The effects of processing on the α ↔ β SiAlON transformation during cycling heat treatments

Typical α/β-SiAlON starting composition of formula Nd0.33Si9.38Al2.62O1.62N14.38 was densified by two different sintering processes, hot pressing (HP) and gas pressure sintering (GPS), using Nd2O3 as a sintering additive. Post-sintering heat treatments have been carried out at 1450 °C for up to 72 h to observe α → β SiAlON transformation and re-sintered at 1800 °C for 4 h to observe reverse transformation. The effects of processing on the α ↔ β SiAlON transformation, microstructure and mechanical properties have been studied. It was found that gas pressure sintered samples have lower amount of grain boundary phase compared to the hot pressed samples. During the post-heat treatment cycles that grain boundary phase promoted α → β SiAlON transformation and grain growth in hot pressed sample more than that of gas pressure sintered. Additionally, after heat treatment cycles gas pressure sintered sample showed that higher hardness and toughness values compare to hot pressed sample.