Cryst. Res. Technol. 51, No. 7, 414–427 (2016) / DOI 10.1002/crat.201500301 Original Paper Mechanical properties of pentaerythritol tetranitrate(PETN) single crystals from nano-indentation: Depth dependent response at the nano meter scale Meiyu Zhai and Gregory B. McKenna ∗ Received 3 November 2016, revised 6 June 2016, accepted 10 June 2016 Published online 6 July 2016 The current work presents the results from an investiga- tion of the mechanical behavior of single crystals of the energetic material pentaerythritol tetranitrate (PETN) us- ing a nano-indentation technique. The indentation tests have been performed on the maximum growth habit face (110) of the PETN, using a spherical, Berkovich and an aniso- metric (wedge-shaped) tip. The load displacement curves along with analysis have been used to extract the mechani- cal properties and identify the anisotropic indentation mod- ulus for the PETN. The indentation moduli of the PETN sin- gle crystal were found to decrease as indentation depth in- creases and become independent of indentation depth for depths greater than 200 nm for the spherical indenter and 300 nm for the anisometric wedge indenter and Berkovich tip. The indentation modulus obtained from spherical tip in- dentation is compared with the results calculated by using literature values of the elastic constants. The wedge inden- ter measurements and Berkovich indentation at various tip orientations are different due to the anisotropy of the PETN. The yield behaviors of the PETN single crystal were also ex- plored using both spherical and wedge tip indentation and the differences are discussed. 1 Introduction Pentaerythritol tetranitrate (C(CH 2 ONO 2 ) 4 ) is a crys- talline molecular explosive with both the fuel and oxi- dizer on the molecule. It is widely used in military and civilian applications [1–3], and research on the perfor- mance of such energetic materials has received intense attention, e.g., kinetic and thermodynamic properties [4, 5], crystal structure and surface morphology have been investigated [1, 6, 7]. Knowledge of the mechanical properties of energetic materials is also of significant in- terest, and several experimental approaches have been used to obtain the elastic constants for PETN single crys- tals, e.g., X-ray diffraction, ultrasonic measurements, and Brillouin scattering [8–11]. As a technological development, nano-indentation has become a powerful method to explore the mechan- ical properties of materials and its capability for testing small amounts of material makes it ideal for the study of PETN single crystals. Handling large samples of energetic materials is potentially dangerous and methods of work- ing with small sample volumes are important. Therefore, instrumented nano-indentation provides a good tech- nique to measure the mechanical properties of small quantities of material [12]. In the present work, nano-indentation was used to capture the mechanical behaviors of the surface of PETN single crystals. Quasi-static ramp loading tests were per- formed using spherical tip, wedge tip and Berkovich tip geometries, and the Oliver and Pharr [13] method was used to extract apparent indentation moduli from the initial slope of the unloading curves. A depth dependent indentation modulus was observed for spherical tip indentation, wedge tip in- dentation and Berkovich tip indentation when the indentation depths were smaller than 200nm to 300 nm. The indentation moduli increased as the indentation depth decreased in the depth dependent region. This unexpected phenomenon is similar to the findings reported by Tweedie et al [2], in which they investi- gated the surface mechanical properties of the glassy, amorphous polystyrene, polycarbonate and poly(methyl ∗ Corresponding author: e-mail greg.mckenna@ttu.edu, Phone: 806.742.3553, Fax: 806.742.3552 Department of Chemical Engineering, Texas Tech University, Box 43121, Lubbock, TX 414 C  2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim