Ionic lubricants: Molecular features and surface protection mechanisms

In this thesis ionic liquids (ILs) are investigated as prospective candidates for lubrication. Three custom synthesized phosphonium orthoborate type ILs were extensively studied from the prospective of molecular structuring both in bulk and at the interface and from the prospective of their lubricating performance as neat lubricants and as additives in oils. A wide selection of contact geometries and surface finishes has been utilized to broaden the applicability of the achieved results. Tribological performance of orthoborate ILs when used as additives in oils was additionally benchmarked against two commercial ILs - phosphonium phosphate, phosphonium phosphinate. Studies on the bulk molecular mobility in the orthoborate ILs performed by means of Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR) clearly showed that the distinct molecular organisation in these systems depends on the chemical structure of orthoborate anion. These results demonstrated a clear correlation with tests of the tribological performance of neat orthoborate ILs, where friction reduction, wear protection and particularities of surface interactions were shown to be clearly defined by anion chemistry. Moreover, this finding was further proven to be relevant when using ILs as additives in oils. A comparison of tribological performance of orthoborate ILs with phosphinate and phosphate ILs showed that a change in anion structure could basically revert the tribological performance of oil solution: from lower to higher friction and wear when compared to the neat oil lubricity. One of the orthoborate ILs - trihexyltetradecylphosphonium bis(mandelato)borate (PBMB) – was selected for a thorough study when used as a sacrificial oil additive. A joint PFG NMR and Quartz Crystall Microbalance (QCM) study demonstrated the build-up of a PBMB rich film on a electrically charged surface. This provided an experimental prove for the possibility of electrostatically driven physisorption of ILs. Tribological tests performed on the same oil composition demonstrated that PBMB when reaching the surface triggered tribochemical reactions and formation of a surface protective tribofilm. Phosphonium orthoborate ILs demonstrated an outstanding performance (decreasing wear by up to 92% and friction by up to 50%) in lubricated mechanical contacts, both when used as neat lubricants and when used as additives. These results are based on an extensive study employing a wide variation in contact geometries, su