Study Of Microstructure And Properties Of Si₃N₄/40CrMo Joint Brazed By Active Brazing Alloy

Joining of ceramics is one of the key technologies for their practical applications. In this paper the AgCuTiPd filler alloys were used for joining Si₃N₄ ceramic and 40CrMo. The influence of brazing parameters and the content of Pd on microstructure and properties of the brazing joints was studied. Si₃N₄/40CrMo interfacial reaction and the effect of the interfacial microstructure on the joining performance was observed and analyzed by means of SEM, XRD and EMPA. The fracture type of the joining was confirmed by analyzing the fracture surfaces.The experimental results showed that a reliable Si₃N₄/40CrMo joint could be obtained by adopting AgCuTi-Pd5% (at.%) as filler meter. When the sample was brazed at 1193K for 30 min, the three-point bending strength of the joint was 352.3MPa. In the joint, there was a TiN reaction layer close to the ceramic and a FeTi and Fe₂Ti mixed reaction layer close to the 40CrMo. The middle part of the joint was an eutectic composed of Ag-riched and Cu-riched solutions. CuPdTi ternary compounds were distributed among the eutectic. The thickness of the FeTi and Fe₂Ti mixed reaction layer incresed with the increase of brazing temperature and the holding time. When the brazing temperature increased, the small fluid-shape Ag-riched and Cu-riched solution transformed into the massive solution, and a new phase of FeSiTi ternary compound was found when the brazing temperature exceeded 1243K. The results of fracture surface indicated that when the content of Pd in the filler alloy was 5%, the joint failed at the location closed to the ceramics where the maximum residual stress existed, and in this case the bending strength of the joint is higher. When the content of Pd in the filler alloy was 20%, the joint failed in the seam, resulting in a lower bending strength.