Formation Mechanism And Mechanical Properties Of SiO₂-BN/Nb Brazed Joint

SiO₂-BN composite ceramic,as a kind of wave-permeable material,due to its excellent thermal shock resistance and good mechanical properties,it is often used in the missile.In the actual assembly process,composite ceramic and metal parts must be connected.The current study often used Invar alloy or Ti alloy as metal transition parts,with active brazing method to achieve bonding of composite ceramic and metal connections.In these studies,SiO₂-BN only reacted with h-BN to generate TiB₂ and TiN,while SiO₂ was found not to participate in the reaction.In this paper,we propose a dual active element system,which uses Nb metal as the base material.Nb dissolves into the liquid solder and acts as a"second"active element,making both of the phases participate in the interfacial reaction and thus enhance the interfacial strength.In this paper,the interface structure of SiO₂-BN/Nb joint brazed with Ag-Cu/Ti alloy was characterized by scanning electron microscopy,transmission electron microscopy and X-ray diffraction.The results show that nanoscale Ti B₂-TiN fine crystalline layer,micron-sized Nb₃Si,Ti₃Cu₃O compound and a variety of Ti-Cu compounds form in the solder joint.At the same time,the possibility of the formation of each phase in the thermodynamic angle is proved by calculation.The formation mechanism of SiO₂-BN/Nb joint interface under double active element system was explained by combining the characterization and the calculation results.The effects of brazing temperature and the holding time on the microstructure and mechanical properties of the SiO₂-BN/Nb joint brazed with Ag-Cu/Ti were studied.The mechanical properties of the joint were affected by the thickness of the composite ceramic reaction layer,the microstructure of the braze joint and the internal residual stress of the brazed joint.The joint shear strength reaches the maximum value of28MPa at 920°C-10min.In order to further improve the mechanical properties of the brazed joint,the content of active element Ti was changed and the corresponding activity was calculated.The optimum parameters with Ti content of 6wt.%were obtained.On the other hand,in order to alleviate the mismatch between the coefficient of thermal expansion of metal and composite ceramic,the composite brazing filler metal with Mo particles was prepared and its influence was studied.Also,the change tendency of the thermal expansion coefficient of brazing filler metal was calculated.When 5wt.%Mo was added,the shear strength of the joint increased to 48MPa,which was 117%higher than that without Mo addition.