Frictional evaluation of halogen-free ionic liquids with low corrosion degree and high decomposition temperature on steel surface

Purpose The purpose of this paper is to investigate the tribological properties, antiwear mechanism and anticorrosion properties of two novel halogen-free borate ionic liquids (ILs) in 500 N base oil. Design/methodology/approach Different qualities of borate ILs were added to 500 N, and their tribological properties were tested on a four-ball machine. The tribological properties of the additives were evaluated by measuring the wear scar diameter (WSD) and average coefficient of friction. The antiwear and antifriction mechanism of ILs was analyzed by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Findings The corrosion degrees of the two borate ILs 1-butyl-3-octyl-imidazole bis(mandelato)-borate ([OBIM][BMB]) and 1-butyl-3-(3-methoxypropyl)-imidazole bis(mandelato)-borate ([MPBIM][BMB]) are 1b and 1a, respectively, suggesting that they both possess significant corrosion resistance and can effectively protect the steel surface. When the concentration of [OBIM][BMB] and [MPBIM][BMB] is 2.5 Wt.%, the friction coefficient of 500 N is reduced by 37.3% and 26.2%, respectively. According to the analysis of the thermo gravimetric analyzer curves, [OBIM][BMB] and [MPBIM][BMB] decomposed at 369.7°C and 374.3°C, respectively, indicate that two additives both can be applied in higher temperature condition. The results of XPS and scanning electron microscope (SEM) reveal that [OBIM][BMB] and [MPBIM][BMB] both can react with the steel surface, thereby forming chemical films composed of iron oxide, B2O3 and organic N-containing compounds. Originality/value Two new environmentally friendly borate ILs were synthesized and their tribological properties in 500 N base oil were investigated for the first time.