Nature of oxide layer formed on NiTi by anodic oxidation in methanol

NiTi was galvanostatically anodized in a methanolic electrolyte to form a thick (more than 10 μm) and uniform oxide layer. The oxide layer was characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), thin-film X-ray diffractometry (TF-XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The layer was composed of a mixture of TiO2 and metallic Ni while the surface of the layer contained non-stoichiometric nickel titanate (NiTiO3−x). The as-prepared coating was free of cracks and composed of very fine crystallites. Vickers indentation showed that the as-prepared layer was ductile, with a low Vickers hardness of about 1.50 GPa. The high room-temperature ductility of the oxide coating was attributable to the nanosized crystallites in the coating.