EBSD sample preparation.docx
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EBSD sample preparation.docx
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EBSDsamplepreparation
SamplePreparationforEBSD
Introduction
Becausediffractedelectronsescapefromwithinonlyafewtensofnanometresofthespecimensurface,specimenpreparationforEBSDiscriticaltoachievegoodresults. Ifmaterialnearthesurfaceisdeformed, orhasanysurfacecontaminant,oxideorreactionproductlayerspresent,thenEBSDpatternformationmaybesuppressed.
Formanymaterials,standardpreparationmethodsmaybeemployedsuccessfullywithcare. Asageneralrulestandardpreparationmethodscanbeprogressedtothefinalpolishingstagewithoutanydeviationfromthenormalrouteemployed.Thereafter,anadditionalpolishingstageusingcolloidalsilicaisallthatisrequiredtoachieveafinishsuitableforEBSD.
However,differentmaterialsresponddifferentlytocommonpreparationmethods.Thereforethematerialunderinvestigationshouldbeconsideredonanindividualbasisandpreparedappropriately.Themanufacturersofpreparationequipmentshouldbeconsultedfortheapplicabilityofagivenapproachforagivenmaterial.
Cuttingthesample
ThefirstthingtorememberwhencuttingsamplesforEBSD,istopreservethesampleaxesorientation. Cutthesampleinsuchamannerthatimportantsampledirections,liketheRollingDirection,TransverseDirectionandSampleNormalarenotlost. Thesecondthingtorememberthatthecuttingprocessmustnotdamageorchangethesampleasthiswouldleadtoerroneousresults.
Avoidaggressivecuttingmethodsthatgenerateheatorcausedeformationatthecutsurface.Severedamageinducedatthisstagemayextendsodeepintothematerialthatitisnotremovedbysubsequentgrindingandpolishing. Heatingcausedduringcuttingmaycausechangestothemicrostructure-phasetransformationsorprecipitation/diffusionsmechanismsmaybecomeactive.Thereforeheatingmustbeavoidedatallcosts.
Abrasivecuttersarecommoninthemetallurgicalindustryandaresuitedtocuttinglargersectionsofmaterial. Oftenthesampleissubjectedtoconsiderableforceandheating,althoughifusedwithcare, goodcutscanbeperformedwithminimaldamage. Heatingisthemainproblem,andusingexcessiveforcewithaninappropriatewheelcancauseasampletolocallyglowredwithheat. Manufacturersofcuttingequipmentandcut-offwheelspublishtablesanddiagramstohelptomakethechoiceofwheeleasier.Theimportanceofobservingthemanufacturer’srecommendationscannotbeover-stressed. Ifover-heatingofthesampleoccurs,itisusuallyduetousingawheelthatistoo‘hard’forthematerialbeingcut.Thewheeldoesnotwearproperlywhichcausestheabrasivetobecomeblunt. Anotherpossibilityisthattheabrasivebecomesclogged. Frictionthencausesexcessiveheatinganddamagetothesample. Heatingoftenresultsinalteredsurfacestructures,whichsubsequentgrindingandpolishingdonotremove. EBSDisverysensitivetodamagecausedinthismannerandgreatercarethanusualiswarrantedtoavoidgeneratingmisleadingresults,orcompromisingpatternquality.
Theselectionofabrasivewheelisthereforeimportanttoavoidintroducingunnecessarylevelsofdamagewhencuttingmaterials.
Precision,LowDeformationCuttingMachines
Therearemanyexamplesofcuttersonthemarketdesignedforprecisionandlowdamagecutting.Suchmachinesnormallyemployboronnitrideanddiamondtypecuttingwheels,althoughothertypesmaybeavailable,dependingonthemachine.Generally,lowdeformationcuttersareparticularlywellsuitedtocuttingsectionsforEBSD.
SampleMounting
Smallspecimensgenerallyrequiremountingsothatthespecimenissupportedinastablemediumforgrindingandpolishing.Themediumchosencanbeeitheracoldcuringresinorahotmountingcompound.
Coldmountingresins
Awiderangeofproductsareavailableonthemarket.Generallyfastersettingproductsincludingacrylicresintypesarelessfavorable,asthesetendtodeveloplowhardnessandoftensufferfrom'shrinkage'. Shrinkageisthetermgivenwhentheresinshrinksawayfromthesamplesurfaceduringcuring. Thisisundesirableasthegapwhichformsharborscontaminants,gritfromgrindingandpolishingstagestocausecrosscontaminationofpolishingsurfaces. Itisdifficulttoobtainagoodpolished,scratchfreesurfacewhengapsinthemountingmaterialarepresent.
Epoxyresintypesgenerallyhavethebestcharacteristicswithrespecttohardnessandshrinkage. However,epoxyresinstendtobeslowercuringandadequatetimeshouldbeallowedtoensurethatthematerialisfullycuredbeforeproceeding. Epoxiesoftentakeaconsiderableperiodoftimeafterinitial'setting'todevelopfullhardness. ItisnotgenerallypossibletomakecoldcuringresinsconductivesuitableforSEMexamination.
HotSampleMounting
Hotmountingusesathermosettingcompound,curedinamountingpresswhichexertsbothheatandhighpressure. Thismountingmethodproduceshardmountsinashortspaceoftime. Howevertheheating(generallyintheorderof120°C)andconsiderablepressureappliedmaybeunsuitablefordelicate,softorlowmeltingpointspecimens. Techniquesmaybeusedtoprotectadelicatesamplefromtheeffectsofpressure,suchasplacingthesampleunderasupportingstructurewithinthemouldingcavity.Suchasupportingstructurecanprotectthesamplefromtheinitialpressureappliedwhenthemountingmaterialisinagranularform,andmostlikelytoinflictdamage. Whenthemountingmaterialbecomesfluid, infiltrationshouldoccurtoencapsulatethesamplewhichwillthenbesubjecttohydrostaticpressure. Hydrostaticpressurecanbeappliedtoallbutthemostdelicateofsampleswithoutproblem. Inthecaseofverysoftorthermallysensitivematerials,hotmountingisnotappropriate.Conductivemountingresinsareavailable,whicharegoodforSEMexamination,althoughtheadhesionandhardnesscharacteristicsarenotasgoodasthoseofepoxyhotsetcompounds. Iftheedgesofthespecimenarenotofinterest,thennonconductivemountingmaterialscanbeused.Ingeneral,hotmountingispreferabletocoldsettingresins,whenthesampleisnotaffectedbytemperatureandpressure(200ºC&50kN).However,notallspecimenscantoleratethis.
Non-conductivemountsmustbecoveredwithadhesiveconductivetapeorcoatedwithaconductivemedium(thesampleareacanbemaskedifsputtercoating,orusinganevaporator.Aluminiumfoilorglasscoverslipsareusefulforthispurpose. Note:
manyadhesivemetaltapeshavenon-conductiveadhesive,sotheuseofcarbon/silverconductivepaint mayberequiredatseams. Whilstverythinfilmsofcarboncanbetoleratedonthesample,theidealisthatthesamplesurfaceshouldbebare.
Characteristicsofthemountingmaterialinclude:
:
∙Goodabrasioncharacteristicsandsufficienthardnesssuchthattheedgesofthesampleareprotected,i.e.,therateatwhichabrasiontakesplaceshouldbeevenacrossthefaceofthemountandthespecimen.
∙Stableandadherenttosample.Thisisimportant.Ifthemountingmaterialhaspooradhesionorhighshrinkage,gapsmayopenupbetweenthemountingmaterialandthesamplesurface.Whenthishappens,itisverydifficulttopreventcross-contaminationofoneabrasivetoanother,causingheavyscratchinginthefinishedsection.Alsoanyfriablesurfacelayers(oxidelayersetc.)shouldbeheldadheredtothesurfaceandnotpulledoff.
∙Propercuring-insufficienttimeandtemperaturecanleadtopartiallycuredspecimenmounts.Undertheseconditionsthepropertiesofthemountingmaterialarenotproperlydevelopedandthematerialmaybelooseandpowdery.Generally,ifthematerialisimproperlycured,thehardnessandabrasioncharacteristicsarepoorandthematerialisadverselyaffectedbyetchesandsolvents.Further,thecharacteristicsundervacuumareverypoorwithout-gassingamajorproblem.Ifthemountingstageissuspectedtobeatfault,itisbesttobreakthesampleoutandstartagain.
∙Stableinvacuum-noout-gassingorvapourtocausecontamination. Thisisarticularlyimportantforhighmagnificationwork,longmapacquisitiontimesandmicroscopeswithhighvacuumrequirement(FEGSEMs)
ToSummarisesampleMounting:
∙HotMountingmaybeunacceptable,iftheeffectoftemperatureandpressureareexpectedtobeinappropriateforthesampleunderinvestigation.
∙Generally,thematerialsemployedforcoldsettingcannotmatchthehardnessofmaterialstraditionallyusedinHotMounting.Thismayleadtocompromisesinthedegreeofedgeprotectionandsupportthatthemountprovidesforthesample.Further,theabrasioncharacteristicsmayneedtobetakenintoaccountduringthepreparation.
∙Thematerialshouldbestableundervacuum.Out-gassingcanbeamajorproblemleadingtohighcontaminationratesonthesample,andevenmicroscopeparts.
GrindingtheSample
CoarseGrinding
Grindingcanbeachievedinavarietyofways,usingavarietyofabrasives.Fixedabrasivesurfacesareavailableusingdiamondorcubicboronnitride(CBN)abrasives.Themethodusedtobindtheabrasivestothewheelaffectsthegrindingcharacteristics,theharderormorerigidthebondingmedium,themoreaggressivethegrindingactionofthesurface.Thereforemetalbondedfixedabrasivewheelsarethemostaggressivegrindingsurfaces,whereasresinbondedwheelsarelessaggressive.
CoarsegritSiliconCarbideorAluminaabrasivesmaybeused,butthedurabilityorcharacteristicsofsuchmaterialsmaybeinappropriateforcertainmaterials.Generally,inordertomaintainsharpabrasiveparticles,grindingpapersneedfrequentcha
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