GB2515580A

GB2515580A - Superhard constructions & methods of making same

Info

Publication number: GB2515580A
Application number: GB1311714.8A
Authority: GB
Inventors: Maweja Kasonde; Leshope Joseph Choenyane; Mehmet Serdar Ozbayraktar
Original assignee: Element Six Abrasives SA
Current assignee: Element Six Abrasives SA
Priority date: 2013-06-30
Filing date: 2013-06-30
Publication date: 2014-12-31
Also published as: GB201311714D0

Abstract

A method of forming a polycrystalline superhard construction comprising: preparing a metastable solid solution to form a catalyst-binder mixture by mechanically alloying a first mass comprising a source of elemental carbon and a second mass of a catalyst comprising an elemental metal powder or metal alloy powder, the first mass comprising between 3 wt% to 15 wt% of source of elemental carbon, and the second mass comprising between 85 wt% to 97 wt% catalyst powder. The catalyst-binder mixture is mixed with a mass of superhard particles to form a green body which is then treated at a pressure of around 5.5 GPa or greater and a temperature to sinter together the grains of superhard material to form a polycrystalline superhard construction comprising a superhard phase and a binder phase, an amount of the superhard phase precipitating in part in the binder phase during the step of sintering. Preferably the source of elemental carbon comprises graphite and/or amorphous carbon powder. Preferably the elemental metal powder or metal alloy powder comprises one or more of the metals cobalt, iron, manganese, nickel, platinum, or ruthenium.

Description

Intellectual Property Office Applicathin No. (lB 1311714.S RTM Dac: 19 Dorcinbcr 2013 The following terms are registered trade marks and should he rcad as such wherever they occur in this document: X'Celerator Inlelleclual Property Office is an operaling name of the Pateni Office www.ipo.gov.uk 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 or polycrystalline cubic boron nitride (PCBN) structures which may be attached to a substrate, and tools comprising the same, particularly but not exclusively for use in rock degradation or drilling, or for boring into the earth.

BACKGROUND

Polycrystalline diamond (PCD) is an example of a superhard material (also called a superabrasive material or ultra hard 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.

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.

Boron nitride exists typically in three crystalline forms, namely cubic boron nitride (cBN), hexagonal boron nitride (hBN) and wurtzitic cubic boron nitride (wBN). Cubic boron nitride is a hard zinc blende form of boron nitride that has a similar structure to that of diamond. In the cBN structure, the bonds that form between the atoms are strong, mainly covalent tetrahedral bonds. Methods for preparing cBN are well known in the art.

One such method is subjecting hBN to very high pressures and temperatures, in the presence of a specific catalytic additive material, which may include the alkali metals, alkaline earth metals, lead, tin and nitrides of these metals. When the temperature and pressure are decreased, cBN may be recovered.

Currently PcBN preparation includes many steps, five of them typically in the binder preparation stage alone. Manipulation of the fine powders between these steps increases the risk of inhalation, adsorption by the skin and ignition. Furthermore, the mixing is not efficient leading to the occurrence of many binary phases present in the sintered materials.

Those binary phases such as Ti-aluminides are brittle, hence detrimental to the properties of the PcBN products. The hexane used as dispersant in attrition milling step is highly hazardous, exposure to this chemical should be minimised.

cBN may also be used in bonded form as a cBN compact. cBN compacts tend to have good abrasive wear, are thermally stable, have a high thermal conductivity, good impact resistance and have a low coefficient of friction when in contact with iron containing metals.

cBN compacts comprise sintered masses of cBN particles. When the cBN content exceeds 80 percent by volume of the compact, there is a considerable amount of cBN-to-cBN contact and bonding. When the cBN content is lower, e.g. in the region of 40 to 60 percent by volume of the compact, then the extent of direct cBN-to-cBN contact and bonding is less.

cBN compacts will generally also contain a binder phase for example aluminium, silicon, cobalt, nickel, and titanium.

When the cBN content of the compact is less than 70 percent by volume there is generally present another hard phase, a secondary phase, which may be ceramic in nature. Examples of suitable ceramic hard phases are carbides, nitrides, borides and carbonitrides of a Group 4, 5 or 6 (according to the new IUPAC format) transition metal, aluminium oxide, and carbides such as tungsten carbide and mixtures thereof. The matrix constitutes all the ingredients in the composition excluding cBN.

cBN compacts may be bonded directly to a tool body in the formation of a tool insert or tool. However, for many applications it is preferable that the compact is bonded to a substrate/support material, forming a supported compact structure, and then the supported compact structure is bonded to a tool body. The substrate/support material is typically a cemented metal carbide that is bonded together with a binder such as cobalt, nickel, iron or a mixture or alloy thereof. The metal carbide particles may comprise tungsten, titanium or tantalum carbide particles or a mixture thereof.

A known method for manufacturing the cBN compacts and supported compact structures involves subjecting an unsintered mass of cBN particles, to high temperature and high pressure conditions, i.e. conditions at which the cBN is crystallographically stable, for a suitable time period. A binder phase may be used to enhance the bonding of the particles. Typical conditions of high temperature and pressure (HTHP) which are used are temperatures in the region of 1100°C or higher and pressures of the order of 2 GPa or higher. The time period for maintaining these conditions is typically about 3 to 120 minutes.

The sintered cBN compact, with or without substrate, is often cut into the desired size and/or shape of the particular cutting or drilling tool to be used and then mounted on to a tool body utilising brazing techniques.

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 superhard 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 HTHF sintering process. The substrate may be in powder form and may solidify during the sintering process used to sinter the superhard material layer.

cBN has wide commercial application in machining tools and the like. It may be used as an abrasive particle in grinding wheels, cutting tools and the like or bonded to a tool body to form a tool insert using conventional electroplating techniques.

cBN compacts are employed widely in the manufacture of cutting tools for finish machining of hardened steels, such as case hardened steels, ball-bearing steels and through hardened engineering steels. In addition to the conditions of use, such as cutting speed, feed and depth of cut, the performance of the cBN tool is generally known to be dependent on the geometry of the workpiece and in particular, whether the tool is constantly engaged in the workpiece for prolonged periods of time, known in the field as "continuous cutting", or whether the tool engages the workpiece in an intermittent manner, generally known in the field as "interrupted cutting".

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 super hard tool inserts may be limited by fracture of the super hard 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 super hard material which forms a cutting layer bonded to the interface surface of the substrate by, for example, a sintering process.

The super 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.

Ever increasing drives for improved productivity in the earth boring field place 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.

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.

Methods used to improve the abrasion resistance of a PCD composite often result in a decrease in impact resistance of the composite.

Furthermore, despite their high strength, polycrystalline diamond (PCD) and PCBN materials are usually susceptible to impact fracture due to their low fracture toughness. Improving fracture toughness without adversely affecting the material's high strength and abrasion resistance is a challenging task.

Sintered PCD has sufficient wear resistance and hardness for use in aggressive wear, cutting and drilling applications. However, a further problem known to exist with such conventional PCD compacts is that they are vulnerable to thermal degradation when exposed to elevated temperature cutting and/or wear applications. It is believed that this is due, at least in part, to the presence of residual solvent/catalyst material in the microstructural interstices which, due to the differential that exists between the thermal expansion characteristics of the interstitial solvent metal catalyst material and the thermal expansion characteristics of the intercrystalline bonded diamond, is thought to have a detrimental effect on the performance of the compact at high temperatures. Such differential thermal expansion is known to occur at temperatures of about 400[deg.] C., and is believed to cause ruptures to occur in the diamond-to-diamond bonding, and eventually result in the formation of cracks and chips in the POD structure. The chipping or cracking in the PCD table may degrade the mechanical properties of the cutting element or lead to failure of the cutting element during drilling or cutting operations thereby rendering the PCD structure unsuitable for further use.

Another form of thermal degradation known to exist with conventional PCD materials is one that is also believed to be related to the presence of the solvent metal catalyst in the interstitial regions and the adherence of the solvent metal catalyst to the diamond crystals. Specifically, at high temperatures, diamond grains may undergo a chemical breakdown or back-conversion with the solvent/catalyst. At extremely high temperatures, the solvent metal catalyst is believed to cause an undesired catalyzed phase transformation in diamond such that portions of diamond grains may transform to carbon monoxide, carbon dioxide, graphite, or combinations thereof, thereby degrading the mechanical properties of the PCD material and limiting practical use of the PCD material to about 750[deg.] C. Attempts at addressing such unwanted forms of thermal degradation in conventional PCD materials are known in the art. Generally, these attempts have focused on the formation of a PCD body having an improved degree of thermal stability when compared to the conventional PCD materials discussed above. One known technique of producing a PCD body having improved thermal stability involves, after forming the PCD body, removing all or a portion of the solvent catalyst material therefrom using, for example, chemical leaching. Removal of the catalyst/binder from the diamond lattice structure renders the polycrystalline diamond layer more heat resistant.

Conventional chemical leaching techniques often involve the use of highly concentrated, toxic, and/or corrosive solutions, such as aqua regia and mixtures including hydrofluoric acid (HF), to dissolve and remove metallic-solvent/catalysts from polycrystall me diamond materials.

An alternative way of increasing the diamond density in such compacts is to include additions such as nano-sized powders in the starting mixture prior to sintering which, when sintered, adhere to the diamond grains to reduce the amount of residual solvent-catalyst remaining in the structure. However, as explained above in the context of PCBN preparation, manipulation and handling of the nano powders carries the risk of inhalation and adsorption by the skin which is highly dangerous and therefore requires the manufacture of the compacts to be carried out in controlled conditions to minimise the danger to the manufacturer.

There is therefore a need for a superhard composite that has good or improved abrasion, fracture and impact resistance and a method of forming such composites in a manner that is potentially safer to the manufacturer than conventional post-synthesis treatment or manufacturing methods involving the handling of nano-particles.

SUMMARY

Viewed from a first aspect there is provided a method of forming a polycrystalline superhard construction comprising: preparing a metastable solid solution to form a catalyst-binder mixture by mechanically alloying a first mass comprising a source of elemental carbon and a second mass of a catalyst comprising an elemental metal powder or metal alloy powder; the first mass comprising between around 3 wt% to around 15 wt% of source of elemental carbon, and the second mass comprising between around 85 wt% to around 97 wt% catalyst powder; mixing the catalyst-binder mixture with a mass of superhard particles to form a green body; placing the green body into a canister to form a pre-sinter assembly; treating the pre-sinter assembly at an ultra-high pressure of around 5.5 GPa or greater and a temperature to sinter together the grains of superhard material to form a polycrystalline superhard construction comprising a superhard phase and a binder phase, an amount of the superhard phase precipitating in part in the binder phase during the step of sintering.

Viewed from another aspect, there is provided a superhard polycrystalline construction comprising a body of polycrystalline superhard material, the body comprising: a mass of superhard grains forming a superhard phase; and a non-superhard phase; the non-superhard phase forming pools in interstitial regions between superhard grains; the body further comprising: a plurality of superhard structures located in the non-superhard phase.

In some embodiments, the mass of superhard grains exhibit inter-granular bonding and define a plurality of interstitial regions therebetween, the non-superhard phase at least partially filling a plurality of the interstitial regions around said superhard structures in said non-superhard phase.

The non-superhard phase may comprise, for example, a binder material.

In some embodiments, the superhard structures in the pools of non-superhard phase may comprise any one or more of nano-particles of superhard material, acicular flakes of superhard material or rosette shaped particles of superhard material.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will now be described in more detail, by way of example only, with reference to the accompanying figures in which: Figure 1 is a schematic perspective view of a PCD cutter insert for a drill bit for boring into the earth; Figure 2 is a schematic cross section view of the PCD cutter insert of Figure 1 together with a schematic expanded view showing the microstructure of the PCD material; Figure 3a is an SEM image of a section through an embodiment of PCD material showing precipitated diamond particles in the poois of residual catalyst-binder between sintered diamond grains; Figure 3b is an enlarged view of a section of Figure 3a; Figure 4a is an SEM image through a different section of the embodiment of PCD material of Figures 3a and 3b showing precipitated diamond particles in the pools of residual catalyst-binder between sintered diamond grains; Figure 4b is an enlarged view of a section of Figure 4a; Figure 5 is an SEM image of a section through another embodiment of PCD material showing precipitated diamond particles in the pools of residual catalyst-binder between sintered diamond grains; and Figure 6 is an SEM image of a further section through the embodiment of PCD material of Figure 5 showing precipitated diamond particles in the form of flakes or rosettes in the pools of residual catalyst-binder between sintered diamond grains.

The same reference numbers refer to the same respective features in the drawings, where applicable.

DETAILED DESCRIPTION

As used herein, a "superhard material" is a material having a Vickers hardness of at least about 28 GRa. 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, thereby forming a non-diamond phase, 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. FCBN 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 ultra hard material layer is formed. For example, a "substrate" as used herein may be a transition layer formed over another substrate.

As used herein, "high speed balling milling" means a process of comminution of powder particles during impact between moving balls and between impacting balls and milling jars/pots rotating at speeds higher than 200 rotations per minutes. Milling balls may be made of dense materials, such as stainless steel grades or tungsten carbide.

As used herein, "mechanical alloying" means diffusion or substitution of solute atoms in strained crystal lattices of a solvent matrix in solid state at relatively low temperature (below melting temperature), leading to the formation of metastable or amorphous solid solutions.

Milling, in general, as a means of comminution and dispersion is well known in the art. Commonly used milling techniques used in grinding ceramic powders include conventional ball mills and tumbling ball mills, planetary ball mills and attrition ball mills and agitated or stirred ball mills.

In conventional ball milling the energy input is determined by the size and density of the milling media, the diameter of the milling pot and the speed of rotation. As the method requires that the balls tumble, rotational speeds, and therefore energy are limited. Conventional ball milling is well suited to milling of powders of low to medium particle strength. Typically, conventional ball milling is used where powders are to be milled to final size of around 1 micron or more.

In planetary ball milling, the planetary motion of the milling pots allows accelerations of up to 20 times of gravitational acceleration, which, where dense media are used, allows for substantially more energy in milling compared to conventional ball milling. This technique is well suited to comminution in particles of moderate strength, with final particle sizes of around 1 micron.

In high speed energy ball milling, speeds of greater than 200 RPM allows for greater energy than conventional planetary ball milling. The type of ball mill used in high speed energy ball milling may be, for example, a planetary ball mill, an attrition mill or a roller mill with horizontal axis.

As used herein, "catalyst material for diamond" is 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 at which diamond is thermodynamically more stable than diamond.

Figure 1 shows a PCD cutter insert 10 for a drill bit (not shown) for boring into the earth, comprising a PCD body 20 bonded to a cemented tungsten carbide substrate 30.

Figure 2 is a cross-section through the FCD cutter insert 10 of Figure 1. The microstructure 21 of the PCD body 20 is also shown and comprises a skeletal mass of inter-bonded diamond grains 22 defining interstices 24 between the diamond grains, the interstices 24 being at least partly filled with a filler material comprising, for example, cobalt, nickel or iron.

A superabrasive compact which may be used to form the PCD body 20 of a PCD cutter insert such as that shown in Figure 1, according to an embodiment may be prepared, for example, by the following method. In a first step, a carbon-supersaturated binder catalyst alloy is prepared by dissolving carbon atoms in a metal system using high energy ball milling and mechanical alloying. The starting materials comprise a source of elemental carbon, such as graphite or amorphous carbon powder, and a catalyst elemental metal powder, such as one or more of cobalt, iron, manganese, nickel, platinum, ruthenium, or an alloy powder formed for example from one or more of said metals. The starting mixture may, for example, be formed of between around 85 wt% to around 97 wt% of catalyst elemental metal powder or alloy powder and between around 3 wt% to around 15 wt% graphite or amorphous carbon.

The catalyst elemental metal powder or alloy powder and the source of elemental carbon are mechanically alloyed by processing in a high energy ball mill machine at a temperature of less than around 200 degrees C in, for example, stainless steel or ceramic milling pots. The milling media may comprise stainless steel or ceramic balls. The ball-to-powder weight ratio is, for example, selected to be between around 10:1 and around 30:1. The milling atmosphere may be inert or reducing, for example, an argon or nitrogen atmosphere. The high speed energy ball milling is undertaken for a duration of up to around 48 hours depending on the metal/alloy powder used and the type of milling machine.

The grains of metal or alloy and the source of elemental carbon such as graphite are heavily strained and fractured into finer particles at the early milling stage. This is observable from the broadening and low intensities of the X-ray diffraction peaks. The mechanical energy input is stored in the strained crystals of the powder mixture constituents. The densities of dislocations increase in the particles of the milled powders. The crystals of graphite are destroyed and carbon atoms diffuse under dislocations and other defects and are dissolved in solid state in the crystals of the metals or alloys. This leads to the formation of a homogeneous solid solution, where the distribution of carbon atoms reaches the atomic scale. The scanning electron microscopy method and elemental mapping may be used to evaluate the completion of the dissolution process through examining the cross sections of the milled powder particles. The formation of a solid solution may also be confirmed by the displacement of the X-ray diffraction peaks toward higher or lower angles depending on the lattice parameters of the starting metal or alloy. It is believed that the relaxation of the lattice strains may be the driving force for dissolution of carbon atoms in the metal or alloy powder particles.

The lattice strains of the milled constituents may be analysed by X-ray diffraction for example using a Philips X'Pert powder diffractometer with an X' Celerator detector, and variable divergence and receiving slits with Ni filtered Co-Ku radiation. These parameters may be calculated by means of a modified Williamson-Hall method. The compositions and distribution of the phases in the cross sections of the powder particles may be analysed in back scattered electron mode under 12kV in an EDAX ESEM instrument equipped with an X-ray energy dispersive detector.

The above-described mechanical alloying process as opposed to a standard mechanical milling process enables the formation of a metastable supersaturated solid solution having more carbon in the metal/alloy than is predicted by the equilibrium phase diagram(s). This supersaturated solid solution forms a mechanically alloyed binder powder which may then be used in the sintering process to manufacture a polycrystalline superabrasive construction or compact of, for example, PCBN or PCD.

In one embodiment, the mechanically alloyed binder powder is admixed with diamond particles, such as synthetic or natural diamond particles, in the proportions of around 1 wt% to around 20 wt% of binder in diamond powder, by ball milling the powders in liquid ethanol or methanol at low milling energy input. This process may assist in achieving a homogeneous mixture of diamond and binder particles in the form of a slurry. In some embodiments, the mean size of the diamond particles in the starting mixture may be from, for example, about 1 to at least about 50 microns. The slurry is dried in a furnace at about 80 degrees C to remove the ethanol or methanol. In some embodiments, the dried powder mixture may be mixed with an organic solution to form a further slurry that may be cast into a diamond/binder paper of desired thickness. PCT application publication number WO2O08/06314 discloses a method of coating diamond particles in binder prior to sintering which may be used in embodiments described herein to achieve the admixing of the diamond and binder powders.

A pre-composite may then be created by forming a layer of the dried diamond/binder powder mixture or paper onto a ceramic substrate such as WC-Co or A1203. The pre-composite may, in some embodiments, be subjected to de-bindering and outgassing before sintering using standard conventional procedures.

The pre-composite may be sintered to form a PCD composite according to standard methods, for example as described in PCT application publication number W02011/141898, using HpHT conditions to produce a sintered PCD table attached to a substrate. The substrate may be pre-formed, for example by pressing a green body of grains of hard material such as tungsten carbide into the desired shape, and sintering the green body to form the substrate element. The substrate may, for example, comprise WC particles bonded with a catalyst material such as cobalt, nickel, or iron, or mixtures thereof. A green body for the superhard construction, which comprises the pre-formed substrate and the particles of superhard material such as diamond particles or cubic boron nitride particles admixed with the binder, may be placed onto the substrate, to form a pre-sinter assembly which may be encapsulated in a capsule for an ultra-high pressure furnace, as is known in the art. In particular, the superabrasive particles/binder mixture, for example in powder form, may be placed inside a metal cup formed, for example, of niobium, tantalum, or titanium. The pre-formed substrate is placed inside the cup and hydrostatically pressed into the superhard powder such that the requisite powder mass is pressed around the interface features, if any, of the preformed carbide substrate to form the pre-composite. The pre-composite is then outgassed at about 1050 degrees C. The pre-composite is closed by placing a second cup at the other end and the pre-composite is sealed by cold isostatic pressing or EB welding. The pre-composite is then sintered to form the sintered body of superhard material bonded to the substrate along the interface therewith.

In one example, the method may include loading the capsule comprising a pre-sinter assembly into a press and subjecting the green body to an ultra-high pressure and a temperature at which the superhard material is thermodynamically stable to sinter the superhard grains. In some embodiments, the green body may comprise diamond grains and the pressure to which the assembly is subjected is at least about 5 GPa and the temperature is at least about 1,300 degrees centigrade. In some embodiments, the pressure to which the assembly may be subjected is around 5.5-6 GPa, but in some embodiments it may be around 7.7GPa or greater. Also, in some embodiments, the temperature used in the sintering process may be in the range of around 1300 to around 2000 degrees C. Embodiments of the nvenhion are described b&ow by way of the foflowing exampes wftch are not intended to be Umiting.

Exampe I A superabrasive construction was prepared by applying a high energy ball milling process to 3wt% graphite, 5wt% aluminium and 92 wt% cobalt powder, at a temperature of less than around 200 degrees C in stainless steel or ceramic milling pots for a duration of up to around 48 hours to form a mechanically alloyed binder powder. The mechanically alloyed binder powder was then admixed with synthetic diamond particles in the proportions of around 1 wt% to around 20 wt% of binder in diamond powder, by ball milling the powders in liquid ethanol at low milling energy input to form a slurry. The slurry was dried in a furnace at about 80 degrees C to remove the ethanol.

A pre-composite was then created by forming a layer of the dried diamond/binder powder mixture on a ceramic substrate of WC-Co. The pre-composite was sintered at a pressure of around 5.5GPa and temperature of around 1400CC to form a PCD composite compact.

Example 2

A superabrasive compact was prepared by applying a high energy ball milling process to 3wt% graphite and 97 wt% cobalt powder, at a temperature of less than around 200 degrees C in stainless steel or ceramic milling pots for a duration of up to around 48 hours to form a mechanically alloyed binder powder. The mechanically alloyed binder powder was then admixed with synthetic diamond particles in the proportions of around 1 wt% to around 20 wt% of binder in diamond powder, by ball milling the powders in liquid ethanol at low milling energy input to form a slurry. The slurry was dried in a furnace at about 80 degrees C to remove the ethanol.

A pre-composite was then created by forming a layer of the dried diamond/binder powder mixture on a ceramic substrate of WC-Co. The pre-composite was sintered at a pressure of around 5.5GPa and temperature of around 1400t to form a PCD composite compact.

The sintered compacts of Examples 1 and 2 were analysed using image analysis. Several images of different parts of a surface or section (hereinafter referred to as samples) through the superabrasive layer were analysed. The results are shown in Figures 3a to 6.

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).

To achieve the SEM images of Figures 3a to 6, a sample piece of each of the PCD sintered bodies was cut using wire EDM and polished. At least 10 back scatter electron images of the surface of the sample were taken using a Scanning Electron Microscope at 1000 times magnifications.

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.

An auto threshold feature was used to binarise the SEM image and specifically to obtain clear resolution of the diamond and binder phases. The binder phase is shown as the grey areas in the SEM images of Figures 3a to 6 and the diamond phase is shown in black.

It will be seen from Figures 3a and 3b which show larger binder pools between interbonded diamond grains forming the PCD material that diamond particles have precipitated in the binder pools during the sintering process.

Figures 4a and 4b show smaller binder pools with the precipitated diamond therein. In the embodiments shown in Figures 5 and 6, in the smaller binder pools shown in Figure 5 of the analysed sample produced using the method of Example 2, diamond particles precipitated in the binder pools during the sintering process. In the section of sample shown in Figure 6, diamond flakes or rosettes and fine nano-sized diamond particles precipitated in the larger binder pool shown, during the sintering process.

Whilst not wishing to be bound by theory, it is believed that the additional presence of superhard phase in the binder pools of the embodiments of PCD illustrated in Figures 3a to 6 is due to simultaneous precipitation-synthesis occurring in the binder phase during sintering. Precipitation-synthesis means the metastable supersaturated solid solution binder formed during high energy ball milling process is thermally decomposed at high temperature.

The carbon atoms thus released precipitate and grow into diamond crystals within the binder phase during the subsequent sintering process.

Mechanically alloying low melting temperature metals such as aluminium, copper, zinc, tin in the proportion of 0.5 to lOwt% during high energy ball -20 -milling preparation of the metastable binders may improve the nucleation rate of fine diamond precipitates in the binder phase, as shown in the comparison of the SEM images of Figures 3a to 4b of Example 1 and those of Figures 5 and 6 of Example 2. The dissolution of low melting temperature metals in the mechanically alloyed binder thereby may assist in controlling the morphology of the diamond particles. The precipitated diamond particles in the binder pools of the sintered product may take the form of, for example, one or more of long acicular flakes, rosette shaped particles and fine particles such as nano-sized particles of sub micron size.

It will be seen from Figures 3a to 6 that the precipitation-synthesis increases the intergrowth of the superhard particles, namely diamond in the examples of Figures 3a to 6. The precipitation-synthesis occurs inside the binder pools during the sintering process to achieve an increase in the volume of diamond particles. This volume increase in diamond particles corresponds to a volume decrease of cobalt in the PCD compact due to the pressure effect generated by the precipitation-synthesis process.

Unlike conventional PcBN materials with attrition milled or wet milled binder materials, the binder materials of some embodiments prepared by high energy ball milling and sintered with PcBN or PCD is homogeneous (single phase) due to increased coefficients of inter diffusion between the high energy milled binder constituents.

In some embodiments, longer milling times may improve the homogeneity and the formation of nanostructured binder pools in the sintered products.

The cBN or diamond may contain multimodal particles i.e. at least two types of cBN or diamond particles that differ from each other in their average particle size. "Average particle size" means the major amount of the particles will be close to the specified size although there will be a limited number of particles further from the specified size. The peak in -21 -distribution of the particles will have a specified size. Thus, for example if the average particle size is 2 pm, there will by definition be some particles which are larger than 2 pm, but the major amount of the particles will be at approximately 2 pm in size and the peak in the distribution of the particles will be near 2 pm.

The use of multimodal, such as bimodal, cBN or diamond in the composition may assist in enabling the matrix to be finely divided to reduce the likelihood of flaws of critical size being present in the pre-sintered composition. This may be beneficial for both toughness and strength in the compact produced from the composition.

Typical conditions of elevated temperature and pressure necessary to produce cBN or diamond compacts are well known in the art. These conditions are pressures in the range of about 2 to about 8 GPa, or greater and temperatures in the range of about 1100°C to about 2000°C.

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. Whilst the examples and Figures illustrate embodiments of PCD, corresponding processes and results may be achieved using other superhard particles such as cBN such that the mechanically alloyed supersaturated binder may be used in the sintering process of FCBN.

Thus it will be seen that some embodiments may assist in providing a method of forming a superhard abrasive composite compact with increased density of the superhard particles which are formed in-situ in the binder phase by thermal decomposition and crystallisation and which may be in the form of a fine dispersion of crystalline particles in the binder pools. This process is achievable without the technological challenges such as agglomeration, oxidation, inhomogeneity, conservation and handling of toxic -22 -starting materials in the form of nano-particles as the nano-particles may be grown in situ in the sintering process. Furthermore, the volume fraction of binder is reduced in the sintering process due to the precipitation-sintering of the superhard particles in the binder pools thereby reducing the need for subsequent post-sintering processes such as acid leaching to remove residual binder-catalyst from the interstitial spaces to improve the thermal stability of the superhard composite, although it may be beneficial still to apply such post-sintering processes in some cases, depending on the intended application of the sintered construction.

Claims

-23 -Claims 1. A method of forming a polycrystalline superhard construction comprising: preparing a metastable solid solution to form a catalyst-binder mixture by mechanically alloying a first mass comprising a source of elemental carbon and a second mass of a catalyst comprising an elemental metal powder or metal alloy powder; the first mass comprising between around 3 wt% to around 15 wt% of source of elemental carbon, and the second mass comprising between around 85 wt% to around 97 wt% catalyst powder; mixing the catalyst-binder mixture with a mass of superhard particles to form a green body; placing the green body into a canister to form a pre-sinter assembly; treating the pre-sinter assembly at an ultra-high pressure of around 5.5 GPa or greater and a temperature to sinter together the grains of superhard material to form a polycrystalline superhard construction comprising a superhard phase and a binder phase, an amount of the superhard phase precipitating in part in the binder phase during the step of sintering.
2. The method of claim 1, wherein the step of preparing the metastable solid solution comprises forming the catalyst-binder mixture by mechanically alloying the first mass comprising graphite and/or amorphous carbon powder with the catalyst.
3. The method of any one of the preceding claims, wherein the step of preparing the metastable solid solution comprises forming the catalyst-binder mixture by mechanically alloying the first mass with the second mass, the second mass comprising one or more of the elemental metals cobalt, iron, -24 -manganese, nickel, platinum, ruthenium, or an alloy powder of one or more of said metals.
4. The method of any one of the preceding claims, wherein the step of mechanically alloying comprises processing said first and second masses in a high energy ball mill machine at a temperature of less than around 200 degrees C.
5. The method of claim 4, wherein the step of mechanically alloying comprises using a high energy ball mill machine in which the milling ball-to-powder weight ratio is between around 10:1 and around 30:1.
6. The method of any one of claims 4 or 5, wherein the step of mechanically alloying comprises milling the first and second masses in a high energy ball mill machine in a milling atmosphere, the milling atmosphere being inert.
7. The method of any one of claims 4 or 5, wherein the step of mechanically alloying comprises milling the first and second masses in a high energy ball mill machine in a milling atmosphere, the milling atmosphere being a reducing atmosphere.
8. The method of claim 7, wherein the milling atmosphere comprises argon or nitrogen.
9. The method of any one of the preceding claims, wherein the step of mechanically alloying comprises applying a high speed energy ball milling process to mill the first and second masses for a duration of up to around 48 hours.-25 -
10. The method of any one of the preceding claims, wherein the step of preparing the metastable solid solution further comprises mechanically alloying between around 0.5 wt% to around 10 wt% of a low melting temperature metal with the first and second masses.
11. The method of claim 10, wherein the low melting temperature metal comprises one or more of aluminium, copper, zinc, or tin.
12. The method of any one of the preceding claims further comprising placing the green body in contact with a substrate to form a pre-sinter assembly, the step of treating the pre-sinter assembly further comprising bonding the substrate to the body of polycrystalline superhard material along an interface.
13. The method of claim 12, wherein the substrate comprises cemented carbide material.
14. The method of any one of the preceding claims, wherein the superhard material comprises natural and/or synthetic diamond grains, and/or cubic boron nitride grains.
15. The method of any one of the preceding claims, further comprising treating at least a portion of body of superhard polycrystalline material to render said portion free of catalyst material for the superhard grains, said portion forming a thermally stable region.
16. A superhard polycrystalline construction comprising a body of polycrystalline superhard material, the body comprising: a mass of superhard grains forming a superhard phase; and a non-superhard phase; the non-superhard phase forming pools in interstitial regions between superhard grains; the body further comprising: -26 -a plurality of superhard structures located in the non-superhard phase.
17. The superhard polycrystalline construction of claim 16, wherein the mass of superhard grains exhibit inter-granular bonding and define a plurality of interstitial regions therebetween, the non-superhard phase at least partially filling a plurality of the interstitial regions around said superhard structures in said non-superhard phase.
18. The superhard polycrystalline construction of any one of claims 16 or 17, wherein the non-superhard phase comprises a binder material.
19. The superhard polycrystalline construction of any one of claims 16 to 18, wherein the superhard structures in the pools of non-superhard phase comprise any one or more of nano-particles of superhard material, acicular flakes of superhard material or rosette shaped particles of superhard material.
20. The superhard polycrystalline construction of any one of claims 16 to 19, further comprising a substrate bonded to the body of superhard material along an interface.
21. The construction of claim 20, wherein the substrate comprises cemented carbide material.
22. The construction of any one of claims 16 to 21, wherein the superhard material comprises natural and/or synthetic diamond grains, and/or cubic boron nitride grains.
23. The superhard polycrystalline construction of any one of claims 16 to 22, wherein the body of superhard material comprises polycrystalline diamond material having interbonded diamond grains and interstices therebetween; wherein at least a portion of the body of superhard material is -27 -substantially tree ot a catalyst material for diamond, said portion torming a thermally stable region.
24. A tool comprising a superhard polycrystalline construction according to any one of claims 16 to 23, the tool being for cutting, milling, grinding, drilling, earth boring, rock drilling or other abrasive applications.
25. A tool according to claim 24, wherein the tool comprises a drill bit for earth boring or rock drilling.
26. A tool according to claim 24, wherein the tool comprises a rotary fixed-cutter bit for use in oil and gas drilling.
27. A tool according to claim 24, wherein the tool is a rolling cone drill bit, a hole opening tool, an expandable tool, a reamer or other earth boring tools.
28. A drill bit or a cutter or a component therefor comprising the superhard polycrystalline construction according to any one of claims 16 to 23.
29. A method of making a polycrystalline superhard construction substantially as hereinbefore described with to any one embodiment as that embodiment is illustrated in the accompanying drawings.
30. A superhard polycrystalline construction substantially as hereinbefore described with to any one embodiment as that embodiment is illustrated in the accompanying drawings.