Computer simulations of laser ablation from simple metals to complex metallic alloys

Abstract

In this work, a method for computer simulations of laser ablation in metals is presented. The ambitious task to model the physical processes, that occur on different time and length scales, is overcome to some extent by the combination of two techniques: Molecular dynamics and finite differences. The former is needed to achieve atomistic resolution of the processes involved. Material failure like melting, vaporization or spallation occur on the atomic scale. Light absorption and electronic heat conduction, which plays the major role in metals, is described by a generalized heat conduction equation solved by the finite differences method. From the so-called Two-Temperature Model temperature, density and pressure evolution - both in time and space - can be derived. With this, various studies on laser heated metals were done. For reasons discussed in more detail later, aluminum was chosen as a model system for most simulations on isotropic materials. As a more complex structure, the metallic alloy Al13Co4 was used because of its special material properties. As an approximant to the decagonal phase of Al-Ni-Co, the alloy shows an anisotropy in its transport properties, e.g. an anisotropic heat conduction.

It will be shown, that the model is able to describe the physics in laser heated solids on time scales from 100 fs up to the ns-scale properly. Great insight was gained about the processes occuring during and shortly after the laser pulse. Many of the quantities interesting for experimentalists can be predicted by the theory. From the simulations relevant parameters like the electron-cooling time or the important ablation threshold were calculated. All values match their experimental counterpart very well.

Die vorliegende Arbeit beschäftigt sich mit der Laserablation in Metallen. Ziel ist es, mit Hilfe von numerischen Simulationen das Verhalten von Metallen nach der Bestrahlung mit intensiven Laserpulsen vorherzusagen. Die Arbeit ist inhaltlich in zwei Teile gegliedert. In der ersten Hälfte werden theoretische Grundlagen, eine qualitative Beschreibung der Ablation und die Implementierung des Modells gegeben. Im zweiten Teil folgen Ergebnisse sowie, falls vorhanden, Vergleiche mit Experimenten. Die Arbeit schließt mit einer Zusammenfassung und einem Ausblick.