Alumina fiber-reinforced alumina ceramic matrix composites (Al₂O_3f/Al₂O₃ CMCs) are strategic for extreme-environment applications, motivating the development of compatible joining methods. In this work, the reactive air brazing (RAB) of Al₂O_3f/Al₂O₃ CMC using Ag-4Nb₂O₅-xTiO₂ (0 ≤ x ≤ 4, in mol%) fillers was performed, and the microstructure evolution, interfacial behavior, and mechanical properties of brazed joints were systematically investigated. The main interfacial reaction product evolves from AgNbO₃ to AlNbO₄ with the TiO₂ content increasing, while the Ag interlayer gradually becomes discontinuous. When brazed with Ag-4Nb₂O₅-2TiO₂, the CMC/Al₂O₃ butt joint exhibits higher average shear strength (45.3 MPa) among the joints tested, outperforming the CMC/CMC butt joint, CMC/CMC flat joint, and CMC/GH3536 butt joint, with shear strengths of 38.1, 7.6, and 27.7 MPa. The fracture behavior of these joints is governed by multiple factors, namely interfacial features (the AlNbO₄ reaction layer, interlocking structure, and pores) and elemental diffusion from GH3536 alloy.

We firstly performed the reactive air wetting and brazing of Al₂O₃ ceramics using Ag–(0.5‒12)Nb₂O₅ fillers, where Nb₂O₅ can react with liquid Ag and O₂ from air to generate AgNbO₃. The contact angle of the Ag–Nb₂O₅/Al₂O₃ system almost linearly decreases from ~71.6° to 32.5° with the Nb₂O₅ content increasing, and the joint shear strength reaches the maximum of ~65.1 MPa while employing the Ag–4Nb₂O₅ filler, which are mainly related to the formation and distribution of the AgNbO₃ phase at the interface. Moreover, the interfacial bonding and electronic properties of related interfaces were investigated by first-principles calculations. The calculated works of adhesion (W_a) of Ag(111)/Ag–O–AgNbO₃(001) and AgNbO₃(001)/Al₂O₃(100) interfaces are higher than that of the Ag(111)/Al₂O₃(110) interface, indicating good reliability of the Ag/AgNbO₃/Al₂O₃ structure. The relatively large interfacial charge transfer indicates the formation of Ag–Ag, Al–O, and Ag–O bonds in the Ag/AgNbO₃/Al₂O₃ structure, which can contribute to the interfacial bonding.