Nanoindentation size effect in single-crystal nanoparticles and thin films: A comparative experimental and simulation study

This work studies the strength dependence of single-crystal metal specimens of submicrometer size on their dimensions. The emphasize is on the plasticity mechanisms controlled by nucleation of dislocations in the presence of free surfaces. We employed a dewetting method to produce an ensemble of faceted, single-crystal, defect-free gold nanoparticles on sapphire substrates. Nanoindentation tests performed on these particles reveal that their deformation compliance increases with decreasing particle size, i.e. as their lateral dimensions decrease. Gold thin films of similar heights, which have no lateral free surfaces, exhibited much higher resistance to plastic deformation than the particles. To understand the role played by lateral free surfaces on the strength of the particles and thin films, we performed atomistic molecular dynamic simulations of the indentation process. The simulations showed that dislocations are nucleated at the interface between the indenter and the particles/films. These dislocations annihilated on the lateral surfaces of the faceted particles, leading to defect-free particles during indentation, while the dislocations accumulated around and beneath the indenter in the thin film, resulting in complex, sessile dislocation structures. Particles elongated in the lateral dimensions showed an intermediate behaviour. The back-stress of the immobile dislocations made the nucleation of new dislocations more difficult and caused hardening both of the film and of the elongated particles.