Investigation of the Lubricated Sliding Friction of Nano-Scale Rough Surfaces

Abstract eng:
Although a lot is known about the factors contributing to friction, a complete physical understanding of the origins of friction is still lacking. At the macroscale several laws describe the relation between load (Amontons, Coulomb), apparent and real area of contact (Bowden and Tabor), and frictional forces. But it is not yet completely understood if these laws of friction extend all the way down to the atomistic level. Because continuum models are not applicable at the atomic scale, particle based modeling techniques are necessary to elucidate the physics of friction at the small scale. In earlier work, we have used molecular dynamics simulations to model dry friction of two rough deformable surfaces and found that friction increases with roughness. Nevertheless, roughness is altered during the sliding motion and we evidenced a surface flattening which follows an exponential decay and reduces the friction to almost zero [1,2]. However, other mechanisms control the evolution of roughness at contacting asperities. For instance, an important factor is the presence of a third body at the contacting points. In order to study this effect, we introduced a polymer fluid in the interstitial volume between the solid contacting bodies. The figure shows a schematic of the model geometry that we used. The normal pressure is enforced by means of a force applied onto the top free surface while the atoms from the bottom free surface are constrained to a fixed position. The sliding motion is created by imposing a constant velocity to the top atoms. Two thermostat layers (one in each body) are set in order to regulate the energy flux created by the deformation of the asperities and/or the fluid. Since the third body is subject to large deformation it is no longer possible to compute a friction coefficient by a direct measure of the forces acting between the two solid bodies. Instead we looked at the total dissipation of the system as a measure of the friction itself. This can be done by making a balance with the work of the external forces, and the energy dissipated by the thermostats. Using this technique, which was validated on dry contact systems, we studied the evolution of roughness and friction under various pressures and temperatures in the presence of a third body.

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