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.