Wear failure behaviour of titanium-based oxide coatings on a titanium alloy under impact and sliding forces

To increase the wear resistance property of a Ti alloy, a plasma electrolytic oxidation (PEO) process was used to form protective oxide coatings. The present work was mainly to study the effects of pure sliding and impact-sliding combined forces on failure behaviours of the oxide coatings with different thicknesses (10-27 μm). A recently-developed impact-sliding test method was used to investigate the combined force effects on the coatings and the uncoated substrate for comparison. In each impact-sliding cycle, the forces comprised a dynamic impact load, Fi, and a pressing load, Fp. The combined loadings were pre-set to be (Fi = 80, 130, 200 N and Fpmax = 200, 300, 400 N, respectively) applied by a steel ball (SI 52100). The test results show that the rough top layers of PEO coatings can cause early failure of the coatings and a large mass-loss on the counterface steel balls. For a relatively smooth coating without polishing, the porous structure can help to cushion the impact force comparing to the exposed dense inner layers of the thick coatings that were ground to have a similar surface finish. Incorporation of Si and P elements into the Ti oxide coatings may alter the surface affinity behaviour and stoichiometry of Ti oxides, causing different wear behaviours under impact and sliding forces.