WO2012019996A1

WO2012019996A1 - Polycrystalline superhard material

Info

Publication number: WO2012019996A1
Application number: PCT/EP2011/063617
Authority: WO
Inventors: Charles Stefan Montross
Original assignee: Element Six Abrasives S.A.
Priority date: 2010-08-13
Filing date: 2011-08-08
Publication date: 2012-02-16
Also published as: GB201113618D0; GB201013615D0; GB2482728A; CN103228821A; EP2603621A1; US20130205678A1; GB2482780A

Abstract

A method for making polycrystalline superhard material comprises providing an electrically conductive substrate defining at least one deposition surface, electrophoretically depositing charged superhard particles or grains on to the deposition surface(s) of the substrate to form a pre-sinter body, and subjecting the pre-sinter body to a temperature and pressure at which the superhard material is thermodynamically stable, sintering and forming polycrystalline superhard material. There is also disclosed a superhard wear element comprising a polycrystalline superhard material produced by such a method.

Description

POLYCRYSTALLINE SUPERHARD MATERIAL

Field This disclosure relates to a method of making polycrystalline superhard material, particularly but not exclusively to a method of making polycrystalline diamond (PCD) material, and to a method of making elements comprising same. Background

Cutter inserts for machine and other tools may comprise a layer of polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN) bonded to a cemented carbide substrate. PCD and PCBN are examples of superhard material, also called superabrasive material, which have a hardness value substantially greater than that of cemented tungsten carbide.

Components comprising PCD are 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. PCD comprises a mass of substantially inter-grown diamond grains forming a skeletal mass, which defines interstices between the diamond grains. PCD material comprises at least about 80 volume % of diamond and may be 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 degrees centigrade in the presence of a sintering aid, also referred to as a catalyst material for diamond. Catalyst material for diamond is understood to be material that is capable of promoting direct inter-growth of diamond grains at a pressure and temperature condition at which diamond is thermodynamically more stable than graphite. Some catalyst materials for diamond may promote the conversion of diamond to graphite at ambient pressure, particularly at elevated temperatures. Examples of catalyst materials for diamond are cobalt, iron, nickel and certain alloys including any of these. PCD may be formed on a cobalt-cemented tungsten carbide substrate, which may provide a source of cobalt catalyst material for the PCD. The interstices within PCD material may at least partly be filled with the catalyst material.

Components comprising PCBN are used principally for machining metals. PCBN material comprises a sintered mass of cubic boron nitride (cBN) grains. The cBN content of PCBN materials may be at least about 40 volume %. When the cBN content in the PCBN is at least about 70 volume % there may be substantial direct contact among the cBN grains. When the cBN content is in the range from about 40 volume % to about 60 volume % of the compact, then the extent of direct contact among the cBN grains is limited. PCBN may be made by subjecting a mass of cBN grains together with a powdered matrix phase, to a temperature and pressure at which the cBN is thermodynamically more stable than the hexagonal form of boron nitride, hBN. PCBN is much less wear resistant than PCD, which may limit its scope of application.

Summary

Viewed from a fi rst aspect there is provided a method for making polycrystalline superhard material, the method comprising providing an electrically conductive substrate defining at least one deposition surface, electrophoretically depositing charged superhard particles or grains on to the deposition surface(s) of the substrate to form a pre-sinter body, and subjecting the pre-sinter body to a temperature and pressure at which the superhard material is thermodynamically stable, sintering and forming polycrystalline superhard material. In some embodiments, the substrate forms the cathode of an electrophoretic cell apparatus, the superhard particles or grains being suspended in a liquid in contact with the deposition surface(s) and an anode, the superhard particles or grains being positively charged so as to be deposited on a deposition surface or surfaces of the substrate upon application of an electric potential between the cathodic substrate and the anode. In some embodiments, the anode defines a complimentary surface or surfaces opposing the deposition surface(s) of the cathodic substrate.

In some embodiments, charged superhard particles or grains are deposited on the substrate in a series of layers or strata.

In some embodiments, the deposition surface(s) of the substrate is/are masked in certain areas or regions, the superhard particles or grains being deposited on exposed portions of the deposition surface(s) so to form discrete three dimensional polycrystalline superhard structures.

The various layers or three dimensional structures of polycrystalline superhard material, in some em bodiments, have at least one different structural characteristic, non-limiting examples of which may include mean superhard grain size, superhard grain content, and content of catalyst for diamond.

In one embodiment, the polycrystalline superhard material is PCD and the superhard particles or grains comprise diamond.

In some embodiments, the diamond particles or grains have an average particle or grain size of from about 5 nanometres to about 50 microns.

In some embodiments, a multimodal mixture of diamond particles or grains of varying average particle or grain size are deposited on the substrate. In some embodiments, the diamond particles or grains are pre-treated with hydrogen or a source of hydrogen ions to render them positively charged. In some embodiments, additional particulate materials are added to the dispersed diamond particles or grains in order to be deposited with the diamond particles or grains on the substrate. Exemplary particulate materials include any compounds containing elements from Groups IA-VIIIA and Groups IB-VIIIB, for example alkali or alkali earth metal, metal or non-metal carbides, nitrides, oxides, carbonitrides, hal ides, borides, sulphates, phosphates, tungstates and the like, or of the unreacted elements, for example metal powders. In some embodiments, the polycrystalline superhard material is PCBN and the superhard particles or grains comprise cBN.

In some embodiments, the superhard grain content of the polycrystalline superhard material is at least about 80 percent, at least about 88 percent, at least about 90 percent, at least about 92 percent or even at least about 96 percent of the volume of the polycrystalline superhard material. In one embodiment, the superhard grain content of the polycrystalline superhard material is at most about 98 percent of the volume of the polycrystalline superhard material.

In some embodiments, the polycrystalline superhard material is PCD material that comprises a catalyst material for diamond, the content of the catalyst material being at most about 1 0 volume percent, at most about 8 volume percent, or even at most about 4 volume percent of the PCD material. In one embodiment, the PCD material comprises at least a region that is substantially free of catalyst material for diamond.

I n one em bodiment, the pre-sinter body comprises deposited diamond particles or grains and the method includes subjecting the pre-sinter body in the presence of a catalyst material for diamond to a pressure and temperature at which diamond is more thermally stable than graphite. In one embodiment, the pressure is at least about 5.5GPa and the temperature is at least about 1 ,250 degrees centigrade.

In one embodiment, the precursor body comprises deposited cBN particles or grains and the method includes subjecting the pre-sinter body to a pressure and temperature at which cBN is more thermally stable than hexagonal boron nitride (hBN). In one embodiment, the pressure is at least about 2GPa and the temperature is at least about 900 degrees centigrade. Viewed from a second aspect there is provided a superhard wear element comprising an embodiment of polycrystalline superhard material made by the above-described method.

In some embodiments, the superhard wear element comprises a plurality of regions, each region comprising polycrystalline superhard material having at least one different structural characteristic, non-limiting examples of which may include mean superhard grain size, superhard grain content, and content of catalyst for diamond. In some embodiments, at least some of the regions are in the form of layers or strata. In one embodiment, at least one of the regions is lean or substantially free of metallic catalyst material for diamond. In other embodiments, at least some of the regions are in the form of discrete three dimensional polycrystalline superhard structures.

In one embodiment, the superhard wear element comprises a structure comprising polycrystalline superhard material joined to a substrate comprising cemented carbide material.

Drawings

Non-limiting embodiments will now be described by way of example and with reference to the accompanying drawings in which: FIG 1 is a schematic side view of an electrophoretic cell apparatus for use in an embodiment;

FIG 2A is a schematic drawing of the microstructure of an element comprising polycrystalline superhard material made by an embodiment;

FIG 2B is a schematic drawing of the microstructure of an element comprising polycrystalline superhard material made by another embodiment; FIG 2C is a schematic drawing of the microstructure of an element comprising polycrystalline superhard material made by a further embodiment; and

F I G 3 is an SEM micrograph of a cross-section of a sintered element comprising polycrystalline superhard material made by an embodiment.

Detailed description of embodiments

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

As used herein, polycrystalline diamond (PCD) material comprises 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 at least partly be 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 P C D m ateria l , interstices or interstitial regions may substantially or partially be filled with a material other than diamond, or they may substantially be empty. Embodiments of 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, polycrystalline cubic boron nitride (PCBN) 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.

As used herein, a superhard wear element is an element comprising a superhard material and is for use in a wear application, such as degrading, boring into, cutting or machining a workpiece or body comprising a hard or abrasive material.

As used herein, a multimodal size distribution of a mass of grains includes more than one peak, or that can be resolved into a superposition of more than one size distribution each having a single peak, each peak corresponding to a respective "mode". Multimodal polycrystalline bodies are typically made by providing more than one source of a plurality of grains, each source comprising grains having a substantially different average size, and blending together the grains.

Electrophoretic deposition (EPD) is a method of forming layers of deposited particles whereby electrically charged particles in liquid dispersion are migrated to one or other electrode by applying an electric potential across two electrodes. Control of the properties of the layers may be achieved by controlling the magnitude and duration of the applied potential, the size and concentration of the suspended particles, and the relative orientation of the two electrodes. With reference to FIG 1 , an embodiment of a method for making an example embodiment of a polycrystalline superhard material is carried out in an electrophoretic cell apparatus 10. The method includes placing a substrate 12, which forms the cathode of the electrophoretic cell 10, with a deposition surface 14 thereof facing towards an anode 16. Very different configurations of the cathodic substrate 12 and anode 16 of the electrophoretic cell could be used. For example, they may be arranged to lie horizontally with one above the other, or they may be cylindrically co-axial. In this embodiment, the cathodic substrate 12 is partially encased in a rubber sleeve 18 such that only deposition surface 14 is exposed. Furthermore, more than one surface, or an alternative surface, may be exposed for providing a number of different deposition surfaces. The exposed deposition surface 14 and at least a portion of the anode 16 is immersed in a stable aqueous suspension 20 containing superhard particles or granules 22, which have been pre-treated in hydrogen to render them positively charged. A magnetic stirrer 24 is used to maintain the superhard particles or granules 22 in suspension. In an example embodiment, a DC potential of about 3V may be applied for about 2 minutes to generate the electric field between the cathode 12 and anode 16 to deposit the superhard particles or grains onto the deposition surface 1 4 of the cathodic substrate 12, to form a layer 26 of deposited superhard particles or grains. The process conditions, such as the acidity of the suspension, the type and quantity of dispersants and the superhard particle or grain surface chemistry may be optimised. The resulting pre-sinter body comprising the superhard layer 26 deposited on the substrate 12 is then removed from the cell 10, dried and sintered.

Referring to Figures 2A to 2C, various configurations of superhard elements are shown. In its simplest form, Figure 2A shows a superhard element 30 comprising a single layer 32 of polycrystalline superhard material, in this case polycrystalline diamond, bonded to a substrate 34, in this case a Co-WC substrate. Figure 2B shows a superhard element 40 comprising a plurality of layers 42 of polycrystalline superhard material, in this case polycrystalline diamond having varying characteristic features, bonded to a substrate 44, in this case a Co-WC substrate. Figure 2C shows a superhard element 30 similar to that of Figure 2A, except that in this case a non-planar interface 36 is defined between the layer 32 of polycrystalline diamond and the Co-WC substrate 34.

In embodiments where the precursor body comprises diamond grains, the pre-sinter body may be sintered by subjecting it in the presence of a catalyst material for diamond to a temperature of about 1 ,350 degrees centigrade and a pressure of about 5.5 GPa to form an embodiment of PCD material.

An ion permeable membrane may be placed between the anode and the cathodic substrate in the electrophoretic cell. This may allow ions to pass through it between the anode and the substrate, while shielding the substrate from gas that may evolve at the anode surface, which may disrupt the electrophoretic deposition process and reduce the efficiency of electrophoretic deposition of the superhard particles or grains on to the substrate.

The liquid used to disperse the powders may consist of organic or inorganic components, or a mixture of the two, for example water, alcohol, or a water- alcohol mixture. The concentration of the dispersed particles or grains in the liquid may vary from 0.1 % to 60%, but preferably 10-20% by weight. The concentration may be maintained at a constant value by gradual or periodic addition of extra particles or grains to the liquid for the duration of the deposition. Alternatively, the concentration may be varied by not replenishing the particles or grains during deposition, so that the concentration of dispersed particles or grains gradually decreases, or the concentration may be increased by adding more particles or grains than has been removed due to deposition. Particles or grains of various compositions and particle sizes may be added at different times to obtain layers of varying composition and particle size. Alternatively, the electrode assembly may be dipped in one suspension for a certain amount of time to achieve deposition, then lifted up and dipped in a different suspension to achieve deposition of a different composition. This sequence may be repeated many times and in different suspensions to build up the required green-body deposit.

The substrate which serves as the electrode onto which the diamond particles or grains or other superhard material is deposited, may be of any material, as long as it is electrically conductive, e.g. electrically conductive polymers such as polypyrrole, or electrically conductive composites e.g. graphite particles or fibres in polymer, or Co-WC. In one embodiment, the substrate comprises a composite of Co-WC and a layer of polycrystalline superhard material, the layer of superhard material defining the deposition surface(s).

The substrate/electrode surface may be planar, non-planar (shaped), and may be of regu lar or irregular shape, sym metrical or non-symmetrical. The substrate may vary in thickness from 0.1 millimetres to 10 centimetres, and the diameter may vary from 0.1 millimetres to 10 centimetres.

The counterelectrode or anode may consist of any electrically conducting material, e.g. metal, graphite or coated metal or graphite. It may have a flat surface, or a shaped non-planar surface, which may be complementary in shape to the deposition surface or surfaces of the cathodic substrate. The reason for this is that the current density at any region on the electrode, and therefore the amount of particles or grains deposited at that region, is a function of the distance between the two electrodes at that point. Regions on the substrate/electrode where it is closer to the anode/counterelectrode will experience a faster rate of deposition, and regions where it is further from the anode/counterelectrode will experience a slower rate of deposition. By controlling the shapes of the substrate/electrode and the anode/counterelectrode, the shape of the surface of the deposited layer, i.e. the thickness of the layer at specific places, may be controlled.

The counterelectrode or anode may also be changed at various stages of the deposition process in order to tailor the deposited layer. For example, a counterelectrode shaped with a dimple or indentation in the centre will cause more electrodeposition to occur on the periphery of the substrate/electrode, or alternatively a counterelectrode with a protrusion in the centre will cause more e l e ct ro d e po s i t i o n i n t h e ce nt re of t h e substrate/electrode. The substrate/electrode may also be blanked off in certain areas using a masking agent, causing the electrodeposited layer to form around these blanked off areas. In this way, by using a modified counterelectrode or substrate/electrode at certain stages of the EPD process, the deposited layer may be tailor-made or functionally graded according to requirements.

Generally the thickness of the electrodeposited layer may vary from 50 nanometres to 5 millimetres.

Although the use of EPD in diamond synthesis has found application in the coating of objects with diamond as separate particles, as discrete nucleation sites for further diamond growth by chemical vapour deposition, or as particles included in metal layers, it has not been used for preparing green bodies for the high-pressure high-temperature production of sintered polycrystalline diamond (PCD).

Thus, DE201 1966 discloses the coating by EPD of an electrically conductive support with non-metallic particles such as carbides, nitrides, borides, silicides and oxides of metals such as W, Mo, Ta, Nb, Ti, Zr; carbides and oxides of Si and B, diamond powder and AI2O3. The problem of adhesion to the support is overcome by first electrodepositing a metal layer to the support, followed by electrophoretic deposition of the non-metallic powder such that the powder particles lodge in the pores of the metal layer. The resulting product is a support coated with a layer of metal containing discrete, non-intergrown hard and wear resistant particles.

Layers of diamond film on silicon find wide application in the semiconductor industry. However, such diamond films, grown by chemical vapour deposition, lack adhesion and uniformity when grown on silicon substrates. US5128006 discloses the use of EPD to obtain discrete and adherent diamond particles on an oxidised silicon substrate. The problem of adherence of diamond films to WC-Co substrates is addressed in J P2002338386, which discloses a method of obtaining a diamond coating on a WC-Co substrate by first acid treating the WC-Co substrate, followed by EPD to deposit discrete diamond particles as seeds onto the acid-treated WC-Co surface, heat treating the seeded substrate and finally growing a diamond film onto the diamond seeds by chemical vapour deposition.

United States patent number 6,258,237 discloses a method of depositing diamond particles on a surface of a substrate, the method comprising the steps of (a) charging the diamond particles by a positive charge to obtain positively charged diamond particles; and (b) electrophoretically depositing the positively charged diamond particles on the surface of the substrate, for obtaining a green diamond particles coat on the surface of the substrate. A different set of problems arise when manufacturing polycrystalline diamond (PCD) cutters, which are used mainly in oil and gas drilling applications. During PC D synthesis, d iamond powder is placed on top of a WC-Co substrate and the assembly is placed in a capsule and pressed at high- pressure high-temperature. Duri ng press i ng , the coba lt i n the WC-Co substrate becomes molten and infiltrates the diamond powder, effectively dissolving some of the outer surface of the diamond particles and precipitating new diamond so that the diamond particles are strongly connected to each other by diamond intergrowth and strongly connected to the WC-Co substrate by the solidified metal infiltrant. Adhesion of the intergrown PCD layer to the substrate therefore does not pose a problem. However, the performance and lifetime of PCD are strongly dependent on managing the stresses in the PCD layer, ensuring thermal stability and obtaining the desired m icrostructure. These desirable properties may be achieved by choosing the appropriate means of combining the various starting materials during green body preparation prior to sintering at high-pressure high-temperature. It has now surprisingly been found that EPD may be used to produce a pre-sinter body comprising diamond or cBN deposited on a substrate that is suitable for making a polycrystalline superhard material and elements comprising same.

Diamond grains typically have a negative charge and would tend to migrate towards a positively charged electrode, the anode. In embodiments of the method, the diamond particles or grains My be provided with a positive charge and the substrate may be employed as a cathode, in order to provide cathodic protection to the substrate, to assist in inhibiting the substrate from undergoing anodic corrosion, which could damage it and possibly make it unsuitable as a substrate for a polycrystalline diamond element. Accordingly, the diamond particles or grains may be contacted with hydrogen or a source of hydrogen ions in order to render them positively charged.

Electrophoretic deposition of diamond particles or grains, with or without additives, offers a simple, quick and controllable method for obtaining thin layers of uniform thickness and homogeneous composition on flat or non- planar surfaces, and it is simple to vary the sequence of the layers according to the material property requirements for managing stresses in the PCD.

Examples

Embodiments are described in more detail with reference to the examples below, which are not intended to be limiting. Example 1

Diamond powder of 2.5 m icron average particle size was treated in a hydrogen atmosphere at 800°C for 1 hour in order to hydrogen-terminate the surfaces of the diamond, thereby positively charging the diamond particles.

An electrophoretic cell, similar to that depicted in Figure 1 , was assembled, consisting of a platinum-coated titanium plate anode, and a cathode cons isti ng of a standard cylindrical cobalt-cemented tungsten carbide substrate as is normally used in high-pressure high-temperature polycrystalline diamond (PCD) synthesis. The substrate was approximately 25 mm high, and had a diameter of approximately 20 mm. The substrate was fitted with a close-fitting non-conducting rubber sleeve which blanked off the sides and the back of the substrate, in order to prevent any deposition of diamond particles on the sides of the cylinder. Electrical contact was made through the rubber sleeve to the back of the substrate.

The electrodes were inserted into a glass beaker containing 200 ml deionised water with the exposed area of the substrate facing the Pt/Ti anode, and the electrodes were electrically connected to a direct current power supply. The beaker was placed on a magnetic stirrer plate and the water was stirred vigorously by means of a magnetic stirrer bar.

Approximately 40 g of hydrogen-treated diamond was added to the deionised water, resulting in an initial dispersed diamond concentration of approximately 20%. The pH of the deionised water was adjusted with small aliquots of 15% nitric acid or 10% aqueous ammonium hydroxide in order to maintain the pH in the range 2-4. The power supply was switched on and a constant potential of approximately 3 V was applied for approximately 2 minutes. A layer of diamond particles less than approximately 0.3 millimetres thick deposited on the Co-WC substrate surface. The layer was surprisingly well adherent to the substrate surface. The substrate with diamond layer was placed in a capsule and sintered under standard high-pressure high-temperature conditions. SEM analysis of a cross- section of the sintered PCD showed a well-sintered layer of approximately 100 micron thick, with strong adherence to the Co-WC substrate.

Example 2

The procedure used in Example 1 was again followed, except that an additional 40 g of hydrogen-terminated diamond powder of average particle size 12 micron was gradually added during the period of 2 minutes when electrical potential was applied to the electrodes. A layer of diamond particles less than approximately 0.7 mill imetres thick deposited on the Co-WC substrate surface.

The substrate with diamond layer was placed in a capsule and sintered under standard high-pressure high-temperature conditions. SEM analysis of a cross- section of the sintered PCD, as shown in Figure 3, showed a well-sintered layer of approximately 400 micron thick, with strong adherence to the Co-WC substrate. The average particle size of the diamond grains in the sintered PC D ranged from approximately 2.5 m icron at the d iamond-substrate interface to an average particle size of approximately 8 micron at the top surface of the PCD. Example 3

The procedure used in Example 1 was again followed, except that the total duration of the applied potential lasted for approximately 10 minutes. During this time, the electrode assembly was raised every 2 minutes and lowered into a different solution. Two solutions were used: Solution 1 contained 40 g of hydrogen-treated diamond powder of average particle size 2.5 micron, and solution 2 contained 30 g of hydrogen-treated diamond powder of average particle size 12 m icron and 1 0 g of hydrogen-treated diamond powder of average particle size 2.5 micron. In this experiment, dispersion was achieved by also inserting an ultrasonic probe to ultrasonically disperse the diamond powder. In this manner, 5 layers of alternating average diamond particle size were deposited, each layer less than approximately 0.3 millimetres thick.

The substrate with diamond layers was placed in a capsule and sintered under standard high-pressure high-temperature conditions. SEM analysis of a cross-sect io n of the s i ntered P C D s howed a we l l-sintered layer of approximately 1 millimetre thick, with strong adherence to the Co-WC substrate.

Example 4 The procedure used in Example 1 was again followed, except that the cathode was Co-WC substrate with a sintered PCD layer on top consisting of 12 micron average diamond grain size. Electrophoretic deposition resulted in a layer of d iamond of average particle size 2.5 m icron and less than approximately 0.3 millimetres thick on top of the sintered PCD layer.

The substrate-PCD cutter with diamond layer deposited on top was placed in a capsule and sintered under standard high-pressure high-temperature conditions. SEM analysis of a cross-section of the sintered PCD showed a well-sintered layer of approximately 100 micron thick of average diamond grain size approximately 2.5 micron, with strong adherence to the PCD layer consisting of 12 micron average diamond grain size.

Example 5 The same procedure was followed as in Example 1 , except that the cell configuration was modified. The anode consisted of a platinum mesh placed horizontally at the bottom of the beaker, with the electrical connection and lead covered in an electrically insulating rubber sleeve. The Co-WC cathode in its rubber sleeve was placed face downwards in the aqueous dispersion so that the substrate surface to be coated was facing the anode. Care was taken to ensure that no gas bubbles were trapped against the Co-WC cathode when placed face downwards, as bubbles would prevent deposition, causing pinholes in the deposited layer. Any agglomerates in the dispersion tended to settle towards the bottom of the beaker, falling through the mesh anode, and the resulting deposit that formed on the Co-WC cathode was of more uniform particle size distribution, without agglomerates being included in the deposited layer. Repeated dip-and-coat in different solutions containing different amounts of diamond powder, each of different particle size range, enabled the build-up of layers of diamond of varying thickness and grain size.

Claims

1 . A method for making polycrystalline superhard material, the method comprising providing an electrically conductive substrate defining at least one deposition surface, electrophoretically depositing charged superhard particles or grains on to the deposition surface(s) of the substrate to form a pre-sinter body, and subjecting the pre-sinter body to a temperature and pressure at which the superhard material is thermodynamically stable, sintering and forming polycrystalline superhard material.

2. A method according to claim 1 , wherein the substrate forms the cathode of an electrophoretic cell apparatus, the superhard particles or grains being suspended in a liquid in contact with the deposition surface(s) and an anode, method further comprising depositing the superhard particles or grains on the deposition surface(s) upon application of an electric potential between the substrate and the anode, the superhard particles or grains being positively charged so as to be so deposited.

3. A method according to claim 2, further comprising positioning the anode to define a complementary surface or surfaces opposing the deposition surface(s) of the substrate.

4. A method according to any one of the preceding claims, wherein the step of electrophoretically depositing charged superhard particles or grains comprises depositing the superhard particles or grains on the substrate in a series of layers or strata.

5. A method according to any one of claims 1 to 3, wherein the deposition surface(s) of the substrate is/are masked in certain areas or regions, the step of depos iting the superhard particles or grai ns comprising depositing the superhard particles or grains on exposed portions of the deposition surface(s) so as to form discrete three dimensional polycrystalline superhard structures.

6. A method according to any one of claims 4 or 5, wherein the step of electrophoretically depositing charged superhard particles or grains comprises to form various layers or three dimensional structures of polycrystalline superhard material, comprises forming the layers or three dimensional structures to have differing structural characteristics from one another.

7. A method according to any one of the preceding claims, comprising forming a polycrystalline superhard material having a superhard grain content of at least 80 percent and at most 98 percent of the volume of the polycrystalline superhard material.

8. A method according to any one of the preceding claims, comprising forming polycrystalline diamond material, the superhard particles or grains comprising diamond.

9. A method according to claim 8, wherein the method comprises forming polycrystalline diamond material comprising at most 10 volume percent of a catalyst material for diamond.

10. A method according to any one of claims 1 to 7, wherein comprising forming a PCBN material, the superhard particles or grains comprising cBN.

1 1 . A superhard wear element comprising a polycrystalline superhard material produced by a method as claimed in any one of the preceding claims.

12. A superhard wear element as claimed in claim 1 1 , comprising a plurality of regions, each region comprising polycrystalline superhard material having at least one different structural characteristic.

13. A superhard wear element as claimed in claim 1 1 or claim 12, in which the superhard wear element is for use in machining, drilling or cutting a workpiece comprising metal.