Mechanical properties and thickness-determined fracture mode of hexagonal boron nitride nanosheets under nanoindentation simulations

Hexagonal boron nitride nanosheets (h-BNNS) is one kind of promising 2D materials due to its outstanding mechanical properties especially at elevated temperature. However, the experimentally measured mechanical properties of h-BNNS have great dispersions due to the intricate experimental measurement conditions. In the present study, the classical molecular dynamics simulations are performed to study the effect of various experimental factors on the mechanical properties of h-BNNS based on nanoindentation, including: size, temperature, loading rate of nanoindenter, ratio of circular-region radius to nanoindenter radius, and thickness (the number of layers). Meanwhile, the evolution of microstructure of h-BNNS up to fracture are investigated to elucidate its fracture mechanism during nanoindentation process. The results indicate that the Young's modulus and fracture strength of h-BNNS gradually decreases with the increasing temperature, and the mechanical properties of h-BNNS remain relatively stable when the ratio of circular-region radius to nanoindenter radius and the circular-region radius is bigger than 10 and 18 nm, respectively. Based on the evolutionary process of the microstructure of h-BNNS, the results not only show the failure process of h-BNNS, but also reveal the conversion of the fracture mode in multi-layer h-BNNS, that is, it will be translated from the bottom-top to the top-bottom fracture with increasing thickness.