Microstructural evolution, mechanical properties, and FEM analysis of the residual stress of sapphire joints brazed with a novel borate glass

Borate glass (80B₂O₃–20Bi₂O₃ in mol.%), which had a coefficient of thermal expansion (CTE) that was exceptionally compatible with that of sapphire, was designed and used as a braze to join sapphire. Prior to joining, various thermodynamic properties of the glass were investigated. Brazing was conducted at 600–750 °C, and the influence of the brazing parameters on the microstructural evolution and mechanical properties of the joints were investigated. The results showed that two crystalline phases, Bi₆B₁₀O₂₄ and Bi₄₈Al₂O₇₅, were formed in the joint due to crystallization within the glass, which could strengthen the joint. The maximum shear strength, 55.5 MPa, was achieved for the joint that was brazed at 725 °C for 20 min. The finite element method (FEM) was utilized to simulate the shear stress during cooling. Analysis of the residual stress in the FEM simulation showed that the shear stress was mainly concentrated near the sapphire/glass interface.