Tailoring Properties And Anti-oxidation Coatings For Carbon-based Composites

High temperature carbon-based composite?carbon fiber reinforced carbon matrix composite:C/C?,as a kind of high performance structural materials,has great application potential in aerospace and civil high-tech technology.However,the brittle fracture of the carbon matrix composite occurs when the interface between the fiber and the matrix is too strong.At the same time,due to the disadvantage of easy oxidation,the long-term use of carbon matrix composites in high temperature oxidation environment is seriously restricted.In order to improve the mechanical properties and oxidation resistance of carbon matrix composites,C/C composites were prepared in a simple and economical way.Through the heat treatment of carbon fiber,the bonding strength of the fiber matrix interface was modified,and the mechanical properties were regulated.Meanwhile,the ultra-high temperature ceramic coating was prepared and its oxidation resistance was evaluated.The main results are summarized as follows:?1?Carbon fiber reinforced carbon matrix composites?C/C?were prepared by Precursor Impregnation and Pyrolysis?PIP?process.The carbon fibers treated at different temperatures?RT-1500??were used as reinforcements,and phenolic resin was used as carbon precursor.Based on the microstructure observation and analyses,it was found that the size and distribution of pores in C/C?after first pyrolysisi?could be modified by fiber heat treatment.Mechanical properties of C/C composites were improved with the increase of heat treatment temperature.Compared with composite reinforced with untreated fibers,the flexural strength and the interlaminar shear strength of 1200°C heat treated fiber reinforced composite increased by 158%and 121%,respectively.Fracture mode of C/C was changed from brittle to pseudoplastic.Oxidation resistance of 600°C heat-treated fiber reinforced composite was improved,while that of 1200°C and 1500°C heat-treated fiber reinforced composites decreased.The main reason why mechanical property was changed was that heat treatment reduced the surface activity of fibers,so C/C composite had a suitable fiber-matrix interface.However,the decrease of interfacial bonding was not favorable to the improvement of oxidation resistance.?2?ZrB₂-SiC coating was prepared on the 1200?heat-treated carbon fiber reinforced carbon matrix composite by brushing method.It was found that there were many cracks in the coating.The reason for the cracks was the thermal stress caused by the mismatch of thermal expansion coefficient between C/C and coating.In order to alleviate the thermal stress,SiC transition layer was prepared by two methods:slurry brushing and pack cementation.In order to strengthen the combination of SiC transition coating and C/C,C/C was pre oxidized.In order to strengthen the combination of SiC transition layer with the external coating,SiC nanofibers were prepared on the surface of SiC transition layer.The obtained ZrB₂-SiC coating was uniform and dense,forming a molten SiO₂ protective film with good fluidity after oxidation after exposing to high temperature oxidative environment.The SiO₂ film inhibited the further diffusion of oxygen into the interior of the material,the ZrSiO₄ was also formed,which embedded in the protective film and improved the stability.After the introduction of SiC transition layer,the cracks in the external ultra-high temperature ceramic coating were obviously reduced,thus the oxidation resistance of the coating was improved.Compared with the brushing method,the pack cementation method was simpler to prepare SiC transition layer,with less damage to C/C substrate,and the anti-oxidation performance of the C/C compsite was improved more significantly.Additionally,due to the introduction of SiC nanofibers,the binding force between the ultra high temperature ceramic coating and the transition layer was increased.SiC nanofibers not only alleviated the thermal stress,but also strengthened and toughened the coating,which further improved the oxidation resistance of the coating.It was also found that the oxidation resistance of the coating was improved by increasing the thickness of the coating.The mass increased by 0.24%after oxidation at 1300?for 60 minutes.