The onset of phase explosion and the role of damage accumulation in ultrafast laser processing of common metals in air

摘要

The aim of the current work is two-fold: First, the aim is to investigate the transition, for a number of metals, from a smooth ablation process to an explosive one. Secondly we aim to study the dependence of the ablation threshold in metals on the applied laser shot number. Ablation of polycrystalline metal samples was performed with multiple pulses from a femtosecond laser (Clark MXR, CPA2001). Morphological investigations of the laser processed areas were recorded using optical and scanning electron microscopies (SEM) and white light interferometry. The investigations have been carried out on sample matrices which were processed for a range of laser fluences and applied laser shots for four metals. Data obtained on ablation rates, ejected particle sizes and crater morphologies prove that ablation changes from a smooth to an explosive process at high fluences, as identified with changes in the material removal mechanisms. Threshold fluences were measured for both the smooth and explosive ablation processes. The ablation threshold fluence depends on the number of pulses applied to the same spot. It was found that the build up of laser induced mechanical stresses, due to the heating and cooling cycles of the samples between consecutive laser shots, plays an important role in the material modification process. It leads to the observed dependence of ablation threshold on shot number, which is described by a power law based on a mechanical fatigue model. The strength of the dependence is governed by the incubation coefficient, 5, which has been measured for all materials studied. It is expected that the build up of laser energy or incubation leads to the accumulation of material defects and residual stresses which has the effect of lowering the energy required to cause ablation using a large number of incident laser shots.