WO2009150626A2

WO2009150626A2 - Coating of particulate substrates

Info

Publication number: WO2009150626A2
Application number: PCT/IB2009/052491
Authority: WO
Inventors: Anna Emela Mochubele; Geoffrey John Davies; Johannes Lodewikus Myburgh
Original assignee: Element Six (Production) (Pty) Ltd
Priority date: 2008-06-12
Filing date: 2009-06-11
Publication date: 2009-12-17
Also published as: GB0810758D0; WO2009150626A9

Abstract

The invention provides a method of increasing the specific surface area of a particulate substrate by coating the particulate substrate with a microporous primary coating material.

Description

COATING OF PARTICULATE SUBSTRATES

BACKGROUND OF THE INVENTION

The present invention relates to the coating of granular and fine particulate materials, hereinafter particulate substrates.

The general methods exploited to apply material coats to particulate substrates are chemical vapor deposition, both in so called packed bed and fluidized bed arrangements, physical vapor deposition or sputtering and many liquid particle suspension methods such as electrolytic deposition, chemical electroless deposition, molten salt methods and so called sol-gel methods and the like. The latter method in particular is taught in PCT applications PCT/IB2007/000234 and WO 2006/032982. CVD methods, sometimes in combination with PVD methods, as applied to abrasive particulate substrates, particularly diamond and cBN, are taught in WO 2005/017227 and WO 2005/078045.

All of these methods involve the interaction of the desired coating material or a precursor chemical species for that coating material with the surface of the particulate substrate. Some methods also require a proportion of the particulate substrate material at the surface to take part in chemical reactions with the precursor material to create the desired coat. Thus the specific surface area (usually expressed in m²g^"1 units) of the particulate substrates plays a defining role in the character of each coating process and often affects the rates of reactions and sub-reactions inherent in the diverse methods. The efficiency, and in some cases the viability, of particular coating processes for particular size ranges can be significantly affected by the specific surface area of the particulate substrate to be coated. The present invention concerns the increasing of the surface area of particulate substrates in order to improve the efficiency, capability and viability of general coating technologies and the novel general particulate or consolidated material structures and compositions that can result therefrom.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a method of increasing the specific surface area of a particulate substrate includes the step of coating the particulate substrate with a microporous primary coat material. The microporous primary coat material has a high degree of open porosity, such that the very high surface area of the open porosity of the primary coated particulate substrate is available for further coating.

This primary coating procedure is employed to provide high surface area coats, such that various other subsequent or secondary coating technologies and procedures can be employed to take advantage of the increased surface area.

The primary coat provides an increase of specific surface area over that of the particulate substrate of up to and approaching that possible for the coating material. Where the coat material is generated using sol-gel techniques to provide micro-porous extremely high surface area material, the specific surface area can be as large as the maximum obtainable for that material type, typically a few to many hundreds of m²g^"1. Typically the specific surface area of the primary coated particulate material is between that of the particulate substrate and several hundreds of m²g^"\ and possibly as high as 1000 m²g^"1- An aspect of the present invention is that the mean of the pore size distributions of the primary coats are in the nano size range, that is less than 100 nm.

Another aspect of the present invention is that the open porosity, e.g. a substantial portion of the open pores, can extend from the external surface of the primary coat to the surface of the particulate substrate. This aspect of the invention allows material from a secondary coating procedure to reach and interact or react with the surface of the particulate substrate.

Where the pore size distribution of the primary coat material is nano sized, i.e. with mean less than 100 nm, the secondary coat material formed will be nano-sized, either in an amorphous or crystalline state. Thus the so formed interpenetrating structures will be interpenetrating on a nano scale.

Preferred particulate substrates are abrasives such as diamond, cubic boron nitride, cBN, silicon carbide, SiC, alumina, AI₂O₃, boron carbide, B₄C, tungsten carbide, WC and the like.

More preferred are the ultra-hard abrasives, diamond and cBN.

Preferred high specific surface area, micro-porous primary coats are those derived from sol-gel procedures and the coat materials derived from these procedures which are heat treated in inert and reactive gas systems

More preferred are such primary coats made of amorphous or nanocrystalline oxides, nitrides, carbides, oxynitrides, oxycarbides, and carbonitrides of titanium, silicon, zirconium, aluminium, tantalum, vanadium, niobium, tungsten, molybdenum and any combination thereof..

Even more preferred are primary coats made of amorphous or nanocrystalline titanium oxide, TiO₂, titanium nitride, TiN, titanium carbide, TiC, silica, SiO₂, alumina, AI₂O₃, zirconia, ZrO₂, tantala, Ta₃O₅, niobia, Nb₃O₅, vanadia, V₂O₃, chromia, Cr₂O₃, tungstic oxide, WO₃, and any combination thereof..

Preferred combinations include titania/tungstic oxide combinations, titania/tantalum oxide combinations, titania/chromia combinations, and mixed nitrides such as Ti_xW_yN, Ti_xTa_yN and Ti_xCr_yN, where the sum of x and y is equal to unity.

The specific surface area of the primary coated particulate material can be between that of the particulate substrate material and up to several hundreds of m²g^"1, and possibly as high as 1000 m²g^"1-

A typical specific surface area of the primary coated particulate material is between that of the particulate substrate material and about 300 m²g^"1.

DETAILED DESCRIPTION OF THE INVENTION

Particle substrates coated with microporous material such that the open porosity of the coat provides a large, greatly increased surface area per particle for further deposition of coat material onto the surface of the open porosity is a key aspect of the present invention. Thus by changing the particle size distribution of the particulate substrate material only by a small amount, due to the thickness of the applied coat, the specific surface area of the material can be increased greatly, even up to the order of several hundreds of m²g^"1. This greatly increased surface area per particle allows the use chemical protocols of normally poor coating efficiency to deposit, nucleate and growth, further or other material into and onto the micro- porosity of the coat.

The particles for coating are considered to be granular when they have a mean particle diameter of between 100 microns and about 2000 microns (2mm) or greater and fine when their mean particle diameters are less than 100 microns. The fine particulate substrates may be further sub-divided on the basis of mean particle diameter as materials in the size ranges tens of microns, 10 to 100 microns, single figure microns, 1 to 10 microns, sub- micron, 0.1 microns (100 nm) to 1 micron and nano sized, less than 100 nm.

The general particulate substrates of interest are materials such as ceramics, both oxide and non-oxide based, metals and intermetallic materials that are required for applications where diverse properties of the particles are exploited, such as their hardness, abrasive resistance, thermal conductivity, chemical resistance, chemical catalysis, electrical and electro chemical behavior and diverse other properties, for example. Of particular interest are particulate substrates for use as abrasives where coatings of particular composition, structure and thickness range can enhance the performance of the abrasives. This may be due to improvements in protection of the abrasive particles from reactions with their environment, such as the bonding materials used to form the abrasives into diverse tools and the gaseous environments present during the manufacture of such tools.

Another great benefit of appropriate coatings on abrasive materials is where the coat provides enhancement of bonding of the abrasive material into the matrix or bonding media of the tool or tool materials. Improved bonding of abrasives into their matrices or tool bond materials usually leads to improvements in abrasive particle retention, which in turn leads to increases in abrasive behavior and efficiency.

Of particular interest as particulate substrates are ultrahard abrasives, made of materials greater then 40 GPa hardness, as measured by hardness measuring techniques known in the art, such as diamond, cubic boron nitride (cBN) and boron suboxides and the like and hard abrasives such as silicon carbide, aluminium oxide, boron carbide and the like.

Of further interest are particulate substrates that together with their coats (and or other added components) can form the main component of hard metal cermets, such as cemented hard metals, including tungsten carbide/cobalt, titanium carbide/nickel and the like.

Further still of interest are coated particulate substrates that on sintering of the coat material lead to composite materials comprising particles in a matrix, the matrix being derived from the sintered coats. Composites where the particulate substrates forming the particles in the matrix are diamond, cBN and combinations of these materials, as taught generally in PCT application WO 2006/032984, are part of the scope of interest of the present invention.

The general coating materials of interest are ceramics, glasses, metals, intermetallic materials, glass/ceramics, cermets and combinations of these material types.

A preferred general method of providing such a primary coat is to use particle suspension methods as disclosed in PCT applications WO2006/032984, WO2006/032982 and PCT/IB2007/000234, the contents of which are incorporated by reference. In these disclosures, particulate substrates such as diamond and cBN have their surface chemistry prepared so that they can efficiently be coated in mainly alcoholic suspensions, using preferably so called sol-gel chemical coating protocols. In particular the latter reference discloses the formation of silica glass coats on to fine particulate ultrahard materials such as diamond and cBN and the like.

The preferred method to produce the primary high surface area coat, includes the employment of metal alkoxide reactions with water in alcoholic solution, in conjunction with often simultaneous polycondensation reactions to coat the suspended particulate substrates in microporous amorphous oxide coats.

These primary high surface area microporous amorphous oxide coats can be conveniently dried and heat treated at low temperatures (typically below 500⁰C dependent upon the particular oxide compounds under consideration), to partially or completely crystallize the coat to form a still highly micro-porous polycrystalline structure with nano sized crystallites (less than 0.1 micron i.e. 100 nm).

As taught in PCT application WO2006/032984, the amorphous and nano- crystalline oxide coats can be converted into non oxide nano-crystalline ceramic materials, such as nitrides, carbides and carbo-nitrides by temperature programmed reactions in reactive gases such as ammonia and hydrocarbon gases respectively.

An aspect of the present invention is that high micro-porosity and specific surface area of these altered primary coats is maintained wholly or in part, and can be utilized for further secondary coating. Usually the specific surface area of such coats is reduced when treated at high temperature to reactively convert them to other material types, due to crystallization and/or sintering effects. However, despite this, substantially increased specific surface areas well above that of the particulate substrates is maintained and can be exploited on secondary coating.

Another important aspect of the invention is the use of the secondary coating technologies and procedures to be applied to the primary coated particulate substrate materials to exploit their high and augmented specific surface areas.

These secondary coating technologies and procedures may be divided into two broad groups, namely 1 ) those which involve exposure to liquid agents and reactants and 2) those which employ gaseous or vapor agents and reactants.

The group 1 ) secondary coating technologies and procedures employed in the present invention involve the exposure of primary coated high specific surface area particulate materials held in liquid suspension to chemical protocols that can provide the nucleation and growth of solid desired material on and at the surface of the open porosity of the primary micro- porous coats.

The protocols include the precipitation and crystallisation of salts and solutes from solution in the liquid suspension media by evaporation of the liquid media or lowering the temperature for crystallization of the solute. The chemical protocols also include the nucleation and growth of solid material on the high surface area of the primary coats by virtue of reactions caused to happen from reactants in solution in the liquid suspension media. The precipitated solid material in the open porosity of the primary coat can fill wholly or in part the porosity. In this way, interpenetrating structures of primary and secondary coat materials result.

The group 2) secondary coating technologies and procedures employed in the present invention involve the exposure of primary coated high specific surface area particulate substrates to gaseous or vapor precursor agents in diverse furnace arrangements and environments. These gaseous or vapor agents are able to penetrate or infiltrate the open porosity of the primary coats, dissociate and/or react with the surface of the porosity and in this way deposit the secondary material in and on the primary coats. In this way the open porosity of the primary coats is filled wholly or in part with the secondary coat material and thus interpenetrating structures can result. A general term for this group of procedures is "chemical vapor infiltration methods" (CVI methods).

Where the gaseous, vapor agent can pass through the open porosity of the primary coat and reach the particulate substrate surface, reactions between the gas or vapor and or its dissociation product can occur with the substrate material to generate further useful material filling the porosity of the primary coat.

The specific surface area of a particulate material where the particles are assumed to be perfectly smooth spheres is given by: S = 6 / pd •(1)

Where S is the specific surface area, p the density of the particulate substrate material and d the diameter of the particles.

Table 1 below gives some exemplary calculated, theoretical specific surface area values using equation (1 ) for a range of mean particle sizes for diamond, a low density particulate substrate material of density 3.5 g cm^"1, and tungsten carbide, WC, a high density particulate substrate material of density 15.8 g em^"1.

TABLE 1

The values in Table 1 may be used as an approximate indication of measured specific surface areas. Real particulate materials are rarely perfect spheres, have surface textures or roughnesses and have particle sizes distributed about some mean. All of these aspects contribute to departures from calculated values such as in Table 1 , mainly on the larger value side. Nevertheless, the measured values are usually within a factor of 2 to 3 of these values.

The specific surface area of a particulate substrate material, with a micro- porous primary coat may be approximately calculated by:

S_op = aS_p + bS_c .(2) Where S_cp is the specific surface area of the particulate material with it's primary coat, and S_p and S_c are the specific surface areas of the particulate substrate material and the primary coat material, respectively, and a and b are the mass fractions of the particle substrates and the coat material, respectively, which make up the composition of the mass of primary coated material.

So, a + b = 1 (3)

The specific surface areas of alkoxide sol-gel derived micro-porous materials can typically range from 50 to 500 m²g^'1 or more, dependent upon the details of the chemical procedures undertaken and the additives employed. For example both titania, TiO₂ and mixed titania alumina microporous materials can be produced with specific surface areas of about 350 m²g^"1 (ref.1).

An exemplary range of coat compositions expressed in mass percentage of coating materials may typically be from 3 to 30%.

It may be seen from Table 1 that diamond and tungsten carbide particulate substrates of the granular type, say of mean diameter close to 1 mm, will have specific surface areas of approximately 1.7 x 10^"3 and 0.4 x 10^"3 m²g^"\ respectively. Table 2 below presents the calculated specific surface areas for such particulate substrates, using equations (2) and (3), for 3 and 30 wt% coats of two exemplary very high specific surface area primary coat materials of 350 and 500 m²g^"1. TABLE 2

Table 2 demonstrates that granular particulates of the order of hundreds of microns to 1000 microns (1 mm), with their normal very low specific surface areas of about 10^~3 m²g^"1 (see Table 1 ), can typically by the method(s) of the present inventions have their specific surface areas increased to values of about 10 to 150 m²g^"\ depending on choice of composition. This is an increase of between four to five orders of magnitude in specific surface area.

Diamond particulate material with high surface area primary coats has particular utility for further secondary coating in order to produce coated granular abrasives for purposes such as stone and concrete sawing applications.

It may be seen from Table 1 that diamond and tungsten carbide particulate substrates of mean diameter of close to about 1 to 2 microns will have specific surface areas of the order of 1 to possibly 5 m²g^"1 if the comments concerning real materials with their real size distributions shapes and surface roughnesses are taken into account. Table 3 below presents the calculated specific surface areas for such particulate substrates, using equations (2) and (3), for 3 and 30 wt% coats of two exemplary very high specific surface area primary coat materials of 350 and 500 m²g^"1.

TABLE 3

Table 3 demonstrates that particulate substrates of about 1 to 2 microns in average diameter may have their specific surface areas increased by one to two orders of magnitude by virtue of the method(s) of the present invention. The resulting coated particulate materials may be of significant utility in their own right and can be of great value for subsequent coating techniques and methods.

It may be seen from Table 1 , taken with the associated comments to Table 1 concerning real size distributions, particle shapes and surface roughnesses, that particulate substrates in the nano size range (10 to 100 nm), can have specific surface areas of about ten to a few hundreds of m²g^"1. Table 4 below presents the calculated specific surface areas for such particulate substrates, using equations (2) and (3), for 3 and 30 wt% coats of two exemplary very high specific surface area primary coat materials of 350 and 500 m²g^'1.

TABLE 4

It may be seen from Table 4 that diamond particulate substrates of about 100 nm size can typically have their specific surface area increased from about 20 to between 30 to 120 m²g^"1 by coating with a high specific area primary coat of up to 500 m²g^"1 and at a level up to 30% by weight (mass fraction of 0.3). Similarly, diamond particulate substrates of about 10 nm size can typically have their specific surface area increased from about 200 to between 205 and 290 m²g^'1 by coating with a high specific area primary coat of up to 500 m²g^"1 and at a level up to 30% by weight (mass fraction of 0.3).

Tungsten carbide particulate substrates of about 100 nm size can typically have their specific surface area increased from about 5 to between 15 to 154 m²g^"1 by coating with a high specific area primary coat of up to 500 m²g^~1 and at a level up to 30% by weight (mass fraction of 0.3). Similarly, tungsten carbide particulate substrates of about 10 nm size can typically have their specific surface area increased from about 50 to between 60 and 185 m²g^"1 by coating with a high specific area primary coat of up to 500 m²g^"1 and at a level up to 30% by weight (mass fraction of 0.3).

Generally, as can be seen from Table 4, the particulate substrate materials, even when they are in the nano size scale of 10 to 100 nm, with associated high specific surface areas, can have their specific surface areas significantly increased by the application of primary coating materials of high specific surface areas as disclosed in the present invention. Clearly even for nano sized particulate substrate materials increases of specific surface areas of up to an order of magnitude can be expected, from tens to hundreds of m²g^~1.

An exemplary, but not comprehensive group of some liquid based secondary coating methods and procedures for application to the high specific surface area primary coated materials includes the precipitation by evaporation of a suspension of the primary coated particulate material in a water or alcohol solution of metal salts such as metal nitrates, chlorides, bromides, iodides, carbonates, hydroxides, sulphates, acetates, oxalates, tungstates, molybdates, and the like. In general all inorganic or organic or complex soluble salts that may be crystallized by evaporation of the solvent may be employed. The metals which form the cations of these salts include, but are not restricted to, Li, Na, Mg, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Zr, Nb, Mo, Pd, Ag, Cd, Ta, W, Pt, Au and many others.

The crystallized salts so formed crystallize in the micro open porosity of the primary coat and so form a very intimate combination of the primary and secondary coat materials, often a nano, nano intimate combination. On further heat treatment in chosen gaseous environments, the metal salts may be converted into other materials. Some salts when heated in oxidizing environments such as air, for example, can be converted into metal oxides, for example nickel nitrate forms nickel oxide. In this way the secondary coat component can be metal oxides. Some salts when heated in reducing gaseous environments such as hydrogen, for example, can be reduced to the metal, for example cobalt chloride will form cobalt metal decorating the surfaces of the micro-porous primary coat material. An exemplary, but not comprehensive list of metals that can be introduced into the open porosity of the primary coats in this way includes Fe, Co, Ni, Cu, Zn, Mo, Pd, Ag, W, Pt, Au and combinations of these metals. The metals so combined within the primary coat are usually nano sized. The internal surfaces of the primary coat can be decorated in very fine or nano metals of this nature and still remain substantially open in porosity with an attendant high surface area. Further, some salts when exposed to reactive gases at appropriate temperatures, such as ammonia or methane, for example, can be converted into nitrides and carbides, respectively.

Another exemplary group of liquid based secondary coating methods and procedures for application to the high specific surface area primary coated materials includes the precipitative reactions of soluble reagents in the suspension liquid of the particulate primary coated high specific surface area material.

An example of a type of reaction of interest includes the hydrolysis reaction of metal salts, such as tantalum chloride (TaCI₅), with water in alcohol suspension, solvents to yield tantalum pentoxide (Ta₂O₅) precipitated in the open porosity of the primary coat. On heat treatment in the appropriate gaseous environment the tantalum oxide may be converted into tantalum metal (reduction in hydrogen), tantalum nitride (reaction in ammonia) or tantalum carbide (reaction in carbonaceous gas, hydrogen mixtures).

Another reaction type of interest is exemplified by the reaction of ammonium paratungstate (H_4ON_1OO_4IW₁₂JH₂O) solution with acids such as nitric or hydrochloric acid in water solution, suspension. This yields tungstic oxide (WO₃) distributed in the open porosity of the primary high specific surface area coat. Similarly the tungstic oxide will convert to tungsten metal, tungsten nitride or tungsten carbide by appropriate reactive gas heat treatment. Yet another type of reaction is exemplified by the reaction of chromium nitrate (Cr(NO₃).9H₂O) with ammonium hydroxide in water suspension, solution to yield hydrated chromium hydroxide (Cr(OH)₃) precipitated into the open porosity of the primary coat. Chromia (Cr₂O₃), chromium nitride (Cr₂N) or chromium carbide (Cr₃C₂) can be generated by heat treatment in the appropriate gaseous atmosphere.

An exemplary but not comprehensive group of secondary coating procedures involving chemical vapor or gaseous means includes those where the primary coated particulate material is heated in a mixture of powders comprising a so called donator and an activator. The donator powders usually consist of a source metal or alloy of the element to be deposited into the porosity of the primary coat. The activator, usually a halide or a halide salt, reacts with the metallic powder(s) producing vapors of volatile compounds which infiltrate the open porosity of the primary coat material (Ref. 2). The volatile compound then dissociates and deposits the metal element into the open porosity. Where the open porosity of the primary coat extends to the surface of particulate substrate materials, the deposited metal may react with some of the substrate material to form other compounds.

A particular example of this method(s) is where the high surface area primary coat is combined with titanium metal power as donator in combination with ammonium chloride powder (NH₄CI) as activator and the mixture heated to a temperature between about 600 and 1100⁰C in a hydrogen, argon gaseous environment. The ammonium chloride reacts with the titanium metal to form volatile titanium chlorides (TiCI₄ and TiCI₃), which then infiltrate the open porosity of the primary coat and dissociate depositing titanium metal into the open porosity.

Where the particulate substrate material is diamond, for example, and the open porosity extends to the surface of the diamond particles, then, at the temperature of deposition of titanium, the titanium reacts with some of the diamond to form titanium carbide. At these temperatures the diffusion of carbon in titanium carbide is sufficiently high for all the deposited titanium in the open porosity to be converted into titanium carbide. In this way a very intimate combinations of the primary coat materials and titanium carbide on diamond particulate substrates can be generated. The intimate interpenetrating primary coat material, titanium carbide combination is usually a nano, nano combination.

References

Ref. 1 , S. R. Kumar et al., "Synthesis of thermally stable, high surface area anatase-alumina mixed oxides", Materials Letters 43, (2000), 286-290.

Ref. 2, G. N. Angelopoulos et al., "Thermochemical aspects on the formation of Ti-bearing coatings in a fluidized bed CVD process", Surface and Coating Technology, 78, (1996), 72-77.

Claims

1. A method of increasing the specific surface area of a particulate substrate comprises coating the particulate substrate with a microporous primary coat material.

2. A method according to claim 1 , wherein the microporous primary coat material has an open porosity characterized by a mean pore size distribution that is less than 100nm.

3. A method according to claim 1 or claim 2, wherein the specific surface area of the primary coated particulate substrate is between that of the uncoated particulate substrate and about 300 m²g^"1.

4. A method according to claim 1 or clam 2, wherein the specific surface area of the primary coated particulate substrate is between that of the uncoated particulate substrate and about 1000 m²g^"1.

5. A method according to any one of claims 1 to 4, wherein the microporous primary coat material has an open porosity characterized by a substantial portion of the open pores extending from an external surface of the primary coat material to the surface of the particulate substrate.

6. A method according to any one of claims 1 to 5, wherein the particulate substrate is selected from diamond, cubic boron nitride, silicon carbide, alumina, boron carbide and tungsten carbide.

7. A method according to any one of claims 1 to 6, wherein the primary coat material is selected from amorphous or nanocrystalline oxides, nitrides, carbides, oxynitrides, oxycarbides and carbonitrides of titanium, silicon, zirconium, aluminium, tantalum, vanadium, niobium, tungsten, molybdenum, and any combination thereof..

8. A method according to any one of claims 1 to 6, wherein the primary coat material is selected from titanium oxide, titanium nitride, titanium carbide, silica, alumina, zirconia, tantala, niobia, vanadia, chromia, tungstic oxide and any combinations thereof.

9. A method according to claim 8, wherein the primary coat material comprises a combination of titania and tungstic oxide.

10. A method according to claim 8, wherein the primary coat material comprises a combination of titania and tantalum oxide.

11. A method according to claim 8, wherein the primary coat material comprises a combination of titania and chromia.

12. A method according to claim 8, wherein the primary coat material comprises a combination of mixed nitrides.

13. A method accordipg to claim 12 wherein the mixed nitrides are selected from Ti_xW_yN, Ti_xTa_yN, or Ti_xCr_yN where the sum of the x and y is equal to unity.

14. A method of producing a coated particulate substrate including the steps of applying a microporous primary coat material to a particulate substrate by a method according to any one of the preceding claims and applying a secondary coat to the microporous primary coat.