Atomic-scale processes of tribomechanical etching studied by atomic force microscopy on the layered material NbSe2

Layered materials play an important role as solid state lubricants. At the same time, they are ideal model systems for the study, of microscopic processes which lead to tribochemical wear. Here, we report on a microscopic study of wear induced by the scanning tip of an atomic force microscope (AFM). For this purpose, freshly cleaved surfaces of the layered dichalcogenide NbSe2 were scanned with the tip of an AFM at force loads of the order of 100 nN. Due to frictional forces between tip and sample, processes of tip-induced wear could be observed. The tribomechanical etching of the sample was stttdied and analysed on the nanometer scale. Three different microscopic processes were identified which contribute to friction-induced wear and layer decomposition: i) tribomechanical etching and wear on the atomic scale due to lateral forces at defects and step edges; ii) lateral force induced cutting of islands of the topmost NbSe2 layer and iii) delamination of smaller islands (diameters < 20 nm) due to lateral forces between the AFM tip and the island.