On the three-term kinetic friction law in nanotribological systems

A post-processing method, which maps the punctiform atoms in molecular dynamics (MD) simulations of boundary lubrication onto smoothed particles, is used to estimate the asperity contact area defined by the minimum cross-section of the formed solid bridges. It is then shown that this asperity contact area excellently agrees with the projected area resulting from a Voronoi tessellation of the corresponding contact zone, and that it can be applied to compute the constitutive system parameters of a three-term friction law, which is found to hold for any boundary-lubricated nanotribological system. Finally, an attempt is made to relate the load-independent friction offset observed in boundary-lubricated nanotribological systems without solid-solid contact to the structural order as measured by the entropy, which is estimated within the single macromolecule approach based on covariance (super)matrices of the carbon backbone atoms in the lubricant.