Analysis of damage mechanisms in fiber-reinforced ceramic matrix composites under thermomechanical loading at intermediate and high temperatures

This paper examines the damage and failure behavior of woven ceramic matrix composites (CMCs) under thermomechanical loading. The study focuses on S200H (Hi-Nicalon™ SiC fiber with a boron nitride coating in a SiNC matrix) and S400N (carbon fiber with a pyrolytic carbon coating in a SiNC matrix) CMCs. Quasi-static and creep-fatigue tests were conducted at various temperatures and stress levels to investigate failure mechanisms at intermediate and high temperatures. Creep-fatigue tests for S200H were performed at 800°C in an oxidative environment at stress levels corresponding to fractions of the ultimate tensile strength (UTS). S400N samples were tested at 600–900°C and 1200°C under low and high stress levels. Residual strength tests were conducted after cooldown to assess mechanical degradation in samples that did not fail. Fracture surface characterization using confocal microscopy, scanning electron microscopy, and energy dispersive spectroscopy provided insights into failure mechanisms. Results indicate that chemical reactions of non-stoichiometric phases in SiC/SiNC, thermal property mismatch, and pyrolytic carbon coating volatilization in C/SiNC govern quasi-static failure mechanisms. Both CMC systems exhibited a 30% reduction in UTS at elevated temperatures, with strain-to-failure decreasing by 17% in SiC/SiNC and 31% in C/SiNC. The SiC/SiNC samples exhibited an increase in yield strength with higher applied stress levels after 100 hours of creep-fatigue testing due to matrix residual compressive stresses, but a 37% reduction in UTS due to oxidation. In contrast, C/SiNC samples experienced significant diffusion-limited oxidation, leading to rapid fracture within 20 hours under creep-fatigue loading.