Brazing Al2O3 to Kovar Alloy with Ni/Ti/Ni Interlayer and Dramatic Increasing of Joint Strength after Thermal Cycles

Abstract

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 995oC 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 Ni2Ti4O, a complex oxide with structure similar to Ti2Ni, 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 200oC-600oC 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.