Densification of silicon nitride powder by spark plasma extrusion

The simultaneous extrusion and sintering of ceramic materials using the spark plasma extrusion (SPE) technique is an alternative densification route for the powders examined in this study. SPE with Y₂O₃ and Al₂O₃ as the additives was employed for the concurrent densification and extrusion of pure α-Si₃N₄ commercial powder. Three compositions of α-Si₃N₄ powders, namely SN-4, SN-12, and SN-20, were studied to understand the effect of additives on the sinter-extrusion process. The number in the mixture name corresponded to the weight percentage of additives in the samples. A mechanical shaker was used to thoroughly mix the selected powder compositions in isopropanol for 24 h at 720 rpm. It was then dried in a furnace at 100 °C for 3 h. The sinter-extrusion treatment was conducted for a holding time of 5 min at a heating rate of 300 °C/min and a sintering temperature of 1500 °C with a pressure in the range of 12–63 MPa. The maximum degree of extrusion was attained when the total amount of additives was at 20 wt% (SN-20 series). As the total amount of additives increased, the capillary pressure gradients led to an inhomogeneous liquid distribution throughout the specimen, which in turn promoted further densification of the Si₃N₄ ceramics. Owing to the conical geometry of the graphite die used for the extrusion process in this study, the largest densification values observed in the extruded ceramics corresponded to the base and the middle sections of the SN-20 series (relative density ~92%). The latter could only take place once the α-to β-Si₃N₄ phase transformation occurred, leading to significant shrinkage of the material.