Phase Transitions in Nanofriction

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For our understanding of friction, it is of considerable importance to recognize the fundamental mechanisms, which facilitate energy dissipation, i.e. which are involved in the conversion from kinetic energy to heat. On the microscopic scale, this problem is closely related to the assumption, that realistic interfaces are formed as multi-asperity contacts, where a single asperity is usually considered as the most fundamental building block of friction. In scanning probe microscopy, such a single-asperity contact is formed between the tip and the sample and the technique is thus considered to provide an ideal model system to study the nanoscale origins of friction.However, while measuring quantitative friction values is relatively straightforward, it still remains a challenge to identify the mechanisms of energy dissipation. Here, a new approach to address this question lies in the analysis of friction phenomena across phase transitions. In solid state physics, phase transitions are usually an area of great interest and phase transitions are already very well analyzed for a large variety of materials. This opens up the possibility to induce well-defined changes of the material properties using an external control parameter, like e.g. temperature, and correlate these changes to the accompanying variations of the interfacial friction. This approach becomes particularly apparent for the case of the so-called electronic friction, which addresses the role of electrons for the energy dissipation process. Here, e.g. superconductors can be analyzed, where the electrical resistance vanishes below a certain threshold temperature and effects relevant for friction like e.g. electron-phonon coupling are changing likewise.At the same time, also structural phase transitions can be analyzed. In this case, scanning probe microscopy can be utilized as a kind of mechanical spectroscopy, where the force interaction between tip and sample allows to probe the changes within the materials during the phase transition. Using this approach, general energy dissipation mechanisms can be analyzed, while also the phase transition itself can be of interest.Despite this potential, phase transitions have so far rarely been targeted by nanoscale friction experiments, which is partially due to the experimental challenge to perform atomic force microscopy measurements as a function of temperature. Up to now, even the fundamental differences between first and second order phase transitions have not yet been addressed experimentally.All in all, this project thus offers new insight into very fundamental questions of nantribology. In addition, applying scanning probe microscopy also allows to characterize the effects related to phase transitions with a spatial resolution in the nanometer range.