  A macroscopically nominal flat surface is rough at the nanoscale level and
consists of nanoasperities. Therefore, the frictional properties of the
macroscale-level rough surface are determined by the mechanical behaviors of
nanoasperity contact pairs under shear. In this work, we first used molecular
dynamics simulations to study the non-adhesive shear between single contact
pairs. Subsequently, to estimate the friction coefficient of rough surfaces, we
implemented the frictional behavior of a single contact pair into a
Greenwood-Williamson-type statistical model. By employing the present
multiscale approach, we used the size, rate, and orientation effects, which
originated from nanoscale dislocation plasticity, to determine the dependence
of the macroscale friction coefficient on system parameters, such as the
surface roughness and separation, loading velocity, and direction. Our model
predicts an unconventional dependence of the friction coefficient on the normal
contact load, which has been observed in nanoscale frictional tests. Therefore,
this model represents one step toward understanding some of the relevant
macroscopic phenomena of surface friction at the nanoscale level.
