Study On The Effects Of Carbon Fiber Preform And Densifying Means On CF/Matrix Interface And Performances Of Carbon/carbon Composites

On the basis of analysis of the structure and performances of the up-to-date carbon/carbon (C/C) composites from abroad for aircraft, needle-punched felts were designed and prepared. The effects of structure of needle-punched felts on the densification rate, structure of pyrocarbon and the properties of as-received C/C composites were performed and a novel way to prepare high quality C/C composites rapidly was explored. The interfacial strength of carbon fiber/carbon matrix was determined by single-fiber-pushing-out method, the micro-structure of carbon matrix was investigated by polarized light microscopy (PLM) and High Resolution Transmission electron microscopy (HRTEM) and the orientation angle (OA) of different carbon was determined.According to the principle of thermo-gradient chemical vapor deposition (CVD), a conductive layer was creatively introduced when preparing needle-punched felts to set up a temperature gradient in the perform, which resulted in much high rate of CVD and simplified the process and obtained C/C composites with higher performances. This is a very effective way to lower cost and improve the performances of C/C composites and is a breakthrough to the processes of C/C composites.A special "buffer" layer structure was found in rough laminar (RL) pyrocarbon by HRTEM for the first time, which is a several-nanometer-thick high-textured layer adjacent to the fiber, then a lower- or mid-textured one and transmit to the bulk high-textured matrix at last. The stack of carbon layers of RL pyrocarbon is much more ordered and thick than that of smooth laminar(SL) pyrocarbon. There are shallow micro-cracks in RL pyrocarbon. The OA Can change in the range of 20?-90?for different type of RL or SL pyrocarbon. The basal carbon layer of pitch-based matrix is also highly-ordered and the OA is small. There is also ordered-layers in resin-based matrix, but most are disordered and the OA is about 180?. The OA of the parts near the interface between carbon fiber and matrix, which demonstrates the stress-induced graphitization.The effects of micro-structure of carbon matrix, type of carbon fibers, pre-heat-treatment temperature of preform and impregnation on the interfacial strength of carbon fiber/pyrocarbon were investigated systematically. RL pyrocarbon and pitch-based matrix adhere much stronger than resin-based matrix to carbon fiber and the interfacial strength of carbon fiber/SL matrix is the lowest. C/C composites with higher interfacial strength can be obtained by using needle-punched felts from oxidized PAN fiber. The pre-heat-treatment temperature of 2100?or 2700?is suitable for preparing C/C composites with higher interfacial strength. Impregnation and carbonization of pitch and resin has hardly any effect on the interface strength of carbon fiber/Pyrocarbon.The influence of type of fibers of non-woven cloth and short-cut web, mass ratio of non-woven cloth to short-cut web, thickness of unit (a layer of non-woven cloth and web is named as a unit)and introducing conductive layer on the preparation, mechanical properties, thermal conductivity, friction and wear performance of C/C composites were carried out. There exists a suitable ratio of cloth to web to obtain high flexural properties, which can be successfully explained by the "hydrid cooperation effect".That frictional performance of RL pyrocarbon is much better than that of SL pyrocarbon can attribute to the effects of the interfacial strength of carbon fiber/pyrocarbon, fracture strength of carbon fiber and the hardness of pyrocarbon on producing debris and the ability of forming frictional film. With the stress of pressure and heat, excess graphitization can be introduced in the frictional face.