Lars Jansen

Temperature dependence of point contact friction on silicon

Point contact friction and adhesion between a silicon tip and an untreated silicon(111) wafer are measured as a function of sample temperature in ultrahigh vacuum by friction force microscopy. While the friction coefficient changes drastically in the temperature range from 50K to room temperature, and shows a reproducible maximum near 100K, the simultaneously recorded adhesion shows much less temperature dependence. Interestingly, the velocity dependence of friction shows a logarithmic increase below 150K although it is nearly constant above 150K. This peculiar behavior has profound consequences for tribological properties of devices manufactured from silicon.

Interfacial friction obtained by lateral manipulation of nanoparticles using atomic force microscopy techniques

Nanometer scale metallic particles have been manipulated on an atomically flat graphite surface by atomic force microscopy techniques and quantitative information on interfacial friction was extracted from the lateral manipulation of these nanoparticles. Similar to conventional friction force microscopy, the particle-surface interfacial friction was extracted from the torsional signal of the cantilever during the particle pushing process. As a model system, we chose antimony particles with diameters between 50 and 500nm grown on a highly oriented pyrolytic graphite substrate. Three different manipulation strategies have been developed, which either enable the defined manipulation of individual nanoparticles or can be utilized to gather data on a larger number of particles found within a particular scan area, allowing for fast and statistically significant data collection. While the manipulation strategies are demonstrated here for operation under vacuum conditions, extensive testing indicated that the pro...

Contact ageing in atomic friction

The stick–slip motion of an atomic force microscope cantilever on highly oriented pyrolytic graphite is investigated experimentally and theoretically. It is shown that the interstitial slips of the cantilever tip at high pulling velocities are thermally activated events and can be described with a single-step rate equation. On the other hand, at slow pulling, the statistics of the slip events deviates from the predictions of the single-step rate approach, signaling the onset of contact ageing, that is, gradual changes of the contact properties in each stick phase. A model is introduced which takes the contact ageing effect into account, and whose predictions are in good agreement with the experimental results for all pulling velocities.

Frictional dissipation in a polymer bilayer system.

Sliding friction between a silicon tip and a polymer bilayer system consisting of a polystyrene (PS) film covered with a few-nanometers-thick capping layer of hard plasma polymer is studied using friction force microscopy. The system was chosen to enable subsurface dissipation channels to be distinguished from surface friction. Frictional energy dissipation in the underlayer can be identified through the kinetics of the polymer relaxation modes that we measured using nanoscale friction experiments as a function of sample temperature, scanning velocity, and applied load. We found a strong nonlinear increase in friction as a function of applied load around the glass-transition temperature of the PS underlayer. This behavior is a clear signature of frictional dissipation occurring in the volume of the polystyrene layer, well below the surface of the sample. The time-temperature kinetics associated with frictional energy dissipation into the PS was found to be in agreement with the known material properties of PS. Moreover, the data was found to support the hypothesis that the observed friction can be understood as the sum of friction resulting from the relaxation process in the polymer underlayer induced by stress due to the sliding of the tip and a second term associated with dissipation due to sliding friction on the capping layer.