Hybrid combinations of graphene nanoplatelets and phosphonium ionic liquids as lubricant additives for a polyalphaolefin

Abstract

Tribological performance of three ionic liquids (ILs), trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate (IL1), tributylethylphosphonium diethylphosphate (IL2) and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate (IL3) combined with graphene nanoplatelets (GnP) as hybrid additives for a polyalphaolefin (PAO 32) base oil was studied. For this purpose, several dispersions were prepared by mixing, stirring, and then sonicating according to the following combinations: PAO 32 + (a wt%) IL + (b wt%) GnP, where a and b represent the concentration of the additives added to the PAO 32 base oil. In this study a is 0 or 1 and b is 0.05 or 0.1. Three PAO 32 + 1 wt% IL mixtures were also prepared. Thermophysical properties and stability against sedimentation of the dispersions were studied by means of a rotational viscometer and visual observation, respectively. Furthermore, friction and wear behaviors were analyzed using a ball-on-disk configuration tribometer operating in rotational mode and both a 3D optical profiler and a scanning electronic microscope, respectively. Confocal Raman microscopy was used to identify compounds in the tribofilms formed on the wear tracks. The hybrid combinations of PAO 32/ILs/GnP improved the friction reduction of the corresponding binary PAO 32/GnP nanolubricants and PAO 32/IL mixtures. Interestingly, the hybrid dispersions with low concentrations of GnP (with 0.05 wt% GnP) are more effective than those of 0.1 wt% GnP. Results also show that the addition of both 0.05 wt% graphene nanoplatelets and 1 wt% IL led to friction reductions up to 36% and wear reductions up to 27%, compared with the capabilities of neat PAO 32. IL1 and IL3, containing the trihexyltetradecylphosphonium cation, generate the hybrid lubricants with the best combined properties (stability, viscosity and tribological properties) of all the lubricants tested.