Depth dependent strain rate sensitivity and inverse indentation size effect of hardness in body-centered cubic nanocrystalline metals

Size effects on hardness (H) and strain rate sensitivity (m) of nanocrystalline (NC) body-centered cubic Mo thin film were examined under nanoindentation testing. Contrast to existing reports that there was no indentation size effect on hardness in NC metals, inverse indentation size effect (ISE) in NC Mo was observed for the first time at penetration depths ranging from 15 to 200 nm, at all the loading strain rates applied. In addition, the strain rate sensitivity of NC Mo exhibited strong dependence on penetration depth, increasing dramatically with decreasing penetration depth. Surface effects related to two deformation mechanisms were proposed to be responsible for the observed inverse ISE on H and depth dependent m. Specifically, the mobility of screw dislocation/component and the diffusion length of interfacial diffusion were altered as the deformed region underneath the indenter was approaching the free surface, resulting in the unusual size effects in NC Mo.