Interface-controlled carbothermal degradation in C/Y2SiO5 composites driven by interfacial amorphous silica enrichment

Carbon fiber-reinforced ceramic composites are widely used in extreme thermal protection systems, where high-temperature failure is generally attributed to oxidative degradation. However, this prevailing view overlooks the critical carbothermal interaction between carbon fibers and silica-derived phases, whose underlying degradation mechanism remains unclear. In this work, we systematically investigate the high-temperature inert stability of C/Y2SiO5 composites, and reveal that trace interfacial enriched amorphous SiO2 is the dominant factor governing composite degradation via an interface-controlled carbothermal reduction mechanism. Using a dense C/Y2SiO5 model composite with low residual SiO2 (~2 vol%), we identify a distinct two-stage degradation process: an incubation stage with full mechanical integrity, defined by grain-boundary migration of residual silica and its interfacial enrichment to form a ~5 nm reactive interlayer, where the composite retains 106.12% of its initial flexural strength after 1 h at 1600 °C. This followed by rapid catastrophic failure, where sustained carbothermal reduction induces severe fiber etching, enhanced interfacial bonding, and suppressed fiber pull-out energy dissipation, leading to a flexural strength retention of only 37.5% after 1600 °C thermal cycling. More broadly, this work establishes that trace amounts of silica can dominate the high-temperature degradation of carbon fiber-reinforced ceramic composites once interfacial enrichment occurs, even at low bulk contents. This work provides important mechanistic insights and practical guidance for improving the high-temperature stability of carbon fiber-reinforced ceramic composites.