Synthesis, crystallization behavior, microstructure and mechanical properties of oxynitride glass-ceramics with fluorine addition

Y–Si–Al–O–N–F oxynitride glasses were prepared by melting batches at 1500–1700 °C for 2 h under N2 atmosphere in a Si–Mo-heated resistance furnace. The appropriate heat treatment temperatures were selected according to the information provided by the differential scanning calorimeter (DSC) measurement. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to study the crystallization behavior of the glass-ceramics. DSC analysis demonstrated that an increase in fluorine content lowers the crystallization temperature of the oxynitride glass at constant N content. XRD and SEM analysis demonstrated that, as the nitrogen content is 6 or 12 equiv. %, Y2Si2O7 began to appear in the base glass first, which reduces the amounts of Y and Si available to form mullite; as the nitrogen content increases to 18 equiv. %, Si3Al6O12N2 and mullite begins to appear first; at 24 equiv.% N, the only crystallization phase is mullite, Y, N and F remain in the residual glassy phase of the glass-ceramic. An increase in nitrogen content favors the formation of Si3Al6O12N2 or mullite (Al6Si2O13) rather than Y2Si2O7, suggesting that which phase may be able to accommodate N into its crystal structure. In terms of mechanical properties, micro-hardness is more sensitive to changes in the nitrogen content of the residual glass phase than the bending strength which is related to the amounts of crystallized phases, phase morphology and crystal size.