Surface-effect territory in small volume creep deformation

It is yet unclear how far surface effects can dominate small volume creep deformation in the surface layer of a metallic solid. We report experimental results of the apparent activation volume of single, ultrafine-grained, and nanocrystalline Cu over a range of nanoscale displacements. The dependence of the apparent activation volume on the depth and grain size was determined using nanoindentation creep tests. The surface-affected deformation regimen, within which interfacial diffusion between the nanoindenter tip and the sample totally dominates the creep behavior, was quantitatively determined to be below ~12 nm. As the initial creep depth is increased, the dominant mechanism is shifted from interfacial diffusion to grain-boundary diffusion as the contribution of the surface effects gradually vanishes when the indenter penetrates deeper into the sample (i.e., further away from the external surface).