Exploring the Multi-Scale Evolution Mechanism of the Ultra-High-Temperature Mechanical Behaviour of Carbon/Carbon Composites

The microstructure and mechanical properties of Carbon/Carbon (C/C) composites were investigated after ultra-high-temperature heat treatment (HTT) in the range of 30 – 2950 °C. The effect mechanism of high temperatures on the mechanical behaviour of C/C composites was proposed for three scales, i.e., graphene sheets, carbon fibre (CF) monofilaments, and C/C composites. The results displayed that the tensile strength of CF monofilaments decreases monotonically as the HTT increases in the range of 2000 – 2950 °C, while C/C composites present a tendency to decrease and then increase, demonstrating that CF/matrix interfacial properties play a decisive role in the tensile properties. The compression and flexural properties showed a trend opposite to the tensile properties, Young's modulus of both CF monofilaments and C/C composites increases. A turning point in the microstructure of C/C composites was observed after HTT at 2700 °C, in which the graphitisation degree decreases and the size of graphene sheets becomes fluctuating, while the high-temperature mechanical properties began to be superior to room temperature (RT). The high-temperature mechanical properties of C/C composites were regulated by the competing mechanisms of the graphite crystallite structure and the skin-core structure of CF monofilament, the CF/matrix interfacial strength and the porosity of C/C composites.