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

SiC fiber (SiC_f) in SiC_f/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 SiC_f with water. KD-II SiC fibers were annealed at 1050 °C and 1350 °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 °C compared to 1050 °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 °C, coalescence between β-SiC crystal and α-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 °C for 16 h is unexpectedly higher than that at 1050 °C for 16 h in the H₂O-O₂-Ar atmosphere.