The effect of surface curvature on the law relating frictional forces F withnormal load L is investigated by molecular dynamics simulations as a functionof surface symmetry, adhesion, and contamination. Curved, non-adhering, dry,commensurate surfaces show a linear dependency, F proportional to L, similar todry flat commensurate or amorphous surfaces and macroscopic surfaces. Incontrast, curved, non-adhering, dry, amorphous surfaces show F proportional toL^(2/3) similar to friction force microscopes. In our model, adhesive effectsare most adequately described by the Hertz plus offset model, as thesimulations are confined to small contact radii. Curved lubricated orcontaminated surfaces show again different behavior; details depend on how muchof the contaminant gets squeezed out of the contact. Also, it is seen that thefriction force in the lubricated case is mainly due to atoms at the entrance ofthe tip.
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