| C/C-SiC composites possess excellent high temperature microstructure stability, good thermo-mechanical properties, stable and superior tribological performance at elevated temperatures. This allows the application of C/C-SiC composites as high-performance structural materials in various fields, such as advanced tribological systems, thermal protection systems and propulsion systems. However, considering fulfilling the different requirements of functional and structural aspects in industrial applications, study on microstructure, composition distribution and properties of C/C-SiC composites is necessary.Currently, C/C-SiC composites are manufactured by either liquid or gas phase routes or alternate derivatives. In recent years, liquid phase processing such as liquid silicon infiltration (LSI) processing has been gaining in popularity because of its near-net shaping technique and good preformance/cost benefits. The LSI process can be divided into three steps:(i) fabrication of a carbon fiber reinforced plastic (CFRP) green body; (ii) carbonization of CFRP green body by pyrolysis of rein matrix to obtain the C/C preform; (iii) siliconization of carbon in C/C preform via the infiltration of liquid silicon. As the end product of the three-step process, C/C-SiC are carbon fiber reinforced multi-phase matrix composites. The constituent phases of matrix include amorphous carbon, silicon carbide and residual silicon.Thus, in this paper, C/C-SiC composites manufactured by the LSI process would be tailored by thermal treatment of carbon fiber. Thermal treatments of the carbon fibers prior to the manufacture of the CFRP green body reduce the fiber-matrix interface bonding strength (FMB) in the polymer stage, which governs the formation of the microcrack pattern. The main work is summarized as follows:Firstly, the T300 carbon fibers were thermally treated at elevated temperatures. After treatment, the thermal stability and microstructure were analyzed by mass change and XRD measurement. The mechanical properties and the surface composition of carbon fibers were measured as a function of temperature by the single filament tensile test technique and ESCA analysis, respectively. The results indicated that T300 fiber had good thermal stability and high temperature mechanical properties. To some extent, the high temperature treatment could lead to a further graphitization of carbon fiber. The surface composition measured revealed that the oxygen concentrations on the surface of fibers significantly decreased. Such composition would result in a decreased surface activity of carbon fibers.Secondly, the CFRP green body was fabricated by resin transfer molding (RTM) technology. The single fiber push-out test and the short-beam shear test were applied to measure the FMB. Resultes indicated that the thermal treatments of the carbon fibers could reduce the FMB in the CFRP green body. Furthermore, the macro-and micro-tests resulted in a similar phenomenon in regard to interface bonding strength of CFRP, but expressed different values. The low value measured from micro-test could be associated with multiple interlaminar shear failures.Furthermore, in order to understand the formation of cracks and the mechanism of their development during pyrolysis, the thermal gravimetric curve was studied, and the interaction of fiber-matrix interface was measured in the different temperature of pyrolysis process. It was found that the microstructure of C/C preform was strongly associated with the thermal treatment temperature of carbon fiber. For the C/C preform reinforced with the low temperature treated fibers, the fiber-matrix bonding strength is high, resulting in a strong interaction between fiber and matrix. In this case, the main crack types are transverse cracks and micro-delamination cracks. For the C/C preform reinforced with high temperature treated fibers, a weak interaction between fiber and matrix is formed and the main crack type is micro-cracks caused by fiber-matrix debonding. In the case of C/C preform reinforced with 1500? treated fibers, a porous structure was formed, because of the small crack size.Finally, the C/C-SiC composites with different microstructure and consisting phase composition were obtained by the reaction of liquid silicon with carbon matrix, which was derived from C/C preform. It was found that the distribution zone of SiC matrix in C/C-SiC composite is very similar to the crack pattern of C/C preform. The constituting properties and interfacial strengthening mechanism were investigated by macro-scale mechanical test combing with microstructure analysis. The oxidation and ablation resistance were also investigated which is useful to elucidate the high temperature microstructural stability of C/C-SiC composites. It was found that the characterization of microstructural stability and mechanical properties such as interlaminar shear strength, flexural strength and fracture toughness are quite dependent on the distribution and volume fraction of constituting phases. The high mechanical properties and excellent microstructural stability of C/C-SiC composites could be attributed to large volume fraction of SiC matrix, moderate fiber-matrix interface bonding and its homogeneous distribution. |
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