Simple but effective: Liquid superlubricity with high load capacity achieved by ionic liquids

Macroscale superlubricity is a new field that can greatly reduce energy consumption due to friction, which accounts for approximately 30% of human nonrenewable energy. However, prior research on liquid superlubricity mostly involved low contact pressures (<600 MPa). Here, we evaluated the lubrication and adsorption properties of ionic liquid alcohol solutions (ILs(as)) with different alkyl chain lengths and monovalent anions between Si3N4 and sapphire surfaces. Robust macroscale superlubricity and ultrahigh load capacity exceeding 1 GPa were obtained by protective adsorption layer caused by tribochemical reactions. Zeta potential measurements and simulation analysis revealed that the ceramic interfaces were enriched with numerous ionic liquid (IL) anions, preventing the direct collision of the asperities of the friction pairs. The relatively low sliding energy barrier and extremely strong electrostatic repulsion developed between these asperities enhanced the superlubricity and ultrahigh load capacity. Meanwhile, long alkyl chains and tetrafluoroborate anions more easily met the hydrodynamic boundary conditions at the solid–liquid interface, and the formation of low energy consumption channels through the smooth potential energy fluctuation surface generated extremely low shear stress. Such macroscale lubrication provides a novel method of achieving extremely high contact pressure in ILs(as), which could enable liquid superlubricity in practical industrial applications.