【作者】 杨振文；

【导师】 冯吉才； 张丽霞；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2013， 博士

【摘要】 SiO₂-BN陶瓷是以SiO₂陶瓷和BN陶瓷为基础发展起来的新一代导弹天线罩材料，表现出优异的协同强化作用，具有较好的力学和介电性能、优越的抗热震性能以及良好的抗烧蚀性能。在天线罩的实际装配应用过程中，陶瓷天线罩需要与金属环进行可靠连接。Invar合金在室温范围内的热膨胀系数较低，成为国内外金属连接环的常用材料。为了解决胶接方法的耐热性差和易老化问题以及消除机械连接带来的附加重量，本文采用活性钎焊方法实现SiO₂-BN陶瓷与Invar合金的可靠连接，基于液态钎料与母材相互作用过程中的界面特征，通过中间层体系优化设计实现对钎焊接头界面组织的控制以及力学性能的提高。设计了两种界面反应模式，采用FIB制样和HRTEM表征方法研究了钎料与SiO₂-BN陶瓷的界面反应。通过在Ag-Cu共晶合金块上放置Ti箔的试验方法研究了液态Ag-Cu/Ti钎料在SiO₂-BN陶瓷表面的润湿铺展行为，揭示了活性元素Ti在反应润湿体系中的作用。研究结果表明，AgCu-Ti/SiO₂-BN体系的润湿性取决于SiO₂-BN陶瓷界面TiN-TiB2反应层的形成；液态钎料中的Ti含量和温度对体系润湿性的影响不明显；陶瓷表面形态仅影响接触角开始降低阶段的速率。采用电子束蒸镀辅助钎焊的方法研究了SiO₂-BN陶瓷自身钎焊过程中的界面特征。钎焊过程中，Ti镀层快速向液态Ag-Cu钎料中溶解扩散直至形成均匀体系。液态钎料中的活性元素Ti在化学势差驱动力作用下向SiO₂-BN陶瓷聚集形成富Ti层。富Ti层与陶瓷反应形成TiN-TiB2纳米晶反应层；并与Cu反应形成不连续的带状Ti-Cu化合物。从热力学角度讨论了润湿铺展以及自身钎焊两种界面反应模式下，活性元素Ti与SiO₂-BN陶瓷的界面反应机制，即解明了厚度为150-200nm的TiN-TiB2纳米晶反应层的形成过程。采用Ag-Cu/Ti活性钎料分析了Invar/SiO₂-BN体系的钎焊性，阐明了Ti含量、工艺参数对接头界面组织和性能的影响。Ti含量和工艺参数影响陶瓷界面反应区域的厚度以及Fe₂Ti-Ni₃Ti脆性化合物的形成和分布。当Ti含量为4.5wt.%时，在钎焊温度880℃，保温10min条件下接头的平均抗剪强度最大为32MPa，并且接头的抗剪强度随着工艺参数的改变急剧降低。两种母材固有的热膨胀系数差异产生的残余应力以及钎焊接头中脆性化合物的形成都弱化接头性能。为了调节中间层组织的热膨胀系数，缓解接头的残余应力，采用机械球磨法制备h-BN颗粒增强的Ag-Cu-Ti+BN复合钎料中间层体系，通过原位反应钎焊Invar合金和SiO₂-BN陶瓷。使用优化的复合钎料中间层钎焊Invar/SiO₂-BN体系，h-BN颗粒完全与活性元素Ti反应形成TiB晶须和TiN颗粒联合增强的接头；Fe₂Ti-Ni₃Ti脆性化合物的形成得到一定程度的抑制，接头的平均抗剪强度达到39MPa。综合考虑钎焊接头的界面结构、连接体系热膨胀系数的匹配性以及中间层组织的塑性变形能力三方面因素，讨论了钎焊接头的强化机制。基于Invar合金向液态钎料中的溶解机制，设计并采用Ag-Cu/Cu/Ag-Cu-Ti软性复合中间层钎焊Invar合金与SiO2-BN陶瓷，实现Fe₂Ti-Ni₃Ti脆性化合物的抑制并缓解接头的残余应力。研究了Cu中间层厚度对Invar/SiO2-BN钎焊接头界面组织和性能的影响，使用软性复合中间层获得的接头强度高于仅使用Ag-Cu-Ti钎料的钎焊接头，当Cu中间层的厚度为100μm时，接头的抗剪强度最大为43MPa。从钎焊过程中的反应次序以及Cu中间层的阻隔作用两方面揭示了接头中Fe₂Ti+Ni₃Ti脆性化合物的抑制机制。结合颗粒增强复合钎料和软性复合中间层两方面的优点，设计Ag-Cu/Cu/Ag-Cu-Ti+BN软性-梯度中间层钎焊Invar合金和SiO2-BN陶瓷。钎焊接头组织既实现对Fe₂Ti+Ni₃Ti脆性化合物的完全抑制，又实现对陶瓷侧热膨胀系数的调节。采用有限元模拟方法评价三种中间层体系钎焊Invar合金和SiO2-BN陶瓷接头的残余应力大小和分布。结果表明，三种中间层体系获得钎焊接头的最大残余应力都在近界面反应层的陶瓷基体中形成。采用Ag-Cu-Ti钎料直接钎焊获得接头残余应力最大值是230MPa；采用BN颗粒增强的复合钎料时，陶瓷基体表面的最大残余应力降低为142MPa；使用软性复合中间层时，陶瓷中的最大残余应力仅为69MPa。结果表明，Ag-Cu/Cu/Ag-Cu-Ti软性复合中间层能够很好的缓解Invar/SiO2-BN钎焊接头残余应力。更多还原

【Abstract】 Based on fused silica and BN ceramic, co-enhanced SiO₂-BN ceramic whichpossess superior thermal and dieletric properites, excellent thermal shock resistanceand good ablation resistance, has been developed to be a new generation of missileradome material. In the practical application of SiO₂-BN ceramic, the assembly ofmissile radome requires joining a SiO₂-BN ceramic radome to a metallic holder.Invar alloy has a very small thermal expansion coefficient around room temperatureand is a commonly used material for metal connecting ring. In order to solve thepoor high-temperature resistence and heat ageing resistance of the joint caused byadhesive bonding, as well as the additional weight produced by mechanical bonding,active brazing is applied to bond SiO₂-BN ceramic to Invar alloy. According to theinterfacial characteriazion during the reaction between liquid filler metal and parentmaterials, interlayer design is used to control interfacial microstructure and improvemechanical properties of an active brazed Invar/SiO₂-BN joint.The interfaicial characterization between liquid filler and parent materials wasstudied by two interfacial reaction modes using FIB sample preparation and HRTEMmethod. The wetting and spreading behaviour of liquid Ag-Cu/Ti filler on SiO₂-BNceramic was researched by the means of a piece of Ti over the Ag-Cu eutectic alloyon the substrate. And the role of active element Ti in the reaction wetting processwas revealed. When the Ti concentration in the liquid/solid interface was enough totrigger the chemical reaction, the reaction drived spreading of the system would takeplace. And Ti content has little effect on the final contact angle of the system. Thewettability of AgCu-Ti/SiO₂-BN system was dependent on TiN-TiB2reaction layerformed on SiO₂-BN ceramic interface. Increasing of Ti content in the liquid fillermainly affected the formation and distribution of Ti-Cu compound. The testingtemperature and surface state of SiO₂-BN ceramic only affected the reduction rate ofcontact angle in the early stage. The interfacial characterization of self-brazedSiO₂-BN ceramic was studied by electron beam evaporation assisted brazing method.During brazing process, the electron beam evaporated Ti layer dissolved into theAg-Cu liquid rapidly until formed a homogeneous system.The active Ti atomsaccumulated at the surface of SiO₂-BN ceramic to form a Ti-rich zone due to thechemical potential difference. Ti reacted with SiO₂-BN ceramic to form a TiN-TiB2reaction layer. On the other hand, it reacted with Cu to form Ti-Cu compounds. Thereaction mechanism between the active element Ti and SiO₂-BN ceramic wasdiscussed from the viewpoint of thermodynamics. The formation mechanism of150-200nm thick TiN-TiB2reaction layer was explained. Invar alloy and SiO₂-BN ceramic was brazed with Ag-Cu/Ti filler metal, therelationship among Ti content, brazing parameters and joint microstruction andproperties was established. Ti content and brazing parameters affected the thicknessof reaction layer as well as the formation and distribution of Ti-Cu compounds in thejoint. When the Ti content was4.5wt.%, shear strength of joints brazed at880℃for10min reached32MPa. Additionally, the joint strength reduced dramatically withthe change of brazing parameters. The bonding properties became weaken becauseof the residual stress yieled by mismatch of thermal expansion coefficient betweenInvar alloy and SiO₂-BN ceramic and the formation of brittle compounds.In order to adjust the thermal expansion coefficient of the interlayer and relievethe residual stress of the joints, Ag-Cu-Ti+BN composite filler which fabricated bymechanical milling was used to braze Invar and SiO₂-BN. The BN content in thecomposite filler was optimized through the joining of Ti/SiO₂-BN, and the reactionmechanism between Ti and BN was studied. When the optimized composite fillerwas used to braze Invar and SiO₂-BN, the h-BN was completely reacted with Ti toform TiB whiskers and TiN nano-grains, meanwhile Fe₂Ti-Ni₃Ti composites wereinhibited in a certain degree. The shear strength of joints reached39MPa. The jointstrength was determinted by the interfacial structure of the joint, the CTE mismatchbetween the joined materials, and the plastic deformability of the brazing seam. Thebalance of the three factors could be achieved by adjusting the BN content.In order to completely inhibit the formation of Fe₂Ti-Ni₃Ti brittle compounds,Ag-Cu/Cu/Ag-Cu-Ti composte interlayer was designed to braze Invar and SiO₂-BN.The effect of Cu-foil thickness on the microstructure and mechanical properties ofthe brazed joints was investigated. It was found from the curve that the shearstrength of the joints brazed with Cu foils of different thickness were higher thanthat of the joints obtained with single Ag-Cu-Ti foil. Additionally, the joint strengthfirst increased dramatically and then decreased slightly with the increase of Cu-foilthickness. The maximum joint strength reached43MPa when a100μm thick Cu-foilwas used, which was more than twice higher than these joints brazed with Ag-Cu-Tifoil. The inhibition of brittle Fe2Ti and Ni3Ti compounds was believed to beinvolved in the reaction sequences and the addition of Cu barrier layer. Based on theabove results, Ag-Cu/Cu/Ag-Cu-Ti+BN interlayer was designed to braze Invar alloyand SiO₂-BN ceramic. The formation of Fe2Ti and Ni3Ti brittle compounds wascompletely inhibited and the CTE of the interlayer adjacent to SiO₂-BN ceramic wasreduced.The size and distribution of residual stresses yieled in Invar and SiO₂-BN jointbrazed with three different types of interlayers were evaluated by finite elementsimulation method. The results showed that the maximum residual stress in thejoints for the three interlayers was yieled in the ceramic substrate adjacent to the reaction layer, and a bowed distribution was observed. When Ag-Cu-Ti filler metalwas used to braze Invar alloy and SiO₂-BN ceramic, the maximum residual stesss inthe joint was as high as230MPa; while Ag-Cu-Ti+BN composite filler was usedbraze Invar and SiO₂-BN, the maximum residual stesss in the joint was142MPa;when Ag-Cu/Cu/Ag-Cu-Ti composite interlayer was used, the maximum residualstesss in the joint was only69MPa. It was found that Ag-Cu/Cu/Ag-Cu-Ti compositeinterlayer has the best effect to relax the residual stress in the Invar/SiO₂-BN brazedjoints. Particularlly, this type of composite interlayer can also be applied to brazeother ceramic-metal systems.更多还原