Microstructure and interfacial morphologies of brazed NiO-YSZ/316 stainless steel using B-Ni2 brazing alloy effect of joint loading during Brazing

Abstract

Joining of NiO-YSZ cermet with 316 stainless steel was carried out using B-Ni2 brazing alloy (7 wt.-% Cr, 4.5 wt.-% Si, 3 wt.-% Fe, 3 wt.-% B, 0.06 wt.-% C, 0.02 wt.-% P, Ni-balance, m. p. 972-999 °C), and moderate loading was applied during brazing normal to the joint surface to realize a reliable sealing of the NiO-YSZ cermet anode/316 stainless steel interconnect structure in a SOFC. In the present research, interfacial (chemical) reactions during brazing at the NiO-YSZ/316 stainless steel interconnect were also promoted by addition of an electroless nickel plate to NiO-YSZ cermet as a coating with relevant process variables and procedures optimized during the electroless nickel plating. A special fixture for static loading was designed for filler metal alloy strips to be pre-placed at the joint surface so that during melting of the brazing filler alloy the liquid alloy layer would be perpendicular to the static load direction during brazing. Brazing was performed in a cold-wall vacuum furnace at 1080 °C. Post-brazing examination of interfacial morphologies between NiO-YSZ cermet and 316 stainless steel was performed using the SEM and EDS procedures. The results indicate that the B-Ni2 brazing filler alloy was fused fully during brazing and a continuous interfacial layer formation and the joint integrity were significantly affected by applied static loading during brazing. The positive loading effect was possibly attributed to the rectification of the joints under load by enhanced diffusion of Fe and Ni atoms from 316 stainless steel to B-Ni2 brazing filler alloy and the consequent increase in the fraction of Fe- and Ni-rich intermetallic phases in the joint. The thickness of the interfacial area, however, remained almost constant at 100 μm before and after brazing. © Carl Hanser Verlag.