Preparation Of Carbon Fiber Reinforced Silicon Carbide Composites And The Thermal-insulation Materials For Aerodynamic Rudder At Low Temperatures

As an important warhead part of new generation hypersonic missiles, aerodynamic rudder must possess thermal-insulation cover, with good ablation and high temperature resistance, to ensure the aerodynamic shape for shooting straight. In this dissertation, C/SiC composites was prepared by precursor impregnation and pyrolysis process, based on the manufacture of thermal-insulation cover for new generation missiles, and the performance, such as mechanical properties, thermal-physical properties, oxidation and ablation properties etc., was tested. Finally, integral formation of complex shaped fabric was realized, and full sized thermal-insulation cover was fabricated.The pyrolysis characteristics of precursor PCS were investigated. When pyrolyzed in inert atmosphere, PCS will transform into ceramic at 750℃, and then crystallize between 880～1050℃. With the temperature rising, the stability of pyrolyzate is enhanced. The pyrolyzate of PCS pyrolyzed at 900℃keeps stabilization at higher temperature as 1200℃, with mass residue just about 0.24%, whereas the pyrolyzate at 700℃is farther decomposed, with mass residue about 4.02%.The effect of pyrolysis temperature on the fiber-matrix interface, microstructure and properties of C/SiC composites was studied. With the pyrolysis temperature rising, the interfacial chemical reaction and element diffusion are aggravated, therefore the interfacial bonding is enhanced, restraining the interface debonding and fiber pulling out, and that degrade the mechanical properties of C/SiC composites. The flexural strength and fracture toughness of C/SiC composites prepared at 700℃is 256.58MPa and 5.63MPa·m^1/2 respectively, with lots of fiber pulling out, while the composite prepared at 900℃exhibits smooth fracture surface with flexural strength of 54.58MPa and fracture toughness 2.25MPa·m^1/2The effect of fiber type on the interface and mechanical properties of C/SiC composites was investigated. Compared with JC-1# carbon fiber, the JC-2# carbon fiber has inert surface and less surface defect, thus the C/SiC composites reinforced by JC-2# carbon fiber with moderate interface has better mechanical properties, with flexural strength 249.84MPa and fracture toughness 9.08MPa·m^1/2.The heat treatment of carbon fiber was studied. The properties of carbon fiber decrease diversely during heat treatment. As the treatment temperature reaches 1200℃, the strength residue of fiber bundles reduces to 79.59%. The 1400℃heat treatment, devoted to lessen the fiber defects and enhance the high-temperature structural stability of carbon fiber, reduces the degradation of fiber strength during heat treatment. And thus the mechanical properties of C/SiC composites reinforced by carbon fiber treated at 1400℃are improved largely, with flexural strength 581.04MPa, and fracture toughness 22.43MPa·m^1/2.The first cycle of the PIP process is the most important for that it is the critical process for interface formation, and influences the interface and microstructure of C/SiC composites remarkably. PCS will expand firstly and then shrink during pyrolysis, and the bulk shrinking will lead to stronger interfacial bonding. After the optimization of PCS pyrolysis process, the high-performance C/SiC composites, pyrolyzed at 700℃in the first cycle and 900℃in the others, was obtained, with flexural strength 600.28MPa and fracture toughness 24.52 MPa·m^1/2.The optimum process of C/SiC composites was determined. Firstly, the fiber is treated in vacuum at 1400℃, and then glued, weaved and unglued. Thus the 1400℃heat treatment is introduced into the process of low-temperature preparation of C/SiC composites, weakening the fiber-matrix interface bonding. Secondly, the sample is prolyzed at 700℃in the first cycle of PIP. Finally, the sample is densified at 900℃in the residual PIP cycles. The room-temperature flexural strength and tensile strength of C/SiC composites, prepared by optimum process, is 643.12MPa and 299.83MPa respectively, and 1600℃flexural strength is 411.01MPa. The axial coefficient of thermal expansion is 0.180×10^-6/K (25～800℃), and the radial coefficient of thermal expansion 2.729×10^-6/K (25～800℃), specific heat 0.98J/g·K, coefficient of thermal conductivity 1.26W/m·K.The oxidation and ablation resistance were researched. The results show that the main reason for oxidation of C/SiC composites is matrix cracks and exposed fibers. With the elevation of oxidation temperature (400～1300℃), the mass residue reduces from 99.76% to 81.83%. In the course of 400～800℃, the strength residue of C/SiC composites are all elevated for the seal of matrix cracks and surface holes by SiO₂ film after oxidation. The mass loss rate and recession rate is 0.0158g/s and 0.0279mm/s respectively after oxyacetylene ablation, and the surface temperature is 2005℃. The ablative product is SiO₂ mostly, and presents two different conformation, keatite and tridymite, due to the dissimilar cooling rate. And the recession rate tested by plasma arc is 0.33mm/s, with heat flux density 35000kW/m², enthalpy 10000kJ/kg, and heat flow pressure 2.8MPa.The integral formation of complex shaped thermal-insulation cover was realized through plane weaving and stitching. After the 1:2 thermal-insulation cover was tested by rocket engine, the structure of cover keeps integrated, and safe and credible. Finally, after the optimization of preparation process, the full sized C/SiC composites thermal-insulation cover was fabricated successfully.