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Direct laser interference (DLIP) combined with laser-induced periodic surface structure (LIPSS) allows the creation of functional surfaces for various materials. The throughput of the process is usually increased by using a higher average laser power. However, this leads to the accumulation of heat, which affects the roughness and shape of the resulting surface pattern. Therefore, it is necessary to study in detail the influence of the substrate temperature on the morphology of the fabricated elements. In this study, the steel surface was line-patterned with ps-DLIP at 532 nm. To investigate the effect of substrate temperature on the resulting topography, a heating plate was used to control the temperature. Heating to 250 \(^{\circ }\)С led to a significant decrease in the depth of the formed structures from 2.33 to 1.06 µm. The decrease was associated with the appearance of different types of LIPSS depending on the orientation of the substrate grains and laser-induced surface oxidation. This study shows the strong effect of substrate temperature, which is also expected when surface treatment is performed at high average laser power to create heat accumulation effects.
Surface treatment methods based on ultrashort pulse laser irradiation are at the forefront of science and industry due to their ability to improve the surface properties of the most important relevant materials1. In particular, laser-induced custom surface functionality is state-of-the-art across a wide range of industrial sectors and application scenarios1,2,3. For example, Vercillo et al. Anti-icing properties have been demonstrated on titanium alloys for aerospace applications based on laser-induced superhydrophobicity. Epperlein et al reported that nanosized features produced by laser surface structuring can influence biofilm growth or inhibition on steel specimens5. In addition, Guai et al. also improved the optical properties of organic solar cells. 6 Thus, laser structuring allows the production of high-resolution structural elements by controlled ablation of the surface material1.
A suitable laser structuring technique for producing such periodic surface structures is direct laser interference shaping (DLIP). DLIP is based on the near-surface interference of two or more laser beams to form patterned surfaces with characteristics in the micrometer and nanometer range. Depending on the number and polarization of the laser beams, DLIP can design and create a wide variety of topographic surface structures. A promising approach is to combine DLIP structures with laser-induced periodic surface structures (LIPSS) to create a surface topography with a complex structural hierarchy8,9,10,11,12. In nature, these hierarchies have been shown to provide even better performance than single-scale models13.
The LIPSS function is subject to a self-amplifying process (positive feedback) based on an increasing near-surface modulation of the radiation intensity distribution. This is due to an increase in nanoroughness as the number of applied laser pulses increases 14, 15, 16. Modulation occurs mainly due to the interference of the emitted wave with the electromagnetic field15,17,18,19,20,21 of refracted and scattered wave components or surface plasmons. The formation of LIPSS is also affected by the timing of the pulses22,23. In particular, higher average laser powers are indispensable for high productivity surface treatments. This usually requires the use of high repetition rates, i.e. in the MHz range. Consequently, the time distance between laser pulses is shorter, which leads to heat accumulation effects 23, 24, 25, 26. This effect leads to an overall increase in surface temperature, which can significantly affect the patterning mechanism during laser ablation.
In a previous work, Rudenko et al. and Tzibidis et al. A mechanism for the formation of convective structures is discussed, which should become increasingly important as heat accumulation increases19,27. In addition, Bauer et al. Correlate the critical amount of heat accumulation with micron surface structures. Despite this thermally induced structure formation process, it is generally believed that the productivity of the process can be improved simply by increasing the repetition rate28. Although this, in turn, cannot be achieved without a significant increase in heat storage. Therefore, process strategies that provide a multilevel topology may not be portable to higher repetition rates without changing the process kinetics and structure formation9,12. In this regard, it is very important to investigate how the substrate temperature affects the DLIP formation process, especially when making layered surface patterns due to the simultaneous formation of LIPSS.
The aim of this study was to evaluate the effect of substrate temperature on the resulting surface topography during DLIP processing of stainless steel using ps pulses. During laser processing, the temperature of the sample substrate was brought up to 250 \(^\circ\)C using a heating plate. The resulting surface structures were characterized using confocal microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy.
In the first series of experiments, the steel substrate was processed using a two-beam DLIP configuration with a spatial period of 4.5 µm and a substrate temperature of \(T_{\mathrm {s}}\) 21 \(^{\circ }\)C, hereinafter referred to as “unheated » surface. In this case, the pulse overlap \(o_{\mathrm {p}}\) is the distance between two pulses as a function of spot size. It varies from 99.0% (100 pulses per position) to 99.67% (300 pulses per position). In all cases, a peak energy density \(\Phi _\mathrm {p}\) = 0.5 J/cm\(^2\) (for a Gaussian equivalent without interference) and a repetition frequency f = 200 kHz were used. The direction of polarization of the laser beam is parallel to the movement of the positioning table (Fig. 1a)), which is parallel to the direction of the linear geometry created by the two-beam interference pattern. Representative images of the obtained structures using a scanning electron microscope (SEM) are shown in Figs. 1a–c. To support the analysis of SEM images in terms of topography, Fourier transforms (FFTs, shown in dark insets) were performed on the structures being evaluated. In all cases, the resulting DLIP geometry was visible with a spatial period of 4.5 µm.
For the case \(o_{\mathrm {p}}\) = 99.0% in the darker area of Fig. 1a, corresponding to the position of the interference maximum, one can observe grooves containing smaller parallel structures. They alternate with brighter bands covered in a nanoparticle-like topography. Because the parallel structure between the grooves appears to be perpendicular to the polarization of the laser beam and has a period of \(\Lambda _{\mathrm {LSFL-I}}\) 418\(\pm 65\) nm, slightly less than the wavelength of the laser \(\lambda\) (532 nm) can be called LIPSS with low spatial frequency (LSFL-I)15,18. LSFL-I produces a so-called s-type signal in the FFT, “s” scattering15,20. Therefore, the signal is perpendicular to the strong central vertical element, which in turn is generated by the DLIP structure (\(\Lambda _{\mathrm {DLIP}}\) \(\approx\) 4.5 µm). The signal generated by the linear structure of the DLIP pattern in the FFT image is referred to as “DLIP-type”.
SEM images of surface structures created using DLIP. The peak energy density is \(\Phi _\mathrm {p}\) = 0.5 J/cm\(^2\) (for a no-noise Gaussian equivalent) and a repetition rate f = 200 kHz. The images show sample temperature, polarization and overlay. The movement of the localization phase is marked with a black arrow in (a). The black inset shows the corresponding FFT obtained from the 37.25\(\times\)37.25 µm SEM image (shown until the wavevector becomes \(\vec {k}\cdot (2\pi )^ {-1}\) = 200 nm). The process parameters are indicated in each figure.
Looking further into Figure 1, you can see that as the \(o_{\mathrm {p}}\) overlap increases, the sigmoid signal is more concentrated towards the x-axis of the FFT. The rest of LSFL-I tends to be more parallel. In addition, the relative intensity of the s-type signal decreased and the intensity of the DLIP-type signal increased. This is due to increasingly pronounced trenches with more overlap. Also, the x-axis signal between type s and the center must come from a structure with the same orientation as LSFL-I but with a longer period (\(\Lambda _\mathrm {b}\) \(\approx \ ) 1.4 ± 0.2 µm) as shown in Figure 1c). Therefore, it is assumed that their formation is a pattern of pits in the center of the trench. The new feature also appears in the high frequency range (large wavenumber) of the ordinate. The signal comes from parallel ripples on the slopes of the trench, most likely due to the interference of incident and forward-reflected light on the slopes9,14. In the following, these ripples are denoted by LSFL \ (_ \ mathrm {edge} \), and their signals – by type -s \ (_ {\mathrm {p)) \).
In the next experiment, the temperature of the sample was brought up to 250 °C under the so-called “heated” surface. Structuring was carried out according to the same processing strategy as the experiments mentioned in the previous section (Figs. 1a–1c). The SEM images depict the resulting topography as shown in Fig. 1d–f. Heating the sample to 250 C leads to an increase in the appearance of LSFL, the direction of which is parallel to the laser polarization. These structures can be characterized as LSFL-II and have a spatial period \(\Lambda _\mathrm {LSFL-II}\) of 247 ± 35 nm. The LSFL-II signal is not displayed in the FFT due to the high mode frequency. As \(o_{\mathrm {p}}\) increased from 99.0 to 99.67\(\%\) (Fig. 1d–e), the width of the bright band region increased, which led to the appearance of a DLIP signal for more than high frequencies. wavenumbers (lower frequencies) and thus shift towards the center of the FFT. The rows of pits in Fig. 1d may be the precursors of the so-called grooves formed perpendicular to LSFL-I22,27. In addition, LSFL-II appears to have become shorter and irregularly shaped. Note also that the average size of bright bands with nanograin morphology is smaller in this case. In addition, the size distribution of these nanoparticles turned out to be less dispersed (or led to less particle agglomeration) than without heating. Qualitatively, this can be assessed by comparing figures 1a, d or b, e, respectively.
As the overlap \(o_{\mathrm {p}}\) increased further to 99.67% (Fig. 1f), a distinct topography gradually emerged due to increasingly obvious furrows. However, these grooves appear less ordered and less deep than in Fig. 1c. Low contrast between light and dark areas of the image shows up in quality. These results are further supported by the weaker and more scattered signal of the FFT ordinate in Fig. 1f compared to the FFT on c. Smaller striae were also evident on heating when comparing Figures 1b and e, which was later confirmed by confocal microscopy.
In addition to the previous experiment, the polarization of the laser beam was rotated by 90 \(^{\circ}\), which caused the polarization direction to move perpendicular to the positioning platform. On fig. 2a-c shows the early stages of structure formation, \(o_{\mathrm {p}}\) = 99.0% in unheated (a), heated (b) and heated 90\(^{\ circ }\ ) – Case with rotating polarization (c). To visualize the nanotopography of the structures, the areas marked with colored squares are shown in Figs. 2d, on an enlarged scale.
SEM images of surface structures created using DLIP. The process parameters are the same as in Fig.1. The image shows the sample temperature \(T_s\), polarization and pulse overlap \(o_\mathrm {p}\). The black inset again shows the corresponding Fourier transform. The images in (d)-(i) are magnifications of the marked areas in (a)-(c).
In this case, it can be seen that the structures in the darker areas of Fig. 2b,c are polarization sensitive and are therefore labeled LSFL-II14, 20, 29, 30. Notably, the orientation of LSFL-I is also rotated (Fig. 2g, i), which can be seen from the orientation of the s-type signal in the corresponding FFT. The bandwidth of the LSFL-I period appears larger compared to period b, and its range is shifted towards smaller periods in Fig. 2c, as indicated by the more widespread s-type signal. Thus, the following LSFL spatial period can be observed on the sample at different heating temperatures: \(\Lambda _{\mathrm {LSFL-I}}\) = 418\(\pm 65\) nm at 21 ^{ \circ}\ )C (Fig. 2a), \(\Lambda _{\mathrm {LSFL-I}}\) = 445\(~\pm\) 67 nm and \(\Lambda _{\mathrm {LSFL-II}} \) = 247 ± 35 nm at 250°C (Fig. 2b) for s polarization. On the contrary, the spatial period of p-polarization and 250 \(^{\circ }\)C is equal to \(\Lambda _{\mathrm {LSFL-I))\) = 390\(\pm 55\) nm and \(\ Lambda_{\mathrm{LSFL-II}}\) = 265±35 nm (Fig. 2c).
Notably, the results show that just by increasing the sample temperature, the surface morphology can switch between two extremes, including (i) a surface containing only LSFL-I elements and (ii) an area covered with LSFL-II. Because the formation of this particular type of LIPSS on metal surfaces is associated with surface oxide layers, energy dispersive X-ray analysis (EDX) was performed. Table 1 summarizes the results obtained. Each determination is carried out by averaging at least four spectra in different places on the surface of the processed sample. The measurements are carried out at different sample temperatures \(T_\mathrm{s}\) and different positions of the sample surface containing unstructured or structured areas. The measurements also contain information about the deeper unoxidized layers that lie directly below the treated molten area, but within the electron penetration depth of the EDX analysis. However, it should be noted that the EDX is limited in its ability to quantify the oxygen content, so these values here can only give a qualitative assessment.
The untreated portions of the samples did not show significant amounts of oxygen at all operating temperatures. After laser treatment, oxygen levels increased in all cases31. The difference in elemental composition between the two untreated samples was as expected for the commercial steel samples, and significantly higher carbon values were found compared to the manufacturer’s data sheet for AISI 304 steel due to hydrocarbon contamination32.
Before discussing possible reasons for the decrease in groove ablation depth and the transition from LSFL-I to LSFL-II, power spectral density (PSD) and height profiles are used.
(i) The quasi-two-dimensional normalized power spectral density (Q2D-PSD) of the surface is shown as SEM images in Figures 1 and 2. 1 and 2. Since the PSD is normalized, a decrease in the sum signal should be understood as an increase in the constant part (k \(\le\) 0.7 µm\(^{-1}\), not shown), i.e. smoothness. (ii) Corresponding mean surface height profile. Sample temperature \(T_s\), overlap \(o_{\mathrm {p}}\), and laser polarization E relative to the orientation \(\vec {v}\) of the positioning platform movement are shown in all plots.
To quantify the impression of SEM images, an average normalized power spectrum was generated from at least three SEM images for each parameter set by averaging all one-dimensional (1D) power spectral densities (PSDs) in the x or y direction. The corresponding graph is shown in Fig. 3i showing the frequency shift of the signal and its relative contribution to the spectrum.
On fig. 3ia, c, e, the DLIP peak grows near \(k_{\mathrm {DLIP}}~=~2\pi\) (4.5 µm)\(^{-1}\) = 1.4 µm \ ( ^{-1}\) or the corresponding higher harmonics as the overlap increases \(o_{\mathrm {p))\). An increase in the fundamental amplitude was associated with a stronger development of the LRIB structure. The amplitude of higher harmonics increases with the steepness of the slope. For rectangular functions as limiting cases, the approximation requires the largest number of frequencies. Therefore, the peak around 1.4 µm\(^{-1}\) in the PSD and the corresponding harmonics can be used as quality parameters for the shape of the groove.
On the contrary, as shown in Fig. 3(i)b,d,f, the PSD of the heated sample shows weaker and broader peaks with less signal in the respective harmonics. In addition, in fig. 3(i)f shows that the second harmonic signal even exceeds the fundamental signal. This reflects the more irregular and less pronounced DLIP structure of the heated sample (compared to \(T_s\) = 21\(^\circ\)C). Another feature is that as the overlap \(o_{\mathrm {p}}\) increases, the resulting LSFL-I signal shifts towards a smaller wavenumber (longer period). This can be explained by the increased steepness of the edges of the DLIP mode and the associated local increase in the angle of incidence14,33. Following this trend, the broadening of the LSFL-I signal could also be explained. In addition to the steep slopes, there are also flat areas on the bottom and above the crests of the DLIP structure, allowing for a wider range of LSFL-I periods. For highly absorbent materials, the LSFL-I period is usually estimated as:
where \(\theta\) is the angle of incidence, and the subscripts s and p refer to different polarizations33.
It should be noted that the plane of incidence for a DLIP setup is usually perpendicular to the movement of the positioning platform, as shown in Figure 4 (see the Materials and Methods section). Therefore, s-polarization, as a rule, is parallel to the movement of the stage, and p-polarization is perpendicular to it. According to the equation. (1), for s-polarization, a spread and a shift of the LSFL-I signal towards smaller wavenumbers are expected. This is due to the increase in \(\theta\) and the angular range \(\theta \pm \delta \theta\) as the trench depth increases. This can be seen by comparing the LSFL-I peaks in Fig. 3ia,c,e.
According to the results shown in fig. 1c, LSFL\(_\mathrm {edge}\) is also visible in the corresponding PSD in fig. 3ie. On fig. 3ig,h shows the PSD for p-polarization. The difference in DLIP peaks is more pronounced between heated and unheated samples. In this case, the signal from LSFL-I overlaps with the higher harmonics of the DLIP peak, adding to the signal near the lasing wavelength.
To discuss the results in more detail, in Fig. 3ii shows the structural depth and overlap between pulses of the DLIP linear height distribution at various temperatures. The vertical height profile of the surface was obtained by averaging ten individual vertical height profiles around the center of the DLIP structure. For each applied temperature, the depth of the structure increases with increasing pulse overlap. The profile of the heated sample shows grooves with mean peak-to-peak (pvp) values of 0.87 µm for s-polarization and 1.06 µm for p-polarization. In contrast, s-polarization and p-polarization of the unheated sample show pvp of 1.75 µm and 2.33 µm, respectively. The corresponding pvp is depicted in the height profile in fig. 3ii. Each PvP average is calculated by averaging eight single PvPs.
In addition, in fig. 3iig,h shows the p-polarization height distribution perpendicular to the positioning system and groove movement. The direction of the p-polarization has a positive effect on the depth of the groove since it results in a slightly higher pvp at 2.33 µm compared to the s-polarization at 1.75 µm pvp. This in turn corresponds to the grooves and movement of the positioning platform system. This effect can be caused by a smaller structure in the case of s-polarization compared to the case of p-polarization (see Fig. 2f,h), which will be discussed further in the next section.
The purpose of the discussion is to explain the decrease in the groove depth due to the change in the main LIPS class (LSFL-I to LSFL-II) in the case of heated samples. So answer the following questions:
To answer the first question, it is necessary to consider the mechanisms responsible for the reduction in ablation. For a single pulse at normal incidence, the ablation depth can be described as:
where \(\delta _{\mathrm {E}}\) is the energy penetration depth, \(\Phi\) and \(\Phi _{\mathrm {th}}\) are the absorption fluence and the Ablation fluence threshold, respectively34 .
Mathematically, the depth of energy penetration has a multiplicative effect on the depth of ablation, while the change in energy has a logarithmic effect. So fluence changes don’t affect \(\Delta z\) much as long as \(\Phi ~\gg ~\Phi _{\mathrm {th}}\). However, strong oxidation (for example, due to the formation of chromium oxide) leads to stronger Cr-O35 bonds compared to Cr-Cr bonds, thereby increasing the ablation threshold. Consequently, \(\Phi ~\gg ~\Phi _{\mathrm {th}}\) is no longer satisfied, which leads to a rapid decrease in the ablation depth with decreasing energy flux density. In addition, a correlation between the oxidation state and the period of LSFL-II is known, which can be explained by changes in the nanostructure itself and the optical properties of the surface caused by surface oxidation30,35. Therefore, the exact surface distribution of the absorption fluence \(\Phi\) is due to the complex dynamics of the interaction between the structural period and the thickness of the oxide layer. Depending on the period, the nanostructure strongly influences the distribution of the absorbed energy flux due to a sharp increase in the field, excitation of surface plasmons, extraordinary light transfer or scattering17,19,20,21. Therefore, \(\Phi\) is strongly inhomogeneous near the surface, and \(\delta _ {E}\) is probably no longer possible with one absorption coefficient \(\alpha = \delta _{\mathrm {opt} }^ { -1} \approx \delta _{\mathrm {E}}^{-1}\) for the entire near-surface volume. Since the thickness of the oxide film largely depends on the solidification time [26], the nomenclature effect depends on the sample temperature. The optical micrographs shown in Figure S1 in the Supplementary Material indicate changes in the optical properties.
These effects partly explain the shallower trench depth in the case of small surface structures in Figures 1d,e and 2b,c and 3(ii)b,d,f.
LSFL-II is known to form on semiconductors, dielectrics, and materials prone to oxidation14,29,30,36,37. In the latter case, the thickness of the surface oxide layer is especially important30. The EDX analysis carried out revealed the formation of surface oxides on the structured surface. Thus, for unheated samples, ambient oxygen seems to contribute to the partial formation of gaseous particles and partially the formation of surface oxides. Both phenomena make a significant contribution to this process. On the contrary, for heated samples, metal oxides of various oxidation states (SiO\(_{\mathrm {2}}\), Cr\(_{\mathrm {n}} \)O\(_{\mathrm { m}}\ ), Fe\(_{\mathrm {n}}\)O\(_{\mathrm {m}}\), NiO, etc.) are clear 38 in favor. In addition to the required oxide layer, the presence of subwavelength roughness, mainly high spatial frequency LIPSS (HSFL), is necessary to form the required subwavelength (d-type) intensity modes14,30. The final LSFL-II intensity mode is a function of the HSFL amplitude and oxide thickness. The reason for this mode is the far-field interference of light scattered by the HSFL and light refracted into the material and propagating inside the surface dielectric material20,29,30. SEM images of the edge of the surface pattern in Figure S2 in the Supplementary Materials section are indicative of pre-existing HSFL. This outer region is weakly affected by the periphery of the intensity distribution, which allows the formation of HSFL. Due to the symmetry of the intensity distribution, this effect also takes place along the scanning direction.
Sample heating affects the LSFL-II formation process in several ways. On the one hand, an increase in sample temperature \(T_\mathrm{s}\) has a much greater effect on the rate of solidification and cooling than the thickness of the molten layer26. Thus, the liquid interface of a heated sample is exposed to ambient oxygen for a longer period of time. In addition, delayed solidification allows the development of complex convective processes that increase the mixing of oxygen and oxides with liquid steel26. This can be demonstrated by comparing the thickness of the oxide layer formed only by diffusion (\(\Lambda _\mathrm {diff}=\sqrt{D~\times ~t_\mathrm {s}}~\le ~15\) nm) The corresponding coagulation time is \(t_\mathrm {s}~\le ~200\) ns, and the diffusion coefficient \(D~\le\) 10\(^{-5}\) cm\(^2 \ )/ s) Significantly higher thickness was observed or required in the LSFL-II formation30. On the other hand, heating also affects the formation of HSFL and hence the scattering objects required to transition into the LSFL-II d-type intensity mode. The exposure of nanovoids trapped below the surface suggests their involvement in the formation of HSFL39. These defects may represent the electromagnetic origin of HSFL due to the required high frequency periodic intensity patterns14,17,19,29. In addition, these generated intensity modes are more uniform with a large number of nanovoids19. Thus, the reason for the increased incidence of HSFL can be explained by the change in the dynamics of crystal defects as \(T_\mathrm{s}\) increases.
It has recently been shown that the cooling rate of silicon is a key parameter for intrinsic interstitial supersaturation and thus for the accumulation of point defects with the formation of dislocations40,41. Molecular dynamics simulations of pure metals have shown that vacancies supersaturate during rapid recrystallization, and hence the accumulation of vacancies in metals proceeds in a similar manner42,43,44. In addition, recent experimental studies of silver have focused on the mechanism of formation of voids and clusters due to the accumulation of point defects45. Therefore, an increase in the temperature of the sample \(T_\mathrm {s}\) and, consequently, a decrease in the cooling rate can affect the formation of voids, which are the nuclei of HSFL.
If vacancies are the necessary precursors to cavities and hence HSFL, the sample temperature \(T_s\) should have two effects. On the one hand, \(T_s\) affects the rate of recrystallization and, consequently, the concentration of point defects (vacancy concentration) in the grown crystal. On the other hand, it also affects the cooling rate after solidification, thereby affecting the diffusion of point defects in the crystal 40,41. In addition, the solidification rate depends on the crystallographic orientation and is thus highly anisotropic, as is the diffusion of point defects42,43. According to this premise, due to the anisotropic response of the material, the interaction of light and matter becomes anisotropic, which in turn amplifies this deterministic periodic release of energy. For polycrystalline materials, this behavior can be limited by the size of a single grain. In fact, LIPSS formation has been demonstrated depending on grain orientation46,47. Therefore, the effect of sample temperature \(T_s\) on the crystallization rate may not be as strong as the effect of grain orientation. Thus, the different crystallographic orientation of different grains provides a potential explanation for the increase in voids and aggregation of HSFL or LSFL-II, respectively.
To clarify the initial indications of this hypothesis, the raw samples were etched to reveal grain formation close to the surface. Comparison of grains in fig. S3 is shown in the supplementary material. In addition, LSFL-I and LSFL-II appeared in groups on heated samples. The size and geometry of these clusters correspond to the grain size.
Moreover, HSFL only occurs in a narrow range at low flux densities due to its convective origin19,29,48. Therefore, in experiments, this probably occurs only at the periphery of the beam profile. Therefore, HSFL formed on non-oxidized or weakly oxidized surfaces, which became apparent when comparing the oxide fractions of treated and untreated samples (see table reftab: example). This confirms the assumption that the oxide layer is mainly induced by the laser.
Given that LIPSS formation is typically dependent on the number of pulses due to inter-pulse feedback, HSFLs can be replaced by larger structures as pulse overlap increases19. A less regular HSFL results in a less regular intensity pattern (d-mode) required for the formation of LSFL-II. Therefore, as the overlap of \(o_\mathrm {p}\) increases (see Fig. 1 from de), the regularity of LSFL-II decreases.
This study investigated the effect of substrate temperature on the surface morphology of laser structured DLIP treated stainless steel. It has been found that heating the substrate from 21 to 250°C leads to a decrease in the ablation depth from 1.75 to 0.87 µm in the s-polarization and from 2.33 to 1.06 µm in the p-polarization. This decrease is due to the change in LIPSS type from LSFL-I to LSFL-II, which is associated with a laser-induced surface oxide layer at a higher sample temperature. In addition, LSFL-II may increase threshold flux due to increased oxidation. It is assumed that in this technological system with high pulse overlap, average energy density and average repetition rate, the occurrence of LSFL-II is also determined by the change in dislocation dynamics caused by sample heating. The aggregation of LSFL-II is hypothesized to be due to grain orientation-dependent nanovoid formation, leading to HSFL as a precursor to LSFL-II. In addition, the influence of the direction of polarization on the structural period and the bandwidth of the structural period is studied. It turns out that p-polarization is more efficient for the DLIP process in terms of ablation depth. Overall, this study uncovers a set of process parameters to control and optimize the depth of DLIP ablation to create customized surface patterns. Finally, the transition from LSFL-I to LSFL-II is entirely heat driven and a small increase in repetition rate is expected with constant pulse overlap due to increased heat buildup24. All of these aspects are relevant to the upcoming challenge of expanding the DLIP process, for example through the use of polygonal scanning systems49. To minimize heat buildup, the following strategy can be followed: keep the scanning speed of the polygonal scanner as high as possible, taking advantage of the larger laser spot size, orthogonal to the scanning direction, and using optimal ablation. fluence 28. In addition, these ideas allow the creation of complex hierarchical topography for advanced surface functionalization using DLIP.
In this study, electropolished stainless steel plates (X5CrNi18-10, 1.4301, AISI 304) 0.8 mm thick were used. To remove any contaminants from the surface, the samples were carefully washed with ethanol before laser treatment (absolute concentration of ethanol \(\ge\) 99.9%).
The DLIP setting is shown in Figure 4. Samples were constructed using a DLIP system equipped with a 12 ps ultrashort pulsed laser source with a wavelength of 532 nm and a maximum repetition rate of 50 MHz. The spatial distribution of the beam energy is Gaussian. Specially designed optics provide a dual-beam interferometric configuration to create linear structures on the sample. A lens with a focal length of 100 mm superimposes two additional laser beams on the surface at a fixed angle of 6.8\(^\circ\), which gives a spatial period of about 4.5 µm. More information on the experimental setup can be found elsewhere50.
Before laser processing, the sample is placed on a heating plate at a certain temperature. The temperature of the heating plate was set at 21 and 250°C. In all experiments, a transverse jet of compressed air was used in combination with an exhaust device to prevent dust deposition on the optics. An x,y stage system is set up to position the sample during structuring.
The speed of the positioning stage system was varied from 66 to 200 mm/s to obtain an overlap between pulses of 99.0 to 99.67 \(\%\) respectively. In all cases, the repetition rate was fixed at 200 kHz, and the average power was 4 W, which gave an energy per pulse of 20 μJ. The beam diameter used in the DLIP experiment is about 100 µm, and the resulting peak laser energy density is 0.5 J/cm\(^{2}\). The total energy released per unit area is the peak cumulative fluence corresponding to 50 J/cm\(^2\) for \(o_{\mathrm {p}}\) = 99.0 \(\%\), 100 J/cm\(^2\) for \(o_{\mathrm {p))\)=99.5\(\%\) and 150 J/cm\(^2\) for \(o_{ \mathrm {p} }\) = 99.67 \(\%\). Use the \(\lambda\)/2 plate to change the polarization of the laser beam. For each set of parameters used, an area of approximately 35 × 5 mm\(^{2}\) is textured on the sample. All structured experiments were conducted under ambient conditions to ensure industrial applicability.
The morphology of the samples was examined using a confocal microscope with a 50x magnification and an optical and vertical resolution of 170 nm and 3 nm, respectively. The collected topographic data was then evaluated using surface analysis software. Extract profiles from terrain data according to ISO 1661051.
The samples were also characterized using a scanning electron microscope at an accelerating voltage of 6.0 kV. The chemical composition of the surface of the samples was evaluated using an energy-dispersive X-ray spectroscopy (EDS) attachment at an accelerating voltage of 15 kV. In addition, an optical microscope with a 50x objective was used to determine the granular morphology of the microstructure of the samples. Before that, the samples were etched at a constant temperature of 50 \(^\circ\)C for five minutes in a stainless steel stain with hydrochloric acid and nitric acid concentration of 15–20 \(\%\) and 1\(-<\)5 \(\%\), respectively. Before that, the samples were etched at a constant temperature of 50 \(^\circ\)C for five minutes in a stainless steel stain with hydrochloric acid and nitric acid concentration of 15–20 \(\%\) and 1\( -<\)5 \(\%\), respectively. Перед этим образцы травили при постоянной температуре 50 \(^\circ\)С в течение пяти минут в краске из нержавеющей стали соляной и азотной кислотами концентрацией 15-20 \(\%\) и 1\( -<\)5 \(\%\) соответственно. Before that, the samples were etched at a constant temperature of 50 \(^\circ\)C for five minutes in stainless steel paint with hydrochloric and nitric acids with a concentration of 15-20 \(\%\) and 1\( -<\)5 \( \%\) respectively.在此之前，样品在不锈钢染色液中以50 \(^\circ\)C 的恒温蚀刻五分钟，盐酸和硝酸浓度为15–20 \(\%\) 和1\( -<\)5 \(\%\)，分别。在此之前，样品在不锈钢染色液中以50 \(^\circ\)C (\%\)，分别。 Before that, the samples were pickled for five minutes at a constant temperature of 50 \(^\circ\)C in a staining solution for stainless steel with a concentration of hydrochloric and nitric acids 15-20 \(\%\) and 1\. (-<\)5 \ (\%\) соответственно. (-<\)5 \ (\%\) respectively.
Schematic diagram of the experimental setup of a two-beam DLIP setup, including (1) a laser beam, (2) a \(\lambda\)/2 plate, (3) a DLIP head with a certain optical configuration, (4) a hot plate, (5) a cross-fluidic , (6) x,y positioning steps and (7) stainless steel specimens. Two superimposed beams, circled in red on the left, create linear structures on the sample at \(2\theta\) angles (including both s- and p-polarization).
The datasets used and/or analyzed in the current study are available from the respective authors upon reasonable request.
Post time: Jan-07-2023