Nanostructured Surfaces Yield Earlier: Molecular Dynamics Study Of Nanoindentation Into Adatom Islands

Using molecular-dynamics simulation we investigate nanoindentation into an fcc metal surface covered by an adatom island. Small islands are pushed into the solid and transported away by prismatic dislocation loops, and no defects remain under the indenter; thereafter indentation proceeds as for a flat surface. For large islands, the island boundaries do not influence indentation. Most interesting is the intermediate case, where the island size is comparable to the contact radius of the indenter at the onset of plasticity. Here plasticity starts immediately at the surface under the weakest step edges bounding the island. The dislocations are pinned to the step edges and dislocation activity remains localized under the island. For this intermediate case, the surface is weakest and yields earliest.