Interfacial structure of the firmly adhered TiO₂ nanotube films to titanium fabricated by a modified anodization

TiO₂ nanotube films firmly adhered to Ti substratewere fabricated by amodified anodization,which involved in a traditional nanotubular structure formation stage in a fluoride-containing electrolyte and an additional fluoride (F^-) ion sedimentation stage by addition of (CH₃COO)₂ Mg during the later period of anodization. The obtained results showed that the adhesion strength of themodified anodization derived TiO₂ nanotube films, as characterized by critical load (Lc) using a scratch test, could be significantly improved from 3.2 ± 0.8 N for the traditional anodization derived one to a maximum value of 17.5 ± 1 N. There was no obvious change in surface morphologies of the amorphous TiO₂ nanotube films prepared by the traditional and modified anodization, respectively. For the traditional anodization derived nanotube film, there was an ultra-thin fluoride-rich layer (about 15nmin thickness) existing between the nanotube bottoms and the Ti substrate,weakening the adhesion strength. However, the modified anodization derived TiO₂ nanotube film with the highest Lc value consisted of two layers: an outer nanotube layer and an inner compact layer adjacent to the Ti substrate composed of amorphous titanium oxide free of fluoride ions. Such structure of TiO₂/Ti interface led to a considerable improvement in the adhesion strength between the nanotube film and the Ti substrate.