Enhancing biocompatibility of NiTi shape memory alloys by simple NH3 treatments

Abstract

This paper presents the treatment of NiTi shape memory alloys (SMAs) in flowing ammonia at 700 °C as a simple and cost-effective nitriding process to provide a protective surface layer hindering Ni ion release in biological environments. Experimental results demonstrated that a smooth protective TiN layer on the NiTi SMAs along with TiOxNy and TiO2 formed on the surface upon treating the as-received NiTi SMA in ammonia at 700 °C. The protective TiN layer and the smooth surface hinder the amount of Ni ion release to artificial saliva (AS) after 28 days of immersion, while the dry air treatment at similar conditions results in a significantly rough surface, leading to about 20 times higher Ni ion release. Overall, the findings presented herein demonstrate that NH3 nitriding is an effective method to eliminate the Ni presence from the surface and to obtain a smooth final surface, which, in turn, restricts the Ni ion release from the NiTi SMA into AS. Consequently, nitriding the surface of NiTi under NH3 at 700 °C turned out as a promising method to lower Ni ion release and thereby contribute to the biocompatibility of NiTi SMAs, which, however; needs to be further validated through further experimentation.