GLASS-TRANSITION AND ALPHA-RELAXATION IN Y-SI-AL-O-N GLASSES AND IN SI3N4 CERAMICS STUDIED BY MECHANICAL SPECTROSCOPY

Y-Si-Al-O-N glasses, with composition approaching that of the intergra nular amorphous phase of Si3N4 ceramics processed with sintering aids, have been studied by mechanical spectroscopy, through the glass trans ition. The cr-relaxation peak of the Y-Si-Al-O-N glasses was found to be similar to the relaxation peak observed in the mechanical loss spec trum of silicon nitride ceramics. The silicon nitride peak is therefor e attributed to the alpha relaxation of the bulk regions of intergranu lar glassy phase present in the material. The viscoelastic behaviour o f the Y-Si-Al-O-N glasses can be well described by a formalism of hier archically correlated molecular mobility through the glass transition. In particular, this formalism explains the observation of an unusuall y high apparent activation energy. This formalism has been applied to simulate the cu-relaxation peak of the Y-Si-Al-O-N glasses and the mec hanical loss peak in the silicon nitride spectrum.