Ionic Polyurethane for Aqueous Lubrication Systems: Synergistic Effect of Multiple Noncovalent Interactions on Friction-Reducing and Load-Bearing Capacity

Water-based lubricants present a promising alternative to oil-based lubricants, addressing issues such as flammability, explosion, and environmental toxicity risks. However, the interactions between lubricant additives and interfaces are prone to being weakened by polar water molecules, thereby compromising the formation and stability of the boundary lubrication layer. In this study, a macromolecular additive based on an ionic polyurethane (PU) is proposed, and the presence of noncovalent interactions within this aqueous lubrication system is verified through comprehensive characterization technologies. Stronger hydrogen bonding interactions, combined with longer hydrophobic chains of the paired cations in polyurethane, significantly contribute to enhancing friction-reducing and load-bearing performance. The resulting lubricant exhibits a low coefficient of friction (≈0.07), high extreme pressure-bearing capacity (450 N), and minimal corrosion to metal surfaces. Different characterization techniques, including quartz crystal microbalance with dissipation (QCM-D), time-of-flight secondary ion mass spectrometry (ToF-SIMS), focused ion beam (FIB) together with aberration-corrected scanning transmission electron microscopy (ac-STEM), and X-ray photoelectron spectroscopy (XPS), confirm pronounced adsorption and active participation of PU in tribo-chemical reactions. Furthermore, this ionic PU exhibits remarkable environmental compatibility and antimicrobial properties, addressing the biological toxicity and susceptibility to microbial proliferation associated with water-based lubricants. This polyurethane-type additive with an ordered structure not only serves as a model for exploring the synergistic effects of multiple noncovalent interactions on lubrication performance but also represents a highly valuable additive with significant practical applications in industry.