Microstructure and deformation behavior of biocompatible TiO₂ nanotubes on titanium substrate

Titanium oxide coatings have been shown to exhibit desirable properties as biocompatible coatings. We report on the quantitative microstructure characterization and deformation behavior of TiO₂ nanotubes on Ti substrate. Nanotubes were processed using anodic oxidation of Ti in a NaF electrolyte solution. Characterization of the as-processed coatings was conducted using scanning electron microscopy and focused ion beam milling. Increases in anodization time had no significant effect on tube diameter or tube wall thickness. Coating thickness, however, increased with time up to 2 h of anodization, at which point an equilibrium thickness was established. Nanoindentation was used to probe the mechanical response in terms of Young's modulus and hardness. Progressively higher values of elastic modulus were obtained for thinner films consistent with increasing effects of the Ti substrate. A possible deformation mechanism of densification of the porous oxide and wear of the dense surface is suggested and discussed.