Observation of Rare-Earth Segregation in Silicon Nitride Ceramics at Subnanometre Dimensions

Shibata N¹, Pennycook SJ¹, Gosnell TR², Painter GS³, Shelton WA⁴, Becher PF³

Nature 428 (6984): 730-733 APR 15 2004

      Silicon nitride (Si₃N₄) ceramics are used in numerous applications because of their superior mechanical properties. Their intrinsically brittle nature is a critical issue, but can be overcome by introducing whisker-like microstructural features. However, the formation of such anisotropic grains is very sensitive to the type of cations used as the sintering additives. Understanding the origin of dopant effects, key to the design of high-performance Si₃N₄ ceramics, has been sought for many years. Here, we show direct images of dopant atoms (La) within the nano-meter scale intergranular amorphous films typically found at grain boundaries, using aberration corrected Z-contrast scanning transmission electron microscopy. It is clearly shown that the La atoms preferentially segregate to the amorphous/crystal interfaces. First-principles calculations confirm the strong preference of La to the crystalline surfaces which is essential for forming elongated grains and a toughened microstructure. While there has been a basis for the micro-scale design of ceramics to improve mechanical properties, this work addresses the atomic-level structural engineering required for the next generation of ceramics.

FIG.3: Pixon reconstructed HAADF image of the IGF/crystal interface. White open circles correspond to the stable positions of La atoms on the N-terminated Si3N4 prismatic surface predicted by the first principles cluster calculations.