Investigation of the effects of surface oxide layer on wear and corrosion in NiTi alloys: A mechanical and chemical perspective

Nickel-Titanium (NiTi) alloys are widely applied in biomedical and industrial domains, mainly because of their unique superelastic characteristics and biocompatibility. However, their performance is often constrained by their tendency to wear and corrode, particularly under repetitive mechanical loading, harsh environmental conditions, and high friction coefficients. This study aims to reduce the wear rate and friction coefficient of NiTi alloys by forming oxide layers on the surface through heat treatment, while also increasing their corrosion resistance. It also examines the effects of high-temperature and prolonged heat treatments on surface performance. NiTi alloys were subjected to heat treatments at 900, 1000, and 1100?C for 2 and 4 hours. Wear tests were conducted using the ball-on-disc method under three different loads (5N, 10N, and 15N), three different test durations (30, 60 and 90 min), and at a constant speed of 0.05 m/s, to measure the friction coefficient and wear rates. Additionally, the alloys mechanical and corrosion performance was evaluated through microhardness and electrochemical corrosion tests. The results showed that samples treated at 1100?C for 4 hours exhibited a 146% increase in microhardness, reaching 640 HV, with the oxide layer thickness increasing to 462 ?m. This oxide layer reduced the wear rate by 60%, bringing it down to 150x10-6 mm?/Nm during a 30 min test under a 15N load, while the friction coefficient decreased by 58%, dropping to 0.30 ?. In contrast, nonheattreated samples exhibited lower hardness (260 HV), higher friction coefficients (0.72 ?), and greater wear rates (370x10-6 mm?/Nm) under a 5N load over a 90 min test. Corrosion analyses indicated that samples treated at higher temperatures showed more positive corrosion potentials and significantly improved corrosion resistance, with the best performance observed in samples treated at 1100?C for 4 hours. These findings suggest that the thick oxide layers formed during high-temperature, long-duration heat treatments significantly improve the wear resistance, reduce the friction coefficient, and enhance the corrosion resistance of NiTi alloys, making them more durable for industrial and biomedical applications.