STRENGTH, FRACTURE AND EROSION PROPERTIES OF CVD DIAMOND

Theoretical and experimental studies have been made on the effect of high modulus coatings on the stress fields generated by indentation and impact onto a flat half-space. The theoretical work used finite-element techniques and it shows that a high modulus coating can have a significant effect on the maximum tensile stresses generated in the substrate providing there is a good bond at the coating/substrate interface. Because it is technically difficult to deposit layers of more than a few micrometres thickness without residual stresses causing debonding, double and multilayer systems have also been examined theoretically. A variety of techniques have been used to determine the strength, modulus, expansion coefficient, thermal conductivity and other physical properties of chemical vapour deposition CVD diamond layers. These are briefly reviewed and data from our own studies using such techniques as the vibrating reed, bulge test and indentation are present. The erosion properties of both CVD Coated substrates and CVD free-standing layers are presented for both liquid drop and solid particle erosion. Finally, a study has also been made of the frictional properties of various CVD diamond layers in a range of environments; data are compared with our earlier work on bulk diamond.