Topography-induced contributions to friction forces measured using an atomic force/friction force microscope

Most friction studies using an atomic force/friction force microscope, while concentrating on material-induced effects, often present users with conflicting and confusing interpretations of the topography-induced friction forces. It has been generally reported that topography-induced contributions are independent of scanning direction and can be removed by subtracting friction data from forward and backward scans. In this article, we present friction studies on samples with well-defined topography variations and find that the above-given statement is not generally true. At surface locations involving significant changes in topography, the topography-induced contributions to friction forces are found to be different between forward and backward scanning directions. This is explained by the ratchet mechanism of friction and due to the additional torsion generated by "collision" of the tip when traversing up an increase in topography, which is absent in the downward travel. Topography-induced contributions to the friction force always correspond to transitions in the surface slope. Moreover, these contributions will be of the same sign in both Trace and Retrace friction profiles of the friction loop whereas changes due to material effects are in opposite directions. These characteristics of topography-induced friction forces will aid in differentiating them from other effects. (C) 2000 American Institute of Physics. [S0021- 8979(00)09520-7].