Tribological properties of adsorbed water layer on silicon surfaces

Tribological properties of adsorbed water layer on solid surface in sliding contact have not yet been fully understood. In this regard, it is important to better understand how surface hydrophilicity and humidity influence the tribological behavior of adsorbed water-mediated microcontact. In this study, we investigated the influence of adsorbed water layer and capillary force on friction as a function of relative humidity for silicon surfaces with different water affinity. Friction of the silicon surface with different water affinity was examined under various humid environments in a wearless sliding condition (low contact pressure) against a glass sphere. Numerical analysis was also conducted to calculate capillary force and interfacial shear strength for each surface as a function of relative humidity. The friction of the hydrophobic Si surface was low and stable, and almost independent of relative humidity whereas that of the hydrophilic surfaces were significantly influenced by relative humidity. These behaviors were explained in terms of capillary wetting and the role of confined water layer in the contact area. The influence of confined water layer became more dominant over capillary force as relative humidity increased. There was a good correlation between the calculated shear strength and the measured friction force for all surfaces regardless of their hydrophilicity and humidity condition.