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

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Applied Surface Science 258 (2011) 290– 296

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Applied Surface Science

jou rn al h om epa g e: www.elsev ier .com/ locate /apsusc

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

J. Stasic a,∗, B. Gakovic a, W. Perrieb, K. Watkinsb, S. Petrovic a, M. Trticaa

a VINCA Institute of Nuclear Sciences, University of Belgrade, P.O. BOX 522, 11001 Belgrade, Serbiab 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 2011Received in revised form 10 August 2011Accepted 10 August 2011Available online 18 August 2011

Keywords:SteelFemtosecond laserLaser processingPeriodic 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 fem-tosecond lasers. These laser systems enable highly precise modifications, which are very important fornumerous applications of metals. The effects of a Ti:sapphire femtosecond laser with the pulse durationof 160 fs, operating at 775 nm wavelength and in two operational regimes – single pulse (SP) and scanningregime, on a high quality AISI 1045 carbon steel were studied. The estimated surface damage thresholdwas 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 ofperiodic 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 sur-face modification of different materials, including metals, is ofgreat interest. With the appearance of femtosecond lasers in themid nineteen–eighties, material/metal surface texturing gained awhole new dimension. From that moment, it became possible totalk about three regimes of modification – femto- (fs), pico- (ps)and nanosecond (ns) regime [1]. Short-pulse laser processing ofdifferent materials gives unique benefits over other available tech-niques therefore it is widely used in microelectronics, optics andbiomedicine. Microelectronic applications include laser masking[2], machining of micro-holes [3], etc. Focusing only on the fs regimeof material surface variation, it can be stated that this regime pos-sesses advantages in relation to ps as well as ns regime. Generally,in this regime of modification: (i) the heat transfer to the mate-rial is drastically reduced; (ii) the transition from solid to vapourphase is quite possible, and finally (iii) high precision of materialmodification 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 ofvarious high quality steels is of high importance. Pulsed lasers usedfor this purpose were ruby [8], Nd:YAG/Nd:YVO4 [9–11], excimer

∗ Corresponding author. Tel.: +38 1113408779; fax: +38 1113408224.E-mail addresses: [email protected] (J. Stasic), [email protected] (B. Gakovic),

[email protected] (W. Perrie), [email protected] (K. Watkins),[email protected] (S. Petrovic), [email protected] (M. Trtica).

[12], TEA CO2 [13] and Ti:sapphire laser [14]. AISI 1045 steel iswithin the class of steels with medium carbon content. The pres-ence of carbon in steel improves its hardness and tensile strength[15]. Due to these and other excellent physical and chemical prop-erties AISI 1045 steel is widely applied from machinery to nuclearindustry. Particularly, in the nuclear industry, it can be used forlow and high-pressure turbine sections, reactor vessels, etc. [16].Interaction of AISI 1045 steel with Ti:sapphire laser beam pulsed inthe femtosecond time domain has not been described in the litera-ture unlike picosecond pulses [17,18]. Our emphasis in the presentpaper is on studying the effects of a femtosecond laser emittingin the near-infrared (775 nm) on AISI 1045 steel surface. Specialattention was paid to morphological surface changes (especiallythe formation of specific periodic structures) of AISI 1045 steel asa function of two operational regimes – single pulse and scanningregime.

2. Experimental

Surface of the AISI 1045 steel was prepared by a standard met-allographic procedure. The samples were mechanically polished(first by using SiC grinding paper (1200–4000 grit) and finallyby using diamond paste (1–0.04 �m)), ultrasonically cleaned anddried in hot air. Prior to laser irradiation they were cleaned inmethanol. Samples had a round shape with 15 mm diameter and2 mm thickness. Their surface was polished to an average roughnessof about 50 nm. The laser employed for irradiation was femtosec-ond laser system, model Clark-MXR CPA2010, based on a chirpedpulse amplification [19]. A mode-locked, diode pumped Erbium

0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2011.08.052


J. Stasic et al. / Applied Surface Science 258 (2011) 290– 296 291

Table 1Typical parameters of Ti:sapphire laser used during irradiation of a steel target AISI1045.

Laser wavelength (�) 775 nmLaser pulse duration (�p) 160 fsLaser pulse energy (Ep) • Ep1 = 10.75 �J

• Ep2 = 6.7 �J• Ep3 = 3 �J

Laser fluence (˚) • ˚1 = 2.37 J/cm2

• ˚2 = 1.48 J/cm2

• ˚3 = 0.66 J/cm2

Laser intensity (I) • I1 = 1.48 × 1013 W/cm2

• I2 = 0.93 × 1013 W/cm2

• I3 = 0.41 × 1013 W/cm2

Laser pulse repetition rate 1 kHzOperational regimes (beamscan. speed (s))

• Single pulse regime (s – 100 mm/s)• Scanning regime (s – 10 mm/s)

Polarization Linear, horizontalMode structure TEM00

doped ring fibre laser at 1550 nm, with the output frequency dou-bled to 775 nm in periodically poled lithium niobate, was used toseed a Ti:sapphire regenerative amplifier (160 fs, 1 mJ, 1 kHz). Beamintensity spatial profile is near Gaussian with a measured 3D Gaus-sian fit parameter ∼0.87. The beam is slightly elliptical and showssome residual astigmatism, probably due to cavity design incor-porating a Brewster cut Ti–sapphire crystal in the regenerativeamplifier [5]. The output from the femtosecond laser system passedthrough a laser safety shutter, a pick-off beam splitter for auto-correlation and then through an attenuator (�/2 plate and Glanlaser polarizer). The beam was further reflected from a periscope,entered a scanning galvo head (GSI Lumonics) and focused on thesample surface by an f-theta lens with a focal length f = 100 mm,AR coated for 775 nm. Transmission through the scan head wasmeasured to be T > 97% and 1/e2 focused spot size at the substratesurface was � ∼ 30 �m. The sample was mounted on a 4-axis (x, y, z,�) motion control system (Aerotech, 0.5 �m repeatability) and thesample surface was carefully brought close to the lens focal plane(z-axis). Focal spot scanning was controlled through a graphicalsoftware interface, (SCAPS GmbH). The angle of incidence (AOI) ofthe laser beam with respect to the sample surface was near normal(�� ∼ ± 3◦) close to the center of the working field of the scan lenswhile the sample could also be translated horizontally (x, y) to min-imise the AOI. Irradiation was carried out in air, at a pressure of 1013mbar and standard relative humidity. Experiments were carriedout at varying pulse energies with scan speeds of s = 100 mm/s and10 mm/s respectively. High speed scanning at s = 100 mm/s resultedin single pulse exposure on the sample surface while lower speedscanning at s = 10 mm/s scan speed resulted in a pulse overlap ofabout 3. Typical parameters used for irradiation of AISI 1045 targetby femtosecond Ti:sapphire laser are given in Table 1.

Various analytical techniques were used for characterizationof AISI 1045 steel prior to and after femtosecond laser irradi-ation. Surface morphology was monitored by optical (OM) andscanning electron (SEM) microscopy. SEM was coupled to anenergy-dispersive analyzer (EDX) for determining the sample sur-face composition (analysis depth 2–3 �m, acceleration voltage20 kV). A white light surface profilometer (WYKO NT1100) wasused for measuring the geometry of ablated/damaged area.

3. Results and discussion

The surface of the AISI 1045 steel had a typical silver–greymetallic colour before irradiation. EDX elemental analysis of anon-irradiated sample surface showed the following content: iron∼90.9 wt.%, balanced to a 100% by C (7.5 wt.%), O (∼0.9 wt.%) andMn (∼0.7 wt.%). Complete elemental analysis was normalized.

Investigations of the morphological changes induced by fem-tosecond laser on the AISI 1045 steel target have shown theirdependence on laser beam characteristics, primarily on the laserfluence, peak power density, number of accumulated pulses andwavelength.

Morphological features of the AISI 1045 steel surface wereobserved at two operational regimes, i.e. single pulse and scan-ning regime. Both regimes induced significant surface changes ofthe steel surface. The results of the induced surface modificationare presented in the following section.

3.1. Single pulse regime

Initially, the effect of femtosecond laser on the target wasstudied in the single pulse regime, at the laser beam scanspeed of 100 mm/s. Three different pulse energies were usedin these experiments, Ep1 = 10.8 �J, Ep2 = 6.7 �J and Ep3 = 3.0 �J.The corresponding fluences, calculated using 1/e2 Gaussian beamradius, were ˚1 = 2.37 J/cm2, ˚2 = 1.48 J/cm2, ˚3 = 0.66 J/cm2, andthe intensities I1 = 1.48 × 1013 W/cm2, I2 = 0.93 × 1013 W/cm2, andI3 = 0.41 × 1013 W/cm2, respectively (Table 1).

Fig. 1 shows SEM micrographs of the target before and afterirradiation with the same number of pulses (N = 10) and differentfluences ˚1, ˚2 and ˚3. It can be concluded that the damage depthon the surface of a solid target increases with higher energy den-sity (Fig. 1A1–A3). After the action of 10 pulses of the lowest laserfluence, the surface is homogeneously textured due to the appear-ance of so called periodic surface structures (PSS), Fig. 1A3 and B3.With increasing fluence, these structures are also present, but aremuch less well defined due to increasing contribution from surfacemelting as fluence increases well above the modification thresholdfluence, even with femtosecond pulses. The appearance of growingtiny globules indicates melting along with a loss of fringe finesseas fluence rises, hence the fringes show a deformed shape in thecenter, Fig. 1B1, B2, C1 and C2.

The phenomenon of periodic surface structures is a subject ofresearch for more than two decades. These investigations wereespecially intensified in the last ten years. The mechanism of theirformation is very complex and generally depends on the numberof parameters, such as the fluence used, the number of accumu-lated laser pulses, polarization of the laser beam, etc. One of thepresumptions is that the PSS occur due to the interference of theincoming laser beam with the so called surface waves scatteredoff the imperfections on the material and running along its sur-face [17]. These structures were also observed in our earlier workwith picosecond laser pulses acting on the same target [17,20]. Thedistance measured between consecutive PSS observed in this workwas ∼750–800 nm (Fig. 1), which is approximately in accordancewith the laser wavelength. The periodicity of such structures �is supposed to depend directly on the laser wavelength, Eq. (1)[21,22]:

� ≈ �laser

cos �ip − polarization (1)

� ≈ �laser

1 ± sin �is − polarization

where �laser denotes the laser wavelength, and �i denotes the inci-dent angle of the beam. In this case, the beam had a linear horizontalpolarization, and the angle of incidence with respect to the samplesurface was near normal, �� ∼ ± 3◦. The measured values for theperiodicity in this work agree with the predictions of Eq. (1) in caseof p-polarized beam.

Based on the micrographs of the damages after the same num-ber of pulses and different fluences, 1/e2 diameter of the Gaussianbeam, as well as the damage threshold of the AISI 1045 steel


292 J. Stasic et al. / Applied Surface Science 258 (2011) 290– 296

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 isdefined as the minimum laser energy/fluence necessary for cre-ating detectable damage on the material surface. These values canbe evaluated using the method given in [23,24]. In this calculation,square diameter of the damage caused by laser action is given as,

D2 = 2ω20 ln

(Fpk

0Fth

)= 2ω2

0 ln(

Ep

Eth

)(2)

where Fpk0 , ω0 and Ep are fluence, laser beam radius and laser pulse

energy, respectively. We used 1/e2 laser beam radius in this work,while Fpk

0 is obtained from the equation,

Fpk0 = 2Ep

ω20

(3)

From three different pulse energies of 4, 6.7 and 10.75 �J, andcorresponding 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. Usingthe slope of the linear approximation of this curve, 1/e2 beam radiusis determined. This value, along with the free term (y-intercept),finally gives the damage threshold pulse energy. The calculatedbeam radius was ω0 = 17.0 ± 0.1 �m and the threshold fluenceFth = 0.22 ± 0.02 J/cm2. There is no data available in the literatureon the damage threshold of carbon steel affected by femtosecond

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

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



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 sur-face 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 occuralready after 5 pulses in the center, as well as periphery, i.e. thetexturing is homogeneous across the whole modified area. In caseof N = 500 pulses, PSS are deformed, and the central zone is ablated.

When ultrashort laser pulse acts on a metal target, various phys-ical processes are generated. The ablation is conducted mostly invapor and plasma phases, with the following scenario: (i) the laser

energy is first absorbed by the free electrons. Shortly after theincoming radiation is absorbed, thermalization of the electron sub-system takes place. Considering the time scale, the absorption oflaser radiation as well as successive thermalization has a durationof the order of several hundreds of femtoseconds. (ii) The next stepcomprises the transfer of energy from the electron subsystem tothe lattice. The energy transfer in this case is also rapid and occurson a picosecond scale. Further, (iii) this results in a formation ofdominant vapor and plasma phases (on the same time scale) fol-



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

lowed by their fast expansion in a surrounding ambient. During allof these processes, thermal diffusion into the bulk is negligible [27],and hydrodynamic effects can be observed only after accumulationof large number of pulses (Fig. 2E2). Generally, depending on thelaser energy/fluence used in irradiation by femtosecond laser, themechanisms of modification and ablation can be (a) normal evap-oration, (b) phase explosion, (c) spallation and (d) fragmentation[28,29]. Irradiation of the AISI 1045 steel resulted mostly in a directsolid–vapor or solid–plasma transition, including probably the phe-nomenon of phase explosion [30]. Melting in the center of modifiedarea occurring after the action of few hundreds of pulses or moreis likely due to heat accumulation in the sample and re-irradiationfrom the evolved plasma plume.

Ablation/modification with femtosecond (ultrashort) laserpulses of the energy close to the damage threshold leads to a reduc-

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 aremarked.

tion of hydrodynamic effects and precise material modification. Thereason for this is that the thermal diffusion length ((�a)1/2, where is a heat diffusion coefficient and �a duration of ablation pro-cess) is smaller than the optical penetration depth (˛−1, where isoptical absorption coefficient). Heat diffusion is then significantlyrepressed, which enables higher precision and minimal heat effectin the material [27]. Laser penetration depth, in case of laser–targetinteraction 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 occur-rence of plasma in front of the target. Contrary to the metal vaporcloud, which develops on the time scale of tens to hundreds ofpicoseconds after the ablation, recent investigations showed thatthe 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.)



Table 2EDX elemental analysis of the AISI 1045 surface. Measuring locations are given inFig. 5.

Spectrum (wt.%) C O Mn Fe Total

Spectrum 1 7.5 0.9 0.7 90.9

∼100

Spectrum 2 6.4 0.6 0.5 92.5Spectrum 3 6.2 0.7 0.4 92.7Spectrum 4 7.6 0.8 0.6 91.0Spectrum 5 5.8 0.4 0.6 93.2Spectrum 6 5.9 0.0 0.5 93.6Spectrum 7 6.1 0.5 0.5 92.8Spectrum 8 8.4 0.5 0.8 90.3Spectrum 9 6.9 0.5 0.4 92.2Spectrum 10 5.7 0.6 0.7 93.0Spectrum 11 5.9 0.9 0.6 92.7Spectrum 12 8.6 0.5 0.5 90.5

this, it can be concluded that the plasma in front of the target cannotinfluence the absorption and propagation of laser radiation in caseof femtosecond laser, since the pulse ends before the formation ofplasma.

The dependence of damage area on the number of deliveredpulses for ˚3 = 0.66 J/cm2 is shown in Fig. 4. At lower number ofaccumulated pulses, Fig. 4, the beam penetrates deeper and the spotsize increases up to N ∼ 100 pulses when the saturation occurs.

Irradiation of the AISI 1045 steel target was performed in airatmosphere, therefore the changes in oxygen content on the sur-face are expected. Monitoring of the target constituents, includingoxygen, before and after the action of 10 pulses at the laser flu-ence ˚1 = 2.37 J/cm2, is shown in Fig. 5. EDX method was used forthis 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 concentrationdecreases from periphery towards the center (Table 2, Fig. 5), whilethe iron content increases. This result implies a relatively efficientremoval of oxide from the steel surface.

3.2. Scanning regime

The surface of the AISI 1045 steel after irradiation with fs laserfluence of 0.66 J/cm2, at a repetition rate 1 kHz and laser beam scanspeed 10 mm/s, is shown in Fig. 6. Taking into account the scan-ning speed, approximately 3 laser spots overlap at every point ofthe target. Dimensions of the scanned areas were approximately1 mm × 1 mm. PSS were formed in both single and triple scanningmode. In case of triple scanning, the structures align along the onesalready formed in previous passages of the laser beam, and they aremore prominent [32].

Investigations of the morphological features induced by laseron AISI 1045 steel surface showed, among other, their dependenceon the characteristics of laser beam [17], especially the number ofpulses (N) and the number of laser beam passages. Generally, theinteraction of femtosecond laser with AISI 1045 steel, at the fluenceof 0.66 J/cm2, resulted in a creation of modified area without thepresence of surrounding material and hydrodynamic shapes.

Periodic surface structures obtained on the steel are, in bothcases (single pulse and laser scanning regime), oriented in the sameway – perpendicularly to the laser beam linear polarization direc-tion (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 forsingle scanning mode at the laser fluence of 0.66 J/cm2. The con-centration of oxygen is approximately the same in both irradiatedand non-irradiated area. This could be due to a smaller numberof pulses, which is not enough for oxide removal, contrary to thesingle pulse regime with ten accumulated pulses.

Table 3Comparative results obtained on the AISI 1045 target by picosecond and femtosec-ond laser pulses.

ps laser(1064 nm)

fs laser

Pulse duration 40 ps 160 fsPulse energy ∼25 mJ ≤11 �JFluence ∼10 J/cm2 ≤2.5 J/cm2

Irradiance ∼1011 W/cm2 ∼1013 W/cm2

Damage shape Craters,depth ∼ 100 �m

Damages andcraters,depth ≤ 20 �m

Damage diameter 300 ± 10 �m ≤40.0 ± 0.2 �mHydrodynamic

(HD) effectsProminent HDfeatures

Almost no HDfeatures

Structure of thecentral damagearea

Granularstructure

PSS, theoccurrence ofablation withhigher numberof pulses

Threshold fluence 0.30 ± 0.02 J/cm2 0.22 ± 0.02 J/cm2

EDX Lower oxygencontent in thecenter, higheron theperiphery(droplet)

Single pulseregime:decrease of theoxygen contentfrom peripherytowards thecenter;Scanningregime:increasedoxygencontent,decreasedconcentrationof iron

Periodic surfacestructures (PSS)

No PSS PSS homoge-neouslydistributedacross thewholemodified area

Plasma Occurrence ofplasma in frontof the target

Occurrence ofplasma in frontof the target

4. Conclusion

A study of morphological changes on the AISI 1045 steel sur-face induced by femtosecond Ti:sapphire laser, operating at 775 nmand in two operational regimes – single pulse and scanning regime,is presented. It was shown that both laser working modes inducemorphological changes of the steel. Laser fluence of = 0.66 J/cm2

(∼3˚th) was found to be sufficient for inducing surface modifi-cations of the sample in both single pulse and scanning regime.Surface modification was in the form of well defined periodic sur-face structures and their periodicity is in agreement with the laserwavelength used.

In our earlier studies [17,18], we have investigated the sur-face of the same material under the action of picosecond Nd:YAGlaser. Comparative results obtained by two lasers in ultrashort timedomain are given in Table 3. The applied fluences are of the sameorder, which justifies the comparison of two irradiation regimes.It can be clearly seen that the femtosecond laser provides moredelicate and precise surface modification, which is of high practi-cal interest in microelectronics, optics, medicine and other fields.The damage level on the metal target is approximately one ordersmaller than in case of picosecond pulses, there are no hydrody-namic features around the modified areas, and periodic surfacestructures are formed. Plasma in front of the target was detectedin both single pulse and scanning regime. The obtained periodicsurface structures, homogeneously distributed across the whole



modified area, can have a number of important applications innanotechnology, industry, nuclear complex, etc. All of the statedpoints out the advantages of using femtosecond laser pulses in themodification of metal surfaces compared to longer pulses (picosec-ond or nanosecond).

Acknowledgments

The research was sponsored by the Ministry of Science andTechnological Development of the Republic Serbia through project,“Effects of Laser Radiation on Novel Materials in Their Synthesis, Mod-ification, and Analysis”, (project No. 172019).

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