Formation of titanium nitride barrier layer in nickel-titanium shape memory alloys by nitrogen plasma immersion ion implantation for better corrosion resistance

Nickel-titanium shape memory alloys (NiTi) are potentially useful in orthopedic implants due to their super-elasticity and shape memory properties. However, the materials are vulnerable to surface corrosion and the most serious issue is out-diffusion of toxic Ni ions from the substrate into body tissues and fluids. In this paper, we describe our fabrication of TiN barrier layers in NiTi by nitrogen plasma immersion ion implantation followed with vacuum annealing at 450 °C or 600 °C. Our results show that the barrier layer is not only mechanically stronger than the NiTi substrate, but also is effective in impeding the out-diffusion of Ni from the substrate. Among the samples, the 450 °C-annealed TiN barrier layer possesses the highest mechanical strength and best Ni out-diffusion impeding ability. The enhancement can be attributed to the consolidation of the Ti-N layer resulting from optimal diffusion at 450 °C.