  The friction of a nanosized sphere in commensurate contact with a flat
substrate is investigated by performing molecular dynamics simulations.
Particular focus is on the distribution of shear stress within the contact
region. It is noticed that within the slip zone, the local friction coefficient
defined by the ratio of shear stress to normal pressure declines monotonically
as the distance to contact center increases. With the lateral force increasing,
the slip zone expands inwards from the contact edge. At the same time, the
local friction coefficient at the contact edge decreases continuously, while at
the dividing between the slip and stick zones keeps nearly invariant. These
characteristics are distinctly different from the prediction of the
conventional Cattaneo-Mindlin model assuming a constant local friction
coefficient within the slip zone. An analytical model is advanced in view of
such new features and generalized based on numerous atomic simulations. This
model not only accurately characterizes the interfacial shear stress, but also
explains the size-dependence of static friction of single nanosized asperity.
