MICROSTRUCTURE OF DIAMOND FILMS NEAR THE INTERFACE WITH WC SUBSTRATE

A diamond-coated cutting tool has been developed by plasma chemical vapour deposition with H2-CH4. The adhesion strength of the diamond film is remarkably improved by predecarburizing the surface of the WC substrate to withstand severe cutting. To investigate the effects of decarburization on the adhesion strength, the microstructure of the diamond film near the interface with the substrate was observed by scanning and transmission electron microscopy. A rugged surface with a large amount of fine roughness is generated on the decarburized substrate after diamond deposition, while the nondecarburized surface remains flat. A thin graphite-like non-diamond carbon layer is observed near the tip of the diamond film contacted to the substrate. No significant difference in the amount of non-diamond carbon is found between substrates with and without decarburization. Therefore the main effect of predecarburization is considered to be mechanical reinforcement of the adhesion strength caused by an increase in contact area between the diamond film and the substrate.