The role of CNT in improving the mechanical strength retention rate of C/C composites during heat treatment

Graphitization is an important post-treatment of carbon/carbon (C/C) composites. Carbon nanotubes (CNT) as a means of reinforcement have been explored extensively in C/C composites. In this study, CNTs were grafted on the carbon fiber and subsequently densified to delve into the effect of CNT on microstructure, residual thermal stress evolution, as well as the mechanical strength retention rate (MSR) in the course of the heat treatment. Pure C/C and CNT-C/C composites were heat-treated at three temperatures, namely, 1600 °C, 2100 °C and 2450 °C. As suggested from the results, CNT facilitated the transition from the turbostratic structure to the ideal graphite structure. According to the results of measurement and finite element analysis, the presence of CNT decreased and concentratedly distributed the residual thermal stress. In pure C/C composites after undergoing 2450 °C heat treatment, the MSR of out-of-plane compression strength (OCS), in-plane compression strength (ICS) and interlaminar shearing strength (ISS) reached 53.07%, 45.27% and 41.16%, respectively, while these in CNT-C/C composites were 69.69%, 66.33% and 69.70%, respectively. CNT noticeably mitigated the effect of heat treatment on mechanical properties; they also reduced the effect on the anisotropy of mechanical properties. By impacting the microstructure and residual thermal stress distribution of the composites, CNT had further effects on the loading capacity and fracture mode, and finally protected the composites during the high-temperature heat treatment and up-regulated the MSR of CNT-C/C composites.