Project I – Universal Friction at low speeds and moderate loads

Contributors: T.Baumberger, B.Persson

The group of T. Baumberger has been involved in friction studies for 8 years. The interest lies in the physics of solid friction at low velocities (typ. < 100 mm/s) and moderate loads (so as to deal with multicontact interfaces). Under such ‘soft’ tribological conditions (in terms of wear and self-heating effects), friction is amenable to an essentially material-independent, i.e. universal phenomenological description. Macroscopic experiments with submicronic displacement resolution, using polymer glasses, now enable us to get insight into the physical mechanisms at work in the rheology of the nanometer-thick adhesive junctions where frictional dissipation localizes. This can be attributed to thermally activated premature depinning of multistable units of nm3 activation volume, akin to Persson’s nanoblocks.
We are now involved in the investigation of a possible ageing/rejuvenation mechanism in the adhesive junction itself, the existence of which has been suggested by the recent identification of a jamming creep regime at an interface submitted to a biased oscillating shear (Bureau, Baumberger & Caroli (2001) submitted to Phys. Rev. Lett.).

Our experimental approach makes it possible to address problems at different scales : nm (junctions, metastable creep units), mm (load bearing asperities), cm (macroscopic slider). It therefore contributes to briging the gap between nano- and macro-tribology. European cooperation would be very useful by permitting a permanent interaction in particular with Bo Persson : modelization of nano-junctions Gunter Reiter: friction of polymers at interfaces, Martin Mueser : multiple scale numerical simulation of tribological layers.