Microstructural evolution and mechanical properties of CVI SiC_f/PyC/SiC composites under high-temperature steam conditions simulating pressurized water reactor accidents

The oxidation behavior of SiC_f/SiC composites and the impact on mechanical properties were investigated under high-temperature steam at 1200 °C and 1 atm. The oxidation-induced weight gain followed a parabolic trend, suggesting that volatile product formation was negligible under these conditions. After oxidation, SiO₂ progressively filled the interface region. Fiber push-in tests revealed significant increases in interfacial bonding strength, IFSS, and debonding energy with prolonged exposure, with a clear transition at 1 h. This transition was due to the accumulation of SiO₂ at the interface, which effectively blocked steam diffusion, causing the oxidation mechanism to shift from chemical reaction control to diffusion control. Steam oxidation led to an increase in defects and disorder in SiC fibers, which, combined with enhanced β-SiC crystallinity. The composites displayed pseudo-plastic fracture behavior without catastrophic failure, and strength decreased linearly, retaining 82.6 % (∼577 MPa) after 5 h, meeting the requirements for loss of coolant accident (LOCA) conditions.