Effect of thermal oxidation on the dry sliding friction and wear behaviour of CP-Ti on CP-Ti tribopairs

Thermal oxidation (TO) has proven to be a cost-effective and efficient technique to engineer the surfaces of titanium and its alloys to achieve enhanced surface properties. The benefits of TO treatment in enhancing the tribological properties of titanium have been demonstrated by many investigators. However, most of the reported tribological studies have been based on the contact between a TO treated titanium specimen and a counter-body made of other materials, mainly ceramics, steels and polymers. Very few studies have been reported on the friction and wear behaviour of TO treated titanium sliding against TO treated titanium. In this work, the effect of thermal oxidation on the dry sliding friction and wear behaviour of commercially pure Ti (CP-Ti) on CP-Ti tribopairs was investigated under loading conditions ranging from elastic contact to plastic contact. Comparisons were made among three contact pairs: (1) untreated Ti on untreated Ti (Ti–Ti), (2) untreated Ti on TO treated Ti (Ti-TO) and (3) TO treated Ti on TO treated Ti (TO-TO). The results show that the TO-TO contact pair presents an ideal material combination to achieve the best tribological performance in terms of low friction and superior wear resistance. On the other hand, the Ti–Ti pair presents the worst combination in terms of tribological performance. While the Ti-TO pair performs better than the Ti–Ti pair tribologically, it is not as good as the TO-TO pair. It is essential to thermally oxidize both specimens in order to achieve optimal tribological performance. It is the oxide layer-on-oxide layer contact that imparts the excellent tribological performance. Failure of the oxide layer in one of the contact bodies can lead to high and unstable friction and increased wear from both contacting bodies. The tribological performance of the three contact pairs and the failure mechanism of the oxide layer are discussed in the paper. The results of this work suggest that the TO treated Ti on TO treated Ti contact pair would have potential tribological applications in engineering.