Joining Si₃N₄ ceramic to Invar using Mo mesh and Cu foil interlayer

Si₃N₄/Invar joints with Mo mesh and Cu foil as interlayers, individually and together, were vacuum brazed using Ag27.5Cu4.5Ti active filler at 1153 K for 10 min with following configurations: Si₃N₄/Ag26.7Cu4.5Ti + Mo/Invar, and Si₃N₄/Ag26.7Cu4.5Ti + Mo/Cu/Ag–Cu/Invar. The correlation between the microstructure of the brazing seam and shear strength of the joints are investigated. Ti segregated at Si₃N₄/filler interface and metallurgical bonding is established by formation of TiN and Ti₅Si₃ reaction products. Vickers microharndess profiles unveiled varying degrees of hardness across the brazing seam, resembling the layered structure. Individually introducing Mo mesh as interlayer suppressed the Fe₂Ti and Ni₃Ti intermetallic compounds (IMC) within the brazing seam, resulting in shear strength of 191.7 MPa. Hybrid combination of Mo mesh with Cu foil as interlayers, yielded 235.5 MPa shear strength in brazed Si₃N₄/Invar joints. This improvement is attributed to simultaneous favorable effects of Mo such as reduced residual stress near the Si₃N₄/filler interface and Cu, which prevents Fe₂Ti and Ni₃Ti IMC in the brazing seam, thereby enhancing the plastic flow.