WO2005078040A1 - Coated abrasives - Google Patents

Abstract

A coated super-hard abrasive comprises a core of super-hard abrasive material, which is typically diamond or cBN based, an inner layer of a metal carbide, nitride, boride or carbonitride chemically bonded to an outer surface of the super-hard abrasive material and an outer layer of a metal carbide, nitride or boride physically deposited on the inner layer, the metal in both layers being the same. The outer layer is preferably applied by physical vapour deposition. The inner and outer layers are both preferably titanium or chromium carbide coatings in the case of a diamond abrasive core, or titanium or chromium nitride or boride coatings in the case of a cBN abrasive core.

Description

COATED ABRASIVES

BACKGROUND OF THE INVENTION

This invention relates to coated abrasives, a process for their production, and to coated abrasives for use in abrasive-containing tools.

Abrasive particles such as diamond and cubic boron nitride are commonly used in cutting, grinding, drilling, sawing and polishing applications. In such applications, abrasive particles are mixed with metal powder mixes, then sintered at . high temperatures to form bonded cutting elements. Typical bond matrices contain iron, cobalt, copper, nickel and/or alloys thereof.

Common problems in applications are retention of particles in the bond matrix, and resistance against oxidative attack on the substrate during the sintering process and the subsequent application.

These problems are commonly addressed by coating the abrasive particles with metals or alloys which bond chemically to the particle, and alloy to the bond matrix. Typically, chemical vapour deposition (CVD) or physical vapour deposition (PVD sputter coating) techniques are used. Titanium carbide is an example of a material that has been proposed as a coating for abrasive particles, because of its good adhesion to diamond. Chromium carbide is a similar coating material that can be used.

A problem with the use of titanium carbide coatings where the bond matrix contains bronze or Cu is that these materials tend to react with the titanium carbide, such that it may be reacted away thus reducing or nullifying the effect of the coating. The diamond particles are then also susceptible to graphitisation of the diamond particle surfaces, where the bond matrix contains metals that are typically used as solvent/catalysts for diamond synthesis. Examples of such metals are Fe, Co and Ni. In the molten state, these metals are capable of dissolving diamond, which precipitates to form graphite. This process of graphitisation of the diamond surface not only weakens the particles but may also result in poorer retention of the particles in the bond.

During manufacture of cutting tools, for example during sintering of saw segments containing diamond particles, oxygen may be present as surface oxides, dissolved oxygen in the metal powders that form the bond matrix, or in gaseous form in the atmosphere or as a consequence of application of the titanium carbide coating itself. At the sintering temperatures this oxygen is liable to attack , the surface of the diamond particles, which weakens the particles.

Further, in order for the coating to protect the diamond particles, it has to form a barrier between the bond matrix and the particles. In other words, it should be impermeable and dense, so that components of the bond matrix are unable to pass through and make contact with the particle surface. One way the components could pass through the coating is by solid-state diffusion through the coating. Alternatively, if the coating is incomplete, cracked or porous, components may pass through the coating to reach the particle surface. A coating may initially be dense and impermeable, but during the sintering process, a phase change may occur due to alloying with the bond matrix, for example, which results in the formation of a less dense alloy, or perhaps a porous coating, which allows passage of the bond matrix components through the coating to the particle surface.

The failure modes mentioned above are typically time dependent. For shorter sintering times, there may not be sufficient time for the problem to develop. Under aggressive sintering conditions, for example lengthy sintering times or high sintering temperatures, these failure modes may become apparent. As a consequence, traditionally applied titanium carbide coatings are limited in their application. SUMMARY OF THE INVENTION

A coated super-hard abrasive comprising a core of super-hard abrasive material, an inner layer of a metal carbide, nitride, boride or carbonitride chemically bonded to an outer surface of the super-hard abrasive material and an outer layer of a metal carbide, nitride or boride physically deposited on the inner layer, the metal in both layers being the same.

The outer layer is preferably applied by physical vapour deposition.

The ultra-hard abrasive material is typically diamond or cBN based, and may include diamond or cBN grit, PCD substrates, thermally stable PCD (TSPCD) substrates, PcBN substrates, CVD diamond film, single crystal diamond substrates.

The inner and outer layers are both preferably titanium or chromium carbide coatings in the case of a diamond abrasive core, or titanium or chromium nitride or boride coatings in the case of a cBN abrasive core, although other metals such as vanadium, molybdenum, tantalum, indium, zirconium and niobium, for example, could also be used.

The inner coating is typically about 0.05 μm to about 6 μm in thickness, preferably about 0.2 μm to about 3 μm.

The outer layer is typically thicker than the inner coating, preferably from about 0.05 μm to about 10 μm, in particular about 0.2 μm to about 6 μm.

DESCRIPTION OF PREFERRED EMBODIMENTS

Whilst the invention extends to various forms of coated abrasive material, it will in the most part be described with reference to the coating of diamond grit for convenience. ^■ Ti in the form of titanium carbide or titanium nitrides and borides have been shown to be useful coating materials for diamond and cBN substrates, respectively. They are particularly useful because of their ability to bind chemically to the substrate and to protect the substrate. However, as has been mentioned previously, they are not suitable in some applications, particularly where they are sintered in aggressive sintering conditions in the presence of bronze or copper, and where the bond matrix contains ferrous metals, for example, or in the presence of oxygen

It has been found that the advantages of titanium based coatings can be extended to other applications utilising diamond grit where a second titanium coating is applied over the titanium based coating layer. This is particularly the case where diamond grit is used in a metal bond matrix containing ferrous metals to form an abrasive tool component upon sintering. It is also useful where the titanium carbide coating, in the case of diamond particles, would be reacted away by a constituent of the metallic material, for example by the bronze and copper phases used in the brazing of the material to another metallic or ceramic material, or by the bronze or copper phases used in sintering or used as an infiltrant in infiltrating a powder to form an infiltrated powder material.

The outer titanium carbide layer is not chemically bonded to the inner layer because such bonding is not necessary. The purpose of the outer layer is not improved adhesion but rather as a barrier for protecting the inner layer and substrate. Careful control of the composition of the inner portion of the outer layer allows acceptable bonding to occur by reducing the buildup of mismatch stresses. The thickness of the outer layer may vary according to requirements, preferably having a thickness of about 0.05 μm to about 10 μm, in particular about 0.2 μm to about 6 μm, giving a total titanium carbide coating thickness of about 0.1 μm to about 16 μm, in particular about 0.4 μm to about 9 μm. In addition, whilst the inner layer may contain oxygen due to the process for applying it, the outer layer is free of oxygen. Further, whilst the inner layer is generally sub-stoichiometric, it is possible to vary the stoichiometry of the outer layer to be the same as that of the inner layer, or to be different. The inner layer could also have gradients of Ti:C due to diffusion of carbon therethrough, whereas the composition of the outer layer would not have gradients unless specifically introduced. These gradients are introduced in order to minimize mismatch stresses occurring at the interface with the inner layer, while simultaneously presenting the most desired composition to the matrix material for coat-to- matrix bonding when sintered into the matrix.

It is especially useful in the making of diamond impregnated tools such as segments for saw blades, drills, beads for diamond wires especially where high amounts of bronze or copper limit the usefulness of traditional titanium carbide coatings, the making of brazed diamond layer tools such as brazed diamond wire beads, the making of diamond containing metal matrix composites, brazing of diamond materials such as affixing TSPCD, PCD and diamond drillstones to a drill body, affixing CVD diamond, monocrystal, TSPCD and PCD to a saw blade, tool post, drill body and the like. The thicker titanium carbide coating is particularly useful as it is possible to tolerate longer reaction times and higher sintering temperatures and greater amounts of diffusion of otherwise problematic reactants. Attempting to obtain such a thick titanium carbide coating in a single process, such as a CVD process, would be practically difficult due to the inherent limitation of carbon diffusion from the diamond substrate, resulting in a very lengthy process. In addition, as the diamond substrate is the source of carbon, such a single step process could result in too much diamond being removed, which may weaken it. Finally, such a thick layer would ordinarily have poor adhesion to the substrate as result of mismatch stresses at the diamond-to-coat interface. The present coated diamond substrates overcome these difficulties. They are therefore rendered viable in almost any application.

The coated abrasive particles are preferably formed using a hot coating process for applying the inner layer and a cold process such as low temperature CVD or a PVD process for applying the outer layer. The diamond grit particles are those used conventionally in the manufacturing of metal bonded tools. They are generally uniformly sized, typically 0.1 to 10mm. Examples of such diamond grit particles include: micron grit 0.1 to 60 micron, wheel grit 40 micron to 200 micron, saw grit 180 micron to 2 millimeter, mono crystal 1 millimeter to 10 millimeter, CVD inserts of a few square millimeter to discs up to 200 millimeter diameter, PCD inserts of a few square millimeter to discs 104 millimeter diameter, cBN grit in micron range 0.1 to 60 micron, in wheel grit range 40 micron to 200 micron, PCBN inserts of a few mm to discs up to 104 mm diameter.

The diamond particles are first coated in a hot coating process to provide an inner layer, which is typically a titanium carbide layer. In the case of cBN, such inner coating would typically be a titanium nitride or boride or boronitride layer. In this hot coating process, the titanium-based coat is applied to the diamond substrate under suitable hot conditions for chemical bonding to take place. Typical hot coating technologies that can be used include processes involving deposition from a metal halide gas phase, CVD processes or thermodiffusion vacuum coating or metal vapour deposition processes, for example. Deposition from a metal halide gas phase and CVD processes are preferred.

In processes involving deposition from a metal halide gas phase, the particles to be coated are exposed to a titanium-halide in an appropriate gaseous environment (e.g. non-oxidising environments containing one or more of the following: inert gas, hydrogen, hydrocarbon, reduced pressure). The titanium halide may be generated from titanium metal as part of the process.

The mixture is subjected to a heat cycle during which the titanium-halide transports the Ti to the surfaces of the particles where it is released and is chemically bonded to the particles.

The outer layer of titanium carbide is deposited using a cold coating technique such as low temperature CVD or PVD, which is preferred. It is a low temperature process in that insufficient heat is generated to cause significant carbide formation. Hence, if used alone, it would result in relatively poor adhesion to the diamond particles. An example of a PVD process for applying the outer coating is reactive sputter coating to form titanium carbide or by direct sputtering of titanium carbide onto the inner layer.

Examples of coated abrasives of the invention include: i) Diamond saw grit coated with titanium carbide by halide gas titanium carbide coating, such as commercially available SDBTCH, followed by physical deposition (PVD) of titanium carbide. Used for producing abrasive segments for saws or drills, especially with high bronze matrices, when infiltration manufacturing process is used or when extreme sintering conditions (high temperatures and long sintering times) are used. ii) Diamond wheel grit or micron grit coated with titanium carbide by halide gas titanium carbide coating, such as commercially available PDA989TCA, followed by physical deposition (PVD) of a further layer of titanium carbide. Used for producing grinding wheels, especially when bronze bonds are used. iii) cBN wheel grit or micron grit coated with titanium nitride or boride (whether singly or in combination) by halide gas titanium based coating, followed by physical deposition of a further layer of titanium nitride. Used for producing grinding wheels especially when bronze bonds are used, or for vitreous bond wheels. iv) PCD cutting tool insert coated with titanium carbide by halide gas titanium carbide coating, followed by physical deposition (PVD) of a further layer of titanium carbide for brazing into a tungsten carbide blank. v) PCBN cutting tool insert coated with titanium nitride or boride (whether singly or in combination) by halide gas titanium based coating, followed by physical deposition of a further layer of titanium nitride for brazing into a tungsten carbide blank. vi) CVD or monocrystal cutting tool insert coated with titanium carbide by halide gas titanium carbide coating, followed by physical deposition (PVD) of a further layer of titanium carbide for brazing into a tungsten carbide blank, or as dresser logs for sintering or brazing into dresser posts.

This invention will now be described, by way of example only, with reference to the following non-limiting example.

EXAMPLE

Diamond grit from Element Six, 40/45 US mesh size, was coated in a CVD process to produce TiC coated diamond according to general methods commonly known in the art. The CVD TiC coated diamond was then used as the substrate for the second coating step.

2,000 carats of this TiC coated diamond, 40/45 US mesh size, was placed in a magnetron sputter coater with a rotating barrel and a large pure titanium metal plate as the target. The coating chamber was evacuated, argon was admitted and the power turned on to form plasma. Sputtering power was increased to 10A (400V) on target while rotating the barrel to ensure an even coating on all the diamond particles at 20sccm argon pressure. Methane gas was admitted to achieve an Optical Emission Measurement of 60%. Sputtering of titanium reacted with carbon was continued for 2 hours. The coated diamond was allowed to cool before removing from the chamber. An analysis of this coated diamond was undertaken, consisting of X-ray diffraction, X-ray fluorescence, Chemical assay of the coating, Optical and Scanning Electron Microscopy image analysis, and particle fracture followed by cross-sectional analysis on the SEM.

Visually, this coating appeared to have a dark grey metallic colour. The coating looked uniform and smooth and without any uncoated areas. Observation on the SEM showed an even coating with a somewhat rough morphology. A two-layer structure was not evident, the complete layer having a thickness of about 1 micron. This particular coating resulted in an assay of 1.6%. The TiC coating in this size used for this batch typically has an assay of 0.77%. The rest of the 1.6% is therefore attributable to the PVD TiC layer on top of the CVD TiC. When analysed using XRD, only TiC was found. XRF analysis showed 100% Ti.

Claims

1. A coated super-hard abrasive comprising a core of super-hard abrasive material, an inner layer of a metal carbide, nitride, boride or carbonitride chemically bonded to an outer surface of the super- hard abrasive material and an outer layer of a metal carbide, nitride or boride physically deposited on the inner layer, the metal in both layers being the same.

2. A coated super-hard abrasive according to claim 1 , wherein the outer layer is deposited by physical vapour deposition.

3. A coated super-hard abrasive according to claim 1 or claim 2, wherein the ultra-hard abrasive material is diamond or cBN based.

4. A coated super-hard abrasive according to claim 3, wherein the super-hard material is selected from the group comprising diamond or cBN grit, a PCD substrate, a thermally stable PCD (TSPCD) substrate, a PcBN substrate, a CVD diamond film, and a single crystal diamond substrate.

5. A coated super-hard abrasive according to claim 3 or claim 4, wherein the inner and outer layers are both titanium or chromium carbide coatiηgs in the case of a diamond based material, or titanium or chromium nitride or boride coatings in the case of a cBN based material.

6. A coated super-hard abrasive according to any one of the preceding claims, wherein the inner coating is from about 0.05 μm to about 6 μm in thickness.

7. A coated super-hard abrasive according to claim 6, wherein the inner coating thickness is from about 0.2 μm to about 3 μm.

8. A coated super-hard abrasive according to any one of the preceding claims, wherein the outer layer is thicker than the inner layer.

9. A coated super-hard abrasive according to claim 8, wherein the outer layer is from about 0.05 μm to about 10 μm in thickness.

10. A coated super-hard abrasive according to claim 9, wherein the outer layer thickness is from about 0.2 μm to about 6 μm.