Defect and surface asperity dependent yield during contact loading of an organic molecular single crystal

Abstract

The onset of plastic deformation was investigated using nanoindentation in single crystals of the explosive cyclotrimethylene trinitramine (RDX). Cleavage and habit planes were tested revealing a range of yielding behaviors. Smooth habit planes of unprocessed single crystals exhibited distinct yield points near the theoretical shear strength; planes produced by cleavage yielded at lower applied stresses. Cumulative probability distributions of maximum shear stresses at yield were used to illustrate the representative yielding behavior for samples prepared by the different methods. A statistically significant difference was observed for cleavage and habit planes. This suggested that structural defects, such as dislocations from growth and sample preparation, were being probed and nanoindentation can be used to correlate the mechanical response of organic molecular crystals with defect density. This capability may help explain the observed range of measurement differences in fundamental properties in this class of materials, such as sensitivity to the initiation of detonation in explosives, and disparate tablet integrity and stability responses in polymorphs of some pharmaceutical materials.