3D-C/SiC的高温弯曲性能和后处理对弯曲性能的影响

FLEXURAL PERFORMANCE OF 3D-C/SiC COMPOSITES AT HIGH TEMPERATURE AND INFLUENCE OF POST-HEAT-TREATMENT ON FLEXURAL PERFORMANCE

3D C SiC用T3 0 0碳纤维编织为三维四向编织体 ,编织角 2 2°。CVI(chemicalvaporinfiltration)法致密 ,纤维体积分数 40 %～ 45 % ,热解碳界面层厚度约 0 .2微米、密度为 2 .0 1g cm3和空隙率为 17% ,最终在试样表面形成 5 0 μm的SiC涂层。在YKM 2 2 0 0超高温试验机上进行三点弯曲试验 ,真空度为 10 - 3Pa ,夹头位移速率为 0 .5mm min。结果表明 ,随试验温度升高 ,弯曲强度在 90 0℃和 170 0℃出现了两个峰值 ;弯曲模量在 110 0℃出现了最大值 ;而弯曲应变在低于 15 0 0℃基本保持不变 ,高于 15 0 0℃不断升高 ,出现了明显的不可逆变性。试样 180 0℃真空保温 1小时后处理 ,弯曲模量明显降低和强度降低 ,而变形能力明显升高。激光拉曼分析得出 :制备的 3D C SiC碳纤维存在残余压应力 ,13 0 0℃处理后碳纤维存在残余拉应力 ,而 2 0 5 0℃处理后碳纤维没有残余应力。高温下因基体和纤维膨胀系数不同 ,易产生界面脱粘和分离 ,使得基体中的裂纹不易扩展到纤维中 ,保持了纤维强度 ,同时纤维易于滑动。这是很高温度下弯曲模量降低、变形能力增加和弯曲强度略增加的原因。

The preform fabricated with T300 carbon fibers weaved into 3-dimensional and 4-directional structure with 22° weaving angle, is deposited pyrocarbon and SiC matrix at 950℃～1*!000℃ with CVI(chemical vapor infiltration) method. Fiber volume fraction is 40%～45%, the thickness of the pyrocarbon is about 0.2*!μm, 3D-C/SiC density is 2.01*!g/cm 3 and porosity is 17%. Finally the thickness 50*!μm of SiC coating is formed on the surface of specimen. Three points flexural experiments are carried out at YKM-2200 super-high-temperature flexural machine with 10 -3 Pa vacuum and 0.5*!mm/min cross head moving speed. The results indicate that flexural strength appeare two peak values, at 900℃ and 1*!700℃ respectively. Maximum value of flexural modulus is found at 1*!100℃. The flexural strains nearly keep constant when temperature lower than 1*!500℃. Above 1*!500℃, the obviously irreversible flexural deformation is obtained. After keeping one hour at 1*!800℃ in vacuum, the flexural modulus decreased obviously at 1*!100℃, whereas flexural deformation increased. Laser Raman spectrum analysis indicates that 3D-C/SiC exists residual compressional stress in carbon fiber, and that residual tensile stress after 1*!300℃ treatment, after 2*!050℃ treatment residual stress disappear. At high temperature the tendency for 3D-C/SiC to destroy interface bond is ascribed to different thermal expansion coefficient between matrix and carbon fiber. It would lead that carbon fiber easily slide, at the same time keep carbon fiber strength because matrix crack unlikely entering into fiber. Therefore flexural deformation increases, flexural modulus decreases and the flexural strength slightly increases when elevating temperature.