Z. Li, O. Morales-Collazo, J.T. Sadowski, H. Celio, A. Dolocan, J.F. Brennecke, F. Mangolini
The University of Texas at Austin,
Keywords: AFM, nanotribology, ionic liquids, surface analysis
Summary:Ionic liquids (ILs) have recently gained considerable attention owing to their unique and tunable physico-chemical properties (e.g., low vapor pressure, high thermal stability), which have made them useful for a range of applications, including energy storage, catalysis, and lubrication. ILs are particularly attractive in lubrication, since their properties make them suitable for components working under extreme conditions, such as those found in engines, gearboxes, and spacecraft. When ILs are used as lubricants, the interface between the IL and the sliding surfaces plays a pivotal role in controlling the friction and wear response. Despite the weight of published literature, remarkably little is still known about the lubrication mechanism of ILs and its dependence on the applied normal pressure. The development of a fundamental understanding of the mechanism by which ILs reduce friction and/or wear requires shedding light on the processes occurring at nanoscale asperities within macroscale contacts. This constitutes a significant challenge that requires the use of complementary surface-analytical techniques to understand the nanoscale mechanisms at play. Here, we used atomic force microscopy (AFM) to visualize and quantify the processes occurring at sliding interfaces in situ, in single-asperity nanocontacts. The combination of AFM experiments, in which a diamond tip was slid on steel in phosphonium phosphate IL (PP-IL), and laterally-resolved ex situ analyses of the surface chemistry of steel allowed for the identification of different mechanisms underpinning the promising lubricating properties of PP-IL: at an applied pressure up to 5.5 ± 0.3 GPa the confined PP-IL molecules undergo a pressure-induced morphological change leading to the generation of a lubricious, solid-like interfacial layer, while at applied pressures between 5.5 ± 0.3 GPa and 7.3 ± 0.4 GPa the removal of the solid-like layer formed by PP-ILs leads to wear of the underpinning steel substrate. The surface adsorption of phosphate ions on metallic iron results in the formation of a densely-packed boundary layer that reduces nanoscale friction. The outcomes of this work provide novel insights into the pressure-dependent lubrication mechanisms of ILs in general, while providing guidelines for tailoring ILs to control the nanoscale structure of solid/IL interfaces and achieve improved tribological performance.