【作者】 李双；

【导师】 张宇民；

【作者基本信息】 哈尔滨工业大学 ， 航空宇航科学与技术， 2013， 博士

【摘要】 随着现代光学技术的发展，航天航空等领域对大口径、轻量化光学系统的需求越来越迫切。反应烧结碳化硅是继铍、玻璃、硅、微晶玻璃后又一种综合性能优异的光学反射镜材料。然而，反应烧结碳化硅陶瓷固有的高脆性制约了材料的机械加工，降低了构件的成品率。复合化是改善单一陶瓷性能的重要途径。本课题针对反应烧结碳化硅陶瓷的高脆性，将短纤维引入SiC/C体系，经注浆成型、反应烧结制备了短纤维增强反应烧结碳化硅复合材料，研究了原料组成、成型工艺、烧结制度对材料性能的影响。短纤维增强反应烧结碳化硅基复合材料克服了反应烧结碳化硅陶瓷脆性大、断裂韧性低的缺点，同时保留了碳化硅陶瓷比刚度高、热膨胀系数低、热导率高的特点，为大口径、轻量化、高精度反射镜的发展提供了必要的保障。为了增强碳纤维的抗氧化性，首先采用溶胶-凝胶法在短切碳纤维表面涂覆致密氧化锆薄膜，研究了原料组成、制备工艺对氧化锆涂层结构与性能的影响。浸渍六次后碳纤维轴向、断口处制备均匀氧化锆涂层。采用EDS法研究了纤维表面的元素组成，表明纤维表面存在致密氧化锆涂层。采用TG-DTA法研究了短纤维的抗氧化性，采用3mol.%YSZ涂覆碳纤维的开始氧化温度提高到650℃，800℃时的氧化失重降低到20%。以双峰碳化硅、炭黑为基体材料，短切碳纤维为增强体制备了短切碳纤维增强反应烧结碳化硅复合材料。克服短切碳纤维的团聚、成球难题，实现了短切碳纤维在SiC/C浆料中的均匀化。研究了短纤维体积分数对反应烧结碳化硅材料显微结构、体积密度、力学性能的影响：短切碳纤维以碳的形式引入材料体系，短纤维的增加提高了材料的体积密度，当短纤维体积含量超过30vol.%时纤维“架桥”效应降低了材料的体积密度；短纤维的增加提高了材料的弯曲强度和断裂韧性，当短纤维体积含量为30vol.%时，弯曲强度和断裂韧性分别为416MPa、5.1MPa·m^1/2，分析认为纤维拔出、界面脱粘、裂纹偏折、沿晶断裂、残余压应力是主要的强韧化机制。研究了碳短切碳纤维经反应烧结、HNO3/HF腐蚀后的形貌变化，分析了形貌变化的原因，提出了硅化纤维的双层结构模型。短切碳纤维在反应烧结时容易与液相硅反应，使增强体的强度降低、强韧化效果下降。实验中采用热稳定性较好的碳化硅晶须为增强体，经湿法球磨、注浆成型、反应烧结制备了碳化硅晶须增强反应烧结碳化硅复合材料。反应烧结后碳化硅晶须保持了表面的竹节结构及初始尺寸，具有优良的热稳定性。晶须的“架桥”效应提高了坯体的孔隙率，增加了材料中游离硅的尺寸、含量，因此材料的体积密度较短切碳纤维增强材料的体积密度低。碳化硅晶须拥有较高的弹性模量和强度，晶须含量为20wt.%时断裂韧性达到极大值4.2MPa·m^1/2。材料断口处有大量的晶须拔出，表明晶须与基体间形成合适的界面结合强度，成为最有效的增韧机制。材料表面存在残余压应力，压应力抑制了表面裂纹的扩展。由于碳化硅晶须平均直径仅为1.5μm，材料力学性能的提高不大。反应烧结碳化硅发生惰性氧化后在材料表面形成致密氧化物薄膜，可以提高材料的可靠性。采用空气中微氧化法在反应烧结碳化硅表面制备致密二氧化硅薄膜，研究了二氧化硅薄膜的形成机制、形貌及对反应烧结碳化硅性能的影响。微氧化后材料表面的凹坑、裂纹、气孔等缺陷减少；游离硅的尖端熔化、尺寸减小；硅化纤维发生显著体积膨胀，显示较高的氧化活性。900℃微氧化形成无定型二氧化硅，1100℃微氧化后无定形二氧化硅发生晶化，晶态二氧化硅衍射峰强度随短纤维体积含量的增加而减少。对短碳纤维增强材料，1100℃微氧化材料的弯曲强度、断裂韧性分别提高34%、49%；对碳化硅晶须增强材料，晶须含量15wt.%、1100℃微氧化时材料断裂韧性达到5.1MPa·m^1/2。碳-硅反应是制备反应烧结碳化硅的基础，针对反应物炭黑，研究了炭黑含量对短纤维增强反应烧结碳化硅陶瓷结构与性能的影响。炭黑硅化后伴随显著的体积膨胀，因此随着炭黑含量的增加，短切碳纤维增强反应烧结碳化硅材料的体积密度逐渐提高。炭黑含量的增加提高了反应烧结碳化硅中碳化硅相的含量，由于碳化硅的热膨胀系数大于硅，材料的热膨胀系数呈增加趋势。当碳纤维体积分数低于20vol.%时，材料的弯曲强度逐步极高；继续增加碳纤维体积分数，炭黑团聚引起材料结构不均匀从而降低了材料的强度。随着炭黑含量的增加，材料的断裂韧性稳步提高。炭黑团聚体反应烧结形成球状SiC-Si多孔结构，空气中微氧化后球状结构表面易于形成致密二氧化硅层。更多还原

【Abstract】 The demands for large scale, light weight optical systems are growing graduallyin the field of space and aviation with the development of modern optics. After theemployments of Be, glass, silicon and crystallite glass, reaction bonded siliconcarbide （RBSC） is becoming mirror materials with excellent comprehensiveperformances. However, the intrinsic brittleness of RBSC limits its machine work,and thus reduces the qualified rate. Composite is a significant approach to improveproperties of monolithic ceramics. To solve the above problems of RBSC, choppedfiber was introduced into SiC/C system to obtain homogeneous slurry. Randomchopped fiber reinforced RBSC composites were prepared by slip casting andreaction sintering. In this study, the influences of starting materials, forming andsintering techniques on the properties of RBSC composites were investigated.Taking advantage of random chopped fiber, the weaknesses of high brittleness andlow fracture toughness of RBSC are relieved; at the same time, the merits of highstiffness, low coefficient of thermal expansion and high thermal conductivity of SiCceramics are retained. This provides a necessary condition for the preparation oflarge scale, ultra-light weight and high precision optical mirror.To protect carbon fiber from destructive oxidation, a Zirconia coating wasdeposited on the chopped carbon fiber surface by sol-gel method. The influences ofmaterial constitutes and process route on the microstructure and properties ofZirconia coating were investigated. When the chopped fiber was dipped for sixcycles, a uniform coating was prepared on the axial and fracture surface. Elementalconstitute of the coating was analyzed by EDS method, which confirmed theexistence of Zirconia coating on the fiber surface. Oxidized performance wasdetermined by TG-DTA method. With respect to the carbon fiber with3mol.%YSZcoating, the destructive oxidation started at650°C and the mass loss arrived only20%until800°C.Random chopped fiber reinforced RBSC composites were prepared withbimodal SiC and carbon as matrix, and chopped carbon fiber as reinforcement. Thisroute solved the aggregation of chopped carbon fiber, giving rise to a homogeneousdispersion of chopped fiber in the SiC/C slurry. The volume fraction of choppedfiber is a decisive factor in the microstructure, bulk density and mechanicalproperties of the RBSC composites. The bulk density of the composites increasedwith increasing of carbon fiber which exists in the form of carbon; whereas, whenthe volume fraction exceeded30vol.%, the bridge effect of chopped fiber led to a decrease of bulk density. The introduction of chopped fiber improved both flexuralstrength and fracture toughness of the composites, reaching the peak values of416MPa and5.1MPam^1/2, at the chopped fiber fraction of30vol.%. Severaltoughening mechanisms, such as fiber pullout, fiber debonding, crack deflection,intergranular fracture and residual stress, were found in the composites. An obviousmorphology change was observed for the chopped fiber in the reactionsintering-acid corrosion test. Based on the morphology change, a bilayer model wasproposed to characterize the structure of the siliconized fiber. The formingmechanism of the bilayer model was analyzed.The chopped carbon fiber reacted with liquid silicon during the sintering, andthe strengthening and toughening performances were damaged. Due to the excellentperformance at high temperature, silicon carbide whisker was chosen asreinforcement. Silicon carbide whisker reinforced RBSC composites were preparedby slip casting and reaction sintering with silicon carbide whisker as reinforcement.After sintering, the whisker maintained the burl profile on the surface and thestarting diameter, indicating excellent high temperature stability. Due to the bridgeeffect of whisker, the content and scale of residual silicon were increased in thewhisker reinforced RBSC composites, leading to a lower bulk density comparedwith the chopped fiber reinforced RBSC composites. The silicon carbide whiskerwas characterized with high elastic modulus and stiffness, so the fracture toughnessof the composites reached4.2MPam^1/2at the whisker fraction of20wt.%. Whiskerpullout was observed on the fracture surface, implying an appropriate bondingstrength between the whisker and the matrix. As a result, whisker pullout wasconsidered as the main toughening mechanism. The compressive residual stress,which can restrain the origin and propagation of micro-cracks, was detected on thepolished surface of the material. The diameter of whisker is approximate1.5μm,thus confining the reinforcing effect.A protective film was prepared on the material surface by mild oxidation inambient air. The formation mechanism, morphology and protective behavior of SiO2film were investigated. Because of the mild oxidation, the defects on the materialsurface, such as holes, cracks, pores, was healed by the SiO2film. At the mildtemperature tips of the residual silicon melted, resulting in a reduction of thediameter. The siliconized fiber underwent an obvious expansion, indicating a highreactivity for the oxidation reaction. At900°C the oxidation product was amorphousSiO2. The amorphous SiO2crystallized to cristobalite at1100°C, whose content wasdetermined by the fraction of the reinforcement. For the chopped fiber reinforcedRBSC composites, the flexural strength and fracture toughness were increased by34% and49%by mild oxidation at1100°C. For the whisker reinforced RBSC composites,the fracture toughness reached5.1MPam^1/2at whisker fraction of15wt.%by mildoxidation at1100°C.The reaction sintering of RBSC is on the basis of carbon-silicon reaction, so theeffect of carbon black in the random chopped fiber reinforced RBSC wasinvestigated. The siliconization of carbon black is accompanied by a volumeexpansion of approximate100%; as a result, the bulk density of the RBSCcomposites was increased with increasing of the carbon fraction. The CET（coefficient of thermal expansion） of SiC is higher than that of Si, so the CET of thecomposites was enhanced with the fraction increase of SiC. When the chopped fiberfraction was lower than20vol.%, the flexural strength was improved with increasingof fiber fraction; if further increasing the fiber fraction, the strength was reduced forthe nonuniform structure resulted from the carbon black aggregation. With respect tofracture toughness, the value increased steadily with increasing of carbon black. Theaggregation of carbon black was turned into porous SiC-Si structure during reactionsintering. By mild oxidation, a dense SiO2film formed on the surface of the spherestructure.更多还原