Unraveling the pathways of tribochemical reactions involving the ZDDP lubricant additive by machine-learning-informed molecular dynamics

Among the most promising lubricant additives developed to date are zinc dialkyldithiophosphates (ZDDPs), which have protected engine components for over eight decades through the formation of tribofilms on the metal surfaces of machines' engines. However, the atomic-scale mechanisms governing this protective film formation remain poorly understood. This thesis investigates the mechanochemical processes underlying ZDDPs tribofilm formation on iron surfaces using molecular dynamics simulations powered by a custom Machine-Learning Interatomic Potential (MLIP). The computational framework integrates Density Functional Theory (DFT) and active-learning techniques to develop an accurate potential capable of capturing complex reaction dynamics. Simulations were conducted on systems containing 700 ZDDPs and linkage isomer LI2 ZDDPs molecules under realistic tribological conditions. The resulting trajectories were analyzed through Coordination Number (CN) and Radial Distribution Function (RDF) methods to track chemical evolution during sliding. This approach enabled detailed investigation of film formation pathways, including chemisorption, iron sulfide layer growth, and polyphosphate network development, allowing for the observation at the atomic-level into how molecular structure influences the protective mechanisms of lubricant additives. The simulations reveal a mechanically-driven multi-phase process: immediate chemisorption upon contact, formation of an iron sulfide interfacial layer, and development of polyphosphate networks. Critically, conventional ZDDPs and their linkage isomers LI2 ZDDPs exhibit markedly different mechanochemical behavior, with LI2 ZDDPs showing enhanced reactivity and faster polyphosphate formation. These results are in agreement and can explain experimental results while providing atomic-resolution insights into how molecular structure influences protective film formation.