CORDIS - EU research results

Science Friction

Final Report Summary - SCIFRI (Science Friction)

The SciFri project has concentrated on fundamental aspects of friction and new strategies to influence frictional forces. One of our strategies to reduce friction involved "superlubricity" between perfectly flat, incommensurate lattices, such as graphene. In this context we have investigated how to produce graphene layers of nearly complete perfection.
With a variable-temperature scanning tunneling microscope (VT-STM) we have followed the high-temperature CVD of graphene on the atomic scale on Rh(111) and Ir(111). One of the surprises was that the moiré patterns between the graphene and the underlying metals, have a dominating effect on the growth mechanism. We also find the orientational and translational misfits between neighboring graphene nuclei make the earliest stages of the CVD process crucially important for the final graphene quality. Starting from our insights, we have designed and further optimized a CVD process for graphene on Cu(111).
First experiments with macroscopic objects sliding over each other with contact areas of 1 cm2 and larger show a significant lowering of the friction force when the objects are covered by a single graphene layer. An additional strategy that has been pursued to reduce friction is to employ thermal fluctuations, a phenomenon called “thermolubricity”. To this end, one of the two surfaces in a macroscopic contact was structured in the form of a large-scale array of micro- or nanopillars. Friction and adhesion measurements on these structures have been surprising. They show that the pillars are extremely flexible and hard to damage, while friction can become excessively high.
We have used a combination of friction force microscopy and various spectroscopies to investigate the origin of the ultralow friction that is introduced by films of diamond-like carbon (DLC). The slipperiness of this material is accompanied by the presence of so-called “third bodies”, worn off the DLC and decorated by a graphite- or graphene-like layer that probably originates from a phase transformation of the material under the high local mechanical stress. We think that the lubrication by DLC works the same way as that by graphite, namely by superlubricity.
Theoretical work has been devoted to a precise study of the energy dissipation in friction force microscopy (FFM). In order to explain typical FFM data, we have to assume a damping rate that is a million times higher than what would seem reasonable. We find the explanation for this in the excessively high velocities that the flexible, atomic-scale tip apex reaches in each slip event, implying that we should expect much lower friction if this flexibility could be avoided.
We have set up a combination of two theoretical treatments of a new mechanism of frictional damping. Special about our approach is that we actually do not assume an explicit, velocity-dependent damping force. Instead, damping ‘emerges’ as the natural dephasing of the phononic eigenmodes of a harmonic lattice that are excited during each slip event. This is a generic effect that applies to most situations of unlubricated friction, irrespective of the precise character of the involved solids, length scales and time scales. It may well be that this ‘explains’ most cases of dry friction.