Analyses of the role of grain boundaries in mesoscale dynamic fracture resistance of SiC-Si3N4 intergranular nanocomposites

Silicon carbide (SiC)-silicon nitride (Si₃N₄) nanocomposites with SiC dispersions as well as Si₃N₄ matrix of mesoscale dimensions (∼1 μm) are considered to have exceptional strength attributed to interactions of SiC dispersions with Si₃N₄ grain boundaries (GBs). However, an account of GBs on the strength of these nanocomposites is not available. In order to analyze this issue, cohesive finite element method (CFEM) based mesoscale dynamic fracture analyses of SiC-Si₃N₄ nanocomposites with an explicit account of length scales associated with Si₃N₄ GBs, SiC particles, and Si₃N₄ grains are performed. Analyses indicate that primary mechanism of fracture in the nanocomposite microstructures is intergranular Si₃N₄ matrix cracking. GBs are responsible for crack deflection and accordingly damage is limited to a smaller geometric region in microstructures with GBs. On an average, a microstructure with GBs present is stronger than the corresponding microstructure with GBs removed. However, in cases where the second phase SiC particles are in the wake of microcracks the microstructure without GB becomes stronger against fracture in comparison to the corresponding one with GBs owing to the crack bridging effect caused by the second phase SiC particles.