Mechanical Properties of Reactive Air Brazed Ceramic-Metal Joints for SOFC

Solid oxide fuel cells (SOFC) consist of ceramic and metallic components which have to be joined and sealed for operation at high temperatures up to 850°C in dual atmospheres. The mechanical integrity of the sealants and their long term stability under operating conditions is a basic requirement for a reliable operation of SOFC stacks. In this respect the use of metallic brazes is considered to have advantages compared to the widely used brittle glasses or glass ceramics. In this study the mechanical properties and the long term stability of ceramic-metal joints are investigated. Fracture experiments are carried out to characterize the 4-point bending strength and failure mechanisms of silver-based reactive air brazes. Characterization of fracture surfaces with SEM reveals details of fracture origin and crack propagation. The cracks in the interfacial layers between braze/ceramic and braze/steel are identified as failure reason for tested samples. The inductive brazed 4-point bending specimens showed higher strength than conventional furnace brazed samples due to thinner interfacial layers. It is shown, that during annealing in air at 850°C the 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.