- Email: [email protected]
Extending ultra-short pulse laser texturing over large area
Applied Surface Science 386 (2016) 65–71
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Extending ultra-short pulse laser texturing over large area G. Mincuzzi ∗ , L. Gemini, M. Faucon, R. Kling Alphanov, Rue Franc¸ois Mitterand, 33400 Talence, France
a r t i c l e
i n f o
Article history: Received 22 January 2016 Received in revised form 30 May 2016 Accepted 31 May 2016 Available online 4 June 2016 Keywords: Ultra-short pulses laser texturing Polygon scanner head Ripples Spikes Metal blackening
a b s t r a c t Surface texturing by Ultra-Short Pulses Laser (UPL) for industrial applications passes through the use of both fast beam scanning systems and high repetition rate, high average power P, UPL. Nevertheless unwanted thermal effects are expected when P exceeds some tens of W. An interesting strategy for a reliable heat management would consists in texturing with a low fluence values (slightly higher than the ablation threshold) and utilising a Polygon Scanner Heads delivering laser pulses with unrepeated speed. Here we show for the first time that with relatively low fluence it is possible over stainless steel, to obtain surface texturing by utilising a 2 MHz femtosecond laser jointly with a polygonal scanner head in a relatively low fluence regime (0.11 J cm−2 ). Different surface textures (Ripples, micro grooves and spikes) can be obtained varying the scan speed from 90 m s−1 to 25 m s−1 . In particular, spikes formation process has been shown and optimised at 25 m s−1 and a full morphology characterization by SEM has been carried out. Reflectance measurements with integrating sphere are presented to compare reference surface with high scan rate textures. In the best case we show a black surface with reflectance value < 5%. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Surface engineering and functionalization by nano and micro structures generated upon short [1–3] and ultrashort pulse (USP) lasers irradiation, has been reported for a wide variety of materials like metals [1–6], semiconductors [7–9], polymers [10–12] and dielectrics [13,14]. In this context, Laser Induced Periodic Surface Structures (LIPSS or ripples), micro-grooves and spikes formation has been intensively studied [15–23] as it allows for tailoring some key surface properties like wettability, tribology and colour [16–18]. It has been observed that for a fixed fluence value ˚ of the laser pulse, the integrated laser fluence, or surface energy dose , has a crucial relevance in determining the final LIPSS morphology [10,11]. depends on several parameters including the inter-pulses distance d, the offset ı between successive scan lines and the number N of successive scans over a given surface area. For instance, for a relatively low number of N, ripples characterized by a spatial periodicity smaller than the laser wavelength, are formed. Increasing N, micro-grooves appear. Finally, for a relatively large number of N, micro-spikes are uniformly induced on the surface. Nevertheless, for the USP surface texturing process to gain an actual foothold into the industrial scene, it is crucial to increase the
∗ Corresponding author. E-mail address: [email protected] (G. Mincuzzi). http://dx.doi.org/10.1016/j.apsusc.2016.05.172 0169-4332/© 2016 Elsevier B.V. All rights reserved.
process throughput in order to reach a takt-time compatible with commercial purposes. A promising strategy would be to keep the values of and ˚ unmodified whilst increasing the scanning speed (up to several tens of m/s) and the repetition rate (in the range of MHz). This issue may be addressed by the use of polygon scanning systems (PSS) which have been proved to be an effective tool to reliably position laser pulses with scanning speeds up to few hundreds of m/s [25,26]. Jointly with high power, high repetition rate, ps lasers, PSS have been employed for material machining processes such as patterning, engraving, structuring and texturing [25]. Nevertheless, at repetition rates ranging from few hundreds of kHz to tens of MHz, the average output power could be high enough for detrimental thermal effects to arise [26]. For instance, it has been estimated that the surface temperature in stainless steel will approach 900 ◦ C for an average output power of about 100 W [27]. Although several laser scanning strategies have been proposed to circumvent heat accumulation and prevent undesired thermal effects [27], a sensible reduction of the energy-per-pulse J, together with the use of sub-ps laser systems, would be the leading choice for optimal USP surface texturing processes in the MHz regime. In this context, spikes formation has been observed recently over stainless steel surface after multiple USP scanning (pulse duration 490 fs) with a fluence-per-pulse value sensibly lower (about 0.2 J/cm2 ) than the ones previously reported (about 1 J/cm2 ) at repetition rate up to 1 MHz and scanning speeds slightly lower than10 m/s [29]. This result opens the way to a remarkable reduc-
66
G. Mincuzzi et al. / Applied Surface Science 386 (2016) 65–71
Fig. 1. Schematic view of the experimental setup. The pulse energy was controlled by the combination of half-wave plate and polarizer. The beam size was expanded of a factor 2.5 before entering the scanning head.
tion of the average power required to carry-out a large scale surface texturing with consequent benefit on the heat management. Here for the first time, a high repetition rate (up to 2 MHz)-high average power (up to 20 W) femtoseconds laser (pulse duration < 400 fs) has been used jointly with a high speed (up to 100 m/s) polygon scanning head for texturing of stainless steel surfaces in low fluence-per-pulse regime. For a fixed value of successive over scans N, the scanning speed v has been varied over a wide range of values from v = 90 m/s to v = 25 m/s. By varying v, ripples, microgrooves and spikes have been obtained. Furthermore, for fixed values of v and offset between scanning lines ı, we show that it is possible to control the surface blackening by varying the number N of successive scans. Our results confirm that a light trapping mechanism induced by the surface morphology is responsible for the reflectivity reduction. We believe that these results not only validate the possibility to induce surface texturing in a low fluence regime, but demonstrate that by a systematic variation of the process parameters, it is possible to obtain ripples, micro grooves and spikes with reduced average power and unprecedented scan speed values. 2. Experimental methods Laser surface texturing tests were carried out on 316 Stainless Steel sheets (RS 559-199) with a thickness of 0.5 mm cleaned in ethanol. An IR fiber laser (emission wavelength = 1030 nm) delivering ultra-short pulses (pulse duration < 400 fs) at 2 MHz repetition rate (Tangerine laser system by Amplitude Systèmes) was used for all tests. The beam was firstly magnified of a factor 2.5 and then delivered on the sample by a polygon scanning head (Next Scan Technology LSE170), which enables to scan 17 cm long lines (being 17 cm the scan field) with a scan rate varying between 100 lines/s and 400 lines/s, that is a scan speed = 25 m/s and = 100 m/s respectively. In our study was varied between 25 m/s and 90 m/s. The focused laser beam spot diameter 2ω on the 316 Stainless Steel samples surface was measured to be 45 m by means of a beam profiler (Win Cam). For every test, the energy-per-pulse J imping on the sample surface was kept constant at 1.77 J, corresponding to a fluence-per-pulse ˚ of 0.11 J/cm2 . A schematic illustration of the experimental setup is shown in Fig. 1. In the first set of tests, the influence of the scanning speed on the surface morphology was studied by increasing the horizontal overlapping, that is varying the spatial distance d between adjacent pulses, for a fixed value of successive scans N = 500. To observe the morphology evolution on a wider energy dose range, the vertical overlapping was also increased by varying the offset ı between scanning lines. This way it was possible to define the set of processing parameters, namely scanning speed and overlaps,
Fig. 2. Schematic illustration of the processed surface and processing parameters involved. The red circles represent the single laser spots. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
for which defined and homogeneous spikes form on the 316 Stainless Steel sample surface. In the second set of tests, the influence of the number N of successive scans on the surface morphology and reflectivity was investigated in order to define the optimized overall process takt-time necessary to induce specific morphology changes. The process takt-time necessary to obtain homogeneous spike morphology, corresponding to complete blackening of the sample surface, was finally evaluated. The textured surface was analyzed by SEM microscopy (Phantom Electronic Microscope by FEI). Finally, reflection spectra measurements in the range 350 nm–1750 nm were carried out by spectrometric analyses (Spectrophotometer Shimadzu UV 3600 plus – Reflectometer x-rite Ci6x). 3. Results and discussion 3.1. From ripples to spikes It is well known that for a fixed fluence-per-pulse ˚ = J/ω2 , the cumulative laser energy dose (J/cm2 ) is the key physical parameter determining the surface morphology [24]. Considering for instance a rectangular surface S = a × b (Fig. 2), can be expressed according to the following formula: =N
E S
=N
J × n × m a×b
= N˚
ω2 d×ı
(1)
where E = J × n × m is the total energy relative to the (n × m) pulses irradiated on S, d = (a/n) the spatial distance between pulses, ı = (b/m) the offset between scanning lines and N the number of
G. Mincuzzi et al. / Applied Surface Science 386 (2016) 65–71
67
Fig. 3. SEM images of the textured surface after N = 500 successive scans with varying scanning speed: (a) v = 90 m/s (d = 45 m, ı = 45 m), ripples – (b) v = 40 m/s (d = 20 m, ı = 20 m), ripples – (c) v = 25 m/s (d = 12.5, ı = 12.5 m), spikes on micro-grooves. White arrows show the orientation of the laser polarization on the surface.
Fig. 4. SEM images of the textured surface after N = 500 successive scans and scanning speed v = 25 m/s (d = 12.5 m). The offset between two successive scan lines was reduced to ı = 3 m (a), 2 m (b) and 1 m (c).
successive scans. A schematic illustration of the processed surface with related processing parameters is shown in Fig. 2. As first step, the influence of the scanning speed v on the final surface morphology was investigated by varying the horizontal overlapping, that is varying the parameter d, whilst keeping the number N of successive scans fixed at 500. With regard to the polygon scanning system parameters, for a fixed laser repetition rate f, it is possible to vary the scanning speed v by varying the distance d between two successive pulses, according to the relation v = f × d. It follows that at f = 2 MHz and d = 45 m, d = 20 m and d = 12.5 m, the resulting scanning speed is v = 90 m/s, v = 40 m/s and v = 25 m/s, respectively. In order to maintain the same overlapping both in the horizontal and vertical direction, the offset between scanning lines ı was also set at 45 m, 20 m and 12.5 m when v = 90 m/s, v = 40 m/s and v = 25 m/s, respectively. It is important to highlight that reducing the scanning speed, the pulses overlapping and therefore the laser energy dose increases. Fig. 3 shows SEM images of the textured surface after N = 500 successive scans with varying scan speed. For v = 90 m/s and ı = 45 m, pulses are separated (Fig. 3a) and ripples perpendicular to the polarization direction are formed only in correspondence of the irradiated areas with a spatial periodicity ranging from 400 nm to 500 nm. For v = 40 m/s and ı = 20 m, pulses overlap (Fig. 3b) and ripples perpendicular to the polarization direction appear with a spatial periodicity of about 700 nm. Finally, for v = 25 m/s and ı = 12.5 m, the pulse overlap increases (Fig. 3c), micro-grooves parallel to the laser polarization orientation are formed and begin to evolve to conical structures (spikes). Generally, the evolution of the surface morphology from LIPSS to spikes may be explained as follows: after the ultra-fast interaction, the surface is found in a superheated liquid state where capillary fluid convection may take place as a consequence of the
Marangoni effect induced by the Gaussian shape of the laser pulse [19]. Exposing the surface to subsequent pulses, the so-induced surface topology may give rise to interference effects between the laser light and surface electromagnetic waves, such as surface plasma polaritons. This in turn will lead to a spatially modulated ablation process and consequently to formation of ripples with a direction perpendicular to the laser polarization orientation [20–23]. Increasing the number of irradiating pulses, the efficacy with which the rough surface absorbs the laser energy is deeply modified by the evolving surface morphology: the conditions for surface plasma polaritons to be excited are not anymore fulfilled and ripples disappear. Subsequently, micro-grooves and spikes are formed as a consequence of the onset of hydrothermal convection effects: hydrothermal waves rise perpendicularly to the previously formed ripples, that is with a direction parallel to the laser polarization, this way defining the direction of the micro-grooves. Increasing further the number of laser pulses, hydrodynamic effects prevail and conical structures start to appear regularly on the top of microgrooves, developing in more defined spikes as a consequence of local variation of the laser energy absorbance [19,29]. A more detailed theoretical description of the above mentioned processes can be found in Ref. [19–23]. In order to increase and to optimize the spikes morphology, d and ı need to be further reduced. With a minimal inter-pulse distance of d = 12.5 m for a scan line of the polygon scanner the re remaining parameter is the offset between two successive scan lines which was reduced to ı = 5, 3, 2, and 1 m. The obtained morphology is shown in Fig. 4. The periodic behaviour of the morphology is visible only for ı > 2 m (Fig. 4a). It is clear that every kind of observed morphology is characterized by a threshold value of laser energy dose over which the specific morphology starts to appear. As previously mentioned, this result is already reported in
68
G. Mincuzzi et al. / Applied Surface Science 386 (2016) 65–71
Fig. 5. Dependence of the values of the conical spikes base diameter on the offset between scanning lines ı.
all the other process parameters constant (v = 25 m/s, d = 12.5 m, ı = 2 m). Fig. 6 shows SEM images of the stainless steel surface after processing with N = 10 (Fig. 6a) and N = 100 (Fig. 6b). For N = 10, an extensive ripple texturing is clearly visible, characterized by an average spatial periodicity of about 500 nm. As expected, the ripples direction is perpendicular to the beam polarization orientation. For N = 100, micro-conical structures appear on the surface characterized by an average size of approximately 8 m. Fig. 7 presents SEM images of the stainless steel surface at scanning speed v = 25 m/s and N = 300 (Fig. 7a), N = 500, N = 600 (Fig. 7b) and N = 800 successive scans (Fig. 7c). For N ≥ 300, typical spikes shape structures are formed with diameter ≈ 18 m. Increasing N, increases reaching a value of about 35 m for N = 500 and N = 600. Moreover, in this case, the spike tips are not sharp but, as already observed, subdivide in smaller structures, starting from the top of the conical structure. Increasing the number of successive scans from N = 500, the spike diameter value stabilize between 30 m and 40 m. It is worth mentioning that, despite of the high scan speed (25 m/s), processes at higher value of N would imply a processing time which is out of interest for any industrial application. 3.2. Surface blackening
literature and it is usually explained as a consequence of the necessary conditions to be fulfilled for surface plasma polaritons to be excited as first, and hydrothermal effects to take place later on, as increases [19,22,23]. For ı = 2 m (Fig. 3b), is high enough to guarantee an uniform texturation of well-defined spikes. For ı = 1 m (Fig. 3c), spikes begin to subdivide in smaller structures, starting from the top of the conical structure. In Fig. 5 the dependence of the conical spikes diameter on ı is shown. In the range 5 m < ı < 12.5 m, the spikes diameter does not increases sensibly and its average value is about 10 m. For ı < 5 m, increases noticeably varying from ≈ 23 m when ı = 3 m to ≈ 47 m when ı = 1 m. It is worth observing that in the last case, beside an increment of the average value, a large error bar was also extracted, underlying not negligible variability. This is probably due to the change observed in the morphology at the top of the spikes in the case ı = 1 m. Table 1 summarizes the processing parameters in relation to the observed morphology. Table 1 highlights the intervals of energy dose for the formation of specific morphology: homogeneous micro-grooves and spikes appear on the surface for in the range 0.22–0.57 kJ/cm2 and 2.37–3.56 kJ/cm2 respectively. These values are on the same order of magnitude of the range reported for spike formation in Ref. [26]. In the previous section all the experiments were carried with a constant number of successive scans N = 500. The laser energy dose was varied by increasing the pulses overlapping both in the scanning direction (d) and in between the scanning lines (ı). Of course, for a fixed pulses overlapping, N determines the process takt-time. This fact highlights the necessity to find the optimal N value for which it is possible to obtain a specific surface texturing in the lowest takt-time. Therefore, the behaviour of the morphology was studied in relation the number of successive scans N, keeping
One of the applications of USP texturing is the control of the surface reflectivity R over a wide range of values. In the visible and near infra-red (NIR) range, textured surfaces with R < 5%, corresponding to an intense blackening of the surface color, have been reported [24]. The reflectivity variation is known to be induced by an efficient mechanism of light trapping within the surface structures [30]. In particular, together with the structures aspect ratio, the surface porosity and morphology at the nanoscale, has been shown to play a crucial role in inducing multiple light reflections which prevent the light to be back reflected [28,29]. This way it is possible to obtain highly absorptive surfaces without the need for special coatings or physical/chemical modifications of the material surface. Fig. 8 presents reflectivity spectra in the range of wavelengths 350 nm–1750 nm relative to stainless steel surfaces textured with number of successive scans N increasing from 10 to 800 (v = 25 m/s, d = 12.5 m, ı = 2 m). The reference reflectivity spectrum of untreated stainless steel is also shown. Measurements have been carried out with a spectrophotometer equipped with an integrating sphere. It can be clearly observed that starting from a reference value of R = 70%, the reflectivity remarkably decreases by increasing N. For N > 300, R is already below 13% and, in the best case N = 600, a value of R < 5% is obtained. For N > 600, no sensible decrease of reflectivity was observed. It is interesting to notice that in all cases the spectrum is quite flat all over the measurement interval. This indicates that the mechanism underlying the reflectivity decrease excludes the possibility of light absorption by dust. Due to Mie scattering, micro-sized dust grains will in fact induce a significant Reflectivity increase in the NIR region. Moreover, as presented in Fig. 9, it is an important result that, while
Table 1 Values of processing parameters (scanning speed , distance between pulses d, horizontal overlapping HO, offset between scanning lines ı, vertical overlapping VO) and laser energy dose in relation to the observed morphology (R: ripples; -G: micro-grooves; S: spikes). (m/s)
d (m)
HO (%)
90 40 25 25 25 25 25
45 20 12.5 12.5 12.5 12.5 12.5
0 56 72 72 72 72 72
ı (m) 45 20 12.5 5 3 2 1
VO (%)
(kJ/cm2 )
Morphology
0 56 72 89 93 96 98
0.04 0.22 0.57 1.42 2.37 3.56 7.12
R R S on -G S on -G S on -G S S
G. Mincuzzi et al. / Applied Surface Science 386 (2016) 65–71
69
Fig. 6. SEM images of stainless steel surface irradiated at scanning speed v = 25 m/s (d = 12.5 m and ı = 2) and N = 10 successive scans (a) and N = 100 successive scans (b).
Fig. 7. SEM images of stainless steel surface irradiated at scanning speed v = 25 m/s (d = 12.5 m and ı = 2) and N = 300 successive scans (a), N = 600 successive scans (b) and N = 800 successive scans (c).
Fig. 8. Reflectivity spectra measured by spectrophotometer in the range 350 nm–1750 nm relative to stainless steel surfaces textured with number of successive scans N increasing from 10 to 800. The reference reflectivity spectrum of untreated stainless steel is also shown in black full squares.
N increases, an increase of the spike diameter corresponds to a decrease of reflectivity: the light trapping mechanism becomes more efficient as the conical structure increases in size. This may be due to a corresponding increase of both the height of the conical
structures and the surface porosity obtained by partial re-melting of nanoparticles generated during the process [30–32] . In Table 2 the average reflectivity values relative to the same surfaces measured by a reflectometer in the visible range are reported. These results
70
G. Mincuzzi et al. / Applied Surface Science 386 (2016) 65–71
where tr is the time required for the sample re-positioning after one raster scanning between two successive scans which is assumed to be tr = 1s. It follows that tp = 8.5 h, which correspond to a processing time for area unit of tu = 30 min/cm2 . Multiple strategies could be addressed to significantly reduce this value:
Fig. 9. Dependence of the spike diameter (squares) and of the average reflectivity Rave (full circles) on the number of successive scans N, at scanning speed v = 25 m/s (d = 12.5 m) and ı = 2. Table 2 Values of the average reflectivity Rave measured by reflectometer in the range of wavelengths 400 nm–700 nm in relation to the number N of successive scans. Rave (%)
N reference 10 100 300 600
confirm the good quality of the values of reflectivity obtained by spectrophotometer analyses. The decrease in reflectivity was also verified by visual inspection of the textured surface (Fig. 10): the surface blackening increases very clearly as the number N of successive scans increases. To conclude, we evaluated the takt-time and in particular, the process total time tp required for blackening a surface S = 17 cm × 1 cm, being 17 cm the maximum working field enabled by the scanner. The process parameters considered here are: scan speed v = 25 m/s corresponding to 100 lines/s, scan lines off set ı = 2 m (corresponding to 5000 lines/cm) and number of successive scans N = 600. The process total time tp can be written as follow:
For instance, a 100 W class, fs laser with repetition rate of nearly 10 MHz could potentially reduce tu < 1 min/cm2 . 4. Conclusions
69.7 23 19 12 4.9
Fig. 10. Progressive surface blackening as the number of successive scans N increases from 10 to 600.
tp = N 5000/100 + tr
A.) Increasing the laser repetition rate to 8 MHz would allow to fully exploit the polygon scanner performances. This way, a scan speed of 100 m/s could be utilized whilst keeping the pulses overlapping unmodified. B.) Increasing the pulse energy values would permit to enlarge the beam size, keeping the unmodified. This, in turn, would permit to increase the distance d between pulses and the offset ı between lines whilst keeping the laser energy dose unmodified. C.) Using a polygon scanner system with larger scan field (from 17 cm to 30 cm), keeping the same value of scanning speed, could be a further interesting option to consider in order to process a larger surface in the same process time. D.) Finally a multi-head approach in which two or more polygon scanners work in parallel might also be considered.
(2)
The possibility to increase the process throughput of USP texturing processes over large area, reaching takt-times compatible with commercial purposes, raises the issue on how to manage the heat induced by the high average power required for processing in the MHz regime. The key concept to address this problem is to utilize low energy-per-pulse and high scanning speed. For the first time a high repetition rate (2 MHz) femtosecond laser has been utilized jointly with a fast (up to 100 m/s) polygon scanner system for stainless steel surface texturing in low fluence-per-pulse regime. The fluence-per-pulse was indeed kept constant at 0.11 J cm−2 , that is almost 1 order of magnitude lower than fluence values conventionally used for this kind of processes. By varying the scan speed from 90 m/s to 25 m/s, three types of morphologies were observed: ripples, micro-grooves and spikes. We demonstrated that it is possible to control the type of morphology by a systematic variation of the process parameters. The possibility to control the type of morphology allows for the control of the surface reflectivity, which was drastically reduced with respect to the reference value of un-textured surfaces. A lowest reflectivity value of R < 5% in the visible spectral range was obtained. The flatness of the reflectivity spectrum in a wide range of wavelength (from 400 nm to 2000 nm) indicates that the surface blackening is induced by a light trapping mechanism. These results open the way for an industrial exploitation of USP surface texturing. Although in our experimental conditions a takttime of nearly tu = 30 min/cm2 was extracted, use of laser sources with tens of MHz and higher pulse energy, jointly with multiple heads processing approach will be beneficial for the final throughput to reach target values suitable for commercial purposes. References [1] I. Ursu, I.N. Mih˘ailescu, A. Popa, A.M. Prokhorov, V.P. Ageev, A.A. Gorbunov, V.I. Konov, Studies of the change of a metallic surface microrelief as a result of multiple-pulse action of powerful UV laser pulses, J. Appl. Phys. 58 (1985) 3909. [2] M.S. Brown, C.B. Arnold, Fundamentals of laser-material interaction and ¨ in: Laser Precision application to multiscale surface modification, Microfabrication, Springer, Berlin Heidelberg, 2010, pp. 91–120.
G. Mincuzzi et al. / Applied Surface Science 386 (2016) 65–71 [3] J.E. Sipe, J.F. Young, J.S. Preston, H.M. van Driel, Phys. Rew. B 27 (1983) 1141. [4] M. Pfeiffer, A. Engel, H. Gruettner, K. Guenther, F. Marquardt, G. Reisse, S. Weissmantel, Appl. Phys. A 110 (2013) 655. [5] S. Sakabe, M. Hashida, S. Tokita, K. Okamura, Phys. Rev. B 79 (2009) (033109). [6] A.Y. Vorobyev, V.S. Makin, C. Guo, J. Appl. Phys. 101 (2007) (034903). [7] L. Gemini, M. Hashida, M. Shimizu, Y. Miyazaka, S. Inoue, S. Tokita, J. Limpouch, T. Mocek, S. Sakabe, J. Appl. Phys. 114 (2013) (194903). [8] A.Y. Vorobyev, C. Guo, Opt. Exp. 19 (2011) A1031. [9] T. Tomita, Y. Fukumori, K. Kinoshita, S. Matsuo, S. Hashimoto, Appl. Phys. Lett. 92 (2008) (013104). [10] Á. Rodríguez-Rodríguez, E. Rebollar, M. Soccio, T.A. Ezquerra, D.R. Rueda, J.V. Garcia-Ramos, M. Castillejo, M.-C. Garcia-Gutierrez, Macromolecules 48 (2015) 4024. [11] M. Castillejo, T.A. Ezquerra, M. Martín, M. Oujja, S. Pérez, E. Rebollar, AIP Conf. Proc. 1464 (2012) 372. [12] S. Yada, M. Terakawa, Opt. Exp. 23 (2015) 5694. [13] A. Rosenfeld, M. Rohloff, S. Hohm, J. Kruger, J. Bonse, Appl. Surf. Sci. 258 (2012) 9233. [14] S. Hohm, A. Rosenfeld, J. Kruger, J. Bonse, Appl. Phys. Lett. 102 (2013) 054102. [15] J. Bonse, J. Kruger, S. Hohm, A. Rosenfeld, J. Appl. Laser 24 (2012) (042006-1). [16] A.Y. Vorobyev, C. Guo, Las. Phot. Rev. 7 (2013) 385. [17] V. Zorba, L. Persano, D. Pisignano, A. Athanassiou, E. Stratakis, R. Cingolani, P. Tzanetakis, C. Fotakis, Nanotechnology 17 (2006) (3234). [18] A.Y. Vorobyev, C. Guo, J. Appl. Phys. 117 (2015) (033103). [19] G.D. Tsibidis, C. Fotakis, E. Stratakis, Phys. Rev. B 92 (2015) (041405).
71
[20] J.E. Sipe, J.F. Young, J.S. Preston, H.M. van Driel, Phys. Rev. B 27 (1983) 1141. [21] J. Bonse, A. Rosenfeld, J. Kruger, J. Appl. Phys. 106 (2009) (104910). [22] T.J.-Y. Derrien, T.E. Itina, R. Torres, T. Sarnet, M. Sentis, J. Appl. Phys. 114 (2013) (083104). [23] J. Bonse, S. Baudach, J. Krüger, W. Kautek, M. Lenzner, Appl. Phys. A 74 (2002) 19–25. [24] M. Faucon, A. Laffitte, J. Lopez, R. Kling, Proc. SPIE 8972 (2014) (89721M). [25] U. Loeschner, J. Schille, A. Streek, T. Knebel, L. Hartwig, R. Hillmann, C. Endisch, J. Laser. Appl. 27 (2015) S29303. [26] R. De Loor, L. Penning, R. Slagle, Laser Tech. J. 11 (2014) 32. [27] S. Nolte, S. Döring, A. Ancona, J. Limpert, A. Tünnermann, High repetition rate ultrashort pulse micromachining with fiber lasers in fiber laser applications, Opt. Soc. Am. (2011) (pFThC1). [28] B. Neuenschwander, B. Jaeggi, M. Zimmermann, G. Hennig, V. Markovic, B. Resan, K. Weingarten, R. de Loor, L. Penning, Power Scaling into the 100 W regime for surface texturing of metals with ultra-short laser pulses, in: The 7th International Congress on Laser Advanced Materials Processing, May 26–29, Kitakyushu, 2015. [29] D.-H. Kam, J. Kim, L. Song, J. Mazumder, J. Micromech. Microeng. 28 (2015) (045007). [30] L.K. Ang, Y.Y. Lau, R.M. Gilgenbach, H.L. Spindler, Appl. Phys. Lett. 70 (1997) 696. [31] H. Huang, L.-M. Yang, S. Bai, J. Liu, Appl. Opt. 54 (2015) 2. [32] A.Y. Vorobyev, C. Guo, Black metals through femtosecond laser pulses international symposium on high power laser ablation 2012, AIP Conf. Proc. 1464 (2012) 397–401, http://dx.doi.org/10.1063/1.4739894.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Extending ultra-short pulse laser texturing over large area G. Mincuzzi ∗ , L. Gemini, M. Faucon, R. Kling Alphanov, Rue Franc¸ois Mitterand, 33400 Talence, France
a r t i c l e
i n f o
Article history: Received 22 January 2016 Received in revised form 30 May 2016 Accepted 31 May 2016 Available online 4 June 2016 Keywords: Ultra-short pulses laser texturing Polygon scanner head Ripples Spikes Metal blackening
a b s t r a c t Surface texturing by Ultra-Short Pulses Laser (UPL) for industrial applications passes through the use of both fast beam scanning systems and high repetition rate, high average power P, UPL. Nevertheless unwanted thermal effects are expected when P exceeds some tens of W. An interesting strategy for a reliable heat management would consists in texturing with a low fluence values (slightly higher than the ablation threshold) and utilising a Polygon Scanner Heads delivering laser pulses with unrepeated speed. Here we show for the first time that with relatively low fluence it is possible over stainless steel, to obtain surface texturing by utilising a 2 MHz femtosecond laser jointly with a polygonal scanner head in a relatively low fluence regime (0.11 J cm−2 ). Different surface textures (Ripples, micro grooves and spikes) can be obtained varying the scan speed from 90 m s−1 to 25 m s−1 . In particular, spikes formation process has been shown and optimised at 25 m s−1 and a full morphology characterization by SEM has been carried out. Reflectance measurements with integrating sphere are presented to compare reference surface with high scan rate textures. In the best case we show a black surface with reflectance value < 5%. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Surface engineering and functionalization by nano and micro structures generated upon short [1–3] and ultrashort pulse (USP) lasers irradiation, has been reported for a wide variety of materials like metals [1–6], semiconductors [7–9], polymers [10–12] and dielectrics [13,14]. In this context, Laser Induced Periodic Surface Structures (LIPSS or ripples), micro-grooves and spikes formation has been intensively studied [15–23] as it allows for tailoring some key surface properties like wettability, tribology and colour [16–18]. It has been observed that for a fixed fluence value ˚ of the laser pulse, the integrated laser fluence, or surface energy dose , has a crucial relevance in determining the final LIPSS morphology [10,11]. depends on several parameters including the inter-pulses distance d, the offset ı between successive scan lines and the number N of successive scans over a given surface area. For instance, for a relatively low number of N, ripples characterized by a spatial periodicity smaller than the laser wavelength, are formed. Increasing N, micro-grooves appear. Finally, for a relatively large number of N, micro-spikes are uniformly induced on the surface. Nevertheless, for the USP surface texturing process to gain an actual foothold into the industrial scene, it is crucial to increase the
∗ Corresponding author. E-mail address: [email protected] (G. Mincuzzi). http://dx.doi.org/10.1016/j.apsusc.2016.05.172 0169-4332/© 2016 Elsevier B.V. All rights reserved.
process throughput in order to reach a takt-time compatible with commercial purposes. A promising strategy would be to keep the values of and ˚ unmodified whilst increasing the scanning speed (up to several tens of m/s) and the repetition rate (in the range of MHz). This issue may be addressed by the use of polygon scanning systems (PSS) which have been proved to be an effective tool to reliably position laser pulses with scanning speeds up to few hundreds of m/s [25,26]. Jointly with high power, high repetition rate, ps lasers, PSS have been employed for material machining processes such as patterning, engraving, structuring and texturing [25]. Nevertheless, at repetition rates ranging from few hundreds of kHz to tens of MHz, the average output power could be high enough for detrimental thermal effects to arise [26]. For instance, it has been estimated that the surface temperature in stainless steel will approach 900 ◦ C for an average output power of about 100 W [27]. Although several laser scanning strategies have been proposed to circumvent heat accumulation and prevent undesired thermal effects [27], a sensible reduction of the energy-per-pulse J, together with the use of sub-ps laser systems, would be the leading choice for optimal USP surface texturing processes in the MHz regime. In this context, spikes formation has been observed recently over stainless steel surface after multiple USP scanning (pulse duration 490 fs) with a fluence-per-pulse value sensibly lower (about 0.2 J/cm2 ) than the ones previously reported (about 1 J/cm2 ) at repetition rate up to 1 MHz and scanning speeds slightly lower than10 m/s [29]. This result opens the way to a remarkable reduc-
66
G. Mincuzzi et al. / Applied Surface Science 386 (2016) 65–71
Fig. 1. Schematic view of the experimental setup. The pulse energy was controlled by the combination of half-wave plate and polarizer. The beam size was expanded of a factor 2.5 before entering the scanning head.
tion of the average power required to carry-out a large scale surface texturing with consequent benefit on the heat management. Here for the first time, a high repetition rate (up to 2 MHz)-high average power (up to 20 W) femtoseconds laser (pulse duration < 400 fs) has been used jointly with a high speed (up to 100 m/s) polygon scanning head for texturing of stainless steel surfaces in low fluence-per-pulse regime. For a fixed value of successive over scans N, the scanning speed v has been varied over a wide range of values from v = 90 m/s to v = 25 m/s. By varying v, ripples, microgrooves and spikes have been obtained. Furthermore, for fixed values of v and offset between scanning lines ı, we show that it is possible to control the surface blackening by varying the number N of successive scans. Our results confirm that a light trapping mechanism induced by the surface morphology is responsible for the reflectivity reduction. We believe that these results not only validate the possibility to induce surface texturing in a low fluence regime, but demonstrate that by a systematic variation of the process parameters, it is possible to obtain ripples, micro grooves and spikes with reduced average power and unprecedented scan speed values. 2. Experimental methods Laser surface texturing tests were carried out on 316 Stainless Steel sheets (RS 559-199) with a thickness of 0.5 mm cleaned in ethanol. An IR fiber laser (emission wavelength = 1030 nm) delivering ultra-short pulses (pulse duration < 400 fs) at 2 MHz repetition rate (Tangerine laser system by Amplitude Systèmes) was used for all tests. The beam was firstly magnified of a factor 2.5 and then delivered on the sample by a polygon scanning head (Next Scan Technology LSE170), which enables to scan 17 cm long lines (being 17 cm the scan field) with a scan rate varying between 100 lines/s and 400 lines/s, that is a scan speed = 25 m/s and = 100 m/s respectively. In our study was varied between 25 m/s and 90 m/s. The focused laser beam spot diameter 2ω on the 316 Stainless Steel samples surface was measured to be 45 m by means of a beam profiler (Win Cam). For every test, the energy-per-pulse J imping on the sample surface was kept constant at 1.77 J, corresponding to a fluence-per-pulse ˚ of 0.11 J/cm2 . A schematic illustration of the experimental setup is shown in Fig. 1. In the first set of tests, the influence of the scanning speed on the surface morphology was studied by increasing the horizontal overlapping, that is varying the spatial distance d between adjacent pulses, for a fixed value of successive scans N = 500. To observe the morphology evolution on a wider energy dose range, the vertical overlapping was also increased by varying the offset ı between scanning lines. This way it was possible to define the set of processing parameters, namely scanning speed and overlaps,
Fig. 2. Schematic illustration of the processed surface and processing parameters involved. The red circles represent the single laser spots. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
for which defined and homogeneous spikes form on the 316 Stainless Steel sample surface. In the second set of tests, the influence of the number N of successive scans on the surface morphology and reflectivity was investigated in order to define the optimized overall process takt-time necessary to induce specific morphology changes. The process takt-time necessary to obtain homogeneous spike morphology, corresponding to complete blackening of the sample surface, was finally evaluated. The textured surface was analyzed by SEM microscopy (Phantom Electronic Microscope by FEI). Finally, reflection spectra measurements in the range 350 nm–1750 nm were carried out by spectrometric analyses (Spectrophotometer Shimadzu UV 3600 plus – Reflectometer x-rite Ci6x). 3. Results and discussion 3.1. From ripples to spikes It is well known that for a fixed fluence-per-pulse ˚ = J/ω2 , the cumulative laser energy dose (J/cm2 ) is the key physical parameter determining the surface morphology [24]. Considering for instance a rectangular surface S = a × b (Fig. 2), can be expressed according to the following formula: =N
E S
=N
J × n × m a×b
= N˚
ω2 d×ı
(1)
where E = J × n × m is the total energy relative to the (n × m) pulses irradiated on S, d = (a/n) the spatial distance between pulses, ı = (b/m) the offset between scanning lines and N the number of
G. Mincuzzi et al. / Applied Surface Science 386 (2016) 65–71
67
Fig. 3. SEM images of the textured surface after N = 500 successive scans with varying scanning speed: (a) v = 90 m/s (d = 45 m, ı = 45 m), ripples – (b) v = 40 m/s (d = 20 m, ı = 20 m), ripples – (c) v = 25 m/s (d = 12.5, ı = 12.5 m), spikes on micro-grooves. White arrows show the orientation of the laser polarization on the surface.
Fig. 4. SEM images of the textured surface after N = 500 successive scans and scanning speed v = 25 m/s (d = 12.5 m). The offset between two successive scan lines was reduced to ı = 3 m (a), 2 m (b) and 1 m (c).
successive scans. A schematic illustration of the processed surface with related processing parameters is shown in Fig. 2. As first step, the influence of the scanning speed v on the final surface morphology was investigated by varying the horizontal overlapping, that is varying the parameter d, whilst keeping the number N of successive scans fixed at 500. With regard to the polygon scanning system parameters, for a fixed laser repetition rate f, it is possible to vary the scanning speed v by varying the distance d between two successive pulses, according to the relation v = f × d. It follows that at f = 2 MHz and d = 45 m, d = 20 m and d = 12.5 m, the resulting scanning speed is v = 90 m/s, v = 40 m/s and v = 25 m/s, respectively. In order to maintain the same overlapping both in the horizontal and vertical direction, the offset between scanning lines ı was also set at 45 m, 20 m and 12.5 m when v = 90 m/s, v = 40 m/s and v = 25 m/s, respectively. It is important to highlight that reducing the scanning speed, the pulses overlapping and therefore the laser energy dose increases. Fig. 3 shows SEM images of the textured surface after N = 500 successive scans with varying scan speed. For v = 90 m/s and ı = 45 m, pulses are separated (Fig. 3a) and ripples perpendicular to the polarization direction are formed only in correspondence of the irradiated areas with a spatial periodicity ranging from 400 nm to 500 nm. For v = 40 m/s and ı = 20 m, pulses overlap (Fig. 3b) and ripples perpendicular to the polarization direction appear with a spatial periodicity of about 700 nm. Finally, for v = 25 m/s and ı = 12.5 m, the pulse overlap increases (Fig. 3c), micro-grooves parallel to the laser polarization orientation are formed and begin to evolve to conical structures (spikes). Generally, the evolution of the surface morphology from LIPSS to spikes may be explained as follows: after the ultra-fast interaction, the surface is found in a superheated liquid state where capillary fluid convection may take place as a consequence of the
Marangoni effect induced by the Gaussian shape of the laser pulse [19]. Exposing the surface to subsequent pulses, the so-induced surface topology may give rise to interference effects between the laser light and surface electromagnetic waves, such as surface plasma polaritons. This in turn will lead to a spatially modulated ablation process and consequently to formation of ripples with a direction perpendicular to the laser polarization orientation [20–23]. Increasing the number of irradiating pulses, the efficacy with which the rough surface absorbs the laser energy is deeply modified by the evolving surface morphology: the conditions for surface plasma polaritons to be excited are not anymore fulfilled and ripples disappear. Subsequently, micro-grooves and spikes are formed as a consequence of the onset of hydrothermal convection effects: hydrothermal waves rise perpendicularly to the previously formed ripples, that is with a direction parallel to the laser polarization, this way defining the direction of the micro-grooves. Increasing further the number of laser pulses, hydrodynamic effects prevail and conical structures start to appear regularly on the top of microgrooves, developing in more defined spikes as a consequence of local variation of the laser energy absorbance [19,29]. A more detailed theoretical description of the above mentioned processes can be found in Ref. [19–23]. In order to increase and to optimize the spikes morphology, d and ı need to be further reduced. With a minimal inter-pulse distance of d = 12.5 m for a scan line of the polygon scanner the re remaining parameter is the offset between two successive scan lines which was reduced to ı = 5, 3, 2, and 1 m. The obtained morphology is shown in Fig. 4. The periodic behaviour of the morphology is visible only for ı > 2 m (Fig. 4a). It is clear that every kind of observed morphology is characterized by a threshold value of laser energy dose over which the specific morphology starts to appear. As previously mentioned, this result is already reported in
68
G. Mincuzzi et al. / Applied Surface Science 386 (2016) 65–71
Fig. 5. Dependence of the values of the conical spikes base diameter on the offset between scanning lines ı.
all the other process parameters constant (v = 25 m/s, d = 12.5 m, ı = 2 m). Fig. 6 shows SEM images of the stainless steel surface after processing with N = 10 (Fig. 6a) and N = 100 (Fig. 6b). For N = 10, an extensive ripple texturing is clearly visible, characterized by an average spatial periodicity of about 500 nm. As expected, the ripples direction is perpendicular to the beam polarization orientation. For N = 100, micro-conical structures appear on the surface characterized by an average size of approximately 8 m. Fig. 7 presents SEM images of the stainless steel surface at scanning speed v = 25 m/s and N = 300 (Fig. 7a), N = 500, N = 600 (Fig. 7b) and N = 800 successive scans (Fig. 7c). For N ≥ 300, typical spikes shape structures are formed with diameter ≈ 18 m. Increasing N, increases reaching a value of about 35 m for N = 500 and N = 600. Moreover, in this case, the spike tips are not sharp but, as already observed, subdivide in smaller structures, starting from the top of the conical structure. Increasing the number of successive scans from N = 500, the spike diameter value stabilize between 30 m and 40 m. It is worth mentioning that, despite of the high scan speed (25 m/s), processes at higher value of N would imply a processing time which is out of interest for any industrial application. 3.2. Surface blackening
literature and it is usually explained as a consequence of the necessary conditions to be fulfilled for surface plasma polaritons to be excited as first, and hydrothermal effects to take place later on, as increases [19,22,23]. For ı = 2 m (Fig. 3b), is high enough to guarantee an uniform texturation of well-defined spikes. For ı = 1 m (Fig. 3c), spikes begin to subdivide in smaller structures, starting from the top of the conical structure. In Fig. 5 the dependence of the conical spikes diameter on ı is shown. In the range 5 m < ı < 12.5 m, the spikes diameter does not increases sensibly and its average value is about 10 m. For ı < 5 m, increases noticeably varying from ≈ 23 m when ı = 3 m to ≈ 47 m when ı = 1 m. It is worth observing that in the last case, beside an increment of the average value, a large error bar was also extracted, underlying not negligible variability. This is probably due to the change observed in the morphology at the top of the spikes in the case ı = 1 m. Table 1 summarizes the processing parameters in relation to the observed morphology. Table 1 highlights the intervals of energy dose for the formation of specific morphology: homogeneous micro-grooves and spikes appear on the surface for in the range 0.22–0.57 kJ/cm2 and 2.37–3.56 kJ/cm2 respectively. These values are on the same order of magnitude of the range reported for spike formation in Ref. [26]. In the previous section all the experiments were carried with a constant number of successive scans N = 500. The laser energy dose was varied by increasing the pulses overlapping both in the scanning direction (d) and in between the scanning lines (ı). Of course, for a fixed pulses overlapping, N determines the process takt-time. This fact highlights the necessity to find the optimal N value for which it is possible to obtain a specific surface texturing in the lowest takt-time. Therefore, the behaviour of the morphology was studied in relation the number of successive scans N, keeping
One of the applications of USP texturing is the control of the surface reflectivity R over a wide range of values. In the visible and near infra-red (NIR) range, textured surfaces with R < 5%, corresponding to an intense blackening of the surface color, have been reported [24]. The reflectivity variation is known to be induced by an efficient mechanism of light trapping within the surface structures [30]. In particular, together with the structures aspect ratio, the surface porosity and morphology at the nanoscale, has been shown to play a crucial role in inducing multiple light reflections which prevent the light to be back reflected [28,29]. This way it is possible to obtain highly absorptive surfaces without the need for special coatings or physical/chemical modifications of the material surface. Fig. 8 presents reflectivity spectra in the range of wavelengths 350 nm–1750 nm relative to stainless steel surfaces textured with number of successive scans N increasing from 10 to 800 (v = 25 m/s, d = 12.5 m, ı = 2 m). The reference reflectivity spectrum of untreated stainless steel is also shown. Measurements have been carried out with a spectrophotometer equipped with an integrating sphere. It can be clearly observed that starting from a reference value of R = 70%, the reflectivity remarkably decreases by increasing N. For N > 300, R is already below 13% and, in the best case N = 600, a value of R < 5% is obtained. For N > 600, no sensible decrease of reflectivity was observed. It is interesting to notice that in all cases the spectrum is quite flat all over the measurement interval. This indicates that the mechanism underlying the reflectivity decrease excludes the possibility of light absorption by dust. Due to Mie scattering, micro-sized dust grains will in fact induce a significant Reflectivity increase in the NIR region. Moreover, as presented in Fig. 9, it is an important result that, while
Table 1 Values of processing parameters (scanning speed , distance between pulses d, horizontal overlapping HO, offset between scanning lines ı, vertical overlapping VO) and laser energy dose in relation to the observed morphology (R: ripples; -G: micro-grooves; S: spikes). (m/s)
d (m)
HO (%)
90 40 25 25 25 25 25
45 20 12.5 12.5 12.5 12.5 12.5
0 56 72 72 72 72 72
ı (m) 45 20 12.5 5 3 2 1
VO (%)
(kJ/cm2 )
Morphology
0 56 72 89 93 96 98
0.04 0.22 0.57 1.42 2.37 3.56 7.12
R R S on -G S on -G S on -G S S
G. Mincuzzi et al. / Applied Surface Science 386 (2016) 65–71
69
Fig. 6. SEM images of stainless steel surface irradiated at scanning speed v = 25 m/s (d = 12.5 m and ı = 2) and N = 10 successive scans (a) and N = 100 successive scans (b).
Fig. 7. SEM images of stainless steel surface irradiated at scanning speed v = 25 m/s (d = 12.5 m and ı = 2) and N = 300 successive scans (a), N = 600 successive scans (b) and N = 800 successive scans (c).
Fig. 8. Reflectivity spectra measured by spectrophotometer in the range 350 nm–1750 nm relative to stainless steel surfaces textured with number of successive scans N increasing from 10 to 800. The reference reflectivity spectrum of untreated stainless steel is also shown in black full squares.
N increases, an increase of the spike diameter corresponds to a decrease of reflectivity: the light trapping mechanism becomes more efficient as the conical structure increases in size. This may be due to a corresponding increase of both the height of the conical
structures and the surface porosity obtained by partial re-melting of nanoparticles generated during the process [30–32] . In Table 2 the average reflectivity values relative to the same surfaces measured by a reflectometer in the visible range are reported. These results
70
G. Mincuzzi et al. / Applied Surface Science 386 (2016) 65–71
where tr is the time required for the sample re-positioning after one raster scanning between two successive scans which is assumed to be tr = 1s. It follows that tp = 8.5 h, which correspond to a processing time for area unit of tu = 30 min/cm2 . Multiple strategies could be addressed to significantly reduce this value:
Fig. 9. Dependence of the spike diameter (squares) and of the average reflectivity Rave (full circles) on the number of successive scans N, at scanning speed v = 25 m/s (d = 12.5 m) and ı = 2. Table 2 Values of the average reflectivity Rave measured by reflectometer in the range of wavelengths 400 nm–700 nm in relation to the number N of successive scans. Rave (%)
N reference 10 100 300 600
confirm the good quality of the values of reflectivity obtained by spectrophotometer analyses. The decrease in reflectivity was also verified by visual inspection of the textured surface (Fig. 10): the surface blackening increases very clearly as the number N of successive scans increases. To conclude, we evaluated the takt-time and in particular, the process total time tp required for blackening a surface S = 17 cm × 1 cm, being 17 cm the maximum working field enabled by the scanner. The process parameters considered here are: scan speed v = 25 m/s corresponding to 100 lines/s, scan lines off set ı = 2 m (corresponding to 5000 lines/cm) and number of successive scans N = 600. The process total time tp can be written as follow:
For instance, a 100 W class, fs laser with repetition rate of nearly 10 MHz could potentially reduce tu < 1 min/cm2 . 4. Conclusions
69.7 23 19 12 4.9
Fig. 10. Progressive surface blackening as the number of successive scans N increases from 10 to 600.
tp = N 5000/100 + tr
A.) Increasing the laser repetition rate to 8 MHz would allow to fully exploit the polygon scanner performances. This way, a scan speed of 100 m/s could be utilized whilst keeping the pulses overlapping unmodified. B.) Increasing the pulse energy values would permit to enlarge the beam size, keeping the unmodified. This, in turn, would permit to increase the distance d between pulses and the offset ı between lines whilst keeping the laser energy dose unmodified. C.) Using a polygon scanner system with larger scan field (from 17 cm to 30 cm), keeping the same value of scanning speed, could be a further interesting option to consider in order to process a larger surface in the same process time. D.) Finally a multi-head approach in which two or more polygon scanners work in parallel might also be considered.
(2)
The possibility to increase the process throughput of USP texturing processes over large area, reaching takt-times compatible with commercial purposes, raises the issue on how to manage the heat induced by the high average power required for processing in the MHz regime. The key concept to address this problem is to utilize low energy-per-pulse and high scanning speed. For the first time a high repetition rate (2 MHz) femtosecond laser has been utilized jointly with a fast (up to 100 m/s) polygon scanner system for stainless steel surface texturing in low fluence-per-pulse regime. The fluence-per-pulse was indeed kept constant at 0.11 J cm−2 , that is almost 1 order of magnitude lower than fluence values conventionally used for this kind of processes. By varying the scan speed from 90 m/s to 25 m/s, three types of morphologies were observed: ripples, micro-grooves and spikes. We demonstrated that it is possible to control the type of morphology by a systematic variation of the process parameters. The possibility to control the type of morphology allows for the control of the surface reflectivity, which was drastically reduced with respect to the reference value of un-textured surfaces. A lowest reflectivity value of R < 5% in the visible spectral range was obtained. The flatness of the reflectivity spectrum in a wide range of wavelength (from 400 nm to 2000 nm) indicates that the surface blackening is induced by a light trapping mechanism. These results open the way for an industrial exploitation of USP surface texturing. Although in our experimental conditions a takttime of nearly tu = 30 min/cm2 was extracted, use of laser sources with tens of MHz and higher pulse energy, jointly with multiple heads processing approach will be beneficial for the final throughput to reach target values suitable for commercial purposes. References [1] I. Ursu, I.N. Mih˘ailescu, A. Popa, A.M. Prokhorov, V.P. Ageev, A.A. Gorbunov, V.I. Konov, Studies of the change of a metallic surface microrelief as a result of multiple-pulse action of powerful UV laser pulses, J. Appl. Phys. 58 (1985) 3909. [2] M.S. Brown, C.B. Arnold, Fundamentals of laser-material interaction and ¨ in: Laser Precision application to multiscale surface modification, Microfabrication, Springer, Berlin Heidelberg, 2010, pp. 91–120.
G. Mincuzzi et al. / Applied Surface Science 386 (2016) 65–71 [3] J.E. Sipe, J.F. Young, J.S. Preston, H.M. van Driel, Phys. Rew. B 27 (1983) 1141. [4] M. Pfeiffer, A. Engel, H. Gruettner, K. Guenther, F. Marquardt, G. Reisse, S. Weissmantel, Appl. Phys. A 110 (2013) 655. [5] S. Sakabe, M. Hashida, S. Tokita, K. Okamura, Phys. Rev. B 79 (2009) (033109). [6] A.Y. Vorobyev, V.S. Makin, C. Guo, J. Appl. Phys. 101 (2007) (034903). [7] L. Gemini, M. Hashida, M. Shimizu, Y. Miyazaka, S. Inoue, S. Tokita, J. Limpouch, T. Mocek, S. Sakabe, J. Appl. Phys. 114 (2013) (194903). [8] A.Y. Vorobyev, C. Guo, Opt. Exp. 19 (2011) A1031. [9] T. Tomita, Y. Fukumori, K. Kinoshita, S. Matsuo, S. Hashimoto, Appl. Phys. Lett. 92 (2008) (013104). [10] Á. Rodríguez-Rodríguez, E. Rebollar, M. Soccio, T.A. Ezquerra, D.R. Rueda, J.V. Garcia-Ramos, M. Castillejo, M.-C. Garcia-Gutierrez, Macromolecules 48 (2015) 4024. [11] M. Castillejo, T.A. Ezquerra, M. Martín, M. Oujja, S. Pérez, E. Rebollar, AIP Conf. Proc. 1464 (2012) 372. [12] S. Yada, M. Terakawa, Opt. Exp. 23 (2015) 5694. [13] A. Rosenfeld, M. Rohloff, S. Hohm, J. Kruger, J. Bonse, Appl. Surf. Sci. 258 (2012) 9233. [14] S. Hohm, A. Rosenfeld, J. Kruger, J. Bonse, Appl. Phys. Lett. 102 (2013) 054102. [15] J. Bonse, J. Kruger, S. Hohm, A. Rosenfeld, J. Appl. Laser 24 (2012) (042006-1). [16] A.Y. Vorobyev, C. Guo, Las. Phot. Rev. 7 (2013) 385. [17] V. Zorba, L. Persano, D. Pisignano, A. Athanassiou, E. Stratakis, R. Cingolani, P. Tzanetakis, C. Fotakis, Nanotechnology 17 (2006) (3234). [18] A.Y. Vorobyev, C. Guo, J. Appl. Phys. 117 (2015) (033103). [19] G.D. Tsibidis, C. Fotakis, E. Stratakis, Phys. Rev. B 92 (2015) (041405).
71
[20] J.E. Sipe, J.F. Young, J.S. Preston, H.M. van Driel, Phys. Rev. B 27 (1983) 1141. [21] J. Bonse, A. Rosenfeld, J. Kruger, J. Appl. Phys. 106 (2009) (104910). [22] T.J.-Y. Derrien, T.E. Itina, R. Torres, T. Sarnet, M. Sentis, J. Appl. Phys. 114 (2013) (083104). [23] J. Bonse, S. Baudach, J. Krüger, W. Kautek, M. Lenzner, Appl. Phys. A 74 (2002) 19–25. [24] M. Faucon, A. Laffitte, J. Lopez, R. Kling, Proc. SPIE 8972 (2014) (89721M). [25] U. Loeschner, J. Schille, A. Streek, T. Knebel, L. Hartwig, R. Hillmann, C. Endisch, J. Laser. Appl. 27 (2015) S29303. [26] R. De Loor, L. Penning, R. Slagle, Laser Tech. J. 11 (2014) 32. [27] S. Nolte, S. Döring, A. Ancona, J. Limpert, A. Tünnermann, High repetition rate ultrashort pulse micromachining with fiber lasers in fiber laser applications, Opt. Soc. Am. (2011) (pFThC1). [28] B. Neuenschwander, B. Jaeggi, M. Zimmermann, G. Hennig, V. Markovic, B. Resan, K. Weingarten, R. de Loor, L. Penning, Power Scaling into the 100 W regime for surface texturing of metals with ultra-short laser pulses, in: The 7th International Congress on Laser Advanced Materials Processing, May 26–29, Kitakyushu, 2015. [29] D.-H. Kam, J. Kim, L. Song, J. Mazumder, J. Micromech. Microeng. 28 (2015) (045007). [30] L.K. Ang, Y.Y. Lau, R.M. Gilgenbach, H.L. Spindler, Appl. Phys. Lett. 70 (1997) 696. [31] H. Huang, L.-M. Yang, S. Bai, J. Liu, Appl. Opt. 54 (2015) 2. [32] A.Y. Vorobyev, C. Guo, Black metals through femtosecond laser pulses international symposium on high power laser ablation 2012, AIP Conf. Proc. 1464 (2012) 397–401, http://dx.doi.org/10.1063/1.4739894.