Silicon nitrides studied by isothermal mechanical spectroscopy

Internal friction spectra (peak and background) measured as a function of frequency, at high temperature (about 950°C), for four silicon nitrides and a partially crystallised oxynitride glass are compared. The characteristics of the peaks (height, width) depend on the microstructure of the materials (grain size, quantity of amorphous intergranular phase). The evolution of the peak with microstructure corresponds to the predictions of Lakkiʹs rheological model, where silicon nitride is considered as the association of a viscous vitreous phase enclosed in a rigid skeleton. After annealing at 1146°C, the peak decreases, leading to a high temperature background. The activation energy deduced from the displacement of this background versus temperature is the same as the energy determined by creep experiments. A correlation parameter related to the mechanisms of relaxation in glassy films was also calculated: its value shows that movements inside glassy films are more correlated than inside bulk glass.