Active Brazing of Single Crystal Al2O3 to Kovar Alloy with AgCuZr Filler Metal

Brazing provides high strength and hermetic joining between metal, ceramic, and composite materials. Origins of the brazing process trace back to 3000BC, yet the technique continues to maintain relevance and demand across aerospace, medical, electronic, optical, and mechanical engineering. Here, single crystal Al2O3 (sapphire) was joined to Kovar in a single step via an AgCuZr active braze filler metal. The microstructure of the completed joint was investigating using microscopy and spectroscopy techniques. A unique braze chamber, paired with an optically clear top substrate (sapphire), allowed further investigation into the dynamics of the active braze process via in situ video monitoring. The collected video data was used to select five quench points in the prescribed thermal cycle ramp. Each quenched sample underwent postmortem microscopy and spectroscopy analysis, making it possible to track microstructure evolution across the thermal cycle. In situ monitoring provided new insight into the reactions that occur in active braze joining, especially at the interface between the filler metal and Al2O3 substrate, and provides a method for real-time investigation of brazing parameters moving forward.