Microstructural Development During Gas‐Pressure Sintering of α‐Silicon Nitride

TLDR

Gas‑pressure sintering of α‑Si₃N₄ was performed at 1850–2000 °C under 980‑kPa N₂, and the diameters and aspect ratios of hexagonal grains were measured on polished and etched surfaces. The sintered Si₃N₄ exhibited a bimodal grain‑size distribution with large, elongated grains whose aspect ratios exceeded the average; these grains formed from large nuclei during the α‑to‑β transformation, and fracture toughness depended on the diameter of these large grains rather than α content or aspect ratio, with crack bridging as the primary toughening mechanism.

Abstract

Gas‐pressure sintering of α‐Si 3 N 4 was carried out at 1850 ° to 2000°C in 980‐kPa N 2 . The diameters and aspect ratios of hexagonal grains in the sintered materials were measured on polished and etched surfaces. The materials have a bimodal distribution of grain diameters. The average aspect ratio in the materials from α‐Si 3 N 4 powder was similar to that in the materials from β‐Si 3 N 4 powder. The aspect ratio of large and elongated grains was larger than that of the average for all grains. The development of elongated grains was related to the formation of large nuclei during the α‐to‐β phase transformation. The fracture toughness of gaspressure‐sintered materials was not related to the α content in the starting powder or the aspect ratio of the grains, but to the diameter of the large grains. Crack bridging was the main toughening mechanism in gas‐pressure‐sintered Si 3 N 4 ceramics.