Nanoindentation of nanoporous tungsten: A molecular dynamics approach

Abstract

Nanoporous metals, also known as metallic nanofoams, offer a wide range of functionalities and improved mechanical properties enabled by nanoscale effects. In particular, tungsten nanofoams are a novel class of materials with potential applications as radiation-resistant coating, and they share some similarities with the fuzz structure arising in fusion devices. We approach their study by nanoindentation tests using molecular dynamics simulations, for a single crystal nanofoam. To help understanding the foam mechanical behavior we also carry out simulations of W nanowire compression, finding elastic moduli of 375–450 GPa, and plastic yielding at 15 GPa. For the nanofoam, we obtain an elastic modulus of 64 GPa, in reasonable agreement with experiments, but our hardness value of 15 GPa is higher, likely due to nanocrystalline effects in the experiment. Atomistic simulations reveal that plastic deformation is caused by a combination of dislocations and twinning in the neighborhood of the indenter surface. It was found that twins also promote complete amorphization of some thin filaments in contact with the indenter tip. Dislocation activity also produces vacancies in the plastic region. Besides, the displacement induced by the indenter also drives changes of the network topology mainly due to densification, filament bending and twisting. Dislocation density is lower in the foam than in bulk indentation, due to the dislocation annihilation on the filament surfaces, but also because of changes in network topology help accommodate strain. Based on the simulation results, a nanoporous bcc foam behaves differently than a fcc foam, but still displays excellent mechanical properties for a low density material, and also offer additional technological advantages.