Stability of intergranular phases in hot-pressed Si3N4 studied with mechanical spectroscopy and in-situ high-temperature XRD

The impulse excitation technique (IET) and high temperature X-ray diffraction (HTXRD) were used to investigate the intergranular glass phase and its crystallisation behaviour in four hot-pressed silicon nitrides. The internal friction or damping peak height measured with IET near the glass transition temperature, Tg, is used as a qualitative indicator for the amount of residual intergranular amorphous phase after sintering. Silicon nitride powder was hot-pressed with different sintering additives. The silicon nitride containing 4 wt.% Al2O3 does not reveal an internal friction peak at Tg, i.e. it does not contain a significant amount of intergranular glass phase. Three other silicon nitrides, containing either 8 wt.% Y2O3, 6 wt.% Y2O3+2 wt.% Al2O3, or 2 wt.% Y2O3+4 wt.% Al2O3+2 wt.% TiN, do show an internal friction peak near Tg. This “Tg-peak” is nearly unaffected by heating up to 1400 °C in the silicon nitride with Y2O3+Al2O3+TiN sintering aids, whereas the amount of intergranular glass in the ceramics containing either Y2O3+Al2O3 or Y2O3 as a sintering aid is strongly reduced by subsequent heating. As observed from HTXRD, the onset temperature of crystallisation of the intergranular glass in the ceramic containing Y2O3+Al2O3 sintering aids is about 1100 °C, with the formation of Y–N-apatite (Y20N4Si12O48) and O-sialon (Al0.04Si1.96N1.96O1.04). The O-sialon phase in the yttria and alumina containing ceramics, formed either during sintering or during heat treatment, is not stable at elevated temperatures and dissolves in the intergranular glass phase between 1300 and 1400 °C. The O-sialon phase in the ceramic without Y2O3 sintering additive, however, is thermally stable. The presence of Ti4+ ions in the intergranular glass phase is suggested to inhibit its crystallisation, resulting in a stable high temperature damping behaviour.