Research On Oxidation Technology Of The Carbon/carbon Composites Technique Modified By Borate

With high temperature mechanical properties and excellent thermalproperties, carbon/carbon (C/C) composites have been widely used as hightemperature structural materials in aerospace and aircraft applications. However,C/C composites will oxidize above450℃in an oxygen containing atmosphere,the properties of the composites are affected seriously which limits theirapplications at high temperature condition. Coating technology is expected to bean effective method for protecting C/C composites from oxidation only at hightemperature. Matrix modification technique is also an effective method toimprove the low temperature oxidation resistance. So the application of C/Ccomposites in the whole temperature range can be expanded obviously. In thisresearch, C/C composites were modified with a borate sol precursor and boroncompound (ZrB2, Boron silicon compounds) by a hydrothermal or solvothermalmethod in order to improve the oxidation resistance property at low temperature.The phases, microstructures and anti-oxidation property of the as-modified C/Ccomposites were characterized by X-ray diffraction (XRD), X-ray photoelectronspectroscopy (XPS), scanning electron microscopy (SEM), energy dispersivespectroscopy (EDS) analyses and isothermal oxidation test. The oxidation kineticof the modified C/C composites was particularly investigated. The main contentsand results are as follows:In order to modify the C/C composites, ZrB2micro-particles were addedinto the borate sol as oxidation inhibitor. The influences of solvothermalmodification temperatue and time on the microstructures and oxidationresistances of the as-modified C/C matrix were mainly investigated. Theoxidation kinetic of the as-modified C/C composites was also studied. Resultsshow that the oxidation resistance of C/C composites will be improved with the The carbon/carbon composite modified by180℃exhibits the bestanti-oxidation ability, which can effectively protect carbon/carbon compositefrom oxidation in air at600℃for6h with a weight loss of26.4mg·cm-2.Theoxidation resistance of C/C composites tends to be increased with thesolvothermal time from24h to72h. Longer solvothermal treatment time isfound to have more ZrB2both on the surface and inside the matrix, resulting inbetter anti-oxidation property. The carbon/carbon composites modified at180℃for60h exhibits the best anti-oxidation property with a mass loss of11.3%afteroxidation in air at800℃for20h. Through the oxidation kinetic analysis of theas-modified C/C composites, the oxidation activation energy is calculated to be161.3kJ/mol below700℃, the oxidation rate is controlled by both the diffusionrate of O2, CO and CO2in the ZrB2/B2O3layer and the oxidation rate of ZrB2;above700℃, the oxidation activation energy is57.7kJ/mol, while the oxidationrate is maily controlled by the volatilization of B2O3and the oxygen diffusionalong the defects of the C/Ccomposites.In order to modify the C/C composites, boron silicon compounds wereadded into the borate sol. Results show that the C/C composits are covered by aglass coating. The oxidation resistance of the C/C composites is improved byincreasing the boron silicon compounds content. When the boron siliconcompounds content is10wt%, the mass loss is16.18mg/cm3after oxidation at600℃for10h.Suspension mixtures prepared by mixing ZrB2and boric acid solution wereused as precursors for the hydrothermal treatment of C/C composites. Theinfluence of ZrB2contentwas mainly studied. Results show that the surface of theC/C composites is covered by a coating composed of B2O3and ZrB2aftermodified by the hydrothermal treatment. The oxidation resistance of the C/Ccomposites is improved along with the content of ZrB2. The mass loss is77.875mg/cm3after oxidation at600℃for6h while the ZrB2content is10wt%.