SUPERLUBRICITY OF MOLYBDENUM-DISULFIDE

We have studied the atomistic origins of the ultralow friction coeffic ient of a molybdenum disulphide (MoS2) coating in ultrahigh vacuum con ditions. A friction coefficient in the 10(-3) range is associated with friction-induced orientation of ''easy shear'' basal planes of the Mo S2 crystal structure parallel to the sliding direction. In addition to this basal plane orientation, an orientation disorder around the c ax is is observed, indicating that frictional anisotropy during intercrys tallite slip could be at the origin of the vanishing of the friction f orce. Experimental HRTEM lattice fringe imaging Of MoS2 Wear particles clearly show the existence of characteristic Moire patterns. We have simulated TEM lattice fringe images of a [0001] MoS2 crystal and produ ced rotational Moire patterns by superimposing two such images. A qual itative agreement between experimental and simulated Moire patterns is demonstrated, which gives credence that ultralow friction of MoS2 in high vacuum can be attributed to a superlubric situation, by frictiona l anisotropy of sulphur-rich basal planes during intercrystallite slip .