Sliding wear of oxygen diffusion hardened Ti-6Al-4V in physiological saline

Abstract

Oxygen diffusion hardening (ODH) may provide a cost-effective means of improving the tribological properties of titanium alloys. In this investigation Ti-6AI-4V balls (8.0 mm in diameter) were subjected to thermal oxidation (TO) with or without subsequent vacuum annealing. The two-stage “boost-diffuse oxidation” (BDO) process afforded a deeper diffusion hardened zone than did TO alone, at the expense of surface hardness. Three specific treatment conditions were tested: TO — oxidation in air at 625°C for 25 hours; BDO1 — oxidation in air at 700°C for 0.5 hours followed by vacuum annealing at 800°C for 25 hours; and BDO2 — oxidation in air at 800°C for 0.5 hours followed by vacuum annealing at 800°C for 25 hours. The case depth of the two BDO samples was between 60 and 80 μm while the case depth for the TO sample was less than 20 μm. The maximum surface hardness of the TO, BDO1 and BDO2 samples was approximately 700HV, 400HV and 600HV respectively compared to 280–320HV for the untreated balls. Unidirectional sliding wear tests were conducted with untreated, TO and BDO balls sliding against chromium oxide coated AISI 316 discs in 9 g/L NaCl solution. The total test duration was 60 minutes. Only the TO treated ball showed a reduction in friction level in the initial 30 minutes after which the friction coefficient increased to levels similar to the other balls. All of the balls caused damage of the ceramic counterface by an adhesive wear mechanism. The BDO treatment decreased the wear of the balls by up to threefold, the wear rate decreasing with increasing hardness. The TO ball showed the lowest wear despite the wear depth exceeding the hardened case depth at the end of the tests. The high wear resistance of the TO-balls seemed to be related to a zone of compacted debris (mostly from the disc) that formed at the trailing edge of the wear scar.