Combined effects of surface stress and strain gradient on instability of laser-excited solid ultra-thin films

Coupled diffusion-mechanical processes play a vital role in surface laser modification technologies. This article proposes an analytical coupled deformation-diffusion model for the dynamics of disorder carriers (atomic point defects) to study the effects of the surface elastic properties and the strain gradients on the instability of a laser-excited solid film with a nano-scale thickness. Dispersion relation of the diffusion-flexural instability (DFI) is obtained and analyzed. It is shown that the analysis, which simultaneously includes the surface elasticity, nonlocal second and fourth order strain gradient effects as well as the bending of the surface layer due-to normal and lateral defect-induced forces, gives two maxima on the instability growth rate curve at sufficiently far above the instability threshold. On the contrary, the growth rate has a single maximum if the excess of the instability threshold is insignificant. The proposed model predicts the dependence of the nano-relief period on the film thickness, the properties of the energy source and material properties. The effect of the process of clustering of atomic defects due to DFI on local or periodic peeling of deposited elastic layers on the substrate is also considered.