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

Gas-pressure sintering of α-Si₃N₄ was carried out at 1850 ° to 2000°C in 980-kPa N₂. 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-Si₃N₄ powder was similar to that in the materials from β-Si₃N₄ 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 Si₃N₄ ceramics.