Effect of heat treatment on the microstructure and tensile strength of KD-II SiC fibers

KD-II SiC fibers, a new type of SiC fibers, were heat treated at elevated temperatures for 1 h in an Ar atmosphere. The microstructure and mechanical properties of the fibers before and after annealing were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and tensile tests, et al. The results show that the as-received fibers consisted of β-SiC nano-crystals, free carbon and a small amount of amorphous intergranular SiCxOy phase. With increasing heat treatment temperature, significant growth of β-SiC grains, ordering of free carbon and decomposition of SiCxOy phase occurred and the fiber surfaces became coarser with development of new defects, such as pores and large SiC particles. It was also found that KD-II SiC fibers maintained high strength with heat treatment at temperatures up to 1500 °C, exceeding which a sharp strength degradation was observed. The strength degradation and microstructural evolution of the fibers due to heat treatment were correlated, and it can be concluded that the β-SiC grains growth, residual tensile stresses as well as surface flaws were the dominating factors for the fiber strength degradation after exposure at high temperatures.