Characterization of bioactive surface oxidation layer on NiTi alloy

To enhance the bioactivity of NiTi alloy, the surface oxidation layers were synthesized by heat treatment in air in the temperature range of 300–800 °C. The surface oxidation layer on NiTi alloy was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The heat treated sample was soaked in simulated body fluid (SBF) to study the bioactivity of the thermally grown oxide layer. Results showed that a protective layer of TiO 2 was formed on the surface of NiTi alloy at heat treatment temperatures of 600 °C or higher with varying degree of anatase and rutile. Small amount of nickel oxide was found on the surface of 300 and 400 °C treated samples by X-ray photoelectron spectroscopy. With further increase in the heat treatment temperature, the nickel concentration on the surface decreased and there was almost no nickel species on the surface after heat treatment at 600 °C or 800 °C. Depth profiling revealed that the amount of TiO 2 (Ti 4+) decreased with depth with a concomitant increase of metallic Ti. In addition, both TiO (Ti 2+) and Ti 2O 3 (Ti 3+) increased initially and then decreased gradually with depth. Ni existed mainly in the oxidized state on the surface of heat treated samples and it changed to metallic state with increasing depth. In vitro test revealed that the titanium oxide layer formed on the 600 and 800 °C heat treated samples was bioactive, and a layer of apatite was formed on the surface of the titanium oxide layer after soaking in simulated body fluid.