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Project X – Friction and Superlubricity
Contributors: J.Frenken, A.De Vita
Friction is recently receiving renewed interest thanks to the availability of atomic force microscopes (AFM) and friction force microscopes (FFM), which are sensitive to forces between individual atoms or small groups of atoms. In many experimental studies, the geometry of a sharp tip in contact with a surface is taken as model for the many possible asperity contacts between two sliding surfaces. At the same time, enormous advances in computer power have stimulated the development of new strategies for large-scale ab-initio simulations, which can be nowadays directly applied to study the interaction of two materials in local contact at the same (nonometric) scale investigated by the experiments.
There is therefore, a timely opportunity for a concerted research approach, where computer simulations and real experiments can be performed simultaneously on the same system to investigate the origin of friction. One of the key issues is the connection between forces and energy dissipation mechanisms active in small (e.g. single-atom or few-atom) contacts, and the forces and dissipation channels at work in more realistic contacts, with e.g. millions of atoms in the contact area. Scaling up the single-atom results to a larger area is indeed not a simple matter. This may seem surprising in the light of the atomic stick-slip motion often observed in atomic-scale friction force measurements obtained with sharp tips, since this motion mimics the stick-slip motion that we are familiar with on a large scale. However, a length scale which plays a crucial role in atomically resolved friction is defined by the crystal lattice over which the tip is sheared. This length scale is independent of the dimensions of the contact, leading to the prediction of a peculiar effect called "superlubricity". Namely, if the lattice spacing or the mutual orientation of the lattices in contact are not identical, the local frictional forces should exhibit a two-dimensional periodicity, with a high degree of mutual cancellation. The total friction force should therefore not increase monotonically with contact area, and for certain combinations of contact area and orientation, it should even, in principle, vanish. We propose to check this important prediction both in carefully designed model experiments and in large-scale ab-initio computer simulations.
The experiments will be set up at the Kamerlingh Onnes Laboratory of the University of Leiden (group of Joost Frenken). They will involve a new combination of microscopies, with which the friction can be measured between single-crystal surfaces under completely controlled conditions. In particular, Field Ion Microscopy (FIM) will be employed to obtain atomically resolved images of a sharp metal tip. The same technique will be used to 'shape' the apex of the tip via field evaporation. We note that in this way, also 'blunt' tips can be produced with flat-end facets with a diameter of more than ten atoms.
If such a facet is brought into gentle contact with another surface, the contact area is limited to the size of that facet. These techniques will thus enable us to control the true contact area. Finally, the new experimental setting will also allow the control of the tip orientation, so that a systematic investigation of
superlubricity will be possible.
The controlled friction experiments will be carried out under ultrahigh vacuum conditions with atomically clean tips on atomically clean surfaces. Typical tip materials that will be used are those for which high-quality FIM-imaging is possible, such as refractory metals (e.g. W). As a counter material, we will use either precisely the same material (e.g. W against W), in order to obtain maximum 'contrast' between friction and superlubricity, or a different material (e.g. W against Au).
The model experiments described above lend themselves very well for model calculations. The key advantage is that the systems studied experimentally (in particular, the contact between the tip and the first few atomic layers of the surface) can be modelled by systems containing a few hundred atoms at most.
This system size is accessible to calculations based on first-principles techniques, provided that massively parallel computing is used. We envisage that the necessary computer time can be made available in the context of the present project on the Swiss parallel platforms of the EPFL and of the Manno Supercomputing Center. If necessary, a grant allocation may be requested to supercomputing centers located outside Switzerland, such as CINECA (Italy). The calculations will be performed using the LAUTREC package for first-principles calculations developed by A.De Vita at the IRRMA Institute in Lausanne. This package is based on Density Functional Theory and performs
first-principles molecular dynamics using the Car-Parrinello method. In developing the package, particular care has been spent to tackle efficiently the severe instability and ill-conditioning problems which affect first-principles computations on metallic systems. Moreover, the package has been carefully optimized for application on large system sizes, and has been already successfully used for studying large low-symmetry systems containing
transition metal surfaces. As a result, the package is particularly suited for studying the systems of interest in the present project.
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