  Zinc dialkyldithiophosphates (ZDDPs) have been commonly used as anti-wear
additives in the automotive industry for the past 80 years. The morphology,
composition and structure of the ZDDPs phosphate-based tribofilm, which is
essential for its lubricant functioning, have been widely studied
experimentally. However, despite their widespread use, a general agreement on
their primary functioning mechanism is still lacking. The morphology and
composition of the ZDDPs phosphate-based tribofilm have been widely studied
experimentally, but the formation process and the relevant driving forces are
still largely debated. In particular, it is unclear whether the stress-induced
molecular dissociation occurs in the bulk oil or on the substrate. In this
work, we employ ab initio density-functional theory simulations to compare ZDDP
fragmentation in vacuum and over a reactive substrate, considering the effects
of surface oxidation on the dissociation path. Our results show that the
molecular dissociation is highly endothermic in the absence of a supporting
substrate, while in the presence of an iron substrate it becomes highly
energetically favoured. Moreover, the presence of the substrate changes the
reaction path. At the same time, surface oxidation reduces the
molecule-substrate interaction. These findings provide valuable insights into
the early stages of the formation of phosphate-based tribofilms.
