Surface texturing of the carbon steel AISI 1045 using femtosecond laser in single pulse and scanning regime

Surface texturing of the carbon steel AISI 1045 using femtosecond laser in single pulse and scanning regime

Applied Surface Science 258 (2011) 290–296 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/loca...

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Applied Surface Science 258 (2011) 290–296

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Surface texturing of the carbon steel AISI 1045 using femtosecond laser in single pulse and scanning regime J. Staˇsic´ a,∗ , B. Gakovic´ a , W. Perrie b , K. Watkins b , S. Petrovic´ a , M. Trtica a a b

ˇ Institute of Nuclear Sciences, University of Belgrade, P.O. BOX 522, 11001 Belgrade, Serbia VINCA Department of Engineering, University of Liverpool, Liverpool L69 3GH, UK

a r t i c l e

i n f o

Article history: Received 8 June 2011 Received in revised form 10 August 2011 Accepted 10 August 2011 Available online 18 August 2011 Keywords: Steel Femtosecond laser Laser processing Periodic surface structures

a b s t r a c t Surface texturing of the metals, including steels, gained a new dimension with the appearance of femtosecond lasers. These laser systems enable highly precise modifications, which are very important for numerous applications of metals. The effects of a Ti:sapphire femtosecond laser with the pulse duration of 160 fs, operating at 775 nm wavelength and in two operational regimes – single pulse (SP) and scanning regime, on a high quality AISI 1045 carbon steel were studied. The estimated surface damage threshold was 0.22 J/cm2 (SP). Surface modification was studied for the laser fluences of 0.66, 1.48 and 2.37 J/cm2 . The fluence of 0.66 J/cm2 , in both working regimes, induced texturing of the material, i.e. formation of periodic surface structures (PSS). Their periodicity was in accordance with the used laser wavelength. Finally, changes in the surface oxygen content caused by ultrashort laser pulses were recorded. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Nearly since their discovery, the application of lasers for surface modification of different materials, including metals, is of great interest. With the appearance of femtosecond lasers in the mid nineteen–eighties, material/metal surface texturing gained a whole new dimension. From that moment, it became possible to talk about three regimes of modification – femto- (fs), pico- (ps) and nanosecond (ns) regime [1]. Short-pulse laser processing of different materials gives unique benefits over other available techniques therefore it is widely used in microelectronics, optics and biomedicine. Microelectronic applications include laser masking [2], machining of micro-holes [3], etc. Focusing only on the fs regime of material surface variation, it can be stated that this regime possesses advantages in relation to ps as well as ns regime. Generally, in this regime of modification: (i) the heat transfer to the material is drastically reduced; (ii) the transition from solid to vapour phase is quite possible, and finally (iii) high precision of material modification is ensured. In the recent years, different types of metals [4,5] and alloys [6,7] have been irradiated by pulsed lasers. In this context, irradiation of various high quality steels is of high importance. Pulsed lasers used for this purpose were ruby [8], Nd:YAG/Nd:YVO4 [9–11], excimer

∗ Corresponding author. Tel.: +38 1113408779; fax: +38 1113408224. ´ [email protected] (B. Gakovic), ´ E-mail addresses: [email protected] (J. Staˇsic), [email protected] (W. Perrie), [email protected] (K. Watkins), ´ [email protected] (M. Trtica). [email protected] (S. Petrovic), 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.08.052

[12], TEA CO2 [13] and Ti:sapphire laser [14]. AISI 1045 steel is within the class of steels with medium carbon content. The presence of carbon in steel improves its hardness and tensile strength [15]. Due to these and other excellent physical and chemical properties AISI 1045 steel is widely applied from machinery to nuclear industry. Particularly, in the nuclear industry, it can be used for low and high-pressure turbine sections, reactor vessels, etc. [16]. Interaction of AISI 1045 steel with Ti:sapphire laser beam pulsed in the femtosecond time domain has not been described in the literature unlike picosecond pulses [17,18]. Our emphasis in the present paper is on studying the effects of a femtosecond laser emitting in the near-infrared (775 nm) on AISI 1045 steel surface. Special attention was paid to morphological surface changes (especially the formation of specific periodic structures) of AISI 1045 steel as a function of two operational regimes – single pulse and scanning regime. 2. Experimental Surface of the AISI 1045 steel was prepared by a standard metallographic procedure. The samples were mechanically polished (first by using SiC grinding paper (1200–4000 grit) and finally by using diamond paste (1–0.04 ␮m)), ultrasonically cleaned and dried in hot air. Prior to laser irradiation they were cleaned in methanol. Samples had a round shape with 15 mm diameter and 2 mm thickness. Their surface was polished to an average roughness of about 50 nm. The laser employed for irradiation was femtosecond laser system, model Clark-MXR CPA2010, based on a chirped pulse amplification [19]. A mode-locked, diode pumped Erbium

J. Staˇsi´c et al. / Applied Surface Science 258 (2011) 290–296 Table 1 Typical parameters of Ti:sapphire laser used during irradiation of a steel target AISI 1045. Laser wavelength () Laser pulse duration ( p ) Laser pulse energy (Ep )

Laser fluence (˚)

Laser intensity (I)

Laser pulse repetition rate Operational regimes (beam scan. speed (s)) Polarization Mode structure

775 nm 160 fs • Ep1 = 10.75 ␮J • Ep2 = 6.7 ␮J • Ep3 = 3 ␮J • ˚1 = 2.37 J/cm2 • ˚2 = 1.48 J/cm2 • ˚3 = 0.66 J/cm2 • I1 = 1.48 × 1013 W/cm2 • I2 = 0.93 × 1013 W/cm2 • I3 = 0.41 × 1013 W/cm2 1 kHz • Single pulse regime (s – 100 mm/s) • Scanning regime (s – 10 mm/s) Linear, horizontal TEM00

doped ring fibre laser at 1550 nm, with the output frequency doubled to 775 nm in periodically poled lithium niobate, was used to seed a Ti:sapphire regenerative amplifier (160 fs, 1 mJ, 1 kHz). Beam intensity spatial profile is near Gaussian with a measured 3D Gaussian fit parameter ∼0.87. The beam is slightly elliptical and shows some residual astigmatism, probably due to cavity design incorporating a Brewster cut Ti–sapphire crystal in the regenerative amplifier [5]. The output from the femtosecond laser system passed through a laser safety shutter, a pick-off beam splitter for autocorrelation and then through an attenuator (/2 plate and Glan laser polarizer). The beam was further reflected from a periscope, entered a scanning galvo head (GSI Lumonics) and focused on the sample surface by an f-theta lens with a focal length f = 100 mm, AR coated for 775 nm. Transmission through the scan head was measured to be T > 97% and 1/e2 focused spot size at the substrate surface was  ∼ 30 ␮m. The sample was mounted on a 4-axis (x, y, z, ) motion control system (Aerotech, 0.5 ␮m repeatability) and the sample surface was carefully brought close to the lens focal plane (z-axis). Focal spot scanning was controlled through a graphical software interface, (SCAPS GmbH). The angle of incidence (AOI) of the laser beam with respect to the sample surface was near normal ( ∼ ± 3◦ ) close to the center of the working field of the scan lens while the sample could also be translated horizontally (x, y) to minimise the AOI. Irradiation was carried out in air, at a pressure of 1013 mbar and standard relative humidity. Experiments were carried out at varying pulse energies with scan speeds of s = 100 mm/s and 10 mm/s respectively. High speed scanning at s = 100 mm/s resulted in single pulse exposure on the sample surface while lower speed scanning at s = 10 mm/s scan speed resulted in a pulse overlap of about 3. Typical parameters used for irradiation of AISI 1045 target by femtosecond Ti:sapphire laser are given in Table 1. Various analytical techniques were used for characterization of AISI 1045 steel prior to and after femtosecond laser irradiation. Surface morphology was monitored by optical (OM) and scanning electron (SEM) microscopy. SEM was coupled to an energy-dispersive analyzer (EDX) for determining the sample surface composition (analysis depth 2–3 ␮m, acceleration voltage 20 kV). A white light surface profilometer (WYKO NT1100) was used for measuring the geometry of ablated/damaged area. 3. Results and discussion The surface of the AISI 1045 steel had a typical silver–grey metallic colour before irradiation. EDX elemental analysis of a non-irradiated sample surface showed the following content: iron ∼90.9 wt.%, balanced to a 100% by C (7.5 wt.%), O (∼0.9 wt.%) and Mn (∼0.7 wt.%). Complete elemental analysis was normalized.

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Investigations of the morphological changes induced by femtosecond laser on the AISI 1045 steel target have shown their dependence on laser beam characteristics, primarily on the laser fluence, peak power density, number of accumulated pulses and wavelength. Morphological features of the AISI 1045 steel surface were observed at two operational regimes, i.e. single pulse and scanning regime. Both regimes induced significant surface changes of the steel surface. The results of the induced surface modification are presented in the following section. 3.1. Single pulse regime Initially, the effect of femtosecond laser on the target was studied in the single pulse regime, at the laser beam scan speed of 100 mm/s. Three different pulse energies were used in these experiments, Ep1 = 10.8 ␮J, Ep2 = 6.7 ␮J and Ep3 = 3.0 ␮J. The corresponding fluences, calculated using 1/e2 Gaussian beam radius, were ˚1 = 2.37 J/cm2 , ˚2 = 1.48 J/cm2 , ˚3 = 0.66 J/cm2 , and the intensities I1 = 1.48 × 1013 W/cm2 , I2 = 0.93 × 1013 W/cm2 , and I3 = 0.41 × 1013 W/cm2 , respectively (Table 1). Fig. 1 shows SEM micrographs of the target before and after irradiation with the same number of pulses (N = 10) and different fluences ˚1 , ˚2 and ˚3 . It can be concluded that the damage depth on the surface of a solid target increases with higher energy density (Fig. 1A1–A3). After the action of 10 pulses of the lowest laser fluence, the surface is homogeneously textured due to the appearance of so called periodic surface structures (PSS), Fig. 1A3 and B3. With increasing fluence, these structures are also present, but are much less well defined due to increasing contribution from surface melting as fluence increases well above the modification threshold fluence, even with femtosecond pulses. The appearance of growing tiny globules indicates melting along with a loss of fringe finesse as fluence rises, hence the fringes show a deformed shape in the center, Fig. 1B1, B2, C1 and C2. The phenomenon of periodic surface structures is a subject of research for more than two decades. These investigations were especially intensified in the last ten years. The mechanism of their formation is very complex and generally depends on the number of parameters, such as the fluence used, the number of accumulated laser pulses, polarization of the laser beam, etc. One of the presumptions is that the PSS occur due to the interference of the incoming laser beam with the so called surface waves scattered off the imperfections on the material and running along its surface [17]. These structures were also observed in our earlier work with picosecond laser pulses acting on the same target [17,20]. The distance measured between consecutive PSS observed in this work was ∼750–800 nm (Fig. 1), which is approximately in accordance with the laser wavelength. The periodicity of such structures  is supposed to depend directly on the laser wavelength, Eq. (1) [21,22]: ≈

laser cos i

≈

laser 1 ± sin i

p − polarization

(1)

s − polarization

where laser denotes the laser wavelength, and  i denotes the incident angle of the beam. In this case, the beam had a linear horizontal polarization, and the angle of incidence with respect to the sample surface was near normal,  ∼ ± 3◦ . The measured values for the periodicity in this work agree with the predictions of Eq. (1) in case of p-polarized beam. Based on the micrographs of the damages after the same number of pulses and different fluences, 1/e2 diameter of the Gaussian beam, as well as the damage threshold of the AISI 1045 steel

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Fig. 1. The view of the AISI 1045 steel surface (SEM analysis) after irradiation with 10 femtosecond laser pulses at different fluences,  = 775 nm. A1–C1: ˚1 = 2.37 J/cm2 ; A2–C2: ˚2 = 1.48 J/cm2 ; A3 and B3: ˚3 = 0.66 J/cm2 ; D – view of the target before irradiation. B – center, C – center and periphery.

target were determined for N = 10 pulses. Damage threshold is defined as the minimum laser energy/fluence necessary for creating detectable damage on the material surface. These values can be evaluated using the method given in [23,24]. In this calculation, square diameter of the damage caused by laser action is given as,



D2 = 2ω02 ln

pk

F0

Fth



= 2ω02 ln

E  p

Eth

laser, however our results are in a good accordance with the results for stainless steel, where Fth is of the order of 0.2 J/cm2 [25,26]. Semaltianos and coworkers also showed that the damage threshold significantly decreases as the number of accumulated pulses rises [24].

(2)

pk

where F0 , ω0 and Ep are fluence, laser beam radius and laser pulse energy, respectively. We used 1/e2 laser beam radius in this work, pk while F0 is obtained from the equation, pk

F0 =

2Ep ω02

(3)

From three different pulse energies of 4, 6.7 and 10.75 ␮J, and corresponding damage diameters acquired from SEM micrographs −28.05, 31.75 and 36.89 ␮m, D2 versus EP diagram is obtained (Fig. 2). The error bars represent standard deviation values. Using the slope of the linear approximation of this curve, 1/e2 beam radius is determined. This value, along with the free term (y-intercept), finally gives the damage threshold pulse energy. The calculated beam radius was ω0 = 17.0 ± 0.1 ␮m and the threshold fluence Fth = 0.22 ± 0.02 J/cm2 . There is no data available in the literature on the damage threshold of carbon steel affected by femtosecond

Fig. 2. Dependence of the square damage diameter on the laser pulse energy for N = 10 accumulated pulses.

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Fig. 3. SEM micrographs of the AISI 1045 steel target after the action of: A – 5, B – 10, C – 20, D – 100, E – 500 pulses of Ti:sapphire femtosecond laser, ˚ = 0.66 J/cm2 ; ∼3 ˚th . (A2–E2: center, A3–E3: center and periphery.)

Micrographs show that the damage depth, as well as the surface of irradiated spot rise with higher pulse number (Fig. 3A1–E1). Also, it can be seen from Fig. 3A2–E2 and A3–E3 that PSS occur already after 5 pulses in the center, as well as periphery, i.e. the texturing is homogeneous across the whole modified area. In case of N = 500 pulses, PSS are deformed, and the central zone is ablated. When ultrashort laser pulse acts on a metal target, various physical processes are generated. The ablation is conducted mostly in vapor and plasma phases, with the following scenario: (i) the laser

energy is first absorbed by the free electrons. Shortly after the incoming radiation is absorbed, thermalization of the electron subsystem takes place. Considering the time scale, the absorption of laser radiation as well as successive thermalization has a duration of the order of several hundreds of femtoseconds. (ii) The next step comprises the transfer of energy from the electron subsystem to the lattice. The energy transfer in this case is also rapid and occurs on a picosecond scale. Further, (iii) this results in a formation of dominant vapor and plasma phases (on the same time scale) fol-

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Fig. 4. Dependence of the damage area on AISI 1045 steel surface on the number of accumulated laser pulses, ˚3 = 0.66 J/cm2 .

Fig. 5. SEM micrograph of the steel surface after irradiation with 10 laser pulses, ˚1 = 2.37 J/cm2 . The locations where line EDX spectral analysis was conducted are marked.

lowed by their fast expansion in a surrounding ambient. During all of these processes, thermal diffusion into the bulk is negligible [27], and hydrodynamic effects can be observed only after accumulation of large number of pulses (Fig. 2E2). Generally, depending on the laser energy/fluence used in irradiation by femtosecond laser, the mechanisms of modification and ablation can be (a) normal evaporation, (b) phase explosion, (c) spallation and (d) fragmentation [28,29]. Irradiation of the AISI 1045 steel resulted mostly in a direct solid–vapor or solid–plasma transition, including probably the phenomenon of phase explosion [30]. Melting in the center of modified area occurring after the action of few hundreds of pulses or more is likely due to heat accumulation in the sample and re-irradiation from the evolved plasma plume. Ablation/modification with femtosecond (ultrashort) laser pulses of the energy close to the damage threshold leads to a reduc-

tion of hydrodynamic effects and precise material modification. The reason for this is that the thermal diffusion length ((  a )1/2 , where

is a heat diffusion coefficient and  a duration of ablation process) is smaller than the optical penetration depth (˛−1 , where ˛ is optical absorption coefficient). Heat diffusion is then significantly repressed, which enables higher precision and minimal heat effect in the material [27]. Laser penetration depth, in case of laser–target interaction near the threshold, is equal to optical penetration depth, and in case of carbon steel is ∼30 nm [31]. Relatively high intensity of the laser radiation of ∼1013 W/cm2 , used in the experiments described here, is sufficient for the occurrence of plasma in front of the target. Contrary to the metal vapor cloud, which develops on the time scale of tens to hundreds of picoseconds after the ablation, recent investigations showed that the initial plasma occurs during the first few picoseconds. Based on

Fig. 6. SEM micrographs of the AISI 1045 steel surface after scanning with femtosecond laser at  = 775 nm; ˚3 = 0.66 J/cm2 ; pulse repetition 1 kHz; scanning rate 10 mm/s. A – after single scanning, B – after triple scanning. (A2, A3, B2, B3 – central zone of the damage.)

J. Staˇsi´c et al. / Applied Surface Science 258 (2011) 290–296 Table 2 EDX elemental analysis of the AISI 1045 surface. Measuring locations are given in Fig. 5. Spectrum (wt.%)

C

O

Mn

Fe

Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4 Spectrum 5 Spectrum 6 Spectrum 7 Spectrum 8 Spectrum 9 Spectrum 10 Spectrum 11 Spectrum 12

7.5 6.4 6.2 7.6 5.8 5.9 6.1 8.4 6.9 5.7 5.9 8.6

0.9 0.6 0.7 0.8 0.4 0.0 0.5 0.5 0.5 0.6 0.9 0.5

0.7 0.5 0.4 0.6 0.6 0.5 0.5 0.8 0.4 0.7 0.6 0.5

90.9 92.5 92.7 91.0 93.2 93.6 92.8 90.3 92.2 93.0 92.7 90.5

Table 3 Comparative results obtained on the AISI 1045 target by picosecond and femtosecond laser pulses. ps laser (1064 nm)

fs laser

Pulse duration Pulse energy Fluence Irradiance Damage shape

40 ps ∼25 mJ ∼10 J/cm2 ∼1011 W/cm2 Craters, depth ∼ 100 ␮m

Damage diameter Hydrodynamic (HD) effects Structure of the central damage area

300 ± 10 ␮m Prominent HD features Granular structure

Threshold fluence EDX

0.30 ± 0.02 J/cm2 Lower oxygen content in the center, higher on the periphery (droplet)

Periodic surface structures (PSS)

No PSS

Plasma

Occurrence of plasma in front of the target

160 fs ≤11 ␮J ≤2.5 J/cm2 ∼1013 W/cm2 Damages and craters, depth ≤ 20 ␮m ≤40.0 ± 0.2 ␮m Almost no HD features PSS, the occurrence of ablation with higher number of pulses 0.22 ± 0.02 J/cm2 Single pulse regime: decrease of the oxygen content from periphery towards the center; Scanning regime: increased oxygen content, decreased concentration of iron PSS homogeneously distributed across the whole modified area Occurrence of plasma in front of the target

Total

∼100

this, it can be concluded that the plasma in front of the target cannot influence the absorption and propagation of laser radiation in case of femtosecond laser, since the pulse ends before the formation of plasma. The dependence of damage area on the number of delivered pulses for ˚3 = 0.66 J/cm2 is shown in Fig. 4. At lower number of accumulated pulses, Fig. 4, the beam penetrates deeper and the spot size increases up to N ∼ 100 pulses when the saturation occurs. Irradiation of the AISI 1045 steel target was performed in air atmosphere, therefore the changes in oxygen content on the surface are expected. Monitoring of the target constituents, including oxygen, before and after the action of 10 pulses at the laser fluence ˚1 = 2.37 J/cm2 , is shown in Fig. 5. EDX method was used for this purpose and the analysis was carried out along the whole spot, at 12 points. The results obtained are given in Table 2. Generally, irradiation of the sample with fs laser, at the fluence of 2.37 J/cm2 , resulted in the changes of oxygen concentration. Its concentration decreases from periphery towards the center (Table 2, Fig. 5), while the iron content increases. This result implies a relatively efficient removal of oxide from the steel surface. 3.2. Scanning regime The surface of the AISI 1045 steel after irradiation with fs laser fluence of 0.66 J/cm2 , at a repetition rate 1 kHz and laser beam scan speed 10 mm/s, is shown in Fig. 6. Taking into account the scanning speed, approximately 3 laser spots overlap at every point of the target. Dimensions of the scanned areas were approximately 1 mm × 1 mm. PSS were formed in both single and triple scanning mode. In case of triple scanning, the structures align along the ones already formed in previous passages of the laser beam, and they are more prominent [32]. Investigations of the morphological features induced by laser on AISI 1045 steel surface showed, among other, their dependence on the characteristics of laser beam [17], especially the number of pulses (N) and the number of laser beam passages. Generally, the interaction of femtosecond laser with AISI 1045 steel, at the fluence of 0.66 J/cm2 , resulted in a creation of modified area without the presence of surrounding material and hydrodynamic shapes. Periodic surface structures obtained on the steel are, in both cases (single pulse and laser scanning regime), oriented in the same way – perpendicularly to the laser beam linear polarization direction (Fig. 6). Periodicity of the PSS is around 700 nm in all cases, which closely approximates to the laser wavelength. Surface compositional analysis by EDX method was done for single scanning mode at the laser fluence of 0.66 J/cm2 . The concentration of oxygen is approximately the same in both irradiated and non-irradiated area. This could be due to a smaller number of pulses, which is not enough for oxide removal, contrary to the single pulse regime with ten accumulated pulses.

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4. Conclusion A study of morphological changes on the AISI 1045 steel surface induced by femtosecond Ti:sapphire laser, operating at 775 nm and in two operational regimes – single pulse and scanning regime, is presented. It was shown that both laser working modes induce morphological changes of the steel. Laser fluence of ˚ = 0.66 J/cm2 (∼3˚th ) was found to be sufficient for inducing surface modifications of the sample in both single pulse and scanning regime. Surface modification was in the form of well defined periodic surface structures and their periodicity is in agreement with the laser wavelength used. In our earlier studies [17,18], we have investigated the surface of the same material under the action of picosecond Nd:YAG laser. Comparative results obtained by two lasers in ultrashort time domain are given in Table 3. The applied fluences are of the same order, which justifies the comparison of two irradiation regimes. It can be clearly seen that the femtosecond laser provides more delicate and precise surface modification, which is of high practical interest in microelectronics, optics, medicine and other fields. The damage level on the metal target is approximately one order smaller than in case of picosecond pulses, there are no hydrodynamic features around the modified areas, and periodic surface structures are formed. Plasma in front of the target was detected in both single pulse and scanning regime. The obtained periodic surface structures, homogeneously distributed across the whole

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modified area, can have a number of important applications in nanotechnology, industry, nuclear complex, etc. All of the stated points out the advantages of using femtosecond laser pulses in the modification of metal surfaces compared to longer pulses (picosecond or nanosecond). Acknowledgments The research was sponsored by the Ministry of Science and Technological Development of the Republic Serbia through project, “Effects of Laser Radiation on Novel Materials in Their Synthesis, Modification, and Analysis”, (project No. 172019). References [1] B.N. Chichkov, C. Momma, S. Nolte, F. von Alvesleben, A. Tunnermann, Appl. Phys. A 63 (1996) 109–115. [2] H.S. Shin, D.K. Chung, M.S. Park, B.H. Kim, C.N. Chu, J. Micromech. Microeng. 20 (2010) 055030. [3] H.Y. Zheng, H. Huang, J. Micromech. Microeng. 17 (2007) N58. [4] Y.C. Lam, D.V. Tran, H.Y. Zheng, Laser Part Beams 25 (2007) 155–159. [5] W. Perrie, M. Gill, G. Robinson, P. Fox, W. O’Neill, Appl. Surf. Sci. 230 (2004) 50–59. [6] M.S. Trtica, B.B. Radak, B.M. Gakovic, D.S. Milovanovic, D. Batani, T. Desai, Laser Part Beams 27 (2009) 85–90. [7] J. Stasic, B. Gakovic, A. Krmpot, V. Pavlovic, M. Trtica, B. Jelenkovic, Laser Part Beams 27 (2009) 699–707. [8] I. Vishniakas, Mechanika 52 (2005) 60–64. [9] Y. Hu, Z. Yao, Surf. Coat. Technol. 202 (2008) 1517. [10] E.G. Gamaly, N.R. Madsen, M. Duering, v.A. Rode, V.Z. Kolev, B. Luther-Davies, Phys. Rev. B 71 (2005) 174405–174416. [11] M.J. Rusu, R. Zamfir, E. Ristici, D. Savastru, C. Talianu, S. Zamfir, A. Molagic, C. Cotrut, J. Optoelectron. Adv. Mater. 8 (2006) 230–234.

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