Unlubricated sliding friction and wear characteristics of thermally oxidized commercially pure titanium

In the present work, the unlubricated sliding friction and wear characteristics of thermally oxidized commercially pure titanium have been investigated. Thermal oxidation of CP-Ti was undertaken at a temperature of 625 °C for durations of 5, 20 and 72 h. This results in a multi-layered structure comprising a titanium dioxide layer (rutile) atop of an α-titanium oxygen diffusion zone (α-Ti(O)). During unlubricated sliding against an alumina counterface, four distinct frictional zones can be observed for each of the oxidized samples. Analysis of wear track morphology shows distinct wear mechanisms for each frictional zone. The observed frictional zones correlate well with the oxide layer, the transition between the oxide layer and the diffusion zone, the oxygen diffusion zone and the substrate. For longer duration oxidation, the rutile oxide layer produced becomes a scale with poor adhesion to the oxygen diffusion zone. Initial attempts have also been made to improve the frictional behavior of the oxide layer, using a prior surface mechanical attrition treatment (SMAT) and controlled slow cooling after oxidation. The results demonstrate that these prior and post treatments have a positive effect on the tribological performance of the oxide layer.