Tribochemistry of n-Alkane Thiols Examined by Gas-Phase Lubrication (GPL)

Abstract

Gas Phase Lubrication (GPL) is an approach allowing to simulate experimentally the reaction mechanism of lubrication additives on a surface thanks to a specific device, an environmentally controlled analytical tribometer (ECAT). Indeed it is shown that GPL is able to simulate the boundary lubrication regime by bringing directly the molecules on the surface in contact. The presence of in situ chemical analysis clearly gives some insights on the chemical dissociation pathway and reaction of additive molecules on the surface under shearing leading to the tribofilm formation. This is a huge advantage compared to standard liquid phase lubrication coupled with ex situ post mortem surface analyses. GPL approach also permits to investigate the critical role of the freshly nascent surfaces obtained by friction in UHV in the decomposition mechanism of the additive molecules. This brings new knowledge of the complex phenomena that can occur in lubricated friction experiments. Moreover, such experimental approach appears particularly suited for combination with numerical simulations, offering unique capabilities for investigating boundary lubricated interfaces. In this chapter, GPL is applied to investigate the tribochemical reactions of sulfur-based compounds, N-alkanethiols. Undoubtedly, the primordial role of nascent tribo-stressed metallic surfaces obtained after shearing in the dissociation of lubricant additives under boundary lubrication conditions is highlighted. The investigation into the reactivity of thiol molecules showed that they suffer dissociative adsorption on the asperities of the tribo-stressed nascent metallic surface, rapidly forming a thin protective film composed of iron sulfide reducing the friction.