Active Brazing Process And Bonding Mechanism Of Silicon Nitrid Ceramic Joint Brazed Using Au-ni-v High Temperature Filler Alloy

Silicon nitride ceramics with excellent mechanical properties at high temperature have been widely used in aerospace and other fields. It is extremely necessary to join the Si₃N₄ ceramics for extending the application of Si₃N₄. In this research, Au-Ni-V filler alloy was used to braze Si₃N₄ ceramics. The effect of filler alloy content and brazing parameters on microstructure and mechnical properties of joint has been studied and the effect of microstructure on mechanical properties was analyzed in detail. The mechanism of interfacial reaction between active filler metal of V and Si₃N₄ ceramics was studied deeply. Furthermore, the fourth element of Pd or Mo was added into Au-Ni-V filler alloy to reveal the effect of addition of fourth element on the microstructure, mechanical properties and the mechanism of the joint formation.The results show that the Si₃N₄/Si₃N₄ joint brazed with Au-Ni-V filler alloy consists of three parts: Si₃N₄ ceramic substrates, filler alloy seam composed of Au[Ni] solid solution and Ni-rich phases, and VN reaction layer between substrates and filler alloy seam. Under brazing parameter of 1373K/30min, with V content in filler alloy increasing in a certain range, the thickness of VN reaction layer increases, resulting in the improvement of bonding strength and mechanical properties of the joints. However, when V content exceeds 15 at.% , the brittle alloy phase Ni₂SiV₃ increases, leading to the rise of residual stress in the joint and decrease of mechanical properties of joint. Under brazing condition of 1423K/30min, with the increase of V content in a certain range, reaction layer thickness increases and the filler alloy seam is composed of Au[Ni] solid solution and granular Ni[Si,V,Au] solid solution distributed uniformly. The mechanical properties of joint can be improved. However, Ni₃Si phases precipitated from Ni-base solid solution, and the properties of joint decreased when V content is too high.According to the study of brazing parameters, it can be found that the VN reaction layer was thickened and the mechanical properties of joint was improved with brazing temperature or holding time increasing in certain ranges. However, when the brazing temperature is too high (1473K) or the holding time is too long (90min), Ni[Si,V,Au] solid solution transformed to an intermetallic compound of Ni₃Si in the filler alloy seam, which leads to the decrease of mechanical properties of joints. Therefore, the average three-point bending strength of Si₃N₄/Si₃N₄ joint brazed using Au58.7Ni₃6.5V4.8 at.% at 1423K for 60min reaches the maximum value of 249MPa at room temperature. The joint has good high-temperature performance. When testing temperature increased to 973K, the bending strength of joint is more than 200MPa and when the joint was annealed in air at 973K for 100h, bending strength of joint increases to 269MPa.In this research, the fourth element of Pd was added into Au-Ni-V filler alloy to improve the mechanical properties of joint. The results show that with increasing Pd content, the thinckness of VN reaction layer increases and the Ni-based solid solution grows because the Si content increases continuously. Finally, Ni-Si compounds are formed in the joint. Therefore, the average three-point bending strength of Si₃N₄/Si₃N₄ joint brazed using Au54.1Ni₃6.1V4.8Pd5 at 1423K for 60min reaches the maximum value of 264MPa at room temperature. When testing temperature increases to 1073K, the average bending strength of joint is 221MPa. As 1 at.% Mo particles is added into Au-Ni-V filler alloy, Ni, Si and V react with Mo to form tetradic compound of Ni₂Si(VMo)₃. The formation of Ni2Si(VMo)3 phase leads to a sharp decrease of the bending strength of joint.This paper illustrates the formation mechanism of Si₃N₄ ceramics joint brazed with filler alloy containing activity of V. During heating process, the filler alloy melted and V at the interface at Si₃N₄ ceramic/liquid filler alloy reacted with Si₃N₄ and formed the VN compound. Si atoms were produced by decomposition from Si₃N₄ and dissolved continuously into the molten brazing alloy. With increasing the brazing temperature or holding time, the VN grain grew and connected with each other. There is no specific crystal orientation relationship between VN and Si₃N₄. The increase of the VN reaction layer with holding time obeys the linear law, indicating that the growth rate of the reaction layer is controlled by the reaction between V and Si₃N₄. Based on the dynamics calculation, the activation energy of VN reaction layer formation of joint brazed using Au58.7Ni₃6.5V4.8 is 329.5 kJ/mol. But when 5 at.% Pd is added into this filler alloy, the activation energy of reaction layer formation reduces to 255.9 kJ /mol, which indicates that the addition of Pd not only improve the melting point of filler alloy, but also change the activation energy of interfacial reaction between V and Si₃N₄, which leads to the effective promotion of interfacial reaction.