WO2015135979A1

WO2015135979A1 - Diamond grains, tools comprising same and methods of using same

Info

Publication number: WO2015135979A1
Application number: PCT/EP2015/055051
Authority: WO
Inventors: Mhlonishwa Cyprian Nzama; Mark Gregory Munday
Original assignee: Element Six (Uk) Limited
Priority date: 2014-03-14
Filing date: 2015-03-11
Publication date: 2015-09-17
Also published as: GB201404633D0; GB2526176A; GB201504095D0

Abstract

A plurality of synthetic diamond grains having a mean size of 5 to 100 microns. The crystal habits of the diamond grains are substantially cubo-octahedral, such that at least 50 per cent of the grains have a morphology index of about 3 to about 6, at least 5 per cent of the grains have a morphology index of about 2 to about 4, and at least about 20 per cent of the grains have a morphology index of about 5 to about 7. There is also disclosed a tool comprising the plurality of synthetic diamond grains.

Description

DIAMOND GRAINS, TOOLS COMPRISING SAME AND METHODS OF USING SAME

This disclosure relates generally to diamond grains, particularly to small diamond grains of up to about 100 microns, to tools comprising same and to methods of using them.

A disclosed method for making the grains includes subjecting a source of carbon in the presence of a suitable metal material to a first ultra-high pressure of at least about 5.2 gigapascals (GPa) and a first temperature of about 1 ,400 degrees Celsius at which the metal is molten, thus inducing the spontaneous nucleation of a plurality of nuclei diamond grains from the carbon source. The pressure and temperature are then reduced to a second pressure and a second temperature at which the nuclei will grow by carbon precipitation to become a plurality of diamond grains, but at which substantial further spontaneous nucleation of diamond grains does not occur. After a period of time, the pressure and temperature are reduced to ambient levels and the grown diamond grains removed from the synthesis assembly comprising the remaining carbon source and metal, in which they are dispersed. The number of grains produced can be influenced by the magnitudes of the first pressure and the first temperature, and the period of time at this first condition, among other factors. The size of the diamond grains can be influenced by the magnitudes of the second pressure and second temperature, and the period of time at this second condition, among other factors. Tools for cutting thin wafers from bodies are needed, particularly but not exclusively bodies comprising or consisting of silicon or sapphire, as well as other materials that tend to be difficult to process. It will likely be preferable for the cutting to result in relatively little wastage of material from the bodies being cut, and for it to be relatively efficient, fast and cost-effective.

Viewed from a first aspect, there is provided a plurality of synthetic diamond grains having a mean size of 5 to 100 microns, the crystal habits of the diamond grains being substantially cubo-octahedral, such that at least 50 per cent of the grains have a morphology index of about 3 to about 6, at least 5 per cent of the grains have a morphology index of about 2 to about 4, and at least about 20 per cent of the grains have a morphology index of about 5 to about 7.

Various distributions of physical characteristics, such as shape, size, inclusion content or impurity content, as well as various combinations of the diamond grains with non-diamond material are envisaged by this disclosure for the plurality of synthetic diamond grains; various tools and methods of using the tools and the diamond grains are also envisaged by this disclosure; non-limiting and non- exhaustive examples of which are described below.

In some examples, at least about 10 per cent or at least about 20 per cent of the diamond grains may have a morphology index of about 2 to about 4; and in some examples, at least about 60 per cent, at least about 70 per cent or at least about 80 per cent of the diamond grains may have a morphology index of about 3 to about 6; or a morphology index of about 4; and in some examples, at least about 30 per cent, at least about 40 per cent may have a morphology index of about 4 to about 7, or about 6 to 7. In some examples, at most about 90 per cent of the diamond grains may have a morphology index of about 3 to about 6; in some examples, at most about 20 per cent or at most about 10 per cent of the diamond grains may have a morphology index of 2 to 4; and or in some examples, at most about 50 per cent or at most about 30 per cent may have a morphology index of 4 to 7 or 5 to 6.

In some examples, at least 10 per cent of the diamond grains may be attached to another diamond grain. In various examples, at least about 12 per cent or at least about 30 per cent of the diamond grains may be attached to at least one other diamond grain. The attached diamond grains may be crystallographically inter- grown, or substantially crystallographically coherent adjacent a boundary at which they are attached. The mean aspect ratio of the diamond grit may be at least 0.70 or at least 0.72; and or the mean aspect ratio may be at most 0.80, 0.76 or 0.74.

In some examples, at least about 50 per cent, at least about 80 per cent or substantially all of the diamond grains may contain an included seed diamond grain. Seed grains may be at least partly surrounded by non-diamond inclusions, which may contain metal, such as iron, nickel, cobalt, manganese, or alloys, intermetallic or other compounds containing one or more of these. In some examples, the included seed diamond grain may be substantially occluded by non-diamond inclusions. In some examples, at least about 50 per cent, at least about 80 per cent or substantially all of the diamond grains may contain non-diamond inclusions. The size distributions and or spatial distributions of the inclusions within the diamonds, and or material composition of the inclusions may result in increased micro-fracture of the diamond grain in use, which used to remove material from a body (silicon or sapphire, for example). In some examples, this may result in continual sharpening or other regeneration of physical characteristics of the diamond grains, suitable for continued use in removing material from the body. This potential effect may arise from the strain distribution within the diamond grains resulting from the size and or spatial distribution and or material composition of the non-diamond inclusions.

In some examples, most of the diamond grains may have relatively rough surface texture, and or may comprise protrusions; this may have the effect of enhancing retention of the diamond grains within a tool, in use. In various examples, the plurality of diamond grains may have a mean size of at least about 10 microns, and or at most about 50 microns; or at most about 30 microns or at most about 20 microns. In some examples, most or substantially all of the diamond grains may be being substantially euhedral or subhedral. In some examples, at least about 50 per cent, at least about 80 per cent or substantially all of the diamond grains may be free from surfaces exposed by cleavage or crushing. In other words, substantially the entire surface are of these diamond grains may be substantially as grown, corresponding to crystallographic facets or planes of diamond.

The diamond grains will have crystal habits (which may also be referred to as crystal shapes) including major surfaces corresponding to (100) and (1 1 1 ) crystallographic planes of diamond. Minor surfaces corresponding to other crystallographic planes may be present, such as the (1 10) or (1 13) crystallographic planes. A relatively small percentage or substantially none of the diamond grains may be generally acicular or plate-like. More than 50 per cent, more than 80 per cent or substantially all of the diamond grains may include minor crystallographic faces on the surface.

In some examples, the diamond grains may be provided combined with non-diamond material, such as material including nickel, cobalt, iron, alloys including iron, nickel or cobalt, braze material, tungsten carbide, resin material, vitreous material, silicon, silicon carbide or silica. In some examples, the diamond grains may be provided within a plurality of pellets, each pellet containing one or more of the plurality of diamond grains encapsulated within a structure comprising the bond material or the precursor material for bond material. In some examples, the diamond grains may be provided as loose powder, or dispersed within slurry, paste, within a solid body comprising a non-diamond matrix, liquid or in some other medium. In some examples, the diamond grains may be provided bonded to body, such as by means of electroplating or adhesive material. In some examples, the diamond grains may be provided within a matrix of non-diamond material, such as metal, alloy, intermetallic, ceramic, hard-metal, cemented carbide, polymer (such as resinous), vitreous material, or combinations of these materials.

In some examples, the diamond grains may comprise at least one coating layer. The coating layer may comprise nickel, boron, silicon, titanium or tantalum, or a combination of any of these in elemental, compound, alloy or other form. In some examples, a first coating layer may comprise carbide compound material including material as comprised in a second coating layer and carbon from the surface of the diamond grain.

In some examples, the diamond grains may have strength in terms of friability index of at least about 50 per cent and or at most about 70 per cent or 60 per cent. In some examples, the strength distribution of the diamond grains may have an effect of enhanced efficiency (such as indicated by speed) of removal of material.

In some examples, the plurality of diamond grains may consist of a sufficient number of diamond grains for the mass of the plurality to be at least 1 gram (g). In some examples, the plurality may consist of at least about 10 or at least about 100 diamond grains. Viewed from a second aspect, there is provided a tool comprising a plurality of synthetic diamond grains according to examples envisaged by this disclosure. In some examples, a tool may comprise a saw blade, a saw wire, a cutter wheel or a slotting tool; the tool may be suitable for removing material from a body comprising or consisting of silicon, sapphire or other materials that tend to be difficult to process. The diamond grains may be attached to a tool body by means of electroplating, adhesive, or by being dispersed within a matrix material.

In some examples, a tool may comprise sintered polycrystalline diamond (PCD) material made using examples of the disclosed diamond grains as raw materials; the diamond grains may be sintered together at an ultra-high pressure and high temperature in the presence of catalyst material for growing and sintering diamond grains. In some examples, the tool may comprise a wire-drawing die, which may comprise PCD material made by sintering the diamond grains together. The diamond grains may be directly inter-bonded with each other in the PCD material.

Viewed from a third aspect, there is provided a method of using a tool according to an example envisaged by this disclosure, the method including using the tool to remove material from a body comprising or consisting of silicon, sapphire or other materials that have relatively high hardness and which are relatively brittle, such as advanced ceramic materials. Non-limiting example methods, arrangements and diamond grains will be described below with reference to the accompanying drawings, of which

Fig. 1 shows a scanning electron microscope (SEM) image of example diamond grit; Fig. 2 shows a schematic representation of the morphology index scale for characterising the crystal habit of synthetic diamond crystals;

Fig. 3 shows a schematic illustration of three kinds of crystal habits;

Fig. 4A and Fig 4B show schematic representations of example substantially cubo- octahedral diamond crystals; Fig. 5 shows a schematic illustration of an example arrangement of a reaction volume for growing diamond crystals in an ultra-high pressure and high temperature synthesis apparatus;

Fig. 6 shows a graph of normal force F, in units of Newtons, generated by an example saw tool (E) and two comparative tools (C1 and C2) at various rates of material removal MRM, in units of cubic millimetre per second (mm³/s); Fig. 7 shows a graph of two measures of surface roughness, R_a and R_z, of an example work-piece consisting of silicon, after being cut by the example saw tool (E) the comparative saw tools (C1 and C2); and

Fig. 8 shows an SEM image of example diamond grit in an example saw tool that had been used in a test involving cutting slots into a silicon work-piece body.

With reference to Fig. 1 , example synthetic diamond grains may have a blocky, generally cubo-octahedral crystal habit. A relatively large percentage of the example grit is evidently attached to at least one other diamond grain, which may be of substantially the same size. In the particular example shown in Fig. 1 , at least about 10 per cent of the diamond grains are attached to at least one other diamond grain. In some examples, the plurality of diamond grains may contain aggregations of relatively well-bonded diamond grains, and aggregations may comprise diamond grains having substantially different sizes. Diamond grains that are attached to each other via relatively strong crystallographic bonds will likely be incapable of being separated from each other by means of milling at low or medium energies. The plurality of diamond grains may comprise or consist of diamond grains having a relatively wide distribution of generally cubo-octahedral shapes, in terms of the morphology index scheme as illustrated in Fig. 2. The example grit shown in Fig. 1 exhibited relatively blocky diamond grains, in which at least about 50 per cent had generally cubo-octahedral habit and a morphology index of about 3 to about 6, in combination with relatively high percentages of diamond grains having morphology index of about 2 to about 4 (at least about 5 per cent of the grains) and of about 4 to about 7 (at least about 20 per cent of the grains). The diamond grains had not been substantially crushed, milled or fractured (in other words, it had not been comminuted) by some other means after their synthesis, which is evident in the substantial absence of fracture surfaces and the prevalence of identifiable diamond crystallographic facets over substantially all of the surface area of substantially all of the diamond grains in the plurality. Fig. 2 presents an idealised representation of a range of cu bo-octahedral shapes associated with respective integer morphology from 0 to 8, in which 0 indicates the presence of only cubic faces, 8 indicates the presence of only octahedral faces and the integers 1 to 7 indicating various combinations of cubic and octahedral faces; each morphology index may be quantifiable in terms of the relative areas of the cubic an octahedral faces. This is described in Bailey and Hedges, "Crystal Morphology Identification of Diamond and ABN", Industrial Diamond Review 1995, 55(1 ), 1 1 -14. Essentially, each number of the morphology index represents 12.5% of the relative proportions of cubic and octahedral faces. A morphology index of 4 represents a particle where 50% of the surface area is formed from cubic faces and the other 50% is formed from octahedral faces. A morphology index of 2 represents a shape where 75% of the surface area is formed from cubic faces and 25% of the surface area is formed from octahedral faces. A morphology index of 6 represents a shape where 25% of the surface area is formed from cubic faces and 75% of the surface area is formed from octahedral faces. A particle having a morphology index of 3 to 6 therefore has a shape where the surface area is made up of between 25 and 62.5% cubic faces. A particle having a morphology index of 2 to 4 has a shape where the surface area is made up of between 50 and 75% cubic faces. A particle having a morphology index of 5 to 7 has a shape where the surface area is made up of between 12.5% and 37.5% cubic faces. Table 1 shows the relative proportions of cubic and octahedral faces as a function of the morphology index.

Morphology Index Proportion cubic % Proportion octahedral %

0 100 0

1 87.5 12.5

2 75 25

3 62.5 37.5

4 50 50

5 37.5 62.5

6 25 75

7 12.5 87.5

8 0 100

Table 1

A diamond crystal may have mixed morphology, in which a part of the crystal may have a different morphology index from another part of the crystal. Diamond crystals may also exhibit minor faces in addition to and between the cubic and or octahedral faces, and may also exhibit crystallographic twinning. The surfaces of synthetic diamond crystals may be smooth or exhibit various kinds of features, such as striations, dendritic micro-formations or pitting, which may arise form etching of the diamond surface or from the environment of the crystal during its synthesis. Crystals may also exhibit deformations arising from growth competition and or physical interaction with other crystals during synthesis, and or from mechanical degradation of the crystal after synthesis.

In some examples, the plurality of diamond grains may be substantially as-grown in an ultra-high pressure furnace apparatus (also referred to as an ultra-high pressure press) for growing diamond crystals from a non-diamond source of carbon such as graphite, under a condition at which diamond is more thermodynamically stable than graphite. In some examples, the grown diamond crystals may not be subjected to substantial crushing or cleavage (apart from potential processing to reduce the frequency of agglomerations of diamond grains that may be weakly attached to each other in clusters). The plurality of diamond grains may be provided by combining pluralities of diamond grains from more than one source, provided that the sources are of synthetic diamond, a minor proportion of which may have exposed cleavage surfaces potentially arising from crushing or other processing of the diamond grains. In other examples, the plurality may be drawn from a single source of diamond grains grown in the same batch crystallisation process.

Fig. 3 shows a schematic illustration of a substantially cubo-octahedral crystal shape A (which may also be referred to as 'blocky'), an example acicular crystal shape B (which may also be referred to as 'needle-like') and an example disc-like crystal shape C (which may also be referred to as 'plate-like'); these shapes may be quantifiable in terms of their relative size dimensions in orthogonal coordinates. The 'blockiness' of the crystal shapes can be expressed as their having a low aspect ratio, with few acicular or plate-like crystals.

Fig. 4A and Fig 4B show schematic representations of example substantially cubo- octahedral diamond crystals, in which major (100), or cubic, and (1 1 1 ), or octahedral crystallographic faces are evident, as well as minor crystallographic faces.

An example method for making diamond grit will be described. With reference to Fig. 5, an example capsule assembly 10 may comprise a reaction volume 12 contained within a ceramic container 14. The reaction volume 12 may comprise a blend of graphite powder as a source of carbon for synthesising the diamond grains, iron (Fe) powder and nickel (Ni) powder, the mass ratio of Fe to Ni therefore being about 2.34 and mass ratio of metal to graphite being at least about 6. In some examples, the mass ratio of metal to graphite may be at least about 4 or at least about 5; and or at most about 7. Small diamond grains were dispersed in the reaction volume to function as seed crystals for growing the example grains, the seed grains having mean size in the range of about 0.5 micron to about 5 microns. In a particular example, the diamond seed grains may have a mean size in the range of about 1 .5 to about 2 microns, at least about 90 per cent of the seed grains being at most about 4.5 or at most about 4 microns, and a specific surface area in the range of about 3.2 to about 3.6 square metres per gram. The reaction volume 12 may be made by blending the graphite, iron and nickel powder with the diamond seeds to achieve a substantially homogeneous dispersion of the diamond seeds within the blended powder, and compacting the blended powder at ambient temperature by means of a uni-axial compaction press. In a particular example, the diamond seed grains may not be coated and may be provided by a method including crushing of coarser diamond grains, the resulting finely sized grains to be used as the seed grains having irregular shape. The capsule assembly 10 can be further assembled into a capsule for a cubic ultra-high pressure furnace (press) and subjected to a pressure of at least about 5.5 gigapascals and a temperature of at least about 1 ,300 degrees Celsius. The pressure and temperature will be such that diamond is more thermodynamically stable than graphite and that the metal will be in molten form. The pressure should be sufficiently low for avoiding substantial spontaneous nucleation of diamond grains. For example, the pressure may be at least about 5.5 gigapascals and at most about 5.8 gigapascals, and the temperature may be at least about 1 ,250 degrees Celsius and at most about 1 ,350 degrees Celsius.

In a particular example, the mass ratio of the metal to the graphite may be such that substantially all of the graphite will be consumed in the growth of the diamond crystals, and the number of diamond seed grains per unit volume of the reaction volume may be such that a plurality of diamond grains having size substantially within a desired size range will be provided once substantially all of the graphite has been substantially uniformly converted into synthetic diamond on the seed grains. This arrangement will likely permit very small diamond grains to be synthesised in a controlled way to achieve a desired mean size. The shape of the synthetic diamond can be controlled to a large degree by adjusting the synthesis conditions.

After the synthesis process, the grown diamond grains can be recovered from the reaction volume by a process that may include crushing the reaction volume by means of a jaw crusher, for example, to reduce the synthesis assembly to a plurality of granules having mean size of about 1 millimetre or less, and digesting the granules in acid to remove the metal. The residue from the acid digestion, which will include the diamond grains and probably some graphite, may then be washed, dried and milled. The residue may be subject to further acid digestion, washing and drying, and then sieved to recover diamond grains in the targeted size range.

Cutting tools comprising example pluralities of synthetic diamond grains may exhibit enhanced performance in cutting silicon or sapphire, for example. Tools comprising the diamond grains may be used to cut wafers from bodies (which may also be referred to as ingots) comprising or consisting of silicon or sapphire, for example. Prior to the cutting of wafers from the ingots, they may be subject to various stages of processing to shape the ingots, which may include using a wire saw to remove material from the ingot. For example, a loose abrasive wire sawing technique may be used to cut a silicon ingot, in which slurry comprising silicon carbide may be used as cutting media entrained on a steel wire. A diamond wire saw device, comprising small diamond grains attached to a wire, may be used to cut through a body comprising or consisting of sapphire, since sapphire is harder and more brittle than silicon. A test was carried out, in which of the performance of the example diamond grit shown in Fig. 1 was compared to that of different grades of diamond grit in an operation involving cutting slots into work-piece bodies consisting of polycrystalline silicon. An electroplated cut-off type circular saw blade was made comprising a plurality of example diamond grains (grit) E, which had not been subjected to substantial comminution subsequent to its synthesis in an ultra-high pressure and high temperature apparatus (in other words, the example grit E was substantially as- synthesised). Two comparative saw blades were also made using different grades of comparative diamond grit C1 and C2, the only substantial differences between the three saw blades being the grade of synthetic diamond comprised in them. Various aspects of the example grit E and the comparative grit C1 and C2 are summarised in Table 2 and described below (in Table 2, the size distributions are characterised in terms of the tenth and ninetieth percentile sizes, the median sizes (d(0.5)) and the volumetric mean (D[4,3]) sizes). The mean aspect ratios of the diamond grit was measured by means of an Ochio™ apparatus, and the closer the aspect ratio is to unit, the blocky or 'round' the grains. Since the aspect ratio of the example grit E is greater than that of the comparative grit (and closer to one), it is has a lower aspect ratio than the latter, which included on average somewhat more acicular or plate-like grains (this may be associated with the fact that the comparative grit had been comminuted when deriving it from larger grains). The friability index of the grit was measured by means of a 'friatester' apparatus, which is expressed as a percentage of grit remaining relatively unfractured by the comminution-based friability test process. The example grit E may be regarded as having been relatively stronger than the comparative grit since it had a somewhat higher friability index value. The strength characteristics of the grades are stated according to the relative friability of the grains, high strength grains having relatively low friability. The shape characteristics of the grades are stated in terms of the regularity of the shapes, as well as whether the crystal habits tended to be relatively blocky or acicular (also referred to as elongate), with reference to Fig. 3. The two pluralities of diamond grains corresponding to comparative tools C1 and C2 were derived from larger grit by comminution. All three samples of diamond grit was processed using a range of techniques for characterising and controlling the quality of micron products at various stages of production; sizing, shape improvement, removal of agglomerated and oversized particles.

£ C1 C2

Product name (not applicable) Micron+ CDA™ Micron+ MDA™

Grade of super-hard As-grown diamond Derived from resin Derived from metal grains grains bond bond

Strength Highly friable High strength characteristic

Shape characteristic Irregular and Irregular and elongated in shape elongated in shape d(0.1 ), microns 16 14 d(0.5), microns 22 19

d(0.9), microns 30 26

D[4,3], microns 23 20

Mean aspect ratio of 0.73 - 0.74 0.68 - 0.70

the grains

Friability index, % 52.2 43.7

Table 2

Slotting may likely provide a useful indication of the likely relative performances of the diamond grit when used in precision wire saw operations, since a wire saw operation may involve cutting a body by means of a length of wire to which small diamond grains have been attached by means of electroplating. The slotting tests were carried out by means of a Jones and Shipman 540X™ surface grinder, in which the cutting forces were measured by means of a Kistler™ dynamometer and recorded for analysis by means of LabView™ data acquisition software. The roughness of the cut surfaces of the work-piece bodies was measured by means of a three dimensional optical measurement system (an Alicona™ apparatus). The test was separated into two parts. The first part was to explore the performance of the diamond grit comprised in the tools over a wide range of material removal rates, and the second part was the roughness of the surfaces of the cut silicon bodies. Different cutting parameters were used for measuring the first and second parts of the test, and are shown in Table 3. In all tests, electroplated cut-off wheels of type 1 A1 R having diameter of 180 mm were used and a coolant liquid was used (6% HoCut 768™ in water).

Grinding Parameter First part Second part

Wheel speed, v_Si m/s 38 38

Work-piece speed , v_Wt m/min 10 8

Depth of cut, a_e, microns (μηη) 50 to 600 1 , 5, 10

Table 3

The results for the first aspect are shown in the graph of Fig. 6 and those for the second aspect are shown in the graph of Fig. 7.

It is evident from Fig. 6 that the example tool E, comprising the example plurality of diamond grit, required substantially lower forces to remove material from the work- piece body than the comparative tools C1 and C2. The forces required for the example tool E were about 51 per cent lower than for the comparative tools C1 and C2. This suggests that the example diamond grit may potentially permit substantially enhanced efficiency when used to cut certain materials such as silicon and sapphire, and consequently higher cutting rates and extended tool life to be achieved, all else being equal.

It is evident from Fig. 7 that the surfaces of the work-piece body cut by the example tool E was substantially rougher than the surfaces cut by any of the comparative tools. This potentially indicates that the work-piece material is removed by a fracture mechanism, being a 'brittle' grinding mechanism. This is likely to be consistent with the observation that the example tool E required lower cutting forces than the comparative tools. Material removal from a silicon work-piece body by means of grinding can be divided into two broad mechanisms, namely ductile and brittle modes. The ductile mode occurs when contact stress is lower than the in-situ contact strength of material. Ductile modes tend to require higher specific energy due to the rubbing of the tips of the grit against work-piece material, associated with higher contact area, and the work-piece material will undergo plastic deformation. For the brittle mode to be achieved, contact stress between the grit comprised in the tool and the work-piece body needs to exceed the in-situ contact strength of work- piece material. The specific energy reduces, which results in more efficient material removal, but the resulting cut surface will likely have higher roughness than in the case of the ductile mode. With reference to the SEM image shown in Fig. 8, the example grit after the tool had been used in the test still displays well defined edges and points, but substantially reduced presence of crystallographic facets. This suggests that grit has undergone micro-fracturing during the test, which is likely to be consistent with the observation of reduced cutting forces and the rougher cut surface of the work-piece body, and the brittle mode of material removal resulting from relatively low contact area between the grit and the work-piece body.

While wishing not to be bound by a particular theory, strongly inter-grown diamond grains particle may have an effect of increasing the probability of fracture of the diamond grains, which may enhance the efficiency of removing material from a body which the diamond grains are used in a tool.

While not wishing to be bound by a particular theory, the strength and shape of synthesised micron promote a brittle grinding mode lowering the grinding forces and resulting in a slightly inferior surface finish.

Disclosed diamond grains may have the aspect of having distributions of shape and size that make them suitable for processing bodies comprising or consisting of sapphire, silicon and potentially various materials that may be difficult to process by other means. The diamond crystals may have the aspect of reduced content of impurities and inclusions surrounding the seed particles on which they are grown. Since impurities and inclusions tend to have a significant effect on the strength of diamond crystals, the crystals made using this method may have substantially enhanced strength.

While wishing not to be bound by a particular theory, a plurality of synthetic diamond grains having relatively 'blocky' shape, in which a substantial area of the surface of each grain corresponds to a (100) (in other words cubic) crystallographic plane, in combination with a relatively wide distribution of cubo-octahedral crystal habits may result in an increased number of points and edges available for removing material from a body when the diamond grains are used in a tool. It may be that relatively infrequent incidence of relatively large, flat surface areas of the diamond grains, particularly the relative infrequency of plate-like or acicular diamond grains may increase the number of edges and points on the diamond grains available for cutting material from a body.

Various concepts and terms as used herein will be briefly explained.

A plurality of abrasive or super-abrasive grains such as synthetic diamond grains may be referred to as 'grit'.

The crystal habit of a crystal describes its visible external shape. Near-perfect to perfectly formed crystals can be described as being euhedral, moderately well- formed crystals can be described as subhedral and poorly formed crystals or crystals with no discernible habit can be described as anhedral. In general, synthetic diamond crystals in the form of grains, which may have mean size of less than about 1 millimetre, tend to have cubo-octahedral crystal habit. Depending on the synthesis conditions, the crystal habit of diamond grains may be cubic, octahedral or both cubic and octahedral facets may be present in various proportions, and minor facets may also be present.

The crystal habits of synthetic diamond and cBN crystals is discussed by Bailey and Hedges (reference above) using a morphology index, which describes the basic characteristics of crystal shapes in terms of the growth of different crystal faces or planes. For example, the shape of synthetic diamond may be substantially defined by various combinations of (1 1 1 ) and (100) surfaces. Eight-sided diamond crystals having only (1 1 1 ) surfaces may be referred to as having octahedral habit and six- sided diamond crystals having only (100) surfaces may be referred to as having cubic habit. Diamond crystals may have both (100) and (1 1 1 ) surfaces and may be referred to as having cubo-octahedral habit. A diamond morphology index has been developed to assign an integer from 0 (completely cubic habit) to 8 (completely octahedral habit) according to the crystal habit.

The size range of abrasive grains may be expressed in terms of U.S. Mesh size, in which two mesh sizes are provided, the first being a mesh size through which the grains would pass and the second being a mesh size through which the grains would not pass. Mesh size may be expressed in terms of the number of openings per (linear) inch of mesh. As used herein, grain sizes can be measured by a technique including laser diffractometry. The diffraction pattern may be interpreted mathematically as if it had been generated by a plurality of spherical grains, the diameter distribution of which being calculated and reported in terms of ECD. Aspects of a grain size distribution may be expressed in terms of various statistical properties using various terms and symbols. Mean values obtained by means of laser diffraction methods may be most readily expressed on the basis of a distribution of grain volumes, the volume mean can be represented as D[4,3] according to a well-known mathematical formula. The median value d(0.5) of a size distribution is the value dividing the plurality of grains into two equal populations, one consisting of grains having size above the value and the other half having size at most the value. Various other values d(y) can be provided, expressing the size below which a fraction y of the plurality of grains reside in the distribution. For example, d(0.9) refers to the size below which 90 per cent of the grains reside, d(0.5) refers to the size below which 50 per cent of the grains reside and d(0.1 ) refers to the size below which 10 per cent of the grains reside.

As used herein, solvent-catalyst material for diamond is capable of promoting the precipitation of carbon in the form of diamond at conditions at which diamond is thermodynamically more stable than graphite. While wishing not to be bound by a particular theory, solvent-catalyst material may promote the growth of diamond crystals principally or exclusively by dissolving a source of carbon and transporting atoms or molecules comprising the source to a seed particle or partially grown crystal to which it becomes attached. Examples of solvent-catalyst material for diamond include iron, nickel, cobalt and manganese or certain alloys including any of these, which are capable of promoting the growth of diamond crystals from a source of carbon such as graphite at ultra-high pressure and high temperature at which diamond is thermodynamically more stable than graphite and the catalyst material is in the liquid state. The solvent and catalytic effects are likely to be particularly strong when the catalyst material is in the liquid state. As used herein, friability is an extrinsic property of solid material indicating the degree to which it can to be reduced to smaller pieces when energy is applied to it. As used herein, comminution is the reduction of the sizes of grains, by means of crushing, grinding or other processes. A comminution device known as a friability tester can be used for providing an indication of the friability of diamond grains. As used herein, a friability tester assembly comprises a cylindrical capsule which can be closed at its ends and a ball consisting of a hard, wear resistant material, which can be accommodated by the capsule and move freely within it. The test process includes introducing a plurality of diamond grains having a certain known mass and size distribution into the capsule with the ball, closing the capsule and causing it to be shaken back and forth in the direction of its cylindrical axis, causing the ball and the diamond grains to be violently agitated and the consequent fracturing the diamond grains. The shaking action is maintained for a certain number of cycles, at a certain amplitude and a certain frequency, after which all the diamond grains are recovered and the size distribution is measured (using the same method as used to measure the initial size distribution of the grains). Based on the differences between the initial and final size distributions, a friability index can be computed as the mass percentage of the unbroken grains in relation to the initial mass. The friability index had been used as an indicator of the likely performance of diamond grains of various particular grades in various industrial applications, such as sawing. Friability testing of diamond grains has been described and discussed in various publications, including by N.G. Belling and H.G. Dyer, "Impact strength determination of diamond abrasive grit" (Industrial Diamond Information Bureau, London, 1964); N.G. Belling and L. Bialy, "The Friability tester™ - 10 years later" (Industrial Diamond Review, August 1974, pages 285 to 291 ); United States patent number 5,140,857; and by Zhou et al. ("Friability and crushing strength of micrometre-size diamond abrasives used in micro-grinding of optical glass", Metallurgical and Materials Transactions A, volume 27A, April 1996, pages 1 ,047 to 1 ,053).

The aspect ratio of a body is the ratio of the maximum length dimension of the body defining a first axis to the minimum length dimension of the body along a second axis perpendicular to the first axis.

In general, various parameters may be used for characterising the roughness of surfaces, based on amplitude parameters derived from measured deviations of the surface normal to the mean plane defined by it (for example, the arithmetic mean roughness R_a).

Claims

A plurality of synthetic diamond grains having a mean size of 5 to 100 microns, the crystal habits of the diamond grains being substantially cubo-octahedral, such that at least 50 per cent of the grains have a morphology index of 3 to 6, at least 5 per cent of the grains have a morphology index of 2 to 4, and at least 20 per cent of the grains have a morphology index of 5 to 7.

A plurality of diamond grains as claimed in claim 1 , in which at least 10 per cent of the diamond grains are attached to another diamond grain.

A plurality of diamond grains as claimed in claim 1 or claim 2, in which the mean aspect ratio of the diamond grains is 0.70 to 0.80.

A plurality of diamond grains as claimed in any one of the preceding claims, in which at least 50 per cent of the diamond grains contain an included seed diamond grain.

A plurality of diamond grains as claimed in any one of the preceding claims, in which at least 50 per cent of the diamond grains are substantially free from surfaces exposed by cleavage or crushing.

A plurality of diamond grains as claimed in any one of the preceding claims, having strength in terms of friability index of 50 to 70 per cent.

A plurality of diamond grains as claimed in any one of the preceding claims, in which the number of diamond grains in the plurality is sufficient for the mass of the plurality to be at least 1 gram (g).

8. A tool comprising a plurality of diamond grains as claimed in any of the preceding claims.

9. A tool as claimed in claim 8, comprising a saw blade or a saw wire.

10. A tool as claimed in claim 8 or claim 9, comprising the plurality of diamond grains attached to a tool body by means of electroplating.

1 1 . A tool as claimed in any one of claims 8 to claim 10, comprising polycrystalline diamond material comprising the plurality of diamond grains sintered together by direct inter-bonding.

12. A tool as claimed in claim 1 1 , comprising a die for wire-drawings.

13. A method of using a tool as claimed in any one of claims 8 to 12, the method including using the tool to remove material from a body comprising silicon or sapphire.