| Chemical vapor infiltration (CVI) is the first candidate for fabricating carbon/carbon(C/C) composites used as aircraft disk brakes where high performance is needed. Among the various kinds of CVI techniques, isothermal CVI remains the most prevalent process for mass production. Though it has the advantage of densifying a large number of complex preforms simultaneously, its main drawback is the very long processing time (usually longer than 800hr), which leads to very low densification efficiency and very high production cost. Therefore, it is essential to develop a rapid CVI process which can be used to scale up in production to make it competitive in technology and economy. The present research work aims to satisfy the foregoing need, and the main contribution of this dissertation is as follows:(1) By employing directional flow thermal gradient CVI as densification or primary densification technique, C/C composites used as aircraft disk brakes with high performance and low cost were manufactured. It was turned out that the self-fabricated directional flow thermal gradient CVI furnace features rapid densification, high effective precursor utilization and good adaptability of scaling up in production.(2) The effects of infiltration conditions (including temperature, pressure, flow rate, temperature gradient and the type of carrier gas) on the densification behavior and microstructure of pyrocarbon (PyC) have been systematically investigated for the first time and the optimum process parameters for densification and rough laminar structure pyrocarbon have been obtained. In addition, mechanisms involving densification and formation of pyrocarbon were discussed.(3) The accurate definition of the gas phase residence time has been given. Furthermore, the present author ascribes the factors which have decisive influences on the densification of preforms and the microstructure of pyrocarbon to the temperature and the gas phase residence time for the first time, which is of great significance to the production.(4) The effects of microstructure of pyrocarbon and high temperature heat treatment on the graphitization degree, the apparentmicrocrystalline size, the hardness and the heat conductivity were systematically investigated, and the real meaning of the graphitization degree measured by the XRD method, together with the limitation of the present XRD method was revealed.(5) The effects of heat treatment temperature of preforms and the final C/C composites obtained, the architecture of the preforms, the microstructure of pyrocarbon and the auxiliary densification by liquid impregnation on the mechanical properties of C/C composites were systematically investigated, and the factors which have strong effects on the mechanical properties of C/C composites were revealed, which offered a substantial basis for design of mechanical properties of C/C composites used as aircraft disk brakes.(6) The effects of bulk density, the final heat treatment temperature, the architecture of the preforms, the braking pressure and the braking energy on the friction and wear properties of C/C composites with rough laminar carbon matrix and smooth laminar carbon matrix were systematically studied, and the mechanism dealing with the friction and wear behavior of CVI-derived C/C composites was well established. It was definitely pointed out that the great differences between the friction and wear properties of RL structure materials and SL structure materials were attributed to the differences in regard to the formation of the friction film on the friction surface which is closely related to the microstructure of pyrocarbon and its mechanical nature. Based on the adequate experimental results, The present author suggested that adhesive wear should be the primary wear mechanism of C/C composites with excellent wear resistance, while other wear mechanisms, especial for grit wear mechanism, should be inhibited.(7) The optimum fabricatin...
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