Machining hardmetal with CVD diamond direct coated ceramic tools: effect of tool edge geometry

A new challenge for chemical vapour deposited (CVD) diamond tools is the machining of hardmetal, one of the most difficult tasks a tool has to accomplish due to the extreme hardness of the workpiece. The development of cutting tool inserts made via direct diamond deposition on silicon nitride ceramic substrates for machining WC-Co materials was recently pointed as an alternative to the conventional brazed CVD diamond tips. In the present work, silicon nitride round inserts having different edge geometry, namely sharp, honed and chamfered edges, were produced by pressureless sintering. Turning of hardmetal containing 25 wt.% Co was conducted in a numerically controlled lathe with ∼15 μm thick CVD diamond coatings. The effects of depth of cut (0.1 to 0.3 mm), feed rate (0.03 to 0.3 mm·rev−1) and wear on the cutting forces were monitored online using a dynamometer and were related to the surface finishing of the workpiece. Honed tools were more prone to diamond film delamination from the cutting edge than the chamfer or sharp edge ones. Adequate finishing quality (Ra<0.2 μm) can be achieved with the sharp edge tools while machining tolerances are respected.