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Electron backscatter diffraction characterization of laser-induced periodic surface structures on nickel surface
Applied Surface Science 302 (2014) 114–117
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Electron backscatter diffraction characterization of laser-induced periodic surface structures on nickel surface Xxx Sedao a,∗ , Claire Maurice b , Florence Garrelie a , Jean-Philippe Colombier a , Stéphanie Reynaud a , Romain Quey b , Gilles Blanc b , Florent Pigeon a a b
Laboratoire Hubert Curien, Université Jean Monnet, 42000 St-Etienne, France Laboratoire Georges Friedel, Ecole Nationale Supérieure des Mines, 42023 St-Etienne, France
a r t i c l e
i n f o
Article history: Received 28 June 2013 Received in revised form 24 October 2013 Accepted 25 October 2013 Available online 1 November 2013 Keywords: LIPSS Ripples Ultrafast laser Femtosecond pulse EBSD
a b s t r a c t We report on the structural investigation of laser-induced periodic surface structures (LIPSS) generated in polycrystalline nickel target after multi-shot irradiation by femtosecond laser pulses. Electron backscatter diffraction (EBSD) is used to reveal lattice rotation caused by dislocation storage during LIPSS formation. Localized crystallographic damages in the LIPSS are detected from both surface and cross-sectional EBSD studies. A surface region (up to 200 nm) with 1–3◦ grain disorientation is observed in localized areas from the cross-section of the LIPSS. The distribution of the local disorientation is inhomogeneous across the LIPSS and the subsurface region. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The formation of laser-induced periodic surface structures (LIPSS), often referred to as ripples, has been observed on the surface of semiconductors, metals, and dielectrics for a broad range of laser parameters [1–4]. On metals, the LIPSS formation may be attributed to an initial optical modulation effect at the surface [1]. A material instability effect upon ultrafast laser irradiation may also contribute to LIPSS formation at the surface [5]. The optic effect of initial modulation gives rise to inhomogeneous energy absorption at the illuminated surface [1,6]. Following localized photo-excitation, a rapid change in the lattice temperature can result in the development of thermal stresses and/or fast phase transition [7]. Evidence of stress loading and fast phase transition, such as formation of lattice defects and amorphization, has been observed in various material systems using transmission electron microscopy (TEM) based techniques [8–11]. However, quantification and spatial distribution of aforementioned microstructural modifications, which are of great importance for understanding LIPSS formation, are still elusive. The purpose of this paper is to analyze, for the first time, lattice deformation and its distribution in the vicinity of LIPSS and its subsurface region. Similar to TEM, high resolution EBSD is one of the few techniques with sufficient
∗ Corresponding author. Tel.: +33650434065. E-mail address: [email protected] (X. Sedao). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.10.152
sensitivity for lattice defect detection and phase discrimination at nanometer scale [12]. The present study benefits from the special strength of EBSD in analyzing the extent of lattice defect, such as plane defects and crystal disorientation, and mapping the locations of orientation gradients [13]. 2. Experimental The material used in this study is a polycrystalline nickel disk Ø 50 mm and 3 mm thick. The sample was prepared by conventional metallographic procedures with a final polish of 0.25 m diamond suspension followed by vibratory polishing in 0.25 m colloidal silica solution for about 45 min. The LIPSS on the nickel substrate were produced using a Ti: Sapphire femtosecond laser system (Legend Coherent Inc.). The laser has a central wavelength of 800 nm with a pulse duration of 50 fs and a repetition rate of 1 kHz. Before delivery onto the surface of the nickel sample, the laser pulses were linearly polarized attenuated through a pair of neutral density filters. A Pockels unit cell is used to control the total number of laser pulses. The laser beam is focused normally, through an achromatic lens onto the sample that is vertically mounted on an X–Y motorized translation stage. The dimension of the beam spot on sample surface, 2w0 = 90 m (1/e2 intensity), is determined by single shot D-square method [14]. In cross-sectional EBSD study, the cross-section sample of the LIPSS was prepared by the site-specific focused ion beam (FIB) technique [15]. A layer of a few 100 nm thick tungsten was deposited to protect the surface of the laser impact
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during FIB process. The sample was then milled with a focused ion beam of 30 keV gallium atoms and lift-out technique was employed to extract the thin lamella from the surrounding milled area. After the extraction, the lamella was welded onto a TEM copper grid, which was later mounted on a SEM sample stub. This allowed morphology observation using SEM and the investigation of the final crystalline state of the material using EBSD. The analysis of surface modification in laser impact area was performed using a scanning electron microscope (SEM, Zeiss Supra55 FEG-SEM), equipped with an HKL-Oxford Instruments EBSD system composed of a Nordlys II camera and Channel 5 software suit. EBSD acquisitions were performed with the accelerating voltage varying from 7 to 20 kV, with a working distance of 15 mm and sample tilt of 70◦ with respect to horizontal. The spatial resolution of the EBSD is typically about 20 × 60 × 10 (horizontal axis x tilted axis x depth, all units in nm) [16]. Direct interrogation of EBSD pattern (EBSP) and local crystal disorientation (LCD) derived from the EBSD analysis are used to analyze the lattice distortions induced by ultrafast irradiation. For cross-sectional study, which will be explained further below, the EBSD data was acquired at 10 nm step size. The local crystal disorientation through texture component function available in the EBSD data analysis software was employed to analyze the crystallographic change after LIPSS formation. The LCD maps illustrate the crystal rotation at each point with respect to a reference site deep into the substrate, providing a good representation of the overall distribution of disorientation induced by lattice distortion [17,18].
3. Results and discussion As discussed before, change in crystalline structure and dislocations at surfaces can be expected upon laser irradiation. These modifications can be revealed from EBSP (also known as Kikuchi diffraction pattern [19]) analysis. For instance, an EBSD study at a flat surface region in the vicinity of the LIPSS can readily reveal the impact of laser irradiation. Fig. 1a shows the SEM image of a nickel surface with its central part (indicated by white-colored circle) irradiated by 20,000 laser pulses at a peak fluence of 10 mJ/cm2 . The laser process condition employed here assures that the LIPSS appear only at the central part while the surrounding of the LIPSS within the laser illuminated area remains flat (therefore surface EBSD analysis can be applied). Due to the Gaussian profile of the laser spot, only at the central part of the irradiated area the local laser fluence was intense enough to cause surface morphology change. In order to visualize the micro-structural change induced by laser irradiation, EBSPs are acquired from two different locations on the surface, on a flat surface area close to the LIPSS and outside the central part of laser irradiated area. These locations are marked with rectangles, number 1- and 2-indexed in Fig. 1a, and their EBSPs are shown in Fig. 1b and c, respectively. The EBSP from the area outside the central part of laser irradiation zone, as shown in Fig. 1b, is the superimposed Kikuchi diffraction bands, generated from different lattice diffracting planes of nickel. It is hard to see any discernible features in the EBSP acquired from the vicinity of the LIPSS (Fig. 1c), indicating a compromise of the diffraction conditions (atomic arrangement of the lattice). The diffuseness of the EBSP could be explained by the accumulation of dislocations (either from plastic deformation or rapid solidification of a melted layer) or the presence of an amorphous phase. The EBSD data can be acquired from the entire sample surface, then indexed and used for generating surface maps for more comprehensive and diverse studies. Analysis through EBSD mapping is applied in the cross-section study, as described in the following. As the backscatter electrons come from a skin layer of ∼10 nm from the surface, EBSD analysis provides information only from
Fig. 1. (a) SEM image of the nickel surface. The circular area marked in the image received laser pulses. The laser polarization is indicated in the figure by the white arrow and letter E. The EBSD diffraction patterns from the number-indexed areas (1) and (2) are shown in (b) and (c).
the very surface. This means EBSD study merely from the surface of LIPSS is not sufficient, as the peak-to-valley amplitude of LIPSS is often in the range of a few tens to 100 nm and crystalline damage could expand a few tens of nm into the subsurface region of LIPSS [11]. In order to obtain micro-structural information from entire LIPSS and their subsurface region, cross-section samples perpendicular to the LIPSS are analyzed. The SEM surface image shown in Fig. 2a is the laser impact prepared for the cross-sectional study. The impact was produced with a peak fluence of 0.47 J/cm2 and 50 laser pulses. Fig. 2b shows a reconstructed EBSD surface map representing surface normal-projected, color-coded inverse pole figure (IPF), of the same area as observed in Fig. 2a,c is the colorcoding of the IPF in Fig. 2b, indicating stereographic projection of
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Fig. 3. (a) SEM image of the lamella. The site marked by the white-colored rectangle is selected for EBSD study. (b) Higher magnification SEM image of the ripples from the selected site. (c) Local crystal disorientation map of the selected site. The inset at top-right corner is the color-coded legend for lattice disorientation, unit in degrees.
Fig. 2. (a) SEM image of the laser impact, plan view. The inset on the upper-right corner shows the LIPSS from the center of the laser impact, with higher magnification. The laser polarization is indicated in the figure by the white arrow and letter E. (b) Color-coded map: inverse pole figure showing grain components from laser impact and its surroundings. (c) Color-coding for the inverse pole figure shown in (b). The black line segments in (a) and (b) indicates the site selected for the lamella extraction.
the surface normal to the projection sphere aligned with the crystal directions. The IPF map indicates that the laser impact is located on an area consisting of multiple grains with different grain sizes. The IPF map also reveals areas with unsuccessful indexing: a dark circle along with black spots inside of it, which appears to coincide with the laser impact. This failure in EBSD indexing could be due to local topography and debris re-deposition, the presence of which cause shadowing effect and compromise EBSD indexing [20]. For the sake of simplicity of the subsequent cross-sectional EBSD analysis, a FIB lamella was extracted from a single grain. The
selected site for lamella extraction is marked by a straight line segment in Fig. 2a and b. The lamella with a dimension of 30 × 10 × 1 (width × height × thickness, all units in m) was extracted from the substrate and then milled using FIB at a reduced current to give a flat cross-sectional surface finishing for EBSD analysis. A SEM image of the lamella is given in Fig. 3a. The LIPSS can be seen on the top surface of the lamella. The left side of the lamella is close to the center of the laser impact and right side is close to the edge. The area marked by the white-colored rectangle, as shown in Fig. 3a, was chosen to be examined in details with EBSD. The local laser fluence during LIPSS preparation at this location, derived using the Gaussian function of the laser spot (the radial distance being the distance between laser impact center and the chosen area), was 0.24 J/cm2 . The selection of the site was based on the high peakto-valley amplitude of the LIPSS (of about 200 nm) and moderate local laser fluence. Fig. 3b illustrates the selected site at a higher magnification and two LIPSS units are shown in Fig. 3b. The voids visible between the LIPSS and tungsten coating were formed during the deposition due to a deficient preparation prior to the FIB milling process. Fig. 3c shows the local disorientation (LCD) map of the selected site. The legend of the LCD map, false-color code representing the lattice rotation in degrees, is added as an inset in Fig. 3c. The LCD map (Fig. 3c) reveals a pronounced crystal rotation within and underneath the LIPSS. The region experiencing lattice
X. Sedao et al. / Applied Surface Science 302 (2014) 114–117
rotations appears to follow the contour of LIPSS, but not in a continuous manner. The disorientation always appears on the crest and in the valley, sometimes on the shoulders of the LIPSS. Small patches without perceptible grain rotation or very little rotation also exist, mainly on some of the shoulders of the LIPSS. The level of rotation is in the range of 1–2.5◦ for the major part of the disorientation pattern with a maximum of 3◦ disorientation in small areas on the outermost surface of the LIPSS. The layer exhibiting high degree of crystal rotation (>1◦ ) has a thickness of 150–200 nm. The disorientation spreads then a few tens of nm further into the subsurface region, with the levels of grain rotation decreasing gradually. A thin amorphous layer may exist at the very surface [9,21]. The identification of amorphous phase in the LIPSS is beyond the scope of this paper. It is worth noting that the existence of the voids and the punctual contact between the tungsten coating and the top of the LIPSS may cause mechanical stress. This stress is thought to be negligible because additional EBSD analysis on the LIPSS in the low local laser fluence area does not show any discernible disorientation. While the lattice rotation and its distribution in the LIPSS and the subsurface have been shown in the local crystal disorientation map, further analysis could give quantitative assessment on the degree of this rotation field and hence is clearly useful for developing a clearer picture of micro-structural evolution during LIPSS formation. Furthermore, since there is little limitation imposed on sample size for EBSD analysis, the characterization approach proposed in this paper can be applied to analyze all the LIPSS units on the same lamella straightforwardly. Correlations can be sought between local laser fluence and lattice deformation, which would also contribute to a better understanding of LIPSS formation. 4. Conclusions We described the micro-structural characterization at surface and subsurface region of the LIPSS on nickel substrate using EBSD. The disappearance of the Kikuchi bands in the EBSD pattern acquired from the surface area near the LIPSS illustrates laser induced damage to the lattice. More detailed information was obtained through cross-sectional study of the LIPSS. The evidence of lattice defect in the form of lattice rotation, the extent and the locations of the local crystal disorientation are visualized. Lattice disorientation (1–3◦ ) is observed in the surface of the LIPSS, extending 150–200 nm into the subsurface region. The present work provides access to the distribution of the local crystal disorientation. In the conditions examined in this paper, the distribution of the disorientation appears to be related to the geometry of the LIPSS: mainly on the crests as well as in the valleys of the LIPSS. Further studies are required to explain the mechanisms involved. Acknowledgement This work was supported by the LABEX MANUTECH-SISE (ANR-10-LABX-0075) of Université de Lyon, within the program
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“Investissements d’Avenir” (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR). References [1] H.M. van Driel, J.E. Sipe, J.F. Young, Laser-induced periodic surface structure on solids: a universal phenomenon, Phys. Rev. Lett. 49 (1982) 1955–1958. [2] J.F. Young, J.S. Preston, H.M. van Driel, J.E. Sipe, Laser-induced periodic surface structure. II. Experiments on Ge, Si, Al, and brass, Phys. Rev. B 27 (1983) 1155–1172. [3] M. Birnbaum, Semiconductor surface damage produced by ruby lasers, J. Appl. Phys. 36 (1965) 3688–3689. [4] J.F. Young, J.E. Sipe, H.M. van Driel, Laser-induced periodic surface structure. III. Fluence regimes, the role of feedback, and details of the induced topography in germanium, Phys. Rev. B 30 (1984) 2001–2015. [5] J. Reif, F. Costache, O. Varlamova, G. Jia, M. Ratzke, Self-organized regular surface patterning by pulsed laser ablation, Phys. Status Solidi (C) 6 (2009) 681–686. [6] F. Garrelie, J.P. Colombier, F. Pigeon, S. Tonchev, N. Faure, M. Bounhalli, S. Reynaud, O. Parriaux, Evidence of surface plasmon resonance in ultrafast laserinduced ripples, Opt. Express 19 (2011) 9035–9043. [7] J.P. Colombier, F. Garrelie, P. Brunet, A. Bruyère, F. Pigeon, R. Stoian, O. Parriaux, Plasmonic and hydrodynamic effects in ultrafast laser-induced periodic surface structures on metals, J. Laser Micro/Nanoeng. 7 (2012) 362–368. [8] T.H.R. Crawford, J. Yamanaka, G.A. Botton, H.K. Haugen, High-resolution observations of an amorphous layer and subsurface damage formed by femtosecond laser irradiation of silicon, J. Appl. Phys. 103 (2008) 053104–053107. [9] M. Couillard, A. Borowiec, H.K. Haugen, J.S. Preston, E.M. Griswold, G.A. Botton, Subsurface modifications in indium phosphide induced by single and multiple femtosecond laser pulses: a study on the formation of periodic ripples, J. Appl. Phys. 101 (2007) 033518–033519. [10] J. Bonse, A. Rosenfeld, C. Grebing, G. Steinmeyer, N. Mailman, G.A. Botton, H.K. Haugen, Ablation and structural changes induced in InP surfaces by single 10 fs laser pulses in air, J. Appl. Phys. 106 (2009), 074907-1–074907-7. [11] M. Schade, O. Varlamova, J. Reif, H. Blumtritt, W. Erfurth, H.S. Leipner, High-resolution investigations of ripple structures formed by femtosecond laser irradiation of silicon, Anal. Bioanal. Chem. 396 (2010) 1905–1911. [12] M. Calcagnotto, D. Ponge, E. Demir, D. Raabe, Orientation gradients and geometrically necessary dislocations in ultrafine grained dual-phase steels studied by 2D and 3D EBSD, Mater. Sci. Eng.: A 527 (2010) 2738–2746. [13] P.J. Hurley, F.J. Humphreys, The application of EBSD to the study of substructural development in a cold rolled single-phase aluminium alloy, Acta Mater. 51 (2003) 1087–1102. [14] J.M. Liu, Simple technique for measurements of pulsed Gaussian-beam spot sizes, Opt. Lett. 7 (1982) 196–198. [15] M.W. Phaneuf, Applications of focused ion beam microscopy to materials science specimens, Micron 30 (1999) 277–288. [16] F.J. Humphreys, Y. Huang, I. Brough, C. Harris, Electron backscatter diffraction of grain and subgrain structures — resolution considerations, J. Microsc. 195 (1999) 212–216. [17] A. Kumar, T.M. Pollock, Mapping of femtosecond laser-induced collateral damage by electron backscatter diffraction, J. Appl. Phys. 110 (2011) 083114–083115. [18] N. Zaafarani, D. Raabe, F. Roters, S. Zaefferer, On the origin of deformationinduced rotation patterns below nanoindents, Acta Mater. 56 (2008) 31–42. [19] K.Z. Baba-Kishi, Review electron backscatter Kikuchi diffraction in the scanning electron microscope for crystallographic analysis, J. Mater. Sc. 37 (2002) 1715–1746. [20] J. Guo, S. Amira, P. Gougeon, X.G. Chen, Effect of the surface preparation techniques on the EBSD analysis of a friction stir welded AA1100-B4C metal matrix composite, Mater. Charact. 62 (2011) 865–877. [21] M. Hashida, Y. Miyasaka, Y. Ikuta, S. Tokita, S. Sakabe, Crystal structures on a copper thin film with a surface of periodic self-organized nanostructures induced by femtosecond laser pulses, Phys. Rev. B 83 (2011) 235413.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Electron backscatter diffraction characterization of laser-induced periodic surface structures on nickel surface Xxx Sedao a,∗ , Claire Maurice b , Florence Garrelie a , Jean-Philippe Colombier a , Stéphanie Reynaud a , Romain Quey b , Gilles Blanc b , Florent Pigeon a a b
Laboratoire Hubert Curien, Université Jean Monnet, 42000 St-Etienne, France Laboratoire Georges Friedel, Ecole Nationale Supérieure des Mines, 42023 St-Etienne, France
a r t i c l e
i n f o
Article history: Received 28 June 2013 Received in revised form 24 October 2013 Accepted 25 October 2013 Available online 1 November 2013 Keywords: LIPSS Ripples Ultrafast laser Femtosecond pulse EBSD
a b s t r a c t We report on the structural investigation of laser-induced periodic surface structures (LIPSS) generated in polycrystalline nickel target after multi-shot irradiation by femtosecond laser pulses. Electron backscatter diffraction (EBSD) is used to reveal lattice rotation caused by dislocation storage during LIPSS formation. Localized crystallographic damages in the LIPSS are detected from both surface and cross-sectional EBSD studies. A surface region (up to 200 nm) with 1–3◦ grain disorientation is observed in localized areas from the cross-section of the LIPSS. The distribution of the local disorientation is inhomogeneous across the LIPSS and the subsurface region. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The formation of laser-induced periodic surface structures (LIPSS), often referred to as ripples, has been observed on the surface of semiconductors, metals, and dielectrics for a broad range of laser parameters [1–4]. On metals, the LIPSS formation may be attributed to an initial optical modulation effect at the surface [1]. A material instability effect upon ultrafast laser irradiation may also contribute to LIPSS formation at the surface [5]. The optic effect of initial modulation gives rise to inhomogeneous energy absorption at the illuminated surface [1,6]. Following localized photo-excitation, a rapid change in the lattice temperature can result in the development of thermal stresses and/or fast phase transition [7]. Evidence of stress loading and fast phase transition, such as formation of lattice defects and amorphization, has been observed in various material systems using transmission electron microscopy (TEM) based techniques [8–11]. However, quantification and spatial distribution of aforementioned microstructural modifications, which are of great importance for understanding LIPSS formation, are still elusive. The purpose of this paper is to analyze, for the first time, lattice deformation and its distribution in the vicinity of LIPSS and its subsurface region. Similar to TEM, high resolution EBSD is one of the few techniques with sufficient
∗ Corresponding author. Tel.: +33650434065. E-mail address: [email protected] (X. Sedao). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.10.152
sensitivity for lattice defect detection and phase discrimination at nanometer scale [12]. The present study benefits from the special strength of EBSD in analyzing the extent of lattice defect, such as plane defects and crystal disorientation, and mapping the locations of orientation gradients [13]. 2. Experimental The material used in this study is a polycrystalline nickel disk Ø 50 mm and 3 mm thick. The sample was prepared by conventional metallographic procedures with a final polish of 0.25 m diamond suspension followed by vibratory polishing in 0.25 m colloidal silica solution for about 45 min. The LIPSS on the nickel substrate were produced using a Ti: Sapphire femtosecond laser system (Legend Coherent Inc.). The laser has a central wavelength of 800 nm with a pulse duration of 50 fs and a repetition rate of 1 kHz. Before delivery onto the surface of the nickel sample, the laser pulses were linearly polarized attenuated through a pair of neutral density filters. A Pockels unit cell is used to control the total number of laser pulses. The laser beam is focused normally, through an achromatic lens onto the sample that is vertically mounted on an X–Y motorized translation stage. The dimension of the beam spot on sample surface, 2w0 = 90 m (1/e2 intensity), is determined by single shot D-square method [14]. In cross-sectional EBSD study, the cross-section sample of the LIPSS was prepared by the site-specific focused ion beam (FIB) technique [15]. A layer of a few 100 nm thick tungsten was deposited to protect the surface of the laser impact
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during FIB process. The sample was then milled with a focused ion beam of 30 keV gallium atoms and lift-out technique was employed to extract the thin lamella from the surrounding milled area. After the extraction, the lamella was welded onto a TEM copper grid, which was later mounted on a SEM sample stub. This allowed morphology observation using SEM and the investigation of the final crystalline state of the material using EBSD. The analysis of surface modification in laser impact area was performed using a scanning electron microscope (SEM, Zeiss Supra55 FEG-SEM), equipped with an HKL-Oxford Instruments EBSD system composed of a Nordlys II camera and Channel 5 software suit. EBSD acquisitions were performed with the accelerating voltage varying from 7 to 20 kV, with a working distance of 15 mm and sample tilt of 70◦ with respect to horizontal. The spatial resolution of the EBSD is typically about 20 × 60 × 10 (horizontal axis x tilted axis x depth, all units in nm) [16]. Direct interrogation of EBSD pattern (EBSP) and local crystal disorientation (LCD) derived from the EBSD analysis are used to analyze the lattice distortions induced by ultrafast irradiation. For cross-sectional study, which will be explained further below, the EBSD data was acquired at 10 nm step size. The local crystal disorientation through texture component function available in the EBSD data analysis software was employed to analyze the crystallographic change after LIPSS formation. The LCD maps illustrate the crystal rotation at each point with respect to a reference site deep into the substrate, providing a good representation of the overall distribution of disorientation induced by lattice distortion [17,18].
3. Results and discussion As discussed before, change in crystalline structure and dislocations at surfaces can be expected upon laser irradiation. These modifications can be revealed from EBSP (also known as Kikuchi diffraction pattern [19]) analysis. For instance, an EBSD study at a flat surface region in the vicinity of the LIPSS can readily reveal the impact of laser irradiation. Fig. 1a shows the SEM image of a nickel surface with its central part (indicated by white-colored circle) irradiated by 20,000 laser pulses at a peak fluence of 10 mJ/cm2 . The laser process condition employed here assures that the LIPSS appear only at the central part while the surrounding of the LIPSS within the laser illuminated area remains flat (therefore surface EBSD analysis can be applied). Due to the Gaussian profile of the laser spot, only at the central part of the irradiated area the local laser fluence was intense enough to cause surface morphology change. In order to visualize the micro-structural change induced by laser irradiation, EBSPs are acquired from two different locations on the surface, on a flat surface area close to the LIPSS and outside the central part of laser irradiated area. These locations are marked with rectangles, number 1- and 2-indexed in Fig. 1a, and their EBSPs are shown in Fig. 1b and c, respectively. The EBSP from the area outside the central part of laser irradiation zone, as shown in Fig. 1b, is the superimposed Kikuchi diffraction bands, generated from different lattice diffracting planes of nickel. It is hard to see any discernible features in the EBSP acquired from the vicinity of the LIPSS (Fig. 1c), indicating a compromise of the diffraction conditions (atomic arrangement of the lattice). The diffuseness of the EBSP could be explained by the accumulation of dislocations (either from plastic deformation or rapid solidification of a melted layer) or the presence of an amorphous phase. The EBSD data can be acquired from the entire sample surface, then indexed and used for generating surface maps for more comprehensive and diverse studies. Analysis through EBSD mapping is applied in the cross-section study, as described in the following. As the backscatter electrons come from a skin layer of ∼10 nm from the surface, EBSD analysis provides information only from
Fig. 1. (a) SEM image of the nickel surface. The circular area marked in the image received laser pulses. The laser polarization is indicated in the figure by the white arrow and letter E. The EBSD diffraction patterns from the number-indexed areas (1) and (2) are shown in (b) and (c).
the very surface. This means EBSD study merely from the surface of LIPSS is not sufficient, as the peak-to-valley amplitude of LIPSS is often in the range of a few tens to 100 nm and crystalline damage could expand a few tens of nm into the subsurface region of LIPSS [11]. In order to obtain micro-structural information from entire LIPSS and their subsurface region, cross-section samples perpendicular to the LIPSS are analyzed. The SEM surface image shown in Fig. 2a is the laser impact prepared for the cross-sectional study. The impact was produced with a peak fluence of 0.47 J/cm2 and 50 laser pulses. Fig. 2b shows a reconstructed EBSD surface map representing surface normal-projected, color-coded inverse pole figure (IPF), of the same area as observed in Fig. 2a,c is the colorcoding of the IPF in Fig. 2b, indicating stereographic projection of
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Fig. 3. (a) SEM image of the lamella. The site marked by the white-colored rectangle is selected for EBSD study. (b) Higher magnification SEM image of the ripples from the selected site. (c) Local crystal disorientation map of the selected site. The inset at top-right corner is the color-coded legend for lattice disorientation, unit in degrees.
Fig. 2. (a) SEM image of the laser impact, plan view. The inset on the upper-right corner shows the LIPSS from the center of the laser impact, with higher magnification. The laser polarization is indicated in the figure by the white arrow and letter E. (b) Color-coded map: inverse pole figure showing grain components from laser impact and its surroundings. (c) Color-coding for the inverse pole figure shown in (b). The black line segments in (a) and (b) indicates the site selected for the lamella extraction.
the surface normal to the projection sphere aligned with the crystal directions. The IPF map indicates that the laser impact is located on an area consisting of multiple grains with different grain sizes. The IPF map also reveals areas with unsuccessful indexing: a dark circle along with black spots inside of it, which appears to coincide with the laser impact. This failure in EBSD indexing could be due to local topography and debris re-deposition, the presence of which cause shadowing effect and compromise EBSD indexing [20]. For the sake of simplicity of the subsequent cross-sectional EBSD analysis, a FIB lamella was extracted from a single grain. The
selected site for lamella extraction is marked by a straight line segment in Fig. 2a and b. The lamella with a dimension of 30 × 10 × 1 (width × height × thickness, all units in m) was extracted from the substrate and then milled using FIB at a reduced current to give a flat cross-sectional surface finishing for EBSD analysis. A SEM image of the lamella is given in Fig. 3a. The LIPSS can be seen on the top surface of the lamella. The left side of the lamella is close to the center of the laser impact and right side is close to the edge. The area marked by the white-colored rectangle, as shown in Fig. 3a, was chosen to be examined in details with EBSD. The local laser fluence during LIPSS preparation at this location, derived using the Gaussian function of the laser spot (the radial distance being the distance between laser impact center and the chosen area), was 0.24 J/cm2 . The selection of the site was based on the high peakto-valley amplitude of the LIPSS (of about 200 nm) and moderate local laser fluence. Fig. 3b illustrates the selected site at a higher magnification and two LIPSS units are shown in Fig. 3b. The voids visible between the LIPSS and tungsten coating were formed during the deposition due to a deficient preparation prior to the FIB milling process. Fig. 3c shows the local disorientation (LCD) map of the selected site. The legend of the LCD map, false-color code representing the lattice rotation in degrees, is added as an inset in Fig. 3c. The LCD map (Fig. 3c) reveals a pronounced crystal rotation within and underneath the LIPSS. The region experiencing lattice
X. Sedao et al. / Applied Surface Science 302 (2014) 114–117
rotations appears to follow the contour of LIPSS, but not in a continuous manner. The disorientation always appears on the crest and in the valley, sometimes on the shoulders of the LIPSS. Small patches without perceptible grain rotation or very little rotation also exist, mainly on some of the shoulders of the LIPSS. The level of rotation is in the range of 1–2.5◦ for the major part of the disorientation pattern with a maximum of 3◦ disorientation in small areas on the outermost surface of the LIPSS. The layer exhibiting high degree of crystal rotation (>1◦ ) has a thickness of 150–200 nm. The disorientation spreads then a few tens of nm further into the subsurface region, with the levels of grain rotation decreasing gradually. A thin amorphous layer may exist at the very surface [9,21]. The identification of amorphous phase in the LIPSS is beyond the scope of this paper. It is worth noting that the existence of the voids and the punctual contact between the tungsten coating and the top of the LIPSS may cause mechanical stress. This stress is thought to be negligible because additional EBSD analysis on the LIPSS in the low local laser fluence area does not show any discernible disorientation. While the lattice rotation and its distribution in the LIPSS and the subsurface have been shown in the local crystal disorientation map, further analysis could give quantitative assessment on the degree of this rotation field and hence is clearly useful for developing a clearer picture of micro-structural evolution during LIPSS formation. Furthermore, since there is little limitation imposed on sample size for EBSD analysis, the characterization approach proposed in this paper can be applied to analyze all the LIPSS units on the same lamella straightforwardly. Correlations can be sought between local laser fluence and lattice deformation, which would also contribute to a better understanding of LIPSS formation. 4. Conclusions We described the micro-structural characterization at surface and subsurface region of the LIPSS on nickel substrate using EBSD. The disappearance of the Kikuchi bands in the EBSD pattern acquired from the surface area near the LIPSS illustrates laser induced damage to the lattice. More detailed information was obtained through cross-sectional study of the LIPSS. The evidence of lattice defect in the form of lattice rotation, the extent and the locations of the local crystal disorientation are visualized. Lattice disorientation (1–3◦ ) is observed in the surface of the LIPSS, extending 150–200 nm into the subsurface region. The present work provides access to the distribution of the local crystal disorientation. In the conditions examined in this paper, the distribution of the disorientation appears to be related to the geometry of the LIPSS: mainly on the crests as well as in the valleys of the LIPSS. Further studies are required to explain the mechanisms involved. Acknowledgement This work was supported by the LABEX MANUTECH-SISE (ANR-10-LABX-0075) of Université de Lyon, within the program
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