SUPERLUBRICITY OF MOLYBDENUM-DISULFIDE

We have studied the atomistic origins of the ultralow friction coefficient of a molybdenum disulphide (MoS2) coating in ultrahigh vacuum conditions. A friction coefficient in the 10(-3) range is associated with friction-induced orientation of ''easy shear'' basal planes of the MoS2 crystal structure parallel to the sliding direction. In addition to this basal plane orientation, an orientation disorder around the c axis is observed, indicating that frictional anisotropy during intercrystallite slip could be at the origin of the vanishing of the friction force. 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 produced rotational Moire patterns by superimposing two such images. A qualitative 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 frictional anisotropy of sulphur-rich basal planes during intercrystallite slip.