Abstract:
A superhard polycrystalline construction comprises a body of polycrystal-line 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 median of the mean free path associated with the non-superhard phase divided by (Q 3 -Q 1 ) for the non-superhard phase is greater than or equal to 0.83 where Q 1  is the first quartile and Q 3  is the third quartile and the median of the mean free path associated with the superhard grains divided by (Q 3 -Q 1 ) for the superhard grains is less than 0.47.

Description:
FIELD 
       [0001]    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 
       [0002]    Polycrystalline superhard materials, such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) may be used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. In particular, tool inserts in the form of cutting elements comprising PCD material are widely used in drill bits for boring into the earth to extract oil or gas. The working life of superhard tool inserts may be limited by fracture of the superhard material, including by spalling and chipping, or by wear of the tool insert. 
         [0003]    Cutting elements such as those for use in rock drill bits or other cutting tools typically have a body in the form of a substrate which has an interface end/surface and an ultra hard 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 ultra hard material layer is typically polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or a thermally stable product TSP material such as thermally stable polycrystalline diamond. 
         [0004]    Polycrystalline diamond (PCD) is an example of a superhard material (also called a superabrasive material) comprising a mass of substantially inter-grown diamond grains, forming a skeletal mass defining interstices between the diamond grains. PCD material typically comprises at least about 80 volume % of diamond and is conventionally made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 Gpa, and temperature of at least about 1,200° C., for example. A material wholly or partly filling the interstices may be referred to as filler or binder material. 
         [0005]    PCD is typically formed in the presence of a sintering aid such as cobalt, which promotes the inter-growth of diamond grains. Suitable sintering aids for PCD are also commonly referred to as a solvent-catalyst material for diamond, owing to their function of dissolving, to some extent, the diamond and catalysing its re-precipitation. A solvent-catalyst for diamond is understood be a material that is capable of promoting the growth of diamond or the direct diamond-to-diamond inter-growth between diamond grains at a pressure and temperature condition at which diamond is thermodynamically stable. Consequently the interstices within the sintered PCD product may be wholly or partially filled with residual solvent-catalyst material. Most typically, PCD is often formed on a cobalt-cemented tungsten carbide substrate, which provides a source of cobalt solvent-catalyst for the PCD. Materials that do not promote substantial coherent intergrowth between the diamond grains may themselves form strong bonds with diamond grains, but are not suitable solvent -catalysts for PCD sintering. 
         [0006]    Cemented tungsten carbide which may be used to form a suitable substrate is formed from carbide particles being dispersed in a cobalt matrix by mixing tungsten carbide particles/grains and cobalt together then heating to solidify. To form the cutting element with an ultra hard material layer such as PCD or PCBN, diamond particles or grains or CBN grains are placed adjacent the cemented tungsten carbide body in a refractory metal enclosure such as a niobium enclosure and are subjected to high pressure and high temperature so that inter-grain bonding between the diamond grains or CBN grains occurs, forming a polycrystalline ultra hard diamond or polycrystalline CBN layer. 
         [0007]    In some instances, the substrate may be fully cured prior to attachment to the ultra hard 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 ultra hard material layer. 
         [0008]    Ever increasing drives for improved productivity in the earth boring field place 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. 
         [0009]    Cutting elements for use in rock drilling and other operations require high abrasion resistance and impact resistance. One of the factors limiting the success 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. 
         [0010]    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 
       [0011]    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 median of the mean free path associated with the non-superhard phase divided by (Q 3 -Q 1 ) for the non-superhard phase is greater than or equal to 0.83, where Q 1  is the first quartile and Q 3  is the third quartile; and   the median of the mean free path associated with the superhard grains divided by (Q 3 -Q 1 ) for the superhard grains is less than 0.47.       
 
         [0016]    In some embodiments, the superhard grains comprise natural and/or synthetic diamond grains, the superhard polycrystalline construction forming a polycrystalline diamond construction. 
         [0017]    In some embodiments, the non-superhard phase comprises a binder phase. 
         [0018]    The binder phase may comprise, for example, 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. 
         [0019]    In some embodiments, the polycrystalline superhard construction further comprises a cemented carbide substrate bonded to the body of polycrystalline material along an interface. 
         [0020]    The cemented carbide substrate may, for example, comprise tungsten carbide particles bonded together by a binder material, the binder material comprising an alloy of Co, Ni and Cr. 
         [0021]    In some embodiments, 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.       
 
         [0027]    The binder in the substrate may additionally comprise between about 2 to 20 wt. % tungsten and between about 0.1 to 2 wt. % carbon. 
         [0028]    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 5.5 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;       
 
         [0032]    wherein the median of the mean free path associated with the non-superhard phase divided by (Q 3 -Q 1 ) for the non-superhard phase is greater than or equal to 0.83, where Q 1  is the first quartile and Q 3  is the third quartile of the mean free path measurements associated with the non-superhard phase; and
       the median of the mean free path associated with the superhard grains divided by (Q 3 -Q 1 ) for the superhard grains is less than 0.47, where Q 1  is the first quartile and Q 3  is the third quartile of the mean free path measurements associated with the superhard grains.       
 
         [0034]    In some embodiments, 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. 
         [0035]    The second fraction may, for example, have an average grain size between around 1/10 to 6/10 of the size of the first fraction. 
         [0036]    In some embodiments, 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. 
         [0037]    In some embodiments, 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 %. 
         [0038]    In some embodiments, the ratio by weight percent of the first fraction to the second fraction is around 60:40, or around 70:30, or around 90:10, or around 80:20. 
         [0039]    The step of providing a mass of grains of superhard material may comprises, for example, providing a mass of grains in which the grain size distributions of the first and second fractions do not overlap. 
         [0040]    The step of providing a mass of grains of superhard material may, in some embodiments, comprise 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. 
         [0041]    In some embodiments, the average grain sizes of the fractions is separated by an order of magnitude. 
         [0042]    In some embodiments, 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 200 nm and a fourth fraction having an average grain size of around 20 nm. 
         [0043]    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. 
         [0044]    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. 
         [0045]    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 
         [0046]    The present invention will now be described by way of example and with reference to the accompanying drawings in which: 
           [0047]      FIG. 1  is a polycrystalline diamond (PCD) structures attached to a substrate; 
           [0048]      FIG. 2  is a plot showing wear scar depth for two embodiments; and 
           [0049]      FIG. 3  is a plot of wear scar area against cutting length in a vertical borer test for an embodiment. 
       
    
    
     DESCRIPTION 
       [0050]    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. 
         [0051]    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. 
         [0052]    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. 
         [0053]    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. 
         [0054]    A “catalyst material” for a superhard material is capable of promoting the growth or sintering of the superhard material. 
         [0055]    The term “substrate” as used herein means any substrate over which the ultra hard material layer is formed. For example, a “substrate” as used herein may 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. 
         [0056]    The superhard construction  1  shown in the  FIG. 1  may be suitable, for example, for use as a cutter insert for a drill bit for boring into the earth. 
         [0057]    Like reference numbers are used to identify like features in all drawings. 
         [0058]    In an embodiment as shown in  FIG. 1 , a cutting element  1  includes a substrate  10  with a layer of ultra-hard material  12  formed on the substrate  10 . The substrate may be formed of a hard material such as cemented tungsten carbide. The ultra-hard 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 ultra-hard material opposite the substrate forms the cutting face  14 , which is the surface which, along with its edge  16 , performs the cutting in use. 
         [0059]    At one end of the substrate  10  is an interface surface  18  that interfaces with the ultra-hard material layer  12  which is attached thereto at this interface surface. The substrate  10  is generally cylindrical and has a peripheral surface  20  and a peripheral top edge  22 . 
         [0060]    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. 
         [0061]    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 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. 
         [0062]    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. 
         [0063]    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. 
         [0064]    The embodiments consists 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, 200 nm and 20 nm. 
         [0065]    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. 
         [0066]    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. 
         [0067]    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. 
         [0068]    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. 
         [0069]    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 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. 
         [0070]    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 ultrahard polycrystalline material. 
         [0071]    In a further embodiment, both the substrate and a body of polycrystalline superhard material are pre-formed. For example, the bimodal feed of ultrahard 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. 
         [0072]    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. 
         [0073]    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 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. 
         [0074]    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. 
         [0075]    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 11 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. 
         [0076]    Solvent/catalyst for diamond may be introduced into the aggregated mass of diamond grains by various methods, including blending solvent/catalyst 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. 
         [0077]    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: 
         [0000]      Co(NO 3 ) 2 +Na 2 CO 3 →CoCO 3 +2NaNO 3  
 
         [0078]    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: 
         [0000]      CoCO 3 →CoO+CO 2  
 
         [0000]      CoO+H 2 →Co+H 2 O
 
         [0079]    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. 
         [0080]    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;. 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.       
 
         [0086]    In some embodiments, the binder additionally comprises between about 2 to 20 wt. % tungsten and between about 0.1 to 2 wt. % carbon 
         [0087]    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.       
 
         [0092]    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. 
         [0093]    The layer of substrate may further comprise free carbon. 
         [0094]    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, a, 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: 
         [0000]        M   s   ∝[C]/[W] ×wt. % Co×201.9 in units of μT.m 3 /kg
 
         [0095]    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. 
         [0096]    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. 
         [0097]    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 micrograph of a polished section of the material. For example, the micrograph may have a magnification of about 1000×. 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. 
         [0098]    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 (μT.m 3 /kg), and the induction of saturation, also referred to as the magnetic saturation, 4πσ, of pure Co is 201.9 μT.m 3 /kg. 
         [0099]    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. 
         [0100]    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 that generate a log normal distribution of particles. Higher matrix (binder) contents would result in higher MFP values. 
         [0101]    Changing grain size by mixing different powder classes and altering the distributions may achieve a whole spectrum of MFP values for the substarte depending on the particulars of powder processing and mixing. The exact values would have to be determined empirically. 
         [0102]    In some embodiments, the substrate comprises Co, Ni and Cr. 
         [0103]    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. 
         [0104]    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. 
         [0105]    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 2×10 −3  g/cm 3  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 Na2CO3; 3 kg carboxymethyl cellulose, 5 litres 
         [0106]    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. 
         [0107]    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. 
         [0108]    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. 
         [0109]    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. 
         [0110]    To assist in improving thermal stability of the sintered structure, the catalysing material may 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. 
         [0111]    Embodiments are described in more detail below with reference to the following examples which are provided herein by way of illustration only and are not intended to be limiting. 
       EXAMPLE 1 
       [0112]    A quantity of sub-micron cobalt powder sufficient to obtain 2 mass % in the final diamond mixture was initially de-agglomerated in a methanol slurry in a ball mill with WC milling media for 1 hour. A fine fraction of diamond powder with an average grain size of 2 μm was then added to the slurry in an amount to obtain 10 mass % in the final mixture. Additional milling media was introduced and further methanol was added to obtain suitable slurry; and this was milled for a further hour. A coarse fraction of diamond, with an average grain size of approximately 20 μm was then added in an amount to obtain 88 mass % in the final mixture. The slurry was again supplemented with further methanol and milling media, and then milled for a further 2 hours. The slurry was removed from the ball mill and dried to obtain the diamond powder mixture. 
         [0113]    The diamond powder mixture was then placed into a suitable HpHT vessel, adjacent to a tungsten carbide substrate and sintered at a pressure of around 6.8 Gpa and a temperature of about 1500° C. 
       EXAMPLE 2 
       [0114]    A quantity of sub-micron cobalt powder sufficient to obtain 2.4 mass % in the final diamond mixture was initially de-agglomerated in a methanol slurry in a ball mill with WC milling media for 1 hour. A fine fraction of diamond powder with an average grain size of 2 μm was then added to the slurry in an amount to obtain 29.3 mass % in the final mixture. Additional milling media was introduced and further methanol was added to obtain a suitable slurry; and this was milled for a further hour. A coarse fraction of diamond, with an average grain size of approximately 20 μm was then added in an amount to obtain 68.3 mass % in the final mixture. The slurry was again supplemented with further methanol and milling media, and then milled for a further 2 hours. The slurry was removed from the ball mill and dried to obtain the diamond powder mixture. 
         [0115]    The diamond powder mixture was then placed into a suitable HpHT vessel, adjacent to a tungsten carbide substrate and sintered at a pressure of around 6.8 Gpa and a temperature of about 1500 ° C. 
         [0116]    The diamond content of the sintered diamond structure is greater than 90 vol % and the coarsest fraction of the distribution is greater than 60 weight % and preferably greater than weight 70%. 
         [0117]    In order to test the abrasion resistance of the finished PCD compacts formed according to the above examples, the diamond layers were polished and a subjected to a conventional granite turning test for 3 minutes. The wear scar progression during the machining process was monitored. The results for both examples are shown in  FIG. 2 . It will be seen that the wear scars of the PCD compacts formed according to examples 1 and 2 above were less than that occurring in a conventional PCD compact shown as Ref 1 which was sintered at a pressure of 6.8 GPa and had an average diamond grain size of around 11.3 microns. 
         [0118]    The PCD compact formed according to Example 2 was then compared in a vertical boring mill test with commercially available polycrystalline diamond cutter elements. In this test, the wear flat area was measured as a function of the number of passes of the cutter element boring into the workpiece. The results obtained are illustrated graphically by  FIG. 3 . The results provide an indication of the total wear scar area plotted against cutting length. It will be seen that the PCD compact formed according to example 2 was able to achieve a greater cutting length and smaller wear scar area than that occurring in a conventional PCD compacts which were subjected to the same test for comparison. The conventional PCD compacts in this test comprised Ref 1 which was sintered at a pressure of 6.8 GPa and had an average diamond grain size of around 11.3 microns and Ref 2 which was sintered at a pressure of 6.8 GPa and had a grain size of around 9.5 microns. 
         [0119]    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. 
         [0120]    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 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. 
         [0121]    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 “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. 
         [0122]    The binder and diamond mean free path measurements 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. 
         [0123]    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. 
         [0124]    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 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.   7. Ten SEM images were analyzed using the grey-scale to identify the binderpools 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. The median value for the non-diamond phase mean free paths and the diamond phase mean free paths were calculated. The term “median” in this context is considered to have its conventional meaning, namely the numerical value separating the higher half of the data sample from the lower half.       
 
         [0139]    Also recorded were the mean free path measurements at Q 1  and Q 3  for both the diamond and non-diamond phases. 
         [0140]    Q 1  is typically referred to as the first quartile (also called the lower quartile) and is the number below which lies the 25 percent of the bottom data. Q 3  is typically referred to as the third quartile (also called the upper quartile) has 75 percent of the data below it and the top 25 percent of the data above it. 
         [0141]    From this, it was determined that embodiments have: 
         [0000]      alpha&gt;=0.83 and beta&lt;0.47, 
         [0142]    where 
         [0143]    alpha is the non-diamond phase MFP median/(Q 3 -Q 1 ), which gives a measure of “uniform binder pool size”; and 
         [0144]    beta=diamond MFP median/(Q 3 -Q 1 ) which gives a measure of “wide grain size distribution” 
         [0145]    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. 
         [0146]    While 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.