Surface Modification of Ni-Ti for Biomedical Applications by Plasma-immersion Ion Implantation

The enormous elasticity of Ni-Ti is becoming integral to the design of a variety of new medical products. The wide spectrum of application in implantology imposes special requirements on the biocompatibility of Ni-Ti. The biological response to implant materials is a property directly related to their surface conditions and an optimum surface layer is thus desired.

The plasma-immersion ion implantation (PIII) technique was used to modify and improve the surface of a Ni-Ti alloy (~ 50.2 at.% Ni, superelastic at body temperature) for biomedical applications. The main goal has been the formation of a Ni-depleted surface, which should serve as a barrier to out-diffusion of Ni ions from the bulk material. Ion implantation of oxygen was carried out. The depth profiles of the elemental distribution in the alloy surface region, obtained by Auger electron spectroscopy (AES), confirm the formation of a Ti-rich oxide layer. The working plan also comprised ion implantation of nitrogen. In this case, the formation of titanium oxynitride (TiNxOy) was observed. The AES depth profiles clearly show a Ni-depleted fraction for experiments performed with 40 keV.

The deposition of a coating by a PVD technique would have disadvantages due to the interface between the coating and the bulk (lower adhesion). PIII creates a graded interface between the modified surface and the bulk. Techniques like thermal oxidation and nitriding could also lead to an improved corrosion resistance and Ni-depleted Ni-Ti surface. However, the high temperature necessary for the experimental procedure would lead to modification of the phase transformation characteristics and loss of specific mechanical properties of the alloy. Heat treatments tests performed at temperatures above 350ºC led to a shift of the transformation temperatures of the Ni-Ti alloy used in this work. Moreover, the R-phase is then present at body temperature, which is not the case for Ni-Ti samples modified by the PIII technique. This technique only changes the properties of the Ni-Ti alloy top layer.