Evaluation of Microstructure, Corrosion, and Surface Properties of Plasma-Assisted Oxy-Nitrided Ti–6Al–4V Alloy

The use of titanium alloys in medical applications has always been controversial due to their corrosion properties. Ti–6Al–4V alloy is usually used due to its good mechanical properties. The poor corrosion properties of this alloy are mainly due to the formation of vanadium oxide, which is soluble in acidic solutions. In this study, to improve corrosion and surface properties and passive behavior, Ti–6Al–4V alloy was plasma-assisted oxy-nitrided in the gas mixtures of 30%N₂ + 70% O₂, 50% N₂ + 50% O₂, and 70% N₂ + 30% O₂ at 450, 500, and 550 °C temperatures for 2, 5, and 8 h. The effect of process temperature, time, and gas mixture on different characteristics of the samples was investigated. Multi-phase surface layers were studied using scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). It was found to increase surface roughness and hardness by the oxy-nitriding process slightly. With increasing process time and temperature up to 8 h and 500 °C, the surface hardness reached 676.1 Vickers in comparison with the untreated sample (139.8 VH). Also, the surface roughness increased from 23.57 to 49.82 nm for 8 h-450 °C-N₂/O₂ = 50/50 sample. Results showed higher resistance to corrosion of the samples treated by glow discharge assisted. The most polarization resistance to corrosion was related to sample 8h-550 °C-N₂/O₂ = 30/70 with a numerical value of 1892.78 Kohm cm² Due to the formation of complex TiN_xO_y and TiN, which was increased compared to the untreated sample with polarization resistance of 1.01 Kohm cm².