Grain growth and interfacial structures in SiC fibers

The understanding of the microstructures is critical to the design of SiC fibers with excellent thermal stability and mechanical properties. We introduce a density-sensitive electron microscopy method, high-angle annular dark field (HAADF) imaging to study the microstructure of SiC fibers processed at a variety of temperatures ranging from 1200 to 1800 °C. It is revealed that, irrespective of their processing temperatures, SiC grains form the skeleton of SiC fibers that are surrounded by a multiple of zones with low HAADF contrast (low-density zones, LDZs). LDZs mainly consist of amorphous SiOC phases and turbostratic graphite. As a result, three important interfaces that dictate grain growth, namely, the SiC/amorphous SiOC interface, the SiC/turbostratic graphite phase boundary and the SiC high angle grain boundary (HAGBs) emerge. We find that, the SiC/turbostratic graphite interfaces and the SiC HAGBs are more effective in suppressing the growth of SiC grains than the SiC/a-SiOC interfaces on a basis of extensive TEM characterization. Aberration corrected TEM reveals marked differences in the atomic structures of those three interfaces, shedding light on how the interfacial structures affect grain growth of SiC fibers.