Brazing of yttria-stabilized zirconia (YSZ) to stainless steel using Cu, Ag, and Ti-based brazes

Copper and silver-base active metal brazes containing Ti (Cu-ABA, Ticusil, and Ticuni) were tested for oxidation resistance to 750–850 C, and for their effectiveness in joining yttria-stabilized-zirconia (YSZ) to a corrosion-resistant ferritic stainless steel. The braze oxidation behavior was characterized using thermogravimetric analysis (TGA), optical and scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). Ticusil and Ticuni at 750 C exhibited sluggish oxidation kinetics whereas Copper-ABA at 850 C displayed the fastest kinetics and relatively large weight gain. The SEM and EDS examination of the steel/braze and YSZ/braze interfaces showed the dissolution of Y and Zr from YSZ in braze, diffusion of Ag in the YSZ, and formation of a thin Ti-rich interphase between YSZ and Ti-base brazes. These compositional changes and interface reconstruction yielded metallurgically sound joints. The Knoop microhardness profiles showed a rather abrupt discontinuity across the YSZ/braze interfaces and a more uniform distribution across the steel/braze interface