Glass transition and α relaxation in Y–Si–Al–O–N glasses and in Si3N4ceramics studied by mechanical spectroscopy

AbstractY–Si–Al–O–N glasses, with composition approaching that of the intergranular amorphous phase of Si3N4ceramics processed with sintering aids, have been studied by mechanical spectroscopy, through the glass transition. The α-relaxation peak of the Y–Si–Al–O–N glasses was found to be similar to the relaxation peak observed in the mechanical loss spectrum of silicon nitride ceramics. The silicon nitride peak is therefore attributed to the α relaxation of the bulk regions of intergranular glassy phase present in the material. The viscoelastic behaviour of the Y–Si–Al–O–N glasses can be well described by a formalism of hierarchically correlated molecular mobility through the glass transition. In particular, this formalism explains the observation of an unusually high apparent activation energy. This formalism has been applied to simulate the α-relaxation peak of the Y-Si-Al-O-N glasses and the mechanical loss peak in the silicon nitride spectrum.