预制体结构对平板CVI过程及材料性能的影响

【作者】 梁世栋；

【导师】 张福勤；

【作者基本信息】 中南大学 ， 材料学， 2011， 硕士

【摘要】 炭/炭(C/C)复合材料具有密度低、高温强度良好、摩擦磨损性能优异等特点,作为一种新型高温结构材料广泛应用于航空航天以及军事领域。炭纤维预制体结构对CVI过程中气体的传质和C/C复合材料的性能有着重要的影响。通过调节预制体结构,寻求制备工艺简单、周期短、成本低及能够提高材料综合性能的化学气相渗透(CVI)方法,对C/C复合材料的研究具有重要的意义。本文采用无纬布/网胎针刺整体毡和针刺全网胎毡为预制体,借助自行研发设计的CVI炉制备了平板C/C复合材料。研究了预制体结构对CVI工艺致密化过程及基体热解炭微观结构的影响；测试了材料的力学性能、石墨化度、导热系数和电阻率,探讨了预制体结构对C/C复合材料力学性能、石墨化度、导热性能及导电性能的影响。主要结论如下：1预制体的孔隙形状、大小和分布状态影响反应气体的传质,对材料致密化过程有重要影响。随着纤维体积分数的增加,材料的增密速率及最终密度值逐渐减小；布毡比对整体毡材料的增密速率及最终密度影响很小,全网胎材料的增密速率及最终密度均大于整体毡；添加Si02晶须后材料的增密速率及最终密度均增大,添加CaSiO3日须后增密速率减小,最终密度相同。所有材料网胎中热解炭圆壳厚度大小在材料厚度方向呈内部小、两侧大的对称分布；增加纤维体积分数或增加布毡比,材料内部的热解炭增厚程度随之减小。2所得C/C复合材料基体热解炭结构全部为光滑层结构,即基体结构没有因预制体结构的改变而发生明显变化。加入晶须后,基体光学活性略有增强。A晶须的加入,会诱导热解炭呈锥状生长。3材料的石墨化度同时受纤维体积分数和基体热解炭含量的制约和影响,无纬布层内的纤维对应力石墨化贡献较大,添加晶须均可提高材料石墨化度。石墨化度是影响材料导热、导电性能的主要因素,预制体结构仅是辅助性的影响。4预制体结构对材料的弯曲和剪切力学性能有相同影响规律：纤维体积分数的过高和过低均会降低材料的强度；布毡比值的增大会提高材料的强度；加入Si02晶须降低材料的强度,加入CaSiO3晶须后强度略有提高。预制体结构对材料压缩性能的影响规律不一：材料的压缩强度随纤维体积分数增大而降低；与全网胎相比,布毡比变为1：1时,材料的垂直压缩强度降低22%,而后随布毡比的增大强度值逐渐增加；平行压缩强度随布毡比值增大而降低；添加晶须后压缩强度增加,添加A晶须的增幅较大。5除添加Si02晶须的整体毡材料,弯曲、剪切、垂直压缩断裂方式均为韧性断裂,添加SiO2晶须的材料和全网胎材料呈脆性断裂；所有材料的平行压缩断裂方式均呈脆性断裂。更多还原

【Abstract】 Carbon fiber-reinforced carbon matrix composites (Carbon/carbon or C/C composites) have been widely used in the applications of aerospace and military fields as new materials associating with high performance, such as low density, high strength in high temperature and excellent friction and wear properties. The structure of the carbon fiber preform has an important influence on the gas transfer during the chemical vapor infiltration (CVI) process and the properties of C/C composites. It is of great significance for the research of C/C composites to find effective CVI method with simple process, short cycle, low cost and improved comprehensive properties of materials by controlling the preform structure.C/C composites plates with needle-punched felt as preforms were prepared in self-designed CVI furnace by methods of CVI. The effect of the preform structure on the densification process and the microstructure of matrix were studied. Meanwhile, mechanical properties, graphitization degree, coefficient of thermal conductivity and resistivity of materials were tested and the effect of the preform structure on the mechanical properties and physical properties of C/C composites were discussed in detail. The following results were obtained.1. The shape, size and distribution of pores in the preform influence the gas transfer and the densification process of composites. The densification rate and the final density of composites decrease with the increase of carbon fiber volume fraction. The densification rate and density of full mat are higher than integral needle-punched felt whose ratio of cloth to mat has little effect on them. The densification rate and density increase after joining SiO2 whisker and the densification rate decreases after joining CaSiO3 whisker whose final density is as the same as before. The distribution of circle thickness of pyrolytic carbon in mat of all materials is symmetrical smaller in the middle and bigger in the laterals. The thickness of pyrolytic carbon in the middle of materials decreases with the increase of fiber content or ratio of cloth to mat. 2. The microstructure of pyrolytic carbon of C/C composites is smooth laminar structure observed by the polarized optical micrographs which suggests that there is no direct relationship between matrix microstructure and preform structure. The optical activity of matrix slightly increases after joining whisker and joining SiO2 whisker may induce pyrolytic carbon with cone growth.3. The graphitization degree of composites is affected by fiber volume fraction and pyrolytic carbon content at the same time. Fibers in cloth have more significant contribution on the stress-graphitization and graphitization degree can be improved after joining whisker. Graphitization degree is the main effect factor on the thermal and electric conductivity properties, and the preform structure only have an auxiliary effect on them.4. The effect of preform structure on the flexural and shear mechanical properties of composites have the same rule. The high and low volume fraction of fiber can decrease the strength of composites and the strength of materials increase with the increase of ratio of cloth to mat. The strength decreases after joining SiO2 whisker but increase a little after joining CaSiO3 whisker. Their compressive mechanical strength decreases with the increase of fiber volume fraction. The vertical compressive strength decreases by 22% compared with full mat composites when the ratio of cloth to mat of integral felt is 1:1 and then increases with the increase of the ratio of cloth to mat, but the parallel compressive strength decreases with the increase of ratio of cloth to mat. The compressive strength increases after joining whisker, and the increase extent is higher after joining SiO2 whisker.5. The fracture model of the flexural, shear and vertical compressive is pseudo-plasticity except integral needle-punched felt of adding SiO2 whisker and full mat felts and composites joined SiO2 whisker have brittle fracture. Fracture model of parallel compressive of all the felts is brittle.更多还原