FEM analysis of the stress response and failure mechanism of SiC-coated C_f/SiC composites during thermal shock

Understanding the thermal stability of ceramic matrix composites is one of the necessary prerequisites for their engineering application. This study aimed to investigate the stress response and failure mechanism of SiC-coated C_f composite during thermal shock in an O₂ free environment, at temperature between 500 and 1800 °C. Composites with a single layer of SiC coating, and a reinforcing phase are carbon fiber yarns with a three-dimensional four-step braided structure. A meso-scale finite element model considering the orientation of fiber yarns was established to analyze thermal residual stress (TRS) and thermal shock stress (TSS) by transient analysis. Results indicated that the most dangerous position generated by the TRS was in the matrix close to the fiber yarn/matrix interface. Initial cracks first nucleated in the matrix, because the matrix was subjected to large residual compressive stress along the braiding direction, which is related to the distribution characteristics of the fiber yarns (braiding method). Fiber yarns with different orientation exhibited almost the same stress state, and interaction between fiber yarns resulted in a similar “twisted” state. For single fiber yarns, the principal axis and fiber yarn/fiber yarn interface were prone to failure. TSS in the composites rapidly increased during the initial cooling stage. A sudden change in temperature and stress gradient led to cracking and peeling at the composites/coating interface. Possible failure modes and mechanisms of the composites with and without the SiC coating are discussed.