| 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. |
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