A preliminary study of TiO2 deposition on NiTi by a hydrothermal method

A simple low-temperature method was employed in depositing an oxide coating on NiTi for enhancing corrosion resistance. Mechanically polished NiTi samples were pretreated in a solution containing Ti4+ ions at 60 °C for 8 h, followed by hydrothermal treatment at 140 °C for 10 h. After hydrothermal treatment, scanning electron microscopy (SEM) indicated the presence of an oxide film of approximately 200 nm on the NiTi substrate. Composition depth profiling of the oxide film by X-ray photoelectron spectroscopy (XPS) revealed that the oxide was TiO2. Thin-film X-ray diffractometry (TF-XRD) confirmed that anatase was the only crystalline phase present in the oxide film. Atomic force microscopy (AFM) recorded a mean surface roughness of 7.3 nm for the coated surface, which was of the same order as that of the polished surface (10.3 nm). Characterization of the corrosion behaviors in Hankʹs solution at 37 °C using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization technique showed that the coated samples had a significantly higher corrosion resistance than samples prepared by mechanical polishing and chemical cleaning. The deposition method employed in the present study meets the important requirements in the surface modification of NiTi implants: (a) the substrate material should not become part of the treated layer in order to minimize the Ni content in the surface; (b) the treatment temperature should be low, preferably not exceeding 300–400 °C, so as not to disrupt the thermomechanical properties of the NiTi implants; (c) the process should not be a line-of-sight one to ensure a uniformly modified surface layer for implants of complex geometry.