Joining of Co coated ferritic stainless steel to ceramic solid oxide cells by a novel Ag-SiO2 braze

A novel Ag-SiO2 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-1SiO2 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-O2 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, Co3O4 and (Co, Fe)3O4 are formed by the oxidation of the prefabricated Co coating. The growth of Cr2O3 was effectively impeded by the formed Co-based spinels. In a second step, Co3O4 reacts with the SiO2 present in braze. The Co2SiO4 formed by the reaction ensures a firm bonding between the steel and Ag-SiO2 braze.