Superlubricity of MoS2: crystal orientation mechanisms

Abstract We have investigated the origin of the extraordinary low friction coefficient (in the 10 -3 range or even less) of pure and stoichiometric sputtered MoS 2 coatings, in ultrahigh vacuum. In these conditions, shear strengths of the interface as low as 1 MPa were measured. Importantly, the tribometer was operating in macroscopic contact conditions, typically at a long time-length scale. Friction-induced orientation of (0001) basal planes of MoS 2 grains parallel to the sliding direction was first verified by means of electron diffraction. Friction-induced rotation of these crystals around the c axis, during intercrystallite slip in the contact, was investigated by high resolution transmission electron microscopy performed on selected wear fragments. Atomic force microscopy at atomic resolution was also carried out on the surface inside and outside the wear scar. The data indicated that the vanishing of the friction force was due to frictional anisotropy in the interface between nanometre-scale domains in rotational disorder (intercrystallite slippage of incommensurate sulphur-rich hexagonal lattices). The term superlubricity was used here because of the zero friction state that could be theoretically predicted in these conditions. Finally, the mechanisms of MoS 2 superlubricity are thought to depend on the proper combination of the grain size, the two crystal orientation effects and the absence of contaminants.