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Applied Surface Science 270 (2013) 49– 57

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

j our nal ho me p age: www.elsev ier .com/ loc ate /apsusc

ffect of dry and wet ambient environment on the pulsed laser ablation ofitanium

isar Ali ∗, Shazia Bashir, Umm-i-Kalsoom, Mahreen Akram, Khaliq Mahmoodentre for Advanced Studies in Physics, GC University Lahore, Pakistan

r t i c l e i n f o

rticle history:eceived 13 August 2012eceived in revised form0 November 2012ccepted 8 December 2012vailable online 11 January 2013

eywords:itaniumaser ablationmbient environmenturface morphology

a b s t r a c t

Surface and structural properties of the laser irradiated titanium targets have been investigated under dryand wet ambient environments. For this purpose KrF Excimer laser of wavelength 248 nm, pulse durationof 20 ns and repetition rate of 20 Hz has been employed. The targets were exposed for various number oflaser pulses ranging from 500 to 2000 in the ambient environment of air, de-ionized water and propanolat a fluence of 3.6 J/cm2. The surface morphology, chemical composition and crystallographical analysiswere performed by using Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy(EDS) and X-ray Diffraction (XRD), respectively. For both central and peripheral ablated areas, significantdifference in surface morphology has been observed in case of dry and wet ambient conditions. Largesized and diffused grains are observed in case of dry ablation. Whereas, in case of wet ablation, small sized,and well defined grains with distinct grain boundaries and significantly enhanced density are revealed.This difference is ascribed to the confinement effects of the liquid. The peripheral ablated area showsredeposition in case of dry ablation whereas small sized grain like structures are formed in case of wetablation. EDS analysis exhibits variation in chemical composition under both ambient conditions. When

the targets are treated in air environment, enhancement of the oxygen as well as nitrogen content isobserved while in case of de-ionized water and propanol only increase in content of oxygen is observed.X-ray diffraction analysis exhibits formation of oxides and nitrides in case of air, whereas, in case ofde-ionized water and propanol only oxides along with hydrides are formed. For various number of laserpulses the variation in the peak intensity, crystallinity and d-spacing is observed under both ambient conditions.

. Introduction

Pulsed laser ablation is a successful and rapidly progressingechnique for material processing and device fabrication [1]. Inecent times, material surface processing in liquid confined envi-onment is becoming popular and emerging area of research [2,3].he main advantages of liquid assisted ablation includes effectiveooling, useful chemical reactions, highly confined plasma pres-ure, debris and pollution free modified surface [2,3]. Bussoli et al.4] investigated laser induced morphological changes on titaniumurface in the air ambient and reported applications of nanosecondnd picosecond pulsed laser ablation of titanium as an implant inedical industry. Schwickert et al. [5] studied the process of laser

ydriding of titanium that leads to the development of TiH2. Theim of the study was to compare the behavior of titanium after

rradiation in hydrogen atmosphere with previous studied processf laser nitriding. Mahmood et al. [6] investigated the effects ofaser treatment on the microstructure, yield stress (YS), ultimate

∗ Corresponding author. Tel.: +92 3344039791.E-mail address: chnisarali@gmail.com (N. Ali).

169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2012.12.049

© 2012 Elsevier B.V. All rights reserved.

tensile stress (UTS) and microhardness polycrystalline titaniumtarget. They found an increase in microhardness with increasingnumber of laser pulses but changes in YS and UTS with laser shotswere found to be anomalous. The growth of nanostructures on thetungsten surface after ablation in liquid environment, improves itsthermionic properties [7] due to reduction in its work function.

The motivation of the present work is to explore the effect ofambient environment (air, de-ionized water and propanol) on sur-face and structural modification of the titanium targets after laserablation. For this purpose titanium targets are ablated with KrFExcimer laser for various number of laser pulses in ambient envi-ronments of air, de-ionized water and propanol. Scanning ElectronMicroscope (SEM) is used to investigate the surface morphology ofablated targets. Energy Dispersive X-ray Spectroscope (EDS) and X-ray Diffractometer are employed to correlate the surface featureswith the change in chemical composition, and crystallinity.

2. Experimental details

Pulsed laser ablation of titanium target was performed by usingKrF Excimer laser (EX 200/125-157 GAM Laser, USA) with the cen-tral wavelength of 248 nm, pulse duration of 20 ns, repetition rate

50 N. Ali et al. / Applied Surface Sc

ohf

wug1

flaetfiFmmfLl

I

wt

fgf

heating also generates thermal and structural stresses related to the

Ffl

Fig. 1. The schematic diagram of experimental setup.

f 20 Hz and pulse energy of 70 mJ. The unfocused rectangular beamaving size 11 mm × 7 mm is focused on the targets by using 50 cm

ocal length lens.Rectangular shaped titanium targets with length of 15 mm,

idth of 10 mm and thickness of 2 mm were grinded, polished andltrasonically cleaned with acetone for 30 min. The prepared tar-ets were placed in quartz cuvette of height of 45 mm and width of0 mm. The schematic of the experimental setup is shown in Fig. 1.

All samples were irradiated with laser 5 cm away from the focus,or a fixed laser fluence of 3.6 J/cm2. The numbers of overlappingaser pulses used for the exposure of targets were 500, 1000, 1500nd 2000. Three set of experiments were performed for the ambi-nt environment of (a) air, (b) de-ionized and (c) propanol underhe same laser parameters. For each exposure quartz cuvette waslled with fresh liquid in case of de-ionized water and propanol.or 248 nm wavelength, the percentage laser energy absorption iseasured by measuring laser pulsed energy before and after trans-ission from both liquids. It comes out to be 6% for propanol and 2%

or de-ionized water. By using following equation of Beer–Lambertaw [8] we calculated the values of absorption co-efficient for bothiquids i.e. de-ionized water and propanol:

(x) = Ioe−˛x (1)

here Io is the intensity of incident beam (W/m2), ̨ is the absorp-ion co-efficient (m−1) and x is the thickness of liquid film.

By substituting values of I(x) = 0.98 for de-ionized water and 0.94

or propanol; x = 4 mm for both liquids (fixed) in above equation weet the values of absorption coefficients that are 5.05 × 10−2/mmor de-ionized water and 1.5 × 10−2/mm for propanol.

ig. 2. SEM images revealing the surface morphology of (a) unirradiated and (b) Excimeruence of 3.6 J/cm2, wavelength of 248 nm, pulse duration of 20 ns and repetition rate of

ience 270 (2013) 49– 57

The surface morphology of ablated targets was investigatedusing Scanning Electron Microscope (SEM-JEOL JSM-6480 LV).Energy Dispersive X-ray Spectroscopy (EDS-S3700N) was used forchemical analysis. X-ray diffractometer (X’Pert PRO (MPD)) wasemployed to determine the crystallographic structure and phaseanalysis.

3. Results and discussion

Fig. 2(a) shows the SEM image of un-irradiated titanium surface.Significantly modified surface of titanium is observed after irradi-ation with 500 accumulative laser pulses at 3.6 J/cm2 as shown inFig. 2(b). The width of ablated area is measured from SEM image for500 pulses is 1.062 mm which increases with increase in number ofpulses. For 2000 number of pulses its value is 1.187 mm. Accumu-lation of resolidified material can be clearly seen at the peripheralablated area. The shock-wave propagation and the recoil momen-tum offered by the vapor plume expulsion causes the surfacedepression in the central ablated area and generate the shoulderat the periphery.

Fig. 3(a)–(d) reveals SEM images of the central ablated area oftitanium in an ambient environment of air for various number ofpulses of (a) 500, (b) 1000, (c) 1500 and (d) 2000. It demonstratesthe appearance of grains with diffused boundaries. Heat generatedduring laser irradiation is responsible for this grain growth [6].The formation of grain-like surface structures involves two mech-anisms: (i) melting and recrystallization of metal resided in theirradiated zone and (ii) cooling and crystallization of metal trans-ferred by hydrodynamic forces owing to intense boiling [9]. Themolten metal is ejected from the central irradiated area and coolson a relatively colder target area. Thus, the surface and under-surface layers exhibit residual thermal tension and compressionstresses, respectively. The high rates of heating and cooling resultinto the tremendous temperature gradients. This is related to thelocal heating, and the plasma-dynamic flows. This is responsiblefor the generation of residual stresses in the surface layer e.g. thedeformation owing to the action of the shock waves. The localized

changes in volume accompanying the phase transformations. Thelocalized heating and cooling results in preferential crystallization,grain growth process and texture development [9]. The number

laser irradiated titanium, under an ambient environment of air, by 500 pulses at a20 Hz.

N. Ali et al. / Applied Surface Science 270 (2013) 49– 57 51

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a

various number of pulses i.e. (a) 500, (b) 1000, (c) 1500 and (d) 2000.Grain growth with sharp boundaries is revealed in Fig. 5 as com-pared to Fig. 3 (air ambient). Fig. 5(a) represents the developmentof grains with wide and distinct boundaries for 500 accumulative

ig. 3. Magnified SEM images of central ablated area revealing the variation in surfaf air for various number of overlapping laser pulses of (a) 500, (b) 1000, (c) 1500, (

f overlapping pulses, heating and environmental conditions areherefore the controlling parameters for the size and orientationf grain growth [9]. Cavities and cracks are also observed alonghe grain boundaries. No significant difference in the surface mor-hology of the grains is observed for increasing number of pulses.owever, both the number and size of cavities formed decreasesith increasing number of laser pulses. Cavities are formed by the

elease of adsorbed gases underneath the surface during irradia-ion, due to volume boiling [10]. Decrease in number of cavitiess observed with increase in accumulative number of pulses up to000. Refilling of cavities by shock liquefied and melted materialauses reduction in the number density and size of cavities [11].

Fig. 3(e) shows SEM images of the peripheral ablated area ofitanium in an ambient environment of air for 2000 number of laserulses. Accumulation of resolidified material over the surface islearly seen. The inset of Fig. 3(e) shows the presence of droplets.he droplets are attributable to hydrodynamical sputtering [12].

Fig. 4 shows the overall surface area of the sample exposed to00 accumulative laser pulses in de-ionized water. Width of theblated area in case of de-ionized water varies from 1.250 mm to.570 mm for varying number of pulses from 500 to 2000. Crater

ormed in de-ionized water environment is shallower and mucharger in size without the redeposition around the crater.

Fig. 5(a)–(d) reveals magnified SEM images of the central ablatedrea of titanium in an ambient environment of de-ionized water for

rphology of titanium after Excimer laser irradiation under an ambient environment0 and (e) peripheral ablated area for 2000 number of pulses.

Fig. 4. The SEM image of titanium sample exposed to 500 overlapping laser pulsesin an ambient environment of de-ionized water.

52 N. Ali et al. / Applied Surface Science 270 (2013) 49– 57

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energy loss in propanol (6%) is greater as compared to de-ionizedwater (2%) and air. The more laser energy absorption by propanolcan be one reason of wavy and spiral shaped modified morphologyas compared to clean crater-formation in case of air and de-ionized

ig. 5. Magnified SEM images of central ablated area revealing the variation in surfaf de-ionized water for various number of overlapping laser pulses of (a) 500, (b) 1

umber of pulses. Pores and cracks are also seen across the grainoundaries. For increasing number of pulses up to 1000, grain size

ncreases and grain boundaries becomes diffusive with reductionn number density of pores (Fig. 5(b)). For further increase in num-er of laser pulses up to 1500, grain size increases along with theppearance of cracks and pores on the grains and more diffusedrain boundaries (Fig. 5(c)). At 2000 number of laser pulses theignificant decrease in size and increase in the compactness withncrease in number density of grains is observed (Fig. 5(d)). Afteraser irradiation, rapid heating and cooling of titanium target isesponsible for grain growth and cracks [11]. Volume boiling duringrradiation may cause the formation of pores due to release of gasesnderneath the surface [13]. Shock liquefied material refills theavities and causes reduction in their number density and size [14].

Fig. 5(e) is the magnified view of the peripheral ablated areaf titanium sample in de-ionized water for 2000 number of laserulses. Grain growth is observed with distinct and wider grainoundaries due to more diffusion [6] (confirmed by EDS analysis).

Fig. 6 is the overall view of sample exposed to 500 number ofaser pulses in propanol medium. For propanol width of the ablatedrea varies from 1.187 mm to 1.875 mm with increasing number of

ulses from 500 to 2000. Here we observed wavy and spiral shapedodified morphology in contrast to elliptical shaped crater forma-

ion in case of air and de-ionized water. No redeposited materials seen at the peripheral ablated area. The measured percentage

rphology of titanium after Excimer laser irradiation under an ambient environment) 1500, (d) 2000 and (e) peripheral ablated area for 2000 number of pulses.

Fig. 6. The SEM image of titanium sample exposed to 500 overlapping laser pulsesin an ambient environment of propanol.

N. Ali et al. / Applied Surface Science 270 (2013) 49– 57 53

F ace mop 0, (d)

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ig. 7. Magnified SEM images of central ablated area revealing the variation in surfropanol for various number of overlapping laser pulses of (a) 500, (b) 1000, (c) 150

ater. The wavy and spiral shaped modified morphology can bessigned to the formation of excessive hydrides which are totallybsent in case of air and are present in small amount in case ofe-ionized water.

Fig. 7(a)–(d) represents the magnified view of the central ablatedrea of the sample treated in propanol for various number of laserulses of (a) 500, (b) 1000, (c) 1500 and (d) 2000. At the centralblated area compact and high density globular grain like struc-ures are observed with distinct grain boundaries at 500 numberf laser pulses. For increasing number of laser pulses up to 1500,rain size significantly increases. The grain boundaries also becomeore clear, distinct and wider, which is attributable to diffusion of

xygen/hydrogen across the grain boundaries [6]. For 2000 overlap-ing pulses, grains size increases further. The appearance of largeumber of pores and cracks along the grain boundaries are seen.

Fig. 7(e) is the magnified view of the peripheral ablated areaf titanium sample in propanol medium for 2000 number of laserulses. Cracks and cavities can be clearly seen.

Comparison of the three media, air (Figs. 2 and 3), de-ionizedater (Figs. 4 and 5) and propanol (Figs. 6 and 7) shows significantissimilarities in surface morphology of ablated titanium. Shallowut large size crater is observed in case of wet ablation. No rede-osited material is observed at the periphery in case of wet ablation.arge sized diffused grains are observed in case of dry ablation,

hereas, in case of wet ablation small size well defined grains withistinct grain boundaries are observed.

When high intensity laser beam is used to irradiate solidarget, high pressure plasma is produced and shock waves are

rphology of titanium after Excimer laser irradiation in an ambient environment of 2000 and (e) peripheral ablated area for 2000 number of pulses.

generated. These shock waves expand adiabatically at supersonicvelocity. When target is immersed in liquids the shock wavesget trapped between the overlaying liquid and the target whichcauses the increase in temperature of the laser induced plasma[15,16]. High density, high pressure, and high temperature plasmais obtained due to confinement effects of liquid. Under this con-finement effect the pronounced heat accumulation takes placewhich stimulates the grain growth. Laser induced heating andmelting stimulates reaction between the molten surface and oxy-gen/hydrogen. During the recrystallization process at solid-liquidinterface oxygen/hydrogen diffused into the target surface causeswider and distinct grain boundaries [6].

Energy Dispersive X-ray Spectroscopy (EDS) is performed forchemical analysis of unirradiated and irradiated titanium tar-gets. Unirradiated sample shows following contents: titanium∼84.19 wt.%, balanced to 100% by Zr (∼1.13 wt.%), C (∼5.57 wt.%),Al (∼6.17 wt.%) and V (∼2.94 wt.%). Table 1 shows the variationin contents at the central ablated area of laser irradiated targetsin air de-ionized water and propanol mediums. Reduction in con-tent of Aluminum is observed in case of ablation in air. Reductionin content of Aluminum is accredited to some extent of evapo-ration after irradiation due to its low evaporation temperature.Furthermore, complete removal of Zirconium and Vanadium is alsoobserved for air ambient. The cavities observed in case of dry abla-

tion (Fig. 3(a)–(d)) can be attributable to reduction of Aluminumand complete removal of Zirconium and Vanadium [17]. For abla-tion in de-ionized water and propanol mediums decrease in contentof Aluminum, Zirconium and Vanadium is observed. Increase in

54 N. Ali et al. / Applied Surface Science 270 (2013) 49– 57

Table 1The comparative EDX analysis of the unirradiated and central ablated area of titanium target after Excimer laser irradiation at a fluence of 3.6 J/cm2, wavelength of 248 nm,pulse duration of 20 ns and repetition rate of 20 Hz for 1000 pulses under ambient environments of air, de-ionized water and propanol.

Elements Untreated (wt%) Air (wt%) Water (wt%) Propanol (wt%)

Ti 84.19 47.78 61.91 61.87Zr 1.13 0.77 0.82 0.67Al 6.17 1.06 2.20 4.19V 2.94 1.27 2.30 2.55

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C 5.57 3.O – 36.N – 9.

ontent of oxygen is observed for all mediums i.e. air, de-ionizedater and propanol, while in case of air ablation increase in con-

ent of nitrogen is also observed. Heating of titanium surface byverlapping pulses diffuses oxygen/nitrogen into the target surfacefficiently, therefore causing the formation of nitrides and oxides6].

Table 2 shows the variation in contents of oxygen and nitrogenn ambient environments of air, de-ionized water and propanol athe peripheral region. For all the three media, i.e. air, de-ionizedater and propanol, decrease in content of Zirconium, Aluminum

nd Vanadium is observed at the periphery. Increase in content ofxygen from 32.14 wt.% to 36.57 wt.% and nitrogen from 8.74 wt.%o 9.25 wt.%, from center to boundary for samples irradiated in air isbserved. For de-ionized water and propanol, increase in contentf oxygen from 31.52 wt.% to 31.77 wt.% in de-ionized water androm 22.65 wt.% to 29.26 wt.% in propanol medium is observed in

oving from center toward the periphery.X-ray diffraction technique is employed to identify variation

n crystallinity, dislocation densities and residual stresses. Fig. 8hows the XRD patterns of un-irradiated titanium target. Presencef oxide and nitride phases is confirmed from XRD diffractograms.ubic TiN (1 1 1) plane appeared at 35.303◦, oxygen rich titanium

ig. 8. XRD patterns of unirradiated and Excimer laser irradiated titanium samplesnder an ambient environment of air for various number of overlapping laser pulsesf 500, 1000, 1500 and 2000.

1.00 1.4631.77 29.26

– –

oxides i.e. TiO2 (0 0 4), Ti3O (−1 −1 2), Ti6O (304), Ti6O (−2 −2 2) at38.363◦, 40.248◦, 76.637◦, and 78.137◦, respectively.

Crystallite size is evaluated for TiO2 (0 0 4) plane reflection byusing Sharer’s formula [14]:

crystallite size (D) = 0.9�

FWHM cos �(2)

where D is crystallite size, � is the wavelength of X-rays (15 nm),FWHM is full width at half maximum, and � is the angle of diffrac-tion.

The residual strains are evaluated by using following relation[14,18]:

strain (ε) = d − d0

d0(3)

where d is the observed and d0 is the standard plane spacing and εis the induced strain.

The dislocation line density is evaluated by following relation[14]:

dislocation density (ı) = 1D2

(4)

where D is the crystallite size.The induced stresses are calculated by the relation given below:

stress (�) = εE (5)

where ε is the induced strain, E is the young’s modulus, for TiO2 itsvalue is 282.76 GPa [19].

Peak intensity of TiO2 (0 0 4) plane reflection (Fig. 8) increaseswith increase in number of laser pulses up to 2000. The increasein the peak intensity is due to increased diffusion of oxygen intothe surface of titanium [6] and enhancement of diffraction of X-rays from target [18]. The crystallite size increases (Fig. 9) withincreasing in number of pulses up to 1500. On further increase innumber of pulses up to a value of 2000, decrease in crystallite size isobserved. The initial increase in crystallite size is due to atomic dif-fusion across the grain boundaries [14,20]. Interstitial diffusion ofoxygen atoms/ions induces microstrain defects and these induceddefects cause reduction in the crystallite size [21].

The variation in induced stresses for various numbers of pulsesis also displayed in Fig. 9. Laser induced thermal shocks and latticedefects generated by nitrogen/oxygen ions incorporation into thelattice may cause residual stress variation. Compressive stressesare generally due to oxygen/nitrogen ion implantation, whereastensile stresses are caused by laser induced thermal shock [22]. At500 accumulated laser pulses, tensile residual stresses are domi-nant. This is the main reason for appearance of large sized crakesand cavities (Fig. 3(a)) [13]. With increasing number of pulses upto 1000 and 1500, relaxation in stresses is observed. This can bedue to annealing effect after increasing multiple laser-irradiations.The defects and stresses produced by initial pulses are annealed

and relaxed after successive pulses. These results are well corre-lated with change in the surface morphology (Fig. 3(b) and (c)).In which reduction in size and number density of cavities andcracks is observed for 1000 and 1500 pulses [18]. When number of

N. Ali et al. / Applied Surface Science 270 (2013) 49– 57 55

Table 2The comparative EDX analysis of the unirradiated and peripheral ablated area of titanium target after Excimer laser irradiation at a fluence of 3.6 J/cm2, wavelength of 248 nm,pulse duration of 20 ns and repetition rate of 20 Hz for 1000 pulses under ambient environments of air, de-ionized water and propanol.

Elements Untreated (wt%) Air (wt%) Water (wt%) Propanol (wt%)

Ti 84.19 52.94 60.00 67.41Zr 1.13 – – 0.57Al 6.17 1.13 3.53 4.79V 2.94 – 2.89 1.16

05 2.06 3.4214 31.52 22.6574 – –

prmw

awTaa7

wIttosgp

iibttsswtfg

Ffa

Fig. 10. XRD patterns of unirradiated and Excimer laser irradiated titanium samples

C 5.57 5.O – 32.N – 8.

ulses are increased to the value of 2000, thermally induced shocksesults into increasing tensile stresses. The enhanced effects of ther-al shock and stresses causes pronounced thermal stress crackinghich is observed in Fig. 3(d) [18].

XRD patterns displayed in Fig. 10 confirm the presence of oxidesnd hydrides of titanium for samples irradiated in de-ionizedater environment. Oxygen rich titanium oxides i.e. TiO2 (0 0 4),

i3O (−1 −1 2) and Ti3O (−2 −1 8) plane reflections are presentt 38.239◦, 40.106◦ and 77.802◦, respectively, whereas TiH (1 3 0)nd TiH2 (1 1 1) plane reflections are also observed at 35.007◦ and0.698◦, respectively, in the XRD diffractograms.

The peak intensity of TiO2 (0 0 4) plane reflection increasesith increase in number of laser pulses to up to 1500 (Fig. 10).

ncrease in peak intensity is due to the atomic diffusion acrosshe grain boundaries and crystal growth [18] (Fig. 11). For fur-her increase in number of pulses a decrease in peak intensity isbserved which is attributable to recrystallization of the irradiatedample after melting and resolidification. The breakup of largerrains into smaller ones after laser irradiation causes decrease ineak intensity (Fig. 10) [23].

Fig. 11 also represents variations in stresses produced afterrradiation. For 500 number of laser pulses tensile stresses are dom-nant. Presence of large number of cracks and pores along the grainoundaries in the SEM images (Fig. 5(a)) confirms the presence ofensile stresses [13]. With increase in number of laser pulses upo 1500, thermal shock results in an increase in residual tensiletresses and crystal growth (Fig. 11). Due to increase in tensiletresses the grain boundaries becomes wider and distinct along

ith grain growth for 1500 laser pulses (Fig. 5(c)). Relaxation in the

ensile stresses with pronounced compressive stresses is observedor 2000 number of laser pulses. This effect is due to diffusion of oxy-en/hydrogen atoms into the surface, which causes the decrease in

ig. 9. The variation in the crystallite size and stresses of laser irradiated titaniumor various number of overlapping laser pulses under an ambient environment ofir.

for various number of overlapping laser pulses of 500, 1000, 1500 and 2000 underambient environment of de-ionized water.

Fig. 11. The variation in the crystallite size and stresses of laser irradiated titaniumfor various number of overlapping laser pulses in an ambient environment of de-ionized water.

56 N. Ali et al. / Applied Surface Science 270 (2013) 49– 57

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ig. 12. XRD patterns of unirradiated and Excimer laser irradiated titanium samplesor various number of overlapping laser pulses of 500, 1000, 1500 and 2000 undern ambient environment of propanol.

rystallite size (Fig. 11) and hence gives rise to compressive resid-al stresses [13]. Compact and small sized grains in SEM imageepresent the presence of compressive residual stresses (Fig. 5(d)).

Fig. 12 shows the XRD patterns of laser irradiated titaniumamples in propanol environment. Titanium oxide: TiO2 (0 0 4),i3O (−2 −2 −2) are found at 38.633◦, and 78.065◦, respectively,hereas, titanium hydride: TiH2 (2 0 0), TiH1.5 (3 1 1), TiH (1 3 0)lane reflections are observed at 35.435◦, 70.939◦, 76.808◦, respec-ively.

Increase in peak intensity of TiO2 (0 0 4) plane reflection isbserved with increase in number of laser pulses up to 2000Fig. 12). Increase in peak intensity of TiO2 (0 0 4) plane reflections attributable to crystal growth with increase in number of laserulses (Fig. 13).

Fig. 13 also demonstrates variations in residual stresses withncreasing number of laser pulses. For samples irradiated inropanol environment, the increasing trend in tensile residualtresses with increasing number of laser pulses is observed. Lessronounced tensile stresses for 500 number of laser are responsi-le for the formation of high density globular grains with distinctrain boundaries (Fig. 7(a)). With increasing number of laser pulsesp to 1500 the increased tensile stresses causes grain growth withuch wider grain boundaries (Fig. 7(c)). For 2000 laser pulses pro-

ounced tensile stresses are responsible for the appearance of highensity pores and cracks along the grain boundaries of irradiatedample (Fig. 7(d)).

Comparison of the three media, air (Figs. 8 and 9), de-ionizedater (Figs. 10 and 11) and propanol (Figs. 12 and 13), shows signif-

cant dissimilarities in chemical composition. In case of treatmentn air, oxides along with nitrides are formed while in case of de-

onized water and propanol oxides are formed along with hydrides.or titanium ablation in de-ionized water environment majorontents are oxides whereas in case of propanol ablation hydrideormation is prominent. In case of air and propanol, variations in

Fig. 13. The variation in the crystallite size and stresses of laser irradiated titaniumfor various number of overlapping laser pulses under an ambient environment ofpropanol.

tensile residual stress are observed, whereas for de-ionized watertransformation of tensile into compressive stresses is observed.

4. Conclusions

The effect of dry and wet ambient environment on surface andstructural properties of the laser irradiated titanium targets fordifferent number of overlapping laser pulses ranging from 500to 2000, has been investigated. Significant dissimilarities betweenablation in air and liquids are found. In the central ablated area,large sized diffused grains are formed in case of dry ablation,whereas, in case of wet ablation small size well defined grains withdistinct grain boundaries are developed. In the peripheral ablatedarea there is a significant redeposition in case of ambient environ-ment of air, whereas in case of wet ablation grains like structuresappear without redeposition. EDS analysis confirmed the increasein content of oxygen and nitrogen in case of air and increase in con-tent of oxygen only, in case of de-ionized water and propanol. XRDanalysis confirmed the diffusion of oxygen and nitrogen, in case ofdry (air) ablation of titanium, which is responsible for the formationof oxides and nitrides on its surface. In case of wet ablation (de-ionized water and propanol), the diffusion of oxygen and hydrogencauses the formation of oxides and hydrides on the metallic surface.For various number of laser pulses variation in peak intensity, crys-tallinity, dislocation densities and residual stresses are evaluatedfor all ambient environments.

Acknowledgement

Authors are thankful to Higher Education Commission (HEC) ofPakistan for funding the project “Upgradation of Laser Lab facilitiesat CASP, GC University, Lahore” and for providing financial support(indigenous scholarship) to first author.

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