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Surface modifications induced by pulsed-laser texturing—Influence of laser impact on the surface properties
Applied Surface Science 288 (2014) 542–549
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface modifications induced by pulsed-laser texturing—Influence of laser impact on the surface properties S. Costil a,∗ , A. Lamraoui a , C. Langlade a , O. Heintz b , R. Oltra b a b
IRTES-LERMPS, Université de Technologie de Belfort - Montbéliard, site de Sévenans, 90010 Belfort Cedex, France ICB, Université de Bourgogne, 21078 Dijon Cedex, France
a r t i c l e
i n f o
Article history: Received 22 January 2013 Received in revised form 8 October 2013 Accepted 13 October 2013 Available online 23 October 2013 Keywords: Laser texturing Thermal spraying Surface modifications Aluminium SEM XPS Microhardness
a b s t r a c t Laser cleaning technology provides a safe, environmentally friendly and very cost effective way to improve cleaning and surface preparation of metallic materials. Compared with efficient cleaning processes, it can avoid the disadvantages of ductile materials prepared by conventional technologies (cracks induced by sand-blasting for example) and treat only some selected areas (due to the optical fibers). By this way, laser technology could have several advantages and expand the range of thermal spraying. Moreover, new generations of lasers (fiber laser, disc laser) allow the development of new methods. Besides a significant bulk reduction, no maintenance, low operating cost, laser fibers can introduce alternative treatments. Combining a short-pulse laser with a scanner allows new applications in terms of surface preparation. By multiplying impacts using scanning laser, it is possible to shape the substrate surface to improve the coating adhesion as well as the mechanical behaviour. In addition, during the interactions of the laser beam with metallic surfaces, several modifications can be induced and particularly thermal effects. Indeed, under ambient conditions, a limited oxidation of the clean surface can occur. This phenomenon has been investigated in detail for silicon but few works have been reported concerning metallic materials. This paper aims at studying the surface modifications induced on aluminium alloy substrates after laser texturing. After morphological observations (SEM), a deeper surface analysis will be performed using XPS (X-ray photoelectron spectroscopy) measures and microhardness testing. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, short-pulse lasers have been shown to be suitable tools for cleaning applications due to their ability to deliver high power per unit area to the localised part surface [1]. Generally, oxides, carbon and oils have to be removed from a metallic surface before its final use. In an ideal case study, the laser energy necessary to remove contaminants should be below a threshold value to prevent substrate modifications and damage. Conversion of absorbed energy via collisional processes into heat is one of the most important effects that occur during the laser interaction. The process can be athermal for the substrate when the layer to be removed is an oxide film (rapid expansion and explosion of gases close to the oxide-metal interface). Another possibility is the vaporisation of micrometric layers through ablation phenomenon corresponding to the fast transition from the overheated liquid to a mixture of vapour and drops. Part of the incident heat can then be absorbed by the material causing microstructure modifications.
∗ Corresponding author. Tel.: +33 03 84 58 32 35. E-mail address: [email protected] (S. Costil). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.10.069
For reactive metallic substrates treated under ambient atmosphere, a limited oxidation is sometimes considered to explain the modifications of surface properties (adhesion, corrosion resistance). In other cases, localised melting (texturing) promoting a small surface roughness can be developed to improve the interface behaviour. These phenomena have been investigated in detail for silicon [2] and tend to develop for metals [3,4] or ceramics especially for the fluence range considered in this work (thermoelastic range of interaction for metallic surfaces). But without new laser technologies (fiber laser, disc laser) which open up new perspectives (significant bulk reduction, no maintenance, low costs), it would have reached its limits. By multiplying laser impacts using a scanner, it is possible to shape the substrate surface to improve applications such as adhesion of coatings as well as surface lubrication against wear, etc. [5]. There is a wide range of texturing techniques commercially available based on different processes such as chemical etching, electro-erosion, sand blasting and laser texturing (Table 1) [6]. Among them, laser technology presents competitive advantages as easy automation, localised treated area, three dimensional treatments and great flexibility. Using laser, topology modifications may occur for all types of materials like glass, ceramic, polymer and
S. Costil et al. / Applied Surface Science 288 (2014) 542–549 Table 1 Texturing processes [6].
Treatment direction 1st
Mechanical texturing Litography Grinding Sand-blasting Printing
543
Thermal texturing Surface coating
Chemical Electro erosion Electrochemical Electron beam Ion beam Laser
PVD CVD Electro deposition
Surface
Textured holes
2nd
3rd Table 2 Characteristics of Al2017. Chemical composition Alloy elements (wt %) Cu/Mn/Fe/Mg/Si/Al 4/0.7/0.7/0.6/0.5/balance
y = 1/R Properties
4th
Value −3
Density (g cm ) Thermal conductivity (W m−1 K−1 ) Mass heat capacity (J kg−1 K−1 ) Melting temperature (◦ C) Absorption @ = 1.06 m, 25 ◦ C (%)
2.79 134 920 587 ∼5
metals. Selecting a specific laser tool adapted to the material to be treated (in terms of wavelength, pulse duration, spot size and pulse frequency), scanner characteristics such as scanning velocity, pulse numbers and structure can influence strongly the surface modifications. Of course, the material behaviours and more precisely their optical and thermal properties as well as surface state induce different responses against laser impact. On top of a surface alteration, material modification can be involved according to the heat flux absorbed during the treatment. In spite of short pulse duration (∼100 ns), the high energy implemented can be effectively absorbed by the material causing local alterations. When considering applications implementing laser texturation, it is necessary to know whether these modifications are beneficial or not. The objective of this work was to investigate the modifications of an aluminium alloy after laser texturation treatment. After surface evaluations including SEM observations and XPS analysis, the material was deeply analysed to consider all the transformations from a mechanical (microhardness) as well as microstructure point of view.
2. Experimental procedure 2.1. Materials The material used for texturing experiments consisted of 10 mm thick cylindrical discs of 25 mm diameter blank machining aluminium alloy Al2017. The chemical composition and some of the properties of the Al2017 are detailed in Table 2.
2.2. Laser treatments The experiments were carried out using a pulsed fiber Nd-Yag laser (Ylia M20, Quantel, France) operating at 1064 nm and generating 100 ns pulses at various frequencies (<250 kHz) with 20 W of power output. The laser beam is circular with 8 mm in diameter and a Gaussian energy distribution. The texturing technique consisted of series of equidistant lines covered with a number of holes as illustrated in Fig. 1. For this, the scanner stop the laser beam at positions and a certain number of pulses is applied to build holes. The resolution (R) sets the number of lines to be textured per millimetre (y = 1/R). In order to obtain a symmetric texture, the distance between holes (x) is equivalent to y. Then, various parameters can be selected like the number of shots per drilled hole, the laser power and the frequency to achieve the surface texturation. In all conditions, experiments were carried out at the focal point of the laser (60 m in diameter).
x=y
Hole diameter Hole depth
Cross section
Fig. 1. Scheme of the texturing treatment.
Fig. 2. Profiles of the indentation lines carried out on the sample cross-sections.
2.3. Surface analyses Several levels of characterizations were implemented during this work. The morphology and the microstructure of aluminium samples were investigated by Scanning Electron Microscopy (SEM) using a JEOL JSM-5800 LV. To investigate the surface composition and more precisely the oxide layer potentially developed through the laser treatment, Xray Photoelectron Spectroscopy (XPS) analyses were carried out. XPS measurements were performed using a PHI Versaprobe 5000 system with 200 m diameter beam. A Al K␣ X-ray source with a power of 50 W and a spectrometer, MAC 2 Riber, with an energy resolution (width of Ag 3d5/2 ) of 2.3 eV for spectra and of 0.8 eV for window were used. All the data were analysed using the software Multipak. In order to estimate the modifications induced inside the aluminium alloy after laser impact, microhardness and microstructure observations were carried out. The Vickers microhardness was measured with a standard device (Miniload-2, Leitz) at 300 g load. Eight measures were carried out to evaluate the mean value with the standard deviation. To estimate the variations after laser treatments, two strategies were developed around the holes. A first indentation line was performed just under the surface between two holes and a second was implemented vertically from the end of the hole to the material bulk (Fig. 2). For measuring the microhardness, samples were cut and polished on the cross section. SEM observations were carried out on cross-sections after mirror polishing and chemical etching. For this, Kellers reagent (2 mL HF, 3 mL HCL, 5 ml HNO3 , 190 mL water) was elaborated and applied on material during 15 s at 9 V of voltage.
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S. Costil et al. / Applied Surface Science 288 (2014) 542–549
Fig. 3. Examples of laser impacts on the surface morphology.
3. Results and discussion Different laser parameters can generate a morphological modification of the surface substrate. It was shown that when implementing an experimental design approach, influent parameters were noticed on the surface [7,8]. It concerned mainly the laser power or the laser density energy (impacting the hole depth) and the laser resolution R (influencing the surface design more and less dense). The other parameters such as laser frequency and scanning velocity demonstrated a lower impact on the surface. Then, several surface morphologies can be developed with holes of different depth and different densities. Fig. 3 illustrates some of textured surfaces elaborated with different laser parameters.
Except the main benefits measured on the coating adherence [7–10], the surface modifications observed after laser texturation present various impacts on the material surface. Concerning holes, a transformation of matter can be noticed around the laser impacts. Ejected matter associated to recast material can be observed all around the holes. An increase of the material temperature can then be suspected during the laser matter interaction until the melting and ejection on the surface due to the pressure drop. Fig. 4 illustrates the surface modifications according to the laser parameters. Several effects can also be observed when using laser drilling process [8,9]. Concerning laser parameters, strong influences have to be noted such as laser power (and then energy density of the laser) and repetition rate. The energy, introduced inside the
Fig. 4. Illustration of textured holes presenting excess of matter around the hole.
S. Costil et al. / Applied Surface Science 288 (2014) 542–549
545
Fig. 5. Effect of solidification according to laser resolution.
Fig. 6. EDS images of laser textured hole.
material on a defined area (imposed by the laser power and the spot diameter on the substrate surface) induces a treatment performed at different temperatures according to the selected parameters. First surface modifications can then be observed on samples if the laser parameters are under the ablation threshold of the treated material. Such remark can be easily observed with the frequency effect where no structuring can be noticed at 80 kHz. Second, the hole density (according to the laser resolution) plays an important role on the surface morphology (Fig. 5). Indeed, the accumulation of laser impacts at a repetition rate introduces several surface
morphologies. If the first configuration (Fig. 5-a) illustrates clearly separated holes with excess of matter around the hole, an accumulation of recast material can be detected when applying shorter distances between the holes. The material thermal response can then be easily detected. But when considering heating effect, different modifications of matter may occur on the material. Considering metallic materials treated under atmospheric environment, an oxidation phenomenon can be expected on the surface. To confirm this, EDS images can be observed Fig. 6.
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S. Costil et al. / Applied Surface Science 288 (2014) 542–549
Fig. 7. Evolution of the surface composition of (a) reference sample, (b) textured samples treated with laser power of 10 W (35 mJ cm−2 , Ech. 1) and 20 W (71 mJ cm−2 , Ech. 6) (other parameters are assumed constant).
Indeed, the element oxygen appears clearly all around the hole as well as between holes when the laser resolution is high. Obviously, this process varies with the laser treatment and tends to increase with the laser power and the pulse number. Nevertheless, such measures are difficult to quantify. In order to estimate more precisely the surface composition, X-ray Photoelectron Spectroscopy (XPS) analyses were operated (Fig. 7). Accordingly, the oxygen content measured on the textured surfaces appears higher than the reference surface (at concentrations around 20–45%). Moreover, considering the laser power (10 and 20 W), an increase of the oxide layer thickness can be noticed. According to the sputtering time of oxygen and silicium elements (10 nm/min), the oxide layer varies from 35 nm (when treated at
10 W or 0.5 mJ) to 160 nm (when treated at 20 W or 1 mJ) at the substrate surface. Considering the laser density energy variations (from 35 to 71 mJ/cm2 ), an increase of the surface temperature can then be suspected. This is the result from the aluminium bond analysis varying between the Al–Al bond and the Al–O bond (Fig. 8). The 2p electronical level of the Al element can be separated at 75.3 eV for the oxide component and 73.2 eV for the metallic component. Then, the intensity variation of Al bonds for both samples (reference and textured samples) can be noted to explain the existence of oxide confirmed by the texturation process. If the previous analysis demonstrated physico-chemical modifications on the substrate surface due to an increase of the material temperature, other modifications can be considered especially
S. Costil et al. / Applied Surface Science 288 (2014) 542–549
547
Fig. 8. Chemical bond analysis—(a) zoom view of the aluminium bond and distinction of the Al–Al bond and the Al–O bond, (b) intensity variation of Al bonds for both samples.
Fig. 9. Microhardness profiles (a) on the cross section (y line), (b) between two holes (x line).
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Fig. 10. Microscopic views of blank machining or textured Al2017 substrates after chemical etching (Keller solution).
from a microstructure point of view, an evolution of the atomic organisation and then a structural hardening can be assumed. To estimate this phenomenon, Vickers micro hardness measurements were implemented on the cross-sections. Two profiles were then drawn (Fig. 2), the one under the substrate surface between two holes (x line) and the other deeply in the bulk material from the end of the holes (y line). The microhardness profiles are illustrated in Fig. 9. Compared to the reference blank machining substrate (160 Hv), a decrease of the material microhardness can be measured all around the textured holes (estimated around 40 and 60 Hv). Whatever the profiles, a regular increase of the aluminium hardness appears during 150 m until the reference one. Considering the space between two holes, the same evolution can be observed close to them to obtain the maximum hardness in the middle area. By this way, these observations confirm the structural modifications implemented during the laser treatment which can be measured whatever the laser parameters carried out. Suspecting a heat treatment carried out with the laser interaction, an evolution of the substrate microstructure and then the atomic organisation can be imagined. This aspect has been noted by microscopic observations after chemical etching of the sample cross sections (Fig. 10). If the initial structure of the blank machining substrate illustrates regular grains with 5 m mean diameter, a modification after laser texturing can be noted on the sample microstructure particularly close to the holes. Enhanced by the laser parameters (inducing deeper holes), a reduction of the grain size (from 3 to 1.5 m mean diameter) seems occurred around the laser interaction area inducing a thinner microstructure. This microstructure modification tends to expand a larger area when the energetic treatment becomes more aggressive. Then, considering all these analyses, it appears a thermal effect induced by the laser texturing treatment illustrated by surface modifications with holes well ordered. Moreover, due to this matter-laser interaction, oxidation phenomena occurred according to the selected laser parameters as well as chemical reactions with the atmosphere [10,11]. Modifications on material surfaces
can then be observed from structural as well as chemical point of view. The intensity of such variations is directly linked to the laser parameters and more precisely to the laser energy delivered to the surface. Similar results were observed after laser ablation with an oxygen (in atmospheric conditions) or nitrogen (under N2 atmosphere) percentage increasing on the metallic substrate surface with the laser characteristics (energy density, pulse number, wavelength) [12–14]. The improvement of the oxide content was associated to the surface temperature up to the melting state. Hence, a micro structural modification of the material inducing thinner grains after laser texturing is reported. A high temperature increase follow by a high kinetic solidification can induce material modifications compared to the initial sample. A structural re-crystallisation can then be noted at the interface close to the holes explaining the microhardness variation. It has been demonstrated that laser welding involves similar phenomena [15–20].
4. Conclusion In this paper, laser texturing has been applied to aluminium alloys in order to explore its applicability to modify the surface behaviours. Under appropriated experimental texturing conditions, the surface structure could be modified inducing holes of different depth and different densities. Nevertheless, modifications of materials can be involved according to the heat flux absorbed during the laser treatment. The high energy implemented can be effectively absorbed by the material causing local alterations. Ejected matter associated to recast material can be observed all around the holes translating localised melting phenomena around the holes. Treatments operated in ambient conditions, a limited oxidation of the cleaned surface can occur. Furthermore, re-crystallisation of the aluminium alloy appears at the interface close to the hole inducing a thinner structure and then an evolution of the microhardness behaviour. In conclusion, laser texturing has demonstrated great potential for surface modification inducing geometric structural variations according to different applications.
S. Costil et al. / Applied Surface Science 288 (2014) 542–549
Acknowledgements The research work is the product of a successful collaborative effort with the Ecosurf project. The authors gratefully acknowledge “Franche comté region” for its financial support. References [1] J.P. Boquillon, P. Bresson, H. Berger, Procedé de nettoyage de surface par laser impulsionnel (Patents: France, no. 8 900 496, USA, no. 5 151 134; European, no. 904 001 229). [2] C. Cohen, J. Siejka, D. Pribat, M. Berti, A.V. Drigo, G.G. Bentini, E. Jannitti, J. Phys. 44 (1983) 179. [3] T. Dimogerontakis, R. Oltra, O. Heintz, Appl. Phys. A 00 (2004) 1, http://dx.doi.org/10.1007/s00339-004-31-7. [4] D. Neves, A.E. Diniz, M.S. Fernandes de Lima, J. Mater. Process. Technol. 179 (2006) 139–145. [5] J.M. Sanchez-Amaya, Z. Boukka, L. Gonzalez-Rovira, J. Navas, J. Martin-Calleja, F.J. Botana, J. Laser Appl. 24 (1) (2012). [6] H. Haefke, Y. Gerbig, G. Dumitru, V. Romano, Proceedings of the International Tribology Conference, Nagasaki, 2000, pp. 217–221. [7] A. Lamraoui, S. Costil, C. Langlade, C. Coddet, Surf. Coat. Technol. 205 ((Dec. 25) Suppl. 1) (2010) 164–167.
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[8] A. Lamraoui, S. Costil, C. Langlade, Rencontres Internationales sur la Projection Thermique 5 RIPT, 7–9 Dec, 2011. [9] A. Lamraoui, S. Costil, C. Langlade, C. Coddet, Thermal spray: global solutions for future application, in: International Thermal Spray Conference (ITSC) DVS, TSS, ASM, IIW, 3–5 May, Singapore, 2010. [10] A. Lamraoui, S. Costil, C. Langlade, accepted by JTST, Nov, 2012. [11] J. Byskov-Nielsen, P. Balling, Appl. Surf. Sci. 255 (2009) 5591–5594. [12] J. Lawrence, L. Li, J.T. Spencer, Surf. Coat. Technol. 115 (1999) 273–281. [13] M. Huber, R.A. Deutschman, R. Neuman, K. Brunner, G. Abstreiter, App. Surf. Sci. 168 (2000) 75. [14] V. Barnier, O. Heintz, D.E. Roberts, R. Oltra, S. Costil, Surface, Interface Anal. 38 (4) (2005) 406–409. [15] H. Li, S. Costil, V. Barnier, R. Oltra, O. Heintz, C. Coddet, J. Surf. Coat. Technol. 201 (3–4) (2006) 1383–1392. [16] A.L. Thomann, A. Basillais, M. Wegscheider, C. Boulmer-Leborgne, A. Pereira, P. Delaporte, M. Sentis, T. Sauvage, Appl. Surf. Sci. 230 (2004) 350–363. [17] A. Pereira, P. Delaporte, M. Sentis, A. Cros, W. Marine, A. Basillais, A.L. Thomann, C. Leborgne, N. Semmar, A. Andreazza, T. Sauvage, Thin Solid Films 453–454 (2004) 16–21. [18] A. Haboudou, P. Peyre, A.B. Vannes, J. Laser Appl. 16 (1) (2003) 20–24. [19] A. Haboudou, P. Peyre, A.B. Vannes, et al., Mater. Sci. Eng. A 363 (1–2) (2003) 40–52. [20] C. Mayer, Optimisation du soudage bord à bord par faisceau laser CO2 de tôles d’alliages d’Al-Mg: étude de la microstructure et du comportement mécanique des soudures (PhD thesis), INSA, Lyon, France, 1996.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface modifications induced by pulsed-laser texturing—Influence of laser impact on the surface properties S. Costil a,∗ , A. Lamraoui a , C. Langlade a , O. Heintz b , R. Oltra b a b
IRTES-LERMPS, Université de Technologie de Belfort - Montbéliard, site de Sévenans, 90010 Belfort Cedex, France ICB, Université de Bourgogne, 21078 Dijon Cedex, France
a r t i c l e
i n f o
Article history: Received 22 January 2013 Received in revised form 8 October 2013 Accepted 13 October 2013 Available online 23 October 2013 Keywords: Laser texturing Thermal spraying Surface modifications Aluminium SEM XPS Microhardness
a b s t r a c t Laser cleaning technology provides a safe, environmentally friendly and very cost effective way to improve cleaning and surface preparation of metallic materials. Compared with efficient cleaning processes, it can avoid the disadvantages of ductile materials prepared by conventional technologies (cracks induced by sand-blasting for example) and treat only some selected areas (due to the optical fibers). By this way, laser technology could have several advantages and expand the range of thermal spraying. Moreover, new generations of lasers (fiber laser, disc laser) allow the development of new methods. Besides a significant bulk reduction, no maintenance, low operating cost, laser fibers can introduce alternative treatments. Combining a short-pulse laser with a scanner allows new applications in terms of surface preparation. By multiplying impacts using scanning laser, it is possible to shape the substrate surface to improve the coating adhesion as well as the mechanical behaviour. In addition, during the interactions of the laser beam with metallic surfaces, several modifications can be induced and particularly thermal effects. Indeed, under ambient conditions, a limited oxidation of the clean surface can occur. This phenomenon has been investigated in detail for silicon but few works have been reported concerning metallic materials. This paper aims at studying the surface modifications induced on aluminium alloy substrates after laser texturing. After morphological observations (SEM), a deeper surface analysis will be performed using XPS (X-ray photoelectron spectroscopy) measures and microhardness testing. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, short-pulse lasers have been shown to be suitable tools for cleaning applications due to their ability to deliver high power per unit area to the localised part surface [1]. Generally, oxides, carbon and oils have to be removed from a metallic surface before its final use. In an ideal case study, the laser energy necessary to remove contaminants should be below a threshold value to prevent substrate modifications and damage. Conversion of absorbed energy via collisional processes into heat is one of the most important effects that occur during the laser interaction. The process can be athermal for the substrate when the layer to be removed is an oxide film (rapid expansion and explosion of gases close to the oxide-metal interface). Another possibility is the vaporisation of micrometric layers through ablation phenomenon corresponding to the fast transition from the overheated liquid to a mixture of vapour and drops. Part of the incident heat can then be absorbed by the material causing microstructure modifications.
∗ Corresponding author. Tel.: +33 03 84 58 32 35. E-mail address: [email protected] (S. Costil). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.10.069
For reactive metallic substrates treated under ambient atmosphere, a limited oxidation is sometimes considered to explain the modifications of surface properties (adhesion, corrosion resistance). In other cases, localised melting (texturing) promoting a small surface roughness can be developed to improve the interface behaviour. These phenomena have been investigated in detail for silicon [2] and tend to develop for metals [3,4] or ceramics especially for the fluence range considered in this work (thermoelastic range of interaction for metallic surfaces). But without new laser technologies (fiber laser, disc laser) which open up new perspectives (significant bulk reduction, no maintenance, low costs), it would have reached its limits. By multiplying laser impacts using a scanner, it is possible to shape the substrate surface to improve applications such as adhesion of coatings as well as surface lubrication against wear, etc. [5]. There is a wide range of texturing techniques commercially available based on different processes such as chemical etching, electro-erosion, sand blasting and laser texturing (Table 1) [6]. Among them, laser technology presents competitive advantages as easy automation, localised treated area, three dimensional treatments and great flexibility. Using laser, topology modifications may occur for all types of materials like glass, ceramic, polymer and
S. Costil et al. / Applied Surface Science 288 (2014) 542–549 Table 1 Texturing processes [6].
Treatment direction 1st
Mechanical texturing Litography Grinding Sand-blasting Printing
543
Thermal texturing Surface coating
Chemical Electro erosion Electrochemical Electron beam Ion beam Laser
PVD CVD Electro deposition
Surface
Textured holes
2nd
3rd Table 2 Characteristics of Al2017. Chemical composition Alloy elements (wt %) Cu/Mn/Fe/Mg/Si/Al 4/0.7/0.7/0.6/0.5/balance
y = 1/R Properties
4th
Value −3
Density (g cm ) Thermal conductivity (W m−1 K−1 ) Mass heat capacity (J kg−1 K−1 ) Melting temperature (◦ C) Absorption @ = 1.06 m, 25 ◦ C (%)
2.79 134 920 587 ∼5
metals. Selecting a specific laser tool adapted to the material to be treated (in terms of wavelength, pulse duration, spot size and pulse frequency), scanner characteristics such as scanning velocity, pulse numbers and structure can influence strongly the surface modifications. Of course, the material behaviours and more precisely their optical and thermal properties as well as surface state induce different responses against laser impact. On top of a surface alteration, material modification can be involved according to the heat flux absorbed during the treatment. In spite of short pulse duration (∼100 ns), the high energy implemented can be effectively absorbed by the material causing local alterations. When considering applications implementing laser texturation, it is necessary to know whether these modifications are beneficial or not. The objective of this work was to investigate the modifications of an aluminium alloy after laser texturation treatment. After surface evaluations including SEM observations and XPS analysis, the material was deeply analysed to consider all the transformations from a mechanical (microhardness) as well as microstructure point of view.
2. Experimental procedure 2.1. Materials The material used for texturing experiments consisted of 10 mm thick cylindrical discs of 25 mm diameter blank machining aluminium alloy Al2017. The chemical composition and some of the properties of the Al2017 are detailed in Table 2.
2.2. Laser treatments The experiments were carried out using a pulsed fiber Nd-Yag laser (Ylia M20, Quantel, France) operating at 1064 nm and generating 100 ns pulses at various frequencies (<250 kHz) with 20 W of power output. The laser beam is circular with 8 mm in diameter and a Gaussian energy distribution. The texturing technique consisted of series of equidistant lines covered with a number of holes as illustrated in Fig. 1. For this, the scanner stop the laser beam at positions and a certain number of pulses is applied to build holes. The resolution (R) sets the number of lines to be textured per millimetre (y = 1/R). In order to obtain a symmetric texture, the distance between holes (x) is equivalent to y. Then, various parameters can be selected like the number of shots per drilled hole, the laser power and the frequency to achieve the surface texturation. In all conditions, experiments were carried out at the focal point of the laser (60 m in diameter).
x=y
Hole diameter Hole depth
Cross section
Fig. 1. Scheme of the texturing treatment.
Fig. 2. Profiles of the indentation lines carried out on the sample cross-sections.
2.3. Surface analyses Several levels of characterizations were implemented during this work. The morphology and the microstructure of aluminium samples were investigated by Scanning Electron Microscopy (SEM) using a JEOL JSM-5800 LV. To investigate the surface composition and more precisely the oxide layer potentially developed through the laser treatment, Xray Photoelectron Spectroscopy (XPS) analyses were carried out. XPS measurements were performed using a PHI Versaprobe 5000 system with 200 m diameter beam. A Al K␣ X-ray source with a power of 50 W and a spectrometer, MAC 2 Riber, with an energy resolution (width of Ag 3d5/2 ) of 2.3 eV for spectra and of 0.8 eV for window were used. All the data were analysed using the software Multipak. In order to estimate the modifications induced inside the aluminium alloy after laser impact, microhardness and microstructure observations were carried out. The Vickers microhardness was measured with a standard device (Miniload-2, Leitz) at 300 g load. Eight measures were carried out to evaluate the mean value with the standard deviation. To estimate the variations after laser treatments, two strategies were developed around the holes. A first indentation line was performed just under the surface between two holes and a second was implemented vertically from the end of the hole to the material bulk (Fig. 2). For measuring the microhardness, samples were cut and polished on the cross section. SEM observations were carried out on cross-sections after mirror polishing and chemical etching. For this, Kellers reagent (2 mL HF, 3 mL HCL, 5 ml HNO3 , 190 mL water) was elaborated and applied on material during 15 s at 9 V of voltage.
544
S. Costil et al. / Applied Surface Science 288 (2014) 542–549
Fig. 3. Examples of laser impacts on the surface morphology.
3. Results and discussion Different laser parameters can generate a morphological modification of the surface substrate. It was shown that when implementing an experimental design approach, influent parameters were noticed on the surface [7,8]. It concerned mainly the laser power or the laser density energy (impacting the hole depth) and the laser resolution R (influencing the surface design more and less dense). The other parameters such as laser frequency and scanning velocity demonstrated a lower impact on the surface. Then, several surface morphologies can be developed with holes of different depth and different densities. Fig. 3 illustrates some of textured surfaces elaborated with different laser parameters.
Except the main benefits measured on the coating adherence [7–10], the surface modifications observed after laser texturation present various impacts on the material surface. Concerning holes, a transformation of matter can be noticed around the laser impacts. Ejected matter associated to recast material can be observed all around the holes. An increase of the material temperature can then be suspected during the laser matter interaction until the melting and ejection on the surface due to the pressure drop. Fig. 4 illustrates the surface modifications according to the laser parameters. Several effects can also be observed when using laser drilling process [8,9]. Concerning laser parameters, strong influences have to be noted such as laser power (and then energy density of the laser) and repetition rate. The energy, introduced inside the
Fig. 4. Illustration of textured holes presenting excess of matter around the hole.
S. Costil et al. / Applied Surface Science 288 (2014) 542–549
545
Fig. 5. Effect of solidification according to laser resolution.
Fig. 6. EDS images of laser textured hole.
material on a defined area (imposed by the laser power and the spot diameter on the substrate surface) induces a treatment performed at different temperatures according to the selected parameters. First surface modifications can then be observed on samples if the laser parameters are under the ablation threshold of the treated material. Such remark can be easily observed with the frequency effect where no structuring can be noticed at 80 kHz. Second, the hole density (according to the laser resolution) plays an important role on the surface morphology (Fig. 5). Indeed, the accumulation of laser impacts at a repetition rate introduces several surface
morphologies. If the first configuration (Fig. 5-a) illustrates clearly separated holes with excess of matter around the hole, an accumulation of recast material can be detected when applying shorter distances between the holes. The material thermal response can then be easily detected. But when considering heating effect, different modifications of matter may occur on the material. Considering metallic materials treated under atmospheric environment, an oxidation phenomenon can be expected on the surface. To confirm this, EDS images can be observed Fig. 6.
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Fig. 7. Evolution of the surface composition of (a) reference sample, (b) textured samples treated with laser power of 10 W (35 mJ cm−2 , Ech. 1) and 20 W (71 mJ cm−2 , Ech. 6) (other parameters are assumed constant).
Indeed, the element oxygen appears clearly all around the hole as well as between holes when the laser resolution is high. Obviously, this process varies with the laser treatment and tends to increase with the laser power and the pulse number. Nevertheless, such measures are difficult to quantify. In order to estimate more precisely the surface composition, X-ray Photoelectron Spectroscopy (XPS) analyses were operated (Fig. 7). Accordingly, the oxygen content measured on the textured surfaces appears higher than the reference surface (at concentrations around 20–45%). Moreover, considering the laser power (10 and 20 W), an increase of the oxide layer thickness can be noticed. According to the sputtering time of oxygen and silicium elements (10 nm/min), the oxide layer varies from 35 nm (when treated at
10 W or 0.5 mJ) to 160 nm (when treated at 20 W or 1 mJ) at the substrate surface. Considering the laser density energy variations (from 35 to 71 mJ/cm2 ), an increase of the surface temperature can then be suspected. This is the result from the aluminium bond analysis varying between the Al–Al bond and the Al–O bond (Fig. 8). The 2p electronical level of the Al element can be separated at 75.3 eV for the oxide component and 73.2 eV for the metallic component. Then, the intensity variation of Al bonds for both samples (reference and textured samples) can be noted to explain the existence of oxide confirmed by the texturation process. If the previous analysis demonstrated physico-chemical modifications on the substrate surface due to an increase of the material temperature, other modifications can be considered especially
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Fig. 8. Chemical bond analysis—(a) zoom view of the aluminium bond and distinction of the Al–Al bond and the Al–O bond, (b) intensity variation of Al bonds for both samples.
Fig. 9. Microhardness profiles (a) on the cross section (y line), (b) between two holes (x line).
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Fig. 10. Microscopic views of blank machining or textured Al2017 substrates after chemical etching (Keller solution).
from a microstructure point of view, an evolution of the atomic organisation and then a structural hardening can be assumed. To estimate this phenomenon, Vickers micro hardness measurements were implemented on the cross-sections. Two profiles were then drawn (Fig. 2), the one under the substrate surface between two holes (x line) and the other deeply in the bulk material from the end of the holes (y line). The microhardness profiles are illustrated in Fig. 9. Compared to the reference blank machining substrate (160 Hv), a decrease of the material microhardness can be measured all around the textured holes (estimated around 40 and 60 Hv). Whatever the profiles, a regular increase of the aluminium hardness appears during 150 m until the reference one. Considering the space between two holes, the same evolution can be observed close to them to obtain the maximum hardness in the middle area. By this way, these observations confirm the structural modifications implemented during the laser treatment which can be measured whatever the laser parameters carried out. Suspecting a heat treatment carried out with the laser interaction, an evolution of the substrate microstructure and then the atomic organisation can be imagined. This aspect has been noted by microscopic observations after chemical etching of the sample cross sections (Fig. 10). If the initial structure of the blank machining substrate illustrates regular grains with 5 m mean diameter, a modification after laser texturing can be noted on the sample microstructure particularly close to the holes. Enhanced by the laser parameters (inducing deeper holes), a reduction of the grain size (from 3 to 1.5 m mean diameter) seems occurred around the laser interaction area inducing a thinner microstructure. This microstructure modification tends to expand a larger area when the energetic treatment becomes more aggressive. Then, considering all these analyses, it appears a thermal effect induced by the laser texturing treatment illustrated by surface modifications with holes well ordered. Moreover, due to this matter-laser interaction, oxidation phenomena occurred according to the selected laser parameters as well as chemical reactions with the atmosphere [10,11]. Modifications on material surfaces
can then be observed from structural as well as chemical point of view. The intensity of such variations is directly linked to the laser parameters and more precisely to the laser energy delivered to the surface. Similar results were observed after laser ablation with an oxygen (in atmospheric conditions) or nitrogen (under N2 atmosphere) percentage increasing on the metallic substrate surface with the laser characteristics (energy density, pulse number, wavelength) [12–14]. The improvement of the oxide content was associated to the surface temperature up to the melting state. Hence, a micro structural modification of the material inducing thinner grains after laser texturing is reported. A high temperature increase follow by a high kinetic solidification can induce material modifications compared to the initial sample. A structural re-crystallisation can then be noted at the interface close to the holes explaining the microhardness variation. It has been demonstrated that laser welding involves similar phenomena [15–20].
4. Conclusion In this paper, laser texturing has been applied to aluminium alloys in order to explore its applicability to modify the surface behaviours. Under appropriated experimental texturing conditions, the surface structure could be modified inducing holes of different depth and different densities. Nevertheless, modifications of materials can be involved according to the heat flux absorbed during the laser treatment. The high energy implemented can be effectively absorbed by the material causing local alterations. Ejected matter associated to recast material can be observed all around the holes translating localised melting phenomena around the holes. Treatments operated in ambient conditions, a limited oxidation of the cleaned surface can occur. Furthermore, re-crystallisation of the aluminium alloy appears at the interface close to the hole inducing a thinner structure and then an evolution of the microhardness behaviour. In conclusion, laser texturing has demonstrated great potential for surface modification inducing geometric structural variations according to different applications.
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