Molecular dynamics simulation study on the friction performance of WS2 coated copper during the scratching process

Tungsten disulfide (WS2) has been assumed to have the excellent tribological property that has the potential to be used in the field of lubricating engineering. In this study, WS2 nanosheet was coated onto the surface of the copper to improve the wear resistance. The tribological properties of WS2 coated copper system were investigated by molecular dynamics (MD) simulation to understand the wear mechanisms from the nanoscale. The mechanical response of the bare and the WS2 coated copper in the indentation process were compared firstly. Then, scratching simulation tests were performed at the various indentation depths from 1.2 Å to 11.2 Å to understand the tribological mechanism from the elastic stage to the fracture stage. The effect of the nanoindentation depth and anisotropy orientation of WS2 on friction behavior were also analyzed during the scratching process. Results show that the load-bearing capacity and wear resistance of the copper substrate were improved by the WS2 coated layer before the fracture during the nanoindentation process, while the protection of the layer failed after rupture that the out-of-plane deformation induced the increase of the friction force. Additionally, the difference of the friction force could be induced by the lattice spacing of WS2 and the energy dissipation during ultralow friction stage. The crack propagation along the Zigzag direction dominated the fracture behavior in both Armchair and Zigzag orientation. This study provides important insights into the friction mechanism of the WS2 when conducting nanoscratching process.