Elsevier

Applied Surface Science

Volume 256, Issue 6, 1 January 2010, Pages 1778-1783
Applied Surface Science

Crater geometry characterization of Al targets irradiated by single pulse and pulse trains of Nd:YAG laser in ambient air and water

https://doi.org/10.1016/j.apsusc.2009.10.003Get rights and content

Abstract

High intensities laser pulses are capable to generate a crater when irradiating metal targets. In such condition, after each irradiation significant ablation occurs on the target surface and as a result a crater is formed. The crater characterization is very important specifically for some applications such as micromachining. In this paper, the crater formation in metal targets was studied experimentally. The planar aluminum 5052 targets were irradiated by frequency doubled (532 nm), Q-switched Nd:YAG (∼6 ns) laser beam in ambient air and distilled water. A crater was produced after each irradiation and it was characterized by an optical microscope. Different laser intensities as well as pulse trains were applied for crater formation. The effects of laser characteristics in crater geometry were examined. The depth of the craters was measured by optical microscope and the diameter (width) was characterized by processing of the crater image. The results were explained in terms of ablation threshold and plasma shielding. The results show that the crater geometry extremely depends on the laser pulse intensity, the number of laser pulses, and ambient.

Introduction

High power pulsed laser are capable to generate plasma when their beam are focused on the surface of solid metal targets. The interaction of such laser pulse with target matter, and the produced plasma have been studied extensively for many years [1], [2]. These studying show that the laser characteristics, target material, and the ambient are very effective for the interactions.

At lower laser intensities (I  106 W/cm2), the target irradiation mostly affects the surface structure and topography [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. There are reports in which some material targets were irradiated by laser pulses and the surface structure was investigated. For example, by using single pulse as well as multi-pulses from Nd:YAG laser, the microstructure of titanium foil were investigated [9], [10]. In these investigations, it was shown that the laser radiation influences the materials surface morphologies and causes some effects such as large-scale periodic structures, surface creases, cones or columns structure. Such structures have been also observed on various target materials, ambient atmospheres, and laser beam characteristics [11], [12].

At higher intensities (I  106 W/cm2), the laser affects the target more significantly. In such condition, after each irradiation significant ablation occurs on the target surface and as a result a crater is formed. The size of the crater and its geometry depends on the removed ablated material. Therefore precise material removal in micromachining tightly gets involved to the crater structure. On the other word the machining can be controlled if the size of crater is known. Therefore, the crater formation is very important specifically for some applications such as micromachining [13], [14], [15], [16], [17].

Pulsed laser micromachining has become a powerful tool in scientific and industrial applications. Micromachining by laser ablation has become a very good alternative to the other traditional micromachining methods. One reason is the extensive developments in laser micromachining. Due to these developments 3-D surface structures on the micrometer scale have been achieved [13], [14], [15], [16], [17]. In laser machining very small portion of material can be removed while traditional machining does not have such advantage. Conventional patterning techniques cannot also provide high aspect ratio features and accurate depth control. In laser machining material is removed in atomic layers by ablation process and crater is formed after each irradiation. For this reason, the kerf in laser machining is usually very narrow, and the depth of laser drilling can be controlled to less than one micron per laser pulse. In such process, material can be saved. This may be important for precious materials or for delicate structures in micro fabrications. Another advantage of laser micromachining with respect to the other conventional patterning or etching techniques is its ability to remove different materials such as ceramics, metals, semiconductors, and polymers without the need to change tooling or chemical processing. Laser micromachining is specifically suitable for complex substrate geometry or when the substrate material cannot be removed by etching [17], [18]. The laser ablation process can be strongly influenced by laser beam parameters, such as wavelength, energy or fluence and pulse duration. It has been also shown that the target material, and ambient conditions are also very effective for the interactions. This process can be also affected by the material properties of the target, such as optical reflectivity, melting temperature, thermal diffusion rate. Optimum combinations of these factors are necessary to process target surface with a complex 3-D geometry. In many applications, the beam energy density (fluence) determines the irradiation effect on the target. This includes the size of the affected area, and consequent effects such as plasma, heat affected zone (HAZ) formation, and the topographical features of the crater. Other properties such as wavelength, and pulse duration, can be also important in some other situations. Lasers with wavelength spectrum from near infrared (1064 nm) to ultraviolet (248 nm), have been used for precision machining of materials. However, short wavelength laser radiation has high photon energy, and can be used for ablation of practically all materials. Both short and ultra-short laser pulses have been also applied in material ablation. However, the main object of this paper is focused on studying the crater formation by irradiating metal targets (namely aluminum 5052) with nanosecond regime laser pulse at intensities I  106 W/cm2. The experiment was performed in air and water ambient (at similar laser intensity and pulse trains) and the results were compared.

The crater formation and its geometry strongly depend on the ablation process. Root [19], Miller and Haglung [20], and Yang [21] studied solid targets irradiated by pulsed lasers and crater formation in ambient vacuum, gas, and air. The irradiation of these targets in ambient water has been also studied due to their wide applications [22].

Pulsed laser ablation of some metals has been studied previously [23], [24]. The ablation process of a metal is initiated by surface absorption of the radiation (in skin depth). The light absorption results in rapid increasing of the surface temperature. The target surface temperature rise is followed by melting, evaporation, and ionization of the vapor. The ablation mechanisms that cause the plasma plume is different for the nanosecond and ultra-short (psec or fsec pulse duration) laser pulses. It has been shown that for the ablation by nanosecond laser, the material ejection (plume) is dominated mostly by thermal processes [25]. The plasma plume contains a lot of neutral atoms, ions, and electrons from the target surface. As the vaporization and plasma formation starts at the surface of the target, the evaporated and plasma material expand toward the laser beam. This expansion of the evaporating vapor (above the surface of the target) induces a gas dynamic flow to the ambient [26]. Moreover, compression of the ambient medium ahead of the expanding mass can result in the formation of a shock wave. It has been shown that ultra-short laser pulse (psec or fsec) can cause non-equilibrium heating [27]. However, during the irradiation time in nanosecond pulse regime, the interaction of electrons and ions (electron-ion collisions) can lead to the equilibrium temperature of the plasma plume [25], [28]. Meanwhile, the vapor plume and background gas (atmosphere air, or any other buffer gas as ambient) interact each other, resulting in the confinement of the plume. Due to the recoil effect, the plasma plume is more ejected from the surface of the target.

Characterization and enhancement of the ablation of materials were experimentally studied by Dupont et al. [29]. Using common industrial laser sources from UV (248 and 308 nm), to visible (532 nm) and infrared (1064 nm) the effects of radiation on some materials (stainless steel, alumina, and silica glass) and the material ablation were studied. Kim and Grigoropoulos [30] showed that no significant surface deformation or ablation plume ejection occur during the laser pulse irradiation. However, they also showed that significant surface deformation occurs after the laser pulse. Ho et al. [31] numerically showed that the ejected high-pressure vapor generates shock waves against the ambient background pressure.

Material removal by single pulse ablation from a substance was also investigated by Yoo et al. [32], [33]. In their experiment, single-crystal silicon target was irradiated by a single-pulse Nd:YAG laser beam (forth harmonic with wavelength 266 nm) with intensities 109–1011 W/cm2.

In this paper we report evaluation of the geometry of the crater at the surface of irradiated metal targets by pulse trains of high intensity nanosecond laser in air and water. In the experiment, aluminum 5052 targets were irradiated by second harmonic of Nd:YAG laser pulse (at 532 nm and maximum repetition rate of 15 Hz) with intensities up to 1011 W/cm2 in atmospheric ambient air and distilled water. Single pulse as well as pulse trains, i.e. 10, 20, 50, 100, 200, and 500 numbers of pulses were also used for irradiations, and the results were compared.

Section snippets

Experiment and data analysis

Fig. 1 shows a schematic setup for the experiment. The laser beam from a Q-switched Nd:YAG (second harmonic at 532 nm, ∼6 ns) was focused by a lens (f  15 cm) at the surface of aluminum 5052 target. The targets were prepared by electro polishing to have average surface roughness of less than 0.5 μm. The irradiation was performed in ambient air and distilled water by using single pulse, as well as 10, 20, 50, 100, 200, and 500 pulses. In order to provide similar condition for the irradiation (in air

Results and discussion

Fig. 3 shows typical image of craters on the target surface. The images are for an aluminum 5052 target irradiated in air (top) and distilled water (bottom) by 1, 10, 20, 50, 100, and 200 pulses (these are indicated by (a)–(f) respectively). The laser intensity at the target surface for both cases was I  8 × 1010 W/cm2. The results show that the crater width (and depth) increases as the pulse numbers increases. The detailed analysis (explained in this section) shows that there are great differences

Conclusion

Planar aluminum 5052 targets were irradiated by frequency doubled Q-Switched nsec Nd:YAG laser pulses and the crater geometry was characterized. By focusing the laser beam, intensities of ∼1010–1011 W/cm2 were provided at the target surface. The targets were irradiated by 1 pulse as well as 10, 20, 50, 100, 200 pulse trains, and the results were compared. The results show that both crater depth and diameter sizes are strongly depends on the laser characteristics, the number of irradiated pulses,

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