Indentation size dependent plastic deformation of nanocrystalline and ultrafine grain Cu films at nanoscale

Nanoindentation creep tests were performed in the depth range from about 28 to 190 nm on nanocrystalline (NC) and ultrafine grain (UFG) Cu films. Pronounced indentation size effects on hardness, creep strain rate (epsilon center dot), and strain rate sensitivity (m(c)) are observed. Both epsilon center dot and m(c) are dependent not only on contact depth (h(c)) but also on grain size. The experiment results and analysis support that the creep deformation of NC and UFG Cu is dominated by grain-boundary-mediated process and diffusion along the interface of tip sample, respectively, under a critical h(c) and dislocation-mediated process begin to work as h(c) increases further.