Influence of diamond characteristics on the tribological behaviour of metals against diamond-coated Ti–6Al–4V alloy

The tribological behaviour in ambient air of the 100Cr6 steel and the Ti–6Al–4V alloy sliding against various diamond coatings deposited at 600°C on Ti–6Al–4V by microwave plasma is reported. It has been studied as a function of the diamond coating type on a rotating pin-on-disk tribometer. As-deposited rough polycrystalline, polished polycrystalline and as-deposited smooth fine-grained diamond coatings were tested. They were characterised by their surface roughness and their diamond quality defined as the relative amounts of sp3- and sp2-hybridised carbons incorporated in the coating. The strong influence of the diamond surface roughness has been evidenced whatever the counterface alloy. The counterface materials present, however, different tribological behaviours either against rough polycrystalline diamond coatings or fine-grained ones. These behaviours also depend on the diamond quality. Different friction and wear mechanisms are discussed as a function of these parameters. The advantages of fine-grained diamond coatings with respect to polycrystalline coatings are emphasised. Very low wear rates of the alloys and low friction coefficients, in the 0.05–0.1 range, were obtained for the smoothest coatings, even under heavy load. The great adhesion of the coatings has been deduced from the absence of coating failure, crack and stress relaxation in the disk track.