Emerging processes for metallurgical coatings and thin films

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Alain Billard, Francis Maury, Pascal Aubry, Fanny Balbaud-Célérier, Benjamin Bernard, et al..

Emerging processes for metallurgical coatings and thin films. Comptes Rendus Physique, Centre

Mersenne, 2018, 19 (8), pp.755-768. �10.1016/j.crhy.2018.10.005�. �hal-02368690�

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This is a Publisher’s version published in:

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To cite this version:

Billard, Alain and Maury, Francis

and Aubry, Pascal and Balbaud-Célérier,

Fanny and Bernard, Benjamin and Lomello, Fernando and Maskrot, Hicham

and Meillot, Erick and Michau, Alexandre and Schuster, Frédéric Emerging

processes for metallurgical coatings and thin films. (2018) Comptes Rendus

Physique, 19 (8). 755-768. ISSN 1631-0705

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Alain Billard

a

,

Francis Maury

b

,

Pascal Aubry

c

,

Fanny Balbaud-Célérier

c

,

Benjamin Bernard

d

,

Fernando Lomello

c

,

Hicham Maskrot

c

,

Erick Meillot

d

,

Alexandre Michau

c

,

Frédéric Schuster

e

,

∗

,

1

a_FEMTO-ST_Institute_(UMR_CNRS_6174),_Université_Bourgogne_–_{Franche-Comté,}_UTBM,_2,_place_{Lucien-Tharradin,}₂₅₂₀₀_Montbéliard_cedex,

France

b_CIRIMAT,_{CNRS/INPT/UPS,}_4,_allée_{Émile-Monso,}₃₁₀₃₀_Toulouse,_France

c_DEN_–_Service_d’études_analytiques_et_de_réactivité_des_surfaces,_CEA,_Université_{Paris-Saclay,}₉₁₁₉₁_{Gif-sur-Yvette,}_France d_DAM_–_Département_des_matériaux,_CEA_Le_Ripault,₃₇₂₆₀_Monts,_France

e_{Cross-Cutting}_program_on_Materials_and_Processes_Skills,_CEA,_Université_{Paris-Saclay,}₉₁₁₉₁_{Gif-sur-Yvette,}_France

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Articlehistory:

Availableonline6November2018 Keywords:

Magnetronsputtering Chemicalvapordeposition Spraying Lasercladding Mots-clés : Pulvérisationmagnétron Dépôtchimique Projection Rechargementlaser

Innovation inthin-filmdepositionprocesses,thermalspraying andcladding technologies mostlyrelyonevolutionsoftheirpreviousiteration.Alongwithotherexamples,fivecase studiesofemergingelaborationprocessesformetallurgicalcoatingsaredescribedcoupled with their applications. In the frame of the lifetime extension of components exposed to aggressive media ortheir functionalization, thisarticle depicts all the developments of the detailed processes. Physical vapor deposition(PVD) of coatings with exceptional properties is possible thanks to sources generating highly ionized metallic vapors. The controloftheaverageenergyperincidentspeciesand particularlymetallicionsstrongly influencesthe characteristics ofthedeposited layerobtained, for example,withHiPIMS (HighPowerImpulseMagnetron Sputtering).WhilePVDtechniquesare mainlydirective regarding thegrowthofthecoating,chemicalvapor deposition(CVD) processesmanage

to homogeneously coat complex 3D shapes. The use of speciﬁc precursors in DLI–

MOCVD (DirectLiquidInjection– MetalOrganicCVD),carefullyselectedfromthe whole metalorganic chemistry, allows one to eﬃciently treat heat-sensitive substrates and broadenstheir application range. Thethird detailed example ofemerging technologyis suspensionplasmaspraying(SPS).Projectionofvarioussolutionscontainingnanoparticles leadstothegrowthofunusualmorphologiesandmicrostructuresandtothegenerationof porous coatingswith multi-scaledporosity. Onthe otherhand, cold-sprayusesmetallic

powders with higher granulometry and does not modify them during the deposition

process.Asaresult,high-purityanddensematerialsaredepositedwithpropertiessimilar tothoseofwroughtmaterials.Whereascold-sprayissuitableonlyforductilemetals,laser cladding canbeappliedtoceramics, polymersand ofcoursemetals.Lasercladding isa key technologyforadvanced metallurgicalengineeringand alloydevelopment duetoits capabilityforfunctionallygradedmaterialsproductionandcombinatorialsynthesis.

*

Correspondingauthor.

E-mailaddresses:alain.billard@femto-st.fr(A. Billard),frederic.schuster@cea.fr(F. Schuster).

1 _Coordinator.

https://doi.org/10.1016/j.crhy.2018.10.005

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L’innovationdanslesprocédés dedépôtde couchesmincesouépaissesetlaprojection thermiquerepose principalementsur lesévolutionsde leursversions précédentes.Cinq étudesde casde procédés émergents d’élaboration de revêtementsmétallurgiques avec leursapplicationssontproposées.Lesdéveloppementsdesprocédéssontdétaillésdansun contexted’améliorationdeladuréedeviedecomposantsexposésàdesmilieuxagressifs. Ainsi, des revêtements aux propriétés exceptionnelles sont obtenus par des procédés

de dépôt physique en phase vapeur (PVD) grâce à des sources de vapeur métallique

fortementionisée.Les propriétésdes couchessont contrôlablespar exemple en HiPIMS (pulvérisation magnétron par impulsions deforte puissance)via l’énergiemoyenne des ions métalliques. Alors que les techniques PVD sont plutôt directives, les procédés de dépôtchimiqueen phasevapeur (CVD)parviennent àrevêtir uniformémentdesformes complexes.L’utilisation en DLI-MOCVD (Direct LiquidInjection – MetalOrganicCVD) de précurseurssoigneusementsélectionnésauseindelachimieorganométalliquepermetde traitereﬃcacement dessubstrats thermosensiblespour élargir le domaine d’application de ces revêtements. Le troisième exemple de technologie émergente est la projection plasmadesuspensions.Cesdernièrescontenantdesnanoparticules,leurprojectionmène àlacroissance de structures inhabituelles etàdes revêtementsàporosité multiple.La projectionà froidutilisequant àelle despoudres métalliques avec desgranulométries supérieures. Celles-ci ne subissant pas de transformations pendant leur projection, des matériaux denses et de haute pureté sont déposés avec des propriétés comparables à cellesdematériauxcorroyés.Alorsquelaprojectionàfroidconvientauxmétauxductiles, lesrevêtementslaserpeuventêtreappliquésauxcéramiques,polymèresetbiensûraux métaux.Cettetechnologieestessentiellepouruneingénieriemétallurgiqueavancéeetle développementdenouveauxalliagesenraisondesacapacitéàproduiredesmatériauxà gradientetàutiliserlasynthèsecombinatoire.

1. Introduction

Metallurgicalcoatingsandthinfilmshavebeendeveloped inthefield ofhigh-performancemetallurgyforavery long time. Theyoftenrely onthe implementationofquite oldprocesses, butemergingtechnologiesare regularlyinvolved. As surface functionalizationstepsareincreasinglyintegratedfromthebeginningintheconceptionandlifecycleofa compo-nent, itis thenpossible toproduce efficientarchitecturescapableof addressingcomplex andextremeenvironments.The coatingsgenerallyoperateundercombinedstressessuchasmechanicalloadingwithcorrosionorirradiationinadditionto oxidation orcorrosion.Thus, themain driverforthedevelopment ofsurface engineeringinthe metallurgyfield isabout improvingcomponentlifetimes.

For mostapplications, the coating design hasto be done on the basis ofprecise specifications when the elaboration method is dictated by a certain number ofscientific, technological, economic,and environmental criteria. Materials effi-ciency, especially whenitcomes totheuse ofcriticalmetalsforexample,aswell astherecyclability ofscarceresources canalsoinsomecasesbecomeamajorcriterion.

Thispaperdoesnotaimatmakingacomprehensivelistofsurfaceengineeringprocessesformetallurgybut,onthe con-trary,toillustratewithafewwell-targetedexamplestheinnovationintermsofmaterialsandprocessesthroughindustrial applications. Thelatter isoftentheresult oftechnological progressmadeby existingprocesses usually knownfora long time.

Surfacetreatmentprocessesfallintofourmaincategories:thin-ﬁlmdepositionprocesseswith,inparticular,PVD (Phys-ical Vapor Deposition) andCVD (Chemical Vapor Deposition) aswell as their hybridized technologies, thermalspraying processes in the broad sense (plasma high velocity oxy-fuel, cold spray...) leading to thick coatings,diffusion processes (among thempackcementationandderivativeprocesses,implementationofpowderslurries,liquidmetals

. . .

),and weld-ing processes.Conversionprocessesandpost-treatmentsalsoneedtobeconsidered.Theycanconferormodifyparticular propertiestotreatedsurfaces(recrystallizationofextremesurfaceforexample,bylaserorelectronbeamtreatment).

Thepurposeofthisarticleistoillustratepotentialvectorsofprocessinnovationthroughseveralexamples,successively PVD, CVD, thermal spraying, and more specifically plasma spraying and cold spraying, and finally laser cladding, which belongs towelding processes.The complementarity betweentheseprocesses isillustrated inFig. 1interms ofsubstrate temperature, ranging from0◦C to 1000◦C, andcoating thickness, ranging from0.1 μm to 10mm. Regarding PVD tech-nologies,the mainrouteforinnovationisthedevelopment ofnewpowersuppliesleading tometallic vaporswithahigh ionizationratethatresultsinspecificproperties,forinstancethecontrolofmicrostructureandinterfaces.InthecaseofCVD, theappropriatechoiceofmetalorganicprecursorsallowsusfirsttolowerthedepositiontemperatures,makingtheprocess compatiblewithheat-sensitivesubstrates,andsecond toaccesstheentirediversityofchemistryintermsofcompositions

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Fig. 1. Coating process comparison (reproduced from [1]).

andcompounds.Finally,whendealingwiththermalsprayingtechnologies,itisveryclearthattheaccesstoinnovativeand on-demandpowdersconstitutesoneofthemainvectorsoftechnologicalrupture.Itis,forinstance,thecasewiththe possi-bilitytoproducegradientcoatingsorviaacontrolledimplementationofnanopowdersusethroughtheformofsuspensions oraggregates.

2. Innovationinthinﬁlmstechnologies

In the ﬁrst part of the review, two innovative vapor deposition technologies are presented, i.e. the HiPIMS and the DLI–MOCVD. Since they are quite different interms of physical chemistry, they are described separately in two distinct sections.

2.1. High-power impulse magnetron sputtering for more eﬃcient PVD

Ifthephenomenon ofsputteringwas highlightedinthemiddleofthe19thcenturyby W.R.Grove,itisinthesecond halfofthe20thcentury,andmoreparticularlyfromtheworkofP.Sigmundduringthe1960s[2],thattheunderstandingof thephysicalandphysicochemicalphenomena involvedandthetechnologicalprogressledtotheindustrialdevelopmentof sputtering.Themicrostructureandpropertiesofthefilmsbeingrelatedtotheaverageenergyperincidentspecies(Fig.1), manystudieswereconductedfromthe1980sonhighlyionizeddischarges(i-PVD)likelytoleadtothesputteringofmetal ions.Duringthe1990s,theInduced CoupledPlasma(ICP) technique involvingtheadditionofaradio frequencypolarized antennaandtheexploitationofelectron–cyclotronresonance(ECR)fromacavitysubmittedtomicrowaveradiationallowed onetopartiallyionize thesputteredmetal vapor.However,itisonlyfromtheworkofW.M.Posadowskietal.[3] onthe effectoftheincreaseofpowerdissipatedonthetargetthatV.Kouznetzovetal.[4] publishedthefirstworkonhigh-power impulsemagnetron sputtering(HiPIMS) in1999, pavingthewayforinnovative applicationsinthefield ofphysicalvapor depositionprocesses.

Diodesputteringconsistsinestablishinganelectricdischargeinararefiedatmosphere(1Pa)ofraregas,generallyargon, byapplyingapotentialdifference(1kV)betweenthetarget,consistingofthematerialattheoriginofthecoating,andthe wallsof thereactor connectedto themass.The Ar+ ions arethen attractedby the negative electricfield applied tothe targetandtheirimpactcausesthesputteringofmetalatomsbyballisticeffect.Themetalvaporthuscreatedcondenseson thesubstratetoformacoating.Thesynthesisofdensecoatingsrequirestoreducethedepositionpressuretoaround0.1 Pa, which isincompatible withthe possibilityof carryingout a diode discharge dueto a mean free path ofelectrons close to that of thetarget-to-substrate distance (10 cm).Maintaining a stablelow-pressure discharge then requires concentric magnetsplacedbehindthetarget,generatingfield linesthatclosewithinthegasphase,trappingtheelectrons,whichcan thencreateionizingshocksintheimmediatevicinityofthegasphase.Thisiscalledmagnetronsputtering.Theintroduction ofachemically activegassimultaneouslywithargonallows thesynthesisofceramic coatingsfrommetal targets,butthe instability ofthe discharge requiresthe implementationof closed-loopcontrol systems toallow the highratedeposition of stoichiometricceramic coatings.High Power Impulse Magnetron Sputtering(HiPIMS) technologyconsists insupplying thetarget withpulsesof10to 200 μs atfrequenciesofthe orderof100to500Hz.The instantaneous powerdissipated during thepulsescanreachabout1MW,whichcorresponds tocurrentsofafew thousandamperesforabias voltageof 600to 1000 V.However, the average powerremains close to afew kW in orderto preserve the integrity of thetarget. Suchconditionsmakeitpossibletoionizethesputteredmetalvaporaccordingtotwomainmechanisms:byimpactofthe electronsofthedischargeorbyPenningionization,producedbythecollisionwiththeexcitedmetastableargonatoms.

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Fig. 2. Schematicmorphologyofcoatingsasafunctionofthegeneralizedtemperature(T *),whichconsidersthetemperatureshiftcausedbythepotential energyoftheparticlesarrivingonthesurface,andnormalizedenergyofimpingingspecies(E*),whichdescribesthedisplacementandheatingeffects causedbythekineticenergyofthebombardingparticles,seeRef.[5] foranexplanation(reproducedfrom[5]).

•

excellentadhesionofthecoatingsobtainedforlimitedetchingtimesofthesurfacetobecoated,

•

a microstructure and a densityof the layers associated witha level of adjustableinternal stressesthrough the bias voltageofthesubstrates,whichmakesitpossibletoperfectlymanagetheaverageenergyperincidentspecies,

•

greaterhomogeneityofcoatingsdepositedonsurfacesofcomplexgeometry,

•

thepossibilityofsynthesizingstoichiometriclayersforreactivegasﬂowrateslowerthanthecriticalﬂowrateatwhich thisinstabilityoccurs,

•

thepossibilityofcrystallizingphases,includingmetastableones,attemperatureslowerthanthosedeﬁnedby thermo-dynamicequilibriumand/orobtainedbyconventionalmagnetronsputteringmethods.

However, itis necessarytoemphasize thepreponderant effectofthemagneticconﬁgurationof themagnetron onthe performanceoftheHiPIMSprocess,particularlyintermsofdepositionrateandpropensitytomaintainasuﬃcientionization rateofthemetalvaporinthevicinityofthesubstratetobecoated.

ThecontroloftheaverageincidentionenergyassociatedwithHiPIMStechnologyallowstheoptimizationofthe proper-tiesofthinfilmsinmanyareascomparedtothesamecoatingsdepositedbyconventionalmethods,orevenbythemethod ofvacuumcathodicarcdepositioninthefield ofhardandultra-hardcoating.AspresentedinFig.2,HiPIMSallowsoneto accesstofilm-growthregimessuchinzonesuchasinzone3.

Intheﬁeldoftransparentconductiveoxides,theimprovementoftheconductivityofn-typecoatingssuchasZnO:Alis believedto berelatedtoepitaxialeffects.Inthe aeronauticalﬁeld,increasedperformanceshavealso beenobservedwith regard toerosion by sandorwater ofAlTiN coatings comparedto the samecoatings depositedby alternative processes. Finally,exceptionalperformance hasbeenobtainedwithrespecttotheprotectionofnuclearfuelcladdingwithchromium coatings,bothundernominalconditionsandunderloss-of-coolantaccidentconditions(LOCA)[6].Fortheselasttwo appli-cations,theperformancesobtainedareassociatedwithexcellentadhesionofthelayersandcompactnessthatconventional oralternativemethodsdonotachieve.

Concerning theprotective coatings on Zr-basedcladding, an enhanced high-temperature steamcorrosion resistance is measured after300sinthe 1000–1200◦CrangeifcomparedwiththeuncoatedZr-basedcladdingaspresentedinFig.3. Moreover, foranextendoxidationtimesup to6000sunderthe maximumLOCA’s(Lost-of-coolantaccident)temperature (1200◦C),ithasbeenobservedthatthecladsegmentisabletoresisttotheﬁnalwaterquenchingdowntoroom tempera-ture.

TheupscalingprocessonFramatome’sM5®_claddings_for_a_further_{industrialization}_is_currently_in_progress_[₅_]._The_latter isthesolutionchosenbyFramatomeintheframeoftheenhancedaccidenttolerantfuel(EATF)competition[7,8].

The HiPIMS technologyallowsone toimprovethe coatings’adherenceandthemicrostructure’s densityon zirconium-basedsubstrates; itwasdemonstratedthatCr–Si–Nnanocompositessystemspresentapromisingbehavior whentheyare submitted tohigh-temperatureoxidation(Fig.4)[10].Thistypeofnanocompositescoatingswillleadtoanincreaseofthe performancesofclassicalchromiumcoatings[11].

Moreover,otherinterestingHiPIMSproductsareTi–Si–Nnanocompositeshardcoatingsthatpresentanenhanced oxida-tionresistanceathightemperatures[12].ThesecoatingsareusuallyprocessedbyHiPIMScoupledandDC-pulsedmagnetron

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Fig. 3. Comparisonbetweentherespectivehightemperaturesteamoxidationbehaviorsat1200◦Cofuncoatedvs.CrcoatedZircaloy-4samples(reproduced from[9]).

Fig. 4. Phase content evolution of Cr–Si–N HiPMS PVD coatings as a function of the oxidation temperature (reproduced from [10]).

sputtering(PDCMS).Thehybridizationofthesetechniquespermitsanaccuratecontrolofthesiliconcontentincoating,thus allowingonetoaddanoptimalsiliconcontentwhichallowsonetoretainthemechanicalpropertiesunderextreme condi-tions.

AmongthepropertiesrequiredforthedevelopmentofcoatingsfortheenhancedATF,agoodbehaviorunderirradiation innominalconditionsisrequired.TheworkachievedbyMamourSal[13] on thestudyofthebehaviorunderion irradia-tionofCr–Tamultilayeredsystemsprovidesinteresting andpromisinganswers.Indeed,inthiscontext,Cr–Tananolayered systemswitharespectiveperiodof15nmand50nmwereirradiatedintheJannusacceleratorwiththeaimofsimulating their behaviorwhenthey are submittedtoneutron irradiation.Theﬁrst resultsshowthat multilayeredstructure remains intact,evenafteraradiationdoseof250dpa2(Fig.5).

Furthermore,regardingtheresponsetoheavy ionsandheliumimplantation,themostsigniﬁcantresultsshowthatthe Cr–Tainterfacesare goodtrapsforhelium,thepost-implantationdiffusionisvery limitedin systemswitha highdensity of interfaces.Cr–Ta systems with a periodof 15 nm allow 20% of helium accommodation, which induces a self-healing behavior of thesecoatings under irradiation. The Cr–Ta interfaces act as defecttraps that allow a recoveryeﬃciency of above80%forthe15-nmperiodsystems.

As showninthe previous examples,HiPIMS is aprocessing routethat opens manypossibilities interms ofadvanced metallurgy. Recently, MAX phases have been synthesized by HiPIMS in one single step [14,15]. These types ofcoatings

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Fig. 5. Cr–Ta nanolayeredsystemaftera250-dpairradiationdose:(A)transmissionelectronmicroscopy and(B)energydispersive X-rayspectrometry micrographs(reproducedfrom[13]).

present an enhanced erosion resistance. Because of their high temperature resistance and self-healing properties, some researchersarefocusingthestudiesonMAXphasesaspotentialcandidatesforthenextgenerationofEATF[16].

Finally, HiPIMS recentlypermitted to depositnew metallic compounds such as high-entropy alloys (HEAs). Some re-searchershaveshownthatthesematerials havehighstrength,excellentcorrosionresistance,goodthermalstability,anda superiorwearresistance[17].

HiPIMS is an innovative technology that is attracting considerable academic interest because ofits abilityto manage the average energy per incident species that conditions the properties of thinfilms. This enthusiasm is also shared by generators manufacturers who have multiplied over the last decade. Its tricky implementation,in particular because of the considerable influence of the magnetic configuration of the magnetron sources on the performance of the process, andthecostofthegeneratorsmeanthattheindustrialdevelopmentsremainconfidential.Nevertheless,thistechnologyis approachingamaturitythatshouldleadtoanamplificationofindustrialapplicationsoverthenextdecade.

2.2. DLI–MOCVD to take advantage of the diversity and richness of organometallic chemistry

TheChemicalVaporDeposition(CVD)processeshavemanyindustrialapplications,inparticularintheﬁeldofcuttingand forming toolsdueto(i) their highproductivity,(ii)thewide rangeofadvancedcoatingsthat can bedeposited (carbides, nitrides, oxides

. . .

),(iii) theperformance ofthesecoatings optimizedthanks to speciﬁc compositions, microstructures or architectures (multilayers, nanocomposites, compositional gradient

. . .

), and(iv)the goodconformal coverage on complex shapesandgeometries. Inthefield offunctionalthinfilms, CVDisalsoan industrialtechnique, forinstanceforepitaxial growthofsemiconductorsinmicroelectronicsorfordepositingfunctionallayersinsolarcellfabricationandglassindustry. For decades, CVD processes were operating at high temperatures because they were based on the chemistry of the halides,whichconsiderablyslowedtheirdevelopment.Theincreasingdemandforhighly diversifiedandhigh-performance coatingsleadstoprocessdevelopmentsforwhichscale-up,lowdepositiontemperature,anduniformthicknessare perma-nentandstringentrequirements.Inthiscontext,newprocessesareemerging.

The familyofCVDprocessesisbased onthegeneration andtransport ofareactive vapor phasethat uponits decom-position inaprocess chamberproduces thegrowthofacoating onthe surfaceto becoveredby heterogeneouschemical reactions.Theenergyrequiredfortheactivationofboththegasphaseandtheheterogeneousdecompositioncanbemainly thermal, photonicorelectricdischarge(plasma). Plasma-enhancedCVDprocessesareundergoingimportantdevelopments forsurfaceengineering, butthey arenotdiscussedhere.Weonlycite,asanexampleinmechanicalengineering,diamond coatingsthatareindustriallydepositedbymicrowaveplasmaCVDaswearresistantcoatings[18].

Onceovercominglow-temperatureandscale-upconstraints,thermalCVDprocessescanachieve challengesthatarenot feasible ordiﬃcult to demonstrate by other techniques.Therefore, they can be implemented into manufacturing in-line or roll-to-roll CVD reactors to deposit protective coatings (BN, TaC

. . .

) on substrates moving at constant speed [19] or for continuousdeposition offunctional layersatan industrial levelasgraphene[20] andphotovoltaic Si[21]. Also, mass productionofceramiccompositematerialsisperformedbylow-pressureCVDinporousbodies,namelybychemical vapor inﬁltration(CVI)[22].

A recent challenge is the development of a process capable of uniformly covering the inner wall of long tubes with a protective coating a few microns thick. Atomic-layer deposition (ALD) belongs to the familyof CVD, and was initially developed principally for manufacturing metal oxide functional thin ﬁlms. ALD is extremelyconformal and it has been recentlyusedforthedepositionofHfO2 ontheinteriorsurfaceinacomplexcoolingcircuitforcorrosionmitigationbythe cooling ﬂuid [23].However, ALD still suffersfromvery low growthratesto bea viableprocess fordevelopingprotective coatings.

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engineeringachievableinmetalorganicchemistryfordesigningsuitableprecursors,manyofthembeingnowcommercially available.

InitiallyDLI–MOCVDhasbeendevelopedandhasgrownrapidlyforthegrowthoffunctionaloxidethinfilmsforoptical andelectronic devices [24]. Foroxide deposition, carboncontamination ofthe films originatingboth from theprecursor andthe organic solventwas preventedby adding an O2 partial pressure, whichinvolves both combustion andpyrolysis reactions.ThepioneeringworkonDLI–MOCVDofcarbides[25],nitrides[26] andmetals[26] frombis(arene)Mprecursors, whereMisatransitionmetalintheoxidationstatezero(Cr,Mo,W,V. . . ),hasrevealedagreatpotentialforapplicationsas metallurgicalprotectivecoatings.FurtherworkonSiC[28] andHfChasconfirmedtheinterestofthisprocess,forinstance toproducenanostructuredmultilayercoatingsbyreadilycontrollingthegasphase[26].

ThemaindisadvantagesoftheDLI–MOCVDprocessresultfromtheuseofhighlyreactiveandsometimestoxicprecursors, andfromtheimplementationofacomplexchemistrythatcanbedifficulttocontrol,inparticularforupscalingtheprocess. However,botharecentexperimentalresultsdemonstratingthepossibilityofclosed-looprecyclingofeffluents[29] and amodelingapproach[30] contributetotheburgeoningindustrializationpotentialofthisDLI–MOCVDprocess.Furthermore, obtaining increasedperformance ofadvancedcoatingssometimesrequirestherealizationofspecificnanostructuresor ar-chitecturessuch as multilayersor nanocomposites.As a result, complexcoatings sometimeshave tobe made by several techniques,andit isobviousthat thereisaconsiderableinterestforhybridprocessesimplementedinasingle deposition chamber.TheDLI–MOCVDmethod,justlikeitsparentMOCVD technique,hasagoodpotentialforhybridization.For exam-ple,III-nitridetunneljunctionswerefabricatedby ahybridMOCVD/MBE(MolecularBeamEpitaxy)process combinedina single chamber [31].Also, plasma-enhancedDLI–MOCVDwas usedforthegrowthofphase changematerials (GeTe)[32], andanovelhybridMOCVD/ALDreactorhasbeenimplementedtofabricateSnOx/TiO2 multilayers[33].

The Cr–C–Nsystem allows aremarkable ﬂexibility.It isboth a modelfor analogousM–C–Nsystems (M

=

Cr, V,Nb, Mo) where similar precursors exist, and a basic system for many metallurgical protective coatings. Bis(arene)chromium precursors decompositionthrough DLI–MOCVDprocessresults inthegrowthof amorphous

a-CrC

x coatings between623

and773K[29]. Theseorganometallic moleculeshavea sandwichstructure withacenterCratom characterizedbya zero oxidation state.Consequently, noCr(VI) derivative isinvolved inthisREACH(the European regulationaboutRegistration, Evaluation,AuthorizationandrestrictionofCHemicals)compatibleprocess.Theadditionofacarboninhibitorleadstothe depositionofcrystallizedmetallicCr[27] whilecubicCrN(andCr2(C, N)intermediatenitride)canbegrownbycontrolling aNH3 partialpressure[26]. NanostructuredCrC/CrN multilayercoatingscanbe readilyobtainedasshowninFig.6a.The Cr(0)chemistry implementedinthisprocess allows aloop-recyclingofeﬄuents untiltheir entiredecomposition,leading toaconversionyieldcloseto100%,whichsigniﬁcantlydecreaseboththeeconomicandtheenvironmentalimpactsofthe process[28].

The dense and amorphous a-CrCx coatings exhibit a glassy-like microstructure without grain and grain boundaries.

They have a high hardness (20 to 25 GPa) and a good abrasive wear resistance. Thermally stable up to about 873 K, they crystallizedabovethistemperaturetoformchromiumcarbides.Si-doping (2at. %)increasesthetemperatureofthis amorphous-to-crystallizedstructuraltransformationto1023K.Finally,theexcellent high-temperatureoxidationresistance of

a-CrC

xcoatingsmakesthemaneﬃcientbarriertoprotecttheinner-surfaceofZr-basednuclearfuelcladdingsinaccident

conditions(Fig.6b)[34].

Numericalmodelingandmulti-scalesimulationoftheprocessareessentialapproachesforupscaling.ADLI–MOCVDpilot iscurrentlycapableofinternallyprotectsabundleof1-m-longfuelcladdingsegmentswithauniformlythick

a-CrC

x coating

of severalmicrometers, as shownin Fig. 1c.Many evaluations specific to nuclear applications are currently in progress. By continuing the coupling between modeling and experimentation, the scaling progresses towards its final extent, i.e. full-lengthnuclearcladdingsof4m.ThiswouldbeafirsttransferofaDLI–MOCVDprocessinthemetallurgyfield.

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Fig. 6. Scanningelectronmicroscope(SEM)cross-section(a)ofaCrCx/CrNmultilayercoatinggrownbyDLI–MOCVDconstitutedof30layerswithaperiod

of53nm,(b)ofaprotectivea-CrCxcoatingafteroxidationathightemperatureasdescribedin[18],and(c)computationalsimulationofadepositionina

bundleof16cladsegments1minlength,validatingthedesignofthegas-phasedistributor.

Fig. 7. Illustration of a nanoparticles suspension sprayed by SPS (courtesy of CEA).

3. Innovationinthermalsprayingandcladdingtechnologies

While thefirstpartofthisreview dealtwithprocessesbelongingtothinfilms depositiontechniques,thesecond part presentsthreetechnologiescapableofcoatingcomponentswiththickerlayers.Theyconsistinsuspensionplasmaspraying, coldspray,andfinallylasercladding.

3.1. Suspension Plasma Spraying (SPS) for the best use of nanomaterials

The SuspensionPlasmaSprayprocess wasdevelopedfromthepioneer studiesonliquidinjectionthroughﬂame(early 1970s) andplasma (1900s) fornanopowder synthesis [35,36]. If the principle remains closeto the initial developments, liquid feedstock thermalspray processes andparticularlysuspension plasma spraying (SPS) are widely studiedsince the 2000sforcoatingproductionduetotheirvariousapplications,asdescribedbythefollowingreviews[35–37].

SuspensionPlasmaSprayingconsistsintheinjectionofthinparticles(

<

5 μmandmostgenerallybetween0.1and1 μm) throughaplasmajet.(SeeFig.7.)MostoftheplasmaguntechnologiesusedfortheSPSprocessarebasedontheionization ofplasma gases(argon,helium,nitrogenoramixtureofthem) byadirectcurrent(DC)discharge betweenacathodeand an anodenozzle,allowingaplasmajetathightemperature(8000–14 000K)andvelocityranges(800–2200m/s)[35].The injectionofsuch asthinparticles requiresacarriermedium heavierthanconventional carriergases usedinAtmospheric Plasma Spraying (APS).Here, solid particles aredispersed in a liquid,usedas acarrier medium. Thissuspension is then injectedthroughtheplasmajetusingacalibratedinjector(usually0.1–0.5mm).Theliquidisfragmentedindropletsbefore its vaporization.In ordertoreduce the energyconsumptionduring liquidvaporization, alcohol-basedsuspensionscanbe used,provoking combustionwithair.FreeparticlesarethenmoltenandacceleratedsimilarlywiththeAPSprocess before flattening onto the substrate. Theuse ofthinner particlesthan forAPS imposes lower standoffdistances (30–50 mm) to prevent anin-flightre-solidificationofmoltennanoparticles[5].Thelow sizeoftheparticlesusedfortheSPS process in-ducesnewcoatingbuild-upmechanismsleadingtounusualmicrostructuresforthermalspray-likecolumnarmicrostructure [38].

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•

SPScancreatethinﬁlmcoatings(afewmicrometers);

•

SPScansprayvariouscompositionsofmaterialsbyusingasolutionofprecursorsorpowdersinsuspension. Ontheotherhand,theSuspensionPlasmaSprayingprocesspresentsalsosomedrawbacks:

•

thedepositionrateissigniﬁcantlylowerthanconventionalplasmasprayprocesses(2–3timeslower);

•

the lower standoff distance could induce an overheating of the coatedcomponent part, requiringa speciﬁc cooling system(forexample,cryogeniccooling), andalsocouldlimit themovementofthe plasmagunin frontofacomplex coatedcomponent.

TheabilityofSPS toperformporous coating(usually10–30%) withmulti-scaledporosity appearstobe interestingfor energyandbiomedicalapplications.Forexample,SPS coatingscouldimprovethe performancesofSolid OxygenFuelCell (SOFC)asanelectrolyteandenhancetheirbiocompatibilityformedicalprostheses.

Intheaeronauticﬁeld, SPSisclosetoitsindustrialexpansionwithSPSthermalbarriercoatings(TBCs)and/or SPS en-vironmentalbarriercoatings(EBCs)usedfortheprotectionofhotsectionpartsofgasturbineengines.Indeed,theunusual columnarstructureobtainedbySPSisparticularlyinterestingfortheprotectionofNickel-basedsuperalloyturbineblades andvanes. The mainaeronautic OEMs(Original Equipment Manufacturers)develop TBCs usingSPS for improved proper-ties(thermal lifetime, thermalinsulation)atlow costcompared tothe standardcoatings performedbyEB-PVD (Electron Beam-PhysicalVaporDeposition).

SuspensionPlasmaSprayingisclosetobecomeamaturecoatingprocess,mainlysupportedbyaeronauticaldevelopments on TBC. Indeed, this process allows various coating morphologies and can be easily added to industrial thermal spray facilities. These technical developments are supported by plasma spray manufacturers withplasma gun and suspension deliverysystemsaswellasthermalspraypowdermanufacturerswithcommercialsuspensionsreadytouse.

3.2. Cold Spray for dense ductile coatings

TheColdSpray(CS)processwasdevelopedinthemid-1980sbyscientistsfromtheInstituteofTheoreticalandApplied Mechanics oftheSiberianBranch ofthe RussianAcademyofSciences (ITAM ofRAS)inNovosibirsk,Russia[39].Thegoal was to deposita large range ofductile materials onto differentsubstrates inan economic way.The principle is still the same,whilethedeviceshavebeenimprovedduring thesenearly fortyyears.Readerswhowanttogofurthercanreferto thefollowingreferences[38,40,41].

ColdSprayisakineticsprayprocess,utilizingsupersonicjetsofcompressedgastoaccelerateparticlestohighvelocities. Fig.9showsthetypicalprinciple.

Issuingfrom thepowder feedstock, the particles(typically 5–50 μm)are injected in ahigh-pressurelow-temperature gas ﬂow (nitrogen,helium or mixturefrom 0.4– low-pressure device – to5 MPa – high-pressuredevice – from300K to1400K)beforetraveling toade Lavaltype nozzle.Atthenozzleexit,thesolid ductileparticles,inmotionatvelocities between300and1500m/s,plasticallydeformonimpactwiththesubstrate,andconsolidatetocreateacoating.However,it impliesthatparticlevelocitiesatimpactarehigherthanaso-calledcriticalvelocity(seeFig.10),whichdependsonparticle material,size,andmorphology.

TheColdSpray(CS)processisofhighinterestduetothefollowingadvantages:

•

CSiswellsuitedfordepositionofalargerangeofmaterials(temperature-sensitive,oxygen-sensitiveandphase-sensitive materials)becauseofthelowdepositiontemperature;

•

CSofmetalsgenerallyenhancesfatigueresistancebecauseofthemicro“shot-peening”effect,withgenerally compres-siveresidualstressesintheresultantdeposits;

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Fig. 9. Principle of Cold Spray, including a de Laval nozzle type.

Fig. 10. Behavior of impacting Cu particles upon substrate depending on the velocity.

•

metal CS deposits havemicrostructures withhighdegreesofconsolidation similar towrought alloysdueto intrinsic high-energylow-temperaturefeatures;

•

CSdepositspresenthigherdensityandreducedpresenceofoxidephases;

•

CShassigniﬁcantlygreatdepositioneﬃciency(DE),dependingonthematerialproperties;

•

CS presentssmallspraybeamandsooffers moreprecisecontrol overthe areaofdeposition ontothe substrate(less needformaskingoftheas-sprayedpart);

•

CSgenerallyincreasesthepossibilityofdissimilarmaterialsjoiningbecauseoflessheatinputintothesubstrate,which makesthesubstratematerialtobeoflesserimportance;

•

ahighvelocityofcoatingbuildingcanbeobtained(upto1mm/pass);

•

astrongbondingbetweenthemetalmatrixandthedispersant. Ontheotherhand,theColdSprayprocesspresentssomedrawbacks:

•

depending onthebothmaterials, powderandsubstrate,thiscouplecanpresentahighlevelofresidual stresses(due toshotpeening),whichrequiresheattreatmentfortensiondecreasing;

•

allthematerialscannotbedepositedduetoatoohighhardness,forexample,leadingtoatoohighcriticalvelocityand soaweakDE.

E. Lugscheider [42] has emphasized the broadvariety of feedstock materials usedin cold spray andso the potential applicationssuccessfullyused:

– againstcorrosionandwear;

– forrepair,especiallymagnesiumparts; – forelectromagneticinterferenceshielding;

– fordemandingelectric,electronic,orthermalapplicationsaswellaseﬃcientdepositionofsolderingorbrazingalloys; – forconductingstructuresonnonmetalcompositelayers.

For some years, the Cold Spray process also appears as an attractive opportunity for free standing near-shape parts by additive manufacturing [43] (because of the highdeposition eﬃciency, highcoating-building velocity, and numerical increasingpossibilitiesbetweendesigncomputerandrobot).

ColdSprayprocessesare emergingtechnologiesthat addressshortcomingsoftheother thermalsprayprocesses.Their main interests lie in their ability to spray high-purity materials, with no modiﬁcation of the oxide content of sprayed metallic powders, andto make coatings with properties closeto those of wrought materials and localized deposits for repairpurposes.

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Fig. 11. Illustration of the laser cladding principle (a) and picture of the cladding nozzle and (b) of the melt pool during operation.

However, such processescan onlyspray ductilematerials (metals, alloys, cermets).Since 2006, newinvestigations are devotedtoceramiccoatingscoldsprayed,achievedbysprayingagglomeratednanometer-sizedparticles.Thebestresultsare obtainedwithheliumasthespraygas,buttheincreasingcostofdepositionleadstosprayinaclosedvessel(forrecycling 90–95%ofthegasused).Thatiswhydevelopmentsarealsocarriedoutwithnitrogenastheworkinggas.

3.3. Laser cladding for combinatorial synthesis and graded materials

Lasercladdingwas developedatthe endof1970salong withthedevelopment oflasertechnologiesformaterial pro-cessingapplications.Withtheincreaseinlaserbeamcapacityenablingthemeltingofmaterials,thisprocesswasoptimized andusedtogeneratethickanddensedepositsofheterogeneousmaterialonasubstratesurface.Theresultingmaterialsare characterized by improvedphysical propertiesoftheir surfaces aswear,corrosionresistance orthermal barriers.Initially developedforcoatingapplications,lasercladdingwasextendedtomanufacture3Dshapes.

Thelasercladdingprocessconsistsofasprayofaconcentratedpowderflowunderalaserbeamonasubstratesurface. The melting of the powder and ofthe upper part of the substratesurface layer generates a melt pool. The progressive movement of the cladding nozzle results in the production of a solidified layer of additional material with a typical thickness from100 μm to some millimeters (see Fig. 11). Instead of powder, a fillerwire can also be used asmaterial provision.

Lasercladdinghasseveraladvantages:

•

ﬁnecontrolofthemeltingzoneandassociatedfusionprocesses,thankstothecapabilityofcontrollingenergy deposi-tion;

•

largetreatedzoneallowingthinorthickdepositsthankstothevariationofthelaserbeamproperties;

•

productionofmaterialswithveryﬁnemicrostructureduetofastsolidiﬁcation;

•

productionofdenseandheterogeneousmaterials;

•

largerangeandtypeofmaterials(metals,ceramics[44] andpolymers).

In the framework of cladding processes, laser cladding can be compared to other welding type processes as Plasma Transferred Arc Welding (PTAW), where the fusion energy is provided by an electric arc. If PTAW is a commonlyused process formanufacturing thickcoatings,therangeofthetreatedzoneislimitedcomparedto laser.Moreover, thePTAW allowsonlyhighdepositionratesfromabout1mmlayerthicknessandinduces deepdilutionzones. Finally,theobtained microstructureistypicallycoarserthanthemicrostructuresynthetizedbylaserprocess.

Thankstothehighversatilityofthelaserfusionenergy,alargerangeofapplicationscanbeenvisaged:

•

depositionofhardfacingmaterialsforwearresistance(Fig.12)[45,46].Usinglasercladdingleadstogeneratecoating ofhighquality:controlledgeometryanddilution,thinmicrostructure,highhardness

. . .

•

depositionofnear-shapegeometries(Fig.13).Laseradditivemanufacturingallowsthindepositionanditscapabilityto build accuratestructuresisadaptedto repairconditions(optimizationoftherepair time,thermalcyclesandinduced stress

. . .

);

•

possibilityofmixingseveralmaterials withvariouschemicalcompositions.Thedifferentpowdersaredirectlyinjected fromdifferentpowderfeedersandaremixedintothenozzlebeforesimultaneousmelting.Thisspeciﬁcprocessallows manufacturingmetallic-matrixcompositematerials;

•

possibilityofacontinuousvariationofthechemicalcompositionofthedepositbyvaryingthefeedingrateofdifferent powders.Thistechniquecanbeusedtoproducegradedmaterialsin3D[48].Thisisparticularlyinterestingfordiﬃcult boundingsituations, as adaptionof differentialdilation coeﬃcientsbetween the substrateandthe coating materials

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Fig. 12. SEM observations of thin Stellite 6 microstructure obtained by laser cladding.

Fig. 13. Laseradditivemanufacturingisusedtobuildthethinwallsforthelatticeonanaerospacecomponent.Thelatticeisﬁlledwithaceramictoform asealagainstthebladethatrubsagainstit(reproducedfrom[47],courtesyofRolls-Royce).

or metallurgical incompatibilities (direct synthesis of a speciﬁc intermediate layer). By extension, the laser cladding processenablestheproductionofmetallicmaterialsbyusingacombinatorialmethodology.Theevolutionofthe com-positionbycontrollingthepowderﬂowsofdifferentpowderfeedersallowsgeneratingalargerangeofalloysinasingle experimentalcampaign.

Although thisprocess hasbeen developedfor about40 yearsand isnow a widely used process to produce material surface withimproved physicalproperties,lasercladding hasto beconsidered asa keyopportunity inthe researchﬁeld dedicated to materials engineering. The adaptation of thisprocess to therecent development performed in the metallic additivemanufacturingdomainallowsconsideringalargerangeofpotentialapplicationsasrepairand3Dgradedmaterials.

4. Concludingremarks

Surfaceengineeringisemergingasastrategicdisciplineinthefieldofhigh-performancemetallurgy.Originallydeveloped toprotect componentsagainst theaggressiveenvironmentswheretheywork,itismoreandmoreoftenusedto function-alize themby activatingtheir surface regardingspecific physicalor chemicalphenomena. Photocatalytic,anti-bacterial or anti-biofoulingsurfacesaredeveloped,forexample,respectivelyinthefieldsofairandwaterdepollutionandmarine ener-gies. Surfaceengineeringalsoprovideslow-costmetalsubstrateswithhighaddedvaluelikemetallicsurfacescoveredwith thin-filmphotovoltaiccellsforsolarthermalapplications.

The few examples of emerging technologies and applications presented in this article demonstrate that through the constant evolution of surface treatment processes, it is possible to propose innovative solutions, including for the most stringent industrialsectorsofhighperformance metallurgy.The development ofHiPIMS PVDprocesses, ortheintegrated

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istheswiftdevelopment of3D printingtechnologiesallowingone togeneratetopologicallyoptimizedcomponents,often withcomplexorporousalveolararchitecturesthatmustbeprotectedfromaggressivemediaormustbesurface functional-ized.Theﬁnishingtechnologiesofadditive manufacturingpartsmustalsobedevelopedtominimizetheimpactofallthe defectsappearingduringthe3Dprintingstep.Itisforinstancethecaseofthedevelopmentofelectrolyticplasmapolishing technologies.

Finally,newmethodologiesandnumericaltoolsenablemuch fasterdevelopment offunctionalcoatingsanddeposition processes. High-throughput approaches such as combinatorial synthesis and artiﬁcial intelligence contribution have just startedtorapidlyoptimizecomplexprocesses,thanksinparticulartotheuseofneuralnetworks.

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