Marker experiments to determine diffusing species and diffusion path in medical Nitinol alloys

The out-diffusion of toxic Ni ions from NiTi (Nitinol), often used for biomedical applications, can be strongly reduced using oxygen plasma immersion ion implantation (PIII). Non-radioactive isotope markers, ⁶⁰Ni and ⁴⁶Ti, were implanted at 180 keV into NiTi prior to the oxygen implantation with fluences between 0.4 × 10¹⁶ and 4 × 10¹⁶ at./cm². Implanting oxygen ions by PIII in the temperature range of 400-550 °C leads to a surface oxide layer consisting of pure TiO₂. The results prove that Ni cations are the mobile species, while Ti is immobile during the oxygen insertion.