Characterisation of nanostructured diamond coatings on various hardmetal surfaces

Thin nanostructured diamond films were deposited onto WC–6%Co hardmetal substrates by microwave plasma chemical vapour deposition (CVD) to get improved wear resistance and tribological behaviour. The substrate surface was always modified by diamond grinding, sandblasting and/or etching. Additionally, different methods of mechanical pre-treatment like ultrasonically induced cavitation or shot peening with Al2O3 and SiO2 particles were used to optimise the adhesion between diamond coating and cemented carbide cutting tool inserts.The influence of cavitation charging and shot peening with hard particles on work hardening effect of the substrate and coating adhesion at the interface diamond/hardmetal was investigated. The effects of the mechanical pre-treatment were examined by measuring Rockwell and Vickers hardness as well as roughness, crack initiation and Co-content using scanning electron microscopy (SEM) and energy dispersive X-ray diffraction (EDX) before and after diamond coating.After cavitation charging no strengthening of the hardmetal surface could be deduced and the hardness was slightly reduced by surface disruption, whereas the peening shows no measurable influence on the hardness values. The enhanced roughness with both treatments modifies the adhesion of the diamond coating to the WC–Co substrate. Because of the influence of hardness, strengthening, roughness, crack building and Co content or residual parts of the peening particles, the optimisation of the coating system diamond/hardmetal is a very complex problem.