On the glass transition domain in some M-SiAlON (M = Y or Ln) oxynitride glasses

The behaviour in the glass transition domain of some oxynitride glasses has been studied by thermoanalytical methods (dilatometry and differential thermal analysis) and mechanical techniques (creep and ultrasonic measurements of Young's modulus). The thermoanalytical data are in good agreement with the glass transition domain defined from viscosity data. The sharp decrease of Young's modulus, that starts at the temperature of the strain point, is compared to results obtained from mechanical spectroscopy by other authors. The difference in the apparent activation energies for viscous flow above and below the temperature of the strain point is used to separate the contribution of the thermal and structural components. The high temperature apparent activation energy of viscosity is in fair agreement with that of the α-relaxation peak described by the formalism of hierarchically correlated molecular mobility.