Mechanical properties of reactive air brazed ceramic-metal joints for SOFC

The mechanical integrity of sealants in solid oxide fuel cells (SOFC) and their long-term stability under operating conditions are a basic requirement for a reliable operation of SOFC stacks. In this respect the use of metallic brazes is considered to have advantages in comparison to the widely used brittle glasses or glass ceramics. In this study the mechanical properties and the long-term stability of ceramic-metal joints prepared by induction heating are investigated. Fracture experiments are carried out to characterize the bending strength and failure mechanisms of silver-based reactive air brazes. Due to thinner interfacial layers the induction brazed 4-point bending specimens showed higher strength than samples brazed in conventional furnaces. Characterization of fracture surfaces wi th SEM reveals details of fracture origin and crack propagation. The cracks in the interfacial layers between braze/ceramic and braze/metal are identified as failure reason for tested samples. It is shown that during annealing in air at 850°C the 4-point bending strength decreases with time while the thickness of the oxide reaction layers increases. Failure mechanisms and possibilities for improvements of the reactive air brazing technology are discussed.