Effect of glass composition on alumina joining: role of Nb2O5 and other additives on seal behaviour and phase formation

Commercial lead-free low-temperature sealing glasses often rely on low-melting oxides like B 2 O 3 and Bi 2 O 3 , which can compromise chemical durability, shelf life, and compatibility with joining matrices. To address these limitations, this study explores six soda-borosilicate glass compositions modified with Al 2 O 3 , ZrO 2 , CaO, BaO, and Nb 2 O 5 for joining alumina ceramics at 700 °C. Glasses containing CaO and BaO formed crack-free seals, likely due to their role as network modifiers that increase non-bridging oxygen content and reduce viscosity. In contrast, Al 2 O 3 - and ZrO 2 -containing glasses showed poor interfacial bonding and cracking. The Nb 2 O 5 -containing glass yielded a visually satisfactory joint and underwent vitroceramic transformation, forming two distinct phases during joining. Raman spectroscopy revealed unique spectral features, though the phases could not be matched to known Nb compounds. FIB-DIC and Nanoindentation revealed that the mechanical properties of alumina and BNS-AZN glass align well with literature values, with increased hardness and modulus near the interface. FIB-DIC analysis showed a predominantly compressive residual stress state in the glass, with slight spatial variations at the microscale. Lap shear strengths of 4.58–5.43 MPa were obtained (DIN EN 1465). These results highlight Nb 2 O 5 ’s potential in enhancing phase separation and bonding performance in glass-ceramic seals. • Six soda-borosilicate glasses joined alumina at 700 °C without PbO or Bi 2 O 3 . • CaO and BaO additions enabled crack-free seals via reduced glass viscosity. • Nb 2 O 5 triggered vitroceramic transformation and phase separation during joining. • Nanoindentation showed increased hardness and modulus near the interface. • FIB-DIC revealed predominantly compressive residual stresses in the glass.