Effect of Aging on Microstructure and Mechanical Strength of Reactive Air Brazed BSCF/AISI 314‐Joints

The perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) is a mixed ionic electronic conductive ceramic and can be used as oxygen transport membrane (OTM) material for oxygen supply in high‐temperature applications. For BSCF, reactive brazing in air atmosphere (RAB) is the only technical applicable solution for a heat resistant and gas tight sealing to metals. As a consequence of brazing BSCF to the heat resistant austenitic steel AISI 314 a change in microstructure in the BSCF appears at the interface due to reactions with the copper oxide (CuO) from the AgCu braze alloy. After aging for 500 h at 850 °C in air atmosphere, a further change in the microstructure appears, which is characterized by an additional increase of porosity in the interface area between ceramic and braze. Also, a needle shaped phase at the grain boundaries in the ceramic can be observed. This kind of structure is known as product phase of a transformation of cubic BSCF into hexagonal perovskite. Mechanical tests with the BSCF/AISI 314‐joints show a decrease of the room temperature strength after aging.