Atomistic and Continuum Understanding of the Particle Clustering and Particle Size Effect on the Room and High Temperature Strength of SiC-Si3N4 Nanocomposites

Abstract

Silicon carbide (SiC)-silicon nitride (Si3N4) nanocomposites are one of the most important high temperature materials. Factors that affect the strength of the SiC-Si3N4 nanocomposites can include the second phase SiC particle placement and clustering along Si3N4 GBs, the SiC particle size, Si3N4 grain size, and Si3N4 matrix morphology. This work presents recent work by our group in analyzing the effect of morphological variations in second phase SiC particle placement and GB strength on the room temperature fracture strength of SiC-Si3N4 nanocomposites using continuum analyses based on a mesoscale (~50 nm) cohesive finite element method (CFEM) and using molecular dynamics (MD) based analyses at nanoscale (~15 nm). The analyses have revealed that high strength and relatively small sized SiC particles act as stress concentration sites in Si3N4 matrix leading to inter-granular Si3N4 matrix cracking as a dominant nanocomposite failure mode under dynamic loading. At high SiC volume fractions that peak at approximately 30%, the CFEM analyses have revealed that due to a significant number of nano-sized SiC particles being present in micro-sized Si3N4 matrix, the SiC particles invariantly fall in the wake regions of microcracks leading to significant increase in fracture resistance. This finding was mechanistically confirmed in the room temperature MD analyses that revealed that particle clustering along the GBs was more effective than particles being placed on GBs in increasing the nanocomposite mechanical strength. The temperature dependent deformation mechanism is found to be a trade-off between the stress concentration caused by SiC particles and Si3N4-Si3N4 GB sliding. The temperature increase tends to work in favor of GB sliding leading to softening of structures. However, microstructural strength increases with increase in temperature when GBs are absent.