Hardness and elastic properties of Bi₂O₃-based glasses

The hardness and elastic properties of 20PbO · xBi₂O₃ · (80 - x)B₂O₃ glasses with x = 20-60 were evaluated through usual Vickers indentation and nanoindentation tests. The glass transition temperature (T_g = 295-421°C), Vickers hardness (H_v = 2.9-4.5 GPa), true hardness (H = 1.5-3.8 GPa) and Young's modulus (E = 24.4-72.6 GPa) decreased monotonously with increasing Bi₂O₃ content. This compositional trend demonstrates that the strength of Bi-O chemical bonds in these glasses is considerably weak compared with B-O bonds and plastic deformations under indentation loading occur easily. The elastic recovery after unloading was about 45% for the glasses with x = 20-50, and the Poisson's ratio was 0.27 for the glass with x = 20. The fracture toughness was evaluated to be 0.37-0.88 MPam^1/2 from the values of H_v and E, and it was proposed that not only weak Bi-O bonds but also boron coord ination polyhedra (BO₃ or BO₄) and their arrangements affect on crack formation. From the temperature dependence of Vickers hardness up to the glass transition region, it was suggested that the glasses with high Bi₂O₃ contents belong to the category to fragile glass-forming liquids.