Deposition, structure, mechanical properties and tribological behavior of polycrystalline to smooth fine-grained diamond coatings

Polycrystalline and smooth fine-grained diamond (SFGD) coatings have been deposited at 600°C on Ti alloys by a two-step microwave PACVD procedure using CO–H2 or CO2–CH4 during the second step. Optical emission spectroscopy revealed important differences between the plasma species produced with different inlet mixtures. As a function of the deposition conditions, the surface roughness could be decreased from approximately 120 nm (RMS) for polycrystalline coatings to 35 nm for the best SFGD coating and to the 30–15-nm range while the sp2-carbon incorporation increased. The different microstructures and the diamond purity are shown to influence the hardness, the Youngʹs modulus and the residual stresses of the coatings. The tribological behavior in ambient air of 100Cr6 steel, Ti-6Al-4V alloy and diamond-coated Ti-alloy sliding against all coating types including some polished polycrystalline coatings evidences the strong influence of the surface roughness. The counterface materials present, however, differ in tribological behaviors, which also depend on the diamond quality. Different friction and wear mechanisms are discussed as a function of these parameters. The advantages of SFGD with respect to polycrystalline coatings are emphasized. 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 fairly heavy loads.