Hot-pressed Si 3N 4 ceramics with Lu 2O 3 additives: Grain-boundary phase and strength

Abstract

The grain-boundary phase of hot-pressed Lu 2Si 2O 7–Si 3N 4 and Lu 4Si 2O 7N 2–Si 3N 4 compositions was characterized using a transmission electron microscopy. In the case of the Lu 2Si 2O 7–Si 3N 4 composition the multiple-grain junction pockets were completely crystalline and in the Lu 4Si 2O 7N 2–Si 3N 4 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 Lu 2Si 2O 7–Si 3N 4 composition and 1.6–2.3 nm for the Lu 4Si 2O 7N 2–Si 3N 4 composition. On the other hand, the two compositions were fractured in four-point flexure, at temperatures between 25 and 1600 °C, in a dry N 2 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 Lu 4Si 2O 7N 2–Si 3N 4 composition, while this response was observed at 1600 °C for the Lu 2Si 2O 7–Si 3N 4 composition. The flexural strength of the Lu 4Si 2O 7N 2–Si 3N 4 composition was greater, compared to that of the Lu 2Si 2O 7–Si 3N 4 composition.