Residual stresses and debonding of diamond films on titanium alloy substrates

This paper is concerned with the adhesion of diamond films to titanium alloy substrates. A study is presented of the effect of prior substrate bombardment with carbon ions on the interfacial toughness. It is shown that this treatment raises the critical strain energy release rate of the interface substantially, from about 20 J m(-2) to about 73 J m(-2). These data have been obtained by depositing relatively thick (approximate to 5 mu m) diamond films on massive (1000 mu m thick) Ti-6Al-4V substrates and monitoring the temperature at which these films spontaneously debonded during post-deposition cooling. The net stress levels in the films at the point of debonding were obtained by summing the calculated thermal stresses and measured intrinsic (deposition) stresses. The intrinsic stresses were measured in parallel experiments using thin (approximate to 125 mu m) tungsten substrates. Diamond films were deposited on these substrates under identical conditions to those for the titanium alloy substrates. The curvatures exhibited after cooling to room temperature were measured and from these the net stress levels were established. Since the tungsten is resistant to inelastic deformation while these curvatures are established, this allows evaluation of the intrinsic stress, which is taken to be the same as that for deposition on the titanium alloy substrates.