Bonding layer microstructures and mechanical behavior of sapphire/sapphire joints diffusion-bonded using MgO-Al₂O₃-SiO₂ interlayer

The bonding layer microstructures and their effects on fracture behaviors of sapphire joined using MgO (0.5%)-Al₂O₃ (94%)-SiO ₂ (5.5%) interlayer are analyzed. The results indicate that the bonding layer is composed by facetted Al₂O₃ grains and pores and they are controlled by joining conditions. The fracture mode is mainly intergranular for the pieces with fine grains and changes into cleavage fracture as the grain size increases. Optimal microstructures can be obtained under the condition of 1700°C, holding time of 1.5 h and pressure of 3 Mpa and the flexure strength corresponding M-{1010̄} plane bonded pieces may be beyond 250 MPa and A-{1120̄} plane bonded pieces may reach above 300 MPa.