Effect of Cu interlayer with different thickness on microstructure and properties of Al₂O₃/Kovar joints brazed with Cu–Ti/Cu/BNi-2 composite filler

In this study, a Cu–Ti/Cu/BNi-2 composite filler was employed to braze Al₂O₃ ceramics and Kovar alloy, aiming to prevent excessive reaction between active Ti and Kovar alloy. The investigation focused on the effects of varying Cu interlayer thickness on microstructure and mechanical properties. The results show that the typical microstructure of the joint is Al₂O₃/Ti–O/Cu₃Ti₃O /Ti-rich + Cu(s, s)/Ni₃Ti + TiB₂/CrB + Ni(s, s)/Kovar alloy. Notably, when the thickness of the Cu layer was 100 μm, the joint strength significantly increased to 128 ± 7 MPa, which represented a remarkable 50.5% improvement compared to the Cu–Ti filler. This enhancement in joint strength can be attributed to the addition of Cu/BNi-2, effectively preventing direct contact between Ti and Kovar alloy, thereby inhibiting their reaction and reducing the formation of brittle Fe-Ti phases within the joint. At the same time, Cu foil also contributed to alleviating the residual stress of the joint. However, as the Cu foil thickness increased, the fracture of the Cu₃Ti₃O reaction layer worsened, resulting in weaker bonding with ceramics and a decrease in joint strength. The study also discussed the microstructure evolution of the Al₂O₃/Cu–Ti/Cu/BNi-2/Kovar joint. This study provided an idea for the application of Cu-based filler.