Braze oxidation behavior and joint microstructure in YSZ/steel joints made using palladium brazes for SOFC applications

Two palladium-base brazes (Palco and Palni), and two silver-base brazes containing palladium as an alloying element (Palcusil-10 and Palcusil-15) were evaluated for oxidation resistance at 750 °C, and used in joining yttria-stabilized zirconia (YSZ) to a corrosion-resistant ferritic stainless steel for possible use in solid oxide fuel cell (SOFC) interconnects. Thermogravimetric analysis (TGA), and optical and scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS) were used to evaluate the braze oxidation behavior and the microstructure of the YSZ/braze/steel joints. Knoop microhardness testing was used to develop hardness profiles across the joint region. The TGA results showed that Palco and Palni exhibited the fastest and the slowest oxidation kinetics, respectively, with Palcusil-10 and Palcusil-15 exhibiting oxidation kinetics intermediate between Palco and Palni. The EDS analysis revealed the dissolution of YSZ and steel constituents in braze, and of braze constituents in the YSZ and steel, which led to metallurgically sound joints. The microhardness profiles exhibited a sharper discontinuity at the YSZ/braze interface than at the steel/braze interface, with a peak hardness of ∼1000 KHN in YSZ and 150–180 KHN in the steel substrate.