Further experimental investigation on fast densification mechanism of bimodal powder during pressureless sintering of transparent AlON ceramics

In our recent work we demonstrated that a bimodal particle size distribution (PSD) AlON powder could be fast densified by pressureless sintering (Shan et al., Ceram. Int. 41 3992-3998 (2015); Shan et al., J. Eur. Ceram. Soc. 36 67-78 (2016)). To obtain a further insight of its fast densification mechanism, this powder was held for 30 min and 60 min at 1500-1700 °C during heating, respectively. However, the added holdings resulted in a decrease in both relative density and transmittance, compared to that of the sample without holding during heating. Further investigation into phase transformation and microstructure evolution of these samples indicates that high heating rate enables the sintering mixtures to keep a near sphere particle shape in bimodal PSD until phase transformation from Al2O3 to AlON is fully completed. Then mass transport between AlON grains of different size can simultaneously happen at the final fast densification stage, which benefits less formation of pores and fast AlON grains growth. Therefore, high heating rate plays a key role to fast and better consolidation of the bimodal AlON powder.