The effect of damage accumulation behaviour on ablation thresholds and damage morphology in ultrafast laser micro-machining of common metals in air

Detailed studies on the ablation of metal targets in air with femtosecond laser pulses are reported. The laser pulses used for the study were 775 nm in wavelength, 150 fs in pulse duration, and the repetition rate used was 100 Hz. Ablation thresholds of copper, titanium, niobium, and stainless steel are measured. The ablation threshold fluence depends on the number of pulses applied to the same spot. The strength of this dependence is governed by the incubation coefficient, S, which has been determined along with the single-shot ablation threshold, phi(th)(1), for all the metals studied. The accumulation behaviour results from the build-up of plastic deformation induced by the laser pulses. Two distinct ablation phases have been identified from ablation rate curves and an SEM investigation of the ablated morphologies. Ripples were observed on the material surface during the gentle ablation phase, whereas much rougher structures occurred during the stronger ablation phase. Morphological changes of the ablated regions were studied as a function of fluence and incident laser shot number. The observed morphologies seem to suggest that normal vaporisation and phase explosion are the most probable physical mechanisms responsible for material removal during ablation. (C) 2004 Elsevier B.V. All rights reserved.