On Coating Techniques For Surface Protection: A Review

more susceptible to a corrosive media. Figure 1 represents a schematic view of different types ofelectron beam PVD machines. In this method, the coating growth is dominated by a physicalevaporation process. The thermal energy needed for evaporation may be supplied by differentsupply units, such as electron beam, heating wire, laser beam, molecular beam, etc. [10]. This thermalenergy heats the atoms of the source material, which can be in the form of solid or liquid, to itsJ. Manuf. Mater. Process. 2019, 3, 283 of 22evaporation point. The vaporized atoms travel a distance through the vacuum and deposit onto thesubstrate.In different studies, the material composition of PVD coatings was investigated and they2.1. PhysicalVapor Deposition (PVD) Coatingclaimed that the base material of the coating significantly affected corrosion properties of the coatedPVD processis famousfor offeringand wearthin protectivefilms on theparts.As an example,Mathewet al. corrosion[11] investigatedtheresistancecorrosion andpropertiesof two differentcompositionsof single-layeredTi-basedto(TiCxOy) andZr-based(ZrCOy) coating layers.surfaceof the materialsthat are exposedcorrosivemedia,andits aredtotheZr-based,oneandin andobjects to industrial parts [9]. The advantage of this method is that the mechanical, c properties of the coating layers could be adjusted on demand. In general, PVD is a processof oxygen in the coating composition. In other research related to the food industry bythat fractionstakes placein a high vacuum and the solid/liquid materials transfer to a vapor phase followed byDamborenea et al. [10], the effect of the acidic environment of artificial casings in an acidic range ofa metal vapor condensation, which creates a solid and dense film. The most known types of PVD are1–3 pH was investigated. They reported that the PVD coating of TiN on the surface of stainless-steelsputtering and evaporation. Since the coating layers created by PVD are thin in nature, there is alwaysequipment increased corrosion resistance and protected the equipment from corrosion failure for aa needfor multilayeredcoatingswhile thematerialsselectionselectionbe consideredcarefully.Apartsignificantlylonger time.In additionto materialforshouldPVD coatingcompositions,manyfromresearchersits decorativeapplications,manypartsserve ascomponentsundergoa high rateinvestigatedthe effectof PVD-coatedcoating quality,porosity,andadhesion onthatdifferentsubstratesof tinglayer.Thisphenomenonreducessuch as stainless steel, Ti-based alloys, and ceramics [12–15]. In summary, PVD coating can be utilizedcorrosionresistancepropertiesof the partsand makesthem moresusceptibleto anda corrosivein manyapplicationssuch as firearms.media.Itprovidesthe advantageof flexibilityusing anytypesorganicand inorganicas a depositionFigure1 representsa schematicview ofindifferentof electronbeammaterialPVD machines.In thislayermethod,while thecoatingoffers highcorrosionprocess.resistance[16].The PVDprocessfor forthe coatinggrowthis layerdominatedby a hardnessphysical ionofthepolymerthatevaporation may be supplied by different supply units, such as electron beam, heating wire, laserreduces the molecular weight of the film. PVD has been used for polyethylene (PE), polyvinylidenebeam,molecular beam, etc. [10]. This thermal energy heats the atoms of the source material, whichfluoride (PVDF), and conductive π-conjugated polymers such as poly(2,5-thienylene) (PTh), andcan be in the form of solid or liquid, to its evaporation point. The vaporized atoms travel a distancepoly(pyridine-2-5-diyl) (PPy) [17,18].through the vacuum and deposit onto the substrate.Figure1. Schematicviewofofa physical(PVD)machinemachineusingelectronbeamas theFigure1. Schematicviewa physicalvaporvapor depositiondeposition (PVD)usingelectronbeamas thesource.heat heatsource.In different studies, the material composition of PVD coatings was investigated and they claimedthat the base material of the coating significantly affected corrosion properties of the coated parts.As an example, Mathew et al. [11] investigated the corrosion properties of two different compositionsof single-layered Ti-based (TiCx Oy ) and Zr-based (ZrCx Oy ) coating layers. They claimed that theTi-based group provides better corrosion resistance compared to the Zr-based, one and in the Ti-basedgroup, the highest corrosion enhancement was provided by samples with 0.55–0.79 fractions of oxygenin the coating composition. In other research related to the food industry by Damborenea et al. [10],the effect of the acidic environment of artificial casings in an acidic range of 1–3 pH was investigated.They reported that the PVD coating of TiN on the surface of stainless-steel equipment increasedcorrosion resistance and protected the equipment from corrosion failure for a significantly longer time.In addition to material selection for PVD coating compositions, many researchers investigated theeffect of coating quality, porosity, and adhesion on different substrates such as stainless steel, Ti-basedalloys, and ceramics [12–15]. In summary, PVD coating can be utilized in many applications suchas aerospace, automotive, biomedical instruments, optics, and firearms. It provides the advantageof flexibility in using any organic and inorganic material as a deposition layer while the coatinglayer offers high hardness and corrosion resistance [16]. The PVD process for polymeric materialsis challenging since the deposition leads to degradation of the polymer that reduces the molecularweight of the film. PVD has been used for polyethylene (PE), polyvinylidene fluoride (PVDF), and

J. Manuf. Mater. Process. 2019, 3, 284 of 22conductive π-conjugated polymers such as poly(2,5-thienylene) (PTh), and poly(pyridine-2-5-diyl)(PPy) [17,18].J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW2.2. Chemical Vapor Deposition (CVD) Coating4 of 212.2.ChemicalVaporDeposition(CVD) CoatingAnothertypeof vapordepositionis called CVD. This process undergoes a high vacuum andis widelyAnotherused intypethe semiconductorsindustrysolid, undergoeshigh quality,andvacuuma high andresistanceof vapor depositionis calledprovidingCVD. This aprocessa ngasolid,highquality,andahighresistancecoating layer on any substrate [19–22]. CVD can be used for mechanical parts in constant contact,coatingon anyagainstsubstrate[19–22]. CVDcan beInusedmechanicalparts in constantwhichneed layerprotectioncorrosionand wear.thisforprocess,the substrate,knowncontact,as a nthisprocess,thesubstrate,knownasawafer,would be exposed to a set of volatile material precursors where a chemical reaction creates a depositionwould be exposed to a set of volatile material precursors where a chemical reaction creates alayer on the surface of the material. However, some byproducts of these chemical reactions, which aredeposition layer on the surface of the material. However, some byproducts of these chemicalremoved by constant airflow of the vacuum pump, can remain in the chamber. A schematic of thereactions, which are removed by constant airflow of the vacuum pump, can remain in the chamber.CVD setup is shown in Figure 2. The vaporized CVD materials are pumped from the right side andA schematic of the CVD setup is shown in Figure 2. The vaporized CVD materials are pumped fromthe heaterskeephigh theenoughto facilitatechemicalreactionsubstratethe rightsidetheandtemperaturethe heaters keeptemperaturehigh theenoughto facilitatethebetweenchemicalthereactionand vaporizedmaterials.between the substrate and vaporized materials.Figure2. Schematicchemicalchemicalvaporvapor depositiondeposition (CVD)parts,andandoperationFigure2. Schematic(CVD) hanism[23].mechanism [23].techniqueprovidesaa wideof ofmaterialsin differentcompositionsand tionmaterialsin sofsuch as carbides, nitrides, oxynitrides, a composition of Si with O and Ge, carbon in forms offluorocarbons, diamond, polymers, graphene, fibers/nanofibers/nanotubes, Ti, and W. In addition,fluorocarbons,diamond, polymers, graphene, fibers/nanofibers/nanotubes, Ti, and W. In addition,these materials could be provided in different microstructures such as monocrystalline,these materials could be provided in different microstructures such as monocrystalline, polycrystalline,polycrystalline, and amorphous [24,25]. Moreover, CVD of polymers has been shown to be a reliableand amorphous [24,25]. Moreover, CVD of polymers has been shown to be a reliable process inprocess in applications such as biomedical device implants, circuit boards, and durable lubriciousapplicationssuchCVDas biomedicaldeviceinimplants,circuitboards, ofanddurable lubriciouscoatingscoatings [26].process performsthree differentcategoriesatmosphericpressure CVD,low- [26].CVDpressureprocess CVD,performsin three differentof atmosphericpressureCVD,and ultra-highvacuum categoriesCVD, and thelast two methodsare themostlow-pressurecommon onesCVD,and [27].ultra-highvacuumand the last twomethodsare themost basedcommonones [27].There areThere aremany CVD,other classificationsrelatedto the CVDprocesson substrateheating,manyother classificationsto plasmathe CVDprocesson ties, andrelatedtypes ofutilizedin basedvaporizingthe materials.Thesesecond-handcategoriesoften includeaerosol-assisteddirect liquidinjectionCVD, plasma-enhancedCVD,and typesof plasmautilizedin vaporizingCVD,the materials.Thesesecond-handcategories oftenincludemicrowave-plasma-assistedCVD,hybrid physical-chemicalCVD, andphoto-assistedCVD [28,29].aerosol-assistedCVD, direct liquidinjectionCVD, rephysical-chemicalarguments on the CVD,advantagesand disadvantagesCVD Thereover PVDbased on theCVD,hybridand photo-assistedCVDof[28,29].are argumentson theapplications. In the CVD process, the substrate is heated up to 900 , which cannot be used foradvantages and disadvantages of CVD over PVD based on the applications. In the CVD process,temperature-sensitive materials. PVD provides a solution for materials of this kind. On the otherthe substrate is heated up to 900 C, which cannot be used for temperature-sensitive materials. PVDhand, CVD has the advantage of less waste of materials since only the heated area can be coated. Inprovidesa solution for materials of this kind. On the other hand, CVD has the advantage of lessorder to enhance this capability, computer-controlled lasers could be utilized to selectively heat thewasteofmaterialssince only the heated area can be coated. In order to enhance this capability,preferred areas [30,31].computer-controlled lasers could be utilized to selectively heat the preferred areas [30,31].2.3. Micro-Arc Oxidation (MAO) Coating2.3. Micro-Arc Oxidation (MAO) CoatingMAO process is known as a flexible process of coating regarding the composition of coatingMAO Theprocessis knowna flexibleprocesscoatingtheutilizescompositionof coatinglayers.schematicof the asprocessis illustratedin ofFigure3. In regardinggeneral, MAOa high voltagedifferencebetweenofanodeand cathodeto generateasgeneral,plasma channels.Whenatheselayers.The schematicthe processis illustratedin micro-arcsFigure 3. InMAO o-arcs.At hitdifference between anode and cathode to generate micro-arcs as plasma channels. When these arcsthe same time, plasma channels release their pressure, which assists the deposition of coating

J. Manuf. Mater. Process. 2019, 3, 285 of 22the substrate, they melt a portion of the surface, depending on the intensity of the micro-arcs. At thesameJ. time,channelstheirpressure, which assists the deposition of coating materialsinManuf. plasmaMater. Process.2019, 3, xreleaseFOR PEERREVIEW5 of 21J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW5 of 21the working electrolyte on the substrate surface. The existing oxygen inside the electrolyte causes amaterialsin theworking electrolyteon oxidesthe thechemicalreactionof oxidationand providesdepositedon thesurfaceof thesubstratematerialsin theworking electrolyteon the substratesurface.Theexistingoxygeninside theelectrolyte causes a chemical reaction of oxidation and provides oxides deposited on the surface ofThe versatilityof thisa processin theofflexibilityof lyte causeschemicalliesreactionoxidation andprovides oxidesdepositedon theofthe substrate materials. The versatility of this process lies in the flexibility of combining desiredas a solutein the materials.working electrolyte.To ofdate,materialscommonlywithMAO arethe substrateThe versatilitythistheprocesslies inmostthe flexibilityof coatedcombiningdesiredelements and compounds as a solute in the working electrolyte. To date, the materials mostelementsandtheircompoundsas aHighsolutecorrosionin the workingelectrolyte.To date,the materialsmost ofAl, Mg,Ti, andalloys [32].resistanceis the mostimportantcharacteristiccommonly coated with MAO are Al, Mg, Ti, and their alloys [32]. High corrosion resistance is [32].Highcorrosionresistanceisthea MAO-treatedlayer.In addition,a porouslayer.structure,this coatingprovideshighmost importantcharacteristicof abeingMAO-treatedIn addition,being sandfixations[33].coatinglayerformedprovidesbone ingrowthwhileformedon biomedicalimplants and fixations [33].coating layer provides high bone ingrowth while formed on biomedical implants and fixations [33].FigureSchematicviewview ofof micro-arcmicro-arc oxidationprocess.Figure3. 3.Schematicoxidation(MAO)(MAO)process.Figure 3. Schematic view of micro-arc oxidation (MAO) process.Advantagesof MAOa hileAdvantagesof MAOcancanbe opertiesAdvantages of MAO can be a coating surface with high hardness and adherence it haswhiledifferentof porositythroughoutits structure.This typeof multi-structuralnaturecomesit hasscalesdifferentscales of porositythroughoutits structure.This typeof requenciesfromcomesthe eunderdifferentfrequenciesresultingfrom the coating itself. Figure 4 illustrates a MAO-treated surface under different frequenciesresulting in porous structures with different porosities. At the first steps of coating, a solid layer ofin sities.the first stepsoffirstcoating,layera ofmetallicoxidesresultingin porousstructuresdifferentAt thestepsaofsolidcoating,solidlayer ofmetallic oxides covers the substrate called barrier inner layer. The porous structure is created on topcoversthe he porousstructureis createdon topisofcreatedthis layerduringmetallicoxides coverssubstrateinner layer.The porousstructureon topof this layer during the next steps of coating with a reported thickness of up to 100 μm [34]. Thisof thislayernexta stepsof coatingwithofa reportedup toporous100 μmstructure[34]. Thisis thethe nextstepsofduringcoatingthewithreportedthicknessup to 100thicknessµm [34].ofThisporous structure is the reason for increased surface adhesion in bio applications. The for increasedsurfaceadhesionin bio ingqualityaffectingthe coatingqualityare voltage,current tthe coatingqualityare ime,type,pulsateare ecurrent,currenti.e.,current, and current type, i.e., AC or DC [35,36]. However, many researchers utilized different ACcurrent, and current type, i.e., AC or DC [35,36]. However, many researchers utilized differentor d itofhasprocessparameterrangesandit has beenutilizedclaimed differentthat in allprocessthe studies,corrosionpropertiesthebeenprocess parameter ranges and it has been claimed that in all the studies, corrosion properties of theclaimedthatsamplesin all thestudies, corrosionpropertiesof the decreasedcoated samplesimprovedwhileThemetalliccoatedimprovedwhile metallicion releasesignificantly[37–39].only ioncoated samples improved while metallic ion release decreased significantly [37–39]. The onlydisadvantageof the MAO processbe itsdisadvantagelimitation in substratematerialsthatmightare mostlyreleasedecreased significantly[37–39].mightThe onlyof the MAOprocessbe its valvelimitationdisadvantage of the MAO process might be its limitation in substrate materials that are mostly valvemetalssuchasAl,Mg,Ti,Zr,Nb,andTa[35].in substratematerialsthat Ti,areZr,mostlyvalvemetals such as Al, Mg, Ti, Zr, Nb, and Ta [35].metals suchas Al, Mg,Nb, andTa [35].Figure 4. SEM micrographs of MAO coating structures under different frequencies of (a) 60 Hz, (b)FigureSEM4. SEMmicrographsofMAOMAOcoatingcoating structuresstructures underfrequenciesof requenciesof60(a)Hz,60 (b)Hz, (b)5004.Hz,(c) 1000Hz, and (d)Hz [40].500Hz,(c)1000Hz,and(d)2000Hz[40].500 Hz, (c) 1000 Hz, and (d) 2000 Hz [40].

Manuf. Mater.Mater. Process.Process. 2019,2019, 3,3, 28x FOR PEER REVIEWJ.J. Manuf.66 of21of 222.4. Electrodeposition Coating2.4. Electrodeposition CoatingElectrodeposition of materials is considered a type of protection utilizing the deposition ofElectrodepositionof materialsconsidereda type ofinprotectiondepositionof metallicmetallicions on a substrate.In this isprocess,a differencepotential utilizingbetween theanodeand cathodepolesionsonana substrate.In thisprocess,a differencein causesion transferin theunit cell.After a while,a coatinglayer formsthesubmergedantransferionsin theunit thecell.otherAfter electrode.a while, a Extensivecoating layerformshaveon thesubmergedsamplebyby ionreceivingfromstudiesbeendone onpopularreceivingions frommaterials.the other electrode.Extensivestudieshave beenon beenpopularelectrodepositionelectrodepositionThe commongroupof scommongroupmetalsthat havebeen Ag/Pd,intensivelystudiedincludes,butis notlimitedincludes, butnot andCo/Pt[41–to,Ni-P/Sn,Ag/Pd,Cu/Ni, Co/Ag,andsignificantlyCo/Pt [41–43].Accordingto these43].Ni-P,Accordingto Ni-P-W,these e corrosionstudies,theofelectrodepositedcoatings significantlyenhancethe showncorrosionpropertiesof thesubstrate.propertiesthe substrate. Moreover,this techniquehas beento bepromisingin producingMoreover,this techniquehas beenshownto bein ssuchaspromisingpolythiophene[44]. In superhydrophobicgeneral, electrodepositioniscoatingssuchas twopolythiophene[44]. Inelectrodepositionis categorizedinto twodepositionprocessescategorizedintoprocesses knownasgeneral,electrolyticdeposition (ELD)and electrophoreticknownas electrolyticdepositionand electrophoretic(EPD), whichare discussedmore (ELD)in the followingsections. deposition (EPD), which are discussedmore in the following sections.2.4.1. Electrolytic Deposition (ELD) Coating2.4.1. Electrolytic Deposition (ELD) CoatingELD is an electrochemical process employed to form a dense metallic coating with a uniformELDan electrochemicalprocessemployedSubstrateto form aanddensemetallicmaterialscoating witha uniformthickness isdistributionon conductivesubstrates.depositionare selectedasthicknessdistributionon conductivesubstrates.Substrate andmaterialsare selectedascathode andanode whileplaced insidean electrochemicalunitdepositioncell. Figure5 illustratesa generalcathodewhile placedinside aanpotentialelectrochemicalunitbetweencell. Figure5 illustratesa generaloverviewandof anodethe process.By applyingdifferenceanodeand tialdifferencebetweenanodeandcathodepoles,metallic ions move toward working electrolyte and from there toward the substrate. The depositionmetallicions movetoward workingelectrolytewhichand fromtheretowardthe substrate.Thephase requiressuper-saturationof electrolyte,occursdueto chargingcurrent inthedepositioncircuit. occursduetochargingcurrentinthecircuit.this technique, the concentration of metallic ions of electrolyte remains constant during the coatingInthis technique,the concentrationof metallicions ofusedelectrolyteremains constantduring the coatingprocess[45]. Althoughthis methodis mostlyfor decorativeand low-corrosion/wearpr

deficiencies of coating techniques by using the benefits of each process in a multi-method coating. In this article, these coating methods are categorized, and compared. By developing more advanced coating techniques and materials it is possible to enhance the qualities of protection in the future.