Single-asperity study of the reaction kinetics of P-based triboreactive films

The reaction kinetics of zinc dialkyldithiophosphate (ZDDP) and ashless DDP antiwear additives were studied using in-situ single asperity AFM experiments. The results show that the ZDDP molecules decompose under high temperature and contact pressure following first order reaction kinetics (n = 0.71 +/- 0.14), whereas the DDP molecules follow a more complex fractional order (n = 0.52 +/- 0.07). The fractional order indicates that the decomposition process includes more side and intermediate reactions that consume part of the additive to form species, e.g. possibly volatile products, other than the antiwear phosphate glass on the contacting surfaces. In the case of ZDDP, the formation rate of the phosphate glass follows an equal exponential dependence on both temperature and contact pressure. However, in the case of DDP, the rate increases exponentially only over contact pressure and linearly over temperature. The findings of this study advance our understanding of the currently widely-used P-based antiwear additives and open future opportunities to develop new green alternatives with similar antiwear capabilities.