Role of ultrafine microstructures in dynamic fracture in nanophase silicon nitride

Using 10(6)-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 zeta = 0.57 and for out-of-plane profiles the exponents zeta(perpendicular to) = 0.84 and zeta(parallel to) = 0.75 are in excellent agreement with experiments.