Microstructure evolution and mechanical properties of ZnAl2O4-reinforced Al2O3/Al2O3 joints brazed with a bismuth borate zinc glass

Abstract

Amorphous bismuth borate zinc glass with a composition of 40Bi2O3–40B2O3–20ZnO (in mol.%) was used as a braze to join 95wt.% purity alumina ceramics (95Al2O3) in air. The interfacial phases in the brazed joints were characterized using scanning electron microscopy, transmission electron microscopy. Zinc aluminate (ZnAl2O4) was identified as the main reaction product in the joints due to the chemical reaction between the Al2O3 substrate and ZnO from the glass. Typical microstructure of the Al2O3/Al2O3 brazed joints was Al2O3 substrate/glassy matrix phase+ZnAl2O4 particles/Al2O3 substrate. The dependence of the microstructure and mechanical properties on brazing temperature was investigated. The size of ZnAl2O4 increased with an increase in brazing temperature. It was found the room lap shear strength of Al2O3/Al2O3 joints firstly increased and then declined with increasing brazing temperature. The maximum lap shear strength of 95MPa was obtained for the samples brazed at 675°C for 30min, which was mainly attributed to the formation of ZnAl2O4.