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Abstract
A better understanding of the formation of femtosecond (fs) laser-induced surface structures is key to the control of their morphological profiles for desired surface functionalities on metals. In this work, with fs laser pulse irradiation, the two stages of formation mechanisms of the columnar structures (CSs) grown above the surface level are investigated on pure Al plates in ambient air. Here, we find that the redeposition of ablated microscale clusters following fs laser pulses of irradiation acts as the nucleation sites of CS formation, which strongly affects their location and density within the laser spot. Furthermore, we suggest their structural growths and morphological shape changes are directly associated with the competition among four laser-impact hydrodynamical phenomena: laser ablation, subsequent molten metal flow, particles’ redeposition, and metal vapor condensation with continued pulse irradiation.
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1. Introduction
Ultrafast laser structuring of metal is one of the most fascinating methods to control surface functionalities [1]. In the past few decades, it has been demonstrated by controlling various laser processing parameters such as the number [2], polarization direction [3], incident angle [4], and fluence [5] of irradiating femtosecond (fs) laser pulses that nano- and micro-scale structures can be spontaneously induced and controlled in their morphologies and surface functionalities through laser-matter interactions.
Among these laser-induced surface structures, laser-induced periodic surface structures (LIPSSs) and cone/spike structures are two notable structures on metals due to their capabilities of exhibiting the unique optical, physical, and mechanical properties of metal surfaces [6–10]. For instance, LIPSSs can generate structural colors [3,11], increase the efficiency of thermal radiation [12], and reduce friction and wear [13], while pillar/cone/spike structures can dramatically enhance the light absorption in visible and infrared wavelengths on metals [14,15], and control the wettability of the metal surface over the broad range from super-hydrophilicity to super-hydrophobicity [16]. These properties are significantly affected by the size, density, and shape of surface structures as well as the adsorption of chemical species at the surface of structures. Therefore, it is essential to have a better understanding of microscopic mechanisms responsible for laser-induced structure formation to better control those of surface structures for manipulating surface functionalities.
In 2013, Zuhlke et al. demonstrated the formation of multiscale surface structures on nickel with two levels of surface growth following multiple fs laser pulses of irradiation [17]. When the laser fluence is relatively low, the growth of the multiscale structures tends to be restricted below the surface level, whereas these structures can grow even higher than the surface level with higher laser fluences. Accordingly, this previous study found that the control of laser fluence regulates the fundamental mechanisms of laser impact processes, including ablation, molten metal flow, and re-deposition on the formation of multiscale surface structures, and eventually determines the level of structure growth. This indicates that the formation and evolution mechanisms of fs laser-induced surface structures can strongly vary with the condition of pulse irradiation as well as the properties of the material [17].
Aluminum (Al) is one of the most important materials in industries due to its high thermal and electrical conductivity with excellent formability while it still has relatively low density [18]. Accordingly, Al and its alloys have been regarded as indispensable materials, particularly in the aerospace, automobile, and construction industries [19,20]. Moreover, recent studies have manifested that the various physical properties of laser-textured Al surfaces can be easily manipulated by additional surface treatments [21,22], and therefore Al is expected to further expand their applications in the future. However, compared to other commonly used industrial materials such as stainless steel and nickel, even fs laser nano- and micro-structuring of Al is known to be very difficult in ambient air due to its rapid oxidization and lower melting temperature as well as a higher thermal conductivity [23]. As a result, there have been only a few attempts to control the shape of nano- and microscale laser-induced surface structures at the surface of Al with fs lasers [24,25].
Yet we note that Zuhlke et al. demonstrated the formation of self-assembled nanoparticle aggregates growing above the original surface level of 2024 T3 aluminum following fs laser pulse irradiation [26], and Tsubaki et. al. reported the formation of comparable aggregated nanoparticle spheres on 2024 T3 aluminum [27]. Based on these previous studies, it appears that the aggregation of ablated nanoparticles at surface defects can initiate the formation of bigger laser-induced surface structures, and monotonically grow with laser pulse irradiation by the condensation of vaporized metal through the vapor-liquid-solid (VLS) mechanism with nanoparticle aggregation [28,29]. However, other growth scenarios or microscopic mechanisms remain largely unexplored, primarily due to the experimental difficulties associated with the structural control over fs laser processing of Al.
In this paper, the formation mechanisms of the columnar structures (CSs) grown above the surface level at their initial stages are investigated on pure Al plates in ambient air. We show that the re-deposition of ablated nano- and microscale particles and clusters plays a crucial role in determining the initial sites of CS growth rather than hydrodynamical processes, which is strikingly different from the previous study on nickel [17]. Based on our observations, we suggest that the structural growths and surface morphology changes of CSs are attributed to four laser impact phenomena, molten metal flow, ablation, nanoparticle/cluster redeposition, and metal vapor condensation with continued pulse irradiation.
2. Materials and methods
The laser used for carrying out this research is an amplified Ti:sapphire fs laser system that generates 33.6 fs pulses with a maximum energy of 1.2 mJ at a central wavelength of 800 nm, while operating at a 5 kHz repetition rate. To fabricate fs laser-induced CSs on Al, we first prepare pure polycrystalline Al plates (99.99%, Goodfellow) that are cut into small square pieces of side 10 mm with a thickness of 1 mm. In order to enhance the accuracy of irradiation and observation of fs laser-induced CSs on Al, the samples and lenses are mounted on a translation stage with three axes of motion and a rotational stage, respectively, controlled by computer software. Linearly polarized femtosecond laser pulses are focused onto the sample surface at normal incidence with a 100 mm focal length plano-convex lens. The structural evolution of CSs with the pulse number is monitored by coaxially imaging the structures with an objective lens, as shown in Fig. 1. The sample surface is positioned at 4.25 mm before the focal plane as shown in Fig. 1. At this defocused distance, the 1/e2 intensity spot radius is about 220 µm, used to calculate the laser fluence. To control the formation and growth of CSs, the laser fluence at the sample surface is adjusted by using a half-wave-plate and polarizer assembly, and the laser pulses are negatively chirped at the output of the laser system to compensate for the dispersion of all these optical components before interacting with the samples. The ablation threshold of our Al sample is estimated at 0.48 J/cm2 and 0.17 J/cm2 with a single irradiating pulse and 500 pulses of irradiation, respectively [30,31]. All our experiments are performed between these ablation thresholds. Accordingly, the incubation effects due to multiple pulses of irradiation are involved in our experiment. For each experiment, a train of fs laser pulses is irradiated to the surface of Al in ambient air, and the number of pulses within the pulse train is controlled with the high voltage applied to the intra-cavity Pockels cell in the regenerative amplifier before the compressor. The detailed texture and morphological profile of CSs are studied by using a scanning electron microscope (SEM) and a UV laser scanning confocal microscope (LSCM), respectively.
Fig. 1. Schematic of fs laser-induced CSs fabrication and visualization system. A plano-convex lens and an objective lens are used to fabricate and visualize CSs, respectively. The numerical aperture (N.A.) of the objective lens is 0.14.
3. Results and discussion
Figure 2 shows the surface texture of Al after a train of 500 pulses of irradiation at fluences (F) of 0.23 J/cm2 and 0.39 J/cm2. Under these conditions, no clear sign of the formation of fs laser-induced CSs is observed in the ablated spot. Instead, microscale clusters are observed in the ablated spot, where fs LIPSSs (Fig. 2(a)) and randomly-oriented nanostructures (Fig. 2(b)) are covered for F = 0.23 J/cm2 and 0.39 J/cm2, respectively. It is also clearly shown that fewer clusters are discovered in the central region of the spot with higher laser fluence. The surface level at the boundary between the clusters and the ablated surface is slightly lower than the ablated surface level by about a few microns, and the lower surface level boundary is described by the microscale dip in the morphological profiles of the clusters and the ablated surface in Fig. 2(b).
Fig. 2. SEM images of the ablated spot after 500 pulses of fs laser irradiation with fluences of (a) 0.23 J/cm2 and (b) 0.39 J/cm2. The morphological profiles of microscale clusters are measured with a UV-LSCM. In (b), the microscale dip in (b) is clearly observed at the boundary between the cluster and the ablated surface with a depth of 6.6 µm with respect to the average of the ablated surface height.
The dependence of the surface textures of Al on the three laser fluences (F) of 0.23 J/cm2, 0.28 J/cm2, and 0.39 J/cm2 with continued pulses of irradiation is described in Fig. 3. The microscale columnar structures (CSs) start to form in the ablated spot with about 1000 pulses of irradiation, as shown in Fig. 3(a), (d), and (g). The CSs exhibit similar morphology to the clusters observed in Fig. 2 but a larger number of columnar structures are produced in the central region of the spot with a lower laser fluence. For F = 0.23 J/cm2 and 0.28 J/cm2, in the central region of the spot, LIPSSs are produced at some portion of the bottom surface between CSs up to N = 3000 similar to those shown in Fig. 2(a); however, with more than 3500 pulses of irradiation, these completely disappear from the central region of the spot. At F = 0.39 J/cm2, LIPSSs are not clearly observed at any pulse number near the center of the spot. Also, under all experimental conditions used in Fig. 3, LIPSSs are not seen at the surfaces of CSs.
Fig. 3. Structural evolutions of the ablated surface with the laser fluence and the irradiating number of pulses. The columnar structures are indicated with single-headed arrows.
Figure 3(a)-(c) and (d)-(f) represent the structural evolution of CSs with the number of pulse irradiation with F = 0.23 J/cm2 and 0.28 J/cm2, respectively. The CSs are completely packed in the central region of the ablated spot, and occupy a larger area with a higher laser fluence. However, at a higher laser fluence of 0.39 J/cm2 described in Fig. 3(g)-(i), some empty region is observed near the central region of the spot, not populated by CS, even though the average size of the 10 largest CSs in the spot is still much bigger than those produced with lower laser fluences.
Figure 4 shows the detailed measurements of the projected area (PA) of CSs, obtained from the SEM images of the 10 largest CSs under the experimental conditions used in Fig. 3. Regardless of the distribution of CS formation in the ablated spot and the laser fluence, with the first 1000 pulses of irradiation, the PA of CS is around 277 µm2. While the PA of CS monotonically increases with the number of pulse irradiation at each laser fluence shown in Fig. 4, the PA of CS expands faster with a higher laser fluence, and the areas eventually reach 547 µm2, 800 µm2, and 2004 µm2 at F = 0.23 J/cm2, 0.28 J/cm2, and 0.39 J/cm2, respectively, with 5000 pulses of irradiation. The microscale clusters evolve to CSs around 1000 pulses of irradiation, from which CSs start to expand their PAs with continued irradiation for all laser fluences shown in Fig. 4.
Fig. 4. Projected area (PA) of CSs versus the number of pulse irradiation with fluences of 0.23 J/cm2, 0.28 J/cm2, and 0.39 J/cm2. The PAs are estimated by the SEM images shown in Fig. 3.
The previous studies on laser-induced cone/pillar structures showed that the initial stage of structure formation potentially originates from laser-induced capillary or surface waves [32–34], laser-induced randomly oriented nanostructures [17], and/or nanoparticle aggregation at the surface defect [27]. According to the dispersion equation of capillary wave [35], with a higher laser fluence, the wavelength of the wave increases due to a deeper depth of melt and a longer lifetime of the molten liquid layer [33–35], leading to the formation of cone/pillar structures with a larger spatial period. In our experiments, for the number of pulse irradiation (N) larger than 1000, the average PA of each CS increases with both laser fluence and the number of pulses as shown in Fig. 3 and 4. Also, at a laser fluence of 0.28 J/cm2, the average PA of CSs gets bigger toward the central region of the ablated spot with N = 3000 and 5000, as shown in Fig. 3(e) and (f).
While the formation of our CSs, based on these aforementioned observations, seems to be consistent with that of cone/pillars suggested by the previous studies on Ti, Ni, and Si [24,32,33], the PA of CSs on Al is nearly independent of laser fluence at the initial stage of CS formation with the number of irradiating pulses up to 1000 (see Figs. 3 and 4). This is in stark contrast to the case when the capillary wave mechanism is dominant since fs laser irradiation at higher laser fluences in this case should have produced CSs that are larger in their lateral size even in the early stage of formation. Also, laser-induced randomly oriented nanostructures are still observed in the central region of the spot at F = 0.39 J/cm2 but CSs are not regularly formed there, as shown in Fig. 3(g)-(i). Consequently, other physical mechanisms need to be carefully examined to better understand the nucleation of CSs at the early stages of growth with fs laser pulse irradiation.
To further test a mechanism in which the aggregation of ablated nanoparticles at defects and scratches may initiate the formation of CSs on Al [26,27], our sample with the early stage of CS formation was sonicated in an ultrasonic bath filled with 25°C water for an hour. As shown in Fig. 5, we found that, when subjected to ultrasonication, some of the initial stage of CSs were detached from the surface, and the microscale circular pits were formed in these regions with nearly the same size as CSs. Accordingly, the presence of circular pits presents one of the critical pieces of evidence supporting that microscale spherical shape structures initiate the CS, and this clearly differs from the initiation mechanism of a laser-induced surface structure suggested by the previous studies [26,27], where the ring patterns are typically observed under the structures. This indicates that our CSs would not be initiated by nanoparticle aggregation at defects and scratches.
Fig. 5. (a) Optical micrograph and morphological profile of CSs fabricated at F = 0.39 J/cm2 with 1000 pulses of irradiation. (b) Optical micrograph and morphological profile of the region in (a) after ultrasonic agitation for an hour. (c) Line profiles of the dash-single dotted line (Profile #1) in (a) and the red solid line (Profile #2) in (b). The circled regions with the red dashed lines in (a) and (b) indicate the location of the pits, where the CSs are detached.
When fs laser-induced surface structures are fabricated with multiple pulses of irradiation, the re-deposition of nanoscale particles and clusters from the plume of ablated materials should have an effect on the structure formation after each pulse of irradiation [36]. In Fig. 2(a), microscale clusters are randomly but quite uniformly located in the center of the ablated spot at F = 0.23 J/cm2; however, the distribution of these clusters changes significantly with the laser fluence, where the number density of the cluster in the central region of the spot significantly decreases at a higher laser fluence of 0.39 J/cm2, as can be seen in Fig. 2(b).
By considering that the volume of the laser-induced plume expands with the laser fluence and this lowers the chance for the ablated material to come back to the center of the ablated spot, a lower number density of the clusters in the center of the spot indicates that the formation of these microscale clusters can be attributed to the re-deposition of ablated materials from the laser plume. Moreover, as mentioned earlier, the formation of the pits with a similar after the detachment of CSs with ultrasonication is a strong indication of the re-deposition of the microcluster. Besides, the presence of microscale dips at the ring-shaped boundary between each cluster and the ablated surface presents another evidence of the re-deposition (collision) of the cluster to the ablated surface. Once the clusters are re-deposited to the surface, the PA of CSs grows with the number of pulse irradiation, but the tendency of CS distribution across the spot does not change much as shown in Fig. 3.
To make sure that the microclusters act as the initiation of CSs, the structural evolution of each cluster is carefully monitored with a coaxial imaging camera by taking each image after every 250 pulses of irradiation with F = 0.3 J/cm2 and 0.42 J/cm2, as shown in Fig. 6 with the red dashed boxes and Visualization 1 and Visualization 2. In both cases, after the clusters are deposited to the surface following fs laser pulse irradiation, the clusters increase their sizes, and eventually become CSs by growing above the surface level with continued pulse irradiation at a nearly fixed location within the ablated spot. Consequently, the re-deposition of the clusters is essential to initialize the CSs formation.
Fig. 6. Optical micrographs of the structural evolution of CSs with the number of irradiating pulses at (a) F = 0.3 J/cm2 and (b) F = 0.42 J/cm2. Optical micrographs are taken every 250 pulses of irradiation by coaxially arranging illumination and imaging systems. See all optical micrographs of the structural evolution of CSs with the pulse number in Visualization 1 Visualization 2. The regions boxed with the red dashed lines indicate specific clusters of interest that display the pronounced structural evolution.
Once the CSs are formed from the re-deposited clusters, the CSs structurally evolve with continued pulse irradiation. Figure 7 shows the detailed comparison of the relative height (RH) of CSs as a function of the number of pulse irradiation for four different fluences of 0.23 J/cm2, 0.28 J/cm2, 0.32 J/cm2, and 0.39 J/cm2. Here, the RHs of CSs are defined as the CS height with respect to the untreated original surface level prior to laser ablation. The RHs of CSs are always positive since they are higher than the original surface level with all pulse numbers and laser fluences used in our experiments. As can be seen from Fig. 7 (a)-(d), the RHs of CSs produced with up to F = 0.32 J/cm2 tend to increase with the number of pulses, and are nearly saturated with about 2500 pulses of irradiation. In Fig. 7(d), the CSs for F = 0.39 J/cm2 do not notably increase their RH after 3500 irradiating pulses. The saturated RH of CSs increases from 14 µm to 32 µm, as the fluence elevates from 0.23 J/cm2 to 0.39 J/cm2.
Fig. 7. Detailed height of CSs with respect to the original surface level prior to the laser treatment (relative height, RH) versus the number of pulse irradiation with fluences of (a) 0.23 J/cm2, (b) 0.28 J/cm2, (c) 0.32 J/cm2, and (d) 0.39 J/cm2. The RHs of CSs are obtained by averaging the heights of the 10 highest CSs under the same laser fluence and pulse number. (e) The ablation depth in the central region of the spot versus the pulse number with fluences used in (a)-(d).
Following fs laser irradiation, the structural growth of our CSs on Al in both their PAs and RHs shortly after their initiation can be understood mainly by hydrodynamical processes such as the condensation of vaporized metal through the VLS mechanism, nanoparticle re-depositions, selective laser ablation, and molten metal flow [17,24–31]. To increase the RH of CSs with the VLS mechanism and nanoparticle re-deposition, the laser beam should be irradiated to the bottom surface of the openings between the CSs to supply the ablated materials toward the top surface of the structures. According to these mechanisms, the structures can monotonically grow with pulses of irradiation until the structures are densely formed at the surface. For F = 0.23 and 0.28 J/cm2 as shown in Fig. 3(a)-(f), CSs are fully packed in the central region of the spot, and therefore the RH of CSs can naturally experience the saturation of the growth as shown in Fig. 7(a) and (b). With a higher laser fluence, the CSs are not fully formed in the central region of the spot as shown in Fig. 3(g)-(i), and can have unlimited chances to grow by the VLS mechanism and nanoparticle re-deposition. However, as depicted in Fig. 7(d), the RH of CSs still exhibits the growth saturation at F = 0.39 J/cm2.
When three-dimensional structures are formed at the surface with fs laser irradiation, the absorbed laser fluence is no longer uniform due to the different local incident angles with respect to the surface of structures, and the surface with a smaller local incident angle can have a higher absorbed laser fluence [37]. Accordingly, the top surface of CSs and the bottom surfaces of narrow gaps between CSs are selectively ablated by fs laser pulse irradiation since the local incident angle at these surfaces is close to normal incidence.
The strong selective ablation at the top surface of CSs on Al makes it relatively flat compared to that of laser-induced cone/spike or multiscale structures in the previous studies [17,34,38], as shown in Fig. 8. Applied to the surface following laser ablation, the recoil pressure can push the molten metal from the top surface of CSs and the bottom surface of the gaps between CSs toward the sidewalls of CSs, where the pressure is relatively low [39]. The molten metal flow from the top of CSs also merges multiple CSs into one CS, as shown in Fig. 8(a). Simultaneously, particles/clusters and vaporized metal in the plume can be re-deposited and condensate over the surfaces of the CSs. An extensive number of particles/clusters (Fig. 8(a-c)) and the mushroom cap-like structures (Fig. 8(b)) at the top of CSs are clear evidence of re-deposition with condensation and molten metal flow, respectively, following the laser ablation. At the highest fluence of 0.39 J/cm2, the molten metal flowing from the top surface completely covers the sidewall of CSs in Fig. 8(c).
Fig. 8. SEM image of CS produced at fluences of (a) 0.28 J/cm2, (a) 0.32 J/cm2, and (c) 0.39 J/cm2. The CSs are viewed at an angle of 35° with 5000 pulses of irradiation.
According to the previous study on Ni [17], laser fluence determines if laser-induced multiscale structures can grow above the original surface level. However, even in a relatively lower laser fluence region near the edge of the CS-formed region, the top surfaces of Al CSs are still located above the original surface level as shown in Fig. 9. Consequently, the CSs induced on Al can grow above the original surface level regardless of the laser fluences used in our experiments as long as those are formed, and this may be due to the initiation of our CSs by the redeposition of the clusters.
Fig. 9. Morphological profile of CSs produced at F = 0.26 J/cm2 with 5000 pulses of irradiation. (a) The relative height (RH) of each CS is positive even at a low local fluence region near the edge of the spot. The relative height indicates the structural height with respect to the untreated original surface level before laser ablation. The surface morphological profile indicated by the red stripe in (a) is plotted in (b).
The evolution of RH in CSs on Al can also be regulated by the competition among the aforementioned hydrodynamical processes. Laser ablation and the molten liquid flow from the top of CSs make the CSs reduce their height by the material removal from the top, whereas the molten liquid flow from the bottom surface in the gaps between CSs, the re-deposition of particles/clusters, and the condensation of vaporized metal tend to increase their height. Therefore, until the RH saturates as shown in Fig. 7, the molten liquid flow from the bottom surface and the re-deposition and condensation of nanoparticles and vaporized metal, respectively, at the top surface are stronger than the laser ablation and subsequent liquid metal flow from the top surface, and eventually these two opposite material transport processes are balanced, resulting in the RH saturation.
As for an increase in the PA of CSs with fs laser pulse irradiation, both the nanoparticles/clusters re-deposition and the molten liquid metal flow in addition to the condensation of vaporized metal play key roles in increasing the PA since these all transport materials toward the sidewall of the CSs. Also, as mentioned earlier, the liquid metal flow can combine multiple CSs into one CS, and increases the PA. The material removed from the sidewall of the CSs by laser ablation is negligible due to a larger incident angle at the sidewall. Accordingly, the PA of CSs continuously increases with the number of irradiating pulses and is not affected by the saturation of RH. It is also worth noting that the effects from the polarization direction of irradiating fs pulses on the shapes of both CS fabricated region and each CS are negligible, and this indicates that the evolution of CSs would be laser-impact hydrodynamical phenomena rather than the direct effect from the electric field of fs laser pulse.
The entire processes of CS formation with fs laser pulse irradiation are summarized in the diagram shown in Fig. 10. First, CSs are initiated by the redeposition of ablated microscale particles/clusters following fs laser pulse irradiation. With continued pulse irradiation, the microscale particles/clusters repeatedly experience the recoil pressure due to selective ablation followed by molten metal flow from both the top and bottom surfaces of the clusters as well as metal vapor condensation and nanoparticles’ redeposition, and the clusters evolve to CSs. Once the amount of material transported by three hydrodynamical processes, molten metal flow, metal vapor condensation, and nanoparticles’ redeposition are well-balanced, the height of each CS is eventually saturated.
4. Conclusion
In conclusion, we have investigated the complete formation mechanisms of the columnar structures (CSs) that grow above the surface level of pure aluminum plates by utilizing femtosecond laser irradiation. Different from the previous study regarding multiscale surface structures induced by femtosecond laser irradiation on nickel, we find that the redeposition of ablated microscale particles/clusters plays crucial roles in initiating the formation of CSs at the very early stages of growth rather than hydrodynamical processes such as the generation of laser-induced capillary, surface waves and the formation of randomly oriented nanostructures, and the aggregation of nanoparticles at surface defects and scratches, as suggested by the previous studies. Also, we have demonstrated that all the CSs are formed above the original surface levels by irradiating the particles/clusters with fs laser pulses with various fluences and pulse numbers, and have suggested that the growth and structural evolution of CSs with laser fluence and the number of irradiating pulses originates from the competition of laser impact hydrodynamical phenomena, selective laser ablation, and subsequent molten metal flow, particles redeposition, and the condensation of vaporized metal. Our study offers a new perspective on the initial formation and structural evolution of fs laser-induced microscale structures above the surface level with different mechanisms at play compared to those suggested by other previous studies, and opens up possibilities for sophisticated control of the surface functionalities for various potential applications including enhanced chemical reactions through a high surface-to-volume ratio and anti-icing & deicing with tailored surface wettability and emissivity. Moreover, this may have significant implications for precisely growing each laser-induced columnar structure by seeding microscale cluster at a desired location prior to fs laser pulse irradiation and even for minimizing the unwanted formation of the minute sizes of randomly distributed structures during laser-based precision machining.
Funding
Korea Institute of Industrial Technology (EO-23-0008); National Research Foundation of Korea (2021R1F1A1063020); Ministry of SMEs and Startups (S3193463).
Acknowledgments
We thank W. P. Hong and J. S. Kim in the R&D department at DAEHAN OK Steel for their technical support. This study has been conducted with the support of the Korea Institute of Industrial Technology as “Development of root technology for multi-product flexible production (EO-23-0008)”. This work was supported by the Technology Development Program (S3193463) funded by the Ministry of SMEs and Startups (MSS, Korea) as “Development of Eco-friendly Color Coated Steel by Femtosecond Laser”. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2021R1F1A1063020).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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