Degradation Mechanisms And Life Prediction Of 3D C/SiC Composite In High Temperature Environments Including Oxidizing Gas And Stress

For the applications in high temperature aero-engine environments, the stressed oxidation of 3D C/SiC composite was investigated in simulating environments following four steps. Firstly, an experimental system simulating high temperature aero-engine environments was set up. Secondly, the degradation process, modes and mechanisms of 3D C/SiC composite were investigated by the change of length and electric resistance recorded during the stressed oxidation test, by the load-displacement curves recorded during tensile and bend test, and by the microstructural observation. Thirdly, the degradation model of stressed oxidation was suggested. Based on the model, the life prediction was achieved finally.An experimental testing system simulating environments of aero-engines was developed by a multiple-step approximated method on basis of similarity theory to simulate the hot section environments of aero-engines. The system included two subsystems: equivalent experiment simulation subsystem and wind tunnel experiment simulation subsystem. Using the simulating system, the materials were tested in the environments with various atmospheres, gas velocity, loads and temperatures or thermal shock parameters. The partial pressure of oxygen, water vapor and molten salt vapor were offered and controlled. Creep, low frequency fatigue and the interaction of them were conducted by a hydraulic servo frame. The evolution of material properties such as length and electric resistance were recorded during the testing. The degradation process of C/SiC composite in high temperature aero-engine environments was recurred effectively by the system.The degradation process of 3D C/SiC was studied in the equivalent experiment simulation subsystem, the degradation mechanisms and the effects of atmospheres and loads were discussed. The continuity and periodicity of the degradation process because of the capability of the composite in remembering the damage were found. The degradation mode of divisional load-bearing and divisional damage is suggested for the first time. The divisional area depended not on the kinds of load but on the max value of load history. Oxygen and water vapor were the major factors to induce the degradation of 3D C/SiC, Sodium sulfate vapor accelerated the degradation by facilitating the oxidation of SiC which resulted in the increase of interface strength. The interface strength affected not only the strength and the toughness of the composite, but also the degradation process. When the interface strength was higher than the appropriate value, the strength of 3D C/SiC will be improved because of the slight degradation of interface. The effect of kinds of load on the degradation speed of 3D C/SiC is carried out by changing the number and distribute of cracks.The degradation process of 3D C/SiC was studied in the wind tunnel, the degradation mechanisms and the effect of temperature, interlayer thickness, structures of fiber preform and kinds of fiber were discussed. It was found that the stressed oxidation mechanisms change with the stress. The oxidation was controlled by the reaction of C/O₂ when the normalization peak strength was higher than 0.4; the oxidation controlled by the gas diffusion through cracks when the normalization peak strength was between 0.25 and 0.4; the oxidation controlled by the gas diffusion through the defects which came from the fabrication and smaller than the cracks when the normalization peak strength less than 0.25. The degradation speed of 3D C/SiC increased with the increasing braiding angle of fiber preforms and decreasing with the increasing oxidation resistance of fibers. The effect of gas velocity on the degradation of 3D C/SiC depended on the oxidation mechanism of the composite. The degradation acceleration of gas velocity was violent when the oxidation was controlled by gas diffusion. The effect of gas flowing velocity was obvious when the oxidation was controlled by the reaction of C/O₂.The degradation process of 3D C/SiC was studied in thermal shock environment, the degradation mechanisms and the effect of load were discussed. The damage saturation occurred after the composite with less interlayer thickness shocked between 600℃and 1200℃for 60 times. The major factor resulting in degradation of the composite was thermal shock, the degradation was accelerated by the oxidation. The oxidation of the composite was accelerated by the load, however, the thermal shock resistance of the composite was improved by the degradation. The acceleration of thermal shock on the degradation of the composite with less interlayer thickness depended on the interface strength.An uniform stressed oxidation degradation model was established on the basis of the similar degradation mode of 3D C/SiC composite in equivalent simulation environments, wind tunnel simulation environments and thermal shock simulation environments. The life prediction equation for equivalent environments was deduced based on the uniform model, in which the parameters included temperature, atmosphere pressure, partial pressure of oxygen, kinds of load, normalization peak strength, properties of fiber, braiding angle of fiber preform and thickness of sample, et al.. The life prediction of wind tunnel environments was also established on the basis of the uniform model, in which the parameters included gas flowing velocity, gas components, partial pressure of oxygen, load, properties of fiber, braiding angle of fiber preform and thickness of sample, et al.. The effect of temperature, load, atmosphere pressure and partial pressure of oxygen were also introduced to the equation by the width of crack. It was proved by the experimental results that the prediction accuracy of the proposed equation was good in the researched environments.