Microstructural development of a-SiALON materials

The stability of a -SiAlON at elevated temperatures (1100-1500°C) depends on the nature of the cations R entering the ot -SiAlON structure. For the trivalent rare-earth cations (i.e. La-Yb Y), the stability increases with decreasing atomic radius [14,81] i.e. the temperature below which a phase transformation a — P can thermodynamically take place decreases. The stability also increases with increasing substitution level. Thermodynamic stabilisation of the a -SiAlON with respect to the p -SiAlON can be achieved by the use of mixed cations [14]. 

Besides the nature of the cation which is incorporated in the a -SiAlON, the rate of a -p -transformation depends on the existence of P -SiAlON nuclei and the amount and viscosity of the amorphous grain-boundary phase. The transformation between ot -SiAlON and p -SiAlON is reconstructive and must occur via a liquid in which one phase can dissolve and the product precipitate. Therefore the transformation can be retarded by the absence of a glassy phase or by a very high viscosity [14,81]. 

Better understanding of the microstructural formation of the oc -SiAlON materials in the future will be necessary for the improvement of the mechanical properties, especially fracture toughness and strength, and the wider application of this group of materials, having higher hardnesses than the P-Si3N4 and P-SiAlON materials (see Fig. 16). 

---