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Innovative technologies such as solid oxide fuel cells for energy generation make high demands on materials that can often be met by embedding ceramic components into metallic structures. A promising and economic method of joining ceramic to metal or to ceramic is reactive air brazing (RAB). A disadvantage of the RAB joints is the high porosity which can lessen the strength of the joint as well as its gas tightness. Different stages of pore development in dependence of filler and base materials could be observed during the brazing process due to the vaporising of the organic binder, the melting process and a reaction between the filler metal and the base material. By adjusting the process parameters the pore formation can be decreased. [ABSTRACT FROM AUTHOR]

Reactive air brazing (RAB) is a promising and economic method for joining ceramic to metal as well as ceramic to ceramic. Hybrid joints gain importance in the field of challenging applications such as gas separation or solid oxide fuel cells. While the ceramic partner is often directly chosen due to its functionality, the metallic partner needs to be chosen carefully in order to avoid high residual stresses due to the differences in the thermal expansion coefficients. This leads to the restriction of the usage of a significant number of potential joining partners. Ceramic/metals components with unadapted thermal expansion behavior (Ba0.5Sr0.5Co0.8Fe0.2O3-δ/Crofer 22 H and 3YSZ/AISI 314) were joined by RAB. Cu-containing as well as Cu-free Ag-based filler metals were used. Analysis of the cross-section of brazed samples showed that the microstructure of the samples depends especially on the used filler metal. The melting process as well as the exothermic reactions leading to a distinct reaction zone could be observed in the differential scanning calorimetric measurements. While the microstructure of the joints is not significantly influenced by the base material, the strength of the BSCF/Crofer 22 H specimens is nearly four times lower than the strength of the specimens with the adapted thermal expansion coefficients (BSCF/AISI 314 and 3YSZ/Crofer 22 H). The reasons are residual stresses caused by the different thermal expansion coefficients. Finite element simulations assuming a visco-plastic material model show that an amount of stress can be reduced by relaxation within the silver based braze. But the volume of the high residual stresses is still larger in the BSCF/Crofer 22 H joints than that of the BSCF/AISI 314 joint resulting in a lower fracture strength. [ABSTRACT FROM AUTHOR]