Why Nanoprojectiles Work Differently than Macroimpactors: The Role of Plastic Flow

Abstract

Atomistic simulation data on crater formation due to the hypervelocity impact of nanoprojectiles of up to 55 nm diameter and with targets containing up to 1.1×10(10) atoms are compared to available experimental data on μm-, mm-, and cm-sized projectiles. We show that previous scaling laws do not hold in the nanoregime and outline the reasons: within our simulations we observe that the cratering mechanism changes, going from the smallest to the largest simulated scales, from an evaporative regime to a regime where melt and plastic flow dominate, as is expected in larger microscale experiments. The importance of the strain-rate dependence of strength and of dislocation production and motion are discussed.

Crater formation by nanoparticle impact: contributions of gas, melt and plastic flow

“…Linear energy dependences above a threshold energy, similar to equation (2), have been observed earlier in the context of crater formation, plastic surface deformation and target atom displacements induced by cluster impact on metal and Si surfaces [15,17,19,36,37]. Such a linear dependence shows that a fixed fraction of the impact energy is used for crater excavation; it is expected to break down if part of the impact energy is deposited deep inside the target-this might be the case for small clusters impinging at high energy.…”

“…The emerging plasticity can be associated with a hardness increase in the near-crater region, as compared to the pristine material before bombardment. Hardness values can be estimated from dislocation densities [17], using a Taylor hardening model, and lead to reasonable agreement with experimental values for macroscopic impactors [14].…”

“…Such a linear dependence shows that a fixed fraction of the impact energy is used for crater excavation; it is expected to break down if part of the impact energy is deposited deep inside the target-this might be the case for small clusters impinging at high energy. It is a salient feature of this and previous investigations [17] that the energy linearity of the crater volume holds over a wide range of impact energies and cluster sizes.…”

Direct Experimental Observation of a New Mechanism for Sputtering of Solids by a Large Polyatomic Projectile: Velocity-Correlated Cluster Emission

“…Sputtering-related phenomena include, for example, the impact-induced formation of periodic erosion patterns (self-organized) at the nanoscale [2,3], enhanced sputtering of nanostructures associated with the explosive ejection of large clusters [4], surface smoothing [5], growth of nanostructured surfaces from size-selected cluster ions [6], and mechanisms of microscopic to mesoscopic crater formation [7,8]. The last one is also relevant for analyzing erosion features of exposed surfaces of a spacecraft [9], or natural astronomical objects [10], due to impact of hypervelocity interstellar dust nanograins, and sputtering erosion inside ion thrusters [11].…”

“…The main recent interest regarding fundamental aspects is related with differences between the characteristics of sputtering induced by polyatomic or cluster projectiles as compared with that induced by atomic projectiles [7,8,[12][13][14][15][16][17][18][19][20][21][22][23][24]. This also reflects an increasing interest in new cluster-surface impact phenomena in general [24].…”