Interfacial behavior in the brazing of silicon nitride joint using a Nb-foil interlayer

Silicon nitride (Si3N4) samples prepared the spark plasma sintering (SPS) technique, which had different amounts of oxide additives, were used as disc-preforms. The surfaces of the materials to be bonded during the brazing process (ceramic, metal and filler alloys) were previously coated with thin layer of silver and then stacked together with these preforms. Sandwich-like specimens of Si3N4/Cu–Zn/Nb/Cu–Zn/AISI-304 combinations were joined at 1000 °C using 5, 20, and 40 min holding times under an inert atmosphere. Analysis by scanning electron microscopy revealed un-joined zones between the ceramic and metallic parts after 5 min of treatment. For holding times >20 min, homogenous and non-porous Si3N4/Cu–Zn/Nb interfaces were obtained. The thicknesses of the resulting ceramic–metal interfaces increased from ∼10 to >25 μm as the holding time was increased. The amount of additives used during the preparation of the Si3N4 ceramics had a direct effect on the decomposition rate of Si3N4 during the joining process. The largest decomposition of Si3N4 was observed at 1000 °C/40 min from the less dense ceramic preforms (4 wt% of additives in this case), which in turn induced the migration of Si atoms through the interface to promote the formation of Si-based components. In contrast, when using larger amounts of additives (8 wt%) during sintering of the ceramic performs, it becomes more difficult for N and Si to dissociate upon brazing. When the diffusion rate of Si is low, it migrates toward the metal part, which limits the formation of Si-based components.