Friction control by engineering the crystallographic orientation of the lubricating few-layer MoS2 films

Highlights: • Crystallographic orientation of MoS₂ influences its nanotribological properties. • Macroscopic coefficient of friction follows the findings on the nanoscale. • The correct growth process for the lubricant layers is crucial for applications. Tribological properties of a surface are controlled by its topography and chemical structure as suggested by theoretical studies. However, the effect of crystallographic orientation on the tribological properties is still controversial. By tuning the heating rate during sulfurization of CVD Mo films, we prepared the few-layer MoS₂ films with similar surface topography but different crystallographic c-axis orientation. This resulted in distinctly different tribological behavior both on the nanoscale and macroscale that can be attributed to different surface chemistry and surface electronic and phonon structures. In particular, horizontally aligned MoS₂ sheets with c-axis oriented along the surface normal favor the weak van der Waals forces which resulted in a twice lower coefficient of friction comparing to the vertically aligned MoS₂ sheets. These results demonstrate the possibility to optimize tribological properties by crystallographic orientation as required for the thin solid-state lubricating layers for extreme conditions.