This work focuses on oxidation-induced embrittlement of SiC/BN/SiC composites at intermediate temperatures and develops models for several critical phenomena involved in the process. The primary phenomena studied include internal oxidation of the constituent materials of the composite, the stress evolution in SiC fibers due to silica scales formation on fiber surfaces, the stress concentration caused by constrained silica growth, and the consequent fiber fractures in the composite.

In the model for internal oxidation of SiC/BN/SiC composite, the primary phenomena considered are: (i) inward diffusion of gaseous oxidants through narrow matrix cracks, (ii) recession of the BN fiber coatings through formation, volatilization and outward diffusion of gaseous boron hydroxide species, and (iii) silica scale growth on the exposed fibers and matrix crack surfaces. Closure of the gap created by BN removal is also taken into account. The model predicts spatial and temporal evolutions of silica scale thickness, BN recession depth and concentrations of gaseous species within the matrix crack. Over the range of temperatures (700°C-900°C) and environments (10-50% H 2O in O2) considered, simulation results indicate that the BN recession rate is essentially controlled by transport of the gaseous reaction products within the matrix crack and gap closure by silica scale formation. One consequence is that, when the opening displacement of the matrix crack is small (ca. 1 microm), recession is confined to regions close to the free surface of the composite. Otherwise, when the opening displacement is large (ca. 100 microm), recession occurs uniformly across the panel thickness. Additionally, throughout the selected parameter space, the terminal extent of BN recession increases monotonically with water content and BN thickness but decreases with temperature.

Another potentially critical step in the embrittlement process is the premature fracture of SiC fibers induced by oxidation. Stress rupture of SiC/SiC composites at intermediate temperatures in oxidizing environments is the result of a series of internal chemical and thermomechanical processes that lead to premature, localized fiber fracture. We have developed analytical models for two potentially critical steps in this process. The first involves the generation of tensile stresses in the fibers due to SiO2 scale formation (following removal of fiber coatings) and the associated reduction in the applied stress needed for fiber fracture. The second occurs once the gaps produced by coating removal are filled with oxide and subsequent oxidation occurs subject to the constraints imposed by the matrix crack faces. In this domain, the failure model is couched in terms of the stress intensification within the fibers caused by constrained oxidation. The models incorporate the combined kinetic effects of oxide growth and viscous flow. The competing effects of increased oxidation rate and accelerated stress relaxation with increasing temperature on fiber stress feature prominently in the results. The results suggest that, in dry air environments, the highest risk of fiber fracture occurs at temperatures in the range 840°-940°C. In this range, the oxide scales grow at appreciable rates yet the resulting growth stresses cannot be mitigated sufficiently rapidly by viscous flow.