Microstructural characterization and high-temperature strength of hot-pressed silicon nitride ceramics with Lu2O3 additives

The microstructures of two hot-pressed Si3N4 ceramics, with 3.33 and 12.51 wt% Lu2O3 additive, have been characterized using transmission electron microscopy. The microstructures of both samples consisted of elongated β-Si3N4 grains and a secondary phase, contained in pockets surrounded by the grains, with a crystalline or amorphous form. In the 3.33 wt% Lu2O3-containing Si3N4 ceramic, all the multiple-grain junctions were completely crystalline while, in the 12.51 wt% Lu2O3-containing Si3N4 ceramic, approximately half the junctions were devitrified. A thin intergranular amorphous film present between the two-grain boundary was common; however, a film-free grain boundary was observed in the 12.51 wt% Lu2O3 sample. The film-free grain boundary was determined to be approximately 35%. Both ceramics fractured in four-point flexure between 1200 and 1600°C. Their high-temperature strength is closely associated with the nature of the grain-boundary phase formed during the sintering process.