Secondary Phase Interaction at Interfaces of High-Strength Brazed Joints made using Liquid Phase Sintered Alumina Ceramics and Ag-Cu-Ti Braze Alloys

Abstract

Alumina-to-alumina brazed joints were formed using 96.0 and 99.7 wt.% Al₂O₃ ceramics using 150 µm thick Ticusil^® (68.8Ag-26.7Cu-4.7 wt.% Ti) braze preforms. Brazing was conducted in a vacuum of 1 × 10^-5 mbar at 850 °C for 10 minutes. Joint strengths were evaluated using four-point bend testing and were compared to the monolithic flexural strengths of standard alumina test bars according to ASTM C1161-13. Brazed joints made using 96.0 wt.% Al₂O₃ consistently outperformed brazed joints made using 99.7 wt.% Al₂O₃, despite similarities in both the flexural strengths of the standard alumina test bars and the microstructures of brazed joints. Secondary phase interaction led to the formation of Ti₅Si₃ reaction products at locations where the triple pocket grain boundaries of the 96.0 wt.% Al₂O₃ surface intersected the Ti-rich reaction layers. It is proposed that due to this interaction, brazed joints made using 96.0 wt.% Al₂O₃, which were relatively cost-effective to produce, achieved higher strengths than brazed joints made using 99.7 wt.% Al₂O₃.