Interactions between components of LSI SiC/SiC in oxidation environment

LSI SiC/SiC composites have garnered significant research interest owing to their low porosity, excellent specific strength, and superior thermal conductivity. However, their ultimate strength declines sharply after high-temperature exposure near or above the melting point of Si, and the underlying degradation mechanisms remain inadequately understood. In this study, a typical LSI SiC/SiC composite reinforced with Cansas 3303 SiC fibers is systematically investigated to elucidate the degradation of mechanical properties and the associated mechanisms following heat treatment at 1500 °C under both inert and oxidizing atmospheres. A multi-scale analysis is conducted for the first time: at the mesoscale, morphology evolutions of the composite surface and interior are characterized; at the microscale, microstructure changes in regions adjacent to the fibers are closely examined. The mechanism of strength degradation can be summarized as that under an inert atmosphere, Si undergoes solid-phase diffusion, causing limited damage to the CVI SiC layer. In an oxidizing atmosphere, oxygen accelerates Si-driven degradation, leading to progressive and extensive damage to the CVI SiC layer, BN interphase, and SiC fibers. These findings offer another way to improve the long-term durability of LSI SiC/SiC composites at high temperatures and in complex environments—not just by reducing residual Si, but also by using protective coatings to limit Si activity and prevent strength loss.