Molecular Dynamics Simulations of the Effect of the Composition of the Intergranular Film on Fracture in Si3N4

Molecular dynamics computer simulations were used to study the fracture behavior of silica, multicomponent silicate, and oxynitride intergranular films (IGFs) between silicon nitride crystals as a function of composition, film thickness, and crystallographic orientation. The maximum fracture strength is higher in the IGF between prism surfaces than between basal surfaces. This is caused by the preferential segregation of specific species to the basal surfaces in contrast to the prism surfaces, effectively modifying the composition within the glassy portion of the IGF, wish a subsequent effect on strength. The ordering observed in the thinner IGFs causes an increase in strength in the linear portions of the stress/strain curves, effectively increasing fracture stress. The simulations show that the force/atom in the 1 nm SiO2 IGF in the direction of tensile strain is more similar to that in beta-cristobalite than to nu-SiO2, whereas such data in the 2 nm SiO2 IGF is more similar to nu-SiO2. Simulations of the multicomponent silicate IGFs show the expected effect of lowering fracture strength with increasing modifier content in the silicate IGF, similar to that of bulk glasses. Similar to experimental studies, a decrease in strength is observed in silicon oxynitride IGFs with increasing oxygen concentration.