Role of Ultrafine Microstructures in Dynamic Fracture in Nanophase Silicon Nitride

Using 10⁶-atom molecular-dynamics simulations, we investigate dynamic fracture in nanophase Si₃N₄. 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 Si₃N₄. We also determine the morphology of fracture surfaces: For in-plane fracture surface profiles the roughness exponent ζ0ex0ex=0ex0ex0.57 and for out-of-plane profiles the exponents ζ_⊥0ex0ex=0ex0ex0.84 and ζ_0ex0ex=0ex0ex0.75 are in excellent agreement with experiments.