Ionic Liquids Based on Phosphonium Cations As Neat Lubricants or Lubricant Additives for a Steel/Steel Contact

Abstract

After doing several miscibility essays with eight ionic liquids (ILs) and four base oils, the ILs tri(butyl)ethylphosphonium diethylphosphate [P4,4,4,2][C2C2PO4] and trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate [P6,6,6,14][(C2F5)3PF3] were selected to be studied as lubricant additives. The neat IL [P4,4,4,2][C2C2PO4], the base oils, and several blends were characterized in terms of density, viscosity, and thermal stability. The tribological performance of the miscible base oil/IL blends (1 wt %) and the neat [P4,4,4,2][C2C2PO4] were evaluated for the lubrication of an AISI 420 steel-100Cr6 steel contact pair. The friction coefficients and wear volumes obtained are also compared with those corresponding to the pure base oils and their mixtures with conventional additive zinc dialkyldithiophosphate (ZDDP). As neat lubricants, [P4,4,4,2][C2C2PO4] showed the best antifriction ability, whereas in terms of wear, better results were obtained with [P6,6,6,14][(C2F5)3PF3]. However, higher improvements in both friction and wear were found for blends containing [P4,4,4,2][C2C2PO4]. XPS analyses of the worn surfaces lubricated with these mixtures indicated the presence of phosphorus in the tribofilm formed on the wear track. However, this compound was slightly detected on tribosamples lubricated with blends containing [P6,6,6,14][(C2F5)3PF3].

Isoconversional kinetic analysis applied to five phosphonium cation-based ionic liquids

“…This fact provoked the introduction of new and more stable ILs based on [FAP] and [NTf 2 ] anions [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53], which present enhanced tribological properties due to the formation of fluoride tribofilms, especially at high loads [54]. Recently, phosphonium cation-based ILs have been introduced in this field because of their growing commercial availability and good tribological performance [26,[55][56][57][58][59][60][61][62]. Finally, taking into account the importance of thermal stability in the lifetime of lubricants and additives, the determination of thermal models has become an interesting research topic from scientific and technical points of view.…”

Physicochemistry aspects on frictional interfaces

“…Except for the polarity-induced surface absorption, under harsh sliding conditions, the active elements [N, F, P(B)] in ionic liquids offer sufficiently rich tribochemistry to form extreme pressure films of lower shearing strength by reacting with the fresh surface, which leads to lower friction coefficient and good wear resistance. It was found that ionic liquids as the additives in base oils also perform very well [60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75]. The tribochemical reactions of ILs have been extensively studied.…”

Tribological Performance of Phosphonium Ionic Liquids as Additives in Lithium Lubricating Grease

“…Several papers have been published on the use of different types of ILs as lubricants, lubricant additives, or base oils to prepare lubricating grease [6][7][8][9][10][11]. However, from the aspect of engine applications, using a small amount of ILs as an additive seems to be an economical option due to the high cost of ILs [12].…”