The Research On Oxidation Properties Of Carbon Fibers And Carbon Fiber Reinforced Carbon Composites

Carbon fibers (CF) and carbon fiber reinforced carbon (C/C)composites have much good properties, such as high specific modulus, high specific strength, corrosion resistance, tiredness resistance, abrasion resistance, light, etc. It is widely used in space flight vehicles, aviation, machinery, chemical engineering, biota stores, etc. At certain temperature, CF and C/C composites will be quicken oxygen in oxidation atmosphere, and the rate of the reaction will step up as the temperature get higher, which lead to the strength of the carbon materials degraded and properties weaken. So the oxidation inhibition of CF and C/C composites is always a focus for many countries' researchers.The three kinds of PAN-based carbon fibers were used as raw materials. Two ways of oxidation inhibition treatment for carbon fibers were boric acid soaking and high-temperature heat treatment with adding boric acid. The oxidation behaviors of untreated CF by boric and treated CF have been investigated. The relations of the oxidation weigh loss rate and graphite crystallite parameter morphology and the construction, composition of CF were investigated by XRD, SEM, LAMAN, XPS. The oxidation inhibition mechanism of CF was clarified. Two kinds of carbon fiber felts were treated separately by high-temperature heat treatment and high-temperature heat treatment with adding boric acid, on subsequence the C/C composites were fabricated with chemical vapor deposition. The influence of the treatment temperature of carbon fiber felt and boric acid on the oxidation behavior of C/C composites have been investigated. The oxidation process and mechanism of C/C composites were clarified by SEM photographs of samples at different oxidation time.The results of the research works show that oxidation behavior of PAN-based CF is related with its morphology and graphite crystallite parameter. After high-temperature heat treatment, crystal dimension of CF get larger, graphitization degree increases, channels, grooves and holes of CF surface decrease, its oxidationinhibition increases.Oxidation inhibition of CF is promoted after CF being soaked by boric acid. H3BO3 dehydrates and then generates melting B2O3, which shelters active sites, fills the defects, obstructs oxidation of CF. High temperature heat treatment with adding boron acid is a better way for oxidation inhibition of CF. Element B can decrease the activation energy for transforming the amorphrous carbon into graphite, promote the graphitization of CF, increase its oxidation inhibition; BN grown by in-situ reacts withoxygen and generates B2O3 which provides oxidation protection at some temperature. Oxidation inhibition of C/C composites increases as that of the carbon fiber felt increases.