Grain size effect on nanomechanical properties and deformation behavior of copper under nanoindentation test

The mechanical properties and deformation behaviors of copper with different grain sizes have been investigated in this study by instrumented nanoindentation. Following the Hall-Petch relation, the hardness of copper specimens increased as the grain size decreased. Dislocations were clearly observed in deformed regions around indent marks, indicating plastic deformation by dislocation formation and sliding. However, the hardness of electroless copper films with an ultrafine grain size of only 10nm dropped. Voiding at grain boundaries and triple grain junctions was observed as a consequence of grain-boundary sliding and grain rotation, which was expected as the dominant deformation mechanism resulting in the reduced hardness. The critical shear stresses for the initiation of plastic deformation in the copper specimens with large grain sizes were close to the theoretical value and comparatively much lower for electroless copper films with an ultrafine grain size.