Vacuum sliding behaviour of thermally oxidized Ti–6Al–4V alloy

In order to improve tribological properties of a Ti–6Al–4V alloy, a thermal oxidation (TO) process was applied at 873 K for 60 h and sliding wear performance of the alloy was tested in vacuum. The surface hardness of the Ti–6Al–4V alloy (382 ± 20 KH) increased to 1631 ± 240 KH as a result of the TO treatment. Dry sliding tests were performed at a pressure of 7 × 10− 6 torr and under ambient conditions, using a ball-on-disc configuration. A normal load of 2 N and a sliding velocity of 0.05 m/s were applied. In vacuum, the wear rate of TO-treated Ti–6Al–4V (no detectable mass loss) was much lower than that of the untreated alloy (2.67 × 10− 4 mm3/m). The worn surfaces of Ti–6Al–4V samples tested in vacuum were rough (Ra = 2.0 μm), and exhibited severe plastic deformation characterized by a “slip-stick” type of wear morphology, but had lower mass loss compared to the ones tested in air that showed oxidational wear. The TO-treated Ti–6Al–4V revealed smoother worn surfaces (Ra = 0.08 μm), and the original oxidized layer remained intact although more wear damage, compared to that caused by Ti–6Al–4V, was inflicted on the 52100 steel counterface. TO-treated samples tested against TO-treated counterfaces in vacuum showed the lowest damage at the sliding surfaces.