Low Temperature Solid-State Bonding of Nickel and Tin with Formic Acid Surface Modifications

The effect of formic acid surface modification process on the bond strength of the solid-state bonded interface of tin and nickel was investigated by SEM observations of fractured surfaces and interfacial microstructures. Tin and nickel surfaces were modified by boiling in formic acid for 600 s. Diffusion bonding was performed at bonding temperatures of 393-493 K under a load of 7 MPa (bonding time for 1.8 ks). The bond strength increased with increasing bonding temperature irrespective of surface modification process. As a result of surface modification process, bonded joints were obtained at a bonding temperature 70K lower than that required for non-modified surfaces used, and the bond strength was comparable to that of the base metal. When the surface modification process was not applied, the fracture mode was brittle. As the bond strength increased with increasing bonding temperature, the fracture mode changed to ductile. With surface modification process, this tendency was observed at a bonding temperature 70 K lower than without it. The results suggest that these changes in the fractured surface between tin and nickel were accompanied by expansion of the metal-to-metal contact area, which contributed to the increase in bond strength.