Effect of Friction Velocity on Tribological Behavior of Coumarin as Mineral Oil Additive

Abstract

Abstract The tribological behavior of lubricants, prepared with a mineral base oil, lauryl alcohol, and different concentrations of coumarin, was examined using a four-ball tester under constant and variable friction velocity conditions. At constant friction velocity, the maximum non-seizure load (PB) increased from 304 N to 392 N at a coumarin concentration of 0.5 wt%. Lubricants with 0.7 wt% coumarin exhibited optimum lubricating properties, and the maximum reductions in friction coefficient (FC) and wear scar diameter (WSD) were 20.0% and 11.88%, respectively. Further investigation of the tribological mechanism implied that the ester group in the coumarin molecule established a connection with the surface atom, resulting in the formation of a tribofilm, which further restricted the adhesion wear regime. Additionally, under variable friction velocity conditions, increasing the coumarin concentration had an obvious effect on the mixed lubrication (ML) and elasto-hydrodynamic lubrication (EHL) regions but not on other lubrication regions. Moreover, a mathematical model was proposed to show the relationship between FC and friction velocity. Importantly, the present work clarifies the effect of friction velocity on the tribological behavior of coumarin and also supports the use of coumarin as a novel additive in mineral oils.