SIZE-DEPENDENT HARDNESS OF SILVER SINGLE-CRYSTALS

The hardness of thick, high-purity, epitaxially grown silver on sodium chloride is found to be dependent on the size of the indentation far sizes below similar to 10 mu m. The measurement of the size effect has been made in two ways. In one, the hardness has been calculated from the load-displacement curve obtained from an instrumented microhardnes s testing machine and assuming a geometric self-similarity in the inde nter shape. In the other measurement, the hardness was obtained from t he load exerted by the microhardness tester divided by the indentation impression area as measured by atomic force microscopy. The observed variation in microhardness with indentation size is consistent with a simplified strain gradient plasticity model in which the densities of the geometrically necessary and statistically stored dislocations are fitting parameters. An equally good fit can also be made with a simple geometric scaling relationship. Transmission electron microscopy obse rvations of a thin (similar to 50 nm) epitaxial gold him embedded in t he silver layers revealed that the deformation was primarily restricte d to the sharp edges of the indentation. In addition, deformation twin ning within the indentation impression was observed on the{111} planes .