Nanotribology of Phosphonium Phosphate Ionic Liquid: a Combined Atomic Force Microscopy and Surface Spectroscopy Study

Ionic liquids (ILs) have recently gained considerable attention owing to their unique and tunable properties (e.g., wide electrochemical window, high thermal stability), which make them potentially useful for a range of applications, including batteries, fuel cells, catalysis, and lubrication. When IL are used as lubricants, the interface between the IL and the solid surface plays a pivotal role to determine the friction/wear response. Despite the weight of the studies published in the literature, remarkably little is still known about the structure of solid/IL interfaces and its relationship with the lubrication mechanism/performance of ILs.
Here, we used atomic force microscopy (AFM) to visualize and quantify the processes occurring at sliding interfaces in situ, in single-asperity nanocontacts. The AFM experiments, in which a diamond tip was slid on steel in phosphonium phosphate ILs, indicated a significant friction reduction only after the removal of the native surface oxide from steel. Based on laterally-resolved ex situ analyses of the surface chemistry of steel by X-ray photoemission and low energy electron microscopy, and time-of-flight secondary ion mass spectrometry, a simple phenomenological model will be proposed to account for the observed lubrication behavior.