Femtosecond-laser-induced nanostructure formed on nitrided stainless steel

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Femtosecond-laser-induced nanostructure formed on nitrided stainless steel Naoki Yasumaru a,∗ , Eisuke Sentoku a , Kenzo Miyazaki b , Junsuke Kiuchi c a

Department of Mechanical Engineering, Fukui National College of Technology, Sabae, Fukui 916-8507, Japan Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan c Eyetec Co., Ltd., Sabae, Fukui 916-0016, Japan b

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Article history: Received 23 May 2012 Received in revised form 8 September 2012 Accepted 12 October 2012 Available online xxx Keywords: Femtosecond-laser ablation Nanostructure Stainless steel Nitriding

a b s t r a c t Periodic surface nanostructures formed on nitrided stainless steel with femtosecond (fs) laser pulses were investigated and compared with those of an untreated surface. The nanostructures are formed in the direction perpendicular to the laser polarization, and the variation of the mean spacing D in the nanostructures was examined as a function of the laser fluence F and the shot number N of fs laser pulses. The minimum value of D observed at F around the ablation threshold was 250 nm, which corresponds to approximately 1/3 of the laser wavelength , and D increased with increasing F. The characteristic phenomenon where an irradiated area on the nitrided surface swells was observed. The size of D formed over a broad area for the nitrided surface was 10–20% smaller than that for the untreated surface. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Austenitic stainless steels are used extensively as anticorrosion materials. Surface hardening of these steels by means of plasmanitriding is well known to improve the wear resistance, fatigue resistance and antiseizure properties [1,2]. Further improvement of its tribological properties may be expected when the nitrided surface is non-thermally nanostructured and covered with a solid or liquid lubricant. In fact, we have demonstrated that the frictional performance of diamond-like carbon (DLC) film is greatly improved by nanostructuring the surface with fs-laser pulses and coating it with sputtered molybdenum disulfide (MoS2 ) [3]. This approach for surface modification using fs laser pulses is versatile and attractive for a variety of solid-state materials. This paper reports the experimental results of the periodic nanostructure formation on stainless steel surfaces with fs laser pulses, where we focus our attention on stainless steel that is surface-hardened by plasma-nitriding, for the purposes of developing machine parts having a high resistance to wear, corrosion and fatigue. The results for the nitrided stainless steel surface are compared with those for the untreated one. Further, a broadly nanostructured nitrided surface required for evaluation of the mechanical properties was also investigated. The fs-laser-induced nanostructuring of untreated stainless steel surfaces have recently been reported from a few groups [4–6].

∗ Corresponding author. Tel.: +81 778 62 1111; fax: +81 778 62 3306. E-mail address: [email protected] (N. Yasumaru).

Concerning the nitrides contained in a nitrided layer, the features of fs-laser-induced nanostructures formed on thin film surfaces of TiN and CrN are presented [7,8]. The present results for nitrided and untreated stainless steels are also compared with those reported so far. We discuss the effect of the surface hardened layer of nitrided stainless steel on nanostructuring, based on the results obtained.

2. Experimental procedure Commercially-available, surface-polished austenitic type 304 stainless steel plates of 3 mm thickness were utilized. These plates were subjected to plasma-nitriding by DC glow discharge plasma in a N2 -H2 mixed atmosphere under a reduced pressure of 700 Pa at approximately 773 K [1,2]. The surface of the plasma-nitrided specimen was progressively removed by emery paper down to #1200 and then polished with diamond paste (3 ␮m). The untreated stainless steel had a Vickers hardness (HV) of 300, whereas the nitrided stainless steel had a much higher HV of 1300. These specimens were irradiated in air with linearly polarized, 800 nm, 180 fs laser pulses from a Ti:sapphire chirped-pulse amplification system operated at a repetition frequency of 1 kHz. The fs laser pulses with pulse energies of 150–600 ␮J were focused on the specimen surface using a 200 cm focal-length parabolic mirror to ablate the specimens. The laser fluence on the target was estimated to be F = 0.06–0.24 J/cm2 . The target plate was mounted on a precise X–Y stage. In the first experiment, the stage was fixed and a number of laser pulses (N = 10–500) was superimposed on the plate at 1 kHz (fixed spot irradiation). In the second experiment, the target plate was

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Please cite this article in press as: N. Yasumaru, et al., Femtosecond-laser-induced nanostructure formed on nitrided stainless steel, Appl. Surf. Sci. (2012), http://dx.doi.org/10.1016/j.apsusc.2012.10.076

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continuously translated at a constant speed of 1.6 ␮m/pulse, where the parallel scan at a period of 120 ␮m was made to ablate the surface over an area of 15 mm × 15 mm (scanned spot irradiation). This method of laser scanning is described in detail in our previous studies [3,9]. Approximately 150 shots of fs laser pulses were effectively superimposed on the target point during the scanned spot irradiation, as estimated from the scan conditions. Optical microscopy, field-emission scanning electron microscopy (SEM; Hitachi Ltd., S-4100) and scanning probe microscopy (SPM; Seiko Instruments Inc., Nanopics 2100) were used to examine the morphological changes of the irradiated specimens. The mean spacing D of the periodic nanostructure formed on the specimen surface was determined from the peak position of the power spectrum obtained by the Fourier transform of an SEM image of the nanostructure using image processing software (Image Metrology A/S, SPIPTM ). 3. Results and discussion

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3.1. Nanostructure formed with fixed spot irradiation

Spatial Frequency (1/µm) Fig. 1 shows SEM images near the center of the nitrided stainless steel plate surfaces irradiated with fs-laser pulses at different pulse numbers of N = 100 (a), 150 (b), 300 (c), and 500 (d) at F = 0.08 J/cm2 , just above the ablation threshold. The periodic nanostructures formed are almost oriented in the direction perpendicular to the laser polarization. At N = 100, the nanostructures are partially generated on the focal spot. At N = 150 the nanostructure with the

Fig. 1. SEM images of nitrided stainless steel surfaces ablated by superimposed fslaser pulses with N = 100 (a), 150 (b), 300 (c) and 500 (d) at F = 0.08 J/cm2 (10,000× magnification). The white scale bar means 2.4 ␮m and the arrow indicates the polarization direction of the fs-laser pulses.

Fig. 2. Example of the power spectrum obtained by the Fourier transform for an SEM image of nitrided stainless steel ablated by superimposed fs-laser pulses with N = 300 at F = 0.08 J/cm2 . The arrow indicates the peak position.

period D ∼ 330 nm is uniformly formed. The period decreases to D ∼ 250 nm at N = 300–500. D values were determined from the Fourier power spectra of SEM images of nanostructures. Fig. 2 shows an example of the power spectrum derived from the 6.5 ␮m × 9.5 ␮m area of the SEM image for nitrided stainless steel ablated with N = 300 at F = 0.08 J/cm2 . D of this nanostructure was estimated at 250 nm from the peak position. Fig. 3 shows SEM images of the nitrided stainless steel surfaces ablated at a higher value of F = 0.16 J/cm2 for different shots of N. At this fluence, the periodic nanostructures with a large D of 670 nm were formed at N = 10, which decreased down to 340 nm at N = 50. D gradually increased up to 430 nm with an increase in the number of laser pulses to N = 500. Fig. 3(a) shows fine periodic structures with a small spacing D of ca. 170 nm that are weakly formed and oriented almost in the direction parallel to the laser polarization. This horizontal fine structure was observed more clearly with a larger D of ca. 300 nm on the untreated stainless steel surface nanostructured with the same irradiation condition of N = 10 and F = 0.16 J/cm2 as shown in Fig. 4. These net-like patterns may suggest thermal melting of the specimen surfaces. The periodic nanostructures formed on untreated stainless steel were also investigated under the same irradiation conditions. Fig. 5 shows examples of SEM images near the center of untreated stainless steel surfaces ablated with N = 300 at different incident fluences of F = 0.08 (a), 0.12 (b) and 0.16 J/cm2 (c). As shown in this figure, arrays of the periodic nanostructure are formed like the case of the nitrided stainless steel and the mean spacing D increased from 280 nm to 520 nm with increasing F. Fig. 6 presents the variation of D derived from the Fourier power spectra for nanostructures formed on (a–c) untreated and (d–f) nitrided stainless steel, as a function of N at different incident fluences of F = 0.08 (a,d), 0.12 (b,e) and 0.16 J/cm2 (c,f). With decreasing fluence, the number of pulses for which the periodic nanostructure begins to be uniformly formed increases from 10 to approximately 200, although D of the resulting nanostructure becomes smaller. The minimum value of D was obtained at 0.08 J/cm2 near the ablation threshold: 250 nm for the nitrided stainless steel and 280 nm for the untreated stainless steel at N = 300,

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Fig. 5. SEM images of the untreated stainless steel surfaces ablated by the fs-laser pulses of N = 300 at F = 0.08 (a), 0.12 (b) and 0.16 J/cm2 (c).

where these values correspond to approximately 1/3 of the laser wavelength (/3). When N is small, D becomes large, i.e., at N = 10 and at a higher fluence of 0.16 J/cm2 , D is 670 nm, which is equivalent to 4/5. However, D decreases with increasing N and almost plateaus. Although D of fs-laser-induced periodic structures formed on metals was known to be larger than /2 in many cases [10–12], it was reported that D was 300–500 nm for austenitic hightemperature steel [4], and the nanostructures of D = 310 nm were formed on stainless steel at low pulse energy [5]. Further, Hou et al. reported that two types of nanostructures are formed on stainless steel [6]. That is, short-periodic ripples of D = 310–270 nm were formed at F = 0.07–0.13 J/cm2 and long-periodic ripples were formed at F > 0.15 J/cm2 , where D decreased rapidly from 600 to 410 nm with increasing N, and only fluctuated at D = 410 nm [6].

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Fig. 3. SEM images of nitrided stainless steel surfaces ablated by superimposed fs-laser pulses with N = 10 (a), 50 (b), 200 (c) and 500 (d) at F = 0.16 J/cm2 .

Our results for the untreated stainless steel are almost the same for the minimum D; however, D increases gradually with increasing F from 0.08 to 0.16 J/cm2 at N > 100. The nanostructure for the nitrided stainless steel had smaller D than that of the untreated stainless steel for the same experimental conditions. This indicates that D is reduced for the nitrided steels. The nitrided surface layer has a high HV of 1300; fine CrN nitrides are precipitated in the nitrided layer and compressive residual stress is generated. The nitrided layer contains ca. 20% CrN at most, and the minimum D formed on the CrN film surfaces is reported to be very small at ca. 110 nm at F around the ablation threshold [8]. Therefore, it could be considered that CrN formation induces a reduction in D.

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Fig. 6. Mean spacing (D) of periodic nanostructures formed on (a–c) untreated and (d–f) nitrided stainless steel, measured as a function of the laser shot number (N) at F = 0.08 (a,d), 0.12 (b,e) and 0.16 J/cm2 (c,f).

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Fig. 7. Crater profiles formed on (a,c) untreated and (b,d) nitrided stainless steel, ablated by superimposed fs-laser pulses with N = 300 at F = 0.08 (a,b) and 0.12 J/cm2 (c,d). Fig. 8. SEM images of nitrided stainless steel surfaces ablated by scanned spot irradiation of fs-laser pulses at F = 0.08 (a), 0.10 (b), 0.14 (c) and 0.22 J/cm2 (d).

Fig. 7 shows cross-sectional crater profiles formed on (a,c) untreated and (b,d) nitrided stainless steel, ablated by superimposed fs-laser pulses with N = 300 and at various incident fluences of F = 0.08 (a,b) and 0.12 J/cm2 (c,d) measured using SPM. For the untreated stainless steel, the crater was formed on an irradiated spot and the depth of crater increased with increasing F. The ablation rate increased from 0.3 to 3.1 nm/pulse with increasing F = 0.08 to 0.16 J/cm2 . From the fluence dependence of the ablation rate, the ablation threshold of 0.07 J/cm2 was estimated for the untreated stainless steel. On the other hand, as shown in Fig. 7(b), the phenomenon where an irradiated spot on the nitrided layer swells approximately 150 nm was evident at a low fluence of F = 0.08 J/cm2 , although the nanostructures with a minimum D of 250 nm are formed on the surface of this swelled area. It should be noted that this swelling is a characteristic phenomenon observed for the nitrided surface, and it is generated by a reduction of the compressive residual stress stored in a nitrided layer by fs-laser irradiation; therefore, it may be considered that the compressive residual stress also induces a reduction in D. 3.2. Nanostructure formed with scanned spot irradiation The periodic nanostructures produced over a broad area of the specimen surface with scanned spot irradiation of fs-laser pulses were analyzed in detail using SEM. Fig. 8 shows SEM images of the nitrided stainless steel surfaces ablated by fs-laser pulses at F = 0.08 (a), 0.10 (b), 0.14 (c) and 0.22 J/cm2 (d). The arrays of fine periodic structures formed are oriented almost in the direction perpendicular to the laser polarization, as was the case for fixed spot irradiation, and D increased from 270 to 310 nm with increasing F. Fine debris was considerably observed on the nanostructured surface compared with the case

for fixed spot irradiation shown in Figs. 1 and 3, and the quantity of debris increased with increasing F. The laser intensity at a specific point of a specimen surface changes with migration of the stage; therefore, the formation of this debris may be due to the variation in laser intensity. The nanostructuring for the untreated stainless steel was conducted under the same processing conditions, and it was found that D increased from 290 nm to 360 nm with increasing F and the fine debris was also generated. The D values for the nitrided surface were smaller than those of the untreated surface, similar to the case for fixed spot irradiation. It was observed that the quantity of debris for the nitrided surface was less than that for the untreated surface at F < 0.1 J/cm2 . Fig. 9 shows the variation of D for the periodic nanostructures formed on (a,b) untreated and (c,d) nitrided stainless steel, measured as a function of F, where (a) and (c) were ablated using scanned spot irradiation, and (b) and (d) were ablated with fixed spot irradiation using superimposed shots at N = 300. D increased with increasing F; however, the rate of increase in D resulting from scanned spot irradiation became smaller than that by fixed spot irradiation. In the case of the fixed spot irradiation, D became smaller at the edge of the ablated spot area. In the case of the scanned spot irradiation, although the laser pulse spot with a constant Gaussian intensity distribution operated at 1 kHz passes a specific point of a specimen surface at a speed of 1.6 mm/s, the true laser intensity irradiated on the point changes with migration of the stage and eventually becomes low. The influence of the lowering of the true intensity becomes large at a higher fluence, so that it may be considered that the rate of increase in D formed with scanned spot irradiation becomes smaller than that formed by fixed spot irradiation superimposed at the same fluence.

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4. Conclusions

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We have compared the features of fs-laser-induced nanostructures formed on plasma-nitrided and untreated stainless steel specimens under various processing conditions. Periodic nanostructures were formed in the direction perpendicular to the laser polarization and D for the nanostructure was a minimum of 250 nm around the ablation threshold, which corresponds to a size of ca. /3, and D increased with increasing F. The characteristic phenomenon where an irradiated area on the nitrided layer swells was observed. Nanostructures were uniformly produced over a broad area by scanned spot irradiation, where D increased with increasing F; however, the rate of increase of D formed over a broad area became smaller than that formed using fixed spot irradiation. It should be noted that the size of D was 10–20% smaller for the nitrided surface than that for the untreated surface. The reduction in D may be due to CrN nitrides and the compressive residual stress generated in the nitrided layer.

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Fluence [J/cm2] Fig. 9. Mean spacing (D) of periodic nanostructures formed on (a,b) untreated and (c,d) nitrided stainless steel, measured as a function of the laser fluence, where (a) and (c) were ablated using scanned spot irradiation, and (b) and (d) were ablated using 300 superimposed laser shots.

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influence of nitriding on the periodic nanostructure by scanned spot irradiation was that D became 10–20% than that for the untreated stainless steel, which was to the case for fixed spot irradiation, as shown in

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

M. Hudis, J. Appl. Phys. 44 (1973) 1489. N. Yasumaru, Mater. Trans., JIM 39 (1998) 1046. N. Yasumaru, K. Miyazaki, J. Kiuchi, Appl. Surf. Sci. 254 (2008) 2364. B. Huis in ‘t Veld, M. Groenendijk, H. Fischer, JLMN 3 (2008) 206. L. Qi, K. Nishii, Y. Namba, Opt. Lett. 34 (2009) 1846. S. Hou, Y. Huo, P. Xiong, Y. Zhang, S. Zhang, T. Jia, Z. Sun, J. Qiu, Z. Xu, J. Phys. D 44 (2011) 505401. N. Yasumaru, K. Miyazaki, J. Kiuchi, Appl. Phys. A 76 (2003) 983. N. Yasumaru, K. Miyazaki, J. Kiuchi, Appl. Phys. A 81 (2005) 933. N. Yasumaru, K. Miyazaki, J. Kiuchi, E. Sentoku, Diamond Relat. Mater. 20 (2011) 542. A.Y. Vorobyev, V.S. Makin, C. Guo, J. Appl. Phys. 101 (2007) 034903. A.Y. Vorobyev, C. Guo, J. Appl. Phys. 104 (2008) 063523. E.V. Golosov, A.A. Ionin, Yu.R. Kolobov, S.I. Kudryashov, A.E. Ligacher, Yu.N. Novoselov, L.V. Seleznev, D.V. Sinitsyn, JETP 113 (2011) 14.

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