Effect of Low-Pressure Gas Oxynitriding on the Microstructural Evolution and Wear Resistance of Ti-6Al-4V Alloy

A Ti-6Al-4V titanium alloy exhibits low hardness and poor wear resistance under sliding contact. This study evaluates the effect of low-pressure gas oxynitriding (LPON) followed by low-temperature oxidation on its microstructure and tribological performance. Specimens were nitrided at 1000 °C for 100 min, then oxidized at 450–600 °C for 120 min. Microstructural and phase changes were characterized by SEM and XRD; surface roughness, hardness, and wear were assessed using 3D laser scanning microscopy, microhardness profiling, and pin-on-disk tests under 2 N and 4 N loads. XRD revealed TiN, Ti2N, Ti2AlN, and TiO2 phases, with oxidation temperature governing TiN grain growth and nitride-to-oxide transformation. Oxidation at 500–550 °C formed a dense TiO2-rich layer over a stable TiN/Ti2N substrate, achieving hardness up to ~670 HV0.025 and the lowest wear volume. At low load (2 N), nitriding alone provided the highest wear resistance, while at higher load (4 N), oxidation yielded only slight improvement due to oxide embrittlement. Excessive oxidation at 600 °C increased roughness, induced spallation, and reduced wear resistance. The optimal condition (550 °C) offered synergistic protection from nitrides and stable oxides, enhancing load-bearing capacity. Overall, duplex nitriding–oxidation is most effective for low-to-moderate load applications, with potential use in biomedical implants, aerospace fasteners, and precision components.