Antiwear Chemistry of ZDDP: Coupling Classical MD and Tight-Binding Quantum Chemical MD Methods (TB-QCMD)

Zinc dialkyl dithiophosphate (ZDDP) is an antiwear additive for steel surfaces currently used in most of engine oils. The mechanism by which the additive is active is based on tribochemical reactions. These reactions occur in the contact zone under the combined effects of pressure and shear. These reactions are predictable on the basis of the HSAB principle (or Chemical Hardness model). We show here that computer simulations can describe the reactions much more accurately than the HSAB principle thanks to the use of a hybrid technique, coupling classical MD and tight-binding quantum chemical MD. In this study, we focused on one of the basic tribochemical reactions of ZDDP: the ability of zinc polyphosphate to react with abrasive metal oxides nanoparticles under pressure and shear. Results show that the driving forces for the reaction are mainly the increases of molecular shearing and entropy, besides temperature. We also studied the case of other metal oxide particles that emanate from elements of addition in steel compositions. We show that manganese and chromium oxides are eliminated in the same way as iron oxides, being in agreement with experimental data obtained by X-ray microanalysis and FIB-TEM characterizations. Eventually, we investigated the case of Al/Si alloys and showed that alumina particles can hardly be digested by zinc phosphate, at the opposite of silica particles. This explains very well why ZDDP is not a good antiwear additive for aluminium alloys and why the presence of silicon grain in the alloy is favourable.