Dynamic Friction Measurements with an Atomic Force Microscope on Polymer Surfaces * Hans-Ulrich Krotil and Othmar Marti Department of Experimental Physics, University of Ulm, D-89069 Ulm, Germany 13.11.1997 The combination of scanning friction force microscopy (SFFM) and lock-in tech- niques leads to dynamic SFFM (DSFFM) and provides great advantages in fric- tion force studies with suh-micrometre resolution. In this paper are presented measurements on thin adsorbed organic films on polymers (polymer blend of 75% poly(allylaminehydrochloride) (PAA) and 25% poly(diallyl-dimethylammonium chlo- ride) (PDDAC)) and on mica (as a reference). The amplitude and phase response as a function of the excitation amplitude can be explained on hard surfaces by a simple static and dynamic friction model. This model allows us further to distin- guish static friction forces and kinetic friction forces in a quantitative way. Further- more, we demonstrate the use of these spectra to determine the correct modulation amplitude of the excitation to achieve the optimal friction contrasts directly. Poly- mer data suggest that the viscoelastic shear flow under the atomic force microscope (AFM) tip is responsible for the shape of the phase and amplitude spectrum. Lastly, we demonstrate that DSFFM is a useful technique for surface characterisation in situations where SFFM may not be adequate. Keywords: friction measurement, atomic force microscopy, AFM, polymer surfaces, scan- ning friction force microscopy, dynamic scanning friction force microscopy 1 INTRODUCTION Tribology is the science of two interacting surfaces; it is a fast growing field especially since in high-technology devices frictional processes become more and more dominant with decreasing lateral dimensions. 1 With scanning friction force microscopy (SFFM) based on atomic force microscopy (AFM), single asperity friction can be investigated at the atomic scale. 2,3 In spite of the great success of SFFM, friction measurements with AFM are still hampered by several difficulties, such as the non-linearity and hysteresis of the scanning stage, and the influence of the topographical slope on the measured data. These drawbacks can be solved to some degree by massive use of image processing techniques. * Published in: J. Synthetic Lubrication 18-1, April 2001 1