Reactive air brazing of YSZ-electrolyte and Al₂O₃-substrate for gas sensor sealing: Interfacial microstructure and mechanical properties

A YSZ/Al₂O₃ layered structure was successfully reactive air brazed using Ag–CuO fillers. The interaction mechanism between the filler and ceramics was systemically analyzed. The effects of filler composition on the interfacial microstructure and joint properties were investigated. Ag–CuO fillers possessed good wettability on the YSZ and Al₂O₃ ceramics. Ag had diffused into the ceramic substrates, but the width of the diffusion layer at the YSZ interface (∼10 nm) was narrower than that at the Al₂O₃ substrate (∼20 nm). Further HRTEM observations confirmed interatomic bonding between the CuO and the ceramics. The lattice mismatches at the Al₂O₃/CuO and YSZ/CuO interfaces were 0.98% and 2.97%, respectively. The filler alloy composition had a major influence on the microstructure and shear strength. The CuO phase preferentially precipitated at the Al₂O₃ interface. Consequently, the continuous CuO and CuAl₂O₄ layers gradually formed at the Al₂O₃ interface with increasing CuO content in the filler. The maximum shear strength (∼45 MPa) was obtained when the composition was Ag-8mol%CuO.