GB2518131A - A method of making a superhard material - Google Patents

Abstract

A method of making a superhard grit comprises preparing a metastable carbon-supersaturated solid solution comprising a first mass of a source of elemental carbon and a second mass of a catalyst comprising an elemental metal powder or metal alloy powder, the first mass comprising between around 10 wt% to 20 wt% of source of elemental carbon, and the second mass comprising between around 80 wt% to 90 wt% catalyst powder. A green body is formed comprising the metastable carbon-supersaturated solid solution and is placed into a canister to form an assembly which is treated at a pressure of around 5 GPa or greater and a temperature of between 1500 to 2000 °C to form a body comprising superhard material in a metal matrix. The body is then treated to dissolve the metal matrix and recover the superhard material. In preferred versions the elemental metal powder may be selected from cobalt, iron, manganese, nickel, platinum, ruthenium, or alloys thereof.

Description

A METHOD OF MAKING A SUPERHARD MATERIAL

FIELD

This disclosure relates to a method of making a superhard material for subsequent use in forming, for example, polycrystalline diamond (PCD) structures which may be used in tools, particularly but not exclusively for use in rack degradation or drilling, or for boring into the earth.

BACKGROUND

Polycrystalline diamond (PCD) is an example of a superhard material (also called a superabrasive material or ultra hard material) comprising a mass of substantially inter-grown diamond grains, forming a skeletal mass defining interstices between the diamond grains. FCD material typically comprises at least about 80 volume % of diamond and is conventionally made by subjecting an aggregated mass of diamond grains, also referred to as diamond grit, to an ultra-high pressure of greater than about 5 GPa, and temperature of at least about 1,200°C, for example. A material wholly or partly filling the interstices may be referred to as filler or binder material.

Traditional diamond grit synthesis from which PCD may be made requires coating of individual diamond seeds with graphite and catalyst materials for its success. Often, the quality is uncontrollable and this results in poor yield as coating of more than one diamond seed per granule may occur or granules with no diamond seed may be formed.

There is therefore a need for a method of synthesising diamond grit with improved quality and yield.

SUMMARY

Viewed from a first aspect there is provided a method of forming a superhard grit comprising: preparing a metastable carbon-supersaturated solid solution comprising first mass of a source of elemental carbon and a second mass of a catalyst comprising an elemental metal powder or metal alloy powder; the first mass comprising between around 10 wt% to around 20 wt% of source of elemental carbon, and the second mass comprising between around 80 wt% to around 90 wt% catalyst powder; forming a green body comprising said metastable carbon-supersaturated solid solution; placing the green body into a canister to form an assembly; treating the assembly at an ultra-high pressure of around 5 GRa or greater and a temperature of between around 1500 to around 2000 degrees O to form a body comprising superhard material in a metal matrix; treating the body to dissolve the metal matrix and recover the superhard material.

DETAILED DESCRIPTION

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

As used herein, a "superhard construction" means a construction comprising a body of polycrystalline superhard material. In such a construction, a substrate may be attached thereto or alternatively the body of polycrystalline material may be free-standing and unbacked.

As used herein, polycrystalline diamond (PCD) is a type of polycrystalline superhard (PCS) material comprising a mass of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume percent of the material. In POD material, interstices between the diamond grains may be at least partly filled with a binder material comprising a catalyst for diamond. As used herein, "interstices" or "interstitial regions" are regions between the diamond grains of PCD material. In PCD material, interstices or interstitial regions may be substantially or partially filled with a material other than diamond, thereby forming a non-diamond phase, or they may be substantially empty. POD 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, "high speed balling milling" means a process of comminution of powder particles during impact between moving balls and between impacting balls and milling jars/pots rotating at speeds higher than 200 rotations per minutes. Milling balls may be made of dense materials, such as stainless steel grades or tungsten carbide.

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

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

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

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

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

As used herein, "catalyst material for diamond" is a material that is capable of promoting the growth of diamond or the direct diamond-to-diamond inter-growth between diamond grains at a pressure and temperature at which diamond is thermodynamically more stable than diamond.

A method of synthesising diamond grit according to an embodiment comprises, in a first step, preparing a carbon-supersaturated catalyst alloy by dissolving carbon atoms in a metal system using high energy ball milling and mechanical alloying. The starting materials comprise a source of elemental carbon, such as graphite or amorphous carbon powder, and a catalyst elemental metal powder, such as one or more of cobalt, iron, manganese, nickel, platinum, ruthenium, or an alloy powder formed for example from one or more of said metals. The starting mixture may, for example, be formed of between around 80 wt% to around 90 wt% of catalyst elemental metal powder or alloy powder and between around 10 wt% to around 20 wt% graphite or amorphous carbon.

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

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

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

The above-described mechanical alloying process as opposed to a standard mechanical milling process enables the formation of a metastable supersaturated solid solution having more carbon in the metal/alloy than is predicted by the equilibrium phase diagram(s).

This supersaturated solid solution forms a mechanically alloyed powder which may then be used to manufacture diamond grit in the following steps.

The mechanically alloyed powder is pressed into a slug. In some embodiments, the slug is outgassed using conventional techniques. In other embodiments, the slug is not outgassed. The slug may be formed into a pre-composite comprising, for example, two ceramic cups. The pre-composite is then subjected to a pressure of at least 5Gpa at a temperature of between around 1500 to around 2000 degrees C to synthesise the diamond grit from the slug.

The metastable supersaturated solid solution formed during high energy ball milling is thermally decomposed at high temperature. The carbon atoms thus released precipitate and grow into diamond crystals in the slug. The slugs are then crushed to partially liberate the synthesised diamond particles without breaking them. The diamond particles may be recovered by dissolving the metal matrix in an acid solution, such as nitric acid or phosphoric acid or a mixture thereof.

Whilst not wishing to be bound by theory, it is believed that the diamond grit may thereby be precipitated-synthesised during the sintering step as a result of thermal decomposition of the supersaturated mechanically alloyed solution and crystallisation. Precipitation-synthesis means the metastable supersaturated solid solution formed during high energy ball milling process is thermally decomposed at high temperature. The carbon atoms thus released precipitate and grow into diamond crystals during the subsequent sintering process. Nano sized and/or micron sized diamond grit may be synthesised in this manner. The size of the diamond grit formed may be controlled by selection of one or more of the initial amount of dissolved carbon in the catalyst material, the pressure and temperature conditions used in the synthesis process and the duration of the HPHT synthesis process.

While various embodiments have been described, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof and that these examples are not intended to limit the particular embodiments disclosed.

Thus it will be seen that some embodiments may assist in providing a method of forming a superhard material such as diamond grit in-situ in a slug comprising a supersaturated solid solution by thermal decomposition and crystallisation.

Claims (11)

1. Claims 1. A method of forming a superhard grit comprising: preparing a metastable carbon-supersaturated solid solution comprising first mass of a source of elemental carbon and a second mass of a catalyst comprising an elemental metal powder or metal alloy powder; the first mass comprising between around 10 wt% to around 20 wt% of source of elemental carbon, and the second mass comprising between around 80 wt% to around 90 wt% catalyst powder; forming a green body comprising said metastable carbon-supersaturated solid solution; placing the green body into a canister to form an assembly; treating the assembly at an ultra-high pressure of around 5 GRa or greater and a temperature of between around 1500 to around 2000 degrees o to form a body comprising superhard material in a metal matrix; treating the body to dissolve the metal matrix and recover the superhard material.
2. 2. The method of claim 1, wherein the step of preparing the metastable solid solution comprises mechanically alloying the first mass comprising graphite and/or amorphous carbon powder with the catalyst.
3. 3. The method of any one of the preceding claims, wherein the step of preparing the metastable solid solution comprises mechanically alloying the first mass with the second mass, the second mass comprising one or more of the elemental metals cobalt, iron, manganese, nickel, platinum, ruthenium, or an alloy powder of one or more of said metals.
4. 4. The method of any one of the preceding claims, wherein the step of mechanically alloying comprises processing said first and second masses in a high energy ball mill machine at a temperature of less than around 200 degrees C.
5. 5. The method of claim 4, wherein the step of mechanically alloying comprises using a high energy ball mill machine in which the milling ball-to-powder weight ratio is between around 10:1 and around 30:1.
6. 6. The method of any one of claims 4 or 5, wherein the step of mechanically alloying comprises milling the first and second masses in a high energy ball mill machine in a milling atmosphere, the milling atmosphere being inert.
7. 7. The method of any one of claims 4 or 5, wherein the step of mechanically alloying comprises milling the first and second masses in a high energy ball mill machine in a milling atmosphere, the milling atmosphere being a reducing atmosphere.
8. 8. The method of claim 7, wherein the milling atmosphere comprises argon or nitrogen.
9. 9. The method of any one of the preceding claims, wherein the step of mechanically alloying comprises applying a high speed energy ball milling process to mill the first and second masses for a duration of up to around 48 hours.
10. 10. The method of any one of the preceding claims, wherein the superhard material comprises diamond grains.
11. 11. A method of making a polycrystalline superhard construction substantially as hereinbefore described with to any one embodiment.