Effect of nitrogen incorporation on the phase structure, thermal behaviour and structural properties of apatite wollastonite (AW) bioactive ceramics

In this research, a powder mixture of SiO₂–P₂O₅–CaO–CaF₂–MgO–Al₂O₃ compound was prepared, and the melt-quenching process was employed to obtain apatite wollastonite (AW) bioactive glasses. The mixture was melted in an alumina crucible at a temperature exceeding 1500 °C for 1 h under both ambient and protective nitrogen (N₂) atmosphere conditions. By using a melt-quenching method, the melted glass was cooled over a critical cooling rate into a graphite crucible. The prepared sample in N₂ condition (G2) and ambient condition (G1) glasses were investigated in terms of chemical content (XRD, EDS and FTIR), microstructure (FESEM), physical (density) and mechanical properties (hardness and indentation fracture toughness). The protective atmosphere (N₂) caused a meaningful increase in the indentation fracture toughness (IFT), from 1.54 to 1.72 MPa m^1/2, 12 %, while the hardness of glasses was slightly increased from 5.98 to 6.00 GPa by the N₂ condition. The findings demonstrate that nitrogen not only contributes to the structural organisation but also plays a critical role in driving the thermodynamic stability and microstructural evolution of the system, significantly influencing the specific phase transformation occurring from β-wollastonite to pseudowollastonite and the thermal behaviour in glass-ceramic systems. This could be considered a critical parameter for future material design and functional applications.