Orthoborate Ionic Liquids for Lubricated Interfaces

Abstract: The life span and efficiency of various machinery that we rely on are often limited by the performance of lubricated interfaces. A lubricated interface is formed by two interacting surfaces and a lubricant that decreases friction and wear by separating the surfaces. There is a constant quest for more efficient lubricants capable of lubricating nonferrous surfaces. Ionic liquids (ILs) possess many unusual physicochemical properties compared to molecular liquids. These include but not limited to high polarity, high conductivity, high thermal stability, and a wide liquid range. Such properties are essential for formulating lubricants for lubricated interfaces in challenging applications such as aerospace, wind turbines, etc. Some ILs have recently shown promising performance in lubricating lightweight non-ferrous alloys, hard coatings, etc. However, most of the widely studied ILs contain anions with fluorine such as [BF4]- and [PF6]- and are prone to hydrolysis releasing toxic HF among other corrosive products.In this work, a number of hydrophobic ILs based on halogen-free and hydrolytically stable orthoborate anions and different classes of cations such as pyrrolidinium and imidazolium were designed, synthesized, and thoroughly characterized. Their potential to efficently lubricate ferrous and nonferrous interfaces was investigated. The work was planned and carried out in the following steps: • Design, synthesis and purification of novel halogen-free boron-based ionic liquids • Physicochemical characterization of the synthesized compounds by liquid-state (1H, 13C, and 11B) and solid-state (13C and 11B) nuclear magnetic resonance (NMR) spectroscopy, Karl Fischer titration, mass spectroscopy, elemental analysis, inductively coupled plasma mass spectrometry (ICP-MS), thermal analysis (TGA, DSC), powder X-ray diffraction, density and rheological measurements • Evaluation of their lubrication performance using ball-on-disc tribometers. • Analysis of the lubricated surfaces using Scanning Electron Microscopy coupled with X-ray Energy Dispersive Spectroscopy (SEM/EDS) and a stylus profilometer. The detailed description of results obtained for selected classes of ILs is given below: o Class I: Nine novel ILs of N-alkyl-N-methylpyrrolidinium bis(salicylato)borate ([CnC1Pyrr][BScB]) were synthesized and physicochemically characterized. They are solids at room temperature and some of them behave as plastic crystals. Some of these compounds were tested as neat lubricants in steel-steel interfaces at 423 K, i.e. above their melting points. The tested compounds showed significantly better antiwear and friction reducing performance compared with 5W40 engine oil. o Class II: Nine novel room temperature ILs of N-alkyl-N-methylpyrrolidinium bis(mandelato)borate ([CnC1Pyrr][BMB]) were synthesized and physicochemically characterized. Their lubrication potential as 3 wt % additives in polyethylene glycol (PEG) was evaluated in steel-steel interfaces at room temperature. Considerably better antiwear and friction reducing properties were achieved compared with neat PEG and 5W40 engine oil. o Class III: Three novel room temperature ILs of 1-alkyl-3-methyl-imidazolium bis(mandelato)borate ([CnC1Im][BMB]) were synthesized and physicochemically characterized. o Class IV: A room temperature IL of trihexyltetradecylphosphonium bis(oxalato)borate, [P6,6,6,14][BOB], was evaluated as a neat lubricant in alumina-steel and sapphire-steel interfaces at room temperature. [P6,6,6,14][BOB] provided lower friction and wear compared with 5W40 engine oil.