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.