Effect and mechanism analysis of free carbon on the oxidation behaviour of SiC fibres

Continuous SiC fibres have received widespread attention in the field of aero-engines due to their excellent mechanical properties and oxidation resistance. Free carbon is one of the main factors affecting the oxidation behaviour of SiC fibres, but its role and influence are still unclear. The oxidation behaviour of two types of SiC fibres with similar oxygen contents and crystallite sizes but different carbon-silicon ratios was investigated in this study. Free carbon did not significantly influence strength retention, but positively affected elastic modulus retention. The oxidised carbon-rich SiC fibres exhibited a thinner, smoother, and denser oxide scale. The oxidation rate of the carbon-rich fibres was lower than that of near-stoichiometric fibres after oxidising at 1200 °C. The oxidation activation energies of both were 170.44 ± 15.80 and 234.45 ± 24.94 kJ/mol, respectively. Free carbon can inhibit oxidation scale crystals at the beginning of oxidation while promoting crystallisation scale as oxidation intensifies. The oxidation scale crystals of carbon-rich fibres are mainly controlled by the interface, whereas those of near-stoichiometric fibres are mainly controlled by diffusion. A stress expansion zone was formed at the interface of the oxidised SiC fibres. Free carbon can enhance the internal stress of the stress expansion zone, resulting in lower residual compressive stress on the surface of carbon-rich SiC fibres. These results are significant for improving the oxidation resistance of SiC fibres by retaining free carbon.