Role of Ultrafine Microstructures in Dynamic Fracture in Nanophase Silicon Nitride

Using ${10}^6$-atom molecular-dynamics simulations, we investigate dynamic fracture in nanophase ${\mathrm{Si}}_3{\mathrm{N}}_4$. The simulations reveal that intercluster regions are amorphous, and they deflect cracks and give rise to local crack branching. As a result, the nanophase system is able to sustain an order-of-magnitude larger external strain than crystalline ${\mathrm{Si}}_3{\mathrm{N}}_4$. We also determine the morphology of fracture surfaces: For in-plane fracture surface profiles the roughness exponent $\zeta =0.57\mathrm{}$ and for out-of-plane profiles the exponents ${\zeta }_{\perp }=0.84\mathrm{}$ and ${\zeta }_{\Vert }=0.75\mathrm{}$ are in excellent agreement with experiments.