Bismuth borate zinc glass braze for bonding sapphire in air

Reliable brazing of sapphire was successfully achieved by using a novel 50Bi₂O₃–40B₂O₃–10ZnO (mol%) glass braze and joining cycles that peak at temperatures of 600 − 700 °C for 20 min in air. The effect of the brazing temperature on microstructure evolution as well as mechanical properties of the sapphire/sapphire joints was investigated. The results showed that Bi₄B₂O₉ and Bi₂₄B₂O₃₉ were formed in the joints brazed at 600 and 625 °C, respectively. ZnAl₂O₄ particles were formed in the joints when the brazing temperature was 635 °C due to a reaction between the glass and the sapphire substrate. With the brazing temperature increasing, ZnAl₂O₄ particles exhibited steady growth, from ~ 160 nm at 635 °C to ~ 455 nm at 700 °C. Besides, ZnAl₂O₄ particles began to aggregate when the temperature was 675 °C. The strength increased sharply when the brazing temperature increased to 635 °C, which was mainly attributed to the formation of ZnAl₂O₄ particles in the joint domain. The maximum shear strength of 85 MPa was obtained for the joints brazed at 650 °C for 20 min. The joint shear strength decreased due to the aggregation of ZnAl₂O₄ particles in the joint.