Indentation of a soft metal film on a hard substrate: strain gradient hardening effects

A new type of nanoindentation experiment showing the effect of a strain gradient on the flow strength of a crystalline material is conducted and analyzed. We show that by indenting a soft metal film (Al) on a hard substrate (glass) with a sharp diamond indenter a strong gradient of plastic strain is created. The true hardness of the film is observed to increase with increasing depth of indentation when the indenter tip approaches the hard substrate, in sharp contrast to the falling hardness with increasing depth in bulk materials. We associate this rise in hardness with the strong gradient of plastic strain created between the indenter and the hard substrate. We use the mechanism-based strain gradient (MSG) plasticity theory to model the observed indentation behavior. The modeling shows that the MSG plasticity theory is capable of describing not only the decreasing hardness with increasing depth of indentation at shallow indentations, as observed in bulk materials, but also the rise in hardness that occurs when the indenter tip approaches the film/substrate interface.