Simulations of the temperature dependence of static friction at the N2/Pb interface

A molecular dynamics approach for studying the static friction between two bodies, an insulator and a metal, as a function of the temperature is presented. The upper block is formed by N2 molecules and the lower block by Pb atoms. In both slabs the atoms are mobile. The interaction potential in each block describes properly the lattice dynamics of the system. We show that the lattice vibrations and the structural disorder are responsible for the behaviour of the static friction as a function of the temperature. We found that a large momentum transfer from the Pb atoms to the N2 molecules misplaces the N2 planes in the proximity of the interface. Around T = 20 K this effect produces the formation of an hcp stacking at the interface. By increasing the temperature, the hcp stacking propagates into the slab, toward the surface. Above T = 25 K, our analysis shows a sharp, rapid drop of more than three order of magnitude in the static friction force due to the misplacing of planes in the stacking of the fcc(111) layers, which are no longer in the minimum energy configuration. Above T = 35 K, we also observe a tendency for the splitting of planes and the formation of steps near the surface. By increasing the temperature we obtain the subsequent melting of the N2 slab interface layer at T = 50 K. The temperature behaviour of the calculated static friction is in good agreement with recent measurements made with the quartz crystal microbalance (QCM) method on the same system.