Rheological Behavior of YSiA1ON Glasses - Its Incidence on the High-Temperature Deformation of Si3N4 Ceramics Sintered with Al2O3 aND Y2O3 Additives

Elasticity and viscosity of oxynitride glasses within the Y-Si-Al-O-N system with different O/N and Y/Al ratios are studied. Young’s modulus (E), glass transition domain (Tg) and viscosity are found to be gradually improved by enhancing the nitrogen content. The Y/Al ratio seems to play a more complex role, with a minimum of Tg at Y/A1=1.76. Highest values of Tg=1000 °C and E=165 GPa have been measured for the lowest aluminum and highest nitrogen contents. The activation energy for the glass viscous flow, ΔHa ≈ 890 kJ/mol., is close to that of steady creep of silicon nitrides. Further, creep ductility of Si3N4 based ceramics appears at temperature just above Tg temperatures of glasses. This suggests that the rate limiting process during creep at elevated temperature is closely related to the Theological properties (diffusional) of the glassy grain-boundary phase.