Microstructural and Mechanical Characterizations of Mo/W and Mo/Graphite Joints with BNi2 Paste

Abstract

Brazing of Mo to W and to graphite is achieved using BNi2 paste (containing Ni, Cr, Si, and B). For the Mo/W or Mo/graphite joint, the joining area consists of a diffusion area and a brazing area. The diffusion area is composed of MoNi and Mo, which is formed by diffusion of the Mo substrate into the braze during brazing. The brazing area of the Mo/W joint contains Ni(ss) (solid solution), Cr(ss), Ni3B, CrB, and Ni4W, while the brazing area of the Mo/graphite joint mainly comprises Ni(ss), MoNi, Ni3B, and CrB. A continuous chromium carbide layer is formed at the brazing area/graphite interface in the Mo/graphite joint due to the reaction of Cr in the BNi2 braze with the graphite. Nanoindentation measurements of the joints show that the diffusion area exhibits the highest hardness and elastic modulus in the joints. The shear strengths of the Mo/W and Mo/graphite joints are 58.1 ± 16.0 and 13.0 ± 4.0 MPa, respectively. The Mo/W and Mo/graphite joints fracture after the shear tests in the W and graphite sides, respectively, near the joining area, indicating that both fractures are caused by the stress concentration in the corresponding areas.