MD simulations [9,8] have concluded that the strongest grain
size for nanocrystal metal, for instance copper, is about 10 nm.
However, it is not easy to conclude from the experimental side
since the experimental data [5,10] for grain size below 100 nm were
largely scattered because of difficulties in preparing high-quality
samples, as well as problems of measurement of the average grain
size. Koch et al. [10] reviewed the literature which gives the inverse
H-P effect at the finest grain sizes, they concluded that only a few
experiments [11] which have reported the inverse H-P effect are
free from obvious or possible artifacts.