Elastic and anelastic properties of silicon nitride at high temperatures by non-destructive impulse excitation

A new impulse excitation technique (IET) device was used to investigate the elastic and anelastic properties of silicon nitride. The test method involves the excitation of vibration of a freely suspended test specimen by gently tapping the specimen with a small projectile. The main (resonance) frequencies of the free vibration of the specimen are related to the elastic moduli through standard equations. The damping or internal friction (Q(-1)) is calculated from the exponential decay of the vibration amplitude. Tests were performed at room temperature on a sintered silicon nitride, both before and after a heat treatment. The heat treatment (1700 K, 5 hours) is shown to result in the crystallisation of initially amorphous grain boundary phases. The effect of the crystallisation on the elastic properties is well detectable (E increases). Tests were also performed in a furnace to measure internal friction as a function of temperature. The difference in Q(-1) between the heat treated and the as-sintered silicon nitride is very pronounced. A Q(-1)-peak is observed at temperatures around 1360 K, the glass transition temperature of the intergranular phases. This peak is caused by vibration energy dissipation in the amorphous triple pockets, which are the residue of the sintering additives. The height of the peak is related to the amount of these pockets in the material. Therefore, it can be concluded that the heat treatment induces a decrease in the amount of amorphous intergranular phase.status: publishe