SOLID PARTICLE IMPACT OF CVD DIAMOND FILMS

Studies of the dynamic impact and static indentation of chemically vapour deposited (CVD) diamond films are described. Polished and unpolished CVD diamond films of thickness approximately 6 to 15-mu-m, deposited on both silicon nitride and silicon substrates, were investigated. In the dynamic impact experiments, sand particles with impact velocities of 34 m s-1 and 59 m s-1 were used. Three stages of impact damage were identified: (I) formation of ring cracks; (II) partial debonding and penetration of the film; and (III) full debonding and film removal. The first stage is similar to that occurring with natural diamond. However, the last stage, i.e. the material loss mechanism for CVD films, is mainly caused by delamination of the film. Examination of fracture surfaces using scanning electron microscopy showed that transgranular fracture of the diamond films was quite common despite the polycrystalline nature of the films. This suggests that the bonding strength between diamond grains is as strong as that within the grains. Finally, static indentation with a tungsten carbide ball of similar size to the sand particles was performed to determine the fracture strength of polished CVD diamond film and to make a comparison with the impact experiments. The fracture tensile strength for CVD diamond films was found to be approximately 2.8 GPa, which is much lower than the reported value of 8.6-12 GPa for natural diamonds.