Atomic Wear

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The wear of mechanical components like bearings or gears limits the life time of machines and results in increased costs of operation and raw materials as well as down times of factories. Avoiding wear and abrasion in various applications is of enormous economical as well as ecological interest. The classic theory of surface wear distinguishes between adhesive and abrasive wear. The former describes the detachment of parts of the surface, while the latter process is characterized by the plowing of the softer surface by the asperities of the harder counter surface. A general microscopic understanding of the underlying atomic wear processes does not exist. A useful approach for the analysis of those processes is to analyze the wear of individual asperities. In particular the atomic force microscope has proven to be a valuable tool, since its geometry closely resembles the ideal single asperity contact.New wear models have in recent years been motivated by experimental results from friction force microscopy studies. The atomic wear for nanoscale contacts has extremely low values of only one atom per millimeter sliding distance. As a consequence recent approaches to describe the process model it as the detachment of individual atoms from the surface, where the rate of overcoming the energy barrier is modelled with the rate equation. Despite the first successful applications of the rate theory, it is only one of several possible descriptions.The key experiment to verify the rate model for atomic wear would be the systematic measurement of the wear of nanoscale contacts as a function of temperature. Such an experimental has not been conducted so far, and is the main focus of this proposal. One central question is: Up to which temperatures is atomic wear dominated by thermal activation? At very low temperatures one would expect a transition to athermal wear, obeying the principles of mechano-chemistry, i.e. the formation/breaking of chemical bonds by force interactions. The measurements will be conducted with three model material systems, to focus on different aspects of atomic wear. Using a friction force microscope in ultrahigh vacuum conditions the atomic wear mechanisms will be studied on ionic crystals, polymers as well as diamond surfaces. For the direct verification of the rate model the wear will be quantified as a function of sample temperature. A sample cooling/heating stage allows assessing a large temperature range from 30 K to 700 K. Our central project goal is to find and understand the anticipated transition from thermal activated to athermal atomic wear processes.