Project XII – Mechanical Properties of Nanoscale Contacts

Contributors: R. Nieminen, A. Foster, R. Bennewitz, E. Meyer

The varying scales and complexity of the phenomena important in nanotribology demand that the problem be approached from several different theoretical levels, from ab initio studies of electronic processes at contacts to atomistic simulations of friction itself. We apply these methods to several outstanding problems in nanotribology:

Direct information on the mechanical properties important in friction can be found by molecular dynamics and statistical simulations of such processes as nanoindentation and sliding friction. For example, molecular dynamics simulations of nanoindentation provide details of the changes in the tip shape, the size of the tip-sample contact area as a function of the indenting force, as well as information on the phononic processes at the surface during contact.

Even with some understanding of the mechanisms of nanoscale friction, it is still difficult to link experimental Friction Force Microscopy data with those mechanisms. Lack of information about the probing tip in friction experiments is especially limiting for clear interpretation. By directly modelling contacts at the atomistic level we can establish how the tip is affected structurally and chemically during scanning, and how the interaction forces are dependent on these properties. This should point to methods for reliably controlling the tip properties, and greatly improve the link between the mechanisms of friction and experimental results.

Success in all these theoretical directions requires direct experimental input in terms of data and discussion. Experiments will be performed (R. Bennewitz, E. Meyer) by state-of-the-art friction force microscopy under well-defined condititions (ultrahigh vacuum) on surfaces, such as ionic surfaces and metallic surfaces. The transition from wearless friction to plastic deformation (nanoindentation) will be investigated.