Tribological performance and degradation of 1-n-butyl-1-methylpyrrolidinium methylsulfate ionic liquid in glycerol as lubricant for steel-steel sliding contacts

The ionic liquid (IL) 1-n-butyl-1-methylpyrrolidinium methylsulfate was studied as additive for glycerol in steel-steel contacts. The mixtures were evaluated in different IL concentrations with a reciprocating sliding tribometer in a ball-on-disc configuration under elastohydrodynamic and boundary conditions at room temperature and 100 degrees C, respectively. At room temperature, friction and wear were mainly determined by the high viscosity of glycerol; hence, little differentiation was found. At 100 degrees C, surface smoothening resulted in decrease of friction and wear with increase of IL concentration, which indicates corrosion as dominant part of the lubricating mechanism. A 0.6-wt% IL turned out as best performing mixture regarding tribological behaviour and additive efficiency. Methylsulfate is reduced to sulphide when in contact with the steel surface. Methylsulfate reacts with glycerol to give glyceryl sulfate already during mixing as revealed by mass spectrometry. Acorrdingly, after triboexperiments, ions composed of sulfate, iron, and glycerol were identified.