Structural evolution and interfacial degradation mechanisms of SiBCNf/SiBCN mini-composites subjected to heat treatment at 1500–1800 °C

SiBCN fiber-reinforced SiBCN matrix composites (SiBCN f /SiBCN) are promising for ultra-high-temperature structural applications, but their degradation mechanisms under thermal exposure remain insufficiently understood. In this work, SiBCN f /SiBCN mini-composites were heat-treated at 1500–1800 °C in N 2 to investigate their structural evolution, interfacial degradation, mechanical response, and failure behavior. The composites showed good overall thermal stability, retaining a flexural strength of 654±29 MPa and a flexural modulus of 66±5 GPa after heat treatment at 1800 °C, corresponding to 71% and 73% of the as-received values, respectively. The most severe degradation occurred around 1600 °C, where rapid mass loss, pore interconnection, and matrix damage markedly reduced the load-bearing capability of the composites. The performance deterioration was governed mainly by matrix evolution and the resulting loss of interfacial stress-transfer capability, rather than by intrinsic instability of the SiBCN fibers. Below 1700 °C, the matrix mainly underwent amorphous network rearrangement with gradual β -SiC development, whereas pronounced phase reconstruction occurred at 1800 °C, including β -Si 3 N 4 precipitation and β -SiC to α -SiC transformation. Meanwhile, the failure mode changed from localized brittle fracture to progressive damage involving interfacial debonding, fiber sliding, and pull-out. These results clarify the matrix–interface–property relationship in SiBCN f /SiBCN composites and provide guidance for the design of precursor-derived ceramic matrix composites for extreme environments.