BN recession and recession cessation in SiC/BN/SiC composites

The internal oxidation behavior of unidirectional SiC/BN/SiC minicomposites was investigated to clarify the mechanisms governing BN interphase recession, silica scale growth, and oxidation‐induced closure under dry and wet oxidizing environments at 1000°C. Using high‐resolution scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses of both pristine and precracked specimens, the evolution of interfacial oxidation was examined as a function of BN coating thickness, crack opening, and environmental water content. Oxidation‐induced closure occurs through multiple mechanisms, governed by crack width and environmental conditions. Four distinct closure pathways were identified: (i) silica scale formation on matrix crack surfaces leading to closure of narrow cracks, (ii) borosilicate glass accumulation filling recession gaps and matrix cracks with minimal volatilization loss, (iii) recession gap filling by silica‐enriched glass under conditions of enhanced boria volatilization, and (iv) oxide buildup on the fibers and matrix at the base of recession channels in the absence of liquid boria. These mechanisms are sensitive to the oxidation environment, the geometry of the crack or recession gap, and the presence of fiber‐coating debonds. In contrast to prior models that assume gas transport‐limited BN recession, current results demonstrate that recession may be reaction limited in some operational domains and may terminate at the reaction front when silica saturation is achieved in the borosilicate glass. Dimensionless closure times for each mechanism are derived, enabling direct comparison under varying crack geometries and environmental conditions. These findings provide new insight into the governing processes of interfacial degradation and help establish a framework for lifetime prediction in SiC/SiC composites.