Thermo-chemo-mechanical Coupling induced Degradation of SiC Fibers and PyC Interphase during NITE-SiCf/SiC process

SiC f /SiC composites are promising for extreme environments, and the NITE process enables their efficient densification. However, the coupled effects of temperature, pressure, and sintering additives on fiber/interphase degradation remain unclear. This study investigates degradation mechanisms in NITE-SiC f /SiC composites fabricated at 1650-1750 °C using Cansas-3303 fibers. Thermal and thermochemical treatments on fibers with 0-700 nm PyC coatings decouple these factors. Results show the pseudo-ductile to brittle fracture transition occurs between 1700 °C and 1750 °C, driven by temperature-induced grain coarsening, additive erosion of fibers, and carbothermal reactions between PyC and additive. These reactions disrupt interphase integrity, induce strong bonding, and inhibit fiber pull-out. Pressure exacerbates both fiber deformation and interphase damage. Monofilament tensile tests reveal that temperature is the dominant degradation factor, with fiber strength dropping from 4.24 GPa (as-received) to 1.82 GPa after heat treatment and further to 1.33 GPa after thermochemical treatment without interphase. The extent of PyC interphase thinning correlates strongly with sintering temperature, suggesting an optimal interphase thickness exists for each processing condition. The findings provide critical insights for optimizing the microstructure and mechanical performance of NITE-SiC f /SiC composites.