Hot-pressed Si3N4 ceramics with Lu2O3 additives: Grain-boundary phase and strength

The grain-boundary phase of hot-pressed Lu2Si2O7-Si3N4 and Lu4Si2O7N2-Si3N4 compositions was characterized using a transmission electron microscopy. In the case of the Lu2Si2O7-Si3N4 composition the multiple-grain junction pockets were completely crystalline and in the Lu4Si2O7N2-Si3N4 case the pockets were partially crystalline. The thin intergranular amorphous film presented at two-grain junctions was common in the two compositions. The thickness of the thin film was 0.5-1.0 nm for the Lu2Si2O7-Si3N4 composition and 1.6-2.3 nm for the Lu4Si2O7N2-Si3N4 composition. On the other hand, the two compositions were fractured in four-point flexure, at temperatures between 25 and 1600 °C, in a dry N2 atmosphere. At and below 1400 °C, only linear stress-strain response was observed for each composition. Nonlinear response was observed in the stress-strain curves for temperatures at and above 1500 °C for the Lu4Si2O7N2-Si3N4 composition, while this response was observed at 1600 °C for the Lu2Si2O7-Si3N4 composition. The flexural strength of the Lu4Si2O7N2-Si3N4 composition was greater, compared to that of the Lu2Si2O7-Si3N4 composition.