Atomic-scale liquid lubrication mechanisms at ZnO interfaces with ultra-low friction coefficient enabled by glycerol-water solutions.

Understanding atomic-scale lubrication mechanisms at polar oxide interfaces is essential for minimizing frictional energy losses and improving the reliability of micro- and nanoscale devices. Zinc oxide (ZnO) exhibits pronounced surface polarity and excellent chemical stability. Glycerol-water solutions can form extensive hydrogen-bonding networks. This combination of properties favors boundary lubrication and friction reduction at solid-liquid interfaces. In this study, reactive force field molecular dynamics (ReaxFF MD) simulations combined with density functional theory (DFT) calculations were used to systematically examine the tribological behavior of a ZnO and glycerol-water solution system under varying normal pressure (1-6 GPa), temperature (300-550 K), and sliding velocity (2.5-15 m s-1). The simulations reveal a stable low-friction regime under nanoscale confinement, with the friction coefficient reaching 0.024 at the reference condition of P = 3 GPa, T = 500 K, and v = 10 m s-1. The friction reduction at the ZnO-glycerol-water interface originates from coupled confinement-induced structuring and interfacial electrostatic interactions. Under applied normal pressure, the lubricant film reorganizes into well-defined layers with reinforced hydrogen bonding, accompanied by increasingly negative Coulomb interaction energies that indicate enhanced electrostatic interactions and interfacial polarization. These coupled effects collectively regulate molecular mobility and shear response, giving rise to the low-friction behavior. These findings provide an atomic-scale understanding of aqueous glycerol lubrication at ZnO interfaces and offer principles for designing sustainable, low-friction lubrication systems.