Microstructure and mechanical properties of carbon/carbon composites densified with pyrocarbon from xylene using LaCl3 as catalyst
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摘要: 采用薄膜沸腾CVI法，以LaCl3为催化剂在1 000~1 100℃下热解二甲苯制备出密度1.67~1.72 g/cm3的C/C复合材料，研究了催化剂含量对其致密化特性、基体微观结构和力学性能的影响。结果表明，催化剂含量由0增加至15 wt%时，热解炭沉积速率升高，其结构由粗糙层（RL）向各向同性（ISO）转变，材料密度和力学性能先升高后降低。含量为3 wt%时材料密度较高，且基体内出现纳米丝状碳（NFC）；含量增大至6 wt%后，NFC数量增多，催化剂表面积碳使基体趋于形成RL和ISO混合结构，高催化剂含量下ISO层较厚。催化剂添加后材料的弯曲和剪切强度分别提高约8.1%~33.0%和15.3%~55.7%，含量为6 wt%时性能较佳，弯曲及剪切强度达230.7 MPa和36.6 MPa。高温处理使材料韧性提高，但强度降低，15 wt%时降低较大，弯曲和剪切强度降低约18.6%和14.4%。Abstract: Carbon/carbon composites with densities of 1.67-1.72 g/cm3 were prepared by film boiling chemical vapor infiltration at 1 000-1 100℃ using xylene and LaCl3 as carbon source and catalyst, respectively. The influence of the catalyst content on the density, pyrocarbon (PyC) microstructure and mechanical properties of the composites was studied by polarized light microscopy, scanning and transmission electron microscopy and mechanical testing. Results show that the PyC deposition rate is remarkably increased by the presence of the catalyst. The microstructure of the PyC changes from a rough laminar (RL) to isotropic (ISO) with increasing catalyst content from 0 to 15 wt%. The density exhibits a maximum with the catalyst content at 3 wt% while the flexural strength and interlaminar shear strength exhibit maxima of 230.7 and 36.6 MPa, respectively, with the catalyst content at 6 wt%. Nanofilamentous carbon is found in the PyC when the catalyst content is above 3wt%. The catalyst particles are encapsulated by the PyC, resulting in the formation of a hybrid matrix consisting of RL and ISO PyCs. The thickness of the ISO layer increases with the catalyst content. The flexural and interlaminar shear strengths of the composites are increased by 8.1-33.0% and 15.3-55.7%, respectively, compared with the values for samples without the catalyst. The toughness of the composites is increased, but the flexural and interlaminar shear strengths are decreased by high temperature treatment. The maximum degradation of the flexural and interlaminar shear strengths is found at acatalyst content of 15 wt%, and these are 18.6 and 14.4% lower than the values for samples with no catalyst.
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