Experiment [28] and simulation [29]
have shown that the hardness at hundred nanometers depth can
reach as high as 2–5 GPa, and further experiment [18] indicated
that the hardness at few nanometers depth can be even as much
as 20 GPa. Our results also agree with results [30] from indentation
test on nano nickel coating, which gave hardness a value of around
20 GPa in a depth of few nanometers.