Effect of water on oxidation behavior of KD-II SiC monofilament

SiC fiber (SiCf) in SiCf/SiC plays an essential role in load-bearing and high-temperature applications with the presence of water. It is important to explore the high-temperature oxidation process of SiCf with water. KD-II SiC fibers were annealed at 1050 degrees C and 1350 degrees C in air and H2O-O2-Ar atmospheres for 0.25 h, 1 h, 4 h, and 16 h to explore the effect of water on the evolution of microstructures and mechanical properties. In the air atmosphere, the fiber strength decreases with the increased temperature and time. However, it responds differently in the H2O-O2-Ar atmosphere since there is no significant reduction of fiber strength after annealing at 1350 degrees C compared to 1050 degrees C. Water can significantly accelerate the oxidation process and results in increased surface defects and roughness, consistent with the continuously decreasing fiber strength. By increasing the temperature to 1350 degrees C, coalescence between beta-SiC crystal and alpha-SiO2 (cristobalite) is observed on the oxide scale (silica). As the result, accompanied by the spallation of the oxide surface, the fiber strength after annealing at 1350 degrees C for 16 h is unexpectedly higher than that at 1050 degrees C for 16 h in the H2O-O2-Ar atmosphere.