Superlubricity of molybdenum disulphide

Abstract

We have studied the atomistic origins of the ultralow friction coefficient of a molybdenum disulphide (${\mathrm{MoS}}_{2}$) coating in ultrahigh vacuum conditions. A friction coefficient in the ${10}^{\mathrm{\ensuremath{-}}3}$ range is associated with friction-induced orientation of ``easy shear'' basal planes of the ${\mathrm{MoS}}_{2}$ 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 ${\mathrm{MoS}}_{2}$ wear particles clearly show the existence of characteristic Moir\'e patterns. We have simulated TEM lattice fringe images of a [0001] ${\mathrm{MoS}}_{2}$ crystal and produced rotational Moir\'e patterns by superimposing two such images. A qualitative agreement between experimental and simulated Moir\'e patterns is demonstrated, which gives credence that ultralow friction of ${\mathrm{MoS}}_{2}$ in high vacuum can be attributed to a superlubric situation, by frictional anisotropy of sulphur-rich basal planes during intercrystallite slip.