  We study the dynamics of the contact between a pair of surfaces (with
properties designed to mimic ruthenium) via molecular dynamics simulations. In
particular, we study the contact between a ruthenium surface with a single
nanoasperity and a flat ruthenium surface. The results of such simulations
suggest that contact behavior is highly variable. The goal of this study is to
investigate the source and degree of this variability. We find that during
compression, the behavior of the contact force displacement curves is
reproducible, while during contact separation, the behavior is highly variable.
Examination of the contact surfaces suggest that two separation mechanism are
in operation and give rise to this variability. One mechanism corresponds to
the formation of a bridge between the two surfaces that plastically stretches
as the surfaces are drawn apart and eventually separates in shear. This leads
to a morphology after separation in which there are opposing asperities on the
two surfaces. This plastic separation/bridge formation mechanism leads to a
large work of separation. The other mechanism is a more brittle-like mode in
which a crack propagates across the base of the asperity (slightly below the
asperity/substrate junction) leading to most of the asperity on one surface or
the other after separation and a slight depression facing this asperity on the
opposing surface. This failure mode corresponds to a smaller work of
separation. those in which a single mechanism operates. Furthermore, contacts
made from materials that exhibit predominantly brittle-like behavior will tend
to require lower work of separation than those made from ductile-like contact
materials.
