Estimation of activation energies based on in-situ observation of tribofilm growth process derived from lubricant additive molecular on single asperity

Abstract

A typical anti-wear additive, zinc dialkyldithiophosphate (ZDDP), has been utilized for many types of lubricants. ZDDP is known to form reaction film by direct contact between sliding two substrates and protects adhesion wear. Especially, the alkyl chain type of ZDDP is important for lubricant design to adjust the anti-wear properties, but there are few reports about the reactivities of ZDDPs with different alkyl chain types using in-situ observation of tribofilm formation. In this study, we conducted in-situ AFM on friction interfaces and investigated the reactivities of ZDDPs possessing different alkyl chain types using a stress-activated Arrhenius model. As experimental results, we found that the growth rate of tribofilm and friction coefficient in primary-ZDDP solution was lower than those in secondary-ZDDP solution. In addition, using a stress-activated Arrhenius model, we estimated the activation energy of primary-ZDDP and secondary-ZDDP, which are 148 and 99 kJ/mol, respectively. The tribofilm formation is known to be influenced by the shear stress value, therefore, we considered that low friction properties of primary-ZDDP influenced the low growth rate of tribofilms.