Surface modification of NiTi for orthopaedic braces by plasma immersion ion implantation

Super-elastic Nitinol (NiTi alloys) possesses superior mechanical properties compared to conventional titanium alloys, which—together with a shape memory effect—should predestine them for modern applications. However, a biocompatibility below the excellent values of titanium, coupled with the lurking shadow of nickel outdiffusion presently precludes a widespread use in biomedical applications. It is shown that oxygen ion implantation, especially using plasma immersion ion implantation for three-dimensional objects, is a highly satisfactory method to obtain both objectives. SE 508 substrates were implanted at elevated temperatures with oxygen. Subsequently, the films were investigated using elastic recoil detection analysis (ERDA) and X-ray diffraction (XRD). A closed oxide layer, containing nearly exclusively only titanium and oxygen, was formed with the nickel anti-segregating towards the bulk into a Ni3Ti layer. Thus, an effective diffusion barrier was formed. Higher temperatures than 400 °C lead to thermal diffusion of Ni, increasing the Ni concentration in the near surface region.