Tribological Properties of Dimer Acid Ionic Liquids as Lubricant Additives in Water-Based Fluid
In this study,four ammonium carboxylate-type ionic liquids(CAILs)were synthesized with four dicarboxylic acids and diisopropanolamine and used as water-based lubricant additives,with their tribological performance and lubrication mechanisms investigated.Infrared spectroscopy and nuclear magnetic resonance(NMR)were used to show that the synthesized CAILs additives had well-defined molecular structures.The four CAILs had been shown to have good solubility and dispersion stability in water-diethylene glycol(WDG)solution.Tribological tests of the four CAILs in WDG were carried out using a TE77 long-range reciprocating friction tester to evaluate the lubrication performance of them as additives in a ball-and-disc configuration for steel-steel sliding contacts.The four CAILs reduced the friction coefficients of the WDG base fluid to varying extents but only the dimeric acid ionic liquids(DC36-DIPA)effectively reduced the wear volume,suggesting that DC36-DIPA possessed the best friction reduction and anti-wear performance under the studied conditions.The influence of concentration of DC36-DIPA was further examined at four concentrations of 0.5%,1.0%,1.5%,and 2.0%(mass fraction).Results showed that the friction coefficients did not show any significant difference,a concentration of 0.5%DC36-DIPA exhibited the smallest wear volume.This suggested that the concentration could significantly affect the anti-wear performance of the water-based lubricants.It may be related to the change of molecular structure and existing status of DC36-DIPA in the base fluid with effects such as self-assembly.To verify this speculation,small angle scattering(SAXS)tests were performed.It was found that DC36-DIPA exhibited scattering differences at different concentrations,which may indicate that DC36-DIPA exists in different molecular morphology in the solution.It may undergo a transition from vesicle to micelle for the four concentrations.This could be the main reason for the large difference in the anti-wear performance of DC36-DIPA at different concentrations.In addition,the DC36-DIPA additive in the WDG could with stand loads of at least 500 N,and it exhibited low and stable friction coefficients in the 50～500 N range.Optical microscopy and scanning electron microscopy were used to analyze the wear surfaces lubricated with the WDG base fluid and 0.5%DC36-DIPA.Compared to the base fluid,the wear track produced by 0.5%DC36-DIPA was narrower,more uniform and more smooth.QCM tests demonstrated that the WDG base fluid produced obvious physical adsorption,and the addition of DC36-DIPA in WDG had negligible effect on the adsorption.In addition,the XPS analysis results also showed that a generation of tribo-film with iron oxides as the main component in the presence of DC36-DIPA.FIB-TEM results showed that the thickness of the film was 60～70 nm.In conclusion,DC36-DIPA had excellent friction reduction and load-carrying capacity performances,which could be mainly attributed to the physical/chemical adsorption of polar groups-COO-in DC36-DIPA.The excellent anti-wear performance was due to the tribo-chemical reaction between DC36-DIPA molecules and the steel disc,which generated a tribo-film of rather uniform thickness.This study provided an example as a reference for the research and application of ionic liquids in the field of water lubrication.
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