A surface topography-independent friction measurement technique using torsional resonance mode in an AFM

The recently introduced torsional resonance mode (TR mode) technique has been applied for friction force measurements at the micro/nanoscale. In this technique, an atomic force microscope (AFM) with a vibrating cantilever tip in torsional mode is used, and the contact torsional vibration amplitude of the tip motion (TR amplitude) is monitored at a contact resonance frequency under constant normal load. In this paper, the TR mode friction force images are compared to the friction force images acquired by conventional contact mode friction force microscopy on three samples: a self-assembled monolayer (SAM) with two phase structure, a silicon ruler, and a metal evaporated (ME) tape. The results from those samples show that the TR mode friction force images are much less affected by the surface topography and are almost independent of the scanning direction of the tip, whereas this is not the case for the contact mode friction force measurements. Mechanisms responsible for these observations are also discussed in the paper. In order to predict coefficient of friction as a function of the change in the TR amplitude, an energy balance model is proposed.