Layered materials have recently attracted great interest for their wide range of applications, including tribology, where they constitute an important class of solid lubricants. They share the common structure of stiff, strongly bound planes held together by weak interlayer forces, which is what favors their sliding capability. Graphite and transition-metal dichalcogenides (TMDs) are regarded as among the most suited compounds for applications where liquid lubricants are not effective, such as, e.g., nanoscale devices. In ultrahigh vacuum the performances of MoS2, the most common TMD, are superior to those of graphite, which on the contrary performs better in humid environments.
We show that intercalated water molecules hinder the sliding motion of MoS2 layers even in the absence of chemical oxidations; that graphene lubricates very effectively steel-on-steel sliding contacts because of its tribochemical passivation of iron surfaces; that an applied uniaxial load in graphene introduces a limited increase in the sliding barrier while in MoS2 it has a substantially different impact on the possible polytypes; that electrostatic interactions, negligible in comparison with van der Waals and Pauli contributions at zero load, progressively affect the sliding motion at increasing loads; and that the calculated interfacial ideal shear strengths which provides the most accurate information on the intrinsic sliding capabilities.