Preparation, microstructure, mechanical and thermophysical properties of short carbon fibre/SiC multilayer composites by tape casting and pressureless sintering

Silicon carbide multilayer composites containing short carbon fibres (Csf/SiC) were prepared by tape casting and pressureless sintering. The short C fibres were firstly dispersed in solvents with the aid of dispersant and then mixed with SiC slurry to limit fibre breakage. The relative densities, mechanical properties, oxidation behavior of Csf/SiC multilayer composites were evaluated. Thermal expansion, diffusivity and conductivity behavior in three directions were tested. The effect of addition of short C fibres on shrinkage, mechanical and thermophysical behavior was discussed. Triton X100 was found to be the best one for Toho Tenax HTC124 (with water soluble coating) among BYK-163, BYK-410, BYK-2150, BYK-9076, BYK-9077 and Triton X100 dispersants. Although the average fibre length (0.5 to 0.6 mm) after mixing was only one-sixth or -fifth of original length (3 mm), it is still much longer than in other Csf/SiC composites using ball-milling, indicating that mixing the SiC slurry with the fibre-predispersed solution is an effective method for adding fibres with limited breakage. Fibres were homogeneously distributed in the tapes and tended to align fairly well along the tape casting direction. The relative density of the composite containing short C fibres decreased with the fibre amount. The Csf/SiC multilayer composites demonstrated significant anisotropic shrinkage behavior in different directions, while the addition of short C fibres hindered the shrinkage in the plane containing the fibres (X and Y directions) during sintering. Elastic modulus, bending strength and fracture toughness decreased with increased porosity, which implies that bending properties are affected significantly by residual porosity. However, due to residual thermal stress, the fracture toughness of SiC/(Csf/SiC) multilayer composites was higher than that of SiC multilayer, which indicates that the fracture toughness of SiC-based thermal protection system could be improved through designing its architecture. The weight loss during oxidation tests was larger for Csf/SiC multilayer composites than for SiC multilayer and increased with the porosity and the fibre amount. Passive oxidation of SiC was still occurred after addition of short fibres. Specific heat of Csf/SiC multilayer composites slightly increased with fibre amount. No significant different in thermal expansion behavior of SiC, 5 and 10 vol.% Csf/SiC multilayer composites in different directions was found. In X direction before oxidation treatment, the thermal conductivity firstly slightly increased after addition of 5 vol.% short C fibre, and then decreased with further fibre addition. However, in Y and Z directions, the thermal conductivities decreased with the increase of fibre amount before oxidation treatment. After oxidation treatment, the thermal conductivities decreased with the increase of fibre amount regardless of test directions. Since fibre aligned along tape casting direction, Csf/SiC multilayer composites demonstrated highest thermal conductivity in X direction regardless of fibre amount. Moreover, due to the presence of the interface between adjacent layers, Z direction showed lowest thermal conductivity. Because of the oxidation of C fibre (in Csf/SiC multilayer composites) and formation of silica (both in SiC multilayer and Csf/SiC multilayer composites), thermal conductivities of SiC multilayer and Csf/SiC multilayer composites decreased after oxidation treatment.