We use atomic force microscopy and grand canonical Monte Carlo simulations to study stick-slip friction of tetrahedral amorphous carbon probes sliding against highly oriented pyrolytic graphite (HOPG) at relative humidities ranging from <1% to near saturation. Friction varies with humidity in a nonmonotonic manner such that water acts as a lubricant only above a threshold humidity; below that threshold, water increases friction substantially relative to dry sliding. Adhesion forces also show a similar nonmonotonic behavior. A nonmonotonic dependence of friction and adhesion on humidity for single-asperity interfaces has previously been attributed to the humidity-dependent adhesion force due to the water meniscus that forms at the contact, which is presumed to increase the solid-solid contact area. However, our simulations show no such increase in solid-solid contact area, but do show a small, continuous increase in tip-sample separation as humidity increases. Experimentally, no significant change in lateral stiffness is observed with humidity. All of this contradicts the hypothesis that the friction increase is due solely to capillary adhesion increasing the contact area. We show that water molecules are present between the tip and sample in increasing numbers as the humidity increases. From this, we attribute the nonmonotonic friction trend to the changing quality of the contact between the water and the substrate, quantified by the number of water molecules in the interface and their registry with the HOPG surface atoms, which in simulations also shows a nonmonotonic trend with humidity. Hysteresis observed in the variation of friction with humidity in both experiments and simulations is explained by the larger energy barrier for surface desorption of water molecules compared to adsorption.

• Received 5 September 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.126001