(cid:105) (cid:105) “Thesis” — 2012/1/28 — 18:31 — page 1 — #1 (cid:105) (cid:105) POLITECNICODIMILANO MECHANICALENGINEERINGDEPRTMENT DOCTORALPROGRAMMEINMECHANICALENGINEERING COLD SPRAY COATING AIMED AT NANOCRYSTALLIZATION: PROCESS CHARACTERIZATION AND FATIGUE STRENGTH ASSESSMENT DoctoralDissertationof: RaminGhelichi Supervisor: Prof.MarioGuagliano Tutor: Prof.MarcoBoniardi Coordinator: Prof.GianpieroMastinu Year2011-CycleXXIV (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) “Thesis” — 2011/12/31 — 18:10 — page 2 — #2 (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) “Thesis” — 2011/12/31 — 18:10 — page I — #3 (cid:105) (cid:105) Dedication Tomylovelyparents PashootanandFatemehGhelichi (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) “Thesis” — 2011/12/31 — 18:10 — page II — #4 (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) “Thesis” — 2011/12/31 — 18:10 — page III — #5 (cid:105) (cid:105) Acknowledgement Itisamyththatadissertationisthesoul-wrenchingcreationsolelyofitsauthor’stime,toilandtenacity. The current thesis is the product of contribution of the people who have been helping me sparingly through these years. Although I am not able to appreciate completely and even mention the value of theirsupport. Iamdeeplythankfultomysupervisor,Prof. MarioGuaglianoforhisguidanceandinfinitesupport ineverysinglestepofthisresearch. Hisintelligence,knowledge,andwisecommentshavealwayslitup newwaysandideas. Nooneshouldbesubjectedtothetortureofreadingmyearlyattemptsattechnical writing,andthankstoMario,noonewill. Hehelpedindifferentaspectsofmylifeduringtheyearsfar fromhomeandkindlytreatedmelikeamemberofhisfamily. It is difficult to overstate my gratitude to Prof. Hamid Jahed from University of Waterloo, who I attributethelevelofmyMaster’sdegreetohisencouragementandefforts;withouthim,too,thisthesis wouldnothavebeencompleted. I owe my sincere gratitude to Prof. Bertrand Jodoin from University of Ottawa. It was an honor toworkwithhisgroupMathieuBolducJean-LouisPelletier,Jean-MichelRoy,JamilAssad,Rubén FernándezUrrutia,andespeciallyDanielMacDonald;withouttheirsupportandfriendlyhelpsIwould notabletogetthisthesisdone. I thank Prof. Ines Inés Fernández Pariente from University of Oviedo, for her kind-great efforts andfriendlysupportwhichgaveuntiringhelpduringdifficultmoments. MygreatappreciationtoProf.MarcoVeranifromMathematicsDepartment,forhelpingmethrough mathematicalapproaches; hisintelligencealongwithinspirationhasopenednewdoorsformeandhas hadaremarkableinfluenceonmyentireresearch. My sincere thanks to Prof. Laura Vergani, Prof. Gianpiero Mastinu, Prof. Bianca Maria Colosimo and especially Prof. Andrea Bernasconi for their detailed review, constructive criticism andexcellentadvicesduringthepreparationofthisthesis. AspecialmentionshouldbemadeofthestaffsofC4,CLASDandOrigoniBLabs,whohavebeen veryhelpfulwithexperimentalset-upsandpracticaladvices:LuigiLanzani,MauroDeMori,Luciano Lucherini,FilippoSpinelli,AndreaDelViscio,SergioBruno,MorenoRiccardi,GiuseppeGhilardi, Marco Ghilardi, Dario Giusti, Pasquale Aquilino, Francesco Cacciatore and Maurizio Pardi; in particularAlessandroTosiandMaurizioGroppiwhoreadilytooktimeoutoftheirbusyschedulesto help me frequently with all the experimental tests and machines. Pietro Pellin and Maurizio Pardi deservespecialthanksfortheirhelpthroughtheXRDmeasurements,microscopysamplepreparations, etc. IwishtothankLucaSignorelliforhiscontributionsoftime,ideasandexperiences. I would like to express my gratitude to Dr. Simone Vezzù from Nanofab Lab. in Civen for his generoussupportandexecutionofcoatingonourspecimens. IhavetoappreciatetheworkofmydearfriendsAntonioMairanoandDanieleVagowhoastheir Masterthesishavecontributedtothisresearch. Thesupportandcareofmyfriendshelpedmeovercomesetbacksandstayfocusedonmygraduate study. I greatly value their friendship and I deeply appreciate their belief in me: Francesco Benzoni, Dr. KhaydarValiullin,Dr. ChiaraColombo,FlaviaLibonati,Dr. MauroMadia,LucaPatriarca, Ermes Tarallo, Gabriella Tarantino, Augusto Sciuccati, Dr. Yonatan Afework Tesfahunegn, and ElisaMarchesin.InparticularIowemydeepestgratitudetoMassimoFossati,DavideCrivelli,Giorgo Vallone, andespeciallyDr. EdoardoConradowhohavebeenalwaysreadytolendahandandassist witheverytypeofproblems. (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) “Thesis” — 2011/12/31 — 18:10 — page IV — #6 (cid:105) (cid:105) IamgratefultoFrancescaMassaro,LilianaMusazzi,AlessandraMiceliandespeciallyCaterina BarbieriforteachingmeItalianlanguage. Theirkindeffortthroughtheseyearsisunforgettable. It is necessary to mention and thank the kind hospitality and support of Dr. Amin Eshraghi, Dr. MorvaridKarimiGhovanlou,Dr. MohammadrezaNoban,andHassanMahmoudiduringmystay inCanadawhichmadeeverythingsimplertome. IamgratefultotheDepartmentSecretary, LiciaSimonelliforhervariousformsofsupportduring mygraduatestudy. The financial support of Politecnico di Milano and Scuola Interpolitecnica which gave me the opportunityandencouragementtoexpandmyresearches,isgratefullyacknowledged. IowemylovingthankstoDr. SaraBagherifard. Herdedication,constantencouragement,sparing assistance,andsupporthavealwaysbeentrulyvaluable. Lastbutnotleast,IwishIwasabletothankmylovelyfamily.IamgratefultomyparentsPashootan andFatemehGhelichi,wholovedmewithinfinitegenerosityandprovidedmewiththeopportunityto bewhereIam. Withoutthem,noneofthiswouldevenbepossible. (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) “Thesis” — 2011/12/31 — 18:10 — page I — #7 (cid:105) (cid:105) Abstract T HE everincreasingresearchoflightness,improvedperformance,safetyandreliabilityispushing tolookformaterialswithsuperiormechanicalpropertiesandtomeetseveredesignrequirements imposedinmanyfieldsofapplication,inordertoobtaincompetitiveproducts.Nanocrystal(NC) materials are experiencing a rapid development in recent years due to their existing and potential ap- plications in a wide variety of technological areas; thanks to their superior mechanical properties that by some means are different from their conventional coarse grained polycrystalline counterparts. The commercialapplicationsofnanomaterials, beyondtheboundariesoflaboratories, lieonthesuccessful production and consolidation of these materials into components preserving the nanostructures. Cold Spray(CS)coatingisaninnovativedepositiontechniquewhichuseselementsofbothphysicaldeposi- tionandsevereplasticdeformationforgeneratingNC.CSisanemergingcoatingprocessinwhich, in contrasttootherwell-knownthermalsprayprocessessuchasflame,arc,andplasmaspraying,powders do not melt before impacting the substrate. This character makes CS process commendable for many different coating applications dealing with various materials not only metals but also polymers, com- posites, etc. Bonding of particles to the substrate occurs due to the high kinetic energy upon impact; therefore, the velocity of the particle plays the most important role in material deposition. During the process,powdersareacceleratedbyinjectionintoahighvelocitystreamofgas.Thehighvelocitystream isgeneratedthroughaconverging-divergingnozzle. Itiswellrecognizedthatparticlevelocitypriorto impact is a key parameter in CS process. It determines what phenomenon occurs upon the impact of sprayparticles,whetheritwouldbethedepositionoftheparticleortheerosionofthesubstrate. Critical velocity(CV)foragivenpowderisdefinedasthevelocitythatanindividualparticlemustattaininor- dertodepositafterimpactingthesubstrate. ExperimentalmeasurementoftheCVispracticallyalmost impossibleduetothefactthatthewholedepositionprocessforasingleparticlelastslessthanfewnano seconds. A finite element (FE) model using commercial Abaqus/Explicit has been developed in order to simulate the phenomenon and broaden the horizon of the physical background of the process. The generalagreementsupportstheideathatadiabaticshearinstabilityplaysanimportantroleinthebonding of the particles. Having used a material model which considers softening phenomenon and strain rate effect, CV, is numerically estimated by characterizing the shear instability that appears in the discrete outputsofthesoftwareasasingularityintheirfunctions. Thesingularityhasbeenfoundbytransferring theresultsinfrequencydomainusingWavelettransformandanalyzingthesmoothnessofthefunctionby calculatingitssecondderivativeinSobolevspace.Theresultsfortestedmaterialshowagoodcorrespon- dencewiththeexperimentalmeasurements. AnotherFEmodelhasbeendevelopedinordertoexamine thepeeningeffectoftheparticlesonthesubstrate. particle’speeningeffect,inducesresidualstressand high plastic deformation. The compressive residual stress generally increases the fatigue endurance of structures;moreover,recrystallizationcanoccurasaresultofhighplasticdeformationofthespecimen’s surface. In CS, particle diameters have a Rosin-Rammler distribution; their velocity and temperature arefunctionsofparticlediameterbasedontheinitialconditionoftheprocess. Although,thenumerical modelconsiderstherandomnessoftheparticlesinsizeandimpactpositionwiththeappropriatedistri- butionandforeachparticle,basedonitssize,usesrelatedvelocityandtemperature,unfortunately,the results of the numerical simulation, in case of residual stress assessment, are not in a good agreement withtheexperimentalmeasurementperformedbyX-raydiffraction(XRD).Itmightbeduetothefact thattheFEsimulationcannotmodelthebondingphenomenonwhichseemsveryimportantinthepro- cess. TheFEsimulationpredictsthepossibilityofnanograingenerationonthesubstrateduetothefact (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) “Thesis” — 2011/12/31 — 18:10 — page II — #8 (cid:105) (cid:105) II ABSTRACT thattheequivalentplasticstraincausedbyparticles’impactismeasuredtobemorethanthethreshold valuesuggestedbyscholarsasacriterionforgrainsizerefinement.Thegrainsizeofthecoatedsamples, notonlyonthedepositedmaterialbutalsoonthesubstratebyremovingthedepositedmaterialusingan electro-polishingdevice,hasbeenmeasuredthroughXRDmeasurements.Thein-situXRDtestmachine hasbeenemployedtoextractthediffractionpeakswhichareusuallyusedforstressmeasurements.Voigt formulationisappliedontheoutputoftheXRDdeviceinordertoseparatethegrainsizeandmicrostrain dataandconsequentlymeasurethegrainsizeofthetreatedsamples. Theresultsapprovetheexistence ofnanograinsonthesubstrate,aswasassessedalsobythenumericalsimulation. TheeffectofCScoat- ing on fatigue strength of specimens coated by different powders has been studied using the specimen design suggested by standard (ASTM593). A load control test machine which satisfies the perquisite requirementsofthestandardhasbeendevelopedintheLabs. ofMechanicalEngineeringDepartment, PolitecnicodiMilano; Themachinehasbeencalibratedusingstraingaugesandcheckingtheeffective distanceandfrequencyonthespecimensunderdifferentloads. Inthisregard,sixaluminumalloyseries havebeenpreparedwithdifferenttreatmentsusingdiversealuminumalloypowders. Theresultsofthe purebendingfatiguetestsindicatethatCScoating,regardlessofthedepositedmaterialtype,increases thefatiguestrengthofthetreatedspecimens. Depositingpowderswithhigherhardnesscomparedtothe substrate, will improve the fatigue strength more than the softer powders. It is also observed that the softerdeposition,insomecases,donotparticipatetotheloadbearingprocessduetopartialdelamina- tionofthedepositedmaterialfromthesubstrate. Thelasttentative,hasbeentoproducenanostructures through severe shot peening after application of CS coating. This combined process was expected to resultinbenefitingfromadvantagesofbothcoatingandshotpeeningonthesamples. Inthiscase, the fatiguestrengthdoesnotincreasewithrespecttothepreviousnotshotpeenedseries. Althoughthegrain size on the samples after shot peening are measured to be less than 100 nm, due to the existence of microcracksandseverelydeformedsurfaceofthesamplesthatactassurfacedefects,itisnotpossible tobenefittheadvantagesofthegeneratedNCtoimprovefatiguestrength. (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) “Thesis” — 2011/12/31 — 18:10 — page III — #9 (cid:105) (cid:105) Contents I Introduction 1 1 IntroductiontoNano-crystallineMaterial 3 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 MechanicalpropertiesofNCmaterial . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.1 Elasticproperties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.2 Hardnessandstrength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2.3 Ductility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.2.4 Fatigueandfracture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.3 ProductionofNCstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.3.1 Electrodeposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.3.2 Inert-gascondensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.3.3 Solidstateprocessingmethods . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.4 Measurementtechniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.4.1 Crystallography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.4.2 Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.5 ApplicationofNCmaterial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.5.1 Nanotechnologyinautomotiveapplications . . . . . . . . . . . . . . . . . . . . 14 1.5.2 Industrialapplicationsofnanocompositecoatings . . . . . . . . . . . . . . . . . 14 1.5.3 Biomechanicsandprosthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2 IntroductiontoColdSprayCoating 19 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.2 Coldspraycoatingprocess . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.2.1 Principalprocessparameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.3 Advantagesofcoldspraycoating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.4 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.4.1 Wearresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.4.2 Corrosionprotection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.4.3 Repairofdamagedcomponents . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.4.4 CoatingandNC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.5 Effectofcoldspraycoatingonfatiguestrength . . . . . . . . . . . . . . . . . . . . . . 30 2.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3 ShotPeeningandSurfaceNano-crystallization 35 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.2 Shotpeeningcontrolparameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.2.1 Almenintensityandsaturationcurve . . . . . . . . . . . . . . . . . . . . . . . 37 3.2.2 Surfacecoverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.3 Shotpeeningprocesstoobtainnano-structuredsurface. . . . . . . . . . . . . . . . . . . 38 (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) (cid:105) “Thesis” — 2011/12/31 — 18:10 — page IV — #10 (cid:105) (cid:105) IV CONTENTS 3.4 Fatiguebehaviorofsurfacenanocrystallizedmaterialobtainedbyshotpeening . . . . . 39 3.5 Numericalsimulationofshotpeening . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 II Numericalsimulations 45 4 Criticalvelocitycalculation 47 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 4.2 ExperimentalmeasurementofCV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 4.3 NumericalsimulationforCVassessment: areview . . . . . . . . . . . . . . . . . . . . 50 4.4 AnewapproachtofindtheCVbyFEM . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.4.1 FEMmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.4.2 Boundarycondition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.4.3 Materialmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 4.4.4 Damagecontrol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 4.4.5 Adiabaticshearinstability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.4.6 Meshconvergencestudy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 4.4.7 Challengesinfindingtheshearinstability . . . . . . . . . . . . . . . . . . . . . 57 4.4.8 DiscretenumbersandSobolevspace . . . . . . . . . . . . . . . . . . . . . . . . 57 4.4.9 Algorithmoftheprocess . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 4.4.10 DenoisingTechnique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 4.5 Evaluationofthenumericalmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.5.1 Particledeformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 4.5.2 ComparisonofCV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 4.6 EffectoftheprocessparametersonCV . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.6.1 Temperatureoftheparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.6.2 EffectofShotpeeningbeforecoating . . . . . . . . . . . . . . . . . . . . . . . 69 4.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 5 Numericalassessmentofthecoatedsurface 75 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 5.2 IntroductiontotheFEMmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 5.2.1 Particlesize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 5.2.1.1 Singleshotmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 5.2.1.2 Particlesizeconsiderations . . . . . . . . . . . . . . . . . . . . . . . 77 5.2.2 Particlevelocity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 5.2.2.1 Singleshotmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 5.2.2.2 Particlevelocityconsiderations . . . . . . . . . . . . . . . . . . . . . 81 5.2.3 Particletemperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.2.3.1 Singleparticlemodel . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.2.3.2 Particletemperatureconsideration . . . . . . . . . . . . . . . . . . . 84 5.3 Numericalmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 5.3.1 Obtainingtheresults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 5.3.2 Processimplementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.4 Resultsofmulti-impactmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 (cid:105) (cid:105) (cid:105) (cid:105)