Role of Ultrafine Microstructures in Dynamic Fracture in Nanophase Silicon Nitride

Using 10{sup 6}-atom molecular-dynamics simulations, we investigate dynamic fracture in nanophase Si{sub 3}N{sub 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 Si{sub 3}N{sub 4}. 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}{sub {perpendicular}}=0.84 and {zeta}{sub {parallel}}=0.75 are in excellent agreement with experiments. {copyright} {ital 1997} {ital The American Physical Society}