Transmission electron microscopy study on crack propagation characteristics of pressureless sintered 15R-β-U Sialon-polytypoid composite

Fabrication of densified 15R-β-U Sialon-polytypoid composite has been reported previously using a pressureless sintering (PLS) method at a relatively low temperature (1650 °C). The mechanical properties of this material are as good as those high temperature heat-pressed Sialon-polytypoids. In this study, the crack propagation characteristics were investigated to interpret the fracture behaviors of this material by transmission electron microscopy (TEM) technique. A schematic diagram was proposed according to the experimental results. The composite mainly fractures via a transgranular mode as indicated by the propagation routes of cracks. Subcrack branches can be induced by deflecting or shifting of cracks intergranularly or within a grain. The bonding strength of interface within grains is higher than the bonding force within the layer structure of 15R, contributing to the excellent mechanical properties of the pressureless sintered composite.