Effects of heat treatment on microstructure and mechanical properties of 2.5D Cf/BN/pyrocarbon composites by chemical vapor infiltration

Abstract In this study, 2.5D carbon fiber reinforced pyrocarbon matrix composites with a BN interphase (Cf/BN/PyC) were prepared by chemical vapor infiltration. Through comparing the distribution of thermal residual stress and the evolution of the interface microstructure of composite before and after heat treatment, the reason why the structure and orientation of the BN interphase in the Cf/BN/PyC composite deviate from the layered structure required for the expected compliant interface was analyzed. The results show that the crystalline skin of Cf affects the order of BN near the Cf/BN interface, thereby forming BN interphase with a structural gradient from the beginning. The infiltration process of pyrocarbon interacts with the growth and rearrangement process of BN, resulting in incomplete crystallization of BN. After heat treatment at 1600°C, BN interphase with high crystallinity in the nano-domain but disordered orientation is formed, although it can also debond the interface according to He and Hutchinson's model. The BN grain orientation deviates from the fiber axis mainly due to multi-factor coupling, in which the axial residual thermal stress generated at the Cf/BN/PyC interface is the main influencing factor. Understanding the influence of the heat treatment (HT) on the atomic-domain orientation change and structural evolution during the preparation of boron nitride (BN) interphase by chemical vapor infiltration paves us an effective way to modify the preparation of interphase with low fracture energy.