Research On Flexible Towpreg Carbon Fiber, Preparation And Oxidation Resistance Of C_f/C Composites

Compared with conventional composites, Cf/C composites possess good qualities such as higher specific strength, higher specific modulus, programmabler material structure and better retentive ratio of strength at high temperature. So Cf/C composites have been widely used in the military, aerospace and other fields. After Cf/C composites anti-oxidation coated are successfully developed, Cf/C composites can be reusable under the condition of oxidized ambience. This makes it possible to become an exellent ablative resistant material and also a high temperature thermal structure material, as today's focus on advanced composite research and development. But some problems limited the application of Cf/C composites. At present the main problem is that commonly-used methods for densification and the property of the impregnant are not satisfactory. This essay explored a new rapid way for preparing Cf/C composites with the powder coated carbon fiber tows, and through the multiple densification process of the pitch impregnation-carbonization and naphthalene in-situ impregnation polymerization-carbonization, few densification cycles and material costs were realized.This article has mainly studied the performance of the samples under the influence of the carbon fiber disposed, BPR powder coating, the formation technology and carbonization of the preforms, the densification and the oxidation protective coating and so on. The spreading of carbon fiber and the coating of BPR powder are the very important aspects to study the performance of Cf/C composites. The carbon fiber dispersed by air-comb and BPR powder coated the carbon fiber results to the flexible towpreg carbon fiber tows. These carbon fiber tows may not only carry on the formation in a direct way, but also product a very flexible towpreg suitable for weaving, braiding, and so on. The densification of Cf/C composites has a very significant effect on its performance, so we focus on three densification methods of Cf/C composites.Due to the inertia feature on the surface, the carbon fiber was hard to be fully soaked, nitric acid oxidized treatment scheme was put forward. The surface treatment of carbon fiber influencing the performance of the composites was deeply explored. The surface shapes, the characteristic groups and surface wettability of surface treated carbon fiber were analyzed by SEM, FT-IR and dynamic contact angle as well. Air-comb spread carbon fiber tows and the powder coating process are firstly introduced to prepare the flexible towpreg carbon fiber tows. When the carbon fiber was spread by the air-comb, the scope and the stability of the spread carbon fiber tow was studied under the gas pressure of air-comb and the processing speed of the carbon fiber, thus the difficulty of the spread carbon fiber tow was overcome. If the routine Roller spreading carbon fiber tow was adopted, the effect was not perfect and it also had the immense breakage to carbon fiber. This essay systematically investigated that the scope of the spread carbon fiber tow, the processing speed, the resin powder particle size and the heating temperature have the influence on the effect of the powder coating. Secondly, the research in boron modified phenolic resin powder coating on the spread carbon fiber tows, focused the powder coating influence on the scope of the spread carbon fiber tow, the processing speed, the resin powder particle size and the heating temperature.The formation technology and the performance relations of the preforms were studied by hot pressing with BPR powder coated carbon fiber tows, and simultaneously the mechanical properties of the preforms and the appearance of cross section were researched in details, finally the preforms with high volume fraction of carbon fiber were triumphantly prepared, which put forward a new train of thought in reducing the cost of Cf/C composites. The BPR impregnation experiment model was established on the basic of Darcy's law, the hot pressing of the powder coated carbon fiber tows and the property of BPR. In addition, it was successfully applied in adjusting parameter in the hot pressing process of the preforms. Cf/C composites were formed after the preforms were carbonized with a reasonable carbonization technology.This article analyzed how the densification methods influenced the performance of Cf/C composites. The PIC, IsIPC and BPR-IC were adopted to densify Cf/C composites, and meanwhile the influence from the densification cycle to the density and open porosity of Cf/C composites was detailedly analyzed. The PIC and IsIPC multiple treatments were used to densify Cf/C composites. The impregnation mechanism, polymerization mechanism and the best conditions of in-situ impregnation polymerization were investigated. The properties of Cf/C composites increased at a certain rate after multiple densifications. The degree of graphitization of Cf/C composites with different densifications have been earnestly analyzed by XRD. The thermal conductivity of Cf/C composites with different densifications were discussed on the thermal conductivity mechanism.In order to study the technology of preparation Cf /C composites to the high temperature oxidation properties resistance, the techniques of powder coating and the methods of densification to Cf/C composites had been researched by the weight loss of Cf/C composites. As a result, poor oxidation resistance of Cf/C composites, Cf/C composites must be treated with oxidation resistance before their application. The SiC/MoSi2 compound coating was prepared by pack cementation technology on Cf/C composites, and the preliminary discussion to its boundary performance and the oxidation resistance performance were explored. The coated Cf/C composites were studied on isothermal oxidation at 1500℃, the research on the thermal shock was done at 1500℃on the condition of the room temperature, and the influence on oxidation resistance at high temperature was analyzed.