Crystallization behaviour of 25Bi2O3–75B2O3 glass braze and strengthening mechanism of crystallization-reinforced sapphire/25Bi2O3–75B2O3/sapphire joints

Borate based glass, 25Bi2O3–75B2O3 (mol.%), was employed as a braze to join sapphire in an atmospheric environment. The crystallization behaviour as well as the crystallization kinetics of the glass during the heating process were investigated in a variety of ways. Wetting tests were conducted to research the wetting and spreading performances of the glass on the surface of sapphire prior to joining. Furthermore, sapphire joints were reliably obtained under various joining parameters. The dependence of the microstructural evolution and mechanical performance of sapphire joints on temperature and dwell time were also explored. The research results indicate that 25Bi2O3–75B2O3 glass possesses outstanding glass-forming ability and excellent wettability on the surface of sapphire. Bi6B10O24 crystalline phases were produced in the seam. The width of the joint as well as the quantity of Bi6B10O24 crystals increased at first and then decreased with the extension of dwell time. Simultaneously, the shear strength of the joints had a tendency to improve gradually and then diminish with the rise in brazing temperature. The maximum strength of the joints (49.3 MPa) was obtained by brazing at 700 °C for 20 min.