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 per-formed at a pressure of 7 x 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 x 10(-4) mm(3)/m). The worn surfaces of Ti-6Al-4V samples tested in vacuum were rough (R-a = 2.0 mu m), 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 (R-a=0.08 mu m), 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. (C) 2007 Elsevier B.V. All rights reserved.