Reactive brazing of silicon nitride to Invar alloy using Ni foam and AgCuTi intermediate layers

Silicon nitride (Si ₃ N ₄ ) ceramic and Invar alloy have been brazed by using AgCuTi active filler and the Ni foam was added to further improve mechanical properties of joints in this study. The microstructure of Si ₃ N ₄ /Invar brazed joint changed obviously after adding Ni foam with different thickness. Ni foam reacted with the AgCuTi active filler during brazing, but it did not completely disappear and still maintained the basic frame structure after brazing. The average shear strength of the brazed joints with 0.2 mm Ni foam could reach 180 MPa, and their thermal cycle lifetime also improved significantly. The addition of Ni foam shifted the fracture location of joints from Si ₃ N ₄ ceramic to brazing seam. These results indicated that the Ni foam could act as a buffer layer to reduce the residual thermal stress, and improve the mechanical properties of Si ₃ N ₄ /Invar joint.