Soft Interface Fracture Transfer in Nanoscale MoS₂

Molybdenum disulfide (MoS₂) nanoflakes, nanotubes, and nanoparticles are used as solid lubricants and oil additives. We investigate the formation of transfer layers due to fracture during sliding on commercially available MoS₂ nanoflakes. The sliding and fracture properties were observed in high-frame-rate videos and high-resolution images captured using in situ transmission electron microscopy. The orientation of the flakes and the adhesion to the surface and to the contact asperity determined the weakest interface, which subsequently determined the fracture transfer layer. The fracture continued until both surface and counter surface lubricant layers were a single sheet. The fractured material created a transfer layer or wear particles. We did not observe the proposed “deck-of-cards” sliding, where the sliding is distributed between all the layers of a MoS₂ flake. Instead, we captured video of an entire flake fracturing at a weak point in the MoS₂ sheets, a “weakest link” soft interface fracture model. The soft interface fracture transfer (SIFT) model is not specific to MoS₂-layered nanoflakes, and we argue it is a general mechanism in the formation of tribolayers.