Ionic liquids as tribological performance improving additive for in-service and used fully-formulated diesel engine lubricants

Abstract

a b s t r a c tIn recent years, several papers have been published that investigate the potential use of ionic liquids (ILs) as additives in lubricants. However, corrosive attack of ILs on lubricated metal surfaces and low miscibility of ILs in the non-polar oils are major obstacles to maintaining an optimum lubrication performance level. High miscibility and no corrosive behaviour of Trihexyltetradecyl phosphonium bis(2,4,4-trimethylpentyl) phosphinate and Trihexyltetradecyl phosphonium bis(2-ethylhexyl) phosphate, as lubricating oil additives have recently been described in literature. This article presents work on using these phosphonium based ILs as an additive in the fully formulated diesel engine lubricants. This approach could allow the used lubricants to recover their tribological performance for further use at the end of service life. This extension of service life has the potential to generate significant economic and environmental benefits. Also it will add to the much needed knowledge about the effect of interaction between ILs and existing additives in engine-aged lubricants on the tribological performance of ringliner tribo-system of diesel engines. Results revealed an improvement in friction and antiwear performance of used lubricant by addition of both ILs. However an increase in wear was noted for new (fresh) and in-service lubricant samples. An interesting interference between existing lubricant additives and added ILs in a boundary film formation process has been observed.

Assessing Boundary Film Forming Behavior of Phosphonium Ionic Liquids as Engine Lubricant Additives

“…In the latter case, the antagonistic interaction between IL2 and already-existing additives in New Oil could have led to the generation of stresses responsible for a higher film removal rate than the film formation rate [6]. The wear results for all the cases of Used Oil are in line with the ECR curves (Figure 3), such that they follow a same trend as the boundary film thickness.…”

“…In addition, the mixtures of Used Oil with both ILs outperformed the fully formulated New Oil (with/without ILs). These observations indicate either the synergistic interaction between the remaining content of already present additives in Used Oil and the later-added ILs (as demonstrated by the authors for high tempertaure testing in the past [6]) or the stronger affinity of ILs to form films on surfaces than the existing additives.…”

“…The XPS analysis was performed using a SPECS Phoibos 100 MCD5 system equipped with a hemispherical electron analyzer, and the spectra were analyzed using CasaXPS software (Casa Software Ltd.). Further details of the equipment used can be found in the previous work of the authors [6].…”

“…Since 2001, many studies have reported on the tribological properties of ionic liquids (ILs) as an additive in mineral and/or synthetic base oils [3,[6][7][8][9][10], and others on their use as engine lubricants [3,4,7,11,12]. ILs have some key inherent properties that make them suitable for an engine environment, such as high thermal stability, very low volatility, non-flammability, detergency in being a solvent, they are non-corrosive, have good wettability, and excellent tribological performances [3,7].…”

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.…”