Nonlinear laser lithography to control surface properties of stainless steel

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Nonlinear

laser

lithography

to

control

surface

properties

of

stainless

steel

L. Orazi

a,

*

,

Ia.

Gnilitskyi

a

,

I. Pavlov

b

,

A.P.

Serro

c,d

,

S. Ilday

b

,

F.O.

Ilday

b,e

a_Department_of_Science_and_Methods_for_Engineering,_University_of_Modena_and_Reggio_Emilia,_Italy b_Department_of_Physics,_Bilkent_University,_Turkey

c

CentrodeQuı´micaEstrutural,InstitutoSuperiorTe´cnico,UniversityofLisbon,Portugal

d

Centrodeinvestigac¸a˜oInterdisciplinarEgasMoniz,InstitutoSuperiordeCieˆnciasdaSau´deEgasMoniz,Portugal

e

DepartmentofElectricalandElectronicsEngineering,BilkentUniversity,Turkey SubmittedbyProf.MarcoSantochi(1)

1. Introduction

Controlofthewettabilitybehaviourofastainlesssteelsurface can be of great interest for several applications. This class of materials is known to be largely used by food, dairy and pharmaceutical production systems owing to their intrinsic chemicalstabilityandstrongresistancetooxidationthatpermit them to be safely cleaned and disinfected by using several chemicalagents.

Inalltheseapplications,controlofsurfacewettabilitycanbeof greatinterest,asitcanleadtoshortercleaningandﬂushingtimes, less organic residuals in pipes and components and minimal retentionofchemicalagentsonthesurface.Severalmethodswere developedtocontrolthewetting behaviourofstainlesssteel.A signiﬁcant part of them investigated the effect of chemical compositionofthesurface,bymodifyingit viasurfacecoatings or through reaction of the base material with other chemical agents. In [1] several approaches were attempted, namely implantationofdifferentions(e.g.SiF3+andMoS22+)intostainless

steel; coating the surface with diamond-like carbon (DLC) via sputtering;plasma chemicalvapourdeposition (PECVD)ofDLC, DLC–Si–O coating onto thesurface and treating thesurface by autocatalytic Ni–P–PTFE and silica coating. In [2] and [3], the authorscarriedoutelectrolyticin-processdressing(ELID)grinding togenerateamirror-qualitysurfaceonferriticstainlesssteelfor surgical applications. This methodis used to functionalize the surfacebyvaryingthetypeandquantityofabrasivematerialsor

theoperatingtemperatureinordertoinducedifferentwettability behaviour.Someoftheabovemethodsareexpensive,complicated and time consuming; others involve chemical treatments and weak coatings that are generally evanescent or fragile. Use of methods that only changethe surface morphology/topography could bethe saferchoiceif chemical stability is critical for an application,wherenosurfacecontaminationsaredesired.

Two different recent examples can be found in [4,5] based respectivelyonavibrationassistedmachiningmethodandonthe useofananosecondUVlaser.

Itiswellknownthatfemtosecondlaserscangenerateregular structures on a material’ssurface calledlaser induced periodic surfacestructures(LIPPS),ofwhicharecentexamplecanbefound in[6].Thisprinciplecanbeappliedtoobtainhydrophobicsurfaces asinvestigatedin[7,8],wherestainlesssteelandtitaniumalloys weretreatedwithaTi:Sapphirefemtosecondlaserandeventually silanized in a low vacuum chamber. Although the results are appealing,themethodscannotbeeasilyscaledformassproduction due tothe low production rate which is a consequenceof the characteristicsofthelasersemployed:highpulseenergiesandlow pulsefrequenciesandscanningspeeds.

Recently,atechniquecallednonlinearlaserlithography(NLL) hasbeenintroducedwhichpermitsproductionofhighlyuniform nanostructuresonlargesurfaceareaswithanelevatedproduction rate.Thistechniqueisbasedontheuseofahighrepetitionrate, well-focusedfemtosecondlaserwithpulseenergyintheorderof nanojoules. Acomprehensivephysicalmodel oftheprocesshas beenpresentedin[9].

Inthiswork,theNLLtechniqueisemployedtomodifythesurface topographyofstainlesssteelandcontrolitswettabilitybehaviour duetoitsuniquefeaturesoverothersurfacemodiﬁcationmethods CIRPAnnals-ManufacturingTechnology64(2015)193–196

ARTICLE INFO Keywords:

Lasermicromachining Nanostructure Surfacemodiﬁcation

ABSTRACT

Inthepresentworkanovelmethodtoimprovethesurfacepropertiesofstainlesssteelispresentedand discussed.Themethod,basedontheuseofahighrepetitionratefemtosecondYbﬁbrelaser,permits generationofhighlyreproducible,robust,uniformandperiodicnanoscalestructuresoveralargesurface area.Thetechniqueischaracterizedbyhighproductivity,which,initsmostsimpleform,doesnotrequire specialenvironmentalconditioning.SurfacemorphologyisscrutinizedthroughSEMandAFManalyses andwettabilitybehaviourisinvestigatedbymeansofthesessiledropmethodusingdistilled–deionized water.Itisshownthatoptimizationofprocessparameterspromotesanisotropicwettingbehaviourofthe materialsurface.

ß2015CIRP.

*Correspondingauthor.

E-mailaddress:leonardo.orazi@unimore.it(L.Orazi).

ContentslistsavailableatScienceDirect

CIRP

Annals

-

Manufacturing

Technology

j o u r n a lh o m e p a g e :h t t p : / / e e s . e l s e v i e r . c o m / c i r p / d e f a u l t . a s p

http://dx.doi.org/10.1016/j.cirp.2015.04.038

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such ascontrolled thermal penetration,chemical cleanness and remotenon-contactprocessing.

2. Experimentalsetup

The material under study was a commonly used AISI 316L stainlesssteelthatissubjectedtoalowtemperaturecarburizing (LTC)processwheretheelevatedcarboncontentinthe302

mm

treateddepthstronglyincreasesthesurfacehardnessfrom200to 1200 Vickers. A cross-section image of the material under metallographicstudyisshowninFig.1.

TheNLLtreatmentswereconductedbyusingacustom-built Ytterbiumﬁbrelaserwithawavelengthcentredat10378nm. Thelaserbeamwaslinearlypolarizedandguidedontothematerial surfacewithagalvanometricscanningheadfocusedbya56mmfocal lengthF-thetalens.

Thesurfaceofthematerialwasﬁrstpolishedandlappedwith diamondpastethenareasof5mm5mmwereNLLtreatedto formperiodicnanostructures.

Codingofthespecimensalongwiththeexperimentalvariables suchas thepolarization angleofthelaserbeam(up),theangle

betweenthepolarizationplaneandthescanningdirection, and pulseenergiesaregiveninTable1. Otherparameterswerekept constant such as the pulse duration (270fs) repetition rate (200kHz), spot diameter (6

mm),

scanning speed (100mm/s) andstepbetweenthescannedlines(2

mm),

whichresultedina highoverlappingbetweenthelaserpulses,aconditionnecessary to generate uniform, periodic surface structures. The process productionratewas0.5cm2_/min.

PP coding denotes tests in which the polarization plane is paralleltothescanningdirection,whereasPTcodingdenotesthose inwhichthepolarizationplaneisperpendiculartothescanning direction.

The surface morphology of the specimens was observed throughVariablePressureScanningElectronMicroscopycoupled with energy dispersive spectroscopy operated in secondary electronsimagingmode.Three-dimensionalimagesofthesurfaces wereobtainedfromstereoscopicpairsofimagesacquiredattilting

anglesof0and458.Surfaceroughnesswasmeasuredbyusingan atomicforcemicroscope(AFM)innon-contactopticalmode.

The wettabilityof thesurfaceswasdetermined throughthe measurementofdistilledanddeionized(DD)watercontactangles bythesessiledropmethod.Dropsof2–4

mL

weregeneratedwith a micrometricsyringe and depositedon thesubstrate surfaces, insidea chamberpreviouslysaturatedwithwater.Dropimages were acquired at pre-deﬁned intervals during 2000s, with the systemcomposedofavideocameraconnectedtoamicroscopeand aframegrabbertosequentiallycapturethedropproﬁle.Longer measurementswerenotdone,sinceevaporationoftheliquidcan beaproblem forlongtimes.Eachimagewasthenanalyzed by softwaretoevaluatethecontactangle.Themeasurementswere doneat258C.Atleast4 dropsweregeneratedandmeasuredfor eachspecimen.

3. Resultsanddiscussions 3.1. Morphologicalresults

Fig. 2 presents a representative SEM image of a PP-coded specimen where the polarization plane is parallel to the scanning direction.As can be seen from the ﬁgure,formation of a quasi-regular linear pattern is evident with all the basic aspectsofNLLpresent: thecoherentpropagationofstructures alongthesurfacethatareobtainedbyscanningthelaserbeam witha smalldiameter[9].Inourcasethebasic mechanismof generationcouldbethe interactionoftheincidentlaser beam with surface plasmon polaritons (SPPs) instead of wave scatteredinthesurface[10].

The periodicity of LSFL wasmeasured as 38050nm. AFM analysesindicatedthatthewidthandtheheightoftheripplesare 32040nmand42030nm,respectively.

ImageinFig. 2bshowsthepresenceofripplesofhighspatial frequency LIPSS (HSFL) inside the valleys between periodic structures.Ananostructureparalleltothepolarizationdirection is evident with a period signiﬁcantly smaller than the laser wavelengthandintheorderof7720nm.

Fig.3showstheAFMmappingofthespecimencodedasPTB, where the scanning direction and the polarization plane are

Fig.1.Across-sectionimageofAISILTCundermetallographicstudy.Thetreated zoneofabout30mmonthesurfaceisindicatedbyarrows.

Table 1

Codingofthespecimensandexperimentalvariables. Codingofthe specimen Polarization angleup(8) Pulse energy(nJ) PPA 0 160 PPB 0 170 PPC 0 180 PTA 90 160 PTB 90 170

PTC 90 180 Fig.2.SEMimagesofthespecimencodedasPPC,wherethescanningdirectionand thepolarizationplaneareparalleltoeachother:(a)alargeareaimage,and(b)the focusedimagetakeninhighermagniﬁcation.

L.Orazietal./CIRPAnnals-ManufacturingTechnology64(2015)193–196 194

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perpendicular to each other. The nanostructures are clearly differentwithrespecttothespecimencodedasPPC,with quasi-hexagonal structures in three principal directions that are mutuallyorientedat608tooneanother.Itseemsliketwosmall ripples are periodically joined in a bigger ripple. The period betweenthesedoublerippleswasmeasuredas79030nm,while the period between the smaller ripples was measured as 35050nm,almosthalfoftheperiodofdoubleripples.

The quasi-hexagonal structures were already obtained and observedinpreviousstudies,wherethestructureswereproduced byscanningthesurfacewithacircularlypolarizedlaserbeamorby usingdoublescanswithtwoorthogonalpolarizations[11].Inour caseitispossiblethatthegalvoscannerperturbsthepolarization state toelliptical depending on the scanning speed and initial polarization. The structures are clearly evident in SEM images showninFig. 4,wheretheyaretakenfroma458tiltedspecimen.

Fig.4bdepictsthehigh-resolutionimageshowingthejointoftwo ripples.

RoughnessdataweremeasuredbyanalysingtheAFMmapsof the different treatments and are presented in Table 2. The untreatedsurfaceisindicatedasUNT,presentingveryhighvalues ofhighorderstatisticalmoments.Thisisprobablyduetonoisein theAFMimages,whiletheotherindicatorsRa,Rqandtherelative

areaareasexpected.PPtreatmentspresentamoderateincreasein allroughnessindicatorswithrespecttoPTtreatments.

3.2. EDSanalysis

Chemical compositionsof the untreated specimenand NLL-treated specimencoded as PTB wereanalyzed through anEDS probeandtheresultsaregiveninTables3 and4,respectively. Althoughthereisanuncertaintyonlowatomicnumberelements likecarbon,thevaluesobtainedfortheuntreatedspecimenareina goodagreementwiththenominalchemicalcompositionofanAISI 316Lthatissubjectedtoalowtemperaturecarburizingtreatment. Ontheotherhand,Table 4showsthatwhenthesurfaceis NLL-treated, it is possibly oxidized as a result of laser-material interactionintheatmosphericconditions.

3.3. Wettabilityanalysis

WettabilitytestswithDDwaterwereconductedthroughthe sessiledropmethodonbothtypesofsurfaces.

Measurementswereperformedintwodirections:paralleland perpendiculartoLIPPS(Fig. 5),toevaluatetheeffectoftheNLL treatmentsonanisotropicwettabilitymeasuredasthedifference betweenthecontactsanglesobservedinthosedirections.

TheresultsarepresentedinFigs.6 and7forthespecimens codedasPPCandPTB,respectively.Dataobtainedfortheuntreated specimen are included for the purpose of comparison. Results scatteringisshownaslightcolouredbands.

BothNLLtreatedsurfacespresentanappreciable hydrophobic-ity.Thisiscoherentwithearlierstudies,wherefemtosecondlaser irradiation is used to treat stainless steel surfaces [12–14]. Depending on theexperimentalconditions, someofthem have

Fig.3.AFMmapsofPTBtreatmentwithaquasi-hexagonalstructureclearlyvisible.

Fig.4.SEMimagesofthespecimencodedasPTB,wherethescanningdirectionand thepolarizationplaneareperpendiculartoeachother:(a)alargeareaimage,and (b)thefocusedimagetakenatahighermagniﬁcation.

Table 2

RoughnessdataasobtainedfromAFManalysis.

Sa(nm) Sq(nm) Skew( ) Kurtosis( ) Relative area( ) PPA 79.7 103.1 0.290 0.765 2.02 PPB 86.7 111.8 0.437 0.919 2.16 PPC 98.4 125.9 0.271 0.811 1.66 PTA 52.0 66.3 0.411 0.492 1.52 PTB 61.9 79.0 0.195 0.509 1.41 PTC 87.0 110.9 0.303 0.712 1.87 UNT 11.9 16.7 2.284 14.106 1.02 Table 3

ResultsoftheEDSanalysisoftheuntreatedspecimen.

Element Series wt.(%) Norm.wt.(%) Norm.at.(%) 3serr.inwt.(%) C K 3.59 4.32 17.31 3.58 Si K 0.29 0.34 0.59 0.15 Cr K 12.84 15.46 14.31 1.16 Mn K 2.66 3.20 2.81 0.36 Fe K 54.79 65.96 56.85 4.40 Ni K 7.22 8.69 7.12 0.76 Mo L 1.68 2.02 1.01 0.33 Sum 83.07 100.00 100.00 Table 4

ResultsoftheEDSanalysisofthespecimencodedasPTB.

Element Series wt.(%) Norm.wt.(%) Norm.at.(%) 3serr.inwt.(%) C K 5.80 6.57 23.95 4.94 O K 1.14 1.30 3.54 1.22 Si K 0.39 0.44 0.68 0.17 Cr K 12.82 14.52 12.22 1.15 Mn K 2.53 2.87 2.28 0.35 Fe K 55.64 63.01 49.38 4.47 Ni K 8.48 9.60 7.16 0.87 Mo L 1.51 1.71 0.78 0.31 Sum 88.32 100.00 100.00

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managedtoachieve super-hydrophobicsurfaces.Obviously, the increaseinhydrophobicitymaybeattributednotonlytochanges inroughnessandtopographyaspredictedbytheWenzelmodel

[15]butalsoduetotheoxidationinducedbythelasertreatmentas evidencedbydatafromTable 4.

The main differencein thewettabilitybehaviourof thetwo surfacesis related withthe degreeof anisotropy. As expected, contactanglesobservedperpendicularlytotheLIPPSdirectionare smaller than those observed in the parallel direction because groovesdrivewaterbycapillaryforce.Thedifferencebetweenthe contactanglesinthetwodirections,however,ismorepronounced inPPCsurfaces(Fig. 6).Fromthemorphologicalanalysiscarried

outabove,itwasconcludedthatNLLtreatmentsledtoastrong linearlystructuredtextureon PPsurfaces anda more isotropic (quasi-hexagonal)structureonPTsurfaces.Thisisreﬂectedbythe wettability results.Moreover, asobserved by otherresearchers

[16],the contact angleanisotropy is stronglycorrelated tothe surfacearithmeticmeanroughness(Sa);roughersamples(asisthe caseofPP,seeTable 2)showlargerwettinganisotropythanthe smootherones.Theaverageincreaseincontactanglemeasuredon theentire setof wettability testsperformed is 15.88while the average anisotropy calculated as the difference between the contactangleinthetwodirectionsappearstobe8.48forPPtest conditionsand5.88forPTones,respectively.

4. Conclusions

TheNLLtechniqueappearstobeaverypromisingmethodfor the control of the wettabilityproperties of stainless steel. The presentedsetuppermittedanincreaseinhydrophobicitybyusing alowpowerandnotparticularlyexpensivefemtosecondlaser.The processseemstobequiterobustandrepeatablealsoifconducted inopenair,whileit canbeeasilyscaledtomassproductionby increasingpowerandscanningspeed.

Allthe resultsconﬁrm it is possible toincrease the surface hydrophobicity with the possibility to tune, in a single pass treatment,thedegreeofanisotropybyvaryingtheexperimental conditions.

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Fig.5.Schematicrepresentationofthecontactanglemeasurementinparalleland perpendiculardirections.

Fig.6.Evolutionofthewatercontactangleswithtimeforthespecimencodedas PPC.PPC_Prepresentsthemeasurementsperformedwhenthecontactangleisina plane perpendicular to the LIPPS direction, while PPC_T represents the measurementsperformedwhenthecontactangleisinaplaneparalleltothe LIPPSdirectionasindicatedinFig. 5.

Fig.7.Evolutionofthewatercontactangleswithtimeforthespecimencodedas PTB.PTB_Prepresentsthemeasurementsperformedwhenthecontactangleisina plane perpendicular to the LIPPS direction, while PTB_T represents the measurementsperformedwhenthecontactangleisinaplaneparalleltothe LIPPSdirectionasindicatedinFig.5.

L.Orazietal./CIRPAnnals-ManufacturingTechnology64(2015)193–196 196