MICROWAVE-PLASMA-CVD OF DIAMOND COATINGS ONTO TITANIUM AND TITANIUM-ALLOYS

CVD-diamond films have been deposited onto pure titanium and Ti-6Al-4V substrates. It was found that with optimized processing conditions, t hese films show very good adhesion, even though they are highly stress ed (-4 to -6 GPa) and relatively thick (4-6 mu m) The reason for this might be the formation of a strong TiC-diamond interface. Calculations for the critical energy release rate G before indentation testing lea d to a value of G = 95 J m(-2), which is comparatively high. The inter nal compressive stresses which developed in the diamond films are abou t 4 GPa lower than expected based on calculations. Several stress reli ef mechanisms were identified. These include the appearance of tensile growth stresses, the formation of a titanium carbide interlayer, plas tic flow in pure titanium substrates and the effect of phase transform ations (beta-titanium-->alpha-titanium + hydride) in both substrates w hile cooling. In addition, the application of a cooled sample stage in duces a thermal gradient in the titanium base substrate during the coa ting process with the result of lower average specimen temperature. Th is leads to smaller contraction and lower film stresses during cooling . The coating process affects the microstructure and mechanical proper ties of the titanium base substrate. Pure titanium seems to be more se nsitive against hydrogen at elevated temperatures than Ti-6Al-4V. The alloy Ti-6Al-4V possesses a CVD-diamond processing window up to deposi tion temperatures of 770 degrees C where no severe changes in the grai n structure occur. In principle, dissolved hydrogen and hydride precip itations in Ti-6Al-4V and in pure titanium can be removed after the co ating process by a vacuum furnace annealing. Using this procedure, mec hanical properties of titanium base substrates can be almost completel y restored after CVD-diamond deposition. (C) 1998 Elsevier Science S.A .