Molecular evidence for sulfurization of molybdenum dithiocarbamates (MoDTC) by zinc dithiophosphates: a key process in their synergetic interactions and the enhanced preservation of MoDTC in formulated lubricants?

Molybdenum dithiocarbamates (MoDTC) are widely used in automotive industries as lubricant additives to reduce friction and to enhance fuel economy. Sulfur-containing additives such as zinc dithiophosphates (ZnDTP) are proposed to play a key role in the improvement of friction reducing properties of MoDTC in formulated lubricants by facilitating the formation of MoS2 tribofilm at the rubbing contacts. This study focuses on the interactions between MoDTC and ZnDTP under conditions comparable with those prevailing in operating engines. The capacity of ZnDTP to sulfurize MoDTC in solution in a hydrocarbon base oil could be demonstrated. Sulfurized Mo complexes bearing one or two additional sulfur atoms (1S-MoDTC and 2S-MoDTC, respectively) which have replaced the genuine oxygen atom(s) from the MoDTC core were detected and quantified using a specifically developed HPLC-MS analytical method. A possible sulfurization mechanism relying on the higher affinity of phosphorus from ZnDTP for oxygen could be proposed. In parallel, the evolution and molecular transformation of the prepared 2S-MoDTC in hydrocarbon base oil under thermal and thermo-oxidative conditions were followed using HPLC-MS and compared with the evolution of their friction coefficients. 2S-MoDTC complexes were shown to exhibit a better retention of friction reducing capability under oxidative conditions than the "classical" MoDTC, although they did not seem to significantly reduce the friction coefficients of lubricants as compared to the "classical" MoDTC. Therefore, sulfurization of MoDTC by ZnDTP might contribute to delay the progressive consumption of MoDTC and the loss of their friction-reducing efficiency in lubricants under thermo-oxidative conditions.