Microstructural length-scale effects in the nanoindentation behavior of thin gold films

Nanoindentation experiments have been conducted on thin gold films of varying thickness and grain size in an effort to study the effects of microstructural length scales on indentation plasticity. We show that the deformation of epitaxial and coarse-grained films is characterized by Hertzian elastic loading, followed by discrete displacement bursts and subsequent softening with increasing depth of indentation. In contrast, the loading response of fine-grained films is continuous, and characterized by elastic–plastic behavior from the earliest stages of indentation, and gradual hardening with increasing indentation depth. We argue that these different behaviors are closely related to the mechanisms of plasticity initiation. Homogeneous dislocation nucleation at theoretical shear stresses is required for indentation into dislocation-free, single-crystalline volumes while grain boundaries provide a ready source for dislocations in fine-grained films. This picture of indentation plasticity is additionally supported by observations of grain boundary proximity effects on the critical load at which plasticity commences.