  Molybdenum dithiocarbamates (MoDTCs) are lubricant additives very efficient
in reducing the friction of steel and they are employed in a number of
industrial applications. The functionality of these additives is ruled by the
chemical interactions occurring at the buried sliding interface, which are of
key importance for the improvement of the lubrication performance. Yet, these
tribochemical processes are very difficult to monitor in real time. Ab initio
molecular dynamics simulations are the ideal tool to shed light into such a
complicated reactivity. In this work we perform ab initio simulations, both in
static and tribological conditions, to understand the effect of surface
oxidation on the tribochemical reactivity of MoDTC and we find that when the
surfaces are covered by oxygen, the first dissociative steps of the additives
are significantly hindered. Our preliminary tribological tests on oxidized
steel discs support these results. Bare metallic surfaces are necessary for a
stable adsorption of the additives, their quick decomposition, and the
formation of a durable MoS$_2$ tribolayer. This work demonstrates the
importance of the catalytic role of the substrate and confirms the full
capability of the computational protocol in the pursuit of materials and
compounds more efficient in reducing friction.
