Brazing Al₂O₃ to Kovar alloy with Ni/Ti/Ni interlayer and dramatic increasing of joint strength after thermal cycles

High purity alumina ceramic and Kovar alloy were brazed by Nickel and Titanium foils stacked as Ni/Ti/Ni layer structure. Airtight joints were achieved with shear strength more than 30MPa, after brazing at 995°C for 30min. A sandwich structure was observed in solder, which means an a-Ti solid solution belt at mid part and Ti₂Ni intermetallics belts at both sides. The main reaction product at alumina/solder interface was Ni ₂Ti₄O, a complex oxide with structure similar to Ti ₂Ni, which is the bonding agent and transition from the solder's metallic crystal lattice to alumina crystal lattice. To simulate the actual serving conditions, a thermal cyclic experiment was undertaken at the 200°C-600°C temperature range. The results showed that shear strength of joint increased dramatically after thermal cycles. This interesting phenomenon is attributed to the annealing effects of thermal cycles, which released residual stress in brazed joints remarkably.