Tribological properties of ionic liquids as lubricants and additives. Part 1: Synergistic tribofilm formation between ionic liquids and tricresyl phosphate

Abstract

The use of imidazolium tetrafluoroborate, IMM + BF − 4 , and hexafluorophosphate, IMM + PF − 6 , ionic liquids as lubricants was investigated at 25 and 100 • C to show lower friction coefficients but higher wear rates than a reference hydrocarbon lubricant. The ionic liquids readily form tribofilms at the lower temperature but have difficulty in forming partial films at the higher temperature. Wear tracks for a Plint TE77 reciprocating ball-on-plate test using ionic liquids show smoother surfaces, with small pits developing, compared to the reference hydrocarbon lubricant test result. Similar ionic liquids not containing fluorine were found to be less effective as lubricants.Addition of ionic liquids to a base grease and a formulated high temperature grease gave surprisingly large increases in the weld load for the Four Ball extreme pressure test. Little difference between base grease and base grease +5 per cent ionic liquid was observed for the TE77 test. There was no clear indication of the effect of alkyl substituent chain length on the imidazolium cation for the Four Ball test wear scar diameter.Addition of 1 per cent tricresyl phosphate (TCP) to ionic liquids rapidly establishes a tribofilm and reduces the wear volume by 64 per cent compared to the same test for the neat ionic liquid or neat TCP. Addition of 1 per cent TCP and 1 per cent ionic liquid to a Group III base oil also establishes a substantial tribofilm and reduces wear volumes compared to the base oil with 1 per cent TCP alone or the base oil with 1 per cent ionic liquid alone.Ionic liquids show promise as neat liquid lubricants by establishing a tribolayer chemically adsorbed to the steel surfaces. They are not as effective as a reference hydrocarbon lubricant in reducing wear of those surfaces by tribocorrosion. The fluorine-free ionic liquids investigated were not as effective as those containing fluorine. The addition of ionic liquids to grease, base or fully formulated, gave a substantial improvement in performance, which indicates a synergistic interaction with the additives present in the formulated grease. There is also clear evidence of a strong synergistic effect between ionic liquids and TCP, both for the neat ionic liquids and for 1 per cent dilution of each respectively in a Group III base oil to give a thick tribofilm and substantially reduced wear in the TE77 ball-on-plate test. The nature of the synergy between ionic liquids and TCP requires further investigation.properties arise from the incompatibility of their component anions and cations, which prevents them crystallizing at conventional operating temperatures. They are suitable for use as lubricants with melting points often well below ambient, with wide liquid temperature ranges, extremely low vapour pressures, thus very low volatilities, very low flammabilities, and high thermal-oxidative stability, better than synthetic base fluids [2]. They also have low reactivities towards JET387

Effect of Imidazolium Ionic Liquid Additives on Lubrication Performance of Propylene Carbonate under Different Electrical Potentials

“…The lubricity of some kinds of ILs has been investigated in recent years [7][8][9][10][11][12][13]. A few studies used ILs as base fluids to provide lubrication for rubbing metal surfaces [8][9][10]13].…”

“…A few studies used ILs as base fluids to provide lubrication for rubbing metal surfaces [8][9][10]13]. However, the use of ILs as base lubricants is still not economical for most of engineering applications.…”

Alkyl Imidazolium Ionic Liquids as Friction Reduction and Anti-Wear Additive in Polyurea Grease for Steel/Steel Contacts

“…Fox et al studied ILs as additives in grease gave surprisingly large increases in the weld load for the four-ball extreme pressure test [38]. However, in addition to research work by Fox et al, alkyl imidazolium ILs as additives in grease is still relatively small.…”

Ionic Liquids as Lubricants

“…In contrast, Fox and Priest reported in a recent study the use of ionic liquids in group III hydroisomerized (high-quality) mineral base oil, mineral oils and different greases as an additive [5]. Of course, for their use in commercial lubricants, ionic liquid additive concepts need to be developed in order to obtain suitable ionic liquid formulations.…”