Mechanisms of MoS2 formation by MoDTC in presence of ZnDTP: effect of oxidative degradation

Friction-reducing properties of MoDTC-containing lubricants have been investigated as a function of their oxidative degradation state. The oxidation test is conducted with oxygen and nitrogen oxide gases simulating aged engine oils during service. The main oxidation effect is to decrease the concentrations of MoDTC and other additives present in the formulation. However, when the ZnDTP concentration is reduced in the lubricant, MoDTC is not so fast consumed because of the formation of less stable MoDTP compound by acid-base reaction is inhibited. The decrease of friction reduction properties appears significant when about 80% of additives content has been consumed by oxidation. The friction reduction mechanism of MoDTC is due to the formation of MoS2 single sheets during the tribochemical reaction. When the concentration of MoDTC decreases in the lubricant due to oxidation, MoS2 sheets are found to be modified both in size, number, shape and chemical composition. Final, at very low MoDTC concentration, sheets cannot grow to a sufficient size to cover the asperities for efficient friction reduction.