WO1990009361A1

WO1990009361A1 - Diamond composites

Info

Publication number: WO1990009361A1
Application number: PCT/AU1990/000052
Authority: WO
Inventors: Colleen Joyce Bettles; Michael Vincent Swain
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 1989-02-13
Filing date: 1990-02-12
Publication date: 1990-08-23
Also published as: GB9117308D0; GB2246773A; DE4090245T

Abstract

Diamond composites comprising diamond particles bound in a ceramic matrix are disclosed. The diamond particles preferably have a particle size of less than 100 micron. The diamond composites contain less than 70 % by volume of the diamond particles. The composites can be formed by densifying mixtures of the diamond particles and a ceramic powder at elevated temperatures and pressures of less than 200 MPa. The ceramic may be an oxide or non-oxide ceramic other than silicon carbide. Included within the scope of the term oxide ceramic are all materials containing oxygen which behave as ceramics at elevated temperatures especially those oxides of elements having an atomic number greater than 11. The term non-oxide ceramic includes all materials containing carbon, nitrogen, boron or a combination thereof that behave as a ceramic at elevated temperatures. Ceramics exemplified include aluminium nitride, chromium carbide, alumina, yttria/zirconia, cordierite (2MgO.2Al2O3.5SiO2) and mullite (3Al2O3.2SiO2). The composites can be used in making abrading wheels and gemstone polishing scaifes.

Description

DIAMOND COMPOSITES The present invention relates to composites of diamonds and ceramics.

Industrial diamonds have been used for many years in applications which employ their unique physical properties. Perhaps the best known physical property of diamonds is that of hardness. As a result diamonds have found increasing use in cutting, shaping and polishing hard substances. Furthermore the thrust for greater and greater productivity has enabled diamonds to find increasing use in high speed machining of hard materials. Diamond drill bits have also permitted advances in geology by enabling core samples of rocks to be cut and brought to the surface for examination. Wire drawing dies have also been made from diamonds especially in applications requiring wire of accurate and consistent diameter made from hard materials. Electric light filaments are made in this way.

Ceramics have been used for thousands of years in a variety of applications. They have been used in the construction industry, and in the production of ornaments, enamels and refractories. The refractory properties of ceramics have generated a new wave of interest in these materials particularly for use in internal combustion engines of high efficiency.

Literature Review There is very little literature pertaining to the oxide ceramic systems and it can be divided broadly into three areas:

(1) single oxide - diamond composites

(2) mixed oxide composites, more specifically a "pottery" body

(3) glass composites.

A brief description of the relevant articles follows. Where possible, these articles have been located on a diamond/graphite stability diagram (Figure 8) using the reference numerals indicated below.

(1) Single Oxide - Diamond Composites

Work in this area was carried out by No a and Sawaoka [1,2]. The area of concern was the alumina - diamond system, with less than 15 vol% diamond. The composites were produced under conditions ensuring diamond stability - a pressure of 6 GPa and a temperature of 1300 degrees C. Subsequently, the composites were annealed to generate controlled graphitisation. De Beers Industrial Diamond Division, U.S. Patent 1,456,765 have made compacts (being greater than 70% diamond) using magnesia, alumina or spinel. • The system utilised a "solvent" in small proportions. The pressure was 4.0 - 6.5 GPa and the temperature was 1200 - 1300 degrees C.

USP 1, 307,713 [3] discloses an article containing in excess of 50% diamond combined with alumina, beryllia or magnesia. Pressures required were in excess of 1 GPa and the temperature was greater than 900 degrees K.

(2) Mixed Oxide - Diamond Composites

Three of the early patents covering ceramic-bonded grinding wheels utilised a mixed-oxide ceramic matrix., These were GB 1 404 956 [5], US 2 132 005 [6] and US 2 3099 453 [7]. The wheels were to be porous, and were simply wet pressed and fired in a conventional kiln. The "ceramic" used was similar to a pottery mix and contained high levels of flux to lower the firing temperature. The compositions of [5] were₎ all required to contain between 1% and 7% barium oxide. [6] is similar but also contained additives such as aluminium and silicon which were reportedly to prevent bloating of the wheels. [7] contained 75% Ball clay and 25% fluxing material.

(3) Glass - Diamond Composites

These contain a mixture of low melting point oxides that combine together to form glasses. Noma and _ι Sawaoka [8] attempted to fabricate a diamond - glass composite (using lead oxide and boric oxide) under microgravity i.e. the processing was carried out in space. Norton Grinding Wheel Co. obtained three U.S. patents involving glass - diamond composites [ 9 ■ - 11]. [9] involved a glass made from boric oxide, zinc oxide, and lead oxide. The temperature was less than 750 degrees C. [10] used a borosilicate glass and Til] a glass from lead silicate and lead oxide. The temperature in the latter was between 500 and 700 degrees C.

Three Russian references [12 - 14] have been located. Two are German patents and the other is British. DE 2 324 111 [12], described a tool and uses an aluminosilicate glass with alkalies as the matrix. DE 2 201 313 [13] describes a binder system based on a borosilicate glass and a lithium-containing mineral. GB 1 418 730 [4] is described as a rectification tool of diamond and glass. It uses a glass which contains small quantities of alumina or zirconia.

USP 2 334 266 to the Carborundum Company [15] discloses glass composites. The glass was made of silica, boric oxide, sodium oxide and a small amount of alumina.

USP 2 566 828 [16] discloses a system involving lead oxide, boric oxide and unvitrified silica (as kaolin, feldspar or quartz). The processing temperature was less than the graphitisation temperature for diamond. European Patent Application No. 0 118 225 [17] discloses a compact involving more than 70% diamond and a glassy phase the melting temperature of which was between 800 and 1400 degrees C.

There has been considerable work in the area of non-oxide ceramic/diamond composites over the years. Most of this has concentrated on the cemented carbide systems, primarily WC-Co. In most cases these materials are fabricated using high temperatures and very high pressures such that during processing the diamond remains in the stable region of the diamond/graphite phase diagram (Fig. 10) . - 5 -

Previous work oh composites of diamonds and non-oxide ceramics can be divided into four sections, namely cemented carbides, general literature, general patents and silicon-infiltrated systems.

Cemented Carbides

The composites found in this area are technically compacts, being usually in excess of 70vol% diamond. They require specialised processing which usually involves very high pressures to keep the diamond in the stable region of the diamond/graphite equilibrium diagram. Typical of these compacts are those disclosed in U.S. Patent Nos. 4,525,178; 3,767,371; 3,944,398 a'hd 4,171,973, all involving abrasive materials and their - manufacture. All quote pressures in GPa or state that they are maintaining diamond stability. Two exceptions disclosed in U.S. Patent Nos. 4,097,274 and 4,164,527 ^" involve unusual processing techniques and rely on high temperature for very short periods of time.

General Literature Very little research work has been published in this area. Noma and Sawaoka "Fracture toughness of high pressure sintered diamond/silicon nitride composites" J. Am. Cer. Soc . _6_8 (10) C271-C273 have produced a silicon nitride composite and examined the effect of diamond addition on the fracture toughness of silicon nitride. Aςain, GPa pressures were involved.

General Patents

Several patents exist involving non-oxide ceramic/diamond composites. Mitsubishi Metal Corporation have eight in this area JKTK 80 62 846/848-852/854 covering heat and abrasion resistant sintered materials. The full range of non-oxide ceramics seems to be covered, with the processing always occurring at 6GPa pressure. De Beers Industrial Diamond Division have British Patent Number 1,456,765 involving primarily silicon nitride, and Megadiamond Corporation in British patent No. 1,307,713 disclose a range of carbides, nitrides and borides. Pressures are in excess of lGPa. Similarly, in French Patent Application No. 2,434,130 Sumitomo Electric Industries disclose a range of materials all processed in excess of 5GPa. Several people have investigated composites based on boron nitride [U.S. Patent No. 3,852,078; JKTK 73 80 617- and 60 63 461]. The processing varies slightly from patent to patent, however all require high pressure to maintain diamond stability.

Silicon Infiltration

A great deal of work has been done in the area of silicon carbide composites. Most of these involve the infiltration of the composite with either silicon or a silicon-rich material to form SiC in situ. This patent application specifically excludes any work on silicon carbide systems and, as such, no references will be quoted.

Furthermore in the presence of a source of oxygen, diamond is readily oxidized to carbon dioxide at moderate temperatures i.e. 600°C. Consequently, in circumstances where the application of high pressure has not automatically excluded air, composites have been formed in an inert atmosphere or a vacuum.

It has now been discovered that composites of diamonds and oxide ceramics can be produced at temperatures and pressures at which diamonds would normally be unstable. It- has also been discovered that the composites produced exhibit synergism with respect to one or more properties. Whilst preparing the samples for testing it was discovered that each was unexpectedly difficult to cut and polish. Unexpected changes in non-brittle failure were also observed with some of the composites. Accordingly, the present invention provides. a diamond composite comprising diamond particles bound together in a matrix of an oxide or non-oxide ceramic other than silicon carbide wherein the diamond particles comprise less than 70 volume per cent of the composite. The term oxide ceramic includes all materials containing oxygen which behave as ceramics at elevated temperatures. The definition also includes any mixtures of these oxides. Preferably the oxide ceramics of this invention are oxides of elements with an atomic number greater than 11.

Oxide ceramics of particular interest include alumina, yttria/zirconia, cordierite (2Mg0.2Al_0-, .5Si0₉) and Mullite (3A1₂0₃.2Si0₂ ) .

Preferably the diamond particles are less than 100 microns in size.

The term non-oxide ceramic includes all materials containing carbon, nitrogen, boron or a combination of these elements that behave as a ceramic at elevated temperatures. Examples of non-oxide ceramics include silicon nitride, aluminium nitride, chromium carbide, titanium diboride, boron carbide and boron nitride. Silicon carbide is specifically excluded as a matrix material f -om this invention. The non-oχiάe ceramics showing greatest promise for use in forming the diamond composites of the present invention are aluminium nitride and chromium carbide.

The composites of the present invention may be formed by techniques such as hot pressing, hot isostatic pressing or preεsureless sintering. Hot pressing involves pressing a mixture of the constituents in a graphite die at temperatures below 1750 C and pressures not exceeding 100 MPa. Hot isostatic pressing utilises pressures of approximately 180 MPa.

The present invention also provides a method of 5 forming the diamond composites. The method of forming the compositions involves taking an intimate mixture of diamond particles and a powder of an oxide or non-oxide ceramic, compacting the mixture and densifying/sintering it in a reducing environment at temperatures below 1750 C 0 and pressures not exceeding 200 MPa. Dies made of graphite provide a suitable reducing environment.

The composites of the present invention have a -variety of potential applications determined largely by the properties that they exhibit which in turn are 5 dependent on their composition. Composites containing between 20% and 40% diamond particles by volume appear to exhibit optimum properties.

Possible uses include machinery components exposed to hard wear and tear, for example, seals, 0 abrading wheels, or gemstone polishing scaifes. On the other hand, the high thermal conductivity of aluminium nitride/diamond composites should enable this material to find applications where high thermal conductivity and hardness are essential. The technique of hot pressing will now be

- explained in more detail by reference to Figures 1 to 7 of the accompanying drawings wherein:

Figure 1 is a schematic elevation of the hot pressing apparatus employed; Figure 2 is an elevation in cross section of the graphite die depicted in Figure 1;

Figure 3 is a schematic elevation of apparatus used in grinding trials;

Figures 4 to 7 illustrate some of the physical properties of composites according to the invention;

Figure 8 is a diamond/graphite phase diagram; Figure 9 is a graph illustrating a property of a composite according to the invention; and

Figure 10 is a diamond/graphite phase diagram.

Figure 1 illustrates hot pressing apparatus 1 which comprises an hydraulic ram 2, a graphite die 5, a copper induction heating coil 3 and insulation 4 between the die and the coils of the induction heater. The die is mounted on a base plate 6. The graphite die 5 depicted in Figure 2 has a central bore 9 into which graphite plunger 7 fits snugly. The bottom of the bore is blocked with a graphite base plug 11. Composite mixtures 10 are placed in the bore above the base plug , and separated one from the other by graphite spacers 12.

The induction coil used in the hot pressing apparatus described above was a 30 Kw R.F. induction heater. The tip of the ram is separated from the graphite plunger by an insulating spacer 8 generally made from zirconia or silicon carbide.

The dies and other graphite components are made of high strength graphite. The maximum pressure which can be applied to the compositions is therefore determined by the co pressive strength of the graphite. The dies employed in preparing composites of the present invention were 150 mm in height, 100 mm in diameter and had bores having a diameter of 12.5 or 15.9 mm.

Composites of the present invention may be prepared using the following method. Diamond particles of appropriate size are selected by using standard sizing techniques. The diamond particles are then mixed with an ceramic powder by a simple shaking technique in the presence of spherical acrylic mixing media. A weighed quantity of powder is poured into a steel die having an., internal bore of either 12.5 mm or 15.9 mm diameter and uniaxially pressed at a pressure of 76 MPa. A pellet having a thickness of about 3 mm is obtained. The pellet is placed in the hot pressing apparatus described above and hot pressed.

The technique of hot pressing involves heating a material to a temperature at which densification can occur and applying an external pressure to increase the rate at which densification occurs. The cycle used in hot pressing is generally such that the die assemblage is heated at a constant rate to the densification temperature, held there for a specified time, then allowed to cool without interruption i.e. the induction heater is turned off and the die is left to cool.

The heating rate employed enables the die assemblage to reach the required temperature in 20/30 minutes. The holding time is generally less than 30 minutes. Usually the pressure is applied once the densification temperature has been reached. However, in some cases this is not possible and the pressure is applied earlier. The pressure applied was 65 MPa. The assemblage is generally cool enough to remove from the heater within 3 hours by which time the pressure drops to about 10 MPa.

Each composite sample was subjected to X-ray diffraction in order to determine the extent to which the diamond particles had transformed to graphite if at all. The physical properties of the composites were measured by determining the Modulus of Rupture, the Vickers Hardness and Fracture Toughness. In some cases grinding evaluations were also performed using the apparatus illustrated in Figures 3a-c.

The methods used in determining the physical properties are set out below 1. Modulus of Rupture (MOR)

A central load was applied to a disc of the composite, the edges of which were simply supported by an annular ring. Equations for determining the Modulus of Rupture may be found in Roark R.J., Young W.C. "Formulae for Stress and Strain" 5th edition page 366 Case No. 16.

2. Vickers Hardness

Standard indentation equipment was used applying loads of 10, 20 and 30 kg.

3. Fracture Toughness

An indentation technique was used which is described in Anstis et al "A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughn©s's: I, Direct Crack Measurements" J.Am.Cer.Soc.' 64(9) 533-538 (1981).

4. Grinding

A 12mm diameter hollow grinding tool 21 was fabricated from the composite and mounted on a spindle 21 using screw 24 as shown in Figures 3a and b. The material being ground was a Mg.PSZ rod 22 (12 mm OD) as shown in Figure 3c.

The rotational speed of the zirconia was 160 rpm, and of the grinding spindle 600 rpm.

The amount of material removed from the Mg.PSZ by the tool, and the weight loss of the tool itself, were measured and a material removal ratio was calculated. weight of zirconia removed MRR *= weight of tool lost

The cut depth was varied from 0.01 mm to 0.12 mm, and always two passes at each depth of cut were taken. The feed rate was maintained at 6 cm/minute.

5. Thermal Diffusivity/Conductivity

The laser-flash technique was used to determine thermal diffusivity. This method is an ASTM technique:- ASTM E.370503 "Thermal Diffusivity by the Flash Method".

Example 1

A diamond powder with a size range of 0.5-3.0 micron was dry mixed with Alumina powder using a polyethylene container and "plastic" milling media. The resulting mix contained 20 vol.% diamond.

The mixture was formed into pellets by pressing uniaxially in a 12.5 mm diameter steel die at a pressure of 76 MPa. These pellets weighed between 1.0 and 1.5 grammes each. They were placed in a graphite die and hot-pressed at 1550 C for 30 minutes at a pressure of 65 MPa.

The resultant composite was a dense material containing Alumina and uniformly dispersed diamond (as determined by X-ray diffraction techniques). The discs were ground to give parallel sides and mechanical properties determined. The modulus of rupture was found to be 50% that of the matrix alone, however, the Vickers hardness using a 30 kg indentation load had increased by 19%. Example 2

Using the method described in Example 1, a ' composite was made containing a 3 mol% Yttria/Zirconia powder. The pellets so formed were hot-pressed at 1450°C for 30 minutes.

The resultant composite was a dense material containing Yttria/Zirconia and dispersed diamond. The modulus of rupture of the composite was found to be 74% that of the matrix material alone. The Vickers Hardness (20 kg load) was increased by 22% and the Fracture Toughness (determined by indentation technique) was • increased by 93%.

Example 3

Using the method described in Example 1, a composite was made using a commercially available cordierite powder. The diamond loading was 30 vol%, and the size fraction was 0.5 - 3.0 micron. The pellets were hot pressed at 1350°C for 35 minutes, a pressure of 35 MPa being applied for the final 10 minutes of the cycle. The resultant composite was diamond in a matrix of better than 90% alpha cordierite, and some silica.

This composite achieved a strength 1.4 times that of the matrix material alone.

Example 4 Using the method described in Example 1 a composite was produced using as the matrix: wt.% Sierralite 24.5 Metatalc 26.5 Metakaolin 49.0 >-' Diamonds in the 0.5-3.0 micron size range were added at a 40 vol% loading. The composite was hot pressed at 1330°C in the same way as Example 3, and contained in excess of 90% alpha cordierite. This composite achieved a strength 2.67 times that of the matrix material alone (which was an extremely difficult material to hot press).

Example 5

Using the same matrix composition as Example 4, a composite was made containing 30 vol% of diamonds, 80% of which were less than 38 micron in size. This material achieved a strength 2.18 times that of the matrix alone.

Example 6

A small grinding tool was fabricated from the matrix material of Example 4 and diamonds with +38-53 micron size range at a 30 vol% loading. The grinding trials were performed against MgPSZ. At a cut depth of 0.0125 mm the MRR was 111. This declined to 50 at 0.0625 mm and increased again to 110 at 0.125 mm.

Example 7

The grinding tool similar to that of Example 6 was fabricated using a commercial cordierite powder. The MRR at a cut depth of 0.012-5 mm was 48.

Example 8 A composite was produced using a commercially available cordierite with a 30% loading of 38-50 micron diamonds. The thermal diffusivity was determined using the laser flash technique. It was found that this addition of diamond increased the diffusivity by a factor of four. - 15 -

Example 9 ^!

A diamond powder with a mean particle size of 40 microns (and a maximum of 60 microns) was dry mixed, with aluminium nitride powder using a polyethylene container and "plastic" milling media. The aluminium nitride contained a sintering aid (calcium oxide at 2 wt.% loading) and the resulting mix was 20 vol.% diamond.

This mixture was formed into pellets by pressing uniaxially in a 12.7 mm diameter steel die at a pressure of 76 MPa. These pellets weighed between 1.0 and 1.5 grammes each. They were placed in a graphite die and hot pressed at 1725 C for 10 minutes at a pressure of 65 MPa.

The resultant composite was a dense material containing aluminium nitride and uniformly dispersed diamond (as determined by X-ray diffraction techniques). A disc of the composite was ground to give parallel sides, and its mechanical properties determined. The - modulus of rupture was found to be 80% that of the matrix material alone and the hardness was comparable to that of Aluminium nitride.

Example 10

Diamonds in the size range 38-53 micron were dry mixed with aluminium nitride powder which had been attrition-milled in isopropanol to reduce the mean particle size to 1.1 micron. The diamond loading was 30 vol%.

The mixture was processed as in Example 1, but at a temperature of 1685 C. The thermal diffusivity of the*,resulting pellets was determined using the laser-flash technique. It was found that:

(a) at room temperature the thermal diffusivity had increased by 84% with the addition of diamonds; -ι (b) at 200 C, the diffusivity had increased by 49%. n.b. The "control" was milled aluminium nitride. Figure 3 shows further results.

Example 11

The material from Example 9 was tested for thermal diffusivity. It was found that:

(a) at room temperature the diffusivity had increased by 79%; (b) at 200°C, the diffusivity had increased by 57%. n.b. The "control" was aluminium nitride with calcium oxide.

Example 12 A chromium carbide powder was mixed with diamond in the same manner as described in Example 1. A sintering aid was again required, being vanadium pentoxide at 1 wt.%.

The pellets were hot pressed at 1700°C for 5 minutes at a pressure of 65 MPa.

The resulting composite as determined by X-ray diffraction was a dense material containing chromium carbide, graphite, and diamond. The modulus of rupture was found to be 50% that of the matrix alone, but the breaking behaviour was not that of a brittle material. Failure was not sudden, but very slow (comparable to yield in a metal). This composite material exhibited good electrical conductivity and could be readily shaped by electric discharge machining or spark machining. Figure 8 is a diamond/graphite phase diagram that illustrates the temperatures and pressures at which composites of diamonds and oxide ceramics according to the present invention are formed by comparison to those of the prior art. The numerals refer to the following publications:

References

[I] Noma, T. and Sawaoka, A. "Toughening in very high pressure sintered diamond - alumina composite". J.

Mat. Sci. _19_ (1984) 2319-2322.

[2] Noma, T. and Sawaoka, A. "Effect of heat treatment on fracture toughness of alumina - diamond composites sintered at high pressures". J. Am. Cer. Soc. _6 (2) C-36 - C-37 (1985).

[3] De Beers Industrial Diamond Division "Bonded abrasive compacts" US 1 456 765.

[4] Megadiamond Corporation "Abrasive articles" US

1 307 713. [5] Leningradskyabrasiwny "Ceramic bonding material for abrasive tools" GB 1 404 956.

[6] Norton Grinding Wheel Company "Article of ceramic bonded abrasive material and method of making the same" US 3 132 005. [7] Norton Grinding Wheel Company "Abrasive article and method of making the same" US 2 309 463.

[8] Noma, T. and Sawaoka, A. "Fabrication of diamond-glass composite under microgravity" Jap. J. App.

Phys. 24_ (10) 1298-1301. [9] Norton Grinding Wheel Company "Abrasive wheels"

US 657 843.

[10] Norton Grinding Wheel Company "Bonded abrasive tools" US 657 563 652.

[II] Norton Grinding Wheel Company "Diamond abrasive article and method of making the same" US 2 334 266.

[12] Leningradskyabraziviny "Ceramic binder for abrasive tools" DE 2 324 222.

[13] As USSR Kola Mine "Ceramic binder for abrasive compositions" DE 2 201 313. [14] Ordena Trudovogo Krasnog "Rectification tool of diamond and glass" GB 1 418 730.

[15] The Carborundum Company "Diamond abrasive article" US 2 334 266.

[16] Raybestos - Manhatten Inc. "Ceramically bonded diamond abrasive products" US 2 566 828.

[17] De Beers Industrial Diamond Division "Diamond abrasive products" EPA 0 118 225.

Figure 10 is a diamond/graphite phase diagram. The reference numbers marked on Figure 10 indicate the approximate temperatures and pressures at which composites of diamond and non-oxide ceramics according to the present invention and those of the prior art are formed. These are as follows: ,525,171

Claims

CLAIMS :

1. A diamond composite comprising diamond particles bound together in a matrix of an oxide or non-oxide ceramic other than silicon carbide wherein the diamond particles comprise less than 70 volume per epnt of the composite.

2. A diamond composite according to Claim 1 wherein the particle size of the diamonds is less than 100 microns.

3. A diamond composite according to either Claim 1 or Claim 2 wherein the composite is formed by densification at an elevated temperature in a reducing environment at a pressure below that at which diamond and graphite are in equilibrium.

4. A diamond composite according to Claim 3 wherein the temperature is less than 1750 C and the pressure is less than 100 MPa.

5. A diamond composite according to any one of Claims 1 to 4 wherein the composites are formed in a die made from graphite.

6. A diamond composite according to any one of * Claims 1 to 5 wherein the composite contains from 20% to 40% by volume of diamond particles.

7. A diamond composite according to any one of Claims 1 to 6 wherein the oxide ceramic is selected from a group consisting of all compounds of oxygen which . behave as ceramics at elevated temperature but which excludes oxides of elements having an atomic number of less than 12.

8. A diamond composite according to any one of Claims 1 to 6 wherein the oxide ceramic is selected from the group consisting of alumina, yttria, zirconia, cordierite and mullite.

9. A diamond composite according to any one of Claims 1 to 6 wherein the non-oxide ceramic is a chemical compound of carbon, nitrogen, boron or a combination thereof that behaves as a ceramic at elevated temperatures.

10. A diamond composite according to any one of Claims 1 to 6 wherein the non-oxide ceramic is selected from the group consisting of silicon nitride, aluminium nitride, chromium carbide, titanium diboride, boron carbide and boron nitride.

11. A process for making diamond composites which process comprises forming an intimate mixture of diamond particles and a powder of a non-oxide or oxide ceramic, compacting the mixture and densifying it in a reducing environment at an elevated temperature and a pressure below that at which diamond and graphite are in equilibrium.

12. A process according to Claim 11 wherein the densifying step comprises hot pressing at temperatures below 1750 C and pressure not exceeding 100 MPa.

13. A process according to Claim 11 or Claim 12 wherein the reducing environment is provided by densifying the mixture in a die made from graphite.

14. A process according to Claim 11 wherein the densifying step comprises hot isostatic pressing at a pressure of up to 180 MPa. ^•

15. A process according to Claim 11 wherein the densifying step comprises sintering.

16. A process according to any one of Claims 11 to 15 wherein the diamond particles have a particle size of less than 100 microns.

17. A diamond composite produced by the process of any one of Claims 11 to 16.