Study On The Graphitization Degree And Structure Of Carbon/Carbon Composites

This thesis reports experimental investigations and theoretical analyses into the micro-distributions of graphitization degree and their effecting factors in C/C composites, the relationships between graphitization degree and such properties as electrical resistivity, thermal conductivity, mechanical property, and friction and wear property of C/C composites for the purpose of manufacturing excellent aircraft brake materials.The graphitization degree of C/C composites was characterized by XRD, Raman spectroscopy and TEM respectively, based on which, the quantitative relationship between XRD parameter g and Raman parameter R-1 has been founded for the first time and successfully used in local analyses of the graphitization degree.The interfacial abnormal graphitization in PAN-based carbon fiber/SL texture pyrolitic carbon matrix composite was found for the first time, of which a forming mechanism was put forward that the interfacial effect of graphitization results from the hot stress induced by the difference of hot expansion between carbon fiber and pyrolitic carbon during graphitization.The quantitative relationships between graphitization degree g and electrical resistivity p ( p=ag+b, where a and b are constants) , graphitization degree g and thermal conductivity λ (λ= λ0+Ae(g/t) where λ0, A and t are constants) of C/C composites were founded for the first time, and the effecting factors of the constants such as pretreatment of carbon fiber, bulk density and orientation of the composite were investigated.The effect of graphitization degree on mechanical property was investigated. It was put forward that the type:; of failure mechanism are different for C/C composites with different graphitization degree-bridged fiber and fiber pull-out for the lower but fiber fracture for the higher.It was found by Raman micro-speclroscopy for the first time that the friction surface has a tendency towards identical graphitization degree. A mechanism was put forward to explain such micro-structural transformation that during braking, the graphite crystals of higher graphitized components on the fiction surface turn gradually direction with their c-axis vertical to the friction surface, flake and cover whole of the friction surface mechanically whereas the lower graphitized components run off as debris.The graphitizablities of several different types of foreign aircraft brake C/C composites were analyzed, and the graphitization treatment temperatures of the compositesand the carbon fibers were determined respectively.It was put forward for the first time that the essential factor determining the properties such as electrical resistivity, thermal conductivity, friction and wear property is not the graphitization degree but the graphitizability of C/C composites. High graphitizability is essential to excellent aircraft brake materials which should consist of PAN-based carbon fiber and RL texture pyrolitic carbon wholly or mainly.