Formation and function of zinc dialkyldithiophosphate anti-wear films: Molecular-level insight through chemical simulation

Zinc dialkyldithiophosphate (ZDDP) anti-wear additives are ubiquitous components of modern motor oil formulations. ZDDPs decompose under engine conditions to form zinc phosphate films which protect engine surfaces from wear. Recently, interest has developed in replacing ZDDPs due to modern environmental concerns and shifts in engine technologies. The rational design of new anti-wear additives would benefit from a detailed knowledge of the mechanisms responsible for the formation and anti-wear functionality of ZDDP films. Unfortunately, decades of experimental research have failed to unravel these mechanisms at the molecular level.; In this thesis, a series of ab initio molecular dynamics simulations and static quantum chemical calculations were performed to explore the behaviour of ZDDP antiwear additives and films. The results of the simulations shed light on the mechanisms through which the ZDDP molecule decomposes and isomerizes, show how the decomposition and isomerization products can react to form (zinc) phosphate precursors to ZDDP anti-wear films, demonstrate how these precursors are transformed into the films and explain how the films inhibit wear. Key results include the identification of two novel phosphate polymerization mechanisms and a pressure-induced cross-linking mechanism that accounts for both the formation and functionality of ZDDP anti-wear films.; Taken together, the results of this study provide a clear pathway connecting the ZDDP additive to the anti-wear films and, moreover, explain how these films work. Such a detailed understanding of these molecular level processes was unavailable prior to this work and, in some cases, defies conventional wisdom regarding ZDDPs. The results of this study are shown to explain a wide body of experimental observations regarding ZDDPs. Perhaps more importantly, the mechanisms developed in this work comprise a predictive theory of ZDDP anti-wear film formation and functionality that can be applied in the rational design of new anti-wear additives.; Keywords: Zinc dialkyldithiophosphates (ZDDPs), Anti-Wear Additives, Lubricants, Tribology, Quantum Chemistry, Density Functional Theory (DFT), Ab initio Molecular Dynamics (AIMD).