Process And Bonding Mechanism For Brazing ZrB₂-SiC Ceramic To Nb Alloy With TiCoNb

Ultra-high temperature ceramics?UHTCs?with unique advantages are attracting more and more researchers.Among them,Zr B₂-based ceramics with high melting point,high hardness,high thermal conductivity,high conductivity,low coefficient of thermal expansion and relatively low density and other advantages are widely used in refractory materials,electrodes and aerospace areas.However,it is difficult to produce complex structure composed by ceramics due to its poor impact resistance and the inherent brittleness.Joining technology is one of the ideas to solve the problem.Besides the ceramic/ceramic components,ceramic/metal composite layer can also expand the application.The reliable joining of Zr B₂-SiC composite ceramic to Nb alloy can make the composite structure more suitable for high temperature application.In this paper,Ti₃₅Co₃₅Nb₃₀ filler was prepared by alloy melting method.Then,the bonding of Zr B₂-Si C/Zr B₂-Si C and Zr B₂-Si C/Nb was studied.The effects of temperature,holding time and the thickness of the brazing filler on the microstructures and mechanical properties of joints were investigated.Finally,the bonding mechanisms of Zr B₂-SiC/Zr B₂-Si C joints and Zr B₂-Si C/Nb joints brazed with Ti Co Nb were studied by thermodynamic analysis.The typical microstructure of ZrB₂-Si C/Zr B₂-Si C joint is: a reaction layer composed of?Nb,Ti?B,?Ti,Nb?C and Nb4 Co Si formed adjacent to Zr B₂-Si C ceramic,meanwhile,the central area of the joints are eutectic structures composed of?Ti,Nb?2Co,TiCo and Nb?s,s?.The thickness of brazing filler decreased and the thickness of reaction layers increased with increasing the brazing temperature or prolonging the holding time.With rising the brazing temperature,prolonging the holding time or increasing the thickness of the filler alloy,the shear strength of the joints first increased and then dropped.The joint with maximum shear strength of 192.4MPa has been obtained when it was brazed at 1280 °C for 10 min and the thickness of the filler foil was 400?m.The typical microstructure of Zr B₂-Si C/Nb joint is: a reaction layer composed of?Nb,Ti?B and?Ti,Nb?C formed adjacent to Zr B₂-Si C ceramic.And a Nb5Si3 layer formed between reaction layer and brazing filler.The central area of the joints are eutectic structures composed of?Ti,Nb?2Co,Ti Co and Nb?s,s?.With rising the brazing temperature or prolonging the holding time,more Nb atoms dissoluted and diffused into liquid filler,causing the thicker of reaction layer and Nb5Si3 layer.Meanwhile,the thickness of the brazing filler decreased gradually.With increasing the thickness of the filler alloy,the thickness of the reaction layers and brazing filler increased.In addition,the amount of ZrC decreased gradually.The joint with maximum strength of 98.4MPa has been obtained when it was brazed at 1270 °C for 10 min and the thickness of the filler foil was 500?m.