Bismuth borate zinc glass braze for bonding sapphire in air

Abstract

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 20min 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 ~160nm at 635°C to ~455nm 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 85MPa was obtained for the joints brazed at 650°C for 20min. The joint shear strength decreased due to the aggregation of ZnAl2O4 particles in the joint.