Elastic Anisotropy of ß‐Silicon Nitride Whiskers

The elastic anisotropy of s-silicon nitride was examined using nanoindentation techniques. A coarse-grained polycrystalline sample composed of whiskers up to 15 µm in diameter and 100 µm in length allowed for the positioning of small indentations near the centers of grains, where neighboring grains minimally affect the property measurements. The crystallographic directions along which measurements were made were determined from the grain geometries in polished sections. The primary directions of interest were parallel to the c-axis (indenter penetration parallel to the whisker axis) and in the basal plane (indenter penetration perpendicular to the whisker axis), for which the indentation moduli were found to be 450 and 310 GPa, respectively. An indentation mechanics model used to estimate the single-crystal elastic constants from the nano-indentation results shows that Young's moduli for s-silicon nitride, which are similar to but not exactly the same as the moduli measured by nanoindentation techniques, vary from 540 GPa parallel to the c-axis to 280 GPa perpendicular to it. Hardness data, which also exhibit a strong anisotropy, are also presented.