Analyses of the role of the second phase SiC particles in microstructure dependent fracture resistance variation of SiC-Si3N4 nanocomposites

One of the primary factors affecting the failure in high strength silicon carbide (SiC)–silicon nitride (Si3N4) nanocomposites is the placement of spherical nano-sized SiC particles in micro-sized Si3N4 grains. In order to analyze this issue, the cohesive finite element method (CFEM) based dynamic fracture analyses of SiC–Si3N4 nanocomposites at two different loading rates (0.5 and 2 m s−1) with an explicit account of the lengthscales associated with Si3N4 grain boundaries (GBs) (sizescale 100 nm), SiC particles (sizescale 200–300 nm), and Si3N4 grains (sizescale 0.8–1.5 µm) are performed. A range of CFEM meshes with selective placement of second phase particles placed exclusively along GBs, exclusively inside Si3N4 grains, and along GBs as well as inside Si3N4 grains are generated for analyses. Analyses of the damage progression and stress distribution as a function of microstructural morphology indicate that high strength and relatively small sized SiC particles act as stress concentration sites in Si3N4 matrix. The dominant mode of fracture in all microstructures, therefore, is intergranular Si3N4 matrix cracking. Crack density evolution and fracture energy dissipation results show loading rate dependence of failure with the role of phase morphology becoming prominent at the lower loading rate. Contrary to the belief that microstructures with second phase particles lying exclusively along GBs are the strongest against fracture, microstructures with SiC particles lying along GBs as well as inside Si3N4 grains in locations near GBs are found to be the strongest