Effect of isothermal aging on the mechanical performance of brazed ceramic/metal joints for planar SOFC-stacks

One of the key requirements for the application of planar solid oxide fuel cells is the hermetical sealing of ceramic and metallic stack-components. Gas tightness and associated mechanical integrity of the sealants are therefore key prerequisites for reliable operation over the lifetime of the stack. To withstand mechanical stresses generated by the differences in thermal expansion of the involved materials as well as non-equilibrium thermal conditions upon heating and cooling, joining with metals rather than brittle glass-ceramics is considered to be advantageous. Hence, as one of the joining possibilities for SOFCs of planar design reactive air brazing of the ceramic cell into a metallic frame has gained increasing interest and proofed its suitability in several mid term full scale stack experiments at Forschungszentrum Jülich. Fracture experiments were carried out to characterize the fracture energy and associated failure mechanisms of silver-based reactive air brazes, used for joining the zirconia electrolyte of an anode supported planar cell with a metallic Crofer22APU frame. The specimens were mechanically tested in notched beam bending geometry at room temperature. Mechanical testing was accompanied by in-situ observation with optical and SEM resolution. Discussion of the results focuses on the influence of isothermal aging and associated interfacial reactions on crack path location and mechanical characteristics. State of the art CuO-containing brazes turned out to be outbalanced by a newly conceived CuO-free braze. The new braze did not only display superior delamination resistance in the as-brazed state, combined with an additional pre-oxidation treatment of the steel but also showed improved degradation resistance against isothermal aging.