Effect of graphitization temperature on microstructure, mechanical and ablative properties of C/C composites with pitch and pyrocarbon dual-matrix

Two graphitized C/C composites with a density of 1.83 g/cm3 were fabricated by CVI, high-pressure liquid-phase carbonization, and graphitization process. After graphitization at 2500 and 3000 °C, the graphitization degree of carbon phases in both composites shows a trend of pitch carbon > pyrocarbon > PAN carbon fiber. With increasing graphitization temperature, the orderly arrangement and growth of the graphite microcrystals cause the improved graphitization degree of the composites. Therefore, the thermal conductivity increases from 68.78 to 126.09 W/(m·K) in the transverse direction and 31.09 to 75.60 W/(m·K) in the thickness direction. Meanwhile, the increase in micro-cracks and pores formed at the interface induced the declined flexural and compressive strength, with the corresponding strength reducing to 104.17 and 113.92 MPa, respectively. During the first 22 s of ablation, the improved graphitization degree and higher thermal conductivity can transfer the heat rapidly, resulting in a lower surface temperature and a higher backside temperature. After 30 s ablation, the mass ablation rate decreases from 0.0030 to 0.0018 g/s, while the linear ablation rate increases from 0.0040 to 0.0093 mm/s due to the increased open porosity and defects under higher graphitization temperature.