Orthoborate Ionic Liquids for Lubricated Interfaces

Sammanfattning: The lifespan and efficiency of various types of machinery is often limited by the performance of its lubricated interfaces, which are formed by two interacting surfaces and a lubricant that separates the surfaces to decrease friction and wear. There is a constant quest for more efficient lubricants capable of lubricating non-ferrous surfaces. Ionic liquids (ILs) possess many unusual physicochemical properties compared to molecular liquids including, but not limited to, high polarity, high conductivity, high thermal stability, and a wide liquid range. Such properties are essential for formulating lubricants for interfaces in challenging applications such as in aerospace and wind turbines. Some ILs have recently shown promising performance in lubricating lightweight non-ferrous alloys and hard coatings, among other materials. 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, we designed, synthesized, and thoroughly characterized a number of hydrophobic ILs based on halogen-free and hydrolytically-stable orthoborate anions and different classes of cations such as pyrrolidinium and imidazolium. The potential of these ILs to efficiently lubricate ferrous and non-ferrous interfaces was investigated.The work was planned and carried out through the following steps: Design, synthesis and purification of novel, halogen-free, boron-based ionic liquids (hf-BILs). 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.A detailed description of the results obtained for selected classes of ILs is given below: 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 anti-wear and friction-reducing performance compared with 5W40 engine oil.II. Eight novel, room-temperature N-alkyl-N-methylpyrrolidinium bis(mandelato)borate ([CnC1Pyrr][BMB]) ILs 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 anti-wear and friction-reducing properties were achieved when compared with neat PEG and 5W40 engine oil.III. Three novel, room-temperature 1-alkyl-3-methyl-imidazolium bis(mandelato)borate ([CnC1Im][BMB]) ILs were synthesized and physicochemically characterized.IV. A room-temperature trihexyltetradecylphosphonium bis(oxalato)borate [P6,6,6,14][BOB] IL, was evaluated as a neat lubricant for alumina-steel and sapphire-steel interfaces at room temperature. [P6,6,6,14][BOB] provided lower friction and wear compared with 5W40 engine oil.