Microstructure evolution and mechanical properties of Co coated AISI 441 ferritic stainless steel/ YSZ reactive air brazed joint

This study investigates the microstructure evolution and mechanical properties of bare and Co coated AISI 441 ferritic stainless steel/YSZ ceramic reactive air brazed joints achieved by Ag–CuO braze for Solid Oxide Fuel/Electrolysis Cells applications. Interfacial microstructure of steel/YSZ joints is analyzed by SEM and TEM with EDS. A thick and porous oxide layer rich in Fe, Cr, and Cu is found in bare steel/YSZ ceramic joints, which is induced by the severe oxidation and its intense reaction with CuO during the brazing process. For the coated steel/YSZ ceramic joint, a comparably dense and uniform (Co, Fe, Cu)₃O₄ spinel layer is formed on the steel surface, which is tightly bonded with Ag–CuO braze without visible bonding defects. Meanwhile, the oxidation of steel substrates and its interaction with CuO is significantly suppressed. Co coated steel/YSZ joints possess reliable mechanical properties with the shear strength of 51 MPa, which is 54.5% higher than that of bare steel/YSZ ceramic joints (33 MPa). Besides, the microstructure evolution of coated steel/YSZ ceramic joints during brazing is schematically illustrated by a physical model.