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Crystals 2017, 7(11), 335;

Nanoindentation of HMX and Idoxuridine to Determine Mechanical Similarity

School of Materials Engineering, Purdue University, West Lafayette, IN 47907, USA
Explosive Science and Shock Physics, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Author to whom correspondence should be addressed.
Academic Editors: Ronald W. Armstrong, Stephen M. Walley and Wayne L. Elban
Received: 28 September 2017 / Revised: 27 October 2017 / Accepted: 28 October 2017 / Published: 1 November 2017
(This article belongs to the Special Issue Crystal Indentation Hardness)
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Assessing the mechanical behavior (elastic properties, plastic properties, and fracture phenomena) of molecular crystals is often complicated by the difficulty in preparing samples. Pharmaceuticals and energetic materials in particular are often used in composite structures or tablets, where the individual grains can strongly impact the solid behavior. Nanoindentation is a convenient method to experimentally assess these properties, and it is used here to demonstrate the similarity in the mechanical properties of two distinct systems: individual crystals of the explosive cyclotetramethylene tetranitramine (HMX) and the pharmaceutical idoxuridine were tested in their as-precipitated state, and the effective average modulus and hardness (which can be orientation dependent) were determined. Both exhibit a hardness of 1.0 GPa, with an effective reduced modulus of 25 and 23 GPa for the HMX and idoxuridine, respectively. They also exhibit similar yield point behavior. This indicates idoxuridine may be a suitable mechanical surrogate (or “mock”) for HMX. While the methodology to assess elastic and plastic properties was relatively insensitive to specific crystal orientation (i.e., a uniform distribution in properties was observed for all random crystals tested), the indentation-induced fracture properties appear to be much more sensitive to tip-crystal orientation, and an unloading slope analysis is used to demonstrate the need for further refinement in relating toughness to orientation in these materials with relatively complex slip systems and crystal structures. View Full-Text
Keywords: nanoindentation; molecular crystals; mechanical properties; hardness; elastic modulus; fracture nanoindentation; molecular crystals; mechanical properties; hardness; elastic modulus; fracture

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Burch, A.C.; Yeager, J.D.; Bahr, D.F. Nanoindentation of HMX and Idoxuridine to Determine Mechanical Similarity. Crystals 2017, 7, 335.

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