Nanomechanics of RDX Single Crystals by Force-Displacement Measurements and Molecular Dynamics Simulations

Nanoenergetic material modifications for enhanced performance and stability require an understanding of the mechanical properties and molecular structure-property relationships of materials. We investigate the mechanical and tribological properties of single-crystal hexahydro-1,3,5-trinitro-s-triazine (RDX) by force-displacement microscopy and molecular dynamics (MD). Our MD simulations reveal the RDX reduced modulus (Er) depends on the particular crystallographic surface. The predicted Er values for the respective (210) and (001) surfaces are 26.8 and 21.0 GPa. Further, our simulations reveal a symmetric and fairly localized deformation occurring on the (001) surface compared to an asymmetric deformation on the (210) surface. The predicted hardness (H) values are nearly equal for both surfaces. The predicted Er and H values are ∼33% and 17% greater than the respective experimental values of 0.798 ± 0.030 GPa and 22.9 ± 0.7 GPa for the (210) surface and even larger than those reported previously. Our experimental H and Er values are ∼19% and 9% greater than those reported previously for the (210) surface. The difference between the experimental values reported here and elsewhere stems in part from an inaccurate determination of the contact area. We employ the parameter √H/Er, which is independent of area, as a means to compare present and past results, and find excellent agreement, within a few percent, between our predicted and experimental results and between our results and those obtained from previous nanoindentation experiments. Also, we performed nanoscratch simulations of the (210) and (001) surfaces and nanoscratch tests on the (210) surface and present values of the dynamic coefficient of deformation friction.