Heat-, Load-, and Shear-Driven Reactions of Di-tert-butyl Disulfide on Fe(100)

Abstract

The surfaces of lubricated mechanical components operating under extreme conditions are protected by films that form in the presence of additives in lubricant formulations. Film formation is believed to be accelerated by heat, load, and shear force in the sliding interface, but the individual contributions of these factors are poorly understood. In this study, we use reactive molecular dynamics simulations to deconvolute the effects of heat, load, and shear force on chemical reactions between di-tert-butyl disulfide, an extreme-pressure additive in lubricants, and Fe(100), a model approximation of the ferrous surfaces of mechanical components. The reaction pathway is characterized in terms of the number of chemisorbed sulfur atoms and the number of released tert-butyl radicals during heat, load, and shear stages of the simulation. Chemisorption is limited by accessibility of reaction sites; thus, shear accelerates the reaction by facilitating movement of radicals to available sites. Analysis of tert-butyl radical release in the context of an Arrhenius-based model for mechanochemical reactions shows that shear lowers the energy barrier for reactions, implying that, in lubricated contacts, the effect of shear will be significant at lower temperatures, which are expected to arise under moderate sliding conditions.