3D-C/SiC复合材料的损伤机理

DAMAGE MECHANISM OF 3D-C/SIC COMPOSITE

陶瓷基复合材料在热结构中潜在着许多用途 ,但对 3D C/SiC材料高温损伤尚不完全清楚。本工作用T3 0 0碳纤维编织为三维四向编织体 ,编织角 2 2° ,用CVI化学气相渗法在 95 0℃～ 10 0 0℃沉积热解碳界面层、SiC基体。最终得到纤维体积分数约为 40 %、热解碳界面层厚度约 0 .2微米和空隙率为 17%的复合材料 ,表面SiC涂层厚度为 5 0 μm。基体由于热应力和外力会产生许多微裂纹 ,用单向陶瓷基复合材料裂纹计算公式可大致估算出 3D C/SiC的基体开裂应力和裂纹间距。纤维束间的孔隙在蠕变中变形 ,孔隙表面基体易产生微裂纹 ,而且纤维束间的夹角不断改变。蠕变是损伤引起的 ,属于损伤蠕变机理。弯曲、断裂韧度、蠕变及疲劳等试验中 ,纤维束力图沿拉应力方向伸直 ,纤维束间相对滑动并产生损伤是细观主要的损伤机理。室温及疲劳循环应力低、循环周次多的断口粗糙度大 ,纤维拔出较长 ;高温及高应力、循环周次少的断口相对齐平 ,纤维拔出较短。纤维束与基体界面和纤维与基体界面的脱粘和滑动产生损伤中 ,以纤维束与基体之间的磨损产生的损伤为主要的 。

CMCs have the potential applications in high temperature structure, but the damage at high temperature of 3D-C/SiC is not very clearly. The preforms used in present work fabricated with T300 carbon fibers which woven into 3-dimension 4-direction structure with 22° weaving angle. 3D-C/SiC was processed by chemical-vapor infiltration(CVI) of SiC into preforms ,before CVI processing fibers were coated with carbon by CVD. Micro-cracks are produced from thermo-stress or external stress. Crack computing formula in unidirectional brittle matrix composites can be used to coarsely estimate matrix crack stress and crack spacing of 3D-C/SiC. Pores inter fiber yarn tows are deformation in creep. On pores surface more likely generated matrix cracks and angles of inter fiber yarn tows were changed, the creep damage concentrated in cross points of fiber yarn tows. Creep is caused by damage, and it can explain by damage creep mechanism. In the bend testing, fracture toughness, creep and fatigue tests, the fiber bundles attempt to extend in load direction. Main mesoscopic damage mechanism of 3D-C/SiC is sliding process inter fiber yarn tows and damage generated by it. Fracture morphology is more rough in room temperature, low fatigue stress and more fatigue cycles, but the length of fiber pullout is longer. The fracture morphology is less rough in the case of high temperature, high stress and less fatigue cycles and the length of fiber pullout is shorter. Debonding and sliding between fiber tows and matrix or between fiber and matrix generate damage, but the main damage caused by wearing between fiber tows and matrix. So the large damage in weaving crossing-point of fiber tows appears.