Microstructure and bond strength of Ni-Cr steel/Si3 N4 joint brazed with Ag-Cu-Zr alloy

Abstract

The interfacial microstructure and bond strength were examined for a Ni-Cr steel/Si3 N4 joint brazed using an Ag-Cu eutectic alloy containing 5 wt-%Zr. The reaction of Si₃N₄ with the brazing alloy formed a very thin ZrN layer with a cubic structure (a = 4.577 Å) at the interface. No Zr silicide (Zr₅Si₃) was present at the interface even though its formation is thermodynamically possible. The reaction product did not contain any of the dilute ceramic phases or intermetallics which are commonly seen in other active metal brazing systems. This strongly implies that the elements of the Ni-Cr steel and Ag or Cu in the brazing alloy, did not participate in the interfacial reaction. The shear strength of the joint was strongly dependent on the thickness of the reaction layer and the morphology of CuZr₂ precipitates in the brazement. A joint with a reaction layer thickness of 0.5 μm, which was formed by brazing at 950°C for 30 min, showed the highest fracture shear strength (∼ 202 MPa).