Photomechanical spallation of molecular and metal targets: molecular dynamics study

Abstract

Microscopic mechanisms of photomechanical spallation are investigated in a series of large-scale molecular dynamics simulations performed for molecular and metal targets. A mesoscopic breathing sphere model is used in simulations of laser interaction with molecular targets. A coupled atomistic-continuum model that combines a molecular dynamics method with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons is used for metal targets. Similar mechanisms of the laser-induced photomechanical spallation are observed for molecular and metal targets. For both target materials, the relaxation of compressive stresses generated under conditions of stress confinement is found to be the main driving force for the nucleation, growth and coalescence of voids in a subsurface region of an irradiated target at laser fluences close to the threshold for fragmentation. The mechanical stability of the region subjected to the void nucleation is strongly affected by the laser heating and the depth of the spallation region in bulk targets is much closer to the surface as compared with the depth where the maximum tensile stresses are generated. Two stages can be identified in the evolution of voids in laser spallation, the initial void nucleation and growth, with the number of voids of all sizes increasing, followed by void coarsening and coalescence, when the number of large voids increases at the expense of the quickly decreasing population of small voids. The void volume distributions are found to be relatively well described by the power law N(V)∼V-τ, with exponent gradually increasing with time. Comparison of the simulation results obtained for Ni films of two different thicknesses and bulk Ni targets suggests that the size/shape of the target plays an important role in laser spallation. The reflection of the laser-induced pressure wave from the back surface of a film results in higher maximum tensile stresses and lower threshold fluence for spallation. As the size of the film increases, the locations of the spallation region and the region of the maximum tensile stresses are splitting apart and the threshold fluence for spallation increases.