Imidazolium Ionic Liquids As Antiwear and Antioxidant Additive in Poly(ethylene glycol) for Steel/Steel Contacts

Three imidazolium-based ionic liquids containing sterically hindered phenol groups were synthesized. The cation was1-(3,5-ditert-butyl-4-hydroxybenzyl)-3-methyl-imidazolium, and the anions were tetrafluoroborates, hexafluorophosphates, andbis(trifluoromethylsulfonyl)imide. The physical properties of the synthetic products and of poly(ethylene glycol) (PEG) with the additivewere evaluated. The oxidative stability of 0.5 wt%1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methyl-imidazoliumhexafluorophosphatesin PEG were assessed via rotating bomb oxidation test (RBOT), thermal analysis, and copper strip test. The tribological behaviors ofthe additives for PEG application in steel/steel contacts were evaluated on an Optimol SRV-IV oscillating reciprocating friction andwear tester as well as on MRS-1J four-ball testers. The worn steel surface was analyzed by a JSM-5600LV scanning electronmicroscopeand a PHI-5702multifunctional X-ray photoelectron spectrometer. RBOT test, thermal analysis, and copper strip test results revealedthat synthesized ionic liquids possessed excellent antioxidant properties. Tribological application results revealed that these couldeffectively reduce friction and wear of sliding pairs compared with the PEG films used without the additives. Specifically, (BHT-1)MIMPF6exhibited better antiwear properties at an optimumconcentration of 1 wt%. At this level, its antiwear property significantly improvedby 100 times with respect to using just the PEG base oil. Boundary lubrication films composed of metal fluorides, organic fluorines,organic phosphines, and nitride compounds were formed on the worn surface, which resulted in excellent friction reduction and antiwear performance.