Water-based lubricants have garnered significant attention due to their environmentally friendly, flame-retardant nature, abundant availability, and cost-effectiveness. However, they commonly suffer from inadequate lubrication performance, susceptibility to corrosion, and low viscosity. In this study, a novel protic poly(ionic liquid), PPDL, was synthesized with the copolymer of poly (ethylene glycol) methyl ether methacrylate and dimethylaminoethyl methacrylate as the cation, and N-lauroyl glutamate containing bicarboxylic groups as the anion and utilized as water-based lubricant additive. The thermal stability, corrosion resistance, and viscosity of PPDL were characterized through thermogravimetric analysis, immersion corrosion tests, viscometer and rotational rheometer analysis. The friction and wear properties of the additive were evaluated using an SRV-V friction and wear tester in conjunction with a fully automated true color confocal microscope. Furthermore, the lubrication mechanisms of the PPDL additive were investigated by scanning electron microscopy (SEM) to examine the micro-morphology of worn surfaces combined with electrical contact resistance (ECR), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary mass spectrometry (ToF-SIMS) analysis.

The PPDL exhibited exceptional thermal stability, with its thermal decomposition temperatures of 5% and 10% reaching 238.5℃ and 250.7℃, respectively. The addition of PPDL effectively mitigated the corrosion caused by water-based lubricating fluids on cast iron sheets, resulting in a change from corrosion grade D (heavy rusting, severe light loss) for water to grade B (no rusting but slight light loss) for PPDL aqueous solution. Furthermore, at a concentration of 4%, PPDL significantly enhanced the kinematic viscosity of water by one order of magnitude. Under the same concentration condition (6%), PPDL exhibits a greatly better viscosity enhancement effect on water-based lubricating fluids compared to commercial viscosity builders SDN45 and APE30.

The addition of PPDL additive to deionized water could significantly decrease the coefficient of friction and wear volume of the water-based lubricating fluid. When PPDL was added at a concentration of 6%, the average coefficient of friction and wear volume were reduced by approximately 71% and 86% respectively, compared to deionized water, demonstrating exceptional friction reduction and anti-wear properties. Furthermore, the lubrication performance of PPDL additive surpassed that of two commercial viscosity builders, SDN45 and 6% APE30. Compared to deionized water, after lubrication of water-based lubricants containing PPDL, the width and depth of abrasive spots decreased, and the adhesive wear and abrasive wear are significantly reduced.

The results of the mechanisms demonstrated that, during the friction process, a complex chemical reaction occured between PPDL and the friction pairs, resulting in the formation of tribochemical reaction films at the interface containing N, C, O, and Fe. These reaction films, combined with the physical/chemical adsorption films formed by the polymer at the interface, synergistically prevent direct contact between sliding pairs, which endowed the water-based lubricant with superior tribological performance. The additive was convenient to prepare and environmentally friendly, and was promising to be utilized as the multifunctional additive in metal working fluids and non-flammable hydraulic fluids.