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 d eposited at 600 degreesC on Ti alloys by a two-step microwave PACVD procedu re using CO-H-2 or CO2-CH4 during the second step. Optical emission spectro scopy revealed important differences between the plasma species produced wi th different inlet mixtures. As a function of the deposition conditions, th e surface roughness could be decreased from approximately 120 nm (RMS) for polycrystalline coatings to 35 nm for the best SFGD coating and to the 30-1 5-nm range while the sp(2)-carbon incorporation increased. The different mi crostructures and the diamond purity are shown to influence the hardness, t he Young's modulus and the residual stresses of the coatings. The tribologi cal behavior in ambient air of 100Cr6 steel, Ti-6Al-4V alloy and diamond-co ated Ti-alloy sliding against all coating types including some polished pol ycrystalline coatings evidences the strong influence of the surface roughne ss. The counterface, materials present, however, differ in tribological beh aviors, which also depend on the diamond quality. Different friction and we ar mechanisms are discussed as a function of these parameters. The advantag es of SFGD with respect to polycrystalline coatings are emphasized. Very lo w wear rates of the alloys and low friction coefficients, in the 0.05-0.1 r ange, were obtained for the smoothest coatings, even under fairly heavy loa ds. (C) 2001 Elsevier Science B.V. All rights reserved.