ELASTIC-ANISOTROPY OF BETA-SILICON NITRIDE WHISKERS

The elastic anisotropy of beta-silicon nitride was examined using nano indentation techniques. A coarse-grained polycrystalline sample compos ed of whiskers up to 15 mu m in diameter and 100 mu m in length allowe d 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 we re determined from the grain geometries in polished sections. The prim ary directions of interest were parallel to the c-axis (indenter penet ration parallel to the whisker axis) and in the basal plane (indenter penetration perpendicular to the whisker axis), for which the indentat ion moduli were found to be 450 and 310 GPa, respectively. An indentat ion mechanics model used to estimate the single-crystal elastic consta nts from the nanoindentation results shows that Young's moduli for bet a-silicon nitride, which are similar to but not exactly the same as th e moduli measured by nanoindentation techniques, vary from 540 GPa par allel to the c-axis to 280 GPa perpendicular to it. Hardness data, whi ch also exhibit a strong anisotropy, are also presented.