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 h igh resolution transmission electron microscopy (HRTEM). Theoretical c alculation of the HRTEM image using the multislice method shows that t he WS2 [2110BAR] images can distinguish between the W and S columns al ong the incident electron beam and enable one to determine not only th e crystal structure but also the fault structure. Observation of a WS2 [2110BAR] crystal, prepared by grinding, discloses stacking faults an d an array of edge dislocations whose extra planes are single S-W-S la yers, revealing the stacking sequence of W and S layers, and also the stepwise structure of its (0001) surfaces as a result of the shear str ain. The lubrication mechanism for solid lubricants such as WS2 and Mo S2 is interpreted microscopically in terms of the crystallography on t he basis of the observation.