Microscopic and atomistic mechanisms of sliding friction of MoS2: Effects of undissociated and dissociated H2O

Highlights: :• Sputtered MoS₂ film showed misoriented layers and intrinsic defect structure. • Sliding tests of MoS₂ showed ultralow friction in N₂ but high friction in humid air. • MoS₂ transfer layers were parallel reoriented with formation of MoO₃ embedded. • DFT calculations showed dissociated H₂O not increasing interlayer binding energy. • DFT calculations showed physisorbed H₂O increase friction by forming H bond with S. Molybdenum disulfide (MoS₂) displays low coefficient of friction (COF) in vacuum and under inert atmospheres, but a higher COF occurs under humid atmospheres. A significant aspect of this increase in COF that is still not well established is the decoupling the effects of dissociated and undissociated water molecules. The MoS₂ thin films used in this study with an intrinsic defect structure incorporating misoriented and fragmented layers as observed by HR-TEM, exhibited an increase in COF from 0.007 in N₂ with 2 (and air) with ~40% RH. Sliding induced transfer layers formed on the counterface revealed a reduction in the spacing between MoS₂ layers oriented parallel to the interface. Effects of dissociated and undissociated H₂O adsorbed between MoS₂ layers on the interlayer binding energy (E_B) were studied using first principles calculations. Accordingly, H₂O can dissociate into 2H and O at a triple vacancy site and form MoOMo bonds, but this process would not change E_B and had a minor effect on friction. An undissociated H₂O molecule physisorbed between these layers formed H bond with S atoms increasing E_B, a tribochemical mechanism that had a pronounced effect on increasing the COF of MoS₂.