Effects of nitrogen and fluorine on crystallisation of Ca–Si–Al–O–N–F glasses

The effect of nitrogen and fluorine substitution on the crystallisation of a new generation of oxyfluoronitride glasses in the Ca–Si–Al–O–N–F system has been studied. Glasses were nucleated for 5 h at the nucleation temperature of Tg + 50 °C and crystallised for 10 h at the maximum crystallisation temperature (900–1050 °C depending on the glass composition) determined from differential thermal analysis. For the oxide glass, crystallisation results in formation of wollastonite (CaSiO3), gehlenite (Ca2Al2SiO7) and anorthite (CaAl2Si2O8) along with a small amount of residual glass. For crystallisation of oxyfluoride glasses (0 equiv.% N), when fluorine content increases, cuspidine (Ca4Si2O7F2) is the major crystalline phase at the expense of gehlenite while in oxyfluoronitride glasses containing 20 equiv.% N, gehlenite is always the dominant crystalline phase at different fluorine contents. At constant fluorine content (5 equiv.%), an increase in nitrogen content favours the formation of gehlenite rather than anorthite or wollastonite suggesting that this phase may be able to accommodate N into its crystal structure. While a small amount of nitrogen substitution for oxygen can be assumed in the gehlenite structure, the residual glass in the glass-ceramic is expected to be very N-rich. In terms of properties, hardness is shown to be more sensitive to changes in microstructure, phase morphology and crystal size compared with elastic modulus which is related to the amounts of constituent phases present.