Atomic Friction - (AFRI)

• Antragsteller: Professor Dr. Roland Bennewitz; Professor Dr. Martin Dienwiebel; Professor Dr. André Schirmeisen
• Fachliche Zuordnung: Physikalische Chemie von Festkörpern und Oberflächen, Materialcharakterisierung ; Physikalische Chemie von Molekülen, Flüssigkeiten und Grenzflächen, Biophysikalische Chemie
• Förderung: Förderung von 2008 bis 2012
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 68749129

In this proposal we intend to understand friction and wear properties in the extreme case of atomic scale friction, where only a few atoms constitute the tip-sample contact. Despite recent successes in the understanding atomic friction processes, where the velocity dependence, load dependence and new effects like superlubricity (structural and externally induced) have been targeted, where many phenomena are still under dispute. At this point, a multitude of experimental and theoretical work exists, however, only a few papers report on the direct overlap of experiments and theory. Atomic scale friction is particularly well suited for direct comparison, since the contact size is as small as possible, and thus is much better defined than in conventional tribology experiments. This invites direct comparison of atomic friction experiments with first principles and molecular dynamics simulations (MD) based on discrete atom geometries. A central question is the role of defects and interfaces. We will investigate atomic friction in the vicinity of defects under ultrahigh vacuum conditions and compare with theoretical studies. The important role of thermal actuation was indirectly derived from the velocity dependence of atomic friction. Temperature studies were performed on glassy polymers, where hindered rotation was found to be the relevant mechanism. However, temperature studies of atomic friction under ultrahigh vacuum studies are still missing. Therefore, an important goal of this project will be the study of atomic friction at temperature from 25K up to 1000K. Recently, high temperature friction was studied theoretically, where a strong reduction is predicted by the skating effect. At very low velocities, a drop of friction due to thermal excitation is also predicted theoretically, the thermolubricity effect. We will try to verify these effects experimentally. Nc-AFM measurements have shown that dissipation of the order of 1 eV per cycle is found in near contact. First principles and MD-simulations with realistic tip geometries were compared with nc-AFM experiments, which demonstrated that adhesion hysteresis due to tip configuration changes is the origin for dissipation. Our aim is to perform nc-AFM experiments with small amplitudes and to directly compare with simulations. The other extreme regime of sliding nanometer-sized contacts is at high normal forces, where the onset of wear occurs. Here, we plan to perform experiments as a function of load and speed and to extend contact areas with the use of a UHV-microtribometer to explore the behavior of multi-asperity contacts.

• DFG-Verfahren: Sachbeihilfen
• Internationaler Bezug: Finnland, Irland, Italien, Schweiz, Spanien, Türkei
• Beteiligte Personen: Professor Dr. Salim Ciraci; Professor Dr. Adam Stuart Foster; Professor Dr. Ernst Meyer; Professor Dr. John Pethica; Professor Dr. Rubén Pérez; Professor Dr. Erio Tosatti