Effect of Atmosphere and Interlayer Thickness on the Mechanical Properties of 3D C/PyC/SiC Composites under Cyclic Stressed and Unstressed Conditions

The effect of atmospheres and interlayer thickness on the mechanical properties of 3D C/PyC/SiC composites was investigated under cycle stressed and unstressed conditions containing oxidizing gases such as water vapor, oxygen and sodium sulfate vapor in a wide temperature range. The damage mechanisms of the composite in various gaseous atmospheres were discussed by weight and strength change. The controlling factors conducing mechanical properties degradation under unstressed conditions were oxygen, water vapor and sodium sulfate vapor were discussed. Oxygen was the dominant factor below 1000°C, water vapor was between 1000°C and 1400°C and sodium sulfate vapor was above 1400°C. However, oxygen was the only dominant factor under cycle stressed conditions. The strength degradation was more serious under cycle stressed conditions than that under unstressed. The study of PyC interlayer thickness showed that C/PyC/SiC samples with about 70-220nm thick interlayer have better flexural strength at room temperature. While in various atmospheres the optimized interlay thicknesses can be selected by a factor S relative to the weight and strength changes of the samples. And the effect of the interlayer thickness on the properties of 3D C/PyC/SiC at low temperatures (<1000°C) is more obvious than that at high temperatures (>1000°C), which is thought due to different microstructure evaluation.