Abstract
ABSTRACTIonic liquids (ILs), both as pure lubricants and as lubricant additives, have been demonstrated extensively to exhibit excellent tribological performance in terms of friction and wear reduction in the boundary lubrication (BL) regime. Because engineering contacts experience boundary and mixed as well as full film lubrication depending on operating conditions, it is crucial to examine whether lubrication regimes other the BL regime can also benefit from the use of ILs. The objective of this work is to investigate the tribological performance of IL additives in the mixed lubrication (ML) and the elastohydrodynamic lubrication (EHL) regimes. Polyethylene glycol (PEG) was used as the base fluid. ILs were synthesized in situ by dissolving lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in PEG. Friction and film thickness measurements were employed to investigate the effectiveness of IL additives at room temperature, 60°C, and 80°C at various loads and slide–roll ratios (SRRs). The effect of IL additives on the rheological behavior of PEG was also investigated. The EHL film thickness increases with increasing IL concentration. EHL friction is, however, only mildly affected by IL additives. In the ML regime, IL additives can reduce friction and metal wear compared to pure PEG in mild conditions. It is conjectured that IL forms sacrificial layers and protects the rubbing surfaces.
Highlights
Room temperature ionic liquids (ILs) are molten salts with low melting temperatures (below 100C; Earle and Seddon (1))
Our results show that IL additives changed the pressure–viscosity coefficient a of Polyethylene glycol (PEG)
We explored the effectiveness of ionic liquids as lubricant additives for mixed lubrication (ML) and elastohydrodynamic lubrication (EHL) regimes
Summary
Room temperature ionic liquids (ILs) are molten salts with low melting temperatures (below 100C; Earle and Seddon (1)) They consist of bulky, asymmetric cations and anions. Pure ILs have demonstrated excellent tribological performance compared to conventional lubricants in numerous studies. Et al (3), Lu, et al (4), Mu, et al (5), and Liu, et al (6) have designed and synthesized various ILs and investigated their tribological properties as pure lubricants using steel–steel and steel–aluminum contacts under oscillatory sliding conditions. Their ILs show superior antiwear performance and load-carrying capacity compared to conventional high-temperature lubricants X-1P and perfluoropolyether (PFPE). Et al (10), (11) demonstrated that ILs have better lubricity than engine oils (15W40 and 0W-10) and polyalphaolefin base oil, with a reduction in friction and wear of steel–aluminum and steel–cast iron contacts
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