Study On Brazing Process And Mechanism Of Si₃n₄ Ceramic Joint Brazed By Au-ni-v-pd High-temperature Filler Alloy

Silicon nitride is one of the most promising structural ceramics for high temperature applications. It is extremely important to investigate the bonding of the Si3N4 ceramic in both theory and practice. In this paper, Si3N4 ceramic was brazed using Au-Ni-V-Pd active filler alloy. The effect of joining parameters on the microstructure and mechanical properties of the joint was investigated by the SEM, EDS, XRD and TEM. And then the brazing mechanism of the interface was discussed.A reliable joint was obtained by brazing the Si3N4 ceramic using Au-Ni-V-Pd filler alloy. According to the analysis of microstructure, the joint consists of the Si3N4 ceramic substrates; the filler alloy composed of Au-based solid solution and Ni-rich phase which distributes uniformly in the Au-based solution; the VN reaction layer between Si3N4 ceramic and filler alloy., With increasing the brazing temperature in a certain temperature range (1323K-1423K), the thickness of VN reaction increased, the Ni[Si,V] solid solution distributed homogenously increased in size, and the bonding strength of joint increased as well. But when the temperature too high (1473K), the thickness of VN reaction layer and the bonding strength of joint both decreased. With increasing the bonding time, the thickness of the VN reaction layer and the size of Ni[Si,V] solid solution increased, and the bending strength of the joint increased as well. However, when the bonding time was too long (90min), the intermetallic compound Ni3Si appeared in the seam, and the bending strength of joint was reduced. In addition, when the brazing temperature is higher or the holding time is longer, the width of seam decreasedWith the V or Pd content increased appropriately, the wetting ability of filler alloy on ceramic was improved, and the interfacial reaction can be improved effectively. But when the composition of V or Pd was too high, the bending strength of joint decreased. The bending strength of the joint can reach 264.4MPa at room temperature, which was brazed with Au54.15Ni36.09V4.76Pd5 filer alloy at the brazing temperature of 1423K for 60min. The bending strength also can reach 221.2MPa at 973K.Fracture surface of the joint shows that the majority of bonding joints was fractured in the ceramic near to the Si3N4 ceramic/filler alloy interface. The main reason is the difference of thermal expansion coefficient between the ceramic and filler metals, resulting in a higher the residual stress in the ceramic during cooling from brazing temperature.