Fracture Mechanics Experiments to Characterize the Failure of Ceramic/Metal Joints in SOFC Stacks

Mechanical integrity of the sealants in planar SOFC stacks is a key prerequisite for reliable operation. In this respect joining with metals rather than brittle glass-ceramics is considered to have advantages. Hence, as one of the joining solutions for SOFCs of planar design the reactive air brazing of the ceramic cell into a metallic frame has gained increasing interest. Fracture experiments were carried out to characterize the fracture energy and 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. In-situ observation with optical and SEM resolution revealed preferential delamination crack propagation near the steel/braze interface. The influence of brazing parameters and associated interfacial reactions on the crack path location is addressed. Discussion of the results focuses in particular on the role of oxide scale formation.