Current and friction transients during tribocorrosion of a biomedical titanium alloy sliding against zirconia

The tribocorrosion mechanisms of a Ti-Nb-Zr-Ta (TNZT) alloy rubbing against a zirconia ball in a Phosphate Buffered Solution (PBS) were analyzed by considering the mechanical and chemical interactions that lead to the material degradation. To understand the governing mechanisms, tribocorrosion tests were carried out in a ball on flat contact and reciprocating tribometer applying two normal loads (6.7 and 11.2 N) and two different anodic potentials (400 and 1500 mV). Worn surfaces were characterized by SEM and FIB cross sections as well as surface analysis. During the tribocorrosion tests, transients consisting in simultaneous abrupt current increase and a decrease in the COF were found at different instants of the tests. Subsurface cracking as well as third body build up were observed. These different phenomena and their temporal sequence could be described by initial sliding tribocorrosion, including metal particle removal and surface oxidation, followed by the build-up of a protective third body. After a certain number of sliding cycles subsurface cracking due to strain accumulation beyond a critical level lead to the detachment of a large particles that reactivated the initial tribocorrosion process. This mechanistic view could be substantiated by a model based on a quantitative analysis of the results.