The Microstructure And Properities Of The Joint Of Si₃N₄ Ceramics By Au-Ni-V Brazing Alloy

Silicon nitride is one of the most promising structural ceramics for high temperature applications. It is extremely important in both theory and practice to investigate the bonding of the Si3N4 ceramic. In this paper Si3N4 Ceramic was brazed using Au-Ni-V active filler alloy, the effect of joining parameters on the microstructure and properities of the joint was investigated, the distribution of element and reactants in the joint was analysed by means of SEM, EDS and XRD. The brazing mechanism of the interface was discussed.The Au-Ni-V filler alloy was used for brazing the Si3N4 ceramic, a realiable joint could be obtained. The joint was composed of reaction layer (V2N) near to Si3N4 ceramic, the Au[Ni] solid solution in the midst of brazing seam, and the Ni[Si,V,Au] solid solution which distributes uniformly in the Au[Ni] solution. New reactants of Ni3Si were formed when the brazing temperature was higher or the holding time was longer.With increasing the brazing temperature, the V2N reaction layer became much thicker, the Ni [Si,V,Au] solid solution distributed more symmetrically, and the bonding strength of joint increased. The bending strength of the joint was up to 222.4MPa when the brazing temperature is 1423K. With further increasing the brazing temperature, the reaction layer did not change anymore, and more and more element Si diffused into the midst of brazing seam. Intermetallic compounds of Ni3Si were formed by the reaction between Si and Ni, leading to the decrease of the bending strength of joint.As the bonding time increased, the thickness of the reaction layer increased, and the Ni[Si,V,Au] solid solution distributed more symmetrically. The bending strength of the joint increased with increase of the holding time. However, when the bonding time was too long, reaction products (Ni3Si) appeared, and the bending strength of joint was reduced.With the V content changed from 4.76 at.% to 10 at.%, the wetting ability of filler alloy for ceramic was improved, thus the bending strength of joint increased.The diffusion of V could be improved effectively by changing the placement of the solder, meanwhile, microstructure of the joint was improved, and bonding rate and properties of the joint was increased. The maximum bending strength, 255MPa, was obtained when the brazing temperature and holding time are 1423K and 60min, respectively.Fracture sureface observation showed that the majority of bonding joints was fractured in the ceramic near the joining interface. The main reason of the fracture 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. The residual stress promoted the initiation and propagation of cracks in the ceramic...