Effect of Treatment Time on the Tribological Behavior of Thermally Oxidized Ti-6Al-4V Under Dry and Oil-Lubricated Conditions

Ti-6Al-4V alloy is a popular metal in engineering, utilized in aerospace and automotive industries because of its mechanical properties. However, Ti-6Al-4V’s poor tribological characteristics cause it to be susceptible to wear due to its low surface hardness and inadequate lubricity. In this study, thermal oxidation (TO) was performed on Ti-6Al-4V under specific conditions of 625 °C for various oxidation durations of 0.5, 1.5, 6, 24 and 96 h and the microstructure, friction, and wear behavior of TO-treated Ti-6Al-4V under dry and oil-lubricated sliding conditions were investigated. Characterization by XRD, SEM, and EDX confirms the development of oxide layers (OL) and oxygen diffusion zones (ODZ) of varying thicknesses. Tribological tests were conducted using a ball-on-disk configuration under a 5 N load against an Al2O3 counterface in both dry and 10W-40 oil-lubricated environments. Under dry conditions, extended oxidation times lead to a deterioration in friction and wear performance due to the increased brittleness and decreased adhesion of the thick OL, leading to brittle failure and interfacial delamination. In contrast, under oil lubrication conditions, all oxidized samples show stable, low-friction (~0.06) and minimal wear, dominated by boundary lubrication. The best performance is achieved at short oxidation durations, where a thin OL and a stable ODZ provide strong adhesion of the OL and high surface hardness. Wear rates up to three orders of magnitude lower than untreated Ti-6Al-4V are observed for short oxidation durations, where oxygen diffusion rather than thick oxide formation dominates the surface-hardening effect. SEM and EDX analyses confirmed the lack of tribofilms or additive-derived elements on the sliding surfaces, indicating that the improved performance results from the oxygen-enrichment in the subsurface and stable boundary lubrication, rather than chemical interactions with oil additives. Overall, oxidation duration is therefore essential to balance oxide growth and OL adhesion, ensuring superior lubricated wear resistance for titanium components.