Diamond and other hard materials in fibre reinforced ceramic matrix composites

In this paper the development and manufacture of ceramic matrix composites (CMCs) which are suitable for grinding and cutting tools is described. Therefore, a long fibre reinforced composite material with a silicon carbide matrix bearing the hard material such as diamond or boron carbide was developed on the basis of the liquid silicon infiltration of carbon/carbon (LSI process). In a first step the hard particles were coated by matrix resin. Then sheets of preimpregnated C-fabrics and coated hard particles were laid up in a mould and subsequently consolidated by pressing or by using autoclave techniques. After pyrolysis of the polymer composite, the resulting porous C/C preform bearing the hard particles was infiltrated with liquid silicon yielding a SiC matrix, which bonds the C-fibres and hard particles tightly. This kind of composite has a sandwich like structure where the long C-fibres provide good strength and fracture toughness, whereas the hard particles offer excellent wear resistance and hardness. It is well known that diamond is prone to graphitization beyond 1200 degreesC and easily reacts with liquid Si. It was found, however, that diamond was still present after LSI processing even at temperatures of 1650 degreesC. Furthermore, the resulting CMCs as well as the intermediate products were characterized by microscopy and mechanical testing.