Surface properties of NiTi alloy modified by plasma immersion ion implantation

The titanium-nickel NiTi alloys is already in use in medical applications due to a good combination of shape memory or superelastic characteristics, biocompatibility and corrosion resistance. However, there are still concerns because of the high nickel content and a possible release of nickel ions, where toxicity and allergic hazards are well documented and carcinogenicity is under discussion. At present the influence of nickel on biocompatibility of the titanium - nickel alloys is an actual and open question for biomedical applications.

The aim of this work is to investigate surface modifications of nickel-titanium alloys and its influence on a biocompatibility and corrosion resistance.

Ar+ and/or N+ implantation in superelastic NiTi alloy with energy of 20-40 keV and fluences of (3-5) x 1017 cm-2 by means of plasma immersion ion implantation was performed.
The near surface layers were studied by grazing incidence X-ray diffraction analysis, Auger electron spectroscopy (AES), scanning electron microscopy and an analysis of the biocompatibility and corrosion resistance of these layers were performed.

The AES analysis of the implanted NiTi showed a reduced Ni content in the near surface (approx. 100 nm). Due to preferential sputtering of Ni and a Ni diffusion in the deeper layers a nickel depleted surface layer was produced, which was sealed by formation of a TiN layer. The deeper layers with a high Ni concentration contain mainly the Ni3Ti or Ni4Ti3 phases. Electrochemical corrosion experiments showed a significantly improved corrosion stability of the ion implanted surfaces.

Bone forming cells grown on untreated NiTi showed in several cases a disturbed cytoskeleton organisation as it was found also on pure nickel. As the effect could not be completely reproduced by nickel ions in the solution, it seems to be a surface effect. On the ion implanted surfaces the cell adherence always was undisturbed.

The results show that by Ar and N ion implantation the corrosion stability and biocompatibility of NiTi can be further improved.