  Friction is the force resisting relative motion of objects. The force depends
on material properties, loading conditions and external factors such as
temperature and humidity, but also contact aging has been identified as a
primary factor. Several aging mechanisms have been proposed, including
increased "contact quantity" due to plastic or elastic creep and enhanced
"contact quality" due to formation of strong interfacial bonds. While proposed
mechanisms for frictional aging have been dependent upon the presence of a
normal force, this factor is not a fundamental prerequisite for the occurrence
of aging. In light of this, we present a novel demonstration of a substantial
frictional aging effect within a cubic silicon carbide system, even when a
normal force is entirely absent. Our observations indicate that the
time-evolution of the frictional aging effect follows a logarithmic behavior,
which is a pattern that has been previously observed in numerous other
materials. To explain this behavior, we provide a derivation that is rooted in
basic statistical mechanics, demonstrating that surface diffusion, a phenomenon
that serves to minimize surface energy in the interface region, can account for
the observed behavior. Upon application of a normal force, the friction force
is enhanced owing to the presence of plastic creep. Although aging resulting
from plastic and elastic creep is widely recognized and incorporated into most
friction laws, diffusion-driven aging has received comparatively less
attention. The ultimate objective is to develop or redesign friction laws by
incorporating the microscopic behavior, with the potential to enhance their
effectiveness.
