Single-Step Induction Brazing of Ceramic–Metal Hybrid Joints: Tailoring Zirconium Content for Microstructure and Mechanical Performance

Dissimilar alumina-to-copper joints are key enabling structures for hermetic ceramic–metal feedthroughs used in vacuum-electronic and X-ray-tube assemblies, yet the large mismatch in thermal expansion and the poor wettability of oxide ceramics by molten braze alloys make reliable joining difficult. In this work, the synergistic effect of zirconium added to a multicomponent Ag–Cu–Ti–Sn active filler metal was investigated by single-step induction brazing of Al2O3 to Cu. Three filler metals, Ag–Cu–Ti–Sn, Ag–Cu–Ti–Sn–3.5Zr and Ag–Cu–Ti–Sn–5.1Zr (wt.%), were produced by vacuum arc remelting, cold-rolled to foil and used to braze the joints for 15 min under a vacuum of 10−6 mbar at 840, 860 and 880 °C, respectively. The microstructure and interfacial reaction products were characterised by optical and scanning electron microscopy with energy-dispersive spectroscopy, while X-ray diffraction was performed on the filler metal foils to identify their crystalline phases. The mechanical response was assessed through shear strength, Vickers microhardness, and fractographic analysis. Every joint developed a brazing zone and a continuous reaction layer. With the Zr-free filler, the reaction layer comprised TiO and Cu3Ti3O, whereas both Zr-bearing fillers produced a TiO-plus-ZrO2 reaction layer, with Ti reacting closer to the alumina interface than Zr, consistent with the more negative Gibbs free energy of TiO formation. The 3.5 wt.% Zr condition, which combined the thickest brazing zone with the thinnest reaction layer, gave the highest mean shear strength at 14 ± 2.5 MPa, about 56% above the Zr-free condition at 9 ± 1.5 MPa, while the 5.1 wt.% Zr condition, with a thicker ZrO2 reaction layer, gave 12 ± 2.0 MPa. With three specimens per condition these differences approach but do not reach significance at the 5% level (one-way ANOVA, p = 0.062), and the ranking is supported by the accompanying microstructural and fractographic evidence. The reaction-layer microhardness increased monotonically with Zr content, from 146 HV (Zr-free) to 248 HV (3.5Zr) and 317 HV (5.1Zr). All joints failed in a quasi-brittle manner along the reaction layer and the adjacent alumina. Because Zr content, brazing temperature and foil thickness varied together, the results identify the 3.5 wt.% Zr condition as the best performing of those investigated rather than an intrinsic optimum Zr concentration, and they clarify the competing roles of reaction-layer chemistry and thickness in Zr-modified active filler metals.