The Tribocorrosion Behaviour of Ti-6Al-4 V Alloy: The Role of Both Normal Force and Electrochemical Potential

The tribocorrosion behaviour of Ti-6Al-4 V exposed to phosphate buffered saline was investigated under a range of normal forces in both cathodic and anodic regions to provide a basis for properly deriving the tribological constants of this alloy. To achieve this, a new customised tribotester was designed and manufactured to rub the Ti-6Al-4 V disks against zirconia balls. The tests were conducted at a sliding frequency of 1 Hz and a sliding distance of 4.8 mm under various normal forces and potentials as 17.5, 10.8, 6, 3.5 N, and - 1.2, - 0.6, 0, 0.4, 0.8 V/V-Ag/AgCl, respectively. The damaged surfaces were characterised by scanning electron microscopy and energy-dispersive X-ray spectroscopy, profilometer, and micro-hardness tester. The post analyses confirmed the appearance of some minor cracks together with third-body wear particles. No significant changes in the hardness were detected after the tribocorrosion tests. The results of profilometry and electrochemical current indicated that in the anodic region the chemical losses accounted for a significant proportion (up to 36%) of the total loss. The proportional chemical loss increased with the potential; however, neither direct nor reverse relationship was found with the normal force. Overall, in the anodic domain, the material loss increased with the potential level due to the formation of oxide layer which may induce more shear cutting. In the cathodic domain, hydrogen embrittlement changed the properties of the interface and thus, the amount of material loss. Both the mechanical and chemical wear were described by an existing tribocorrosion theory; thereby, the theory was equipped with its tribocorrosive constants for future analyses on the tribocorrosion of this alloy extensively used in various applications including biomedical implants.