Reaction sintering and the α-Si3N4/β′-sialon transformation for compositions in the system Si-Al-O-N

Abstract

The reaction sintering of three powder compositions corresponding to points near the β′-phase line in the region of z=0.75 has been studied using apparatus which allowed the continuous monitoring of the densification kinetics. The powder compositions were prepared from mixtures of α-Si3N4, Al2O3 and AIN, and the weight changes in the compacts could be kept to less than ∼ 1% over a two hour period. The densification rate is sensitive to small changes in powder composition and decreases markedly as the β∼'-sialon phase is approached from the oxygen-side. The α-Si3N4 to β′-sialon conversion rate, on the other hand, is almost independent of the powder composition. The sintering and transformation kinetic data, combined with surface area measurements and observations on the microstructure of the sintered compacts, indicate that the sintering behaviour is controlled by at least two processes, namely the vapour phase transport of material and a solution-diffusion-reprecipitation process involving a grain boundary liquid phase. Both processes result in the conversion of α-Si3N4 to β′-sialon and result in microstructural coarsening but only the second process leads to overall densification in the powder compact.