Quaternary Ammonium Salt-Based Deep Eutectic Solvents As Alternative Base Lubricants: Interfacial Tribological Performance

As functional solvents, deep eutectic solvents (DESs) have been widely applied in various fields. However, their application in the lubrication field remains in an exploratory stage. Existing studies are often limited by directly transplanting formulations from other domains into lubrication without a thorough investigation of formulation design. To address this gap, this study focuses on hydrogen bond acceptor quaternary ammonium salts (QASs) as a research object. By introducing long alkyl chains and dual-long-chain substituents and incorporating the hydrogen bond donor menthol (DL), three novel QAS-DL DESs were designed and synthesized. Experimental results demonstrated that with increasing alkyl chain length and number of long chain substituents, the viscosity of DESs significantly increased. In terms of tribological performance evaluation, the high-viscosity dual-long-chain DES exhibited optimal friction-reducing and antiwear properties under 60 N load due to its effective separation of friction pairs. Compared to conventional ChCl-DES, it achieved a 53.7% reduction in the average friction coefficient and a 31.1% decrease in the wear rate. The increment in the length of alkyl chains and the quantity of long-chain substituents can elevate the system viscosity, thereby enhancing the load-bearing capacity of the oil film and effectively isolating the friction interfaces. In conjunction with XPS characterization, the lubrication mechanism of QAS-DL DESs was revealed: QAS-DL DESs achieve friction reduction and wear resistance via the synergistic effect of a physically adsorbed film and the chemical reactions of iron oxide and iron chloride.