The polycarbosilane derived KD-II SiC fibers were heat treated in argon atmosphere at temperatures from 1000 °C to 1800 °C and in oxidation atmosphere at temperatures from 1000 °C to 1500 °C for 1 hour, respectively. The effect of heat treatment on microstructure of all fibers were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Atomic Force Microscopeand, and the tensile strength of fibers were evaluated by monofilament tensile test and Weibull model. The results reveal that the fibers of heat treatment in argon atmosphere can maintain their original tensile strength (~2.5 GPa) up to the temperature at 1200 °C. After that, their tensile strength starts to decline, especially when the temperature exceeds 1400 °C a sharp strength degradation is observed. Through detailed characterization, we find that the reasons for the strength degradation of fibers heat treatment in argon atmosphere include the appearance of surface defects, growth of β-SiC grains and decomposition of SiCxOy amorphous phase. Correspondingly, the fibers heat treated in oxidation atmosphere have lower tensile strength compared with those fibers heat treatment in argon atmosphere at the same temperature. Furthermore, their tensile strength starts to degenerate at 1000 °C, and only 30% of original strength is retained when the temperature reached 1500 °C. Obviously, for fibers heat treatment in oxidation atmosphere, their tensile strength degradation starting temperature is lower and degradation speed is faster than those fibers heat treatment in argon atmosphere. This can be attributed to the interface stress existed between silica layer with internal fibers, the surface cracks caused by the thermal stress and the growth of β-SiC grains

Keywords: SiC fibers, Heat treatment, Microstructure, Tensile strength, Argon atmosphere, Oxidation atmosphere