Investigation on the role of interfacial water on the tribology between graphite and metals.

We investigated the role of interfacial water on the atomic-scale tribology of graphite by contact atomic force microscopy. Upon the approach of Au and Pt tips toward graphite in water, the hydration layers on the respective surfaces interact with each other. This results in a discontinuous motion of the metallic tips towards the graphite surface. Snap-in forces measured with Au and Pt tips scale with their respective water adsorption energies. Moreover, we observed significant differences for the atomic-scale friction between the Au and Pt tips and graphite in water. The atomic-scale sliding friction between an Au tip and graphite is characterized by low friction forces ( < 1 nN in the range of normal force values = 1-10 nN) and by a periodic stick-slip that corresponds to the honeycomb structure of graphite. With a Pt tip, the sliding friction on graphite in water is characterized by high friction forces ( ≈ 5 nN in the range of normal force values = 1-10 nN) and by an atomic-scale stick-slip whose characteristic lengths may correspond to an ordered water adsorption layer between platinum and graphite.