Original And Service Properties Of C/sic Composites By Pip Method

Carbon fibre reinforced silicon carbide(C/SiC) composites which show several unique properties, such as low density, high strength, high temperature stability, and so on, were among the most promising materials for high-temperature structural applications, which have expansive prospect in the applications of aerospace propulsion system. The original properties and properties in service environment of C/SiC composites fabricated by precursor infiltration pyrolysis (PIP) have been investigated.The mechanical properties of C/SiC composites at room temperature were investigated. It is found that the the flexual properties mearsured by 4-point-bending test method have less standard deviation, higher modulus. In 3-point-bending test method, the width of the specimens according to GB6569-86 is narrower than that according to ASTM C1341-00, so the flexural strengths mearsured by GB6569-86 have higher standard deviation. The flexural strength(according to GB6569-86) of C/SiC composites fabricated by PIP is 463.00MPa, and the statistical distribution of the strengths was in good agreement with a two-parameter Weibull distribution, characterized byλ=478.18 and the Weibull modulus is 15.52. Tensile strength, modulus and poisson ratio are 324.1MPa, 169.3GPa and 0.32, respectively. The ratio of flexual strength and tensile strength is about 1.5. The compressive strength in X direction is 57.6MPa, while that in Y direction is 108.1MPa.Thermophysical properties of C/SiC composites were systematically researched. The coefficient of thermal expansion (CTE) in Y direction is higher than than in X direction, and in Y direction, the CTE at high temperature is a little higher than that at low temperature. But in X direction, the CTE at high temperature is much higher than that at low temperature. With the increase of temperature, thermal diffusivity (α) decreases gradually, at room temperature,αis 2.102mm²/s, while at 1000℃,αis 1.407mm²/s. Above 1100℃,αis as the equivalent level as that at 1000℃. From room temperature to 1400℃, the specific heat capacities(Cp) are between 0.363J/(g^-1K^-1) and 0.827J/(g^-1K^-1), and the thermal conductivities are between 1.25 J/(g^-1K^-1) and 2.67J/(g^-1K^-1) by calculation.Compared to pure SiC, the oxidation behavior of SiC pyrolyzed from polycarbosilane(PCS) was researched. The results show that the masses of SiC powder and SiC pyrolyzed from PCS at 1200℃are increased after oxidation at 1000℃, whereas the mass of SiC pyrolyzed from PCS at 1800℃is decreased. After oxidated at 1600℃,the crystal degree of SiO₂ is higher.The results of the high temperature oxidation behavior of PIP C/SiC show that oxidation corrasion at 1400℃is the strongest while the degree of oxidation at 1200℃is the same as that at 1600℃. The flexual strength after oxidation is the exponentialfunction of mass loss ratio, the remaining flexural strength can be predicted by massloss ratio. The oxidation mechanism of PIP C/SiC composites is the diffusion controlledmechanism.The effects of space environment on structure and properties of PIP C/SiCcomposites have been investigated. It includes that after 8 hours in the vacuum of6.0×10^-3Pa at 1200℃, mass loss didn't happen, the flexural properties and fracturemode have no obviously change. After 60 days in cold environment at -196℃, there isno significant change in flexural properties and fracture mode. The original flexuralstrength, modulus and interlaminar shear strength are 449.5MPa, 89.8GPa and 73.7MParespectively. After different thermal cycling times from -196℃to 200℃, theseproperties have little change, the fluctuation ranges are 30MPa, 4.7GPa and 10.7MPa,respectively. And compared with original composites, the fracture mode and fibre andmatrix bond have no visible change.The propellent combustion gas characteristic of methylhydrazine and nitrogendioxide and the C/SiC composites thruster failure mode were analysed. It is found thatthe oxidation components in propellent combustion gas are H2O,CO2,NO,O2,O etal, while the temperature and pressure of the gas are up to 1800℃and 3MPa,respectively. The failure mode of the thruster with the wall temperature of 1580℃ismechanical scouring. But the failure mode of the thruster with the wall temperature of1740℃is mechanical scouring together with oxidation.