On the surface topography of ultrashort laser pulse treated steel ...

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1556 J. Vincenc Obona et al. / Applied Surface Science 258 (2011) 1555–1560subsequent grating-assisted surface plasmon–laser coupling. It canprovide a view of LSFL growth [13] on various materials includingmetals, semiconductors and dielectrics for multiple exposures.However, it is still unable to give a full explanation of HSFL creation.In this work, we show that surface features created by laserpulses on alloyed and stainless steel start at low fluence with smallsphere-like objects (bubbles) followed with increasing fluence byelongated HSFL parallel to laser beam polarization vector. At evenhigher fluence the coexistence of orthogonal system of HSFL andLSFL, was observed. Surprisingly, as shown in this paper, HSFLlooses its polarization vector dependency in the presence of LSFLat high fluences. The HSFL becomes perpendicular to sidewalls ofthe LSFL. For explanation of this phenomenon, effects beyond thepurely electromagnetic have to be taken into account. Observationspublished in ref. [6] suggest that also relaxation of the absorbedenergy plays a significant role in the surface objects creation.The objective of this work is to compare two surfaces of steelwith slightly different chemical compositions. Two pulse durations(fs and ps), were used in order to compare influence ofpulse duration and electron–phonon relaxation time on the surfacetopography. Detailed inspections by SEM have been made. Crosssectionsof the surface objects have been prepared by focused ionbeam technique to allow measurement of spatial dimensions of thesurface features.2. ExperimentalIn the experiments two different materials as well as laser pulsegenerating systems were used.A titanium sapphire based laser system (Coherent RegA) witha wavelength of 800 nm was applied for machining of 800H hightemperature alloy. The laser system generates of 210 fs laser pulseswith a Gaussian distribution. The system delivers the pulses at afrequency of 50 kHz. Average powers of 5, 10, 25 and 30 mW wereapplied. The experiment was performed as an exposure in sets ofparallel lines under various conditions. Laser beam scanning speedwas set to 50, 100, 200, 400 and 800 mm/s. The effect of multipleenergy delivery to the same surface area was realized by applying2, 5, 10 and 20 overscans. Diameter of the laser beam was 20 m.800H steel was used as a substrate. Its chemical composition islisted in Table 1. The surface was chemically etched prior to laserprocessing to highlight grain boundaries on the surface [14].An ytterbium-doped YAG (Yb:YAG) system (Triumph TruMicro)with a central wavelength of 1030 nm was used for machining ofstainless AISI 304L steel. Its composition is presented in Table 1. Thelaser system generates laser pulses (repetition rate 50 kHz) witha maximum of 500 mW average power. We used 50 mW powerfor the samples treatment. The duration of Gaussian shaped laserpulse was 6.7 ps in all experiments. A combination of a rotary /2wave plate and a beam splitting cube served as a power attenuator.The laser light was linearly polarized. Manipulation of the beamover the sample was accomplished by a two-mirror galvo-scannersystem (Intelliscan 14 of ScanLab, Germany). A 100 mm telecentricf-Theta lens (Ronar of Linos, Germany) for 1030 nm wavelengthfocused the beam to a circular spot with diameter of 28 m. Theaverage power was measured at the exit of the scanner system by apower meter. In all experiments the normal incidence of laser lightwas used. The processing conditions used in the experiments arelisted in Table 2.Two different microscopy techniques were used to investigatethe sample surfaces treated by ultrashort laser pulses. A PhilipsXL30 SEM equipped with a field emission gun offers a lateral resolutionof the surface objects at a level of few nanometers. The lackof height information of the objects was partially compensated byobservations on tilted surfaces. The exact profile of treated surfaceFig. 1. SEM micrograph of the 800H steel sample treated by a single 210 fs laser pulsewith the 0.1 J pulse energy. The sample is tilted vertically in 55 ◦ from normal view.The centre of application of laser beam is located at the centre of the micrograph.RI, B and HSFL area denote random indents, bubbles and area covered by HSFL,respectively.was observed by SEM after cross-sectioning it using focused ionbeam (Tescan Lyra FIB-Field Emission Gun) with Pt depositedprotection.3. ResultsScanning electron microscopy observations of treated surfaceswere performed on both materials. In order to avoid confusionwhen using the term fluence for overlapped and overscanned laserpulses in the following, we mention rather the delivered energy asa combination of pulse energy, overlap (Eq. (1)) and the numberof overscans instead of overall, accumulated or absorbed fluence.Overlap is defined as:O = D − pD × 100% (1)where D is focus spot diameter and p is the pitch (pulse to pulsedistance) expressed as scanning speed v, divided by the repetitionrate f:p = v fIn the case of 800H steel treated by 210 fs pulses, the lowestenergy delivered on the surface was reached by setting of0.1 J pulse energy and 20% overlap without subsequent overscans.Detailed inspection of Fig. 1 reveals predominance of randomindents (RI) at the margin of the beam and many bubble-like objects(B) within the whole observed area. At the highest energy (centreof Fig. 1), the protrusions started to be organized into aligned linearelongated objects (HSFL).The energy delivered in the experiments was increased in threedifferent ways: (i) by increasing of laser pulse energy, (ii) bydecreasing of laser beam scanning speed at fixed frequency leadingto an increase of overlap of subsequent laser pulses and (iii) by multiplescanning over the same laser tracks (2, 5, 10 and 20 overscans).The laser track presented by SEM pictures in Fig. 2 was obtained byscanning of the 800H sample surface at 0.6 J pulse energy and90% overlap. Fig. 2a shows the end of such laser track. The evolutionof the surface objects as a function of the amount of energycan be observed in the middle of this micrograph from the rightto the left side. Appearance of the surface is continuously changingfrom (i) slightly modified surface at the right laser track marginfollowed by (ii) poorly aligned HSFL (vertically oriented), (iii) wellaligned HSFL and (iv) well aligned HSFL on poorly developed LSFLat the area of the steepest increase of the delivered energy. Finally,the surface gets also (v) well-defined LSFL (horizontally oriented)covered by discontinuous HSFL in area with highest energy input.Close-up of Fig. 2a displayed on Fig. 2b reveals the transition from(2)
J. Vincenc Obona et al. / Applied Surface Science 258 (2011) 1555–1560 1557Table 1Chemical composition of 800H and AISI 304L stainless steel substrates.C (wt.%) Cr (wt.%) Fe (wt.%) Ni (wt.%) Mn (wt.%) Si (wt.%) P (wt.%) S (wt.%) Al (wt.%) Ti (wt.%) Al/Ti (wt.%)AISI 304L 0.03 18.0–20.0 Bulk 8.0–12.0 2.0 1.0 0.045 0.03 – – –800H 0.06–0.1 19.0–23.0 Bulk 30.0–35.0 – – – – 0.15–0.6 0.15–0.6 0.85–1.2Table 2Summary of the processing conditions: from left to right: – laser light wavelength, – pulse duration, ss – spot size, f – repetition rate, P – average laser power, E p – pulseenergy, – incidence angle, v – scanning speed, O – overlap and OS – number of overscans. (nm) (fs) ss (m) f (kHz) P (mW) E p (J) ( ◦ ) v (mm/s) O (%) OS800H 800 210 20 50 5–30 0.1–0.6 90 50–800 20–95 2–20304L 1030 6700 28 50 50 10 90 50, 1000 29, 96 5, 50region covered by HSFL (the right side) to the area of coexistenceof HSFL and LSFL together. The rounded-ridge shape of the HSFL aswell as plenty of globular objects on the surface (Fig. 2b) suggestthe formation of the HSFL from the liquid state when using 210 fslaser pulses.The orientation and spacing of HSFL and LSFL were quantifiedusing 2D Fourier transform of SEM images (not shown here).Spacing and height of the surface features were investigated oncross-sections prepared with the aid of FIB technique. Spacing ofHSFL in the areas with low energy delivered is ranging over anaverage distance ∼200–400 nm, and height ∼40–60 nm (Fig. 3a).The LSFL showed an average periodicity of ∼650 nm and height of200–300 nm (Fig. 3b).The final increase of delivered energy in this experiment wasreached by making an overscan of already existing laser tracks.Fig. 4 shows a wavy surface of the same 800H steel sample treatedwith laser pulse energy 0.6 J, 80% overlap and 10 overscans. Herethe LSFLs are oriented vertically (laser track follows up-downdirection on the micrograph, polarization vector is horizontal) andHSFL are again perpendicular to these. The spacing of LSFL is comparableto the one in Figs. 2 and 3b (∼670 nm). The difference tothe previous energy level (Fig. 2) lies in a stronger development ofLSFL with deeper valleys between them. The HSFL bridges the LSFL.Again, the globular objects are present at the surface and togetherwith round-ridge shapes of HSFL (like frozen liquid) suggest thatthe formation of the liquid state is the last step in process.Another experiment with wavelength of 1030 nm and pulseduration of 6.7 ps (see Section 2) shows at high delivered energy(1 J pulse energy, 96% overlap and 5 overscans) a slightly differentpicture of HSFL and LSFL (Fig. 5). In this case, LSFL (orientedperpendicular to polarization vector) are not continuous butinterrupted quite frequently. The direction of HSFL is not anymorecontrolled by the orientation of the polarization vector butthey are always located inside valleys between the LSFL andoriented perpendicularly to their sidewalls. Therefore, in someplaces where LSFL are interrupted one may observe HSFL withthe direction even perpendicular to the vector of polarization.This observation is the clear evidence that a formation of HSFLstrongly depends on the roughness of treated surface. When LSFLare already present, the direction of HSFL is not controlled bypolarization direction anymore. The observations suggest that aprocess related to melting of the material caused by laser pulseFig. 2. SEM micrograph of the 800H steel sample treated by 210 fs laser pulses with 90% overlap and pulse energy 0.6 J. (a) Areas with different delivered energies aremarked at the end of laser track. (b) Detail from the area with highest delivered energy shows coexistence of small and big ripples. Sample was tilted vertically 55 ◦ from thenormal view.Fig. 3. SEM picture of (a) HSFL and (b) LSFL on cross-sections produced by FIB. Process parameters of the laser track preparation are: (a) 0.6 J pulse energy, 20% overlapand one scan; (b) 0.6 J, 96% overlap and 20 overscans. Images are tilted 55 ◦ in vertical direction. Pt-IBID and Pt-EBID are protection Pt layers applied by so-called ion beaminduced deposition and electron beam induced deposition, respectively.
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