Effects of Sulfur Vacancies on Friction of Two-Dimensional MoS₂

Molybdenum disulfide (MoS₂) has attracted considerable attention for its outstanding tribological performance, excellent mechanical properties and thermal stability which make the material one of the most popular solid lubricants. Despite extensive studies devoted on the tribological properties of MoS₂, most studies have focused on pristine MoS₂ without considering the effect of defects such as sulfur vacancies, which are inevitable in the fabrication process. Since vacancies change atomic structures and influence the interaction between sliding layers, it is essential to understand the effects of vacancy defects on the tribological property. To investigate the effects of sulfur vacancies on the frictional behavior of MoS₂, perfect MoS₂ and defected MoS₂ with various vacancy concentrations are studied using the molecular dynamics simulation with the atomic interactions described by ReaxFF potential. To further investigate the influence of vacancy distribution on the frictional property of MoS₂, two arrangements of vacancies are also considered; (i) the straight-line arrangement and (ii) the random distribution. The simulation results reveal different frictional behaviors with line and random distribution models as vacancy concentration changes resulting from different interlayer distance and surface roughness, providing a further understanding of the role of vacancies in the frictional properties of MoS₂.