Bioactive oxynitride glasses: Synthesis, structure and properties

The low mechanical strength of bioactive glasses restricts their use to non load-bearing applications. One way to increase strength of glasses is to introduce nitrogen into the silicate network. Oxynitride bioactive glasses in the SiO₂-CaO-Na₂O-Si₃N₄ system were successfully prepared. Structural characterization using ²⁹Si MAS-NMR shows that, as nitrogen is substituted for oxygen, formation of [SiO₃N] tetrahedra and Q⁴ units occurs with extra bridging anions at the expense of Q³ units. Glass transition temperature, hardness, Young's modulus and fracture resistance increase with nitrogen content because of the extra cross-linking of the glass network.CaF₂ was substituted for CaO in these glasses which results in decreases in glass transition temperature. Thus, glasses can be melted at lower temperatures while addition of fluorine allows higher solubility of nitrogen and has no effect on the significant increases in mechanical properties induced by nitrogen substitution. Initial assessment of bioactivity suggests that varying the N content may allow tailoring of the level of bioactivity of these glasses.