Fracture Mechanisms in Multilayer Phosphorene Assemblies: From Brittle to Ductile
The outstanding mechanical performance of nacre has stimulated numerous works in the design of artificial nacres. Phosphorene, a new two-dimensional (2D) material, has crystalline in-plane structure and non-bonded interaction between adjacent flakes. Therefore, multi-layer phosphorene assemblies (MLPs), in which phosphorene flakes are piled up in a staggered manner, may exhibit outstanding mechanical performance, especially exceptional toughness. Therefore, molecular dynamics simulations are performed to study the dependence of the mechanical properties on overlap distance between adjacent phosphorene layers and the number of phosphorene flakes per layer. Results indicate that when the flake number is equal to 1, a transition of fracture pattern is observed by increasing the overlap distance, from a ductile failure controlled by interfacial friction to a brittle failure dominated by breakage of covalent bonds inside phosphorene flakes. Moreover, the failure pattern can be tuned by changing the number of flakes in each phosphorene layer. Results imply that the ultimate strength follows a power law with the exponent -0.5 in terms of the flake number, which is in good agreement with our analytical model. Furthermore, the flake number in each phosphorene layer is optimized as 2 when the temperature is 1K in order to potentially achieve both high toughness and strength. Moreover, our results regarding relations between mechanical performance and overlap distance can be explained well with the shear-lag model. However, it should be pointed out that increasing the temperature could cause the transition of fracture patterns from ductile to brittle. Therefore, the optimal flake number depends heavily on temperature in order to achieve both its outstanding strength and toughness. Overall, our findings unveil the fundamental mechanism at the nanoscale for MLPs as well as provide a way to design phosphorene-based structures with targeted properties via tunable overlap distance and flake number in phosphorene layers.