WO2007088461A1 - Glass coated hard and ultra-hard abrasive particles and a method of making them - Google Patents

Abstract

Hard and ultra-hard particles, typically ultra-hard abrasive particles, are provided with glass coats under sol-gel conditions and heat treatment. The glass coats may be porous or fully dense. The glass types and compositions may be chosen and 'tailored' to have thermal expansion, thermal shock, color and chemical properties to suit specific applications. The glass is bonded via chemical bonds to the hard or ultra-hard particle surface atoms and the glass composition and structure, and resultant properties of such glass, are chosen to match and be compatible with both the bulk hard or ultra-hard particle properties and eventual bond matrices into which a plurality of the coated particles are to be incorporated.

Description

GLASS COATED HARD AND ULTRA-HARD ABRASIVE PARTICLES AND A METHOD OF MAKING THEM

BACKGROUND OF THE INVENTION

THIS invention relates to glass coated hard and ultra-hard abrasive particles and to a method of making them.

Vitreous bonded grinding wheels and tools containing ultra-hard abrasive particles such as diamond and cubic boron nitride, for example, are widely used in general grinding operations. Typically, the abrasive particles are held in a porous glass matrix. The tools are made by mixing or combining the ultra-hard abrasive particles with glass frits and/or glass forming starting materials, compacting or forming a required shape for the grinding wheel or component of said wheel, and then heat treating it to a temperature sufficient for the glass to sinter to a desired degree such that a crushable porous matrix is obtained for the ultra-hard abrasive particles. There are several problems that limit the making and use of such abrasive wheels and articles.

In the case where diamond is the required ultra-hard abrasive particle, the temperatures, heat treatment times, and furnace environments used or ideally required are such that significant degradation of the diamond particles can occur due to oxidation. It is well known that diamond oxidation reactions can detectably commence at temperatures as low as 52O⁰C in air and can become very rapid at temperatures exceeding 800⁰C. This limits the fabrication procedures to the use of inconvenient and sometimes expensive gaseous environments. Moreover, the oxidation reactions of diamond, being surface area dependent, become extremely rapid as the diamond abrasive particle size becomes small. This tends to limit the convenient use of diamonds in vitreous bonds to the coarser sizes, such as about 100 to 150 micrometers (μm) in diameter, whilst diamond sizes as fine as 1 to 10 μm may be desired for some applications.

In addition, it is often desired to incorporate organic compounds and agents into the glass compact so that controlled porosity may be generated by the pyrolysis and thermal degradation of such organics. Even though inert gas environments may be employed, this pyrolysis of the organic components leads to highly oxidative products that can oxidize and damage the diamond abrasive particles.

In the case where cubic boron nitride particles are the desired ultra-hard abrasive, certain glass making components or compounds, which could be desired fluxing agents for the glass sintering and formation, can inappropriately react with the cubic boron nitride leading to large amounts of gas evolution and foaming that can disrupt and damage the wheel or abrasive article. Examples of these glass components are alkali oxides, such as lithium oxide (Li₂O), sodium oxide (Na₂O) and potassium oxide (K₂O). Lithium oxide is known to easily react with cubic boron nitride at elevated temperatures with the evolution of nitrogen gas (N₂). This gas evolution and resultant foaming can disrupt the fabrication of vitreous bonded grinding wheels or articles. The glass, vitreous bond choices are thus limited to those that do not contain significant amounts of compounds that can catastrophically react with cubic boron nitride. This problem is also magnified as the cubic boron nitride particle size becomes smaller due to a large increase in surface area and resultant reactive surface and so there is also a tendency not to employ fine cubic boron nitride particle size distributions.

When mechanical mixtures of the ultra-hard particles and the glass frits and/or glass starting material combinations are subjected to the glass sintering and formation conditions, bonding and keying of the abrasive particles into the vitreous matrix can be problematic due to inadequate wetting and contact between the abrasive particle and the glass. Often slow cooling rates are necessary during the manufacture of vitreous bonded tools to minimize cracking damage, which can occur due to thermal expansion miss-match between the abrasive grains and the porous glass bond matrix.

EP 0 4 003 22 and its corresponding US patent 4,951 ,427 disclose abrasive particles, including diamond and cBN particles, having a refractory metal oxide substantially covering the surface of said particles. The preferred methods in this patent involve first applying a metal coat in an elemental form to the particles followed by converting said coat into oxides by heat treatment. The preferred oxide disclosed is titania (TiO₂), which under the procedures outlined will form a polycrystalline ceramic either of the anatase or rutile structure. Although an alternative method for TiO₂, involving a sol-gel procedure is suggested in one of the examples, such method is believed to be non-enabling, to have insufficient details and does not provide a means of coating individual fine particles in glasses, in particular pluralities of fine sized (100μm and smaller, including nano-sized) particles.

A method of encapsulating abrasive particles in substantially spherical glass globules by forming a mixture of glass powder, abrasive particles and binder, fusing the binder to produce a solid mass, attriting the mass into particles and finally fusing the particles to create the globules, is disclosed in US 5,125,933 and its European counterpart EP 0 530 983. The disclosed method is limited in that it is required to heat the particulate materials to sufficient temperatures to fuse the glass. These high temperatures are of concern as damage to diamond and cBN abrasives particles may well ensue. Individually encapsulated abrasive particles are also unlikely to be generated. The method is also not applicable to fine sizes of abrasive particles, particularly those of less than 100 μm in diameter.

EP 0 608 062 and corresponding US 5,300,129 describe a double layered glass coating on cBN abrasive particles for use in vitreous bond matrices. The inner layer, adjacent the surface of cBN particles, is generated by chemical reaction of a glass with the cBN itself and the outer layer by spraying the so coated particles with a vitreous material. The disclosure is limited to glass types that can suitably react with cBN, such as glasses containing alkali metal oxides. It is expected that the desired control of the reaction stipulated in the patent will be extremely difficult to exercise, particularly for fine cBN particle sizes with large surface areas. This method is not applicable to diamond or other abrasive types where the appropriate chemical reactions with glasses do not take place. The high temperatures necessary are also of concern and limiting. The spraying of a vitreous material to form the outer layer is also problematic for fine cBN particles, particularly those less than 100 μm in diameter.

The glass coated ultra-hard abrasives, preferably diamond and cBN, in the current invention significantly alleviate or mitigate these problems.

SUMMARY OF THE INVENTION

According to one aspect of the invention a method or coating hard or ultra- hard particles with a silica based glass coat material includes the steps of providing a plurality of hard or ultra-hard particles, contacting the particles with a silicon based compound under sol-gel reactive conditions for silica based compounds, thereby coating the particles, recovering the coated particles and heat treating them under conditions and at a temperature suitable to vitrify the coat material to form a glass coat material on the hard or ultra-hard particles.

In particular, the sol-gel method of producing the glass coated hard or ultra- hard particles of the invention provides that hard or ultra-hard particles, preferably diamond or cBN, are coated in micro-porous glass network coats by progressive addition of alkoxide (or mixed alkoxide) alcoholic solutions to vigorously stirred suspensions of the ultra-hard particles in alcohol water aliquots, with optional acid or alkali agents present, such that the hydrolysis and polycondensation reactions so caused operate such that nucleation and growth of the porous hydroxilated glass network only or largely occurs on the particle surfaces.

The surface chemistry of the particles, preferably diamond or cBN particles, must advantageously be prepared in order that the so generated functional groups can take part in the hydrolysis and polycondensation reactions. The water to alkoxide ratio, alkoxide and alcohol types and concentration, rate of addition of reagent solutions, type and concentration of acid or base catalyst or pH agent, must be chosen to suit each glass type desired.

The coated particles are typically removed from the suspension by settling, sedimentation or filtering.

The coated particles are then dried and heat treated in order to produce glass coats with chosen degrees of porosity up to and including coats of full density. The drying to remove excess alcohol and water may be done at temperatures up to about 100⁰C in vacuum, air or inert atmosphere. This is followed by an initial heating stage up to about 500⁰C with carefully controlled heating rates and duration in order to largely remove hydroxyl groups (-OH) from the internal porosity, create further polycondensation and remove the finest porosity (where the pores are about one to several nanometers) and thereby partially densify the glass coat. If further densification of the glass coat is required the temperature may be further increased.

The temperatures preferably approach the glass transition temperature (Tg) of each type of glass composition, such that fully dense glass coats are produced. Tg for otherwise pure silica is about 900⁰C, dependent upon hydroxyl (-OH) impurity contents.

The heat treatments for cBN particulate substrates may be carried out in air or inert atmosphere. In the case of diamond particulate substrates inert or reducing atmospheres are preferred in order to avoid oxidation of the diamond above about 600⁰C.

According to a further aspect of the invention there is provided a glass coated ultra-hard particulate material, characterised in that individual ultra- hard particles have glass coatings.

The ultra-hard particles are preferably diamond or cBN particles.

The average size of the diamond and cBN particles can be nano-sized (10 to 100 nm), near nano-sized (100 to 200 nm), submicron (0.1 to 1.0 μm), 1 to 10 micron, and tens to hundreds of microns (1.0 to 1000 μm).

The average thickness of the coatings is preferably from about 10 nm to several microns, and is preferably less than 2 microns, more preferably less than 0.5 microns.

The coatings are preferably crack free and exhibit complete or partial coverage of the particles.

The porosity of the coatings typically varies from about 50% to 0% (fully dense glasses), preferably close to 0%, pore sizes typically being in the range of a few to 100 nm.

The composition of the glass coatings is preferably selected from pure silica (SiO₂), binary, tertiary and multi-component silica based glasses and germania, GeO₂ containing glass coatings.

Additional glass components, including those known in the art to be possible to combine with sol-gel derived silica using sol-gel reactions or subsequent metal salt solution treatments, may be included in the glass coatings. A non restrictive set of examples of these are, TiO₂, ZrO₂, B₂O₃, AI₂O₃, Na₂O, K₂O, Li₂O, and M_mO_n where M is Cr, Mn, Fe, Co, Ni, Cu, or V and m and n can typically be 1 , 2, 3 etc, determined by the valence state of the metallic element.

The thermal expansion coefficient of the fully dense silica glass coats can be varied from that of pure fused silica glass (0.5 x 10^{"6 0}C) to essentially zero by additions of TiO₂ in the range of about 3 to 11 weight percent. Thermal expansion miss-match between the ultra-hard particles and the glass coatings can thus be chosen.

Similarly, the thermal expansion coefficient of the fully dense silica glass coats can be increased from that of pure silica by additions of B₂O₃.

Thermal expansion miss-match between the ultra-hard substrate particles and the glass coatings can thus be chosen.

Where low concentrations of M_mO_n (where M is Cr, Mn, Fe, Co, Ni, Cu, or V) composition glasses are used, the coatings are colored, particularly when fully dense.

The glass coatings may be chosen to be compatible with the vitreous bond matrices of tools such that good glass "alloying" results with appropriate matching of glass properties. The ultra-hard particles may thus exhibit better bonding and keying to the matrices with resulting enhanced performance in applications.

The invention further extends to a glass coated ultra-hard particle, characterised in that the ultra-hard particle is a fine particle, preferably having a particle diameter of less than 10 microns, more preferably less than 1.0 micron, even more preferably less than 200nm, even more preferably less than 100nm.

The invention also extends to a tool component comprising the glass coated ultra-hard particulate material of the invention as defined above embedded in a matrix. The matrix is preferably comprised_, of, at least in part, the glass coating material.

The tool component may be any appropriate cutting or grinding tool component such as, for example, grinding wheels, polishing pads and honing sticks.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides for hard and ultra-hard particles, typically ultra-hard abrasive particles, with glass coats and to a general method for generating the coats involving a specific combination of sol-gel and heat treatment technologies. The method utilises temperatures that are low compared to alternative methods and does not employ melting. The ultra- hard particles include, but are not restricted to, diamond and cubic boron nitride (cBN) from nanometer (nm) to millimeter (mm) dimensions. The glass coats may be porous or fully dense. The glass types and compositions may be chosen and "tailored" to have thermal expansion, thermal shock, color and chemical properties to suit specific applications. The thickness range of the glass coats extends from nanometer (nm) to micrometer (μm) dimensions.

The hard or ultra-hard particles have tailored glass coatings, whereby the glass is bonded via chemical bonds to the hard or ultra-hard particle surface atoms and the glass composition and structure, and resultant properties of such glass, are chosen to match and be compatible with both the bulk hard or ultra-hard particle properties and eventual bond matrices into which a plurality of the coated particles are to be incorporated.

Ultra-hard particulate materials are usually defined as having hardness values exceeding 40 GPa as measured using conventional hardness techniques. This includes diamond, cubic boron nitride (cBN), boron carbide (B₄C), boron suboxide (B₆O) and various structures of carbon and nitrogen. This invention conveniently refers to diamond and cubic boron nitride, but does not preclude other hard or ultra-hard particulate materials. The method of the invention is based generally on the use of sol-gel techniques for coating the ultra-hard abrasive particles. It is known from the teaching of US 3,640,093 that general sol-gel techniques, particularly by the controlled hydrolysis and condensation of alkoxides, can be used to prepare multi-component glasses of most compositions. Alkoxides are metalorganic compounds of general formula M(OR)_n, M being metalloids such as Si, Ge or B and/or metals such as Ti, Zr, Al, Na, K, R usually being an alkyl such as methyl (-CH₃₎, ethyl (-C₂H₅), and many others, and n being the valency of the metal/metalloid. Metal salt solutions could also be used as sources of some metal oxide components of the glasses, by incorporating the solutions into the gels. High purity glasses could then be produced by appropriate drying and then heat treatment.

The glasses in the scope of the present invention are mainly those based on sol-gel generated silica (SiO₂), but also include more unusual glasses based upon germania (GeO₂) or other glass types possible using appropriate sol-gel methods. Also included are the multifarious compositions possible by combining the many glass modifying components such as B₂O₃, AI₂O₃, MgO, Na₂O, K₂O, TiO₂, ZrO₂ and others.

Conventionally, multi-component glasses are formed by cooling melts of combinations of these components. The glass coats produced by the method of this invention are those possible by using sol-gel processing. An exemplary, but not restrictive, list of binary, ternary and multi-component glass compositions possible using the general method is given in Table 1 below. Table 1

Glass Compositions from Sol-gel

Mono Component Glasses

SiO₂

GeO₂

Binary Component Glasses

SiO₂ - GeO₂

SiO₂ - TiO₂

SiO₂ - B₂O₃

SiO₂- ZrO₂

SiO₂- AI₂O₃

SiO₂ - Fe₂O₃

SiO₂ - Na₂O

SiO₂ - Li₂O

SiO₂ - M_mO_n (M is Cr, Mn, Fe, Co, Ni, Cu, or V.)

Ternary Component Glasses

SiO₂ - TiO₂- ZrO₂

SiO₂ - B₂O₃ - TiO₂

SiO₂ - B₂O₃ - Na₂O

SiO₂-AI₂O₃ - B₂O₃

SiO₂- AI₂O₃ - Na₂O

SiO₂ - TiO₂- M_mO_n (M is Cr, Mn, Fe, Co, Ni, Cu, or V.)

Multi-component Glasses

SiO₂ - AI₂O₃ - B₂O₃ - K₂O - Na₂O

SiO₂ - AI₂O₃ - TiO₂ - Li₂O

SiO₂ - B₂O₃ - Na₂O - AI₂O₃

SiO₂ - M_mO_n (M is any combination of Cr, Mn, Fe, Co, Ni, Cu, or V.)

The above additives allow a wide scope of properties of the resultant glass coatings to be chosen, such as large ranges of thermal expansion coefficient, thermal shock resistance, thermal conductivity, mechanical properties, optical properties and chemical resistance. In the case of cBN as the ultra-hard abrasive material, it is preferable to use glass compositions which do not include components which readily react with cBN, such as the alkali metal oxides, particularly Li₂O.

Other components that at relatively low concentrations, typically less than a few mole%, can modify the light wavelength transmission or color of the glass coats are also included. Examples of these components are CeO₂, TiO₂, (colorless, ultra violet absorbing); Co₃O₄, Cu₂O, CuO, (blue); Mn₂O₃ (purple); Cr₂O₃, V₂O₃, CuO, (green); Fe₂O₃, (brown); Na₂S, (amber); CdS, Ag, (yellow) and various combinations. Pure silica glass itself is colorless and ultra violet transmitting. Thus the method of the invention can yield colored glass coatings on the ultra-hard particles.

Dry sol-gel derived materials such as silica (SiO₂) consist of micro-porous amorphous structures. This micro-porosity may be controllably reduced by choice of heat treatment at chosen temperatures and times until fully dense SiO₂ glass (2.2 g cm^'3) is generated at temperatures above about 800⁰C (at temperatures approaching the glass transition point Tg). The density and pore size of the sol-gel silica or silica based glass coats of this invention may be chosen and controlled by such means. Tg in each case is highly dependent upon the detailed glass composition, and thus the temperature and time conditions to control the porosity of each glass composition is usually specific to that composition.

There are two alternative pathways for producing sol-gel derived silica materials known in the art: (1) gelation of colloidal suspensions of discrete solid particles in the range 1 to 100 nm or (2) simultaneous hydrolysis and condensation of alkoxide chemical precursors. The method of this invention includes both general approaches, the preferred and most versatile approach, however, being pathway (2). Colloidal systems generally result in larger, submicron scale porosity and structures while alkoxide systems exhibit smaller nanometer (nm) scale porosities and structures and also, under certain circumstances allow molecular mixing of specific chosen additives.

A general reaction scheme for pathway (2) for silica is:

≡Si-OR + H₂O → ≡Si-OH + ROH (1)

=Si-OH + HO-Si≡ → ≡Si-O-Si≡ + H₂O (2)

≡Si-OH + RO-Si≡ → ≡Si-O-Si≡ + ROH (3) where R is an alkyl (-CH₃, -C₂H₅ etc.). Equation (1) represents a hydrolysis reaction and equations (2) and (3) represent condensation reactions to produce — Si— O— Si— linkages. Equations (2) and (3) normally occur simultaneously and are in general not complete. Theoretically four moles of water are required per mole of silicon alkoxide, Si(OH)₄ and two moles of water are consumed for complete (net) conversion to SiO₂ in accordance with:

Si(OR)₄ + 4H₂O = Si(OH)₄ + 4 ROH (4)

Si(OH)₄ = SiO₂ + 2H₂O (5)

The preferred alkyl group for the method of this invention is ethyl, -CH₂CH₃. Equation (4) describes the overall hydrolysis of the silicon alkoxide, which is followed by, or is simultaneous with equation (5), which describes the polycondensation of silicon hydroxide to a micro-porous three-dimensional — Si— O— Si— linkaged network. These reactions may be carried out in alcohol solution.

Almost all typical glass components are available as alkoxides ( M(OR)_n ), where M is a metal such as Al, Ti, Zr, Na, K, Li, Cr, Co, Fe, rare earths and others and n is the valency of the metal. Generally these alkoxides can undergo the reactions indicated for Si alkoxides in equations (1) to (5). The reactions for each component can sometimes and preferably be arranged to occur simultaneously with the silica forming reactions, though sequential reactions for the different alkoxides can also be exploited if the required conditions are too dissimilar. In this way — Si— O— M— linkages can be formed together with the — Si— O— Si— linkages in the micro-porous oxide network.

The kinetics of these reactions, hydrolysis and polycondensation, are influenced by the pH and concentration of the solutions, the nature of the acid or base used to change the pH, the type and molecular weight of the alkoxide, the rate of addition of the reagents, the order of the additions, temperature, and most importantly the amount of water.

If the reactivity of the desired metal alkoxide is too dissimilar to that of the chosen silicon alkoxide for simultaneous hydrolysis and polycondensation reactions to occur then it is possible to prepare mixed alkoxides, which then allow molecular mixing of the desired oxides to be potentially possible involving the — Si— O— M— linkages. A general formula for a mixed alkoxide with silicon may be represented by, ₃(OR)Si— O— M(OR')_n-1. R' may be a different alkyl group to R.

The heat treatment temperatures required to produce the final desired glass coated ultra-hard particles, is much lower than that required in conventional glass making. In the case of the more refractory compositions such as silicate glasses containing titania, zirconia, or rare earth oxides, the sol-gel methods require temperatures one-third to one half lower than when melting mixed oxide batches. This provides one of the great advantages of the method of the present invention over the prior art methods for forming glass coatings and encapsulations.

The method of this invention concerns itself with a general technique for the silica (or germania) and silica (or germania) based glass coating of pluralities of ultra-hard particles held in suspension in the alcoholic solutions. It is known from the teaching of a process published by W. Stober and co-workers in 1968 (Stober W., Fink A. and Bohn E., 1968, J. Colloidal Interface ScL, vol 26, p 65) that by control of tetraethylorthosilicate (a silicon alkoxide, Si(OCH₂CH₃)₄) hydrolysis with water in ethyl alcohol solution, using ammonia as catalyst, it is possible to generate a monodispersion (equi-sized particles) of micron and submicron micro-porous silica spheres. In order to coat suspended particles in accordance with this invention, this technique has advantageously been adapted by slowly adding a dry alcoholic solution of silicon alkoxide to a suspension of particles in an aliquot of water in alcohol, in the presence of a chosen concentration of ammonia (or other pH modifying agent). In this way, the conditions of heterogeneous nucleation and growth of a silica coating only on the suspended particles may be generated, without the homogeneous nucleation and growth of independent particles of silica in the suspension.

It is known in the art that this approach can be used to form porous silica coats on suspended oxide particles such as hematite (M. Ohmori and E. Matijevic, J. Colloid Int. ScL, vol. 150, p 594, 1992), alumina for the formation of mullite ( M. D. Sacks, N. Bozkurt, and G.W. Scheiffele, J. Am. Ceram. Soc, VoI 74(10), p. 2428, 1991) and even nano-sized metal particles, such as 15 nm gold particles, (LM. Liz-Marzan,M. Giersig and P. Mulvanet, Langmuir, 1996,vol.12, p.4329) and silver nanowires (Y. Yin, Y. Lu, Y. Sun and Y. Xia, Nano Letters, 2002, Vol.2, No. 4, p 427). It is also known in the art that particles such as alumina, can be coated by such means with borosilicate glasses, (M. A. Marmer, H. Bergna, M. Saltzberg and Y. H. Hu, J. Am. Ceram. Soc. Vol. 79 (6), p.1546, 1996). However, the present invention surprisingly provides for the coating of non-oxide (and non-metallic) hard and ultra-hard material particles, in particular ultra-hard particles such as diamond and cBN, with silica and silica based glasses. It includes the requirement of generating an appropriate surface chemistry on the diamond and cubic boron nitride particles such that processes are found whereby the silica and/or silica with chosen glass modifying components can nucleate, grow and chemically bond to the diamond and/or cBN surfaces.

An example of an appropriate surface chemistry is to treat the diamond and cBN particles so that oxygen related species dominate the surface chemistry and can take part in the sol-gel hydrolysis and condensation reactions. Hydroxyl (-OH) terminated surface chemistries are preferred.

The method of the invention, as set out schematically in the accompanying Figure 1 , provides process parameters that allow the controlled coat formation on particle sizes of diamond and/or cBN from nano-size diameters through to 100's of micrometers. This allows for the avoidance of (or minimization of) spontaneous nucleation of oxide polymers in the suspending alcohol and the formation of oxide polymer coats on the surfaces of the suspended ultra-hard particles only (or predominantly). These parameters for each case of diamond, cBN, and desired glass composition include specification of particle surface chemistry, particle suspension concentration, alcohol suspension/reaction media type, water concentration, alkali or acid catalyst type and concentration, type and concentration of silicon alkoxide (or other alkoxide) alcoholic solution and the rate of slow addition of said solution. The temperature at which the reactions are conducted are also chosen and specified. Typical values for these parameters are provided in the examples below.

The reactivity of alkoxide compounds (M(OR)_n) with water and their resultant polycondensation reactions are highly dependent upon both the metal or metalloid M, and the size and structure of the alkyl group R. Moreover, there is also a big variation in solubility in alcohols.

For pure silica glass variations of coat, R may be chosen from methyl, ethyl, propyl, isopropyl, butyl alkyl groups and others. Preferably R is chosen to be methyl (-CH₃) or ethyl (-CH₂CH₃). More preferably R is chosen to be ethyl (-CH₂CH₃). Thus the preferred silicon alkoxides are silicon tetramethoxide Si(OCH₃)₄ (TMOS) and silicon tetraethoxide Si(OC₂Hg)₄ (TEOS). More preferably silicon tetraethoxide Si(OC₂H₅)₄ (TEOS) is used. Alternatively these silicon alkoxides are called silicon tetramethoxysilane (TMOS) and silicon tetraethoxysilane (TEOS), respectively.

For binary, tertiary and multi-component glasses to combine with the silicon alkoxide, M may be chosen from Ti, B, Zr, Al, Fe, Na, K, Li, and others. In each case the particular alkoxide may be chosen from those with a large range of R types, again such as methyl, ethyl, propyl, butyl and others. The choice is made on the basis of reactivity with water and solubility in convenient alcohols, such as methyl, ethyl, iso-propyl alcohols.

In the case of Ti, the preferred alkoxide is titanium iso-propoxide, Ti(OCH(CH₃)2 )_A- In the case of B, the preferred alkoxide is boron iso- propoxide, B(OCH(CH₃)₂)₃. In the case of Zr₁ the preferred alkoxide is zirconium n-propoxide, Zr(OCH₂CH₂CHs)₄. In the case of Al, the preferred alkoxide is aluminium sec-butoxide, AI(OC₄Hg)₃.

After coating of the particles they may be removed from the suspension by appropriate means such as filtering, sedimentation and decanting of excess liquid. Several steps of washing and removal of the coated particles from suspension may be employed. The coated particles can then be dried by warming in air or vacuum, at temperatures up to and including 10O⁰C. Excess water, alcohol in the micro porosity of the coat and -OH chemical species, which populate the surfaces of the micro-porosity of semi-dense silica or silica glass coat, may then be removed by low temperature heating up to and including a few hundred degrees centigrade in air or inert gas. Finally, the density/porosity of the glass coat is determined by accurately heating the coated particles to chosen temperatures and times up to typically 400 to 1100⁰C, depending upon the glass composition. The temperatures required for fully dense glass coats are much lower than those required when conventional glass fusion techniques are employed as in the prior art. In the case of diamond particulate substrates the heat treatment can take place in inert or reducing atmospheres to prevent diamond oxidation while the silica glass coat porosity decreases. Diamond oxidation typically occurs at temperatures around 600⁰C in air depending upon the diamond type and particle size.

A significant advantage of the present invention is that when a fully dense, coherent silica based glass coat is generated covering diamond particles, the diamonds are rendered oxidation resistant up to temperatures in excess of 85O⁰C. This can alleviate the first limiting problem of using diamond abrasive particles in vitreous bonds as previously outlined. The glass coatings also provide a means of preventing or inhibiting undesired reactions with reagents, such as fluxing agents, that can react with cBN during vitreous bond tool manufacture. Porosity of the coats can controllably be reduced along with average size of the pores (down to a few nanometers). This occurs predominantly by viscous flow dominated sintering. At temperatures of about 900⁰C and above for heating times of a few hours, fully dense pure silica glass (2.2 g cm^"3) coats can be produced.

The thicknesses of glass coats on diamond and cBN that can be produced by this general method can range from a few tens of nanometers to many microns, dependent upon the average size of the diamond and cBN particles and the duration and rate of deposition of sol-gel coating. The range of coating thicknesses, size of particles to be coated, and density of coat on diamond and cBN particles made possible by the method of this invention is far greater than any prior art method.

A notable feature of the invention is the capability of producing a large scope of glass coat compositions and types that allows the tailoring of the properties of the coats to suit particular desired purposes. These purposes include:

(1). Matching the thermal expansion coefficient of the glass to that of diamond or cBN particle substrate. This can minimize any thermal expansion mismatch between the coat and particle substrate and thereby manage any possible undesired stress situation which could lead to coat cracking and de-lamination when the coated particles are used in any desired subsequent application or process, particularly those involving further heat treatment.

(2). The prevention of chemical attack of the ultra-hard particle by gaseous or liquid agents prevalent in the environment of the particle in subsequent processes and applications. This includes the prevention of oxidation of a diamond particulate substrate in oxidizing gases at temperature, thus the coated diamond is rendered oxidation resistant and thermally stable. Also included is the protection of cBN particle substrates from liquid glass fluxing agents, such as Li₂O. (3). The choice of glass types and properties to be compatible with the bonds of abrasive wheels, tools and bodies. This can facilitate the bonding and keying of the coated abrasive particle into the tool, thereby improving the performance of said tool.

(4). The generation of colored glass coats by the incorporation of glass coloring agents. This allows the identification of particular glass coating types and thus can be used as a "labeling" technique for product types.

The thermal expansion coefficient of diamond increases from about 0.5 x Kr^{6 0}C^"1 at room temperature to about 5 x 1CT^{6 0}C^-1 at 1000⁰C and that of cBN over the same temperature range from about 1 to 6 x 10^{"6 0}C^"1, (G. A. Slack and S. F. Bartram, Journal of Applied Physics, Vol. 46, No. 1, p. 89, 1975). Silica and some silica glasses tend to have low thermal expansion coefficients and as a result usually exhibit good thermal shock resistance. Fully dense fused silica glass has a thermal expansion coefficient of 0.5 x 10^{'6 0}C^"1. The thermal expansion coefficient of silica glass can be manipulated by combining with glass components such as B₂O₃, TiO₂, etc. It is well known that the thermal expansion coefficient of silica glass increases in proportion to B₂O₃ additions, whereas additions of TiO₂ (in the range of 3 to 11 wt. %) lowers the expansion coefficient and can even produce glass types with negative coefficients.

Example 1

A 2Og sample of facetted synthetic diamond of average particle size of 105 to 125 μm was suspended in 1.25 liters of ethyl alcohol of purity greater than 99%. The diamond sample had previously been heated at 48O⁰C for 10 min in a flowing stream of 20% oxygen in argon, in order to produce surfaces with predominantly oxygen containing functional groups. To this vigorously stirred suspension 250 ml of de-ionised water and 30ml of 25% by volume aqueous ammonium hydroxide solution were added. 40 g of tetraethoxysilane, TEOS, (Si(OC₂H₅)₄) was dissolved in 100ml of the 99% pure ethyl alcohol. This solution was slowly added at a constant rate to the stirred suspension maintained at room temperature over a period of 8 hrs. Stirring was continued for a further 1hr. Stirring was stopped and the coated diamond particles were allowed to settle. The supernatant liquid above the settled plurality of diamond particles was predominantly clear and was decanted. The coated particles were then washed three times with the pure ethyl alcohol. After filtering the plurality of diamond particles was dried in a vacuum oven at 6O⁰C for 24 hrs.

A sample of the coated particles was then examined in a scanning electron microscope (SEM), which showed that the particles were completely covered with a coat, shown to be composed of silicon and oxygen using energy dispersive x-ray analysis (EDAX). The thickness was estimated to be about 0.4μm.

Half of the sample was then heated in a stream of pure argon in a tube furnace to a temperature of 67O⁰C (sample A) and maintained at that temperature for 3 hrs. The heating rate was 3⁰C per min. On further examination in the SEM it was observed that a degree of coalescence of the coat had taken place and that some shrinkage of the coat had occurred.

The other half of the sample was heated in a stream of pure argon to a temperature of 900⁰C (sample B) for 3 hrs, again at a heating rate of 3⁰C per min. On examination in the SEM it was observed that the coat had a glass appearance and fully covered all parts of the facetted diamond surfaces. The estimated thickness was about 0.2 to 0.3 μm and had an appearance consistent with being a dense silica glass. Under the optical microscope the coat was transparent. No cracks were observed in the coats, which indicated that the thermal expansion mismatch between the diamond particulate substrate and the silica coat was low.

The 67O⁰C (sample A) and 900⁰C (sample B) heat treated coated samples were then tested in comparison to an uncoated sample of the same diamond on a thermo-gravimetric analyser in a stream of air at a heating rate of 2O⁰C per min. The onset of oxidation reaction of the uncoated diamond was measured to be 781⁰C and for sample A and B, 791⁰C and 893⁰C respectively. This indicates that sample A still had significant open porosity, giving a minor inhibition of oxidation whereas sample B, with an onset temperature delay of about 110⁰C, exhibited substantial oxidation protection of the diamond. This result also indicates that the 90O⁰C heat treated material, sample B was completely covered in a predominantly dense SiO₂ glass coat as suggested by the SEM and optical images.

Example 2

A plurality 0.75 to 1.5 μm sized diamond particles, which had been produced by well known crushing and size classification processes, was treated in fuming concentrated sulphuric acid to which was added potassium nitrate. This procedure ensured that the diamond surface chemistry was dominated by oxygen related functional groups, which allows the surface chemical species to take part in the sol-gel reactions. 2Og of this diamond was dispersed with the aid of an ultrasonic probe in 2.5 liters of 99% pure ethyl alcohol, to which had been added 500 ml of de- ionised water and 60 ml of 25 vol% aqueous ammonium hydroxide solution.

The suspension was vigorously stirred using a mechanical paddle and maintained at room temperature (25⁰C). A solution of 80 g of silicon tetraethoxysilane.TEOS, (Si(OC₂H₅)₄) was dissolved in 100 ml of 99% pure ethanol. This solution was slowly added to the suspension over a period of 12 hrs. The stirring was then continued for another hour. The plurality of coated diamond particles was then removed from suspension, washed, dried as described in example 1.

SEM examination showed that each approximately 1 μm sized diamond particle was completely coated. By weighing before and after coating, the coat was estimated to be 37% weight of the total mass.

The coated material was divided into three approximately equal amounts and labeled sample C, D and E. Sample D was heated in a stream of pure argon to a temperature of 67O⁰C for 3hrs, the heating rate was 3⁰C per minute. Similarly, Sample E was heat treated at a top temperature of 1000⁰C, again for 3 hrs at a rate of 3⁰C per minute. Sample C was retained in the dried state and was not further heat treated. On SEM examination, the particles of sample E were completely covered in a crack free coat with the appearance of a fussed glass.

The specific surface area of an uncoated sample of the diamond and Samples C, D and E were measured using the well established Brunauer, Emmet and Teller (BET) nitrogen adsorption method. The results are presented in Table 2.

Table 2

It is noted from Table 2, that the silica coat of Sample C provided a twenty- fold increase of specific surface area as compared to the specific surface area of the uncoated diamond powder. This demonstrated that the coat indeed had a highly micro-porous open structure. After heat treatment in argon at 670⁰C of Sample D, sufficient viscous flow of the silica of the coat had occurred such that the open porosity had substantially been removed as indicated by the specific surface area now being lowered back to that similar to the uncoated powder. The specific surface area of Sample E, after heat treatment at 1000⁰C, had been lowered to slightly below that of the uncoated powder. This indicated slightly more closing of the open porosity and possibly a slight smoothing of the powder surface, consistent with the formation of a dense silica glass coat. These results indicate that the porosity and density of the silica coats can be manipulated by choice of heat treatment procedure subsequent to sol-gel coating. Example 3

20 g of well facetted, highly crystalline, 105 to 125 μm diameter cBN grit particles were treated in boiling, 32 vol% hydrochloric acid, washed in water and dried. This material was suspended by vigorous stirring in a mixture of 1 ,8 liters 99% pure ethanol, 350 ml of de-ionised water and 40 ml 25 vol% aqueous ammonium hydroxide solution. A 30% by weight solution of tetraethoxysilane, TEOS, (Si(OC₂Hs)₄) in dry pure ethanol was then slowly and consistently added to the stirred suspension over a period of 10 hrs. Stirring was continued for a further hour. The material was allowed to settle, the supernatant liquid removed and the coated cBN particles washed in pure dry ethanol. The coated material was then dried at 6O⁰C in a vacuum oven for 24 hrs. This coated material was then heated in dry, pure argon at 3⁰C per min, up to a temperature of 800⁰C and maintained at this temperature for 3 hrs. On subsequent SEM examination, it was found that the grit particles were completely covered in a crack free silica coat of smooth featureless appearance, indicating the formation of a dense silica glass.

Example 4

The coating procedure detailed in example 2 for 0.75 to 1.5 μm diamond was applied to the coating of a cBN micron powder of average size 1.25 μm. The sol-gel coated fine powder after drying for 24 hrs in vacuum at 60⁰C was heat treated at 800⁰C for 3 hrs. On SEM examination, it was shown that complete coverage with a dense silica coat had been achieved for each individual cBN particle.

Example 5

A plurality 1 to 2μm sized diamond particles, commercially available as a very fine abrasive with a specifically designed particle shape and surface topography to accentuate relative high friability in applications, were treated in fuming concentrated sulphuric acid to which was added potassium nitrate. This procedure ensured that the diamond surface chemistry was dominated by oxygen related functional groups, which allowed the surface chemical species to take part in the sol-gel reactions.

Four 2Og samples of these diamond particles were coated in silica, washed and dried as described in detail in Examples 1 and 2. SEM analysis was carried out on each of the four samples, and it was confirmed that each diamond particle had been completely coated in silica, with no apparent uncoated areas. By weighing before and after coating, the coat was estimated to be 37% weight of the total mass.

To vitrify and densify the silica coat, one batch of silica coated diamond was then heat treated in a stream of pure argon to a temperature of 100O⁰C for 3hrs, at a heating rate of 3⁰C per minute.

From the estimated mass increase of coating (37% of total mass), an average coating thickness of dense silica glass was calculated, assuming spherical geometry, an average diameter of diamond particles of 1.5μm, and a silica glass density of 2.2cm^'3. The calculated thickness estimate was 0.19 μm using these assumptions. This batch was designated as batch F and was considered to have a "single coating". The "single coating", therefore, was considered to have a thickness of approximately 0.2μm.

The three remaining batches were then separately re-suspended, with the aid of an ultrasonic probe in 2.5 liters of 99% pure ethyl alcohol, to which had been added 500 ml of de-ionised water and 60 ml of 25 vol% aqueous ammonium hydroxide solution. As described in Example 2, the suspensions were vigorously stirred using a mechanical paddle and maintained at room temperature (25⁰C). A solution of 80 g of silicon tetraethoxysilane, TEOS, (Si(OC₂H₅)₄) was dissolved in 100 ml of 99% pure ethanol. This solution was slowly added to the suspension over a period of 12 hrs. The stirring was then continued for another hour. The coated diamond particles were then removed from suspension, washed and dried. In this way three separate batches of "double coated" 1 to 2μm diamond particles were produced. It was estimated that the so-called double silica coats amounted to 70% weight of the total mass of each batch. These three double coated batches were designated as batches G, H and I, respectively.

Each batch was then subjected to a heat treatment in flowing Argon to 1000, 1050 and 110O⁰C, respectively, to vitrify and densify the coats.

Assuming the silica coats were now close to, or at, silica glass full density, then the average coat thickness for these double coated batches was estimated to be approximately 0.4μm.

Samples of the uncoated 1 to 2μm diamond, together with samples taken from batches F, G, H and I₁ were each then heated in static air in a furnace at three temperatures, namely, 550, 600 and 65O⁰C, for time periods of 30min for each furnace run at each temperature.

The sample weights were measured for each sample before and after each furnace run and the percentage mass loss measured for each heat treatment. Table 3 presents the weight loss results.

Table 3

As can be seen from Table 3, uncoated diamond particles of this size and type oxidizes very readily in air at temperatures of 55O⁰C and above, and that after 30 min at 650⁰C only about 10% of the diamond remained.

The single silica glass coat after vitrification/densification at 1000⁰C (Batch F), provided sufficient oxidation protection at 65O⁰C to increase the retained diamond to about 89%.

The double silica glass coat after vitrification/densification at 1000⁰C (Batch G), provided sufficient oxidation protection at 65O⁰C to increase the retained diamond further to about 98%.

The double silica glass coat after vitrification/densification at either 1050⁰C (Batch H) or 1100⁰C (Batch I), provided complete oxidation protection at 650⁰C, as 100% of the diamond sample was retained as shown by the zero measured weight change.

Thermo-gravimetric analysis (TGA) was then carried out on small samples of uncoated diamond and silica glass coated diamond particles from batches F, G and I. The experiments were carried out in a stream of flowing air, with a heating rate of 10⁰C per min, with each sample continuously weighed on a microbalance.

The sample weights were constant until the onset of oxidation of the diamond particulate material, when loss of weight was registered, due to the formation and removal of gaseous carbon oxides. The relative oxidation onset temperatures of the different samples were compared by recording the temperatures at which 1%, 2% and 3% weight losses were registered. Table 4 presents the results of the TGA experiments. Table 4

As can be seen from Table 4, uncoated diamond of this size and type starts to oxidize and rapidly lose weight in flowing air at close to 524⁰C. Both single (approx 0.2μm thickness) and double (approx 0.4μm thickness) coats of silica glass formed on the diamond particles provided efficient oxidation protection for the diamond and delayed the onset of significant oxidation by up to 200⁰C. Moreover it may be seen that thick rather than thin coats and the higher temperature vitrification/densification of the coat was preferable, yielding better protection for the fine diamond particles. Batch I gave the best result with the onset of oxidation being elevated by just over 200⁰C.

Example 6

25g of extremely fine diamond particulate material, designated as 0 to 0.1 μm, was dispersed in 700ml of AR ethanol, with the aid of a high intensity ultrasonic probe to "break up" agglomerated or stuck together particles. The diamond particles had been previously acid cleaned in fuming concentrated sulphuric acid to which was added potassium nitrate as previously described and thus the diamond surface chemical species were dominated by oxygen types such as hydroxy! (-OH), and related functional groups.

The diamond had been examined using a scanning electron microscope (SEM) which showed that the diamond particles ranged in size from about 0.2μm (200nm) down to about 50nm with the predominant size being about 0.1μm (100nm).

After allowing the diamond suspension to cool to room temperature, 25ml of 25mol% aqueous ammonium hydroxide was added, to increase the pH and render the system substantially alkaline. 110mI of cold de-ionised pure water was also added.

28.9g of silicon tetraethoxysilane, TEOS, (Si(OC₂Hs)₄), was dissolved in 200ml AR ethanol and then slowly added to the diamond suspended in the ethyl alcohol, water and ammonium hydroxide over a period of a few hours. The suspension was continuously and vigorously stirred, the stirring continuing for a subsequent 12hrs after the addition of the tetraethoxysilane solution.

The suspension was then allowed to settle naturally and the clear supernatant liquid decanted. The coated diamond particles were then dried in a heated, rotating vessel at 75⁰C under a continuously pumped reduced pressure.

The dried diamond particles were then examined with a SEM. It was seen that the material consisted of masses of partially agglomerated coated diamond and that each and every diamond had been coated. The coated particles ranged in size from about 50nm to about 0.2 μm (200nm), but were mainly about 0.1 μm (100nm). By weighing before and after coating, the silica coat was estimated to be 24% weight of the total mass, which is close to the theoretical expected value of 24.9%.

Example 7

Two 50 gram batches of 1 to 2μm diamond particles coated in amorphous silica coat (single coats of about 37% by weight of the total coated masses), as described in Example 5, were produced. The dried coated diamond particle batches were then separately suspended in 2 litres of AR ethyl alcohol. 120ml of de-ionised pure water was added to each suspension followed by 10ml of 32% hydrochloric acid aqueous solution to acidify it. Two lots of 60.4g of aluminium tri(sec)butoxide (AI(OC₄Hg)₃, were dissolved in 200ml of isopropyl alcohol, and these solutions slowly added to the two separate suspensions over a period of about 2hrs while the suspensions were continuously and vigorously stirred. The stirring was continued overnight and then the suspensions allowed to settle.

After decanting the supernatant liquids, the coated diamond particles were dried under vacuum at 75⁰C.

On examination of each batch with the SEM it was seen that each batch was made up of masses of well coated diamond particles and that on energy dispersive x-ray analysis (EDAX), on the SEM, carbon, oxygen, silicon and aluminium were present, together with a trace of chlorine. No separate layers of different coats could be distinguished. This is consistent with the contention that the deposition of the second material, amorphous hydrated alumina took place mainly in the open porosity of the initial amorphous silica coat.

One of the batches was then heat treated in a pure stream of pure argon at top temperature of 1050⁰C for 3hrs. The heating rate to attain the top temperature was slow, at about 3⁰C per min. This batch was designated as batch J. The second batch (designated as batch K), was heat treated in the same way except that the top temperature chosen was 1100⁰C. On subsequent SEM examination both batches had the appearance of well coated particulate masses with each diamond particle individually coated.

Samples of uncoated 1 to 2μm diamond particles and both batch J and K were then each subjected to heat treatment at temperatures of 550, 600 and 650°C in static air for 30mins. The samples were weighed before and after each heat treatment. Table 5 below presents the results of these measurements.

Table 5

It may be seen that the silica/alumina combined coat, vitrified and densified at 105O⁰C provided a substantial degree of oxidation protection reducing the oxidative weight loss from about 89 to about 3.5% for the 65O⁰C heat treatment. The higher vitrifying and densifying temperature of 1100⁰C improved this behavior further in that the weight loss was reduced further to about 1.5%.

Thermo-gravimetric (TGA) analysis of a sample of Batch K was carried out under the same conditions of flowing air as described above in this example. The onset of oxidation results in comparison to that of the uncoated sample are given in Table 6. -fUI/lϋiU U / / U U U Z

Table 6

It is shown in Table 6 that the onset of oxidation of the 1 to 2μm diamond in flowing air, was significantly delayed (by about 175 to 19O⁰C) by a silica/alumina binary glass coat.

Example 8

A solution of 80 g of silicon tetraethoxysilane, (Si(OC₂Hs)₄), was dissolved in 100ml of pure anhydrous ethyl alcohol. To this solution was added 9.Og of titanium ethoxide (Ti(OC₂H₅)₄. In this way the titanium ethoxide was reacted with some of the silicon tetraethoxysilicate to form a mixed silicon, titanium ethoxide compound or complex, which remained in solution, together with remaining unreacted tetraethoxysilane.

2Og of 1 to 2μm diamond particles was suspended, with the aid of an ultrasonic probe in 2.5 liters of 99% pure ethyl alcohol, to which had been added 500 ml of de-ionised water and 60 ml of 25 vol% aqueous ammonium hydroxide solution. The combined titanium, silicon tetraethoxide solution was then slowly added over 2hrs to diamond suspension, while the suspension was continuously and vigorously stirred. The suspension was then stirred for a further 12 hrs and subsequently the now coated diamond particles allowed to settle. After decanting the supernatant liquid the coated material was dried under vacuum at 75⁰C.

The dry diamond particles were then heated under a stream of pure argon gas, with a heating rate of 3⁰C per minute to a top temperature of 1100⁰C, which was maintained for 3hrs.

After cooling, a sample of the coated diamond particles was examined in a scanning electron microscope (SEM) and it was observed that each diamond particle was now covered in a uniform coat. Using energy dispersive x-ray analysis (EDAX), the coat was shown to consist of silicon, titanium and oxygen. The intensity of the silicon peak relative to the titanium and oxygen peaks in the so derived EDAX spectrum was consistent with the expected result of a binary glass coat with a 95%/5% silicon /titanium composition.

It is expected the method exemplified here for binary glass compositions, utilizing the generation of mixed silicon, metal alkoxide complex solutions, may be extended to general multicomponent glass systems by the appropriate reaction of multiple metal alkoxide solutions. In this way glass coatings on fine particulate substrates, made up of appropriate ratios of SiO₂, TiO₂, ZrO₂, AI₂O₃ etc. may be generated.

Claims

1. A method of coating hard or ultra-hard particles with a silica based glass coat material including the steps of providing a plurality of hard or ultra-hard particles, contacting the particles with a silicon based compound under sol-gel reactive conditions for silica based compounds, thereby coating the particles, recovering the coated particles and heat treating them under conditions and at a temperature suitable to vitrify the coat material to form a glass coat material on the hard or ultra-hard particles.

2. A method according to claim 1 , wherein the silicon based compound is a silicon alkoxide compound.

3. A method according to claim 2, wherein the silicon alkoxide is either silicon tetraethoxide (Si(OC₂H₅)₄ or silicon methoxide (Si(OCH₃)₄.

4. A method according to any one of claims 1 to 3, wherein the hard or ultra-hard particles are suspended in a suspension liquid.

5. A method according to claim 4, wherein the suspension liquid is an alcohol or alcohol solution.

6. A method according to any one of claims 1 to 5, wherein the heat treatment is carried out below the vitrifying glass softening point for the glass coat material such that it remains as an amorphous and microporous material.

7. A method according to any one of claims 1 to 5, wherein the heat treatment is carried out above the vitrifying glass softening point for the glass coat material such that it densifies and becomes a fully dense or substantially dense material.

8. A method according to any one of claims 1 to 7, wherein the hard or ultra-hard particles are abrasives selected from diamond, cubic boron nitride (cBN), silicon carbide (SiC), boron carbide (B₄C) and alumina (AI₂O₃).

9. A method according to claim 8, wherein the ultra-hard abrasive particles are diamond or cubic boron nitride (cBN) particles.

10. A coated particulate hard or ultra-hard material comprising coated hard or ultra-hard particles produced according to the method of any one of claims 1 to 9.

11. A coated particulate material according to claim 10, wherein the coat on each particle is constituted as a whole or in part by amorphous, porous silica (SiO₂) or fully dense silica (SiO₂) glass.

12. A coated particulate material according to claim 10, wherein the coat on each particle is constituted as a whole or in part by amorphous, porous binary or multi-component silica (SiO₂) metal oxide compositions or fully dense binary or multi-component silica (SiO₂) metal oxide composition glasses.

13. A coated particulate material according to claim 10, wherein the coat on each particle is constituted as a whole or in part by amorphous, porous, binary or multi-component silica (SiO₂) in combination with one or more oxide compositions selected from B₂O₃, TiO₂, ZrO₂, AI₂O₃, and M_mO_n, where M is Cr, Mn, Fe, Co, Ni, Cu or V, and m and n are integers determined by the valence state of the metallic element.

14. A coated particulate material according to claim 10, wherein the coat on each particle is constituted as a whole or in part by a dense glass made up of binary or multi-component combinations of silica (SiO₂) and oxide compositions selected from B₂O₃, TiO₂, ZrO₂, AI₂O₃, and M_mO_n, where M is Cr, Mn, Fe, Co, Ni, Cu or V, and m and n are integers determined by the valence state of the metallic element.

15. A coated particulate material according to claims 13 or claim 14, wherein the composition of the coat on each particle is a binary composition of silica (SiO₂) with AI₂O₃, TiO₂ or ZrO₂.

16. A coated particulate material according to claim 15, wherein the binary composition comprises silica (SiO₂) and 3 to 11 weight percent TiO₂.

17. A coated particulate material according to any one of claims 10 to 16, wherein the particles have an average particle diameter of 1OO's of microns, 1O's of microns, 1 to 10 micron, sub-micron (0.1 to 1.0 micron), near nano-size (100 to 200nm) or nano-sized (less than 100nm).

18. A coated particulate material according to any one of claims 10 to 16, wherein the ultra-hard particles are diamond particles of average particle diameter of 1 to 10 micron, sub-micron (0.1 to 1.0 micron), near nano-size (100 to 200nm) or nano-sized (less than 100nm).

19. A coated particulate material according to any one of claims 10 to 16, wherein the ultra-hard particles are diamond particles of average particle diameter of nano-sized dimensions (less than 100nm).

20. A coated particulate material according to any one of claims 10 to 19, wherein the coating thickness on each particle is less than 2 microns.

21. A coated particulate material according to claim 20, wherein the coating thickness is less than 0.5 microns.

22. A coated, ultra-hard abrasive particulate material comprising silica coated diamond or cubic boron nitride particles having an average particle diameter of less than 10 microns.