Phosphonium ionic liquids as lubricants for aluminium-steel

The performance of a series of novel room temperature ionic liquids (ILs) based on the trihexyl(tetradecyl)phosphoniumcation (P66614) and a number of novel anions have been studied in pin-on-disk tests using a 100Cr6 steel ball on AA2024 aluminium disks. The anions coupled to the (P66614) cation include diphenyl phosphate (DPP-), dibutyl phosphate (DBP-), bis (2,4,4-trimethyl pentyl) phosphinate (M3PPh) and bis(2-ethyl hexyl) phosphate (BEH-). More traditional anions such as bis(trifluoromethanesulfonyl) amide (NTf2) and bromide (Br-) were also investigated. Experiments were conducted at various loads to assess the IL film forming abilities. The results suggest that the structure of the anion is important in forming a surface film that reduces the friction and wear of the aluminium disk. At 30N five of the six ILs tested showed a 30-90% reduction in wear, as determined from wear scar depth measurements, compared to fully formulated diesel oil. The IL lubricant with a diphenyl phosphate anion achieved the lowest wear coefficient, showing a better performance than a typical fluorine-containing IL anion, NTf2. To further investigate wear mechanisms and surface interactions the wear scars were analysed using a scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS).