Friction mechanisms of WS2 in humid environments: Investigating H2O adsorption via DFT computations and sliding friction experiments

Tungsten disulfide is increasingly used for space and nuclear applications owing to its low friction in vacuum, but the friction increases drastically in humid environments. The increased friction has been conventionally attributed to the oxidation of WS2. This study investigated the frictional behaviour of WS2 in humid atmospheres through sliding friction experiments combined with density functional theory computations. The results showed that the increased friction can be attributed to the formation of O-H···S type interfacial hydrogen bonds between WS2 layers by adsorbed H2O, without significant oxidation. This interaction led to enhanced interlayer bonding and adhesion, consequently elevating friction. This study provides atomistic insights into the friction mechanism of WS2 and design guidelines for thin films operatable in humid environments. [Display omitted] •Adsorption energy value of H2O is highest at VWS2 defect of WS2 and W-O-W bonds are formed.•H2O will form H···S type interfacial hydrogen bonds at the WS2 defects, hindering WS2 interlayer shear.•The defects on WS2 will help H2O to adsorb onto WS2 in both dissociative and undissociative ways.•Dissociated water molecules at the WS2 interface have little effect on the binding energy of WS2.