Phase formation and microstructural evolution of Ca α-sialon using different Si3N4 starting powders

Silicon nitride has two polymorphous structures, α-Si3N4 and β-Si3N4. In this study three different Si3N4 starting powders (∼100%α, 40%α+60%β, ∼100%β) were used to prepare Ca α-sialon with the composition Ca1.8Si6.6Al5.4O1.8N14.2 by pressureless sintering. Comparison was made concerning the densification process, reaction sequence and microstructure of the corresponding materials. The sluggish reactivity of β-Si3N4 resulted in poorer densification during sintering. All the three starting powders produced a similar final phase assembly, namely α-sialon together with a small amount of AlN and AlN polytypoid except that traces of unreacted β-Si3N4 remained until 1800° in samples prepared with ∼100%β-Si3N4 powders. Elongated α-sialon grain morphology has been identified in the samples prepared using all the three different Si3N4 starting powders. Coarser elongated α-sialon grains with lower aspect ratio were found in samples using higher β phase starting powders.