【作者】 夏存娟；

【导师】 王浩伟；

【作者基本信息】 上海交通大学 ， 材料学， 2013， 博士

【摘要】 碳纤维增强镁基复合材料(C/Mg复合材料)，具有密度低、比强度、比模量高、热膨胀系数可以接近于零、在较大温度范围内具有很好的尺寸稳定性等突出的特性，在航空、航天、国防等领域将具有广阔的应用背景。然而C/Mg复合材料体系本身的一些特性因素制约了它的发展，主要包括（1）碳纤维和镁基体不润湿给C/Mg复合材料的制备造成很大困难；（2）严重界面反应使得C/Mg复合材料无法获得理想的界面结合状态，同样制约着复合材料的力学性能。本文通过对C/Mg复合材料界面结构的理论分析，制备工艺及界面的优化设计，成功制备出碳纤维增强镁基复合材料，并对其微观组织结构、界面反应层的形成规律及机理和力学性能进行了系统的研究；揭示了不同纤维涂层造成的界面反应产物对界面结构和断裂行为的影响规律，并在TiO2涂层C/Mg复合材料中实现了设计的新界面结构；在此基础上，研究了基体中铝含量以及涂层厚度对新界面结构及力学性能的影响规律，初步探讨了TiO2涂层C/Mg复合材料的界面形成机制及其断裂行为。为了实现C/Mg复合材料的理想界面结合，本文提出了一个具体的界面结构：微反应层界面结构来获得适度界面结合的状态。在这个结构中，首先碳纤维周围存在一层扩散阻挡层以保护碳纤维不受基体合金的反应侵蚀而造成力学性能下降。其次，涂层材料和镁基体之间反应产物不是以有针状界面相存在，而是以极薄的纳米级厚度反应物层的形式存在于界面区域，这样可以避免针状界面产物在界面区域造成应力集中而导致整体复合材料力学性能急剧下降的现象。本文通过溶胶凝胶法实现碳纤维表面MgO，SiO2和TiO2涂层的涂敷，通过控制溶胶、涂胶过程、烧结过程可以实现连续、厚度均匀、无突起、不开裂的纤维涂层。SiO2和TiO2等润湿涂层能够显著降低C/Mg复合材料的浸渗压力。采用具有规则球形外形的空心微珠作为混杂颗粒，在降低浸渗压力的同时又提高了碳纤维的分散均匀性。MgO涂层不能改善碳纤维和镁之间的润湿问题，所制备的涂层C/Mg复合材料中很容易生成明显的浸渗缺陷，其弯曲模量和弯曲强度分别为86GPa和643MPa；SiO2涂层改善了C/Mg的润湿性，复合材料的弯曲模量提高到116GPa，但由于SiO2涂层和镁基体之间的界面反应严重，在碳纤维周围生成大量的针状界面相，其弯曲强度为676MPa，没有显著提高。TiO2涂层C/Mg复合材料的界面洁净，未发现针织界面产物，其弯曲力学性能显著提高，弯曲模量和弯曲强度分别提高到132GPa和1009MPa。TEM分析结果表明，TiO2涂层和镁基体的反应产物并没有以颗粒或者针状的形式聚集在界面区域，而是以一层20-30nm界面反应层的形式存在于界面区域。这是因为Ti和Mg之间不互溶也不反应，因此被基体Mg置换出的单质Ti不会和Mg生成金属间化合物，由界面反应产物Ti和MgO组成的反应物层隔绝了TiO2涂层和镁基体之间的继续接触，界面反应进度得以控制，最终在C/Mg复合材料中形成了微反应层界面结构，并且这种新的界面结构提高了复合材料的力学性能。镁基体中的铝含量以及涂层厚度对TiO2涂层C/Mg复合材料的界面结构及力学性能具有显著影响。当基体从AZ91（9%Al）变成Mg-2%Al合金时，C/Mg复合材料的弯曲强度了26%，从1009MPa提高了到1277MPa（对比中涂层厚度为150nm）。在TiO2涂层C/Mg复合材料中存在最小界面层厚度。最小界面层厚度受基体中铝含量的影响。本研究中Mg-2%Al和AZ91的最小界面层厚度分别为30nm和40nm。本质上说，界面层能够阻止基体中Al扩散到碳纤维生成Al4C3相。因此，界面层需要达到最小界面层厚度才能发挥作用。当涂层厚度大于最小界面层厚度时，复合材料力学性能随涂层厚度的降低而增加。碳纤维表面TiO2涂层厚度分别为150nm，60nm和30nm时，Cf-TiO2/Mg-2%Al复合材料的对应弯曲强度随涂层的增加而提高，分别为1277MPa，1309MPa和1391MPa；当TiO2涂层厚度小于最小界面层厚度时，TiO2涂层过薄，无法形成有效的扩散阻挡层保护碳纤维，在界面区域生成的Al4C3相会显著降低复合材料的力学性能。如TiO2涂层厚度从50nm降低到30nm时，Cf-TiO2/AZ91复合材料的弯曲强度没有提高，反而从1108MPa降低到874MPa，降幅超过20%。更多还原

【Abstract】 Carbon fiber reinforced magnesium alloy composites (C/Mg composites) have exhibitedhigh mechanical properties, good thermal conductivity and zero or near-zero coefficient ofthermal expansion. It has been suggested that composites based on ultra-high modulus carbonfibers possess specific stiffnesses higher than any known metallic material. Unfortunately, theintrinsic disadvantage of C/Mg composites restrict its development, including:(1) Poorwettability of C-Mg system, the fabrication process of C/Mg composites are suffered from thepoor wettability.(2) Harmful interfacial reactions, these reactions between carbon fiber andmagnesium matrix can introduce unexpected interface bonding.In present work, C/Mg composits have been successfully fabricated with optimizated oninterfacial structure using squeeze casting method. The interfacial reactions andmicrostructure in C/Mg composites have been studied. The results showed that the interfacialstructure can be tuned by different fiber coatings. The optimized interfacial structure has beenachieved in TiO2coated C/Mg composites. Further, the effect of Al content in Mg matrix andthickness of fiber coating on the interfacial structures and mechanical properties ofCf-TiO2/Mg composites have been investigated.A specific model of interface structure:“the thin reaction layer interface structure” isproposed to accomplish the improved interface bonding in C/Mg composites. In this novelinterface structure, a diffusion barrier around carbon fiber is existed to protect the fiber fromdegradation for harmful interface reactions. The interface reaction products can form acontinue uniform transition layer under nanoscale between fiber coating and metal matrix,rather than aggregating around interface to form needle-like interphases or particles, which can avoid stress concentration caused by those harmful interphases.Carbon fibers were coated with MgO, SiO2and TiO2films respectively using the sol-gelmethod. The uniform, adherent, crack-free and non-bridging fiber coatings can be achieved bycontrolling sol-gel precursors, coating processes and heating parameters.MgO coated C/Mg composites are tend to progegate infiltration defect due to the poorwettability between Mg and C. flexural strength are86GPa and643MPa. SiO2coating canimprove the wettability of C/Mg composites. The needle-like Mg2Si phase is formed as theinterfacial react product at the same time. The flexural modulus and flexural strength are116GPa and676MPa. The interfacial zone of TiO2coated C/Mg composites is clean and withoutany brittle phase observed. The flexural modulus and flexural strength increase to132GPaand1009Mpa respectively. The TEM results showed that the interfacial structure is formed a20-30nm thick reaction layer. Since Ti is highly compatible with Mg, and the characteristic ofTi and Mg is benefit to control the extent of interface reaction, the designed interfacialmicrostructure is thus achieved. This novel interface structure can improve the mechanicalproperties of C/Mg composites.The interfacial structure and mechanical properties are found on dependence of both Alcontent in Mg-Al alloy matrix and the thickness of TiO2coating.When the Al content increases in Mg alloy from Mg-2%Al matrix to AZ91matrix, theflexural strength decreased from1277MPa in to1009MPa (The thickness of TiO2coating is150nm in this comparison).A minimum thickness of interfacial layeris found existed in C/Mg composites in ourstudy. This minimum thickness is affected by Al content in matrix. The minimumthicknesses of C/Mg-2%Al and C/AZ91composites are30nm and40nmrespectively in the experiments. Fundamentally，this interfacial layer is a barrier layerfor the Al diffusing from Mg matrix to carbon fiber to form Al4C3phase. Thus, aminimum thickness of interfacial layer is required to take into effect. When the thickness of TiO2coating is larger than minimum thickness, the mechanicalstrength is decreasing. For example, it is found if the thickness of TiO2coating inCf-TiO2/Mg-2%Al composites is150nm,60nm and30nm, the flexural strength ofcomposites is1277MPa,1309MPa and1391MPa, respectively.When the thickness of TiO2coating is smaller than minimum thickness, the TiO2fibercoating is too thin to work as diffusion barrier to protect carbon fiber. The brittleAl4C3phase reacted in interface zone decreases mechanical properties of C/Mgcomposites. When the thickness of TiO2coating decreases from50nm to30nm inCf-TiO2/AZ91composites, the flexural strength decreases by21%from1108MPa to874MPa.更多还原