Compressive strength degradation of SiC fibers exposed to high temperatures due to impurity-induced internal oxidation

SiC fiber oxidation is a potential factor limiting the operating temperature of SiC_f/SiC composites owing to the strength degradation after oxidation. Herein, we fabricated 1-μm-diameter pillars at the core of the fiber cross-sectional surface after SiO₂ removal to eliminate surface effects caused by external oxidation. The fiber strength significantly decreased during the first hour of oxidation in dried air at 1400 °C, but this deterioration became less pronounced after 10 h. Simultaneously, the oxidation lowered the Young's modulus and Weibull modulus. Oxidation considerably increased the porosity and the alterations in the mechanical behavior were primarily caused by the variations in porosity. Oxidation-induced pores were frequently detected at the fiber core and were partially filled with SiO₂. Compared with those of the as-received fibers, O impurities in the oxidized fiber core were significantly reduced. Thus, the fiber strength was potentially degraded by the internal oxidation reaction between residual C and O.