DIAMOND CONTAINING MATERIAL

BACKGROUND OF THE INVENTION

This invention relates to a diamond-containing material.

Diamond-containing material (DCM), used extensively in cutting, milling, grinding, drilling and other abrasive operation; may take many forms, for example:

• a diamond matrix tool material, where the diamond particles are held together in a metallic or intermetallic matrix. These are typically formed at atmospheric pressure by sintering the diamond-matrix mixture. Diamond volume densities tend to be less than 60 volume %;

• Abrasive compacts, that consist of a mass of ultrahard particles, typically diamond, bonded into a coherent, polycrystalline conglomerate. The abrasive particle content of these abrasive compacts is high, generally in excess of 70 volume %; and more typically in excess of 80 volume %. There is generally an extensive amount of direct particle-to-particle bonding or contact. Abrasive compacts are generally sintered under high pressure, high temperature (HpHT) conditions at which the diamond is crystallographically or thermodynamically stable. Diamond compacts are also known as PCD.

Abrasive compacts also usually have a second or binder phase. In the case of certain types of polycrystalline diamond compacts, this second phase is typically

a metal such as cobalt, nickel, iron or an alloy containing one or more such metals. When diamond particles are combined with a suitable metallic solvent/catalyst, this solvent/catalyst promotes diamond-to-diamond bonding between the diamond grains, resulting in an intergrown or sintered structure. This mechanism occurs in part because of the solubility of carbon in the solvent/catalyst which allows carbon from the diamond to dissolve and re- precipitate on other diamonds while in the diamond stable field during manufacture. This results in extensive diamond-to-diamond bonding, hence producing a strong diamond composite. In the final sintered structure however, solvent/catalyst material necessarily remains within the interstices that exist between sintered diamond grains.

A well-known problem experienced with this type of diamond compact (PCD) however, is that the residual presence of solvent/catalyst material in the microstructural interstices has a detrimental effect on the performance of the compact at high temperatures. This decrease in performance under thermally demanding conditions is postulated to arise from two different behaviours of the compact.

The first arises from differences between the thermal expansion characteristics of the interstitial solvent/catalyst and the sintered diamond network. At temperatures much greater than 400 ⁰ C, the metallic component expands far more than the intergrown diamond network and can generate micro-fracturing of the diamond structure. This micro-fracturing significantly reduces the strength of the bonded diamond at increased temperatures.

Additionally, the solvent/catalyst metallic materials which facilitate diamond-to- diamond bonding under high-pressure, high-temperature sintering conditions can equally catalyse the reversion of diamond to graphite at increased temperatures and reduced pressure with obvious performance consequences. This particular effect is mostly observed at temperatures in excess of approximately 700 ⁰ C.

As a result, PCD sintered in the presence of a metallic solvent/catalyst, notwithstanding its superior abrasion and strength characteristics must be kept at

temperatures below 700 ⁰ C. This significantly limits the potential industrial applications for this material and the potential fabrication routes that can be used to incorporate them into tools.

Potential solutions to this problem are well-known in the art. One type of approach focuses on the use of alternative or altered sintering aid materials. These materials, when present in the final sintered structure, exhibit much reduced retro-catalytic efficacy at high temperatures and typically have thermal expansion behaviours better matched with those of the sintered diamond phase.

One of the methods of altering the binder phase material is through the use of complex metallic systems that can still facilitate the consolidation of the diamond compact but have reduced thermal degradation effects in the final product. Certain classes of intermetallics are examples of these, lntermetallic compounds are typically defined as solid phases that contain two or more metallic elements, with optionally one or more non-metallic elements, whose structure is distinct from that of any of the constituents. They usually have a characteristic crystal structure and usually a definite composition. In common use the research definition, including poor metals (aluminium, gallium, indium, thallium, tin and lead) and metalloids (silicon, germanium, arsenic antimony and tellurium), is extended to include compounds such as cementite, Fe ₃ C. The latter compounds, sometimes termed interstitial compounds can be stoichiometric, and share similar properties to the classical intermetallics.

US 4,793,828 describes a diamond compact with a matrix phase that consists of silicon and/or silicon carbide. This compact is produced by infiltration from a silicon powder or foil source at elevated pressures and temperatures. This compact was found to be capable of withstanding temperatures of 1200 ⁰ C under a vacuum or in a reducing atmosphere without significant graphitisation or evidence of thermal degradation occurring.

US Patent 4,534,773 teaches the formation of a diamond compact with a binder phase comprising nickel suicides. This intermetallic binder phase is generated through the interaction/reaction between molten nickel and silicon at HpHT

conditions. The material produced is claimed to be an improved thermally stable polycrystalline diamond compact.

US Patent 4,789,385 teaches silicon, silicon-nickel, and silicon-cobalt combinations that will form intermetallics during sintering such as silicon carbide or nickel suicides or cobalt suicides while bonding diamond in the diamond stable field. These suicides are stated to provide thermal stability to the polycrystalline diamond compact.

US 2005/0230156 revisits this topic of intermetallics with a focus on cobalt suicide (CoSi), and particularly cobalt disilicide (CoSi ₂ ). It is claimed that these compounds, formed in situ, improve thermal stability behaviour due to having a lower thermal expansion coefficient than the cobalt metal binder commonly used. It is to be noted that this patent relies on consumption of SiC to form these intermetallics; and that certainly in the case of the disilicide, the reaction is not likely to proceed for thermodynamic reasons. The use of suicide intermetallics as binders for DCM's can have significant disadvantages. Suicides are known to be very brittle and can be a source of micro-cracking and flaws when used in an environment which is impact-prone, such as drilling or machining. The patent further discloses the proposed use of other intermetallics or alloys such as cobalt aluminides, borides, niobides, tantalides etc. Particularly in the case of Al-Co intermetallics, it is observed that in the Al-Co-C system, AI ₄ C ₃ is the thermodynamically preferred phase rather than Al-Co intermetallics. AI ₄ C ₃ is highly reactive with water or moisture, resulting in decomposition and degradation in a water-containing environment:

AI ₄ C ₃ + H ₂ O -→ CH ₄ + AI ₂ O ₃ (and/or AI(OH) ₃ ) (reaction i)

There are literature references to the use of particular Ni-Al intermetallics in forming diamond matrix tool pieces at low pressure. In particular, Ni ₃ AI is preferred because of its high hot strength. Hwang (1) further teaches that the exothermic reaction of nickel with aluminium to form the Ni ₃ AI intermetallic itself can be of assistance in producing a matrix that can hold diamond grit. This was

accomplished at moderate pressures (i.e. pseudo-hipping) using a hot isostatic press system.

US2006/0280638 discloses a diamond matrix tool material with an Ni ₃ AI binder system, where the diamond content is between 20 and 70 volume %, but more typically about 40 volume % of the structure. This material is further described by Wittmer (2). Essentially, the material . comprises diamond crystals cemented together by a pre-formed intermetallic Ni ₃ AI binder phase at atmospheric pressure by hot-pressing and by vacuum / pressure-less sintering at temperatures in excess of 1200 ⁰ C. This composite material does not exhibit diamond-to-diamond bonding. The heating cycle for manufacturing the composite material does not appear to thermally compromise the diamonds as no graphite was detected in the final product using XRD; however there is a lower useful limit on the size of diamond crystals which can be incorporated into the product because of oxidation attack.

A particular problem with the usefulness of Ni ₃ AI-diamond composites lies in the thermodynamic stability of the Ni ₃ AI phase. Under the more extreme conditions of demanding industrial applications, it is expected the Ni ₃ AI will react with the diamond,

4 Ni ₃ AI + 3 Cdia _m oπd → 12 Ni + AI ₄ C ₃ (reaction //)

producing the thermodynamically more stable aluminium carbide. As previously discussed, this material is highly reactive with water/moisture which will adversely affect performance (according to reaction i).

SUMMARY OF THE INVENTION

The present invention, according to a first aspect, provides the use of a nickel aluminide intermetallic containing material wherein the NhAI ratio is 2:3 in a diamond-containing material (DCM). The nickel aluminide intermetallic containing material preferably constitutes substantially all of or part of the second phase

(also known as matrix or binder phase) of the material. The exact phase composition of the nickel aluminide intermetallic containing material will be dependent on the conditions of synthesis i.e. may not achieve equilibrium composition. The elemental stoichiometry will, however, be consistent with the 2 Ni: 3 Al intermetallic composition. Thus, the material may contain some equilibrium phases, i.e. Ni ₂ AI ₃ , and some non-equilibrium phases, e.g. NiAI and complex aluminium rich phases.

The DCM may be PCD. In this form of the invention, the nickel aluminide intermetallic containing material comprises at least a part, and generally substantially all of, the second or binder phase. The nickel aluminide intermetallic containing material is in contact with the diamond in the PCD and is thermodynamically more stable than AI ₄ C ₃ . This means that during tool use at increased temperatures, there will be minimal reaction between the material and the diamond. Experimental results have indicated that the material is stable in the presence of diamond under HpHT diamond synthesis conditions. The PCD ₁ as discussed above, will contain in excess of 70 volume % diamond and preferably in excess of 80 volume % diamond.

The DCM may contain not more than 70 volume % diamond. Such materials may be made under diamond synthesis conditions or under milder hot pressing conditions.

The DCM may further contain other abrasive particles such as carbide particles. When other abrasive particles are present, the DCM is preferably one containing not more than 70 volume % diamond.

According to yet another form of the invention, the DCM is a diamond composite abrasive compact comprising a layer of a diamond-containing material of the invention, preferably PCD, bonded to a cemented carbide substrate. The cemented carbide substrate preferably has a nickel aluminide intermetallic containing material as defined above as at least part of the binder and more preferably the binder consists essentially of nickel aluminide intermetallic

containing material _, i.e. any other elements are present in minor or trace amounts only.

According to another aspect of the invention, there is provided a method of producing a diamond containing material as described above including the steps of providing a reaction mass comprising a source of diamond particles and nickel aluminide intermetallic containing material or reactants suitable to produce such a material and subjecting the reaction mass to diamond synthesis conditions.

Still further according to the invention, a method of producing a diamond containing material comprising diamond particles and a second phase of a nickel aluminide intermetallic containing material as defined above includes the step of reacting nickel with aluminium carbide (AI ₄ C ₃ ) under diamond synthesis conditions.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 represents a scanning electron backscatter image of material generated by example 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention provides the use of nickel aluminide intermetallic containing material wherein the NkAI molar ratio is 2:3 in a DCM that also has a second phase. The exact phase composition of the nickel aluminide intermetallic containing material can be complex because of the formation of non-equilibrium phases during cooling from sintering conditions. Hence, although the overall elemental stoichiometry will be consistent, i.e. the NhAI molar ratio will be 2:3, the existence of non-equilibrium phases may occur. For example, the existence of a NiAI and complex aluminium-rich (non-equilibrium) phases in addition to the Ni ₂ AI ₃ phase is common, albeit that the overall elemental stoichiometry is 2 Ni : 3 Al. This variation in local material composition is not seen as detrimental to the material performance, as long as the phases formed are not reactive to diamond

(on heating). It is preferred that the material contains as much Ni ₂ AI ₃ phase as possible.

The invention has particular application to DCM where the nickel aluminide intermetallic containing material comprises at least part, and generally substantially the all of, the second phase. When the nickel aluminide intermetallic containing material forms substantially all of the second phase, any other components will be present in trace or minor amounts only, not affecting the abrasion resistance and toughness of the DCM. During manufacture of the DCM, the nickel aluminide intermetallic containing material is in contact with the diamond particles and does not result in any significant synthesis of aluminium carbide. Even the variation in phases due to non-equilibrium conditions mentioned previously is not seen as detrimental to the material performance, as long as the phases formed are not reactive to diamond (on heating).

In a preferred form of this invention, the nickel aluminide containing material is formed in situ by reacting nickel metal with AI ₄ C ₃ in the correct proportions, so that it will preferentially form Ni ₂ AI ₃ while precipitating carbon, as per reaction Hi.

8 Ni + 3 AI ₄ C ₃ → 4 Ni ₂ AI ₃ + 9 C (reaction Hi)

Preferably, the reaction is controlled to ensure that AI ₄ C ₃ reagent is not present in excess, as incompletely reacted aluminium carbide will have detrimental affects on material properties.

If this reaction is carried out under diamond synthesis conditions, which are the conditions used for producing PCD, the excess carbon will precipitate as diamond. From chemical calculations, approximately 15 volume % additional diamond will form.

Where this reaction is complete in the presence of existing diamond grains, this new diamond precipitation aids in facilitating diamond-to-diamond bonding between the existing diamond grains; hence generating a high strength PCD Compared to the formation of other Ni-Al - diamond materials; DCM of this

invention formed at HpHT with pre-existing diamond can use fine diamond, for example, diamond having a particle size of less than 10μm or even diamond that is submicron in size, as this is not thermally compromised, oxidised or consumed, as would usually be the case at low pressures and the temperatures required for sintering a material such as the Ni ₃ AI-diamond containing material.

It is believed that the physical properties of the nickel aluminide intermetallic containing composition at higher temperatures are important in facilitating the formation of a suitable DCM. It seems that this intermetallic system has a peritectic melting point significantly lower than other nickel aluminides such as Ni ₃ AI. Hence at the manufacturing conditions, sufficient molten binder should exist in the nickel aluminide intermetallic containing material to effectively facilitate liquid phase sintering.

In another form of the invention, the nickel aluminide intermetallic containing material is used as at least part of, preferably substantially the whole of, the binder for a cemented carbide substrate for a layer of diamond containing material, particularly PCD. It is possible to make the PCD and substrate in one step in the diamond synthesis apparatus resulting in an industrially efficient process. For example, the capsule for the diamond synthesis apparatus can be filled with a tungsten carbide and the reactants for producing the nickel aluminide then a layer of diamond and the reactants for producing the nickel aluminide can be added. The contents of the capsule are then subjected to diamond synthesis conditions. A structurally or compositionally graded cutter can be made by varying the tungsten carbide to diamond ratio as the capsule is filled.

Diamond may be admixed with the appropriate amounts of nickel and aluminium compounds, and then combined with carbide, admixed with appropriate amounts of nickel and aluminium compounds, and processed under diamond synthesis or milder hot pressing conditions. The product produced is a diamond-carbide composite that is thermally stable. The diamond content of such a composite will generally be not more than 70 volume %. This diamond/carbide/nickel aluminide containing material composite would be superior to any cobalt-based diamond tungsten carbide composite with respect to thermal stability, corrosion, wear

resistance and high temperature mechanical properties and forms another aspect of the invention.

The diamond containing material of the invention may be produced under diamond synthesis conditions. These conditions must be used when the diamond containing material is a diamond compact (PCD). Such conditions are well known in the art. Typically, the high pressure/high temperature conditions are a temperature of at least 1300 ⁰ C and a pressure of at least 5 GPa.

When the diamond containing material contains not more than 70 volume % diamond, milder hot pressing conditions may be used. Typically such conditions are temperatures in excess of 1200 ⁰ C, more preferably around 1400 ⁰ C. These are typically carried out in a vacuum or non-oxidising atmosphere to ensure that there is little or no degradation of the diamond. In order to ensure that there is sufficient of the nickel aluminide intermetallic-based material mixed with the diamond particles to ensure a coherent, robust body, nickel aluminide intermetallic containing material levels in excess of 25 volume % and more preferably 30 volume % are preferred.

EXAMPLES

Example 1

In an example of the invention Ni and AI ₄ C ₃ binder reactants were mixed with diamond grit and the mixture subjected to diamond synthesis conditions. A Ni ₂ AI _3- based diamond compact (PCD) containing a second phase of a complex intermetallic with the stoichiometry of Ni ₂ AI ₃ (i.e. 2 Ni : 3 Al) was formed, according to the reaction

8 Ni + 3 AI ₄ C ₃ → 4 Ni ₂ AI ₃ + 9 C

In order to achieve roughly 20 volume % Ni ₂ AI ₃ intermetallic-based binder with diamond, 11.11g Ni and 10.24g AI ₄ C ₃ were mixed with 78.65g of diamond grit of approximately 12μm in size.

Subjecting this mixture of components to diamond synthesis conditions resulted in an intergrown diamond compact containing a second nickel aluminide intermetallic-based phase with a stoichiometry of 2Ni : 3 Al being produced. The compact was analysed using SEM (scanning electron microscopy) and evidence of precipitated diamond formation in the interstices between the larger diamond grains was observed. This can be seen in the region designated A in Figure 1 , a backscatter SEM image of the material generated in this example.

The diamond compact was analyzed by X-ray diffraction before and after heat treatment at 850 ⁰ C under vacuum for 2 hours. In the as-synthesised sample, the intermetallic-based binder had an overall elemental stoichiometry of 2:3 (Ni:AI) with the presence of some equilibrium Ni ₂ AI ₃ phase and some non-equilibrium NiAI and complex aluminium-rich phases. In the heat-treated sample, there was no evidence of thermal degradation of the diamond compact and no AI ₄ C ₃ was detected from reaction of the intermetallic-based binder with the diamond.

Example 2

A diamond containing material was produced by reacting nickel with aluminium carbide under diamond synthesis conditions. No diamond powder was added to the mixture prior to synthesis. The reaction is based on

8Ni + 3AI ₄ C ₃ → 4 Ni ₂ AI ₃ + 9C

where 52.09g of Ni would be mixed with 47.91 gms of AI ₄ C ₃ . The nickel and aluminium carbide reactants when reacted under diamond synthesis conditions of 5.5 GPa and a temperature preferably greater than 1450 ⁰ C, yielded an intermetallic-based binder that had an overall elemental stoichiometry of 2:3 (NhAI) with the presence of some equilibrium Ni ₂ AI ₃ phase and some non- equilibrium NiAI and complex aluminium-rich phases; together with a fine dispersion of diamond crystals in the binder phase. It is preferable that the AI ₄ C ₃ was well mixed with the Ni powder to ensure a complete and homogeneous reaction while avoiding moisture that would decompose the AI ₄ C ₃ . The presence of diamond was confirmed by X-Ray diffraction analysis.

Example 3

A diamond containing material was produced by reacting nickel with aluminium carbide under diamond synthesis conditions, in the presence of tungsten carbide powder. No diamond powder was added to the mixture prior to synthesis. The reaction is based on

8Ni + 3AI ₄ C ₃ + WC → 4 Ni ₂ AI ₃ + WC + 9C

The nickel, aluminium carbide and tungsten carbide powder reactants in the appropriate ratios (as per the reaction scheme above) were reacted under diamond synthesis conditions, yielding:

• an intermetallic-based binder that had an overall elemental stoichiometry of 2:3 (NhAI) with the presence of some equilibrium Ni ₂ AI ₃ phase and some non-equilibrium NiAI and complex aluminium-rich phases; together with

• a fine dispersion of diamond crystals in the binder phase

• additional WC phase

The presence of diamond was confirmed with X-Ray diffraction.

Example 4

Ni powder and Al powder reactants were mixed to achieve the 2Ni : 3Al stoichiometry and then pre-reacted under vacuum at a temperature of about 850 ⁰ C to form Ni ₂ AI ₃ . This material was then mixed with diamond grit particles of approximately 20 μm in size in sufficient quantity to achieve a volume concentration of 30 volume % of intermetallic in the diamond. The mixture was subjected to pressure-less, high temperature sintering conditions, at approximately 1200°C in a non-oxidising atmosphere. A coherent Ni ₂ AI _3- based intermetallic diamond matrix tool material containing a second phase with the stoichiometry of Ni ₂ AI ₃ (i.e. 2 Ni : 3 Al) was formed. No thermal degradation of the diamond particles was observed during the sintering cycle, illustrating the stability of the diamond in the presence of the intermetallic composition.

REFERENCES

(1) Hwang et al; "Diamond Cutting Tools with M ₃ AI Matrix Processed by Reaction Pseudo-Hipping"; Metallurgical Transactions A, V36A, no. 10, (2005), pp 2801.

(2) Wittmer; "Processing and Friction Properties of lntermetallic Bonded Diamond Ceramic Composites"; Southern Illinois University; American Ceramic Society, 30th International Conference on Advanced Ceramics and Composites, pp 22-27 January, 2006 : ICACC 2006 Cocoa Beach, Florida.