Dynamics and Morphology of Brittle Cracks: A Molecular-Dynamics Study of Silicon Nitride

Crack propagation in amorphous S${\mathrm{i}}_3$ ${\mathrm{N}}_4$ films is investigated with large-scale molecular-dynamics simulations on parallel machines. We observe a correlation between the speed of crack propagation and the morphology of fracture surfaces. Initially, as the crack propagates slowly, the roughness exponent for fracture surfaces is found to be 0.44. However, beyond a certain speed of crack propagation, the exponent crosses over to 0.8. This crossover behavior is similar to the recent experimental finding by Bouchaud and Navéos.