Monolayer Transfer Layers During Sliding at the Atomic Scale

One of the fundamental issues in friction is understanding the atomic details of how two materials slide against each other and start to wear. Whether this involves single-atom processes or the collective motion of atoms has been open to debate for some time. Here we report direct observations of this via in situ studies within a transmission electron microscope. We observed for both graphite and molybdenum disulfide that single atomic layers are transferred from the material to a sliding tip to form a transfer layer, and subsequent sliding takes place by sliding of single layers of graphite or molybdenum disulfide against each other. Despite the similarity of the end result, how the single layers are formed is quite different; with graphite, it involves buckling/wrinkling ~3 nm ahead of the tip, whereas with molybdenum disulfide it is via direct transfer of single sheets. Graphite is more like plastic wrap, molybdenum disulfide more like a pack of cards. This difference is attributed to the large difference in the bending modulus and strength of monolayers in the two cases. In both cases, collective processes are taking place.