Abstract
Two ionic liquids, tributylmethylphosphonium dimethylphosphate (PP) and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP), as lubricant additives in polyalphaolefin (PAO8) were studied under boundary lubricating conditions on two types of steel (AISI 52100 bearing steel and AISI 316L stainless steel). The tribological behavior of these ILs was compared with dodecanoic acid, a well-known organic friction modifier. This study employs a ball-on-disk tribometer with an alumina ball as a counterpart. A range of advanced analytical tools are used to analyze the tribofilms, including scanning electron microscopy equipped with a focused ion beam, scanning transmission electron microscopy equipped with X-ray energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy. A quartz crystal microbalance with dissipation was used to study the surface adsorption of the additives on iron- and stainless steel-coated sensors to reveal the adsorption kinetics, adsorbed layer mass, and bonding strength of the adsorbed layer on the metallic surfaces. The most important factors controlling friction and wear are the thickness and viscoelastic properties of the adsorbed layer, the thickness and chemical composition of the tribofilm, and the hardness and chemical composition of steel. Among all additives studied, BMP on stainless steel gives a strongly adsorbed layer and a durable tribofilm, resulting in low friction and excellent antiwear properties.
Highlights
Friction between moving parts and their associated wear is estimated to be directly responsible for 23% of the world’s energy consumption.[1]
The friction evolution results on AISI 52100 steel and AISI 316L stainless steel are presented in Figure 1A,B, respectively
Similar trends are observed for PAOPP and PAO-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP), in which PP and BMP delayed the increase of the coefficient of friction (COF) to 30 and 110 m, respectively
Summary
Friction between moving parts and their associated wear is estimated to be directly responsible for 23% of the world’s energy consumption.[1] Road transport is responsible for 22% of Europe’s CO2 emissions. An electric car charging on the European electricity grid corresponds to about 20 g/km of CO2 emissions. In electric cars, moving parts work at a higher speed than in internal combustion engine cars, making the lubricants function more as a torque transfer than as a load-bearing.[2] The higher the speed of the tribological component, the higher the temperature generated in the lubricant. Low-viscosity lubricants with better cooling properties and higher temperature stability are the trend for meeting the UN sustainable goals.[3,4]
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