Mechanical properties, bioactivity and corrosion resistance of oxygen and sodium plasma treated nickel titanium shape memory alloy

Abstract

Nickel titanium (NiTi) shape memory alloy is a unique material displaying the shape memory effect and superelastic property making it attractive to the orthopedic field. However, with its high nickel content of ∼ 50%, there is concern on health and safety when this material is implanted inside the human body for a prolonged period of time as toxic nickel ions may leach into the body due to corrosion under physiological environment. Previous work demonstrated that the corrosion resistance of this material could be enhanced by implanting a packed oxide layer on the substrate surface using oxygen plasma immersion ion implantation (PIII). The present study aims at improving its bioactivity by further implanting sodium ions into the modified surface using PIII. The chemical composition of the modified surface is characterized by X-ray photoelectron spectroscopy (XPS). Simulated body fluid (SBF) immersion tests for 21 days indicate that implantation of sodium successfully enhances the accumulation of calcium/phosphorus-rich deposits on the modified surface. Anodic polarization scans suggest that sodium PIII does not affect the corrosion resistance of the oxygen plasma implanted surface. Three-point bending test reveals changes in the bulk mechanical property after PIII. However, superelasticity can be retained in the implanted NiTi materials. Differential scanning calorimetry (DSC) suggests a change in the transition temperature of the substrate which likely causes the change in mechanical property. In conclusion, oxygen and sodium PIII can enhance the bioactivity and corrosion resistance of NiTi, but care must be exercised because this treatment may change the bulk mechanical property.