The mechanical properties of as-grown noncubic organic molecular crystals assessed by nanoindentation
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John D. Yeager and Daniel E. Hooks Explosive Science and Shock Physics, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Teresa M. Carvajal School of Agricultural and Biological Engineering, Purdue University, West Lafayette, Indiana 47907, USA
David F. Bahra) School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, USA (Received 17 March 2017; accepted 18 May 2017)
Organic molecular crystals are often noncubic and contain significant steric hindrance within their structure to resist dislocation motion. Plastic deformation in these systems can be imparted during processing (tableting and comminution of powders), and the defect density impacts subsequent properties and performance. This study measured the elastic and plastic properties of representative monoclinic, orthorhombic, and triclinic molecular crystalline structures using nanoindentation of as-grown sub-mm single crystals. The variation in modulus due to in-plane rotational orientation, relative to a Berkovich tip, was approximately equal to the variation of a given crystal at a fixed orientation. The onset of plasticity occurs consistently at shear stresses between 1 and 5% of the elastic modulus in all three crystal systems, and the hardness to modulus ratio suggests conventional Berkovich tips do not generate fully self-similar plastic zones in these materials. This provides guidance for mechanical models of tableting, machining, and property assessment of molecular crystals.
I. INTRODUCTION
Molecular crystals are a broad category of materials that include energetics, foods, nonlinear optics and pharmaceuticals. Many have noncubic structures and assessing their physical and mechanical properties is of wide interest.1,2 The formability of powders into tablets3,4 is often evaluated using qualitative or semiquantitative mechanical properties such as compactibility and brittleness, which are related to industrial processes like comminution. Quantifiable mechanical properties (e.g., elastic modulus, yield strength) must be measured and understood if molecules are ever to be engineered to provide an optimal range of performance.5 However, these are difficult to measure for low-symmetry, fragile molecular crystals. Nanoindentation is a common method for evaluating the mechanical properties of small volumes. In addition to properties extracted from load–depth curves, such as hardness and modulus,5 the technique can provide direct observations of slip systems,6 phase changes, dislocation nucleation and/or activation,7 and fracture.8 For Contributing Editor: Linda S. Schadler a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.219
molecular crystals, specific advantages include probing polymorphism, anisotropy, and crystal orientation.9 The elastic–plastic transition during nanoindentation can also be used to determine a “window of plasticity”; the expected range in which plastic deformation will begin to occur in low-defect materials. This allows for an estimation of the yield p
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