Consider a thin film which is homogeneously energized to an energy of E0 (eV/atom); for comparison among different materials, we refer this energy to the cohesive energy U of the material, ε=E0/U. Fig 1. shows temporary void creation in Cu at E0=0.8 eV/atom, ε=0.23 [movie]. The film is irradiated at the top surface; laterally the slab is assumed to continue periodically.
Several movies show the processes in a model Lennard-Jones film. Below the ablation threshold, [ε=0.20, movie], the material will expand due to thermal expansion; oscillations induced by the relaxation of the high thermoelastic pressure are visible. If the temperature reached is high enough, the film may melt. Around the threshold, [ε=0.235, movie], voids (vapour bubbles) are formed temporarily, but decay again. Beyond the threshold [ε=0.27, movie], the material tears. This spallation occurs when the tensile pressure exceeds the yield strength of the material.
These processes occur universally in any material. The [movie] shows the same process in a Lennard-Jones solid for ε=0.33. Note that the threshold is higher in the smaller system than in the larger system described above. The processes in this Lennard-Jones system for other energizations are displayed in [ε=0.17, movie] (only evaporation) and [ε=0.39, movie] (spall).
Further information: Upadhyay, Inogamov, Rethfeld, Urbassek, Phys Rev B (2008).