Design And Implementation Of Precise Molding For Large And Thin C_f/SiC Composites Components

Thin Cf/SiC composite component were widely used in the thermal protection system of aerospace vehicle. In order to improve the safety and reliability of the thermal protection system, also reduce the difficulty of connection, installation and maintenance in the system, near net forming and preparation of the large size and thin Cf/SiC composite component is essential. In this dissertation, the manufacturing of thin Cf/SiC composite component via PIP included two steps:the precision molding of preforms; the densification of the composites component. Firstly, the precision forming process of the preforms was explored, as well as the auxiliary molding glues were designed and chosen. Secondly, the parameters of molding process about different glues were investigated. Also the forming effect, microstructures and properties of Cf/SiC composite component with different glues were analyzed. The mechanism of the dimensional change in densification process was discussed. Finally, according to the results of the experiment, the large size and thin Cf/SiC composites component was successfully fabricated.Through the comparative analysis of vacuum bag molding, compression molding and RTM, it was obvious that the maximum pressure of the vacuum bag molding process was limited and the best quality of preforms were obtained by the compression molding process. According to the molding process, metal mold was designed and produced. The aim of the mould design is to reduce the impacts of molding pressure and improve the molding precision and repeatability.Based on the practical application of the thin component, the PCS/DVB and phenolic/ethanol were chosen as auxiliary molding glues. The viscosity, differential thermal analysis and thermal gravimetric analysis of different glues were investigated. According to the results, the parameters of molding process were obtained and2D preforms of carbon fiber were successfully shaped.The forming effect of2D preforms with different auxiliary molding glues was studied. PCS/DVB glues cross-linked and changed into white hard block solids. The binding force was weak between the solids and fiber bundles. Good morphology and size accuracy of2D preforms were obtained with25wt%and50wt%phenol/ethanol glues. The glues cross-linked to solids which is continuous and has the strong binding with carbon fiber upon pyrolysis. The phenomenon of resin starvation occurred on the surface of specimen when10wt%phenol/ethanol glues was used.The changes of samples’thickness in densification process with different auxiliary molding glues were investigated. It was evident that the dimensional change of the sample associated with weight gain rate. Excessive weight gain rate can cause the fluctuation of size. During densification treatment, the sample appeared warpage with10wt%phenol/ethanol glues. The warpage was mainly caused by excess SiC matrix content of top surfaces. After PIP process, the specimens exhibited better dimensional stability with25wt%and50wt%phenol/ethanol glues. But when fiber volume fraction of the sample was higher (over60%), the binding force was relatively lower with25wt%phenol glues, leading to delamination on12-layers specimen.50wt%solution of phenolic/glue cross-linked to product solids of which can fabricate carbon matrix by the decomposition at elevated temperature. Carbon matrix and fiber could adhesion together. This structure played a very good role in stereotypes. Duting densification treatment, the thickness of sample was changed greatly with PCS/DVB glues. When the glues cross-linked to solids, than decomposed to silicon carbide upon pyrolysis, it led to shrinkage and fracture. So large voids and weak combination were found in preforms.The mechanical properties of samples with different auxiliary molding glue were investigated. The results show that the tensile strength and flexural strength of components were235.7MP and392.4MPa respectively, with25wt%phenol/ethanol glue. Carbon matrix could protect carbon fiber from high temperature and improve the interface bonding between fiber and silicon carbide.According to the findings of precision molding, preliminary study was carried out about the large size and thin component (450mm X200mm X3mm). The results show that by designing the mold, shaping the preform and densification, high accurate dimensional of Cf/SiC composites heat panel can be obtained.