Sliding properties of MoS2 layers: load and interlayer orientation effects

Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disulfide has the most consolidated application outcomes in tribological fields. However, despite the growing usage as nanostructured solid lubricant due to its lamellar structure, little is known about the atomistic interactions taking place at the interface between two MoS2 sliding layers, especially at high loads. By means of ab initio modeling of the static potential energy surface and charge distribution analysis, we demonstrate how electrostatic interactions, negligible in comparison with van der Waals and Pauli contributions at zero load, progressively affect the sliding motion at increasing loads. As such, they discriminate the relative stability and the frictional behavior of bilayers where the two monolayers defining the interface have a different relative orientation. In particular, for antiparallel sliding layers we observed a load-induced increase of both the depth of the minima and the height of the energy barriers compared to parallel ones, which may have important consequences for the fabrication of more efficient ultralow friction devices at the nanoscale