Structure and properties of oxygen implanted NiTi surfaces

The memory shape and superelastic NiTi alloy for biomedical implants and devices has the drawback of the high nickel concentration of 50 at%. Plasma immersion ion implantation (PIII) for improved corrosion resistance and surface nickel depletion of NiTi has been suggested. In the recent work it could be shown that oxygen PII implantation (ion energy of 20 keV and fluence of 1e17 – 1e18 cm-2) can reduce the Ni concentration in 50-100 nm surface layers to concentration below 1 at%. This effect is combined with a Ni enriched layer below the Ni depleted surface layer, what suggests a reactive process as mechanism. Grazing incidence X-ray diffraction analysis revealed the formation of rutile TiO2 phase in the surface layer and perhaps Ni4Ti3 in the Ni enriched layer. The effect of ion fluence and implantation temperature on element profiles, phase composition, layers thickness has been systematically investigated.
The nickel depleted TiO2 layer prevents corrosion and out-diffusion of Ni ions. The oxygen ion implanted NiTi surface had a superior blood compatibility in vitro compared to untreated NiTi.