3D-C/SiC复合材料试样在空气介质中600℃、900℃和1300℃热曝露不同时间后,采用三点弯曲法测试了以室温弯曲弹性模量表征的损伤变化规律,并进行了SEM和EDS分析.结果表明:3D-C/SiC在热曝露15 h后,损伤变化可分为急剧上升(阶段Ⅰ)和平稳上升(阶段Ⅱ)两个阶段.阶段Ⅰ归因于炭纤维和炭层界面在空气中的直接氧化,阶段Ⅱ由复合材料内部氧的扩散所致.在复合材料制备过程的冷却阶段,因基体和炭纤维热膨胀系数不同所产生的基体微裂纹提供了氧化反应的表面与氧扩散的途径.在同一热曝露时间下,损伤随温度的上升而减少的原由可能是由于高温下裂纹收缩导致氧化表面减少,并降低氧向复合材料内扩散所致.
3D-C/SiC composites, exposed in air at 600, 900, and 1 300 ℃ for 0 to 15 h, were investigated by three point bend tests at room temperature, SEM, and energy dispersive spectroscopy. The results show that the damage curves, expressed as a relative change of elastic modulus, of the composites for a 15h exposure, could be divided into a sharply increasing stage (stage Ⅰ) and a steady increasing stage ( stage Ⅱ). Stage Ⅰ may be caused by a direct oxidation of the carbon fibers and interface carbon layers by the oxygen in air, and stage Ⅱ may be caused by a diffuse controlled oxidation of the inner part of the composites. The matrix micro-cracks, induced by a difference of coefficients of thermal expansion between matrix and carbon fibers in the cooling process after composite preparation act as oxygen diffuse paths and are where the oxidation takes place. The fact that the damage decreases with temperature for the same exposure time may be caused by the crack shrinking at high temperature, which decreases the oxidizable surface area and inhibits the diffusion of oxygen into the composites.
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