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

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