Friction Anisotropy of MoS₂ Investigated via Atomic Force Microscopy

Two-dimensional (2D) materials are of particular interest as solid lubricants for nano- and micro-scale devices as traditional fluid-based lubrication schemes fail at such small length scales due to problems including but not limited to surface tension. Among various 2D materials investigated as solid lubricants, MoS₂ is of special importance for space applications since its lubricative properties do not degrade but rather improve under vacuum conditions. A particular aspect that has so far not been addressed in detail within this context is the direction dependence, i.e. anisotropy, of the frictional properties of MoS₂, which could be an important design parameter for various applications. Here, we perform atomic force microscopy (AFM) measurements on CVD-grown and mechanically-exfoliated MoS₂ to investigate the potential occurrence of friction anisotropy. Results indicate that (i) both CVD-grown and mechanically-exfoliated MoS₂ exhibit strong friction anisotropy and remarkably, (ii) the periodicity associated with the anisotropy is not exclusively determined by the atomic structure of MoS₂, but is a function of the AFM probe employed in the experiments.