Interfacial characterization of YSZ-to-steel joints with Ag–Cu–Pd interlayers for solid oxide fuel cell applications

Yttria-stabilized zirconia (YSZ)/stainless steel joints made using two commercial silver-based interlayers containing palladium (58Ag–32Cu–10Pd and 65Ag–20Cu–15Pd), were systematically analyzed for the microstructures of the interlayer matrices, interlayer–steel interface, and interlayer–YSZ interface using scanning electron microscopy (SEM) and transmission electron microscopy coupled with energy dispersive spectroscopy (TEM/EDS). In the interlayer matrix, a face-centered cubic (FCC) Cu-rich phase formed in the vicinity of the YSZ and dissolved a significant amount of Zr from the ZrO2 and minor amounts of Fe and Cr from the steel. The Cu-rich phase in the interlayer matrix in the vicinity of the steel substrate was the ordered phase Cu3Pd with antiphase boundaries (APBs) and the L12 crystal structure. Silver particles precipitated within the Cu3Pd phase in 58Ag–32Cu–10Pd; a Fe(Cr) needle-like phase, instead of Ag particles, precipitated within the Cu3Pd phase in 65Ag–20Cu–15Pd. Although no reaction products at the interlayer–steel interface were found, dislocations appeared within the Ag- and Cu-rich phases. At the interlayer–YSZ interface, two reaction products, SiO2 (impurity in YSZ) and Ti3O5 (from the reaction of YSZ with Ti impurities in the steel) were observed. Diffusion and chemical reactions led to the compositional changes and interface reconstruction, thereby yielding metallurgically sound YSZ/steel joints.