HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY OF LAYER STRUCTURE AND STACKING-FAULTS IN TUNGSTEN DISULFIDE LUBRICANTS

Tungsten disulphide crystals used as lubricants have been studied by high resolution transmission electron microscopy (HRTEM). Theoretical calculation of the HRTEM image using the multislice method shows that the WS2 [2110BAR] images can distinguish between the W and S columns along the incident electron beam and enable one to determine not only the crystal structure but also the fault structure. Observation of a WS2 [2110BAR] crystal, prepared by grinding, discloses stacking faults and an array of edge dislocations whose extra planes are single S-W-S layers, revealing the stacking sequence of W and S layers, and also the stepwise structure of its (0001) surfaces as a result of the shear strain. The lubrication mechanism for solid lubricants such as WS2 and MoS2 is interpreted microscopically in terms of the crystallography on the basis of the observation.