Nanosecond pulsed lasers have been widely applied to interact with and characterize many different materials. For the purpose of a broader application, the current challenge is to achieve a speedup of ablation process, which is commonly thought to be possible by raising the on-target laser intensity. But the use of high intensity lasers results in severe laser-matter-plume interaction, leading to unwanted effects (e.g. saturation, shielding and thermal damage), which further affect the ablation process and ablation quality. However, laser-matter-plume interaction and its effects on ablation characteristics during laser scanning ablation processes are not well understood. In this paper, shadowgraph images and optical images during a laser ablation process were taken with a pump-probe shadowgraph imaging setup and an ultrahigh-speed camera. The results demonstrate that, under a high incoming laser density, laser-matter-plume interaction presents a periodical process, and thus cause a major impact on ablation regimes and microstructure formations. Moreover, the characteristics of micromorphologies and ejected particles suggest that the laser-matter-plume interaction has a significant influence on the ablation process, which, in turn, provides a more comprehensive understanding of the influence of laser-matter-plume interaction on the scanning ablation process. Consequently, laser-matter-plume interaction and its influence on the ablation process were summarized and clarified.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Over the past decades, nanosecond pulse laser machining has been used in a broad range of applications, such as material removal and material modification, due to the high precision and adaptability of micro/nanoscale processing [1–3]. Nevertheless, thermal effects, such as heat accumulation and heat conduction, are prone to occur during nanosecond laser machining, which can create a large heat-affected zone and heat defects, such as chipping and cracks [4–6]. In addition, a shielding effect can arise due to the formation of severe plumes (free electrons, self-trapped excitons, vapor and nanoparticles) above the irradiated spot, leading to complex, difficult-to-monitor and difficult-to-control interactions between successive laser pulses, the material and the plume [7,8].
With the purpose of enhancing the controllability of laser micromachining, an understanding of the reactions between laser beams and substrate materials is of interest for both academic and engineering prospects. In the past decades, substantial studies have been carried out to expound the laser-matter interaction during pulsed laser ablations as a function of the variable parameters and their effects on ablation quality [9–11]. By using a low intensity laser, Sokolowski-Tinten et al. investigated macrotextures on a Si wafer after irradiation and clarified the fundamental thermodynamic and hydrodynamic processes that occur without plasma formation . Marla et al. reported that for nanosecond pulsed laser micromachining of Al, vaporization is a more dominant mechanism than other mechanisms, such as vaporization and melt flow . Miotello and Kelly reported that when a target material is irradiated with a sufficiently high intensity laser, it can be heated above the equilibrium boiling temperature, and explosive boiling will become the main ablation mechanism . Moreover, for nanosecond pulse laser ablations, other mechanisms have been explored that are related to laser-matter interactions, including phase explosion [15,16], chemical reactions , incubation effects , and fragmentations or heterogeneous vapor nucleations .
In addition to the experimental research, mathematical modeling has also been developed to explain laser-matter interactions. To exploit the vaporization mechanism in the laser machining process, Martynyuk et al. first employed the Hertz-Knudsen equation to calculate the flux of atoms leaving a target’s surface . The Hertz-Knudsen equation has since been widely used in evaluating the velocity of surface recession during laser irradiations [21,22]. By using an inconstant boundary heat conduction equation, Peterlongo et al. ascertained the downward motion of the top surface due to vaporization . With the intent of achieving a higher prediction accuracy, Bulgakova et al. employed a similar model to predict the ablation rate, and further incorporated the moving boundary heat conduction equation with the latent heat of fusion . Instead of modeling the mechanism of material removal, Cadot et al. reported that based on empirical calibrations, a precise prediction of the removal rate of a target material can be realized, and that real-life machining can be mathematically simulated for pulsed laser micromachining .
In the meantime, the plume and relative shielding effects during high intensity laser irradiations have been studied in previous publications [26,27]. Bulgakov et al. focused on an analysis of the generation of ablation plumes and clarified that the dynamics of plume formation are similar to an underexpanded jet . By observing material ejections during nanosecond laser machining of fused silica, Stavros et al. summarized that there were five distinct phases of the material response . Yuan et al. employed a YAG laser incident on cement to demonstrate that complex laser-plume interactions play an essential role in material ablation processes . Moreover, Balaz et al. established a model to elucidate the plume shielding effect, which considered radiation absorption by inverse Bremsstrahlung mechanisms . Amoruso et al. reported that, in addition to inverse Bremsstrahlung, radiation absorption by photoionization should also be considered . Furthermore, Rozman et al. demonstrated that the effect of Mie absorption due to clusters inside the plume is also significant . Pangovski et al. investigated the plume dynamics during picosecond laser ablation of H13 steel using high-speed digital holography, and suggested that the effect of an increased plume absorption and a concurrent shift to an energetically costlier, vapour-driven ablation mechanism resulted in the decrease of ablation efficiency . These studies mainly focus on the plume dynamics and laser-plume interactions during the irradiation of a single laser pulse, or multiple laser pulses on the same spot. However, the laser-matter-plume interaction and its effects on ablation characteristics during the laser scanning ablation process, which are the pivotal quest to realize better controllability in micron/nanoscale fabrication, have barely been investigated.
In this paper, the dynamics of laser-matter-plume interaction and its corresponding ablation characteristics during laser scanning ablation processes have been systematically studied. First, the dynamics of laser-matter-plume interaction and the ablation process under different laser parameters were observed using a time resolved shadow graphic imaging system. The results provide detailed information about the evolution of laser-matter-plume interaction on the irradiated spot during a nanosecond laser scanning ablation process. Moreover, the characteristics of the micromorphologies affected by laser-matter-plume interaction were analyzed, which correspondingly map the ablation details of a moving workpiece at any given moment while the workpiece passes through the laser irradiation. A qualitative analysis of the ejected particles was conducted to determine their characteristics. Consequently, the effects of scanning speed and mono-pulse energy on laser-matter-plume interaction were clarified.
2. Test details
The test setup is schematically illustrated in Fig. 1. A pulsed fiber laser (IPG, No: YLP-1-100-20-20-CN, Germany) with a wavelength of 1064 nm, a pulse width of 5-7 ns, a maximum repetition rate f of 200 kHz and a maximum mono-pulse energy E0 of 1.0 mJ was used for the pulsed laser scanning ablation test. The laser beam has a near Gaussian profile, a divergence close to the diffraction limit, and a focused diameter of 31.5 µm. In the pre-experiment of ablation on WC-Co, it is found that with the laser fluence above 1.2 $\times$ 109 W/cm2, the formation of plume is severe, and the plume shielding can be not ignored during laser ablation process. Based on this consideration, the selected laser fluence in this study is above 1.2 $\times$ 109 W/cm2, i.e., E0 $\approx$ 0.04 mJ. The sample was mounted on a microdistance worktable in order to translate the laser spot in perpendicular directions in the plane of the target. The scanning speed vs of the microdistance worktable ranges from 1-3000 mm/s. The mono-pulse energy was measured with a thermopile power meter Gentec model ED-200. Moreover, a particle collector was pre-set to collect the ejected particles orthogonal to the sample surface and offset by approximately 5 mm to avoid exposure to the laser beam. In this study, the incoming laser energy per irradiated length E along the scanning path was used as the major variable parameter during laser scanning ablation, which can be calculated as following:1. A Coherent Verdi V10 CW Nd:YAG laser with a wavelength of 532 nm was used to illuminate the workpiece area. The laser beam is able to pass through the sample parallel to the workpiece surface. The image was then captured by a high-speed intensified CCD camera. The camera was controlled by the pulsed laser to change the delay between the image acquisition and the laser shot, and its acquisition gate was set at 10 ns, which makes it possible to image various time points of the dynamics of laser-matter-plume interaction during the entire timeline. The time delays for photography were measured after the peak intensity of the laser pulse, i.e., 0 delay represents the peak of the pulse arriving on the target surface.
WC-Co, used as the workpiece material in this study with a size of 20 mm × 20 mm × 2 mm, has a high melting point and a high volatilization point, which makes it suitable for wide average laser powers to easily adjust laser parameters . After machining, the morphology of the ablation traces was observed by scanning electron microscopy (SEM, Nova Nano430, FEI, USA) and a 3D measuring laser scanning confocal microscope (LSCM, Olympus OLS4000, Japan). Moreover, the geometric size of the morphological characteristics of the ablation trace were measured with a white light interferometer (Wyko NT9300, Veeco Inc., USA).
3. Experimental results
3.1 Dynamics of laser-matter-plume interaction
Optical images of the ablation process with different mono-pulse energies and scanning speeds are illustrated in Fig. 2. It should be mentioned that since the ultrahigh-speed camera obtains these images from the top side of the sample, the position of the sample is behind the light spot of scattering effects. But due to the high light intensity of scattering and non-luminescence of sample surface, it is hard to obtain the information of the position of the sample on these images. The scattering effect, which commonly occurs inside the plume due to the presence of small particles [36,37], is present near the sample surface under all laser parameters. This phenomenon can be used to infer the temporal and spatial distribution of the ejected particles generated during a pulsed laser scanning ablation. From Figs. 2(a), 2(b), 2(c), 2(d) and 2(f), it is clear that the size of the scattering periodically increases and decreases. For instance, with E0=0.8 mJ, the scattering of the particles is about 2∼3 mm at 200 µs, while at 400 µs, it increases to 5 mm. This demonstrates that the scattering effect periodically attenuates and enhances, suggesting an uneven particle ejection process. By comparing Figs. 2(a), 2(b), 2(c), 2(d) and 2(f), it can be found that with a lower scanning speed and a higher pulse energy, the time interval between two severe particle ejections decreases. In contrast, the intensity and distribution of the scattering effect in the low laser intensities in Figs. 2(e), 2(g) and 2(h) remain uniform and mild, implying a uniform particle ejection process.
To directly compare the dynamics of the ejected particles under vs=100 mm/s and vs=1000 mm/s (E0=0.2 mJ), the corresponding shadowgraph images of the ejected particles after the 20th and 60th pulse were taken and are presented in Fig. 3. It can be found that the particle ejections for vs=100 mm/s [Figs. 3(a) and 3(b)] are generally more severe than that those for vs=1000 mm/s [Figs. 3(c) and 3(d)]. In addition, for vs=100 mm/s, the ejected particles in the earlier pulses [Fig. 3(a)] are milder than those in the latter pulses [Fig. 3(b)]. For vs=1000 mm/s, however, the ejection of particles remains uniform and mild. This result suggests that the particles left by previous pulses can be reheated and further excited by latter pulses for vs=100 mm/s, while for vs=1000 mm/s, the particles generated are mostly mild with a short duration, which minimally affects the latter pulses. It should be mentioned that there is certain correlation between the scattering on Fig. 2 and the ejected particles on Fig. 3. A severer particles ejection can result in a stronger scattering effect, so the spatial distribution of the ejected particles can be inferred from the scattering effect . Thus, the periodical attenuation and enhancement of the scattering effect suggest that the particles ejection change periodically.
To conduct a further study of the dynamics of laser-matter-plume interaction, magnified shadowgraph images with a higher sampling frequency for nanosecond laser scanning with a train of pulses under vs=100 mm/s and vs=1000 mm/s (E0=0.2 mJ) are captured and shown in Fig. 4. Under both laser parameters, the initial ablation process presents similar phenomena. After the irradiation of the 20th pulse (500 ns) a hemispherically shaped shockwave occurs due to the pressure difference between the ambient and dense plumes [Figs. 4(a12) and 4(b12)]. Next, after a 5 µs delay, a dense plume is generated above the target surface [Figs. 4(a13) and 4(b13)]. Successive laser pulses interact with the plume, resulting in a further expansion of the plume. An additional shockwave can then be generated, which is the reason for the generation of a bulge at the center of the shockwave.
Nevertheless, the ablation processes for the two laser parameters present different characteristics. For vs=100 mm/s and E0=0.2 mJ, Fig. 4(a13) clearly shows a large amount of ejection particles after a 5 µs delay of the 20th laser pulse irradiation. Additionally, from Fig. 4(a21), it can be found that the ejection of these particles lasts over ten microseconds (the time gap between two pulses). For vs=1000 mm/s, however, there are hardly any ejected particles inside the plume [Fig. 4(b13)]. These results indicate that the ejected particles inside the plume are only generated under a high enough incoming laser energy, which is coincided with the report that sufficiently high laser fluences is needed for the formation of the plume . After the irradiation of the 40th laser pulse it is obvious that for vs=100 mm/s the plume containing denser ejected particles in Fig. 4(a23) is optically denser than that in Fig. 4(a13), which suggests that for vs=100 mm/s the plume can be reinforced, in terms of strength and distribution, with the previous laser irradiation. This result may be because, in addition to the plume being induced by the current pulse, the plume left by previous pulses can be reheated and further excited by latter pulses. In the meantime, with an increasing intensity and distribution of the plume, the laser beam cannot pass through the high-density background plume [Fig. 4(a31)], which suggests that the laser ponderomotive force is not strong enough to push the high-density background plume away . With the decay of a plume and the forward motion of the laser beam, the laser beam can pass through the background plume in the 80th pulse and irradiate the target surface again [Fig. 4(a42)]. Both the shockwave and the plume recur [Figs. 4(a42) and 4(a43)]. Since the plumes generated by previous pulses are able to completely shield the target surface from the irradiation of the 60th laser pulse, the next plumes are also guaranteed, as well. So, for vs=100 mm/s, there will be a periodic laser-matter-plume interaction during the nanosecond pulsed laser scanning ablation process, resulting in a reciprocating ablation and shielding process.
In contrast, for vs=1000 mm/s and E0=0.2 mJ, the dynamics of shockwaves and plumes after the 40th, 60th and 80th irradiations are similar, and the plumes generated after being irradiated by different pulses remain mild and even. The laser-matter-plume interaction and plume shielding are insignificant. Thus, there is an even and consecutive ablation process.
Based on the above results in Fig. 4, it can be inferred that under the influence of severe laser-matter-plume interaction, directly increasing the incoming laser energy will not be an efficient way to achieve a higher ablation rate. The dynamics of the plumes suggest that one effective method to avoid laser-matter-plume interaction is to use a low pulse frequency laser. After the decay of a plume left by previous pulses, material ablations with latter pulses display a high energy efficiency and a highly uniform consistency of the micromorphology. Moreover, another possible strategy for minimizing the effects of laser-matter-plume interaction is to control plume dynamics by temporally and spatially shaping the laser pulses and to further modify the localized transient material properties. Next, one could adjust material phase changes, and eventually circumvent the plume shielding effect in micro/nanofabrication .
3.2 Characteristics of ablation traces
SEM micrographs of microgrooves with different E, i.e., a different incoming laser energy per irradiated length, after the first scanning pass are presented in Fig. 5. Generally, the ablation morphologies under different scanning speeds and mono-pulse energies can be summarized into three types, including intermittent microgrooves (type A in Fig. 5), continuous microgrooves (type B in Fig. 5), and isolated ablation spots along the scanning path (type C in Fig. 5). With approximately E >5 mJ/mm, i e., a high incoming laser energy, type A ablation morphologies can be generated. With decreasing mono-pulse energy or increasing scanning speed, there is a transformation for the ablation morphology from uneven microgrooves to the even microgrooves formed by overlapping ablation spots (type B), and the transformation trends of microgroove morphologies are similar for both approaches.
Type A and B ablation morphologies can be distinguished based on the geometric characteristics of the ablation traces. The type A ablation morphology in Fig. 5 is characterized by a succession of intermittent and deep dimples. It is worth noticing that, with an extremely high E, the microgrooves in Figs. 5(d), 5(e) and 5(j) present as isolated ablation microholes with their ridges pushed back. Figs. 5(l) and 5(q) shows the type B ablation morphology, which is characterized by a series of even overlapped shallow pits. The spatial displacement of adjacent pulses l0 that result from the scanning speed was applied as the reference size. In the pulsed laser scanning process l1 can be calculated by:5 are shown in Fig. 6. It is obvious from Fig. 6 that with decreasing incoming laser energy, the morphology of laser scanning traces changes from intermittent to continuous. The minimum scanning speed that still yields an intermittent ablation trace is a characteristic parameter to describe the transition between type A and B ablation morphologies. We will refer to this in the following as the critical scanning speed vc. (we will discuss this later).
Further, when the scanning speed is high enough, adjacent pulses will no longer overlap, so type C ablation morphologies can be generated, as shown in Fig. 5(u). Moreover, it should be mentioned that the width of the microgrooves may exceed the laser spot diameter (see Figs. 5(d) and 5(j)). For the scanning machining, the time interval between pulses is close to the heat diffusion time. The energy delivered by each pulse will accumulate at the laser spot before diffusing out, forming a point source of heat [39,40]. Due to this ensuing diffusion, the width of a microstructure can significantly exceed the laser spot diameter.
The critical scanning speed vc under different mono-pulse energies and repetition rates is summarized in Fig. 7. To present the trend of the critical scanning speed vc, the experimental data in Fig. 7 is fitted using boltzmann fit equations. With increasing mono-pulse energy E0, the critical scanning speed vc increases monotonically and nonlinearly for all repetition rates. Moreover, it can be observed that when the mono-pulse energy is 0.6 mJ, the critical scanning speeds for the repetition rates of 50, 100 and 200 kHz are 600 mm/s, 1150 mm/s and 2140 mm/s, respectively, which increase exponentially. This effect can be applied to almost all mono-pulse energies, which means that the threshold for the incoming laser energy per irradiated length along the scanning path for the critical state is approximately constant. This threshold may depend on the physical properties of the target material.
Representative ablation morphologies of type C with an extremely high incoming laser intensity (E0=0.75 mJ and vs=100 mm/s) on the (a) middle and (b) end of an ablation trace after removal of the recast layer during postprocessing are illustrated in Fig. 8. This result shows material mechanical damage and a crack network generated around the ablation trace by fragmentation. It can be inferred that this region is exposed to pressures above the mechanical failure threshold of the material. The formation of the crack formation around the microgroove may be due to the fact that the stresses exceed the tensile strength of the material during the ablation process [41,42].
3.3 Characteristics of ejected particles
The ejected particles are collected with a scanning speed vs=100 mm/s under various mono-pulse energies. Based on the shape and size of particles, the collected debris are sorted into three types. Representative examples are shown in Fig. 9. Figure 9(a) shows small, spherical particles (type 1) with diameters on the order of several micrometers. Due to the small size of these particles, it can be inferred that these kinds of particles are ejected in a conical pattern in the presence of expanding gaseous materials near the central region. Based on the appearance of these particles, it can be inferred that these types of particles are ejected from the melted material while in the liquid phase, and they resolidify into spheres or near spheres due to the surface tension on the collector.
The particles shown in Fig. 9(b) contain fractured and melted materials. The type 2 particles [Fig. 9(b)] are clearly associated with fractured materials that extend from the base of the molten material [Fig. 9(c)]. It is reasonable to infer that these types of particles (type 2) are produced by mechanical failures but involve materials at the interface between the melted volume and the surrounding material. Fibers are also observed in type 2 particles in Fig. 9(b), indicating that the separation occurs while the material on one side of the interface is still in liquid phase, thus producing a fiber as it is pulled away. Figure 9(c) shows type 3 particles generated by fragmentation. The fractured edge of the type 3 particles suggests that these particles (type 3) originate from the surroundings of the melting material volume during the process, and this unmelted region is exposed to pressures above the mechanical failure threshold of the material.
The number of different types of particles per unit length under different mono-pulse energies is summarized in Fig. 10. The changing number of ejected particles can help to better understand the material ejection process. It can be observed that with an increasing laser mono-pulse energy, the number of particles pronouncedly increases and then remains basically stable. The qualitative results support the fact that laser-matter-plume interaction leads to the shielding effect which result in the attenuation of the laser intensity irradiating the target. Thus, even if the mono-pulse energy still increases, the particle number will remain constant. Moreover, the qualitative analysis demonstrates that these particles (type 2 and 3 particles) will occur only after the laser intensity reaches a certain threshold (the pressure induced by the shockwave and plume above the mechanical failure threshold of the material). Therefore, the threshold may be related to the mechanical properties of the material . Thus, these above results and analyses suggest that particles with specific morphologies can be generated by parameterization, which can guide the fabrication of nano/microparticles with desired microstructures.
The experimental results presented in this study suggest that laser-matter-plume interaction under different laser parameters during laser scanning processes results in different material removal processes and ablation regimes that give rise to ablation traces with distinctive morphologies.
If the incoming laser energy E is below the threshold of laser-matter-plume interaction, approximately E < 5 mJ/mm, laser-matter-plume interaction and the plume shielding effect are insignificant during the laser scanning ablation process. When a nanosecond laser pulse irradiates a surface, melting, vaporization and particle ejections occur in the irradiated zone, thus realizing the ablation of the target material. In the meantime, a shockwave is generated. The melting materials in the center of the irradiated zone are pushed outside and blasted away by the shockwave, resulting in more materials being redeposited in the periphery . Thus, the modified area looks like a crater with the periphery higher than the center. (Fig. 5(u)). In the meantime, the melted materials are ejected and resolidified into spheres or near spheres, i.e., the type 1 particles in Fig. 9(a).
During the laser scanning process the thermal melting and material depositions repeat with the irradiation of each successive pulse. In the overlapping area the microstructure formed by the former pulse can be erased and reformed again by the successive pulses. The original microstructure in the nonoverlapping area still exists and forms microripples. This is the formation mechanism of morphology type B in Fig. 5. The overlapping area reduces with an increase in the scanning speed. Moreover, with a high enough scanning speed (vc/f > w0), the modified area will not overlap, resulting in the formation of type C ablation morphologies in Fig. 5.
For a sufficiently high incoming laser energy E, the physical ablation is much more severe and forms a much deeper microhole. In the meantime, with the irradiation of a train of pulses, the plume becomes optically denser and more difficult for successive pulses to pass through, which is called the shielding effect. The laser intensity that irradiates the target surface will attenuate due to the plume shielding. When the laser is attenuated below the ablation threshold of a target material, the ablation will not occur. With the decay of a plume and the moving laser beam, the next ablation occurs in a newly irradiated spot, resulting in a new microhole. Thus, the ablation trace presents as a series of intermittent microholes along the scanning path [Figs. 5(b), 5(c), 5(g) and 5(h)].
Moreover, the mechanical damage and cracks of the ablation traces on the middle and end of an ablation trace in Fig. 8 shows that mechanical damage and crack networks are generated by fragmentation, from which can be inferred that the pressure induced by a shockwave under high laser intensity can be extremely strong. When this pressure is above the mechanical failure threshold of the target material, the ridge between two microholes can be pushed back to form the ablation morphology of Fig. 8. In the meantime, mechanical damage leads to the generation of fragments, such as the type 3 particles shown in Fig. 9(c). Moreover, the high energy threshold for the formation of type 3 particles is in accordance with the results in Fig. 10. As the incoming laser energy decreases but is still high enough to trigger laser-matter-plume interaction, the pressure can be lower than the mechanical failure threshold of the material, thus avoiding the occurrence of cracks and ridge damage between two microholes. There will be isolated microholes with complete ridges along the machining path [Figs. 5(b), 5(c), 5(h), 5(n) and 5(o)]. In summary, due to the periodical laser-matter-plume interaction, the morphologies of microstructures under a high incoming laser intensity E appear to be locally irregular and discontinuous, but the whole appears as a series of interval microholes.
National Natural Science Foundation of China (NSFC) (51575193); Guangzhou Municipal Science and Technology Project (201904010239); Natural Science Foundation of Guangdong Province (2018B030311051).
The authors thank the National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials for the use of their equipment.
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