| Carbon/carbon composites have been used as new material with high performance, and have been widely used in the applications of Aeronautics and Space, because of its excellent performance. The Chemical Vapor Infiltration (CVI) process, which is used to prepare C/C composites, is one of the hot subjects both domestic and international research. The structure of the fiber carbon perform have important influence to the properties of the C/C composite. On the basis of fully understanding of the influence of the pore structure of fiber carbon preform on the gas transfer during the CVI process and the properties of the C/C composites, it has important significance to select and design the suitable preform in order to improve the densification rate, reduce cost and improve performance according to difference demand.This study adopted total fiber fleece needle felt, aligned fiber bundles/ fiber fleece and finely-Woven 3D C/C materials(The composites which were made by these preform were replaced by A,B and C respectively.) through preparing C/C composites by ways of CVI, the effect of the pore structure of preform on the process of densification and the microstructure of matrix were studied. Meanwhile, flexural properties, shear properties and heat conduction performance were tested to study the effect of the structure of preform on the mechanical properties and thermal conductivity of C/C composite. The geometric model of pore structures and mathematical model of gas transfer of the three-dimensional orthogonal, total fiber fleece needle felt, aligned fiber bundles/fiber fleece needle felt were built. The influence of pore structure of preform to the densification process were studied theoretically, and verified the calculation results through the experimental results.The main contents and conclusions of this work are as follows:(1 )The pore structure and shape of the preform are the main factors that determine the gas transfer and the final density of the composite.The average gain rate per unit time of A was the maximum among them, B took the second place, and C was the minimum; When the density of A and B equal to that of C as the with the deposition continue, The average gain rate per unit time of B was the maximum, C took the second place, and A was the minimum.For the density, C was the maximum,B took the second place, and A was the minimum.(2)The microstructure of pyrolytic carbon has no direct relationship with the pore structure of the preform. The thickness of pyrolytic carbon within A is big than that of B and C. The thickness of pyrolytic carbon within fiber fleece is thicker than that of fiber bundle fiber.(3)The flexural strength and shear strength of composite A was far lower than that of B and C;A displayed brittle failure, B and C displayed good plasticity and toughness.And the stress-strain curve and stress-displacement curve decreased in a step-like manner after reached maximum,and displayed the characteristics of delamination fracture.The pull out of fiber and fiber bundle and interfacial debonding between fiber and matrix are helps to improve the fracture toughness of composites.(4)The thermal conductivity of three materials in the parallel direction had significant difference. The thermal conductivity of C in vertical direction is lower than that of A and B.(5)The physical and mathematical models built in this paper are reasonable.the process of gas transfer is one of the main factors which influence the deposition rate of pyrolytic carbon; The permeability of finely-Woven 3D C/C materials change with the diameter of fiber boudle in the X,Y and Z direction. The permeability of total fiber fleece needle felt is big at the beginning, but its permeability lower than that of the aligned fiber bundles/ fiber fleece needle felt when their density is equal. The final density of the composites can not reached 100%. |
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