Abstract
Friction force microscopy was performed with oxidized or gold-coated silicon tips sliding on Au(111) or oxidized Si(100) surfaces in ultrahigh vacuum. We measured very low friction forces compared to adhesion forces and found a modulation of lateral forces reflecting the atomic structure of the surfaces. Holding the force-microscopy tip stationary for some time did not lead to an increase in static friction, i.e., no contact ageing was observed for these pairs of tip and surface. Passivating layers from tip or surface were removed in order to allow for contact ageing through the development of chemical bonds in the static contact. After removal of the passivating layers, tribochemical reactions resulted in strong friction forces and tip wear. Friction, wear, and the re-passivation by oxides are discussed based on results for the temporal development of friction forces, on images of the scanned area after friction force microscopy experiments, and on electron microscopy of the tips.
Highlights
Contact ageing, the strengthening of contacts after formation, is an important phenomenon in tribology, with impact ranging from the nano-scale (NEMS and MEMS) [1,2] to the macroscale [3,4]
We conclude that contact ageing on Au(111) that results from Au atom diffusion is too fast to be observed with our experiment, or that the neck formation by Au diffusion does not result in increased static friction as the neck follows the AFM tip movement without additional friction
We did not observe any contact ageing for oxidized silicon tips sliding on gold surfaces or on oxidized silicon surfaces, we found evidence for atomic rearrangement in the course of scanning
Summary
The strengthening of contacts after formation, is an important phenomenon in tribology, with impact ranging from the nano-scale (NEMS and MEMS) [1,2] to the macroscale (sliding of rock in earthquakes) [3,4]. Different microscopic mechanisms for contact ageing have been identified, e.g., material creep [5], structural changes in the interfacial contact [6,7] or an increase in number of chemical bonds [8,9]. Friction force microscopy (FFM) is a key method to investigate the microscopic mechanisms underlying friction, wear, and lubrication as it allows for measurements of static and kinetic friction of single nanometer-scale contacts. In FFM, an ultrasharp tip is scanned across the surface line by line probing a square frame. Lateral forces acting on the sliding contact are determined as deflection of a cantilever spring holding the tip
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