Most of GBs have finite dislocation absorption ability, which could
accommodate few pairs of dislocations. After saturated by enough
dislocations, GBs either are taken place as shear planes for rigid
translation, or considered as barrier for formation of dislocation
pileup [38]. In atomic shallow indentation, the finite penetration
depth ensures that the number of dislocation (partial or full) generated
is much less compared to deep indentation or severe bulk
deformation, thus the dislocation pileup can hardly be formed,
which could be one of the reasons why Fig. 3 does not show the H-P
effect above the transition size dc.