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An experimental study of the elastic and plastic properties of sucrose single crystals, which can be considered to be a model material for both pharmaceutical excipients and explosives, has been carried out using nanoindentation. Instrumented indentation was used to characterize the properties of both habit and cleavage planes on the (100) and (001) orientations; the elastic modulus on the (100) is 38 GPa, while the modulus on the (001) is 33 GPa. The hardness of sucrose is approximately 1.5 GPa. Nanoindentation enables assessment of the onset of plastic deformation on cleaved surfaces, and a maximum shear stress of 1 GPa can be supported prior to plastic deformation. The deformation in this material is crystallographically dependent, with pileup around residual indentation impressions showing evidence of preferential slip system activity.

I. INTRODUCTION

Brittle molecular crystals, such as sucrose, cyclotrimethylenetrinitramine (RDX), cyclotetramethylene– tetranitramine (HMX), and pentaerythritol tetranitrate (PETN), are inherently challenging to test mechanically with traditional test techniques due to their mechanical and chemical fragility. These materials cannot withstand extensive sample preparation or large-scale strain without fracture,1,2 and the use of extensive electron microscopy for quantitative microscopy is problematic due to chemical disassociation.3,4 Due to this fragility and the complex crystal structures that molecules form, relatively little is known about fundamental deformation mechanisms within molecular crystals compared with metals or alkali halides. The pharmaceutical and explosive industries both use this class of materials in mechanical applications. Molecular crystals are used as pharmaceutical excipients to aid in the formation of tablets and are the energetic component within plastic-bonded explosives. An increased understanding of deformation during compaction would assist in the formation of solid dosage tablets with increased physical integrity and desirable density-dependent dissolution rates for bioavailability.5 Knowledge of fundamental deformation mechanism would allow for assessment and progression of proposed dislocation-based initiation mechanisms pertinent to the safety and performance of explosives.6–9

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0249 J. Mater. Res., Vol. 22, No. 7, Jul 2007

http://journals.cambridge.org

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Much of what is known about deformation in molecular crystals has come from indentation studies. The scale of indentation experiments in terms of both size and strain is conducive to testing molecular crystals. The small strains accessible through indentation allow for the evaluation of deformation with minimal or no fracture, while the relatively small amount of material necessary for indentation is beneficial, as large molecular crystals are difficult to obtain. Previous studies have used microhardness indentation and instrumented nanoindentation to assess material properties su