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 (Si(3)N(4)) nanocomposites is the placement of spherical nano-sized SiC particles in micro-sized Si(3)N(4) grains. In order to analyze this issue, the cohesive finite element method (CFEM) based dynamic fracture analyses of SiC-Si(3)N(4) nanocomposites at two different loading rates (0.5 and 2ms(-1)) with an explicit account of the lengthscales associated with Si(3)N(4) grain boundaries (GBs) (sizescale 100 nm), SiC particles (sizescale 200-300 nm), and Si(3)N(4) grains (sizescale 0.8-1.5 mu m) are performed. A range of CFEM meshes with selective placement of second phase particles placed exclusively along GBs, exclusively inside Si(3)N(4) grains, and along GBs as well as inside Si(3)N(4) 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 Si(3)N(4) matrix. The dominant mode of fracture in all microstructures, therefore, is intergranular Si(3)N(4) 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 Si(3)N(4) grains in locations near GBs are found to be the strongest.