Ceramic matrix composites (CMCs) based on SiC/BN/SiC are promising candidates for high-temperature thermostructural applications in aerospace propulsion systems due to their low density and high strength and toughness. However, their long-term durability appears to be limited by oxidation and volatilization of the boron nitride (BN) interphase coatings at intermediate temperatures (700-1100°C), which can lead to significant strength and toughness degradation. Understanding the mechanisms governing microstructure evolution during oxidation is crucial for predicting long-term performance of these composites in service environments.

This dissertation investigates the oxidation behavior of SiC fibers, BN interphase coatings, and SiC matrices in SiC/BN/SiC composites exposed to oxidizing environments containing water vapor. Experiments are conducted on bare fibers, polished minicomposite surfaces, and minicomposites with matrix cracks, all at 1000°C. Advanced characterization techniques, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and custom image analysis codes, are employed to characterize and quantify microstructural changes, especially those in the vicinity of the fiber-matrix interface.

The results reveal that small differences in fiber composition can significantly affect the kinetics of scale growth and crystallization as well as the cracking behavior of these scales. The BN coating thickness and uniformity in pristine minicomposites are found to depend on the local fiber volume fraction and distance from the tow exterior, with closed fiber clusters being a major source of coating non-uniformity. Exposure of minicomposites to oxidizing environments lead to formation of oxide plugs and/or open channels, with the extent of BN recession dependent on coating thickness and as well as the transport distances for reaction product removal. Comparisons of oxidation behavior of composites with three types of BN coatings (crystalline, amorphous, and Si-doped) reveal distinct phase evolution and recession mechanisms.

This work yields new insights into the connections between microstructure, composition, and oxidation behavior of SiC/BN/SiC composites. The findings may in turn provide useful guidance in the development of coating designs and processing methods that lead to improved oxidation-resistant composites. Future research directions are proposed, including investigation of the role of oxidation in mechanical behavior, effects of cyclic oxidation, influence of gases representative of real combustion environments, and evolution of properties of borosilicate glasses during their formation and subsequent boria volatilization.