The Simultaneous Enhancement of Rheological and Tribological Properties of Lubricating Grease at High Temperatures by the Boron-Containing Ionic Liquid

This study synthesized an environmentally friendly halogen-free bis(mandelato)borate ionic liquid ([C₁₀MIm][BMB], BMB) as an additive for lithium-based grease. A comprehensive evaluation of its rheological and tribological properties was conducted combined with molecular dynamic simulations to probe the interaction mechanism between the additive and soap molecules. A marked increase of 22 °C in the dropping point was observed with the addition of 2 wt % BMB, but the cone penetration was unaltered. At an elevated temperature of 125 °C, the thixotropic loop area was reduced by 51% compared with that of the base grease. Rheological analysis confirmed that BMB effectively enhanced the structural recovery capability of the grease at high temperatures, albeit with a moderate reduction in its structural strength. Based on tribological testing, the incorporation of BMB resulted in a 16.8% lower average coefficient of friction (COF) and 71% less wear volume compared to the base grease. Furthermore, it maintained stable lubrication under severe operating conditions (100 N and 130 °C), whereas the base grease failed. Mechanistic studies indicated that the anions of BMB adsorb electrostatically onto the friction interface and decompose to form a high-hardness B₂O₃/iron oxide composite boundary lubrication film. Molecular dynamics simulations further confirmed that BMB promotes the formation of a denser soap fiber skeleton structure by mitigating the intermolecular interactions between soap fibers. BMB significantly enhances the thixotropic recovery performance of the grease at elevated temperatures by facilitating a more stable and reversible three-dimensional soap fiber network, while the generated densely structured and rigid B₂O₃ boundary lubrication film ensures excellent high-temperature lubrication and antiwear capabilities. This work offers an effective strategy for developing high-performance environmentally friendly greases suitable for high-temperature and high-load applications.