Plasma immersion ion implantation for NiTi surface modification

The shape-memory and superelastic nickel-titanium alloy (NiTi, nitinol) has a high nickel concentration of 50 at% which may result in nickel ion release in biological solutions, preventing from application in biomedical implants and devices. Recently, it was demonstrated that the nickel surface concentration may be reduced significantly using plasma immersion ion implantation (PIII) [1, 2]. Our previous biocompatibility tests show a superior in vitro blood compatibility of the oxygen ion implanted NiTi surfaces compared to the untreated ones [3]. Understanding of the mechanisms of surface modification and nickel depleted layer formation during PIII as well as its phase composition evolution is to date rather limited.
In present paper we consider the effect of both ion fluence and implantation temperature on the phase composition, surface morphology, and electrochemical properties of PIII processed NiTi. The samples were examined by grazing incidence X-ray diffraction analysis (GIXRD), Auger electron spectroscopy, elastic recoil detection analysis, atomic force microscopy, spectroscopic ellipsometry and corrosion test.
Oxygen PIII from an inductively coupled RF plasma at an ion energy of 20 keV, an ion fluence of E17-E18 cm-2, and a substrate temperature below 250°C results in the formation of a transparent rutile TiO2 surface layer with a thickness of 50-200 nm and a Ni content below 1 at%. In contrast, the underlying alloy is enriched with Ni. The GIXRD analysis indicates the presence of the Ni4Ti3 and Ni3Ti phases in addition to NiTi. The oxide layer thickness as well as the surface roughness are controlled by the balance of reactive ion-induced diffusion and ion sputtering. The nickel-depleted TiO2 layer prevents from corrosion and out-diffusion of Ni ions.
The results of this study show that PIII is a promising technique for the surface modification of NiTi alloy for biomedical applications.