Effect of Contact Load on the Oil lubrication Behavior of Thermally Oxidized Ti-6Al-4V Sliding against Alumina.

Ti-6Al-4V is widely used in engineering, aerospace, and automotive applications due to its excellent mechanical properties, but its low surface hardness and poor tribological performance limit its use in sliding contacts. Ti-6Al-4V specimens were thermally oxidized (TO) at 625 °C for 0.5 h, and their tribological performance under oil-lubricated sliding was investigated across a wide range of contact load. Characterization by XRD, SEM, and EDX confirmed the development of an oxide layer (OL) and an oxygen diffusion zone (ODZ) on the surface of the alloy after the TO treatment. Tribological tests were conducted using a ball-on-disk configuration at different loads of 5, 10, 20, 40, 60 and 100 N against an Al2O3 counterbody in 10W-40 oil-lubricated environments. The results show that the engine oil failed to lubricate the untreated Ti-6Al-4V, leading to unstable high friction from the sliding contact. TO treatment at 625 °C for a short time of 0.5 h was very effective in enhancing the oil lubrication behavior of the contact pair, resulting in much increased wear resistance of Ti-6Al-4V compared with the untreated alloy. Under oil-lubricated conditions, the TO treated surfaces maintained a low and stable COF of 0.12 and a low wear rate in the order of 10-6 mm3m-1N-1 over the investigated load range. Increasing the applied load from 40 to 100 N caused plastic deformation of the oxide layer without severe material removal or catastrophic failure. Despite localized damage, the oxide layer maintained low friction and effective wear resistance. These findings demonstrate that short-duration thermal oxidation (TO) enhances the tribological performance of Ti-6Al-4V under lubricated sliding conditions and reveals the effect of contact load on oxide layer deformation and durability.