn lines through the microstructure. This binder thickness measurement is also referred to as the "binder mean free path" by those skilled in the art. For two materials of similar overall composition or binder content and average diamond grain size, the material which has the smaller average thickness will tend to be more homogenous, as this implies a "finer scale" distribution of the binder in the diamond phase. In addition, the smaller the standard deviation of this measurement, the more homogenous is the structure. A large standard deviation implies that the binder thickness varies widely over the microstructure, i.e. that the structure is not even, but contains widely dissimilar structure types.

The binder mean free path measurements and standard deviations therein were obtained for various samples formed according to embodiments in the manner set out below. Unless otherwise stated herein, dimensions of mean free path within the body of PCD material refer to the dimensions as measured on a surface of, or a section through, a body comprising PCD material and no stereographic correction has been applied. For example, the measurements are made by means of image analysis carried out on a polished surface, and a Saltykov correction has not been applied in the data stated herein.

In measuring the mean value of a quantity or other statistical parameter measured by means of image analysis, several images of different parts of a surface or section (hereinafter referred to as samples) are used to enhance the reliability and accuracy of the statistics. The number of images used to measure a given quantity or parameter may be, for example between 10 to 30. If the analysed sample is uniform, which is the case for PCD, depending on magnification, 10 to 20 images may be considered to represent that sample sufficiently well.

The resolution of the images needs to be sufficiently high for the inter-grain and inter-phase boundaries to be clearly made out and, for the measurements

34 stated herein an image area of 1280 by 960 pixels was used. Images used for the image analysis were obtained by means of scanning electron micrographs (SEM) taken using a backscattered electron signal. The back-scatter mode was chosen so as to provide high contrast based on different atomic numbers and to reduce sensitivity to surface damage (as compared with the secondary electron imaging mode).

1 . A sample piece of the PCD sintered body is cut using wire EDM and polished. At least 10 back scatter electron images of the surface of the sample are taken using a Scanning Electron Microscope at 1000 times magnifications.

2. The original image was converted to a greyscale image. The image contrast level was set by ensuring the diamond peak intensity in the grey scale histogram image occurred between 10 and 20.

3. An auto threshold feature was used to binarise the image and specifically to obtain clear resolution of the diamond and binder phases.

4. The software, having the trade name analysis Pro from Soft Imaging System® GmbH (a trademark of Olympus Soft Imaging Solutions GmbH) was used and excluded from the analysis any particles which touched the boundaries of the image. This required appropriate choice of the image magnification:

a. If too low then resolution of fine particles is reduced.

b. If too high then:

i. Efficiency of coarse grain separation is reduced.

ii. High numbers of coarse grains are cut by the boarders of the image and hence less of these grains are analysed.

iii. Thus more images must be analysed to get a statistically-meaningful result.

5. Each particle was finally represented by the number of continuous pixels of which it is formed.

6. The AnalySIS software programme proceeded to detect and analyse each particle in the image. This was automatically repeated for several images.

35 7. Ten SEM images were analyzed using the grey-scale to identify the binder pools as distinct from the other phases within the sample. The threshold value for the SEM was then determined by selecting a maximum value for binder pools content which only identifies binder pools and excludes all other phases (whether grey or white). Once this threshold value is identified it is used to binarize the SEM image.)

8. One pixel thick lines were superimposed across the width of the binarized image, with each line being five pixels apart (to ensure the measurement is sufficiently representative in statistical terms). Binder phase that are cut by image boundaries were excluded in these measurements.

9. The distance between the binder pools along the superimposed lines were measured and recorded - at least 10,000 measurements were made per material being analysed. Mean values were reported for the non-diamond phase mean free paths.

Also recorded were the standard deviations in the binder mean free path measurements.

From this, it was determined that embodiments have a ratio of the standard deviation of the non-diamond phase mean free path to the mean of the non- diamond phase mean free path of greater than 80% when the average grain size in the diamond phase is less than or equal to 25 microns, at a magnification of 1000x.

In some embodiments, the ratio of the standard deviation of the non- diamond phase mean free path to the mean of the non-diamond phase mean free path is greater than 80% but less than 150% and in other embodiments is greater than 80% but less than 120%.

It has also been found that multimodal distributions of some embodiments may assist in achieving a very high degree of diamond intergrowth while still maintaining sufficient open porosity to enable efficient leaching. Also, it have been found that the combination of the following elements in embodiments provides unexpected additional advantages over any of the individual components:

1 . A multimodal grain size distribution with an average pre-sintered grain size of approximately less than or equal to 25 microns to give a good wear resistance.

36 2. A tungsten carbide substrate having additions of Ni and Cr to provide erosion resistance.

3. An interface design that minimizes interfacial stresses.

4. Sintering conditions greater than 6GPa for improved composite densification and sintering.

Whilst various embodiments have been described with reference to a number of examples, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof and that these examples are not intended to limit the particular embodiments disclosed.

37

Claims

Claims:

A superhard polycrystalline construction comprising a body of polycrystalline superhard material formed of:

a mass of superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, the superhard grains having an associated mean free path;

wherein:

2. A superhard polycrystalline construction according to claim 1 , wherein the superhard grains comprise natural and/or synthetic diamond grains, the superhard polycrystalline construction forming a polycrystalline diamond construction.

3. A superhard polycrystalline construction according to any one of the preceding claims, wherein the non-superhard phase comprises a binder phase.

4. A superhard polycrystalline construction according to claim 3, wherein the binder phase comprises cobalt, and/or one or more other iron group elements, such as iron or nickel, or an alloy thereof, and/or one or more carbides, nitrides, borides, and oxides of the metals of Groups IV-VI in the periodic table.

5. A superhard polycrystalline construction according to any one of the preceding claims, further comprising a cemented carbide substrate bonded to the body of polycrystalline material along an interface.

6. A superhard polycrystalline construction according to claim 5, wherein the cemented carbide substrate comprises tungsten carbide particles bonded together by a binder material, the binder material comprising an alloy of Co, Ni and Cr.

7. A superhard polycrystalline construction according to claim 6, wherein the tungsten carbide particles form at least 70 weight percent and at most 95 weight percent of the substrate; the binder material comprising between about 10 to 50 wt.% Ni, between about 0.1 to 10 wt.% Cr, and the remainder weight percent comprising Co; and wherein the size distribution of the tungsten carbide particles in the cemented carbide substrate has the following characteristics:

fewer than 17 percent of the carbide particles have a grain size of equal to or less than about 0.3 microns;

between about 20 to 28 percent of the tungsten carbide particles have a grain size of between about 0.3 to 0.5 microns;

between about 42 to 56 percent of the tungsten carbide particles have a grain size of between about 0.5 to 1 microns;

less than about 12 percent of the tungsten carbide particles are greater than 1 micron; and

the mean grain size of the tungsten carbide particles is about 0.6+0.2 microns.

8. A superhard polycrystalline construction according to claim 7, wherein the binder additionally comprises between about 2 to 20 wt.% tungsten and between about 0.1 to 2 wt.% carbon.

9. A superhard polycrystalline construction according to any one of the preceding claims, wherein the average grain size of the superhard grains is between around 8 to 20 microns.

10. A superhard polycrystalline construction according to any one of the preceding claims wherein the ratio of the standard deviation in the mean free path associated with the non-superhard phase to the mean of the mean free path associated with the non-superhard phase is less than 150% when measured using image analysis techniques at a magnification of 1000.

1 1 . A superhard polycrystalline construction according to any one of the preceding claims wherein the ratio of the standard deviation in the mean free path associated with the non-superhard phase to the mean of the mean free path associated with the non-superhard phase is less than 120% when measured using image analysis techniques at a magnification of 1000.

12. A superhard polycrystalline construction according to any one of the preceding claims, wherein the body of polycrystalline superhard material comprises a first region and a second region adjacent the first region, the second region being bonded to the first region by intergrowth of grains of superhard material; the first region comprising a plurality of alternating strata or layers, each stratum or layer having a thickness in the range of around 5 to 300 microns; the second region comprising a plurality of strata or layers, one or more strata or layers in the second region having a thickness greater than the thicknesses of the individual strata or layers in the first region, wherein:

the alternating layers or strata in the first region comprise first layers or strata alternating with second layers or strata, the first layers or strata being in a state of residual compressive stress and the second layers or strata being in a state of residual tensile stress; and

one or more of the layers or strata in the first or second regions comprises: the mass of superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween; and

the non-superhard phase at least partially filling a plurality of the interstitial regions and having an associated mean free path; the ratio of the standard deviation in the mean free path associated with the non-superhard phase to the mean of the mean free path associated with the non-superhard phase is greater than or equal to 80% when measured using image analysis techniques at a magnification of 1000.

13. A superhard polycrystalline construction according to claim 12, wherein each stratum or layer in the first region has a thickness in the range of around 30 to 300 microns, or around 30 to 200 microns.

14. A superhard polycrystalline construction according to any one of claims 12 or 13, wherein the strata or layers in the second region have a thickness of greater than around 200 microns.

15. A superhard polycrystalline construction according to any one of claims 12 to 14, wherein the layers or strata in the first region comprise two or more different average diamond grain sizes.

16. A superhard polycrystalline construction according to any one of claims 1 to 1 1 , wherein the body of polycrystalline superhard material comprises a first region and a second region adjacent the first region, the second region being bonded to the first region by intergrowth of diamond grains; the first region comprising a plurality of alternating strata or layers, each layer or stratum in the first region having a thickness in the range of around 5 to 300 microns; one or more of the layers or strata in the first region and/or the second region comprises:

a mass of superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween,; and a non-superhard phase at least partially filling a plurality of the interstitial regions and having an associated mean free path; wherein:

the ratio of the standard deviation in the mean free path associated with the non-superhard phase to the mean of the mean free path associated with the non-superhard phase being greater than or equal to 80% when measured using image analysis techniques at a magnification of 1000.

17. A superhard polycrystalline construction according to any one of claims 12 to 16, wherein the first region comprises an external working surface forming the initial working surface of the PCD structure in use.

18. A superhard polycrystalline construction according to any one of claims 16 or 17, wherein the second region has a thickness greater than the thickness of the individual strata or layers in the first region.

19. A superhard polycrystalline construction according to any one of claims 16 to 18, wherein the second region comprises a plurality of layers or strata.

20. A superhard polycrystalline construction according to any one of claims 16 to 19, wherein the alternating layers or strata comprise first layers or strata alternating with second layers or strata, the first layers or strata being in a state of residual compressive stress and the second layers or strata being in a state of residual tensile stress.

21 . A superhard polycrystalline construction according to any one of claims 12 to 20, wherein layers or strata in the first region and/or the second region comprise one or more of:

up to 20 wt% nanodiamond additions in the form of nanodiamond powder grains;

salt systems;

borides or metal carbides of at least one of Ti, V, or Nb; or

at least one of the metals Pd or Ni.

22. A superhard polycrystalline construction according to any one of claims 12 to 21 , wherein the PCD structure has a longitudinal axis, the layers or strata in the first region and/or the second region lying in a plane substantially perpendicular to the plane through which the longitudinal axis of the PCD structure extends.

23. A superhard polycrystalline construction according to any one of claims 12 to 22, wherein the layers or strata are substantially planar, curved, bowed or domed.

24. A superhard polycrystalline construction according to any one of claims 12 to 21 , wherein the PCD structure has a longitudinal axis, the layers or strata in the first region and/or the second region lying in a plane at an angle to the plane through which the longitudinal axis of the PCD structure extends.

25. A superhard polycrystalline construction according to any one of claims 12 to 24, wherein the volume of the first region is greater than the volume of the second region.

26. A superhard polycrystalline construction according to any one of claims 12 to 25, wherein one or more of the strata or layers intersect a working surface or side surface of the PCD structure.

27. A superhard polycrystalline construction according to any one of claims 12 to 26, wherein each strata or layer is formed of one or more respective PCD grades having a TRS of at least 1 ,000 MPa; the PCD grade or grades in adjacent strata or layers having a different coefficient of thermal expansion (CTE).

28. A superhard polycrystalline construction according to claim 27, wherein one or more of the strata or layers comprise a PCD grade or grades having a CTE of at least 3 X 10-6 mm/oC.

29. A superhard polycrystalline construction as claimed in any of claims 12 to 28, wherein at least a portion of the first region is substantially free of a catalyst material for diamond, said portion forming a thermally stable region.

30. A superhard polycrystalline construction as claimed in claim 29, wherein the thermally stable region extends a depth of at least 50 microns from a surface of the PCD structure.

31 . A superhard polycrystalline construction as claimed in any one of claims 29 or 30, wherein the thermally stable region comprising at most 2 weight percent of catalyst material for diamond.

32. A superhard construction according to any one of the preceding claims further comprising:

a substrate comprising a periphery and an interface surface and a longitudinal axis; wherein the body of polycrystalline superhard material is formed over the substrate and having an exposed outer surface, a peripheral surface extending therefrom and an interface surface; wherein one of the interface surface of the substrate or the interface surface of the body of polycrystalline superhard material comprises:

a plurality of spaced-apart projections arranged to project from the interface surface, the interface surface between the spaced-apart projections being uneven.

33. The superhard construction of claim 32, wherein all or a majority of the interface surface between the spaced-apart projections is non-curved and extends in one or more planes which are not substantially parallel to the plane of the exposed outer surface of the body of polycrystalline superhard material.

34. The superhard construction of any one of claims 32 or 33, the substrate having a central longitudinal axis, wherein all or a majority of the interface surface between the spaced-apart projections extends in one or more planes which are not substantially parallel to a plane through which the central longitudinal axis of the substrate extends.

35. The superhard construction of any one of claims 32 to 34, wherein the projections are arranged in one or more substantially radial arrays around the central longitudinal axis of the substrate.

36. The superhard construction of claim 35, wherein the projections are arranged in a first array and a second array, the second array being positioned radially within the first array.

37. The superhard construction of claim 36, wherein the first and second arrays are substantially concentric with the substrate.

38. The superhard construction of any of claims 36 or 37, wherein the first array comprises substantially double the number of projections than the second array.

39. The superhard construction of any one of claims 36 to 38, wherein the projections in the first and second arrays are staggered relative to each other.

40. The superhard construction of any of claims 32 to 34, wherein the projections are randomly arranged on one of the interface surface of the substrate or the interface surface of the ultra hard material layer.

41 . The superhard construction of any one of claims 32 to 40, wherein one or more of the surfaces of all or a majority of the projections extend in one or more planes which are not substantially parallel to the plane of the exposed outer surface of the body of polycrystalline superhard material and/or in one or more planes which are not substantially parallel to a plane through which the central longitudinal axis of the substrate extends.

42. The superhard construction of any one of claims 32 to 41 , wherein the thickness of the body of polycrystalline superhard material about the central longitudinal axis of the substrate is substantially the same as the thickness of the body of polycrystalline superhard material at the peripheral surface.

43. The superhard construction of any one of claims 32 to 42, wherein the exposed outer surface of the body of polycrystalline superhard material is substantially planar.

44. The superhard construction of any one of claims 32 to 43, wherein the projections are of equal height.

45. The superhard construction of any one of claims 36 to 38, wherein the projections in the first array are of a greater height than the projections in the second array.

46. A superhard construction according to any one of claims 1 to 31 further comprising:

a substrate comprising a periphery and an interface surface and a longitudinal axis; wherein

the body of polycrystalline superhard material is formed over the substrate and has an exposed outer surface, a peripheral surface extending therefrom and an interface surface;

wherein one of the interface surface of the substrate or the interface surface of the body of polycrystalline superhard material comprises:

a plurality of projections arranged to project from the interface surface, the projections abutting one or more adjacent projections along an edge thereof on the surface from which the projections project, and extending over all or a majority of the interface surface; and

wherein one or more of the surfaces of all or a majority of the projections extend in one or more planes which are not substantially parallel to the plane of the exposed outer surface of the body of polycrystalline superhard material and/or in one or more planes which are not substantially parallel to a plane through which the central longitudinal axis of the substrate extends.

47. The superhard construction of claim 46, wherein any interface surface between any projections or not covered by the projections is uneven.

48. The superhard construction of any one of claims 46 or 47, wherein the projections are substantially pyramidal.

49. The superhard construction of any one of claims 45 to 48, wherein the projections are arranged in an ordered array over the interface surface.

50. The superhard construction of any one of claims 32 to 49, wherein the interface surface of the substrate is a negative or reversal of the interface surface of the body of polycrystalline superhard material such that the two interface surfaces form a matching fit.

51 . The superhard construction of any one of claims 32 to 50, wherein the superhard construction is a cutter element.

52. An earth boring drill bit comprising a body having the superhard construction of any one of the preceding claims mounted thereon as a cutter element.

53. A method of forming the superhard construction of any one of claims 1 to 51 .

54. A method of forming a superhard polycrystalline construction, comprising: providing a mass of grains of superhard material comprising a first fraction having a first average size and a second fraction having a second average size, arranging the mass of superhard grains to form a pre-sinter assembly; and treating the pre-sinter assembly in the presence of a catalyst/solvent material for the superhard grains at an ultra-high pressure of around 6 GPa or greater and a temperature at which the superhard material is more thermodynamically stable than graphite to sinter together the grains of superhard material to form a polycrystalline superhard construction, the superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, a non-superhard phase at least partially filling a plurality of the interstitial regions; wherein the non-superhard phase has an associated mean free path; and

wherein:

the average grain size of the superhard grains is less than or equal to 25 microns; and the ratio of the standard deviation in the mean free path associated with the non-superhard phase to the mean of the mean free path associated with the non- superhard phase is greater than or equal to 80% when measured using image analysis techniques at a magnification of 1000.

55. The method of claim 55, wherein, the step of providing a mass of grains of superhard material comprises providing a mass of diamond grains having a first fraction having a first average size and a second fraction having a second average size, the first fraction having an average grain size ranging from about 10 to 60 microns, and the second fraction having an average grain size less than the size of the coarse fraction.

56. The method of claim 54, wherein the second fraction has an average grain size between around 1/10 to 6/10 of the size of the first fraction.

57. The method of any one of claims 54 to 56, wherein the average grain size of the first fraction is between around 10 to 60 microns, and the average grain size of the second fraction is between about 0.1 to 20 microns.

58. The method of any one of claims 54 to 57, wherein the weight ratio of the first fraction to the second fraction ranges from about 50% to about 97%, the weight ratio of the second fraction ranging from about 3% to about 50 weight%.

59. The method of claim 58, wherein the ratio by weight percent of the first fraction to the second fraction is around 60:40.

60. The method of claim 58, wherein the ratio by weight percent of the first fraction to the second fraction is around 70:30.

61 . The method of claim 58, wherein the ratio by weight percent of the first fraction to the second fraction is around 90:10.

62. The method of claim 58, wherein the ratio by weight percent of the first fraction to the second fraction is around 80:20.

63. The method of any one of claims 54 to 62, wherein the step of providing a mass of grains of superhard material comprises providing a mass of grains in which the grain size distributions of the first and second fractions do not overlap.

64. The method of any one of claims 54 to 63, wherein the step of providing a mass of grains of superhard material comprises providing three or more grain size modes to form a multimodal mass of grains comprising a blend of grain sizes having associated average grain sizes.

65. The method of any one of claims 54 to 64, wherein the average grain sizes of the fractions is separated by an order of magnitude.

66. The method of any one of claims 54 to 65, wherein the mass of superhard grains comprises a first fraction having an average grain size of around 20 microns, a second fraction having an average grain size of around 2 microns, a third fraction having an average grain size of around 200nm and a fourth fraction having an average grain size of around 20nm.

67. A tool comprising a superhard polycrystalline construction according to any one of claims 1 to 50, the tool being for cutting, milling, grinding, drilling, earth boring, rock drilling or other abrasive applications.

68. A tool according to claim 67, wherein the tool comprises a drill bit for earth boring or rock drilling.

69. A tool according to claim 67, wherein the tool comprises a rotary fixed- cutter bit for use in the oil and gas drilling industry.

70. A tool according to claim 67, wherein the tool is a rolling cone drill bit, a hole opening tool, an expandable tool, a reamer or other earth boring tools.

71 . A drill bit or a cutter or a component therefor comprising the superhard polycrystalline construction according to any one of claims 1 to 50.

72. A superhard polycrystalline construction for a rotary shear bit for boring into the earth, or for a percussion drill bit, comprising a superhard polycrystalline construction as claimed in any one of claims 1 to 50 bonded to a cemented carbide support body.

73. A superhard polycrystalline construction substantially as hereinbefore described with reference to any one embodiment as that embodiment is illustrated in the accompanying drawings.

74. A method of making a superhard polycrystalline construction, substantially as hereinbefore described with reference to any one embodiment as that embodiment is illustrated in the accompanying drawings.