HIGH-TEMPERATURE ANELASTIC BEHAVIOR OF SILICON-NITRIDE STUDIED BY MECHANICAL SPECTROSCOPY

Silicon nitride materials have been studied by mechanical spectroscopy in parallel with creep tests and TEM. A mechanical loss relaxation peak was observed (similar to 1270 K, 1 Hz). The associated activation energy and limit relaxation rate (>I000 kJ/mol and >10(45) S-1) are unusually high, indicating a transformation of the material, i.e. ''glass transition'' of the amorphous grain boundary phase. The evolution of the spectra during thermal cycling could be associated with the crystallization respectively reamorphization of this phase. Effectively, TEM observations carried out at different stages of thermal treatment were found in direct correlation with the spectra. As the anelastic phenomena appear at temperatures below the ones at which plasticity begins at a macroscopical range, the analysis of the mechanical loss spectra could provide Valuable information concerning the mechanisms of microplasticity in silicon nitride. In fact, a good qualitative agreement is found between creep and anelastic behaviour of these materials.