Joining of Co coated ferritic stainless steel to ceramic solid oxide cells by a novel Ag-SiO braze

A novel Ag-SiO braze for Solid Oxide Cells was successfully developed and applied to join Co coated AISI 441 ferritic stainless steel to YSZ-NiO half cells. The braze showed an improved wettability on the coated steel and the YSZ compared to pure Ag. A defect-free steel/YSZ joint was obtained using an Ag-1SiO braze jointed at 1000 °C for 30 min in air. The microstructure of the joints was characterized by scanning electron microscopy and transmission electron microscopy with energy-dispersive X-ray spectroscopy. Furthermore, a Co-Si-O phase diagram was calculated and used to explain the experimental results and formulate a reaction mechanism. It was found, that during the air brazing process, CoO and (Co, Fe)O are formed by the oxidation of the prefabricated Co coating. The growth of CrO was effectively impeded by the formed Co-based spinels. In a second step, CoO reacts with the SiO present in braze. The CoSiO formed by the reaction ensures a firm bonding between the steel and Ag-SiO braze.