  It has been shown that contact aging due to chemical reactions in single
asperity contacts can have a significant effect on friction. However, it is
currently unknown how chemically-induced contact aging of friction depends on
roughness that is typically encountered in macroscopic rough contacts. Here, we
develop an approach that brings together a kinetic Monte Carlo model of
chemical aging with a contact mechanics model of rough surfaces based on the
boundary element method to determine the magnitude of chemical aging in
silica/silica contacts with random roughness. Our multi-scale model predicts
that chemical aging for randomly rough contacts has a logarithmic dependence on
time. It also shows that friction aging switches from a linear to a non-linear
dependence on the applied load as the load increase. We discover that surface
roughness affects the aging behavior primarily by modifying the real contact
area and the local contact pressure, whereas the effect of contact morphology
is relatively small. Our results demonstrate how understanding of chemical
aging can be translated from studies of single asperity contacts to macroscopic
rough contacts.
