Oxygen-diffusion shielding: A novel mechanism governing the thermo-mechanical-oxidative behavior of C/SiC composites via In-situ CT

While numerous studies have examined C/SiC composites under ultra-high temperatures in air, inert, or vacuum atmospheres, the actual service environment of hypersonic vehicles features ultra-high temperatures and low pressure. To address this gap, we analyzed the residual mechanical properties, surface morphology, and pore-structure evolution of C/SiC composites after exposure to different temperatures, pressures, and loads. Mechanical testing, SEM, and in-situ CT were jointly employed. The results show that, when the temperature increases from 1200 °C to 1400 °C, the average residual strength and modulus exhibit increases of approximately 4 % and 18 %, respectively. Increasing pressure from 5 kPa to 15 kPa and 25 kPa leads to progressive reductions in residual strength by about 8 % and 30 %, while the residual modulus increases modestly by approximately 3 % and 12 %, indicating distinct controlling mechanisms. Notably, at 1200 °C and 5 kPa, an oxygen-diffusion shielding effect was observed: low loads enhance residual strength and modulus by closing pores and suppressing oxygen diffusion, whereas high loads reopen pores and accelerate crack-tip oxidation. In-situ CT analysis further reveals that porosity and fractal dimension decrease at low stress levels before increasing again with increasing stress. Based on these observations, an oxygen-diffusion shielding model that incorporates pore closure and fractal characteristics and a coupled thermo-mechanical-oxidation predictive expression was developed. It accurately captures the three-dimensional dependence of degradation on temperature, pressure, and load, and successfully predicts experimental results at 1400 °C under different loading conditions.