Improved mechanical properties and fracture mechanism of C/C composites with salt treatment monitored by synchrotron-based in-situ tensile XRD

Abstract

Due to the excellent mechanical and thermal properties, carbon fiber reinforced carbon matrix (C/C) composites have been used for wide applications in wear devices, aerospace, and nuclear reactor. Microstructure optimization is a critical way for obtaining a champion C/C composite. Based on the synchrotron-based tensile X-ray diffraction (XRD), we performed an in-situ observation of how microstructure of C/C composite responds to molten salt (FLiNaK) and applied load, a field where current study is limited, helping to reveal the actual microstructure evolution induced by the salt during applied load process. Molten salt impregnation in C/C composites can increase the microstructure stability of graphite and extend their structure relaxation process due to the synthetic interaction between applied load stress and salt squeeze. Notably, fractures of both composites are mainly due to a higher crystallographic strain shrinkage behavior along the tensile direction. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) of fractures observed many F and K elements existence in fibers, leading to the fracture of fibers much difference. The mechanical properties including modulus and hardness of C/C composite are improved simultaneously by salt treatment, which should be attributed to the infiltration of F and K elements into fibers and the changed microstructure towards better-ordered and higher-crystalline graphite. These present results would facilitate to understand the mechanism of improved mechanical properties of C/C composite induced by salt, therefore promoting the reliable and safe operation of graphite class material at a wide field.