Tribochemical Reactions of MoDTC Lubricant Additives with Iron by Quantum Mechanics/Molecular Mechanics Simulations

The remarkable lubricant properties of molybdenum dithiocarbamates (MoDTCs) make this class of oil additives well-known in the automotive industry. However, the mechanism of function of these compounds is still not completely understood at the atomistic level. We provide new insights into the dissociation of MoDTCs in tribological conditions, which are the key to describe the debated mechanism to form MoS2. Quantum mechanics/molecular mechanics (QM/MM) dynamic simulations allowed us to monitor in real time the tribochemical reactions occurring at the iron interface and revealed that the presence of the iron substrate and the mechanical stresses alter the dissociation path with respect to what is expected for the isolated MoDTC molecules. Moreover, they uncovered the important role of molecular oxidation on the dissociation pattern: the presence of oxygen atoms in the ligand position of MoDTCs favors the release of the central units of the molecules, containing just Mo and S atoms with the correct stoichiometry to form MoS2. This work demonstrates how the predictive power of ab initio simulations can be very valuable to design new lubricant additives.