The hardness measured by the nanosize indenter under atomic indentation is examined for the cases of
nanocrystalline nickel by means of molecular dynamics (MD) simulations. The grain size effect observed
is different from the one by uniform deformation or deep indentation. The results show that hardness
can only show inverse Hall–Petch (H-P) effect, no H-P effect is observed with the grain size up to 40 nm.
Grain boundary (GB) absorption of the localized strain is the main deformation mechanism when the
indenter size and the depth both come to nano size. The area of plastic zone generated beneath the tip is
strongly dependent on the GB density, sample with small grain size results in larger plastic area, which
leads to the softer response of hardness.