Abstract
In boundary lubrication, formation of tribofilms and subsequent friction and wear properties are strongly influenced by lubricant affinity to the lubricating surfaces. Ionic liquids (ILs), which are composed of ions, are expected to provide enhanced surface affinity compared to molecular liquids. This is also of interest for non-ferrous surfaces such as diamond-like carbon (DLC) coatings. DLC technology is widely utilised for sliding components, but DLC performance with conventional lubricants does not always fulfil the requirements. In this study, we compare lubricating performance of two non–halogenated ILs with a common cation, trihexyl(tetradecyl)phosphonium, but different anions, bis(oxalato)borate and bis(mandelato)borate, in the steel/steel and steel/DLC contacts. The tribofilms formed and their lubrication mechanism are investigated using a wide range of surface analysis techniques: Atomic Force Microscopy (AFM), nano-indentation, SEM-EDX and X-ray Photoelectron Spectroscopy (XPS). It is demonstrated that both ILs exhibited better lubricating performance than PAO oils of similar viscosity. It is also shown that the lubricating performance is controlled by the chemical stability of ILs, surface hardness and topography. The phosphonium bis(oxalato)borate IL exhibited excellent lubricity but its performance was adversely affected by the rough and very hard DLC surface that disrupted a delicate balance between the rates of tribofilm formation and removal. A higher chemical stability of the phosphonium bis(mandelato)borate IL facilitated formation of a non-sacrificial ionic tribofilm that reduced friction to a lower extent than the phosphonium bis(oxalato)borate IL but without accelerated tribochemical wear. The results demonstrate the potential for controlling lubrication properties by tuning functional groups of anion structure and subsequent chemical reactivity of ILs with lubricated surfaces.
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