New design of sapphire joints brazed with bismuth-borate glass

Bismuth-borate glass, 50Bi₂O₃–50B₂O₃ (mol%), was prepared and employed to braze sapphire in air. Prior to the joining process, the structure and mechanical and thermal parameters of the glass were investigated. Additionally, the wettability of the glass on sapphire was found to be good. Subsequently, sapphire was successfully brazed at 600–750 °C with a dwelling time of 20 min. The microstructure evolution of the joint with respect to the temperature was investigated. The results showed that Al₄B₂O₉ whiskers were formed in situ in the joint as a result of the chemical reaction between the glass and sapphire. The size of the Al₄B₂O₉ whiskers increased with the temperature. The mechanical integrity of the joints was also studied. With the increase in the temperature, the shear strength first increased and then decreased; the maximum value of 95 MPa was observed at 650 °C. Additionally, the effects of the dwelling time on the microstructure and shear strength of the joint brazed at 700 °C were studied, revealing that the size of Al₄B₂O₉ increased and the shear strength decreased with the increase in the dwelling time.