New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study

Molybdenum DialkyldiThioCarbamate (MoDTC) is a friction modifier that has been used in automotive engines for many years. However, its exact decomposition mechanism within tribocontacts is not fully understood. In this study, an attempt has been made towards understanding the mechanism of MoDTC decomposition in steel/steel contacts by employing Raman spectroscopy. The results show that the main MoDTC decomposition products are MoS2, FeMoO4 and sulphur-rich molybdenum compounds, MoSx (x > 2), in contrast to the previously reported MoS2 and MoO3. The formation of these products is dependent on tribological parameters. Raman results from this study indicate that the Mo6+ species previously observed in X-ray photoelectron spectroscopy (XPS) analysis are probably from FeMoO4 and not MoO3. This paper presents an alternative reaction pathway for MoDTC decomposition in steel/steel contacts with MoSx as an intermediate product and MoS2 as the final product. FeMoO4 is formed from a side reaction of iron oxides with molybdenum compounds at low temperatures and low MoDTC concentrations. The results also show that friction is dependent on the nature of the decomposition products at the tribocontact. Knowledge of the MoDTC decomposition reaction pathway will enable the friction performance of MoDTC lubricants to be optimized.