Unraveling the mechanism to form MoS2 lubricant layers from MoDTC by ab initio simulations

The morphology of molybdenum disulfide (MoS2) is a crucial aspect to ensure the functionality of this remarkable 2D-material both in electronic and tribological applications. Indeed, molybdenum dithiocarbamates (MoDTCs) can be tribochemically transformed into MoS2, which is able to reduce the friction coefficient of metallic moving parts. However, this transformation is influenced by temperature, sulfur/oxygen ratio, normal and shear stresses, making the mechanism of this process particularly challenging to explain. Ab initio simulations based on density functional theory (DFT), including a quantum mechanics/molecular mechanics (QM/MM) approach, are used here to shed light on the crystallization of MoS2 promoted by mechanical stresses. Chemistry plays an important role during the reorganization of the units of MoSx obtained from MoDTC, because sulfur and oxygen atoms tend to move outside of the amorphous layer, surrounding the molybdenum atoms and creating a structure that can crystallize into MoS2. Normal load and sliding have a synergistic effect in rearranging the amorphous units into a crystalline structure, as the former helps overcoming the energy barriers associated to bonds breaking and forming, while the latter allows misplaced atoms to be pulled towards the crystalline sites. A crystalline MoS2 was obtained by ab initio calculations below 1000 K.