Effect of pyrolytic carbon content on properties of carbon/carbon-polyimide composites

Carbon/carbon-polyimide composites were prepared by chemical vapor infiltration and vacuum-pressure impregnation curing process. Firstly, the carbon felt was deposited with pyrolytic carbon by chemical vapor infiltration method to prepare porous carbon/carbon materials with pyrolytic carbon contents of 7.33%(mass fraction, the same below), 14.55% and 22.00%, respectively. Then, through a specific impregnation curing process, the carbon/carbon-polyimide composites P1, P2 and P3 and the control group P0 without pyrolytic carbon were obtained in turn. Through SEM, linear expansion coefficient test and mechanical properties test, the effects of different pyrolytic carbon contents on the microstructure, average linear expansion coefficient and mechanical properties of carbon/carbon-polyimide composites were investigated. The results show that in the temperature range of 20-300℃, the XY-direction average linear expansion coefficients of P0, P1, P2 and P3 are between (0.67-0.79)×10-6℃-1, and the XY-direction average linear expansion coefficients of four groups are basically the same. The Z-direction average linear expansion coefficients of P1, P2 and P3 decrease by 41.6%, 41.8% and 24.1%, respectively compared with P0. Pyrolytic carbon significantly reduces the Z-direction average linear expansion coefficient of materials. The XY-direction compressive strengths of P0, P1, P2 and P3 are 243.91, 244.73, 216.65 MPa and 210.79 MPa, respectively, and the Z-direction compressive strengths are 269.22, 258.80, 246.68 MPa and 219.20 MPa, respectively. The compressive strength of P1 is equivalent to that of P0. The Z-direction bending strengths of P0, P1, P2 and P3 are 92.77, 77.11, 80.71 MPa and 86.06 MPa, respectively. Different pyrolytic carbon contents have different effects on the mechanical properties of materials. The lower pyrolytic carbon content can basically maintain compressive strength of materials, while the higher pyrolytic carbon content can basically maintain bending strength of materials.