Nanoindentation of polycrystalline Pd hollow nanoparticles: Grain size role

Abstract

Polycrystalline hollow nanoparticles present a unique combination of strength and flexibility. However, the exact role displayed by their grain structure in mechanical properties has not been yet fully understood. Here, by means of molecular dynamics simulations, the role of grain boundary structure during the nanoindentation of metallic hollow nanoparticles with a polycrystalline shell was investigated. Our simulations were performed for a range of grain sizes and shell thicknesses, including the large strain regime. Our results show that hNP mechanical properties can be controlled by tuning the grain size of the polycrystalline shell, following an inverse Hall-Petch type dependence with the grain size. Deformation involves dislocation activity, twin hardening, grain boundary sliding, coalescence, and rotation. For single crystal shells at large strain there is hardenning following the closure of the internal cavity. For nanocrystalline shells at large strains a constant flow stress regime is observed even for deformations as high as 80%, thanks to grain boundary activity. Surprisingly, some particular grain size not only leads to an improvement in strength, but also a flow stress higher than the observed in their single-crystalline counterparts. Our work, suggest that grain boundary structure can be employed to improve and tailor desired mechanical properties in hollow nanostructures.