Abstract
• The wettability of Ag-SiO 2 braze on the YSZ and Co coated steel surface was studied. • Defect-free steel/YSZ joints was achieved and its microstructure was characterized in detail. • The Co-Si-O 2 phase diagram was calculated to explain the interfacial reaction. • A physical model was proposed to describe the interfacial microstructure evolution. A novel Ag-SiO 2 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 2 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 2 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, Co 3 O 4 and (Co, Fe) 3 O 4 are formed by the oxidation of the prefabricated Co coating. The growth of Cr 2 O 3 was effectively impeded by the formed Co-based spinels. In a second step, Co 3 O 4 reacts with the SiO 2 present in braze. The Co 2 SiO 4 formed by the reaction ensures a firm bonding between the steel and Ag-SiO 2 braze.
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