Microwave-plasma-CVD of diamond coatings onto titanium and titanium alloys

Abstract

CVD-diamond films have been deposited onto pure titanium and Ti-6Al-4V substrates. It was found that with optimized processing conditions, these films show very good adhesion, even though they are highly stressed (−4 to − 6GPa) and relatively thick (4–6 μm). 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 lead to a value of G=95 J m −2, which is comparatively high. The internal compressive stresses which developed in the diamond films are about 4 GPa lower than expected based on calculations. Several stress relief mechanisms were identified. These include the appearance of tensile growth stresses, the formation of a titanium carbide interlayer, plastic flow in pure titanium substrates and the effect of phase transformations (β-titanium→α-titanium + hydride) in both substrates while cooling. In addition, the application of a cooled sample stage induces a thermal gradient in the titanium base substrate during the coating process with the result of lower average specimen temperature. This leads to smaller contraction and lower film stresses during cooling. The coating process affects the microstructure and mechanical properties of the titanium base substrate. Pure titanium seems to be more sensitive against hydrogen at elevated temperatures than Ti-6Al-4V. The alloy Ti-6Al-4V possesses a CVD-diamond processing window up to deposition temperatures of 770 °C where no severe changes in the grain structure occur. In principle, dissolved hydrogen and hydride precipitations in Ti-6A1-4V and in pure titanium can be removed after the coating process by a vacuum furnace annealing. Using this procedure, mechanical properties of titanium base substrates can be almost completely restored after CVD-diamond deposition.