Prefabricated Structure Tablet Cvi Process And Material Properties

Carbon fiber-reinforced carbon matrix composites (Carbon/carbon or C/C composites) have been widely used in the applications of aerospace and military fields as new materials associating with high performance, such as low density, high strength in high temperature and excellent friction and wear properties. The structure of the carbon fiber preform has an important influence on the gas transfer during the chemical vapor infiltration (CVI) process and the properties of C/C composites. It is of great significance for the research of C/C composites to find effective CVI method with simple process, short cycle, low cost and improved comprehensive properties of materials by controlling the preform structure.C/C composites plates with needle-punched felt as preforms were prepared in self-designed CVI furnace by methods of CVI. The effect of the preform structure on the densification process and the microstructure of matrix were studied. Meanwhile, mechanical properties, graphitization degree, coefficient of thermal conductivity and resistivity of materials were tested and the effect of the preform structure on the mechanical properties and physical properties of C/C composites were discussed in detail. The following results were obtained.1. The shape, size and distribution of pores in the preform influence the gas transfer and the densification process of composites. The densification rate and the final density of composites decrease with the increase of carbon fiber volume fraction. The densification rate and density of full mat are higher than integral needle-punched felt whose ratio of cloth to mat has little effect on them. The densification rate and density increase after joining SiO2 whisker and the densification rate decreases after joining CaSiO3 whisker whose final density is as the same as before. The distribution of circle thickness of pyrolytic carbon in mat of all materials is symmetrical smaller in the middle and bigger in the laterals. The thickness of pyrolytic carbon in the middle of materials decreases with the increase of fiber content or ratio of cloth to mat. 2. The microstructure of pyrolytic carbon of C/C composites is smooth laminar structure observed by the polarized optical micrographs which suggests that there is no direct relationship between matrix microstructure and preform structure. The optical activity of matrix slightly increases after joining whisker and joining SiO2 whisker may induce pyrolytic carbon with cone growth.3. The graphitization degree of composites is affected by fiber volume fraction and pyrolytic carbon content at the same time. Fibers in cloth have more significant contribution on the stress-graphitization and graphitization degree can be improved after joining whisker. Graphitization degree is the main effect factor on the thermal and electric conductivity properties, and the preform structure only have an auxiliary effect on them.4. The effect of preform structure on the flexural and shear mechanical properties of composites have the same rule. The high and low volume fraction of fiber can decrease the strength of composites and the strength of materials increase with the increase of ratio of cloth to mat. The strength decreases after joining SiO2 whisker but increase a little after joining CaSiO3 whisker. Their compressive mechanical strength decreases with the increase of fiber volume fraction. The vertical compressive strength decreases by 22% compared with full mat composites when the ratio of cloth to mat of integral felt is 1:1 and then increases with the increase of the ratio of cloth to mat, but the parallel compressive strength decreases with the increase of ratio of cloth to mat. The compressive strength increases after joining whisker, and the increase extent is higher after joining SiO2 whisker.5. The fracture model of the flexural, shear and vertical compressive is pseudo-plasticity except integral needle-punched felt of adding SiO2 whisker and full mat felts and composites joined SiO2 whisker have brittle fracture. Fracture model of parallel compressive of all the felts is brittle.