Tribological Properties of Tetrabutylammonium Chloride-Polyethylene Glycol Deep Eutectic Solvents

The development of technology and industry has necessitated new performance standards for lubricant, which requires that the lubricant not only superior friction-reducing anti-wear properties, but also non-flammability, low saturation vapor pressure, low toxicity, simple preparation. Over the past two decades, ionic liquids have received widespread attention due to their exceptional tribological properties, high temperature resistance, non-volatile nature, and designable structure. However, their application in industrial settings has been somewhat constrained by the complex synthesis process and elevated cost. Deep eutectic solvent (DES), as a novel class of functional liquid exhibit physical properties akin to ionic liquids while offering the benefits of a more straightforward synthesis process and reduced cost. Consequently, they are poised to meet the performance demands of industrial lubricant development and anticipated to emerge as a new type of lubricant. Deep eutectic solvent (DES) is solvent composed of hydrogen bond donors (HBD) and hydrogen bond acceptors (HBA) in a certain molar ratio, which is subject to hydrogen bonding so that their melting point is lower than the melting points of the individual components. In this study, three deep eutectic solvents (TP2, TP4, and TP6) were successfully prepared by using polyethylene glycol with molecular weight of 200, 400, and 600 as the hydrogen bonding donor, and tetrabutylammonium chloride (TBAC) as the hydrogen bonding acceptor. Shift of characteristic peaks in NMR and FTIR spectra verified the successful preparation of DESs. The physicochemical properties of DESs had been characterized. It was shown that the prepared DESs had low pour point, good thermal stability and great viscosity-temperature performance. In oscillating sliding contacts at 25 ℃ and 100 N (2.17 GPa), compared with the ester base oil, three prepared DESs exhibited excellent friction reduction and anti-wear performances. The friction coefficients of TP2, TP4 and TP6 were 64.77%, 60.64% and 65.96% of those of an ester base oil, and the wear volume was 16.93%, 15.92% and 18.85% of the ester base oil. In the step-loading experiment conducted at 25 ℃ with a load range of 100~800 N, the tribological performance of TP4 was found to be on par with that of TMTC base oil. Notably, it surpassed the performance of PAO6 base oil. Wear surface morphology and elemental composition were analyzed using scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) to gain insight into the lubrication mechanism. The good tribological performance of the DESs was attributed to the adsorption film and the tribochemical films formed on the rubbing interfaces. These films work effectively to reduce the friction and wear in boundary lubrication. This study contributed partly to the application of deep eutectic solvents in the realm of tribology. Furthermore, it facilitated the exploration of novel deep eutectic solvents possessing superior tribological properties.