Evolution of microstructure and tensile strength of Cansas-II SiC fibers under air oxidizing atmosphere

The influence of oxidation on the microstructure and tensile strength of Cansas-II SiC fibers at 900-1500 degrees C in the air was investigated in depth. The growth of beta-SiC grains ordering as well as the increase of the size of free carbon in the SiC core occurred due to the thermal exposure. The thickness of the amorphous SiO2 layer increases with the temperature, starting to transform to cristobalite at 1200 degrees C. The activation energy in the ambient air is determined as 148KJ/mol, similar to that of Hi-Niaclon fibers (107 similar to 151 KJ/mol). With the growth of the SiO2 layer, lots of bubbles appeared in the SiO2 layer due to the release of excess CO gas. Moreover, many cracks occurred on the fiber surface caused by the residual stress. The mean tensile strength decreased from initial 2.7 GPa to 0.3 GPa after the treatment at 1500 degrees C, which could be mainly attributed to the SiO2 layer.

Automated summary

The study found that oxidation of Cansas-II SiC fibers at high temperatures can lead to ordered growth of beta-SiC grains and an increase in the size of free carbon in the SiC core, as well as an increase in the thickness of the SiO2 layer, ultimately affecting the tensile strength of the fibers.