High-temperature structural evolution and strength retention of SiC grain stabilized Si–C–B–N fibres

High-temperature–resistant ceramic fibres are key reinforcements in accident-tolerant ceramic matrix composites designed to survive harsh environments. SiC grain stabilized Si–C–B–N fibres with thermal endurances up to 2100 °C under nitrogen have been developed by our group. In this work, the high-temperature evolution of the Si–C–B–N fibres with layered surfaces is investigated in an argon environment. Without the assistance of nitrogen, carbothermal reduction reactions occur between 1500 and 1700 °C, generating significant defects that degrade the fibre strength from 1.79 to 0.91 GPa. The metastable SiC x N y phase is eliminated, which makes the fibre more stable. After heat treatment at 1800 °C, surface healing and densification occur, restoring the strength to 1.53 GPa. The U-shaped retention curve of tensile strength suggests that the SiC grains and layered surfaces played key roles in the microstructural evolution. Benefiting from the optimized microstructure stabilization, the fibre strength is maintained at approximately 1.5 GPa throughout the 1500–1700 °C range, in argon atmosphere. This work provides a controllable heat treatment for high-temperature–resistant Si–C–B–N fibres, which can be applied to the other Si-C-B-N multinary ceramics also.