Effect of Y/Al ratio on crystallization, microstructures and mechanical properties of Y-Si-Al-O-N-F glass-ceramics

Y-Si-Al-O-N-F glasses with various Y/Al ratios were prepared by melt-quenching the liquid under high-pure N-2 atmosphere inside a glove box. Nominal compositions of the oxynitride glasses in equivalent percent (equiv.%) are xY: (40-x)Al: 60Si: 86O: 12N: 2F (x = 10, 15, 20, 25, 30). X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to study the crystallization behaviors and microstructure evolution of the oxynitride glass-ceramics. XRD analysis of the Y-Si-Al-O-N-F glass-ceramics after adequate heat-treatment revealed that increasing Y/Al ratio led to increasingly higher contents of Y2Si2O7 at the expense of the mullite crystal phase. In terms of mechanical properties, the elastic moduli of the oxynitride glass-ceramics were in the range from 107 to 152 GPa, while the bending strengths were 167-201 MPa. The micro-hardness of the glass-ceramics remained almost unchanged, varying from 8.4 to 8.9 GPa. The role of Y/Al ratio is that of altering the mullite and Y2Si2O7 phase-transformation processes and influencing the final microstructures. Rod-like mullite crystals with high aspect-ratio are helpful to improve the mechanical properties of the oxynitride glass-ceramics. This work certainly has implications for other oxynitride glass-ceramics with high aspect-ratio morphologies.