Crater formation caused by nanoparticle impact: A molecular dynamics study of crater volume and shape

We present molecular-dynamics simulations of cratering induced by projectiles containing N congruent to 10-10(6) atoms in the velocity regime of 1-70 km/s. Self-bombardment of a condensed Ar and a Cu target are studied. We corroborate the earlier finding that for small clusters, N less than or similar to 1000, above a threshold regime, the crater volume scales linearly with the total impact energy E; by scaling energies to the target cohesive energy U, crater volumes of such diverse materials as condensed Ar and Cu coincide. At threshold E-th, craters are shallow. They become hemispheric at energies similar to 5E(th). Part of the material excavated from the crater is sputtered. This fraction decreases with cluster size N. Relatively less material is sputtered from an Ar target than from a Cu target. Larger cluster impact, which we simulate up toN = 3 x 10(6), shows a stronger size effect, such that the resulting craters increase slightly more than linearly with total energy. This finding is discussed in light of available experimental data for mu m- and mm-sized projectiles. Simulations on ductile samples containing pre-existing defects (nanocracks) show that such pre-existing damage plays a negligible role for crater formation and size in metals.