The formation of the passive oxide surface layer accounts
for the
superior biocompatibility of NiTi alloy based implant materials. However,
the usage NiTi alloy is limited by the long-term release of biotoxic
Ni ion from the bulk, facilitated by the formation of defects and
vacancies in the surface oxide layer during conventional processing.
To aid the improvement of the biocompatibility of NiTi alloys, extensive
first-principles based calculations were performed to uncover the
microscopic mechanism for the temperature controlled oxidation of
NiTi alloy. We show that the oxygen adsorption and diffusion on the
NiTi surface are the elementary steps for the formation of a surface
oxide layer. Oxygen will bind strongly with the alloy surface and
even induce surface reconstruction, and the adsorption energy can
be as high as −6.14 eV. The requested surface diffusion for
formation of TiO2 surface terminations is thermodynamics
driven, but the corresponding kinetics is strongly affected by temperatures.
These result in formation of TiO at low temperatures and TiO2 at elevated temperatures.