Water-based lubrication has gained significant attention in tribology due to its availability, eco-friendliness, non-flammability, high thermal conductivity, and excellent cleaning properties. Replacing oil-based lubricants, which pose environmental risks, is an effective way toward achieving green tribology. However, water-based lubricants typically face challenges such as low viscosity, susceptibility to corrosion, and inferior lubrication performance. Water-soluble poly(ionic liquid)s, which combine the benefits of polymers and ionic liquids, offer potential as multifunctional water-based lubricant additives to enhance the physicochemical and tribological properties of water-based lubricants.

Through rational molecular structure design, we developed protic poly(ionic liquids) (PPILs) water-based lubricating additives, PPD-N, by combining a polymer chain visco-enhancing structure with a proton-type ionic liquids lubricating structure. PPD-N demonstrates excellent viscosity enhancement, corrosion resistance, and lubricating properties. Kinematic and dynamic viscosities of different water-based lubricating fluids were investigated at 25 ℃ and 40 ℃ using a Pinkevitch Viscometer and a rotational rheometer, with the commercial viscosity builder, Koreox W55000, serving as the control. Following the national standard GB/T 6144—2010, we evaluated corrosion inhibition performance on first-grade gray cast iron using immersion corrosion tests, comparing deionized water, 6% PPD-N, and Koreox W55000 aqueous solution. The friction reduction performance of PPD-N additives was assessed using the SRV-V tester, while its anti-wear properties were characterized using a fully automated real-color confocal microscope. Elastohydrodynamic lubrication properties of PPD-N were investigated by optical interferometry. The surface micromorphology of wear patches was observed using scanning electron microscopy. We also investigated the lubrication mechanisms of the additives using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS).

The results of ¹H NMR, FT-IR, GPC and TGA tests confirmed the successful synthesis of PPD-N, which demonstrated excellent thermal stability with 5% and 10% thermal decomposition temperatures of 249.7 ℃ and 268.9 ℃, respectively. Adding PPD-N can significantly improve the viscosity of water-based lubricants, with viscosity increasing proportionally to the PPIL additive content. PPD-N also effectively reduced the corrosion of cast iron sheets caused by water-based lubricants, outperforming commercial water-based viscosity builders at equivalent concentrations. At a 6% concentration, the PPD-N aqueous solution achieved a coefficient of friction and wear volume of 0.106 and 22.89×10^-5 mm³, respectively, a reduction of about 66% and 85% compared to water's coefficient of friction (0.314) and wear volume (148.20×10^-5 m³). Elastohydrodynamic lubrication tests revealed that the PPD-N-containing aqueous solution increased the film thickness at the friction interface with rising velocity. Both the central film thickness and minimum film thickness at the lubricant outlet were significantly higher than those of deionized water. Based on XPS and ToF-SIMS analyses, the lubrication mechanism of PPD-N can be attributed to the formation of tribochemical reaction films and adsorption films at the friction interface. These films effectively prevent direct contact between friction surfaces, endowing the water-based lubricant with superior tribological performance.

Compared to the commercial viscosity builder, Koreox W55000, PPD-N additives demonstrate superior lubricity and anti-wear properties. They significantly enhance the viscosity of water-based lubricants and effectively inhibit cast iron corrosion in water. Free of phosphorus, sulfur, and halogens, PPD-N is simple to synthesize, environmentally friendly, and has great potential as a viscosity-building lubricant additive for non-flammable hydraulic fluids and fully synthetic water-based metalworking fluids.