Mechanical properties and damage of 2.5D braided SiC_f/SiC ceramic matrix composites under different thermal shock conditions

The effects of different thermal shock conditions (times) on the mechanical properties and damage of 2.5D braided SiC_f/SiC (f represents fiber) ceramic matrix composites (CMCs) were analyzed by uniaxial tensile and three-point bending experiments, combined with SEM and EDS. The results show that the tensile stress-strain curves of 2.5D braided SiC_f/SiC CMCs exhibit nonlinear changes at room temperature without thermal shock and under different thermal shock conditions at 1200℃. The tensile strength first decreases gradually, and then increases slightly. The tensile strength of the material decreases to 48.39% under 10 thermal shocks, and increases to 54.11% after 30 thermal shocks; The three-point bending displacement-load curves of the material without thermal shock at room temperature and under different thermal shock conditions at 1200℃ also show nonlinear changes, with the increase of the number of thermal shocks, the flexural strength decreases rapidly, the bending strength of the material rapidly drops to 26.06% under 10 thermal shocks, and decreases to 10.77% after 30 thermal shocks. From the macroscopic fracture analysis, it can be seen that the tensile and bending fractures of the material at room temperature without thermal shock show pseudo-brittle fracture characteristics, while the tensile and bending fractures show ductile fracture characteristics under thermal shock conditions. From the microscopic fracture, damage behaviors such as fiber pullout, fiber debonding, interface debonding, crack propagation and fiber fracture are observed, and with the increase of the number of thermal shocks, the interface bonding force is gradually weakened, and the above-mentioned damage behaviors increase significantly.