Effect of surface roughness on the ultrashort pulsed laser ablation fluence threshold of zinc and steel

Laser ablation is a subtractive micromachining technique, which can be employed to improve the surface functionality of a product by applying a laser-induced texture to the surface. It is a flexible and precise manufacturing process compared to other techniques like electric discharge texturing, chemical etching, shot blasting and electron beam texturing [1,2]. The absorption of laser light and subsequent heating of the material being machined depends not only on the optical properties of the material, but also on its initial surface condition. This means features such as roughness, oxidation or defects etc., of the targeted material play a major role in the efficiency of material removal and ultimately the resulting quality of the machined surface. For continuous wave (cw) laser processing, a higher surface roughness typically results in better absorption of laser energy through scattering from surface irregularities [3,4,5]. In the case of ultra short laser pulses, with pulse durations in the picosecond regime, or shorter, the absorption of laser energy (photons) and modification of the material such as ablation take place at different time scales. That is, absorption of photons typically takes place on the femto- to picosecond time scale, whereas modification of material takes place on the nanosecond timescale or longer [6]. To study the effect of surface roughness on the fluence threshold above which ablation occurs in metals in ultrashort pulse regimes, ablation of bulk zinc, galvanized steel and mild steel is performed with single, as well as multiple picosecond laser pulses at wavelengths of 1030 nm and 515 nm and at different preliminary surface roughness (Ra) values ranging from 0.03 µm to 1.5 µm. Using confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM), the morphologies and crater dimensions of thousands of ablated craters is studied, by varying the laser fluence over a wide range, as well as varying the number of pulses (N) on the same location between N=1 and N=50. From the analysis, it can be concluded that, within the boundaries of our experimental (laser) conditions, the ablation threshold increases with increasing surface roughness. In addition, the surface roughness of the underlying substrate of a coated material (galvanized steel) also affects the crater morphology and ablation threshold.