How Water Makes the Surface Slippery; Friction on Water/Au(111) Investigated with Variable-Pressure Friction Force Microscopy

Understanding the influence of water on tribological properties is crucial for advancing lubrication science, yet precise control of water vapor pressure and nanotribological measurements remain challenging. In this study, we investigated the effect of water on the friction and surface morphology of Au(111) using variable-pressure atomic force microscopy. Our results show a decrease in friction with increasing water vapor pressure, attributed to water molecule adsorption. By monitoring the dependence of the onset load of the plastic deformation on changing the vapor pressure of water, we show that the water layers suppress the plastic deformation induced by AFM tip loading. Near-ambient pressure X-ray photoelectron spectroscopy revealed the extent of water adsorption under varying pressure conditions. To further elucidate these findings, molecular dynamics simulations were performed, demonstrating that a higher water vapor pressure reduces dislocation density and length while facilitating bond breaking in water networks under load. This bond-breaking mechanism explains the reduction in the onset load of plastic deformation, as energy applied by the AFM tip is dissipated into hydrogen bond disruption. Our findings reveal the nanoscale passivation of metal surfaces in humid environments and highlight the lubricating role of water in tribological behavior.