MoS₂ formation induced by amorphous MoS₃ species under lubricated friction

Amide molybdate has been recently introduced as a friction modifier for tribological applications. Combined with zinc dithiophosphate (ZDDP) and fatty amines, it provides an ultralow friction coefficient. The ultimate product of Mo compound transformations in tribological contact, due to frictional heating and shearing, as well as chemical interactions with oil additives, is molybdenum sulfide (MoS₂). Understanding the decomposition of amide molybdate leading to MoS₂ is of primary importance to the optimization of the design of lubricant formulations. This study focuses on the investigation by Raman spectroscopy of amide molybdate decomposition intermediates. Raman spectra of tribofilms, obtained after friction tests under different temperatures and pressures, revealed the formation of an amorphous MoS₃ intermediate coexisting with MoS₂. However, under severe conditions, the tribofilms are mostly composed of MoS₂.