The effects of processing on the alpha <-> beta SiAlON transformation during cycling heat treatments

Abstract

Typical alpha/beta-SiAlON starting composition of formula Nd(0.33)Si(9.38)Al(2.62)O(1.62)N(14.38) was densified by two different sintering processes, hot pressing (HP) and gas pressure sintering (GPS), using Nd(2)O(3) as a sintering additive. Post-sintering heat treatments have been carried out at 1450 degrees C for up to 72 h to observe alpha -> beta SiAlON transformation and re-sintered at 1800 degrees C for 4 h to observe reverse transformation. The effects of processing on the alpha <-> beta 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 alpha -> beta 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