Nanotribology
Our research area studies the mechanical properties of materials. In particular we aim to understand microscopic mechanisms of friction through dedicated experiments. How exactly is the kinetic energy transferred into heat? What is the role of the molecular bonds? What is the role of the surface structure? High-resolution friction force microscopy is our most important method to answer these questions. Here a fine tip is scanned over the material surface. The microscope is sensitive enough to detect the friction force of each single molecular bond broken. Furthermore, we develop novel experimental methods in order to contribute to the understanding of the complex world of friction.
Friction on ultrathin films
In this project we grow ultrathin films – just a few atomic layers – on surfaces and study the change of the frictional response. Graphite is a solid lubricant of great technological importance. What thickness of a graphite film will efficiently reduce friction? We find a dramatic decrease of friction for a single atomic layer of graphite on a silicon carbide crystal. However, significant differences between friction of bulk graphite and single layer graphite lead us to new ideas about the physical mechanisms of friction.
Ultrathin film of potassium bromide on a copper crystal
Atomically resolved image of an ultrathin film of potassium bromide on a copper crystal. An island and three holes in the films can be recognized, which play an important role for the friction and the stability of the surface (from Phys. Rev. B77, 035430 (2008)).
Friction under electrochemical control
Liquid environments are of great importance for many frictional processes. We look into the molecular mechanisms of these processes by means of a friction force microscope which is built into an electrochemical cell. For example, we study the interplay of mechanical stress and corrosion. Furthermore, we can deposit ultrathin metal films on the surface and explore their role in the friction process. With this setup we also plan to study friction, which is most relevant to humans: the aqueous lubrication of biologically relevant system.
Friction of rough surfaces
A significant difficulty for the understanding of friction is the complexity of rough surfaces. In technically relevant systems billions of microscopic contacts are continuously formed, deformed and ruptured. In order to gain fundamental understanding of these processes, we have fabricated the simplified version of a rough surface. A hundred thousand identical pyramids in a regular pattern serve as model system. Its collective frictional properties are studied by means of light scattering.
Regular pattern on a rubber surface
Regular pattern on a rubber surface serve as an experimentally accessible model system for a rough surface (from J. Phys.: Condens. Matter 20 (2008) 015004).
Nanometer-scale plasticity
The mechanical stability of crystalline materials strongly depends on so-called dislocations, i.e. extended defects in the crystalline order. With the sharp tip of the force microscope we can not only introduce single dislocations by indentation of perfect crystals, we also image the defects where they appear at the surface. Single dislocation experiments provide insights into the physics of plasticity and in particular of the extraordinary hardness of nano-structured materials.
