BACKGROUND

This invention relates to ultra-hard composite materials of diamond.

Ultra-hard diamond composite materials, typically in the form of abrasive compacts, are used extensively in cutting, milling, grinding, drilling and other abrasive operations, and also may be used as bearing surfaces and the like. They generally contain a diamond phase, typically diamond particles, dispersed in a second phase matrix or binder phase. The matrix may be metallic or ceramic or a cermet. These particles may be bonded to each other during the high pressure and high temperature compact manufacturing process generally used, forming polycrystalline diamond (PCD).

Polycrystalline diamond (PCD) is used extensively due its high abrasion resistance and strength. In particular, it may find use within shear cutting elements included in drilling bits used for subterranean drilling.

A commonly used tool containing a PCD composite abrasive compact is one that comprises a layer of PCD bonded to a substrate. The diamond particle content of these layers is typically high and there is generally an extensive amount of direct diamond-to-diamond bonding or contact. Diamond compacts are generally sintered under elevated temperature and pressure conditions at which the diamond particles are crystallographically or thermodynamically stable.

Examples of composite abrasive compacts can be found described in US patents 3,745,623; 3,767,371 and 3,743,489.

The PCD layer of this type of abrasive compact will typically contain a catalyst/solvent or binder phase in addition to the diamond particles. This typically takes the form of a metal binder matrix, which is intermingled with the intergrown network of particulate diamond material. The matrix usually comprises a metal exhibiting catalytic or solvating activity towards carbon such as cobalt, nickel, iron or an alloy containing one or more such metals.

PCD composite abrasive compacts are generally produced by forming an unbonded assembly of the diamond particles and solvent/catalyst, sintering or binder aid material on a cemented carbide substrate. This unbonded assembly is then placed in a reaction capsule, which is then placed in the reaction zone of a conventional high pressure/high temperature apparatus. The contents of the reaction capsule are then subjected to suitable conditions of elevated temperature and pressure to enable sintering of the overall structure to occur.

It is common practice to rely, at least partially, on binder originating from the cemented carbide substrate as a source of metallic binder material for the sintered polycrystalline diamond. In many cases, however, additional metal binder powder is admixed with the diamond powder before sintering. This binder phase metal then functions as the liquid-phase medium for promoting the sintering of the diamond portion under the imposed sintering conditions.

The preferred solvent/catalysts or binder systems used to form PCD materials characterised by diamond-to-diamond bonding, which include Group VIIIA elements such as Co, Ni, Fe, and also metals such as Mn, are largely due to the high carbon solubility of these elements when molten. This allows some of the diamond material to dissolve and reprecipitate again as diamond, hence forming intercrystalline diamond bonding while in the diamond thermodynamic stability regime (at high temperature and high pressure). This intercrystalline diamond-to- diamond bonding is desirable because of the resulting high strength and wear resistance of the PCD materials.

The unfortunate result of using such solvent/catalysts is a process known in the literature as thermal degradation. This degradation occurs when the diamond composite material is subjected, in the presence of such solvent/catalyst material, to temperatures typically greater than 700 ⁰ C either under tool application or tool formation conditions. This temperature can severely limit the application of diamond composite materials generally, and PCD materials particularly in areas such as rock drilling or machining of materials.

Thermal degradation of PCD materials is postulated to occur via two mechanisms:

® The first mechanism is assumed to result from differences in the thermal expansion coefficients of the metallic solvent/catalyst binder and the intergrown diamond. Differential expansion at elevated temperature is assumed to cause micro-cracking of the intergrown diamond. It may become of particular concern even at temperatures exceeding 400 ⁰ C.

β The second mechanism more commonly accepted by people experienced in the field is due to the inherent catalytic activity of the metallic solvent/catalyst in a carbon system. The metallic binder begins converting the diamond to non-diamond carbon when heated above approximately 700 ⁰ C. This effect occurs appreciably even though the binder is still in the solid state. At low pressures, i.e. in the graphite stability regime, this results in the formation of non-diamond carbon, in particular graphitic carbon, the formation of which will ultimately cause bulk degradation of mechanical properties, leading to catastrophic mechanical failure. This second mechanism applies more generally to diamond composite materials comprising solvent/catalyst material, even where such material is absent significant diamond intergrowth.

One of the earliest methods of addressing this thermal degradation problem was disclosed in US 4,224,380 and again in US 6,544,308, comprising the removal of the solvent/catalyst through leaching by acids or electrochemical methods, which resulted in a porous PCD material that showed an improvement in the thermal stability. However, this resultant porosity caused a degradation of the mechanical properties of the PCD material. In addition, the leaching process is unable completely to remove isolated solvent/catalyst pools that are fully enclosed by intercrystalline diamond bonding. Therefore, the leaching approach is believed to result in a compromise in properties.

A further method for addressing thermal degradation involves the use of non- metallic or non catalyst/solvent binder systems. This is achieved, for example, through infiltration of the diamond compact with molten silicon or eutectiferous silicon, which then reacts with some of the diamond to form a silicon carbide binder in situ, as taught in US Patents 3,239,321 ; 4,151 ,686; 4,124,401 ; and 4,380,471, and also in US 5,010,043 using lower pressures. This SiC-bonded diamond shows a clear improvement in thermal stability, capable of sustaining temperatures as high as 1200°C for several hours as compared with PCD materials made using solvent/catalysts, which cannot tolerate temperatures above 700 ⁰ C for any appreciable length of time.

Other methods of addressing the thermal degradation problem are taught by US Patents 3,929,432; 4,142,869 and 5,011 ,514. Here, the surface of the diamond powder is first reacted with a carbide-former such as tungsten or a Group IVA metal; and then the interstices between the coated diamond grit are filled with eutectic metal compositions such as suicides or copper alloys.

Another approach taken is to attempt to modify the behaviour of standard metallic solvent/catalysts in situ. US 4,288,248 teaches the reaction of solvent/catalysts such as Fe, Ni, and Co with Cr, Mn, Ta, and Al to form intermetallic compounds. Similarly, in US Patent No. 4,610,699, standard metal catalysts are reacted with Group IV, V, Vl metals in the diamond stability zone resulting in the formation of unspecified intermetallics.

A more recent teaching using intermetallic compounds to provide thermal stability but still achieve high strength materials through diamond intergrowth is discussed in US2005/0230156. This application discusses the necessity of coating the diamond grit with the cobalt catalyst to allow polycrystalline diamond intergrowth before allowing interaction with the admixed intermetallic forming compounds. After the desired diamond intergrowth, it is postulated that the cobalt catalyst will then form an intermetallic which renders it non-reactive with the intergrown diamond.

US 4,518,659 discloses an HpHT process for the manufacture of diamond-based composites where certain molten non-catalyst metals (such as Cu, Sn, Al, Zn, Mg and Sb) are used in a pre-infiltration sweepthrough of the diamond powder in order to facilitate optimal catalytic behaviour of the solvent/catalyst metal. Here, although low levels of residual non-catalyst presence are anticipated to remain within the PCD body, these are not anticipated to be in sufficient quantities to result in significant intermetallic formation.

WO 2009/027948 discloses ultra-hard composite materials comprising a diamond phase and a binder phase, the binder phase comprising a ternary carbide such as Co-Sn-C and Fe-Sn-C ternary carbides.

US patent application US2003/0186636 discloses the use of a 312 type MAX phase, specifically Ti ₃ SiC ₂ , as a carrier phase (or part of a carrier phase) for a cutting tool which contains abrasive bodies embedded in the carrier material. The abrasive bodies may be diamond; but the cutting tool is not produced under HpHT conditions.

The use of Ti ₃ SiC ₂ as a binder in HpHT sintered diamond composites was investigated [2]. Sintering conditions for these materials were high, requiring pressures of approximately 8GPa and temperatures of approximately 1800° C. These conditions were found to produce compacts where the optimal MAX phase content exceeded 30 volume %. Analysis of the diamond - binder interfaces found a mixture of titanium suicides, titanium carbide and Ti ₃ SiC ₂ infiltrated between the diamond grains.

The problem addressed by the present invention is therefore the identification of a suitable binder system that provides for thermally stable diamond composite materials, which allows diamond dissolution and reprecipitation under diamond synthesis conditions, but does not facilitate thermal degradation when the resultant composite material is used at elevated temperatures (above 700 ⁰ C) under ambient pressure conditions.

SUMMARY OF THE INVENTION

According to the invention, there is provided an ultra-hard composite material, in particular a polycrystalline diamond material, comprising a mass of diamond grains exhibiting inter-granular bonding and a binder comprising a material of the formula M _n+1 AX _n ; where M is one or more elements selected from the group consisting of Sc, Ti, V, Cr, Zr, Nb, Mo, Hf and Ta; A comprises at least one element selected from the group consisting of Al, Si, Sb, Bi, P, S, Se, Te, Po, Ga, Ge, As, Cd, In, Sn, Tl and Pb; X is carbon or nitrogen or a combination of carbon and nitrogen; and n is about 1 , 2 or 3.

An ultra-hard material is understood to mean a material having a Vickers hardness of at least 25 GPa.

In some embodiments, the binder comprises a material of the formula M ₂ AX, where M, A and X are as defined above. A material having the formula M ₂ AX is referred to as a MAX phase of type 211.

In one embodiment, M is at least one element selected from the group consisting of Sc, Ti, V, Cr, Zr, Nb, Hf and Ta; and A is one or more elements selected from the group consisting of Al, P, S, Ga, Ge, As, Cd ₁ In, Sn, Tl and Pb.

In one embodiment, M is selected from the group consisting of Ti, Cr, Ta and a combination of Ti and Cr. In one embodiment, M is selected from Ti and Cr. In one embodiment M includes Ti.

In one embodiment, A comprises one or more elements selected from the group consisting of Ge, Sn, In, Pb and a combination of any of these. In another embodiment, A comprises one or more elements selected from the group consisting of Ge, Sn ₁ In, Pb and a combination of In and Sn. In one embodiment, the material of the binder is selected from Ti ₂ GeC, Ti ₂ (Sn ₁ In)C, Ti ₂ Pb(C ₁ N), (Ti ₁ Cr) ₂ Ge(C ₁ N), Cr ₂ GeC and Ta ₂ AIC.

In one embodiment, the binder may also contain a known diamond catalyst/solvent material such as cobalt, iron or nickel. In another embodiment, the binder consists of the material of formula M _n+ iAX _n , apart from unavoidable impurities and phases.

The ultra-hard diamond composite materials of some embodiments have a binder content of less than 40 volume %, less than 30 volume % or in the range 20 volume % to 5 volume %.

According to another aspect of the invention there is provided a compact comprising an ultra-hard composite material according to an aspect of the invention, the ultra-hard composite material being bonded to a cemented carbide substrate.

In some embodiments the cemented carbide is cobalt-cemented tungsten carbide.

A cemented carbide is understood to mean a material comprising grains of metal carbide, especially tungsten carbide (WC) or less typically titanium carbide (TiC) or other carbides, dispersed within a binder comprising a metal, especially cobalt (Co) or less typically nickel (Ni) or metal alloy. The binder may be said to cement the grains together as a sintered compact, typically having negligible porosity. The most common cemented carbide is Co-cemented WC.

According to another aspect of the invention, a cutter element for a tool comprises a PCD structure having a cutter surface and a surface region extending from the cutter surface to a depth, the surface region comprising an ultra-hard composite material according to an aspect of the invention.

In some embodiments, at least a portion of the surface region is a layer or stratum and the depth is at least about 5 microns, or at least about 10 microns, or at least about 20 microns, or at least about 50 microns. In some embodiments, at least a portion of the surface region is a layer or stratum and the depth is at most about 100 microns, or at most about 200 microns or at most about 500 microns.

In some embodiments the cutter element is suitable for use in boring into the earth to extract oil or gas from the earth.

According to a further aspect of the invention, a drill bit for boring into the earth comprises a cutter element according to an aspect of the invention.

According to yet another aspect of the invention, a method for making an ultra- hard composite material includes providing a source of diamond particles and a binder, or the components for making the binder, the binder comprising a material of the formula M _n+1 AX _n ; where M, A, X and n are as defined above, forming a reaction volume of the diamond particles and the binder, or the diamond particles and the components for making the binder, and sintering the reaction volume under ultra-high pressure and temperature (HpHT) conditions at which diamond is thermodynamically stable.

In one embodiment, the binder comprises a material of the formula M ₂ AX, where M, A and X are as defined above.

In one embodiment, the binder is generated by pre-reacting M, A and X and is then either admixed with the diamond particles or infiltrated into the diamond particles under HpHT conditions.

In one embodiment, the components for making the binder are provided in elemental form as a homogeneous mixture. In one embodiment, the homogeneous mixture is mixed with the diamond particles to form the reaction volume. In one embodiment, the homogeneous mixture forms a layer or bed adjacent a layer of the diamond particles in the reaction volume. In one embodiment, a mixture of M, or a sub-carbide and/or sub-nitride of M, and diamond particles is provided, and A is subsequently infiltrated into the resultant reaction volume from an external infiltration source.

According to another aspect of the invention, a method for making an ultra-hard composite material according to the invention includes providing an at least partially porous PCD material, introducing elements M and A, and optionally element X, into at least some of the pores of the PCD material, and treating the infiltrated PCD material under conditions suitable for element A, element M and, as appropriate, element X and/or carbon from the diamond of the PCD to form a compound having the formula M _n+ iAX _n , where M, A, X and n are as defined above.

In one embodiment, the binder comprises a material of the formula M ₂ AX, where M, A and X are as defined above.

In some embodiments, the method includes introducing a slurry containing particles comprising element M or element A, or containing particles comprising element M and particles comprising element A, dispersed within a slurry carrier material, into the pores of the PCD material, and then removing slurry carrier material from the pores. In some embodiments, the slurry may be introduced into the pores by pressure infiltration or vacuum infiltration.

In one embodiment, element M may be introduced into the pores in the form of a nitride compound or in elemental form, or in both the form of a nitride compound and in elemental form. In one embodiment, about 50% of the atoms of element M are introduced in the form of a nitride compound MN, and about 50% of the atoms of element M are introduced in elemental form. In one embodiment, elements M/MN, A and carbon from the diamond of the PCD material may react to form a phase having the formula M _n+ iA(C,N) _n , where the (C ₁ N) component is a carbonitride in solid solution, and n is 1 , 2 or 3.

In one embodiment, the method includes infiltrating a material comprising an organometallic compound containing element M into the pores, treating the material to form a porous M-containing material within the interstices of the PCD material, and introducing a material containing element A into the pores of the M- containing material, element M and element A reacting with carbon from the diamond of the PCD material.

In one embodiment, the method includes infiltrating the pores of the porous PCD material with a mixture of a material comprising an organometallic compound containing element M and a material comprising an organometallic compound containing element A. In one embodiment, the organometallic compounds may be in solution in a carrier medium.

In some embodiments, the method includes causing the reaction between element M and element A to occur in the presence of a nitrogen-containing substance, such as ammonia or hydrazine, to generate a M _n +iAN _n or M _n+1 A(C ₁ N) _n phase within the PCD material, where n is 1, 2 or 3.

In one embodiment, the at least partially porous PCD material comprises a porous region and a non-porous region.

In one embodiment, the at least partially porous PCD material defines a working surface, the region extending a depth from the working surface. In some embodiments, the depth of the region is at least about 5 μm or at least about 10μnn or at least about 20 μm or at least about 50 μm. In one embodiment, the depth of the region is up to about 100μm, up to about 200μm or up to about 500μm.

In one embodiment, the at least partially porous PCD material may be provided by removing catalyst material for diamond from the interstices between the diamond grains of a PCD material by, for example, treating the PCD material in acid liquor.

According to yet another aspect of the invention there is provided an insert for a tool, wherein the insert comprises an ultra-hard composite material according to an aspect of the invention. In some embodiments, the insert comprises a compact according to an aspect of the invention. In some embodiments the insert is for an earth boring or rock drilling tool or bit, as may be used in the oil and gas industry.

The invention extends to a diamond abrasive compact comprising the diamond composite material of an aspect of the invention, and to a tool comprising the diamond abrasive compact, which is capable of use in a cutting, milling, grinding, drilling or other abrasive application.

In some embodiments the diamond composite material may also be for a bearing surface, owing to the low friction of MAX phases.

DRAWING CAPTIONS

Embodiments of the invention will now be described in more detail, by way of example only, with reference to the accompanying figures in which:

Figure 1: is a scanning electron micrograph of an embodiment of a PCD material sintered with Ti ₂ GeC - based and Cr ₂ GeC - based binders.

Figure 2: is a scanning electron micrograph of an embodiment of a PCD material sintered with a Ti ₂ (Sn ₁ In)C based binder. >

Figure 3: is a scanning electron micrograph of an embodiment of a PCD material sintered with a Ti ₂ Pb(CN) based binder.

Figure 4: is a scanning electron micrograph of an embodiment of a PCD material sintered with a (Ti ₁ Cr) ₂ Ge(CN) based binder.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is directed to an ultra-hard composite material comprising diamond having increased thermal stability over conventional solvent/catalyst sintered diamond composite materials. In some embodiments, the binder contains a 211 MAX phase.

MAX phase materials are a new type of tough ceramic that was investigated and developed in the 1990's [1]. They have highly desirable properties such as oxidation resistance, inherent thermal stability, high stiffness and increased machineability. They have the general formula M _n+1 AX _n where;

o M is one or more of the early transition metal elements;

•> A is one or more elements selected from the elements of Groups MIA to

VIA, typically a Group INA or IVA element, and Cd • X is carbon and/or nitrogen β n is 1 ,2 or 3

These variable stoichiometries lead to the characteristic formation of subsets of MAX phases usually referred to as :

e 413 type, such as Ti ₄ AIN ₃ o 312 type, such as Ti ₃ AIC ₂ © 211 type, such as Ti ₂ AIC and / or Ti ₂ AIN

MAX phases exhibit a characteristic nano-laminate crystal structure - where the transition metal carbide (M ₂ X, M ₃ X ₂ , M ₄ X ₃ ) is interleaved with layers of pure group A element. It is hence quite different in structure to standard mixed or ternary carbide materials which are also refractory materials.

It has also been observed in this invention that X can consist of a mixture of C and N, typically with at least some carbon. MAX based phases in the diamond microstructure with pure nitrogen have not been found. The N in the MAX based phases has been found to improve the oxidation resistance of the MAX - diamond microstructure. The use of 211 MAX phases or 211 MAX-based phases as a binder for PCD may produce a well-sintered compact under much more moderate sintering conditions - for example, approximately 5.5 GPa and 1400 ⁰ C. Furthermore, in some instances these binders may produce PCD compacts with increased diamond content. This is surprising given that it has been reported that 211 MAX phases tend to dissociate in the 1250 - 139O ⁰ C temperature range into the MX phase and the A element due to molar volume constraints. Suitable A elements tend to have low melting points, and so melt under these conditions. Furthermore, they do not exhibit solvent/catalytic behaviour in diamond-based systems and typically have very low carbon solubility.

Without being bound by theory, it is believed that it is this very dissociation which may facilitate diamond intergrowth. The molten A phase may act as the liquid transport medium for the carbide-forming M element, such that carbon transport can be effected and diamond intergrowth may occur.

Diamond-to-diamond bonding or intergrowth between adjacent diamond grains may be observed using SEM (scanning electron microscopy), although ultrasonic elastic modulus measurements may be more indicative of the presence or absence of diamond-to-diamond intergrowth. A possible problem with such ultrasonic modulus measurements, however, may be caused by the absorption of the ultrasonic energy by the nano-laminate structure of the MAX phase material. Intergrown diamond skeletons are by definition more contiguous in structure than non-intergrown structures - this parameter can be determined using conventional image analysis techniques. Furthermore, intergrown structures may exhibit characteristic microstructural features, which may have the appearance of "necklace" structures in which entrapped binder between adjacent intergrown diamond grains is evident. This may be due to extensive intergrowth occurring between adjacent grains so rapidly that fine pools of binder phase are occluded within the diamond skeleton.

Intergrown diamond skeletons exhibit vastly superior mechanical and behavioural properties due to the high strength of the sintered diamond network, when compared to diamond grains bound within a non-intergrown structure. Diamond compacted under HpHT conditions, known as hot compacted diamond, without any metallic or intermetallic binders and without diamond intergrowth will have diamond to diamond contact. However, hot compacted diamond may have poor strength and wear resistance, and may disintegrate when faced with the severe conditions found while drilling through rock.

Embodiments of PCD structures according to the invention may exhibit thermal stability as well as desirable mechanical properties that make it suitable to be employed in application requiring thermally stable cutting materials.

Embodiments of diamond composite materials of the invention may be generated by sintering diamond powder in the presence of a MAX-based binder phase, or the constituents for making such a MAX-based binder phase, at an ultra-high pressure and temperature (HpHT) at which diamond is thermodynamically more stable than graphite. They may be generated on a substrate comprising cemented carbide material, and in some embodiments, a barrier is placed between the substrate and the diamond to control infiltration of, for example, catalyst material from the substrate into the diamond.

Embodiments of ultra-hard diamond composite materials may have a binder content of less than 40 volume %, less than 30 volume %, less than 20 volume %, or less than 10 volume %.

The diamond powder employed may be natural or synthetic in origin and may have a multimodal particle size distribution.

The surface chemistry of the diamond powder may have reduced oxygen content in order to ensure that the MAX phase constituents do not oxidise excessively prior to formation of the diamond composite material, which may reduce their effectiveness. Hence both the metal and diamond powders should be handled during the pre-sintering process with sufficient care to ensure minimal oxygen contamination. The MAX-based binder can be formed by several generic approaches, for example:

• pre-reaction of M, A and X to generate the MAX phase, which may be done under vacuum at temperature, which is then either admixed or infiltrated into the diamond powder feedstock under HpHT conditions;

• in situ reaction under HpHT sintering conditions, preferably using an intimate homogenous mixture of the required components, which may be elemental. This may be provided within the diamond powder mixture or from an infiltration layer or bed adjacent to it, and may include the carbon/nitrogen component, or in the case where X is carbon, this carbon component may be sourced from the diamond powder;

• a staged in situ reaction under HpHT sintering conditions using a mixture of M and diamond powder and subsequent infiltration and in situ reaction with A from an external infiltration source;

• a staged in situ reaction under HpHT sintering conditions can also be achieved where M is mixed as a sub-carbide and/or sub-nitride with the diamond powder and subsequent infiltration and in situ reaction with A from an external infiltration source.

In some embodiments, the MAX-based binder species or precursors may be introduced into the diamond powder mixture by powder admixing, thermal spraying, precipitation reactions, vapour deposition techniques, or an infiltration source can also be prepared using methods such as tape casting or pre-alloying, for example.

In one embodiment, the compact may be heat treated to maximise the amount of MAX phase present.

Embodiments of diamond composite materials of the invention may be generated by introducing a suitable metal into a pre-sintered, at least partially porous, diamond composite material or PCD body. The pre-sintered PCD body can be produced in an ultra-high pressure furnace by sintering together diamond grains in the presence of a catalyst material for diamond at a pressure of at least about 5.5GPa and a temperature of at least about 1,300°C. The catalyst material may comprise a known metal catalyst material for diamond, such as cobalt, iron or nickel, or certain alloys thereof. The sintered PCD body, as a whole or at least a region thereof, is then treated to remove the majority of binder catalyst material from the PCD body or desired region thereof.

The catalyst material present in the PCD body may be removed by any of various methods known in the art, such as electrolytic etching, evaporation techniques, acid leaching (for example by immersion in a liquor containing hydrofluoric acid, nitric acid or mixtures thereof) or by means of chlorine gas, as disclosed in international patent publication number WO2007/042920, or by another method (e.g. as disclosed in South African patent number 2006/00378).

The diamond powder used in producing the pre-sintered PCD body may be natural or synthetic in origin and will typically have a multimodal particle size distribution.

In some embodiments, the MAX-based binder may be formed by introducing M, A and, where appropriate, X in elemental form into the partially porous PCD structure under vacuum and reacting the elements to generate the MAX phase. In some embodiments, the MAX-based binder may be formed by pre-reacting elements M, A and, where appropriate, X to generate the MAX phase under vacuum at temperature and then introducing the MAX phase into the partially porous PCD material.

In one embodiment, MAX-based binder species or precursors may be introduced into a pre-sintered PCD body by removing binder material from the PCD body to generate porosity and then infiltrating the pores with a slurry containing dispersed particles comprising element M and element A in the desired ratio of M to A. Known methods such as vacuum infiltration or pressure filtration may be used to achieve the infiltration. The pores are filled with these particles and the slurry carrier medium is removed along with ambient atmosphere that may be present. The M and A element particles are then reacted together and, with the diamond acting as a carbon source, the M _n+1 AC _n type of MAX phase is generated.

In one embodiment, MAX-based binder species or precursors may be introduced into an at least partly porous pre-sintered PCD body by infiltrating the pores with particulates of the M element as a nitride with particulates of the A element in the appropriate ratio of MN, M, and A. Reaction of these elements can form the M _n+ iA(C,N) _n MAX phase where the X element is now a solid solution carbonitride.

In one embodiment, MAX-based binder species or precursors may be introduced into an at least partly porous pre-sintered PCD body by infiltrating the pores with an organometallic compound including the M element, which can be reacted and precipitated on the porous surface of the intergrown diamond. This M element compound can be left as is or reacted with the diamond to form a carbide or sub- carbide. The porosity can then be further infiltrated with either particulate A element or molten A element to be reacted with the M element compound to form the MAX phase in the porous diamond body.

In one embodiment, MAX-based binder species or precursors may be introduced into an at least partly porous pre-sintered PCD body by infiltrating the pores with a mixture of M element and A element organometallic compounds, either as a solution in a carrier media, or directly as organometallic compounds. These M and A element compounds are in the desired correct ration of M to A. These compounds are reacted together with the diamond to generate the M _n+ iAC _n MAX phase within the diamond body.

In one embodiment, MAX-based binder species or precursors may be introduced into an at least partly porous pre-sintered PCD body by reacting the M and A elements or compounds containing the M and A elements within the diamond body in a nitrogen rich environment, using ammonia, hydrazine, or other nitrogen compound, for example, to generate a M _n+1 AN _n or M _n+ iA(C,N) _n MAX phase within the diamond body. In some embodiments, the ultra-hard diamond composite materials may have a binder content of less than about 40 volume %, less than about 30 volume %, less than about 20 volume %, or less than about 10 volume %.

In view of the diamond-to-diamond bonding present in the pre-sintered partially porous PCD material, obtaining additional diamond intergrowth may not be necessary. Accordingly, various other MAX-based phases other than the 211 MAX phase are envisaged. Accordingly, 312 (e.g. Ti ₃ SiC ₂ or Ti ₃ AIC ₂ ) type MAX phases can be introduced into the porous diamond body. (Ti and Si based organometallics, such as titanium isopropoxide and tetra-ethyl ortho silicate, for example, are commonly used and diamond with TiC and TiN have been made). Additionally, the porous surface can be coated with Ti and Ti compounds (either by particulate infiltration or organometallics or vapor or organometallic liquid or from vapour) and the silicon infiltrated by either organometallics or silane vapour, which then reacts with each other to form the Ti ₃ SiC ₂ compound.

It is also envisaged that a 413 (e.g. Ti ₄ AIN ₃ ) type MAX phase can be introduced into a porous diamond material.

In order to evaluate the diamond composite materials of the invention, in addition to electron microscopy (SEM) and XRD analysis, thermal stability and thermal wear behaviour application-based (milling) tests were used.

A thermal stability test may be used to measure the effective thermal stability of a small, PCD sample. The suitably-sized sample to be tested is thermally stressed by heating under vacuum at about100°C/hour to 850 ⁰ C, held at 850°C for 2 hours, and then slowly cooled to room temperature. After cooling, Raman spectroscopy is conducted to detect the presence of graphitic carbon or non-sp ³ carbon resulting from the thermal degradation of the diamond. This type of heat treatment is considered to be very harsh, where a commercially available Co- based PCD showed a significant graphite peak after such treatment. A reduced conversion of diamond to graphite is indicative of an increase in thermal stability of the material. Embodiments of MAX phase diamond composite materials may show no detectable graphitic or non-sp ³ carbon after such a test.

A thermal wear behaviour application-based test can also be used as an indicator of the degree to which a PCD-based material will survive in a thermally demanding environment. The test is conducted on a milling machine including a vertical spindle with a fly cutter milling head at an operatively lower end thereof. Rock, in particular granite, is milled by way of a dry, cyclic, high revolution milling method. The milling begins at an impact point where the granite is cut for a quarter of a revolution, the granite is then rubbed by the tool for a further quarter revolution and the tool is then cooled for half a revolution at which point the tool reaches the impact point. For an unbacked cutting tool, a shallow depth milling of the rock is carried out - typically a depth of cut of about 1mm is used. For a backed tool, the depth of cut is increased, typically to about 2.5mm.

The length of the rock that has been cut prior to failure of the tool is then measured, where a high value indicates further distance travelled and a good performance of the tool, and a lower value indicates poorer performance of the tool. As the test is a dry test, the failure of the tool is deemed to be thermally induced rather than abrasion induced. Hence this test is a measure of the degree to which the tool material will wear in a thermally stressed application.

MAX phase 211 based binder diamond compacts of the invention had milling test results with distances typical of binderless carbonado (natural polycrystalline diamond) and fully leached polycrystalline diamond. These results clearly show the thermal stabilty of the MAX phase binder diamond compacts without compromise of the wear resistance.

The invention will now be described in more detail, by way of example only, with reference to the following illustrative examples.

EXAMPLES

Example 1 A bed of multimodal diamond powder of approximately 20 microns in average diamond grain size was placed into a niobium metal canister and a layer of Ti metal powder and a layer of Ge metal powder in the correct (2:1) atomic ratio were placed onto this diamond powder bed. The metal powder layers were sufficient to provide a binder constituting about 11 volume %. The canister was then evacuated to remove air, sealed and treated under HpHT conditions at approximately 5.5 GPa and 1500 ⁰ C to sinter the PCD.

The sintered PCD compact was then removed from the canister and examined using:

β scanning electron microscopy (SEM) for evidence of intergrowth; β XRD analysis to determine the phases present in the binder; and o a milling test as described above.

The PCD material produced appeared to show evidence of intergrowth between the diamond grains when examined under the SEM, as is evident from the high magnification micrograph shown in accompanying Figure 1. XRD analysis confirmed the presence of Ti ₂ GeC in the binder phase. Similar behaviour was seen substituting Cr for Ti, forming Cr ₂ GeC in the binder phase, also shown in Figure 1.

The performance of the PCD material was then investigated using an application- based milling test. Milling test results had distances typical of binderless carbonado (natural polycrystalline diamond) and fully leached polycrystalline diamond.

Example 2

A sample was prepared according to the method described above for Example 1, save that the A element used was a solid solution of Sn and In; and the diamond and metal powders were mixed together using a planetary ball mill (with the metal powder mixture constituting 11 volume % of the mixture). The sample was then examined using:

® scanning electron microscopy for evidence of intergrowth; • XRD analysis to determine the phases present in the binder.

The PCD material produced appeared to show evidence of intergrowth between the diamond grains when examined under the SEM, as is evident from the micrograph shown in accompanying Figure 2. XRD analysis confirmed the presence Of Ti ₂ (Sn ₁ In)C, Sn and TiC in the binder.

Example 3

A sample was prepared according to the method described above for Example 1 , save that A was Pb and the X element used was a solid solution of C and N; and the diamond and metal powders were mixed together using a planetary ball mill (with the metal powder mixture constituting 11 volume % of the mixture).

The sample was then examined using scanning electron microscopy for evidence of intergrowth. The PCD material produced appeared to show evidence of intergrowth between the diamond grains when examined under the SEM, as is evident from the micrograph shown in accompanying Figure 3.

Example 4

A sample was prepared according to the method described above for Example 1 , save that the M element used was a solid solution of Ti and Cr and the X element was a mixture of C and N; and the diamond and metal powders were mixed together using a planetary ball mill (with the metal powder mixture constituting 14 volume % of the mixture).

The sample was then examined using scanning electron microscopy for evidence of intergrowth. The PCD material produced appeared to show evidence of intergrowth between the diamond grains when examined under the SEM, as is evident from the micrograph shown in accompanying Figure 4.

Example 5

A pre-reacted Ta ₂ AIC MAX phase binder in powder form was blended directly with multimodal diamond powder of approximately 22 microns average diamond grain size and the blended powder was sintered at about 6.5GPa at 1,500 ⁰ C for about 20 minutes.

Optical micrograph analysis showed what appeared to be contiguity between the diamond grains due to the absence of boundaries, possibly indicating diamond intergrowth. XRD analysis of the microstructure showed the presence of diamond, TaC, Ta ₂ AIC and Ta ₅ AI ₃ C.

Example 6

A pre-sintered, at least partially porous PCD composite material can be infiltrated with Ti ₂ GeC as an alternative to the method disclosed in Example 1. In order to prepare the Ti ₂ GeC, an organometallic compound of titanium isopropoxide can be co-precipitated with tetraethyl ortho germanate or tri-ethyl ortho germane within the pores of the porous intergrown diamond by reaction with water (either as a vapor or liquid). The precipitated compound can be readily reduced into the elemental metal form with a flowing Argon 10% Hydrogen gas mixture (preferably not more than 10% hydrogen for safety reasons) to yield an oxygen partial pressure (pθ ₂ ) of approximately 10 ^"30 atmospheres. The temperature during reduction should be greater than 1000 ⁰ K or about 725 ⁰ C, but not greater than about 94O ⁰ C.

A cycle of evacuation and refilling with the Ar-10%H ₂ mixture will aid the removal of oxygen from the precipitated elements within the porosity of the intergrown diamond. The thicker the intergrown diamond, and the more MAX phase elements precipitated within the porosity, the more time during reduction and the more cycles of evacuation and refilling with Ar-10%H ₂ is desired. When the MAX phase elements are suitably reduced, these MAX phase elements can be reacted under a strong vacuum (preferably 10 ^"6 Torr) with the diamond to form Ti ₂ GeC within the porous intergrown diamond body.

Example 7

In a similar method to that envisaged in Example 6, Ti ₂ SnC and Ti ₂ InC MAX phases can be precipitated within the porous intergrown diamond body using commercially available tin isopropoxide and indium isopropoxide. Either the tin or indium isopropoxide (or both) can be thoroughly mixed with titanium isopropoxide in the correct ratios to generate a 2 M element to 1 A element ratio. Tetraethyl stannate can also be used for providing the Sn element, instead of tin isopropoxide, and it will behave similarly to tetraethyl ortho germinate described in Example 6. The titanium and tin or indium compounds can be co-precipitated through hydrolysis within the porosity of the intergrown diamond using water as described in Example 6.

The precipitated compounds can be reduced into elemental form using the methods described in Example 6 for the titanium germanium system. A cycle of evacuation and refilling with the Ar-10%H ₂ mixture will also be helpful for the removal of oxygen from the precipitated elements within the porosity of the intergrown diamond. During the reduction phase, care should be taken that an appropriate temperature is used, given the low melting points of the Sn and In elements, which can result in pre-reaction and formation of the MAX phase before the complete reduction of the precursors within the porosity of the intergrown diamond.

When the MAX phase elements are suitably reduced, these MAX phase elements can be reacted under a strong vacuum (preferably 10 ^"6 Torr) with the diamond to form Ti ₂ SnC or Ti ₂ InC within the porous intergrown diamond body. REFERENCES

[1] M.W. Barsoum, T. El-Raghy, "The MAX Phases: Unique New Carbide and Nitride Materials", American Scientist, (2001 ), volume 89, number 7-8,p334-343.

[2] L. Jaworska et al, "Ti ₃ SiC ₂ as a bonding phase in diamond composites", Journal of Materials Science Letters, (2001), volume 20, pages 1783-1786.