GB2543032A

GB2543032A - Faceted diamond grains

Info

Publication number: GB2543032A
Application number: GB1517193.7A
Authority: GB
Inventors: Cyprian Nzama Mhlonishwa; Gregory Munday Mark
Original assignee: Element Six UK Ltd
Current assignee: Element Six UK Ltd
Priority date: 2015-09-29
Filing date: 2015-09-29
Publication date: 2017-04-12
Anticipated expiration: 2035-09-29
Also published as: GB201517193D0; GB2543032B; CN106552555B; CN106552555A

Abstract

A method of producing faceted diamond grains having an average particle size from 1 µm to 20 µm, at least 75% of the source diamond grains having an average particle size and being substantially free of facets is disclosed. The source diamond grains are mixed with metal particles to form a reaction mass. The metal particles have an average particle size no greater than the average diamond particle size. The reaction mass is subjected to a temperature of greater than 1100°C and a pressure of greater than 4.5 GPa to form faceted diamond grains, such that fewer than 50% of the faceted diamond grains form a part of an agglomerate comprising at least two inter-bondedfaceted diamond grains. The faceted diamond grains are recovered from the reaction mass. Carbon may be added to the reaction mass and a tool including euhedral diamond grains formed with the method may be such that the diamond grains are orientated in the tool at an orientation relating to the orientation of the longest dimension of the euhedral grains.

Description

FACETED DIAMOND GRAINS

FIELD

The invention relates to the field of faceted diamond grains and in particular to methods of forming powders of diamond grains.

BACKGROUND

Polycrystalline super hard materials, such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) may be used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. 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. A particular challenge in synthesising very small diamond grains is that the desired grain size is achieved rapidly, in a matter of seconds or a few minutes, and the timing of the load and heat cycle is critical but difficult to control. This may be why most commercially available diamond grains less than about 80 microns in size are manufactured by crushing coarser diamond grains rather than by direct synthesis. Crushed diamond grains tend to be irregular in shape and substantially free of facets, and are difficult and costly to sort. This presents problems for using fine diamond grit in polishing surfaces, such as opto-magnetic storage media, to sufficient tolerance. It would be better to provide substantially euhedral, well-faceted and “rounder” diamond grains for such applications. In principle, direct synthesis of such grains could provide such grains if the challenges mentioned above could be overcome.

It is possible to re-facet crushed facet-free diamond grains. US 6,835,365 describes such a technique in which a source of substantially facet-free source diamond grains are mixed with a solvent/catalyst and subject to a temperature in the range of 1100°C to 1500°C and a pressure in the range of 4.5 to 7.0 GPa. The resultant diamond grains comprise facets and are substantially euhedral with clearly distinguishable facets. A problem with re-faceting diamond grains with a small grain size (for example, up to 20 pm) is that the diamond grains tend to agglomerate, as shown in Figure 1, where at least four diamond grains are inter-bonded. As described in US 6,835,365, such agglomerates can be broken up by mechanical techniques, but this can damage the surface of the grains.

There is a need for a method of preparing high quality faceted diamond grains having mean size of the order of magnitude of less than about 20 pm.

SUMMARY

It is an object to provide a method of synthesizing high quality diamonds having a size of up to around 20 pm.

According to a first aspect there is provided a method of producing a plurality of diamond grains having an average particle size from 1 pm to 20 pm, at least 75% of the source diamond grains having an average particle size and being substantially free of facets. The source diamond grains are mixed with a plurality of particles of metal particles to form a reaction mass, wherein the metal particles have an average particle size no greater than the average diamond particle size. The reaction mass is subjected to a temperature of greater than 1100°C and a pressure of greater than 4.5 GPa to form faceted diamond grains, such that fewer than 50% of the faceted diamond grains form a part of an agglomerate comprising at least two inter-bonded faceted diamond grains. The faceted diamond grains are then from the reaction mass. An advantage of using metal particles with an average particle size no greater than the average diamond particle size is that is reduces agglomeration.

The temperature and pressure are optionally selected to be suitable for re-faceting the source diamond grains. Alternatively, a source of carbon is mixed with the reaction mass, wherein temperature and pressure are selected to be suitable for growing faceted diamond grains.

As an option the elevated temperature is in the range 1100 to 1500°C.

As an option the elevated pressure is in the range 4.5 to 7 GPa.

Optional examples of the metal particles are any of iron, nickel, cobalt, manganese, and alloys thereof.

The method optionally further comprises adding a further source of carbon to the reaction mass in a quantity no higher than a quantity suitable to form a saturated solution in the metal particles during the conditions of elevated temperature an elevated pressure. As a further option, the further source of carbon comprises graphite.

The method optionally further comprises mixing the source diamond grains with the metal particles in a dispersant, the dispersant comprising water and a water-soluble polymer. The resultant dispersion is dried at a temperature below 100°C to remove water. The water soluble polymer is decomposed at a temperature below 400°C.

As an option, the volume percent of source diamond in the reaction mass is no more than 30%.

As an option at least 80% of the source diamond grains have an aspect ratio of 1.5:1, such that the longest dimension of a source diamond grain is at least 1.5 times greater than the next longest dimension of the source diamond grain.

As an option, at least 80% of the source diamond grains have an aspect ratio of 2:1, such that the longest dimension of a source diamond grain is at least 2 times greater than the next longest dimension of the source diamond grain.

The method optionally further comprises selecting the source diamond grains by any of table sorting and sieving using a slit with slotted apertures.

As an option the source diamond grains are metal-clad. An advantage of this is that cladding helps to further reduce agglomeration.

According to a second aspect, there is provided a plurality of re-faceted diamond grains having an average particle size from 1 pm to 20 pm, wherein fewer than 50% of the diamond grains form a part of an agglomerate comprising at least two inter-bonded diamond grains.

As an option the plurality of euhedral diamond grains have an average particle size selected from any one of the ranges 1 pm to 5 pm, 5 pm to 10 pm and 10 pm to 20 pm.

As an option at least 80% of the diamond grains are substantially euhedral.

As an option, at least 80% of the diamond grains have an aspect ratio of 1.5:1 such that the longest dimension of a diamond grain is at least 1.5 times greater than the next longest dimension of the diamond grain. As a further option, at least 80% of the diamond grains have an aspect ratio of 2:1 such that the longest dimension of a diamond grain is at least two times greater than the next longest dimension of the diamond grain.

According to a third aspect there is provided a mixture comprising a plurality of euhedral diamond grains as described above in the second aspect, and any of a bond material for a tool and a precursor bond material for a tool.

As an option the euhedral diamond grains are dispersed in any of a solid matrix; vitreous material; resinous material; polymer material; within an electroplated layer; dry powder, slurry or paste comprising the bond material or precursor bond material.

According to a fourth aspect there is provided a tool comprising a plurality of euhedral diamond grains as described above in the second aspect.

As an option the diamond grains are oriented in the tool at an orientation relating to the orientation of the longest dimension of the euhedral diamond grains.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 is a micrograph showing agglomerated diamond grains;

Figure 2 is a micrograph showing source diamond grains mixed with metal powders; Figure 3 is a micrograph showing diamond grains sintered in a metal solvent;

Figure 4 is a flow diagram showing exemplary steps;

Figure 5 shows a morphology index for synthetic diamond crystals;

Figure 6 is a micrograph showing source diamond grains mixed with metal powders according to an exemplary embodiment;

Figure 7 is a micrograph showing diamond grains sintered in a metal solvent according to an exemplary embodiment;

Figure 8 is a micrograph showing re-faceted diamond grains prepared according to an exemplary embodiment;

Figure 9 is a micrograph showing source diamond grains having a high aspect ratio; Figures 10a and 10b are micrographs showing exemplary re-faceted diamond grains prepared using source diamond grains having a high aspect ratio; and Figure 11 illustrates schematically a cross-section side elevation view (not to scale) of a grinding tool having a surface comprising faceted diamond grains having a high aspect ratio.

DETAILED DESCRIPTION

As described above, preparing diamond grains with a size of less than around 20 pm using existing techniques leads to agglomerated diamond grains which can be damaged when they are lightly milled to break up the agglomerates.

Figure 2 is a micrograph showing source diamond grains mixed with metal solvent particles before sintering using a known re-faceting process, such as that described in US 6,835,365. The source diamond grains are the dark particles and the metal powders are the lighter particles. The average particle size of the metal powder is much greater than the average particle size of the source diamond grains. This leads to the source diamond grains aggregating together in the interstices between the large metal particles.

Figure 3 is a micrograph showing the re-faceted diamond grains after sintering. The diamond grains have formed agglomerates (as shown in the white circle) because many of the source diamond grains were aggregated together in the interstices between the metal particles. Once the metal solvent is removed, the resultant refaceted diamond grains are agglomerated, as shown in Figure 1. The inventors have realised that reducing the aggregation of source diamond grains in the interstices between metal particles would lead to a reduction in the agglomeration of the resultant re-faceted diamond grains. By making the average particle size of the starting metal powders no greater than the average grain size of the source diamond, the likelihood of source diamond grains aggregating in the interstices between metal powders is reduced.

As used herein, grain sizes expressed in length units such as micrometres (microns) refer to the equivalent circle diameters (ECD), in which each grain is regarded as though it were a sphere. The ECD distribution of a plurality of grains can be measured by means of laser diffraction, in which the grains are disposed randomly in the path of incident light and the diffraction pattern arising from the diffraction of the light by the grains is measured. 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. Particular examples of such terms include mean, median and mode. The size distribution can be thought of as a set of values Di corresponding to a series of respective size channels, in which each Di is the geometric mean ECD value corresponding to respective channel i, being an integer in the range from 1 to the number n of channels used.

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 result can be converted to surface area distribution, the mean of which being D[3,2] according to a well-known mathematical formula. Unless otherwise stated, mean values of size distributions as used in the present disclosure refer to the volume-based mean D[4,3]. The median value D50 of a size distribution is the value dividing the plurality of grains into two equal populations, one consisting of grains having ECD size above the value and the other half having ECD size at most the value. The mode of a size distribution is the value corresponding to the highest frequency of grains, which can be visualised as the peak of the distribution (distributions can include more than one local maximum frequency and be said to be multi-modal).

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 ECD size below which 90 per cent of the grains reside, d(0.5) refers to the ECD size below which 50 per cent of the grains reside and d(0.1) refers to the ECD size below which 10 per cent of the grains reside.

Figure 4 is a flow diagram showing exemplary processing steps for preparation of euhedral diamond grains. The following numbering corresponds to that of Figure 4: 51. Source diamond grains are selected. 52. Metal particles for forming a solvent are provided. The metal particles have an average particle size no greater than the average particle size of the source diamond grains. Typical metals used for the solvent include iron, nickel, cobalt, manganese, and alloys of these metals. It will be appreciated that other metals or alloys thereof may be used. The solvent may also act as a catalyst material, which is capable of promoting the precipitation of carbon in the form of diamond at conditions at which diamond is thermodynamically more stable than graphite. 53. The metal particles and the source diamond grains are mixed together to form a reaction mass. A typical mixing regime is to disperse the metal particles and the source diamond grains in an aqueous polymeric dispersant. A suitable dispersant is a water-based polymer such that it can be dried at less than 100°C after mixing and the polymer decomposes in an air or gas atmosphere at less than around 400°C. Once the reaction mass has been thoroughly mixed in the dispersant, the dispersant is removed by drying and burning off any residual polymer. 54. The reaction mass is sintered at high pressure and high temperature. A typical temperature range is 1100°C to 1500°C, and a typical pressure range is 4.5 to 7 GPa. 55. Euhedral diamond grains are recovered from the sintered reaction mass.

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 (M.W. Bailey and L.K. Hedges, 1995, “Crystal morphology identification of diamond and ABN”, Industrial Diamond Review, vol. 1/95, pages 11 to 14) 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 (111) and (100) surfaces. Eight-sided diamond crystals having only (111) 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 (111) 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. This index is shown in Figure 5.

Euhedral diamond grains have substantially well-defined facets, and may comprise a relatively small number of relatively large facets, and may have relatively low specific surface area. Source diamond grains prepared by crushing larger grains tend to have irregular, non-faceted shapes and are described as anhedral.

The technique of using metal powders with an average particle size no greater than the average particle size of the source diamond grains has been found to reduce agglomeration for re-faceted diamond (in which the anhedral source diamond grains grow new facets) and for synthesized diamond grains (in which an additional source of carbon is added to the reaction mix and the source diamond grains grow to form faceted diamond grains.

Table 1 shows exemplary source diamond grain sizes and metal powder particle sizes for producing re-faceted diamond grains. All of the resultant euhedral refaceted diamond grains were found to be substantially unagglomerated. TABLE 1

Figure 6 is a micrograph showing the mixture of source diamond grains (examples shown in white circles) and metal powders where the source diamond grains were Grade 2 PCDMA (polycrystalline diamond micron abrasive) with a mean size of 4 pm and the starting metal powder was a 70/30 mixture of iron and nickel with a mean particle size in the range of 0.5 to 4pm. Owing to the small size of the metal particles, the source diamond grains do not cluster together in the interstices between metal powders, as shown in Figure 2.

Figure 7 is a micrograph showing the reaction mass of metal solvent and diamond particles after sintering. Again, when compared to Figure 3, it can be seen that the diamond grains (examples highlighted in circles) have not formed agglomerates.

Figure 8 is a micrograph showing the recovered euhedral diamond grains. Compared to Figure 1, there is substantially no agglomeration of the euhedral diamond grains evident. On average, the diamond grain size ranges from 5 to 10pm.

As mentioned above, the technique works for both synthesized diamond grains and re-faceted diamond grains. However, it has been observed that some diamond loss occurs when preparing re-faceted diamond grains. This is thought to be because some carbon dissolves into the metal solvent during sintering and is therefore not available for the re-faceted diamond grains. In a further embodiment, a further source of carbon is added to the reaction mass before sintering to compensate for the solubility of carbon in the metal solvent. Table 2 shows the relative mass of graphite, iron, nickel and source diamond material for five different samples. TABLE 2

The properties of the graphite powder added to the reaction mass are shown in Table 3: TABLE 3

In the examples of Table 2, the amount of graphite added was still below the saturation level of carbon in the metal solvent. The mass of carbon shown in Table 2 corresponds to 3% (m/m) carbon, and this may be increased further to 4.5 % (m/m) carbon. In all cases, the addition of graphite prior to re-faceting was found to improve the yield of euhedral diamond grains and the additional graphite was exhausted in the process of re-faceting the source diamond grains.

Further investigations were carried out to determine the effect of the ratio of source diamond grains to metal powder on the yield of re-faceted diamond grains. Grade 4 PCDMA (with an average particle size of 4 pm) and Grade 12 PCDMA (with an average particle size of 12 pm) were used as a starting material with a 70/30 iron/nickel mixture. The yield of re-faceted diamond grains is shown in Table 4. It can be seen from Table 4 that there is no significant diamond loss during the refaceting process under HPHT conditions. TABLE 4

The techniques described above to prepare substantially un-agglomerated micron sized euhedral diamond grains can also be used where control of the euhedral diamond grain shape is required. In order to achieve this, source diamond grains are carefully selected with the required elongation characteristics (for example, an aspect ratio of 2:1). One way to select the required source diamond grains to table source crushed diamond source grains. For finer source powders, sieving using a slotted sieve aperture may be used. This selects diamond source powders in which a substantial number of the source diamond grains have the required aspect ratio. The source diamond grains are then subject to a synthesis or re-faceting sintering process as described above.

Figures 10a and 10b are micrographs showing euhedral diamond grains prepared using a re-faceting process. Figure 10a in particular shows a euhedral diamond grain having an aspect ratio of around 2:1. Similarly shaped euhedral diamond grains have been synthesized.

Elongated euhedral diamond grains can be used in tools such as grinding wheels. Figure 11 illustrates a grinding wheel 1 with elongated abrasive diamond grains 2 embedded into the binder matrix 3 in a preferred orientation such that substantially the same faces of each of the diamond grains 2 are exposed to a work piece 4. An advantage of using elongated diamond grains 2 in the grinding wheel 1 is that it may reduce the diamond consumption during a grinding operation.

While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims. For example, while the above concepts are described when applied to re-faceting diamond grains, it will be appreciated that similar concepts equally apply to synthesis of diamond powders from seeds.

Claims

Claims:

1. A method of producing a plurality of diamond grains having an average particle size from 1 pm to 20 pm, the method comprising: providing a plurality of source diamond grains having an average particle size, at least 75% of the source diamond grains being substantially free of facets; mixing the source diamond grains with a plurality of particles of metal particles to form a reaction mass, wherein the metal particles have an average particle size no greater than the average diamond particle size; subjecting the reaction mass to a temperature of greater than 1100°C and a pressure of greater than 4.5 GPa to form faceted diamond grains, such that fewer than 50% of the faceted diamond grains form a part of an agglomerate comprising at least two inter-bonded faceted diamond grains; and recovering the faceted diamond grains from the reaction mass.

2. The method according to claim 1 wherein the temperature and pressure are selected to be suitable for re-faceting the source diamond grains.

3. The method according to claim 1, further comprising mixing a source of carbon with the reaction mass, wherein temperature and pressure are selected to be suitable for growing faceted diamond grains.

4. The method according to claim 1, 2 or 3, wherein the elevated temperature is in the range 1100 to 1500°C.

5. The method according to any one of claims 1 to 4, wherein the elevated pressure is in the range 4.5 to 7 GPa.

6. The method according to any one of claims 1 to 5, wherein the metal particles comprise any of iron, nickel, cobalt, manganese, and alloys thereof.

7. The method according to any one of claims 1 to 6, further comprising adding a further source of carbon to the reaction mass in a quantity no higher than a quantity suitable to form a saturated solution in the metal particles during the conditions of elevated temperature an elevated pressure.

8. The method according to claim 7, wherein the further source of carbon comprises graphite.

9. The method according to any one of claims 1 to 8, further comprising: mixing the source diamond grains with the metal particles in a dispersant, the dispersant comprising water and a water-soluble polymer; drying the resultant dispersion at a temperature below 100°C to remove water; and decomposing the water soluble polymer at a temperature below 400°C.

10. The method according to any one of claims 1 to 9, wherein the volume percent of source diamond in the reaction mass is no more than 30%.

11. The method according to any one of claims 1 to 10, wherein at least 80% of the source diamond grains have an aspect ratio of 1.5:1, such that the longest dimension of a source diamond grain is at least 1.5 times greater than the next longest dimension of the source diamond grain.

12. The method according to any one of claims 1 to 10, wherein at least 80% of the source diamond grains have an aspect ratio of 2:1, such that the longest dimension of a source diamond grain is at least 2 times greater than the next longest dimension of the source diamond grain.

13. The method according to any one of claims 11 or 12, further comprising selecting the source diamond grains by any of table sorting and sieving using a slit with slotted apertures.

14. The method according to any one of claims 1 to 13, wherein the source diamond grains are metal-clad.

15. A plurality of re-faceted diamond grains having an average particle size from 1 pm to 20 pm, wherein fewer than 50% of the diamond grains form a part of an agglomerate comprising at least two inter-bonded diamond grains.

16. The plurality of euhedral diamond grains according to claim 15, having an average particle size selected from any one of the ranges 1 pm to 5 pm, 5 pm to 10 pm and 10 pm to 20 pm.

17. The plurality of euhedral diamond grains according to claim 15 or claim 16, wherein at least 80% of the diamond grains are substantially euhedral.

18. The plurality of euhedral diamond grains according to any one of claims 15 to 17, wherein at least 80% of the diamond grains have an aspect ratio of 1.5:1 such that the longest dimension of a diamond grain is at least 1.5 times greater than the next longest dimension of the diamond grain.

19. The plurality of euhedral diamond grains according to claim 18, wherein at least 80% of the diamond grains have an aspect ratio of 2:1 such that the longest dimension of a diamond grain is at least two times greater than the next longest dimension of the diamond grain.

20. A mixture, comprising a plurality of euhedral diamond grains as claimed in any one of claims 15 to 19, and any of a bond material for a tool and a precursor bond material for a tool.

21. The mixture as claimed in claim 20, wherein the euhedral diamond grains are dispersed in any of a solid matrix; vitreous material; resinous material; polymer material; within an electroplated layer; dry powder, slurry or paste comprising the bond material or precursor bond material.

22. A tool comprising a plurality of euhedral diamond grains according to any one of claims 15 to 19.

23. A tool comprising a plurality of euhedral diamond grains according to claim 22, wherein the diamond grains are oriented in the tool at an orientation relating to the orientation of the longest dimension of the euhedral diamond grains.