Grain boundary character distributions in Si<sub>3</sub>N<sub>4</sub> ceramics

The structure of grain boundary has an essential influence on the physical and mechanical properties, especially the high-temperature mechanical properties of silicon nitride ceramics. In the present work, the five-parameter analysis method which is established based on electron backscatter diffraction (EBSD), and stereology and statistics are used to study the grain boundary character distributions in the two commercial silicon nitride ceramic ball samples. These two samples are both fabricated by hot isostatic pressing but made in China and abroad, respectively. The results indicate that the misorientation distributions of the two samples radically deviate from the random distribution at a rotation angle of 180°, showing that the frequency of the relevant grain boundaries is dramatically higher than that of the randomly distributed ones. These grain boundaries are mainly those possessing the misorientations of [0 1 –1 0]/180° and [–1 2 –1 0]/180°, corresponding to <i>Σ</i>2 and <i>Σ</i>3 boundaries, respectively. The grain boundary inter-connections (GBICs) of the <i>Σ</i>2 boundaries are basically the same in the two samples, and they are dominated mainly by the {0 0 0 1}/{0 0 0 1} basal-to-basal inter-connection. However, the GBICs of <i>Σ</i>3 boundaries are quite different in the two samples. they primarily the {–1 2 –1 0}/{–1 2 –1 0} prismatic-to-prismatic inter-connection for the <i>Σ</i>3 boundaries in the sample made in China, but it is mainly the {1 0 –1 0}/{1 0 –1 0} prismatic-to-prismatic inter-connection for the <i>Σ</i>3 boundary in the sample made abroad. Crystallographic analysis shows that the planar coincidence site density (PCSD) for the {–1 2 –1 0}/{–1 2 –1 0}, {0 0 0 1}/{0 0 0 1} and {1 0 –1 0}/{1 0 –1 0} GBIC are 2.45 /nm², 7.95 /nm² and 9.10 /nm², respectively, implying that the degree of the structural ordering increases in turn. Further discussion emphasizes that the grain boundaries possessing {1 0 –1 0}/{1 0 –1 0} GBIC and {0 0 0 1}/{0 0 0 1} GBIC should be one type of special boundaries due to their high degrees of structural ordering.