Adaptive oxidation protection in C/Y2Si2O7 composites enabled by defect sealing and bilayer surface-structure evolution

Limiting oxygen ingress through surface-connected defects is critical for improving the oxidation durability of carbon-fiber-reinforced ceramic-matrix composites at high temperatures. Conventional strategies based on external coatings or passive oxide scales may be compromised during thermal cycling because thermal mismatch, cracking, and spallation can reopen pathways for oxygen ingress. This study reveals a coating-independent and intrinsically adaptive oxidation-protection mechanism in Y 2 Si 2 O 7 -based composites, primarily enabled by a sol-gel derived intergranular amorphous SiO 2 nanophase that assists in situ defect sealing during high-temperature oxidation. At 1400 °C, the amorphous grain-boundary SiO 2 undergoes viscous relaxation and local redistribution, which helps seal surface-connected pores and microcracks, reduces open-defect connectivity, and confines oxidation mainly to the subsurface region. During thermal cycling, the surface further evolves into a bilayer structure with a compliant porous outer zone and a relatively dense inner layer, helping relieve thermal strain while suppressing deep oxygen permeation. After oxidation at 1400 °C for 2 h and thermal-shock, the composite retained 99.69% and 98.42% of its initial mass, respectively. Unlike conventional protection strategies that rely on external coatings or passively formed oxide scales, the present composite exploits temperature-responsive intergranular amorphous SiO 2 to assist the in situ closure of surface-connected defects, thereby limiting oxygen ingress during high-temperature exposure. This work demonstrates a coating-free adaptive design strategy for improving the durability of carbon-fiber-reinforced ceramic-matrix composites in high-temperature.