Tribological performance of diamond-coated Ti–6Al–4V alloy with respect to diamond characteristics

Polycrystalline and smooth fine-grained diamond coatings with different characteristics were deposited on Ti–6Al–4V at 600°C in a microwave plasma. Surface roughness of as-deposited polycrystalline and fine-grained diamond coatings was in the 15–140 nm (r.m.s.) range. Some polycrystalline coatings were polished after deposition to a final roughness of 5 nm. Sliding experiments were conducted in ambient air with a rotating pin-on-disk tribometer. The friction coefficient of uncoated Ti–6Al–4V pins during sliding against the two types of diamond coatings was in the 0.05–0.1 range after a run-in period. The fine-grained diamond coatings provided low friction coefficients and low wear rates to the titanium counterbodies. All the results showed the great influence of the surface roughness and the influence of the diamond quality on the tribological results. Sliding tests between two diamond-coated counterfaces evidenced a self-polishing mechanism leading to ultra smooth wear tracks and a low final wear rate. The advantages of the fine-grained diamond coatings with respect to the polycrystalline coatings were also pointed out in that case. The great adhesion of the coatings was deduced from the absence of coating failure, crack and stress relaxation in the disk track even at high load.