Ti6Al4V interacting with Al2O3 under ambient and vacuum conditions: Effect of plastic deformation and tribo-chemical reactions on wear mechanisms

Abstract

Tribological interaction is a synergy between several mechanisms. In this paper, those pivotal mechanisms are studied by investigating the sliding interaction of Ti6Al4V (referred to as Ti64) pins against an Al2O3 disc under both ambient and vacuum environments. The impact of sliding speed was examined by altering the rotation rate (measured in ms−1) of the disc. This assessment utilized a 6.6 mm diameter pin, while keeping the load and environmental conditions consistent across all experimental trials. The findings reveal that in ambient environment, the frictional characteristics and wear rate of the tribological pair are intricately influenced by a synergistic interplay between plastic deformation and tribo-chemical reactions (TCR). A transition in wear mechanism occurs from abrasion to composite type (abrasion + oxidative) to oxidative with step-wise increment in sliding velocity (0.01–1.5 m/s). Coupled action of the TCR sub-mechanisms, viz. tribo-oxidation and formation of mechanically mixed layers (MML) play a pivotal role in the aforementioned wear transition. For vacuum-based experiments, the prevailing wear mechanism is exclusively dictated by plastic deformation. This phenomenon is in turn regulated by adiabatic shear banding (ASB)/strain rate response (SRR) at lower speeds, while at higher speeds, temperature-induced effects (phase transformation assisted thermal softening) come into action. Adhesion takes precedence in governing the material removal process, given that the development and activity of tribo-oxides and MML are impeded in the absence of air. Further, additional tests to study the effect of change in the pin-diameter (2.1, 4.6, and 6.6 mm) on the tribological response are performed and analysed.