Mechanical spectroscopy connected to creep and stress relaxation in a high resistant silicon nitride

Silicon nitride processed by gas pressure sintering contains a very small amount of glassy phase and consequently exhibits a strong resistance to deformation until 1450 degreesC. Above this temperature, both relaxation kinetics and creep rate rapidly increase. To explain such a behaviour, the formation of a liquid phase by dissolution of YSiAlON phases was proposed. The present paper shows that mechanical spectroscopy argues for the existence of such a liquid phase at high temperature. The mechanical loss is very low in the as-sintered material. Nevertheless, the internal friction peak generally observed in silicon nitride, and attributed to the glass transition in the glassy pockets, is also observed in the gas pressure sintered silicon nitride. Moreover, the peak is much higher in annealed and "quenched" specimens and it increases with annealing time. These results show that the annealed and "quenched" material contains much more glassy phase and so argues for the dissolution of crystalline phases at high temperature. (C) 2002 Elsevier Science Ltd. All rights reserved.