Journal of Manufacturing and Materials Processing

Review On Coating Techniques for Surface Protection: A Review

Behzad Fotovvati 1,* , Navid Namdari 2 and Amir Dehghanghadikolaei 3

1 Department of Mechanical Engineering, The University of Memphis, Memphis, TN 38152, USA 2 Mechanical, Industrial and Manufacturing Engineering Department, The University of Toledo, Toledo, OH 43606, USA; [email protected] 3 School of Mechanical, Industrial and Manufacturing Engineering, Oregon State University, Corvallis, OR 97331, USA; [email protected] * Correspondence: [email protected]



Received: 21 February 2019; Accepted: 18 March 2019; Published: 25 March 2019


Abstract: A wide variety of coating methods and materials are available for different coating applications with a common purpose of protecting a part or structure exposed to mechanical or chemical damage. A benefit of this protective function is to decrease manufacturing cost since fabrication of new parts is not needed. Available coating materials include hard and stiff metallic alloys, ceramics, bio-glasses, polymers, and engineered plastic materials, giving designers a variety freedom of choices for durable protection. To date, numerous processes such as physical/chemical vapor deposition, micro-arc oxidation, sol–gel, thermal spraying, and electrodeposition processes have been introduced and investigated. Although each of these processes provides advantages, there are always drawbacks limiting their application. However, there are many solutions to overcome 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.

Keywords: surface modification; sol–gel; thermal spray; vapor deposition

1. Introduction Mechanical parts and structures are designed for specific applications. Prior to fabricating these parts, some extensive material selection constraints have to be met. These constraints include body materials, mechanical properties (e.g., tension, compression, yield, torsion, fatigue, bending, and creep), desired functionality (e.g., friction properties, hydrophobicity, wear resistance), thermal properties (e.g., thermal expansion and conductivity to transfer heat flux), electrical conductivity, dynamic load bearing (e.g., vibrations, high-speed rotation), and corrosion resistance. In addition, other parameters such as availability, cost of materials, safety, and toxicity of these materials must be considered. The latter category plays an important role in finalizing the material selection processes in advance to manufacturing mechanical parts and structures. For instance, silver is known to offer high electrical conductivity values, but fabricating a huge bulk of silver for electrical conductivity applications is too costly [1]. NiTi alloys are well-known for showing the shape memory effect (SME) and superelasticity (SE), which are useful in designing new actuators. Moreover, these alloys provide high biocompatibility as they are used as bone implants that could be combined with SME, SE, or both, to develop new biomedical devices for micro-surgeries inside the human body. However, the corrosion process of NiTi in physiological environments releases Ni ions as byproducts, which are a toxic and harmful category of materials for living organs [2]. Copper is a material with high thermal and electrical conductivity with many applications such as brazing advanced materials; however, it suffers from low stiffness

J. Manuf. Mater. Process. 2019, 3, 28; doi:10.3390/jmmp3010028 www.mdpi.com/journal/jmmp J. Manuf. Mater. Process. 2019, 3, 28 2 of 22 and wear resistance. In the case of copper rotary cooling fins, the durability of the mechanical parts decreases significantly due to the high susceptibility of copper to wear mechanism [3]. To overcome these issues and to enhance material properties for specific applications, there have been different methods offered, such as heat treatment, alloying processes, and coatings. Among these solutions, coating processes have the highest portion of material enhancement since coating layers can reduce the cost and neglect scarcity of materials as the thickness of coating layers rarely pass micrometers. This means less material is needed to form coating layers on a bulk of substrate materials. Coatings can offer different properties such as corrosion/wear resistance, enhanced surface hardness, modified surface texture, thermal/electrical insulation, enhanced wettability, hydrophobicity, etc. [4]. Coating methods are available in a wide variety due to the enormous diversity of applications and needs in different fields. These processes consist of many different on-line/off-line parameters while giving way to many different outcomes in the form of material microstructure, effectiveness, suitability, and durability. However, coating methods are useful in specific applications according to the desired functionality among which corrosion and wear protection are the most important [5]. Mechanical properties of the materials decrease by corrosion process whereas the corrosion products are released in different forms that may cause a more extreme corrosive environment or harmful side effects in different applications [6]. Coating materials have deferent deposition mechanisms that needs to be investigated for the revelation of their pros and cons for the desired application. There are many processes available, but only a few are among the most effective and applicable, including physical vapor deposition (PVD), chemical vapor deposition (CVD), micro-arc oxidation (MAO), sol–gel, thermal spray, and polymer coatings. Each of these methods is suitable for different applications as they offer different deposition methods, different materials, second phases, different thicknesses, and densities. As a result, mechanical stability, corrosion properties, biocompatibility (for biomedical applications), and enhancement of material behavior for a specific type of coating have to be considered carefully [7]. Although coating processes are applied to provide the abovementioned benefits, they suffer from disadvantages that degrade their reliability. Of these adverse effects, negative thermal effects (e.g., distortion, crack, delamination, etc.), destructive effects of loose atmospheric protection (e.g., penetration of inclusions and contaminations into the substrate) and coating materials properties (e.g., melting point, availability in different forms of foils/powders/rods, biocompatibility, etc.,) are the most crucial ones to be considered. Materials selection is the key parameter in having a successful coating as they provide all protection purposes. Many different materials, including metals, ceramics, and polymers, can be used to form a protective layer [8]. However, the diversity of coating processes and material properties may cause difficulties in choosing the best composition of the deposited layer. To overcome this issue, the most popular candidates such as Al, Ti, Hf, Zr, Ni, Co, Pt, MgO, ZrO2, Al2O3,Y2O3, BeO, PEEK, and PTFE must be considered while any probable new candidates should not be neglected. Although each of the feedstock materials offer corrosion or wear resistance properties, they possess different melting points, mechanical behavior, and chemical properties. Combined with their availability in different forms of powders, rods, plates, and wires for specific uses, these parameters keep the material selection in a narrower range. This review briefly covers common coating methods, materials, and their surface modification quality whereas there are plenty of other protection processes such as heat treatment, mechanical treatment, mechanical/chemical finishing, and polishing, which have not been covered in this review.

2. Reliable Coating Methods Coating processes provide protection to a specific part or area of a structure exposed to harsh and corrosive environments in different fields ranging from aerospace and the automotive industry to tiny biomedical devices and implants inside the human body. J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 3 of 21

PVD process is famous for offering corrosion and wear resistance and thin protective films on the surface of the materials that are exposed to corrosive media, and its applications range from decorative objects to industrial parts [9]. The advantage of this method is that the mechanical, corrosion, and aesthetic properties of the coating layers could be adjusted on demand. In general, PVD is a process that takes place in a high vacuum and the solid/liquid materials transfer to a vapor phase followed by a metal vapor condensation, which creates a solid and dense film. The most known types of PVD are sputtering and evaporation. Since the coating layers created by PVD are thin in nature, there is always a need for multilayered coatings while the materials selection should be considered carefully. Apart from its decorative applications, many PVD-coated parts serve as components that undergo a high rate of wear that causes abrasion on the surface and removes the coating layer. This phenomenon reduces corrosion resistance properties of the parts and makes them more susceptible to a corrosive media. Figure 1 represents a schematic view of different types of electron beam PVD machines. In this method, the coating growth is dominated by a physical evaporation process. The thermal energy needed for evaporation may be supplied by different supply units, such as electron beam, heating wire, laser beam, molecular beam, etc. [10]. This thermal energy heats the atoms of the source material, which can be in the form of solid or liquid, to its J. Manuf. Mater. Process. 2019, 3, 28 3 of 22 evaporation point. The vaporized atoms travel a distance through the vacuum and deposit onto the substrate. 2.1. PhysicalIn different Vapor Deposition studies, the (PVD) material Coating composition of PVD coatings was investigated and they claimed that the base material of the coating significantly affected corrosion properties of the coated parts.PVD processAs an example, is famous Mathew for offering et al. corrosion[11] investigated and wear the resistance corrosion andproperties thin protective of two different films on the surfacecompositions of the materials of single-layered that are exposed Ti-based to (TiC corrosivexOy) and media, Zr-based and (ZrC itsx applicationsOy) coating layers. range They from claimed decorative objectsthat to the industrial Ti-based group parts [provides9]. The advantagebetter corrosion of this resistance method compared is that the to the mechanical, Zr-based, corrosion,one and in and aestheticthe Ti-based properties group, of thethe coatinghighest layerscorrosion could enhanc be adjustedement was on provided demand. by In sa general,mples with PVD 0.55–0.79 is a process thatfractions takes place of inoxygen a high in vacuumthe coating and composition. the solid/liquid In other materials research transfer related to to a vaporthe food phase industry followed by by Damborenea et al. [10], the effect of the acidic environment of artificial casings in an acidic range of a metal vapor condensation, which creates a solid and dense film. The most known types of PVD are 1–3 pH was investigated. They reported that the PVD coating of TiN on the surface of stainless-steel sputtering and evaporation. Since the coating layers created by PVD are thin in nature, there is always equipment increased corrosion resistance and protected the equipment from corrosion failure for a a needsignificantly for multilayered longer time. coatings In addition while theto material materials selection selection for should PVD coating be considered compositions, carefully. many Apart fromresearchers its decorative investigated applications, the effect many of PVD-coatedcoating qualit partsy, porosity, serve asand components adhesion on that different undergo substrates a high rate of wearsuch that as stainless causes steel, abrasion Ti-based on the alloys, surface and cerami and removescs [12–15]. the In coating summary, layer. PVD This coating phenomenon can be utilized reduces corrosionin many resistance applications properties such as aerospace, of the parts automotive, and makes biomedical them more instruments, susceptible optics, to and a corrosive firearms. media.It Figureprovides1 represents the advantage a schematic of flexibility view of in different using any types organic of electron and inorganic beam material PVD machines. as a deposition In this layer method, the coatingwhile the growth coating is layer dominated offers high by a hardness physical and evaporation corrosion process.resistance The [16]. thermal The PVD energy process needed for for evaporationpolymeric may materials be supplied is challenging by different since the supply depos units,ition leads such to as degradation electron beam, of the heating polymer wire, thatlaser beam,reduces molecular the molecular beam, etc. weight [10]. of This the film. thermal PVD energy has been heats used the for atomspolyethylene of the (PE), source polyvinylidene material, which fluoride (PVDF), and conductive π-conjugated polymers such as poly(2,5-thienylene) (PTh), and can be in the form of solid or liquid, to its evaporation point. The vaporized atoms travel a distance poly(pyridine-2-5-diyl) (PPy) [17,18]. through the vacuum and deposit onto the substrate.

FigureFigure 1. Schematic 1. Schematic view view of of a physicala physical vapor vapor depositiondeposition (PVD) (PVD) machine machine using using electron electron beam beam as the as the heatheat source. source.

In different studies, the material composition of PVD coatings was investigated and they claimed that 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 compositions of single-layered Ti-based (TiCxOy) and Zr-based (ZrCxOy) coating layers. They claimed that the Ti-based group provides better corrosion resistance compared to the Zr-based, one and in the Ti-based group, the highest corrosion enhancement was provided by samples with 0.55–0.79 fractions of oxygen in 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 increased corrosion 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 the effect of coating quality, porosity, and adhesion on different substrates such as stainless steel, Ti-based alloys, and ceramics [12–15]. In summary, PVD coating can be utilized in many applications such as aerospace, automotive, biomedical instruments, optics, and firearms. It provides the advantage of flexibility in using any organic and inorganic material as a deposition layer while the coating layer offers high hardness and corrosion resistance [16]. The PVD process for polymeric materials is challenging since the deposition leads to degradation of the polymer that reduces the molecular weight of the film. PVD has been used for polyethylene (PE), polyvinylidene fluoride (PVDF), and J. Manuf. Mater. Process. 2019, 3, 28 4 of 22 conductive 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 REVIEW 4 of 21 2.2. Chemical Vapor Deposition (CVD) Coating 2.2.Another Chemical type Vapor of vaporDeposition deposition (CVD) Coating is called CVD. This process undergoes a high vacuum and is widelyAnother used in type the semiconductorsof vapor deposition industry is called providing CVD. This aprocess solid, undergoes high quality, a high and vacuum a high and resistance is coatingwidely layer used on in any the substrate semiconductors [19–22 industry]. CVD canproviding be used a solid, for mechanicalhigh quality, parts and a in high constant resistance contact, whichcoating need layer protection on any against substrate corrosion [19–22]. CVD and wear.can be In used this for process, mechanical the substrate,parts in constant known contact, as a wafer, wouldwhich be exposed need protection to a set ofagainst volatile corrosion material and precursors wear. In this where process, a chemical the substrate, reaction known creates as a depositionwafer, layerwould on the be surface exposed of theto a material. set of volatile However, material some precursors byproducts where of these a chemical chemical reaction reactions, creates which a are deposition layer on the surface of the material. However, some byproducts of these chemical removed by constant airflow of the vacuum pump, can remain in the chamber. A schematic of the reactions, which are removed by constant airflow of the vacuum pump, can remain in the chamber. CVD setup is shown in Figure2. The vaporized CVD materials are pumped from the right side and A schematic of the CVD setup is shown in Figure 2. The vaporized CVD materials are pumped from the heatersthe right keep side the and temperature the heaters keep high the enough temperature to facilitate high theenough chemical to facilitate reaction the betweenchemical thereaction substrate and vaporizedbetween the materials. substrate and vaporized materials.

FigureFigure 2. Schematic2. Schematic chemical chemical vaporvapor depositiondeposition (CVD) (CVD) setup, setup, mechanical mechanical parts, parts, and and operation operation mechanismmechanism [23 ].[23].

CVDCVD technique technique provides provides aa wide selection selection of ofmaterials materials in different in different compositions compositions and forms and such forms suchas as carbides, carbides, nitrides, nitrides, oxynitrides, oxynitrides, a composition a composition of Si of with Si with O and O andGe, Ge,carbon carbon in forms in forms of of fluorocarbons,fluorocarbons, diamond, diamond, polymers, polymers graphene,, graphene, fibers/nanofibers/nanotubes,fibers/nanofibers/nanotubes, Ti, and Ti, andW. In W. addition, 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 reliable and amorphous [24,25]. Moreover, CVD of polymers has been shown to be a reliable process in process in applications such as biomedical device implants, circuit boards, and durable lubricious applicationscoatings [26]. such CVD as biomedical process performs device in implants, three differe circuitnt categories boards, of and atmospheric durable lubricious pressure CVD, coatings low- [26]. CVDpressure process CVD, performs and ultra-high in three different vacuum categoriesCVD, and the of atmosphericlast two methods pressure are the CVD, most low-pressure common ones CVD, and[27]. ultra-high There are vacuum many CVD,other classifications and the last tworelated methods to the CVD are the process most based common on substrate ones [27 heating,]. There are manymaterial other classificationsproperties, and related types of to plasma the CVD utilized process in basedvaporizing on substrate the materials. heating, These material second-hand properties, and typescategories of plasma often include utilized aerosol-assisted in vaporizing CVD, the materials. direct liquid These injection second-hand CVD, plasma-enhanced categories often CVD, include aerosol-assistedmicrowave-plasma-assisted CVD, direct liquid CVD, injection hybrid physical CVD, plasma-enhanced-chemical CVD, and CVD, photo-assisted microwave-plasma-assisted CVD [28,29]. CVD,There hybrid are physical-chemicalarguments on the CVD,advantages and photo-assisted and disadvantages CVD of [28 CVD,29]. Thereover PVD are argumentsbased on the on the ℃ advantagesapplications. and In disadvantages the CVD process, of CVD the substrate over PVD is heated based onup theto 900 applications. , which cannot In the be CVD used process,for temperature-sensitive materials. PVD provides a solution for materials of this kind. On the other the substrate is heated up to 900 °C, which cannot be used for temperature-sensitive materials. PVD hand, CVD has the advantage of less waste of materials since only the heated area can be coated. In provides a solution for materials of this kind. On the other hand, CVD has the advantage of less order to enhance this capability, computer-controlled lasers could be utilized to selectively heat the wastepreferred of materials areas [30,31]. since only the heated area can be coated. In order to enhance this capability, computer-controlled lasers could be utilized to selectively heat the preferred areas [30,31]. 2.3. Micro-Arc Oxidation (MAO) Coating 2.3. Micro-Arc Oxidation (MAO) Coating MAO process is known as a flexible process of coating regarding the composition of coating layers.MAO The process schematic is known of the asprocess a flexible is illustrated process in ofFigure coating 3. In regardinggeneral, MAO the utilizes composition a high voltage of coating layers.difference The schematic between ofanode the processand cathode is illustrated to generate in micro-arcs Figure3. In as general, plasma channels. MAO utilizes When a these high arcs voltage differencehit the betweensubstrate, anode they melt and a cathode portion of to the generate surface, micro-arcs depending as on plasma the intensity channels. of the When micro-arcs. these arcsAt hit the same time, plasma channels release their pressure, which assists the deposition of coating

J. Manuf. Mater. Process. 2019, 3, 28 5 of 22 the substrate, they melt a portion of the surface, depending on the intensity of the micro-arcs. At the sameJ. time,Manuf. plasmaMater. Process. channels 2019, 3, x release FOR PEER their REVIEW pressure, which assists the deposition of coating materials 5 of 21 in J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 5 of 21 the working electrolyte on the substrate surface. The existing oxygen inside the electrolyte causes a materials in the working electrolyte on the substrate surface. The existing oxygen inside the chemicalmaterials reaction in the of oxidationworking electrolyte and provides on oxidesthe substrate deposited surface. on the The surface existing of theoxygen substrate inside materials. the electrolyte causes a chemical reaction of oxidation and provides oxides deposited on the surface of The versatilityelectrolyte causes of this a processchemical lies reaction in the of flexibility oxidation and of combining provides oxides desired deposited elements on the and surface compounds of the substrate materials. The versatility of this process lies in the flexibility of combining desired as a solutethe substrate in the materials. working electrolyte.The versatility To of date, this theprocess materials lies in most the flexibility commonly of coatedcombining with desired MAO are elements and compounds as a solute in the working electrolyte. To date, the materials most Al, Mg,elements Ti, and and their compounds alloys [ 32as]. a Highsolute corrosion in the work resistanceing electrolyte. is the most To date, important the materials characteristic most of commonly coated with MAO are Al, Mg, Ti, and their alloys [32]. High corrosion resistance is the commonly coated with MAO are Al, Mg, Ti, and their alloys [32]. High corrosion resistance is the a MAO-treatedmost important layer. characteristic In addition, of a being MAO-treated a porous laye structure,r. In addition, this coating being a layer porous provides structure, high this bone most important characteristic of a MAO-treated layer. In addition, being a porous structure, this ingrowthcoating while layer formed provides on high biomedical bone ingrowth implants while and formed fixations on biomedical [33]. implants and fixations [33]. coating layer provides high bone ingrowth while formed on biomedical implants and fixations [33].

FigureFigure 3. 3.Schematic Schematic view view ofof micro-arc oxidation oxidation (MAO) (MAO) process. process. Figure 3. Schematic view of micro-arc oxidation (MAO) process. AdvantagesAdvantages of MAOof MAO can can be abe coating a coating surface surface with wi highth high hardness hardness and and adherence adherence properties properties while Advantages of MAO can be a coating surface with high hardness and adherence properties while it has different scales of porosity throughout its structure. This type of multi-structural nature it haswhile different it has scalesdifferent of porosityscales of porosity throughout throughout its structure. its structure. This typeThis type of multi-structural of multi-structural nature nature comes comes from the coating itself. Figure 4 illustrates a MAO-treated surface under different frequencies fromcomes the coating from the itself. coating Figure itself.4 illustrates Figure 4 illustrate a MAO-treateds a MAO-treated surface surface under differentunder different frequencies frequencies resulting resulting in porous structures with different porosities. At the first steps of coating, a solid layer of in porousresulting structures in porous with structures different with porosities. different At porositi the firstes. steps At the of first coating, steps aof solid coating, layer a ofsolid metallic layer of oxides metallic oxides covers the substrate called barrier inner layer. The porous structure is created on top coversmetallic the substrate oxides covers called the barrier substrate inner called layer. barrier The porous inner layer. structure The porous is created structure on top is of created this layer on top during of this layer during the next steps of coating with a reported thickness of up to 100 μm [34]. This the nextof this steps layer of during coating the with next a steps reported of coat thicknessing with ofa reported up to 100 thicknessµm [34 ].of This up to porous 100 μm structure [34]. This is the porous structure is the reason for increased surface adhesion in bio applications. The parameters porous structure is the reason for increased surface adhesion in bio applications. The parameters reasonaffecting for increased the coating surface quality adhesion are voltage, in bio applications.current density, The electrolyte parameters type, affecting process the time, coating pulsate quality affecting the coating quality are voltage, current density, electrolyte type, process time, pulsate are voltage,current, currentand current density, type, electrolyte i.e., AC or type, DC process[35,36]. However, time, pulsate many current, researchers and currentutilized type,different i.e., AC current, and current type, i.e., AC or DC [35,36]. However, many researchers utilized different or DCprocess [35,36 parameter]. However, ranges many and researchers it has been utilizedclaimed differentthat in all process the studies, parameter corrosion ranges properties and it of has the been process parameter ranges and it has been claimed that in all the studies, corrosion properties of the claimedcoated that samples in all the improved studies, corrosionwhile metallic properties ion rele ofase the decreased coated samples significantly improved [37–39]. while The metallic only ion coated samples improved while metallic ion release decreased significantly [37–39]. The only releasedisadvantage decreased significantlyof the MAO process [37–39 ].might The onlybe its disadvantage limitation in substrate of the MAO materials process that might are mostly be its valve limitation disadvantage of the MAO process might be its limitation in substrate materials that are mostly valve metals such as Al, Mg, Ti, Zr, Nb, and Ta [35]. in substratemetals such materials as Al, Mg, that Ti, are Zr, mostly Nb, and valve Ta [35]. metals such as Al, Mg, Ti, Zr, Nb, and Ta [35].

Figure 4. SEM micrographs of MAO coating structures under different frequencies of (a) 60 Hz, (b) Figure 4. SEM micrographs of MAO coating structures under different frequencies of (a) 60 Hz, (b) Figure500 4. Hz,SEM (c) 1000 micrographs Hz, and (d of) 2000 MAO Hz coating[40]. structures under different frequencies of (a) 60 Hz, (b) 500 Hz,500 (Hz,c) 1000 (c) 1000 Hz, Hz, and and (d )(d 2000) 2000 Hz Hz [40 [40].].

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2.4. Electrodeposition Coating 2.4. Electrodeposition Coating Electrodeposition of materials is considered a type of protection utilizing the deposition of metallicElectrodeposition ions on a substrate. of materials In this isprocess, considered a difference a type of in protection potential utilizingbetween theanode deposition and cathode of metallic poles ionscauses on an a substrate. ion transfer In thisin the process, unit cell. a difference After a while, in potential a coating between layer forms anode on and the cathode submerged poles sample causes anby ionreceiving transfer ions in the from unit the cell. other After electrode. a while, a Extensive coating layer studies forms have on the been submerged done on sample popular by receivingelectrodeposition ions from materials. the other electrode.The common Extensive group studies of metals have been that done have on been popular intensively electrodeposition studied materials.includes, but The is common not limited group to, ofNi-P, metals Ni-P/Sn, that have Ni-P-W, been Ag/Pd, intensively Cu/Ag, studied Cu/N includes,i, Co/Ag, but and is notCo/Pt limited [41– to,43]. Ni-P, According Ni-P/Sn, to Ni-P-W,these studies, Ag/Pd, the Cu/Ag, electrodepos Cu/Ni,ited Co/Ag, coatings and significantly Co/Pt [41–43 enhance]. According the corrosion to these studies,properties the of electrodeposited the substrate. Moreover, coatings significantlythis technique enhance has been the shown corrosion to be properties promising of thein producing substrate. Moreover,superhydrophobic this technique polymeric has been coatings shown such tobe as promising polythiophene in producing [44]. In superhydrophobicgeneral, electrodeposition polymeric is coatingscategorized such into as two polythiophene processes known [44]. In as general, electrolytic electrodeposition deposition (ELD) is categorized and electrophoretic into two deposition processes known(EPD), which as electrolytic are discussed deposition more (ELD)in the following and electrophoretic sections. deposition (EPD), which are discussed more in the following sections. 2.4.1. Electrolytic Deposition (ELD) Coating 2.4.1. Electrolytic Deposition (ELD) Coating ELD is an electrochemical process employed to form a dense metallic coating with a uniform thicknessELD isdistribution an electrochemical on conducti processve substrates. employed Substrate to form aand dense deposi metalliction materials coatingwith are selected a uniform as thicknesscathode and distribution anode while on conductive placed inside substrates. an electroc Substratehemical and unit deposition cell. Figure materials 5 illustrates are selected a general as cathodeoverview and of anodethe process. while placedBy applying inside a an potent electrochemicalial difference unit between cell. Figure anode5 illustrates and cathode a general poles, overviewmetallic ions of themove process. toward Byworking applying electrolyte a potential and from difference there toward between the anode substrate. and The cathode deposition poles, metallicphase requires ions move super-sa towardturation working of electrolyte, electrolyte wh andich from occurs there due toward to charging the substrate. current in The the deposition circuit. In phasethis technique, requires super-saturationthe concentration ofof electrolyte,metallic ions which of electrolyte occurs due remains to charging constant current during in the the coating circuit. Inprocess this technique, [45]. Although the concentration this method of metallic is mostly ions ofused electrolyte for decorative remains constant and low-corrosion/wear during the coating processapplications, [45]. Although there have this been method reports is mostlyof development used for decorative of other applications and low-corrosion/wear such as optics, applications, electronics, therebiomedical, have been high-temperature, reports of development and solid-oxide of other fuel applications cells [46,47]. such By as further optics, increasing electronics, the biomedical, potential high-temperature,difference in electrolytic and solid-oxide unit cells, ceramic fuel cells materials [46,47]. can By furtherbe deposited increasing on metallic the potential substrates difference that is inmore electrolytic similar to unit the MAO cells, ceramicprocess. Tian materials et al. [48] can deposited be deposited Ni-Co-Al on metallic2O3 on steel substrates pipes and that reported is more similara notable to enhancement the MAO process. of corrosion Tian of et substrate al. [48] deposited exposed to Ni-Co-Al oil sand2 Oslurry.3 on steelYang pipeset al. [49] and deposited reported aNi-Co-SiC notable enhancement on carbon steel of corrosion pipes exposed of substrate to oil exposedsand slurry to oil and sand reported slurry. significant Yang et al. [corrosion49] deposited and Ni-Co-SiCerosion-enhanced on carbon corrosion steel pipes resistance. exposed The to same oil sand result slurrys were and reported reported by Fayomi significant et al. corrosion [50] on a andZn- erosion-enhancedNi-Al2O3-coated mild corrosion steel substrate. resistance. In addition, The same Redondo results wereet al. [51] reported deposited by Fayomi a corrosion et al. resistant [50] on apolypyrrole Zn-Ni-Al2 O(PPy)3-coated coating mild on steel a copper substrate. substr Inate addition, from a dihydrogen Redondo etphosphate al. [51] deposited solution. a corrosion resistant polypyrrole (PPy) coating on a copper substrate from a dihydrogen phosphate solution.

Figure 5. Schematic setup for electrodeposition of copper metal particles over aluminumaluminum oxideoxide [[52].52].

2.4.2. Electrophoretic Deposition (EPD)(EPD) CoatingCoating EPD is anotheranother form ofof electrodepositionelectrodeposition that providesprovides thicker coating layers with a colloidalcolloidal nature. Using an electric field field in a unit cell, similar to that of ELD, thin filmsfilms form on substrates by coagulation ofof colloidalcolloidal particles.particles. EPD is a multi-phase technique,technique, inin which:which:

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1. External electric field forces suspended particles in electrolyte toward one electrode called 1. External electric field forces suspended particles in electrolyte toward one electrode electrophoresis. called electrophoresis. 2. The moving particles gather in one electrode and form a larger coagulated particle. 2. The moving particles gather in one electrode and form a larger coagulated particle. 3. The larger particles deposit on the surface of the electrode, which is a to-be-coated substrate. 3. The larger particles deposit on the surface of the electrode, which is a to-be-coated substrate. Finally, a thick coating layer will be created on the substrate having a powder-shaped structure. FigureFinally, 6 represents a thick coating a schematic layer will of bethe created workin ong themechanism substrate of having the EPD a powder-shaped process. Densification structure. Figureprocesses6 represents (e.g., furnace a schematic curing, of light the workingcuring, sinterin mechanismg, etc.,) of are the recommended EPD process. Densification to increase the processes quality (e.g.,of the furnaceprotective curing, layer. light Up to curing, now, numerous sintering, applications etc.,) are recommended have been introduced to increase for the EPD quality that include of the protectivecoating, selective layer. Up deposition, to now, numerous graded material applications deposition, have been porous introduced structure for EPDdeposition, that include and coating,biomedical selective applications deposition, [53,54]. graded The materialmaterials deposition, used in EPD porous are commonly structure deposition, borides, carbides, and biomedical oxides, applicationsphosphates, [and53,54 metals]. The [53,55]. materials Castro used et in al. EPD [56] are reported commonly fabricating borides, corrosion carbides, resistant oxides, phosphates,coatings by andsol–gel metals and [EPD53,55 ].on Castro stainless-steel et al. [56 ]AISI reported 304 and fabricating reported corrosion two and four resistant times coatings increases by in sol–gel corrosion and EPDresistance on stainless-steel for each of these AISI 304processes, and reported respectively. two and In four another times study increases by Gebhart in corrosion et al. resistance[57], an AISI for each316 L of stainless these processes, steel was respectively. coated with In chitosan another for study biomedical by Gebhart applications. et al. [57], They an AISI reported 316 L stainlesspositive steeleffects was of coatedthis coating with chitosanon corrosion for biomedicalbehavior of applications. the substrate. They They reported also asserted positive that effects the applied of this coatingelectric onfield corrosion in EPD behavioris the key of thefactor substrate. in controlling They also coating asserted features, that the such applied as hydrophobicity, electric field in EPDthickness, is the and key factorstructure. in controlling TC4 Ti-alloy coating orthopedic features, im suchplants as were hydrophobicity, coated by graphene thickness, by and Chen structure. et al. TC4[58]. Ti-alloyThey reported orthopedic that implantsthe graphene-coated were coated artificia by graphenel joint byimplants Chen et show al. [58 a ].considerable They reported increase that the in graphene-coatedlife span. They found artificial that joint the implantsreason any show corrosion a considerable on substrates increase occurred in life span. was Theymicro-cracks found that in thecoating reason surfaces. any corrosion Fei et al. on[59] substrates studied the occurred wear resistance was micro-cracks of EPD coatings in coating and surfaces.successfully Fei deposited et al. [59] studiedSiC particles the wear on paper-based resistance of friction EPD coatings materials and an successfullyd achieved depositedan excellent SiC wear particles enhancement on paper-based of this frictionmaterial. materials Table 1 and summarizes achieved an ELD excellent and wearEPD enhancement processes regarding of this material. their Tablecharacteristics1 summarizes and ELDcomponents. and EPD processes regarding their characteristics and components.

Figure 6. Sketch of the electrophoretic deposition process [[60].60].

Table 1. Characteristics of electrodeposition techniques [61]. Table 1. Characteristics of electrodeposition techniques [61]. PropertyProperty ELDELD EPD EPD CoatingCoating elements elements Ions Ions Solid particles Solid particles SurfaceSurface charge charge Medium Medium High High Preferred electrolyte Water Organic Ionic electrolyticPreferred strength electrolyte High Water Organic Low ElectrolyticIonic conductivityelectrolytic strength High High Low Low ApproximateElectrolytic rate of deposition conductivity 0.1 µm/min High Low 1000 µm/min Approximate rate of deposition 0.1 μmmin⁄ 1000 μmmin⁄

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J.2.5. Manuf. Sol–gel Mater. Coating Process. 2019, 3, 28 8 of 22 Sol–gel coating is one of the most successful coating processes of biomedical devices. The wide 2.5.range Sol–gel of investigations Coating on this process and its applications can ease the setup and performance of experimentsSol–gel coatingwhile keeping is one of the the outcomes most successful reliable coating [62]. processesOn the other of biomedical hand, sol–gel devices. is capable The wide of rangeenhancing of investigations previously existing on this coating process layers and from its applications corrosion and can ion ease release the setuppoint of and view. performance Due to its ofliquid-permeating experiments while nature, keeping sol–gel the outcomescan easily reliable seal porous [62]. Oncoating the other structures hand, sol–gelor damaged is capable layers. of enhancingCalcium phosphorous previously existing (CaP) precursors coating layers dissolved from corrosion in ethanol/distilled and ion release water point are ofused view. to Duemake to the its liquid-permeatingsolution called as nature,Sol. In sol–gel order canto make easily seala gel porous phase coating out of structures the solution, or damaged the prepared layers. Calciummixture phosphorousundergoes heating (CaP) at precursors different dissolvedtemperatures in ethanol/distilled to facilitate the aqueous water are portion used to of make the solution the solution and calledincrease as the Sol. viscosity In order to makethe desired a gel phase level. outThis of phase, the solution, which thetransforms prepared it mixturefrom a liquid undergoes solution heating to a atgel different phase, is temperatures where sol–gel to gets facilitate its name. the aqueous After pr portioneparation, of thethe solutionparts or anddevices increase are dipped the viscosity in the tosol–gel the desired medium level. at a This constant phase, and which controlled transforms speed. it from This a process liquid solution may be to repeated a gel phase, to achieve is where a sol–gelmultilayered gets its coating name. or After thicker preparation, coating of the the parts same or material. devices In are addition, dippedin the the coated sol–gel samples medium can at be a constantbaked to anddry controlledout faster or speed. to provide This process intentional may dehydrating be repeated tocracks achieve on the a multilayered surface of the coating coating or thickerlayer for coating next processing of the same steps. material. Figure In addition,7 represen thets coateda schematic samples of an can example be baked of to a dry sol–gel out fastercoating or toprocess. provide intentional dehydrating cracks on the surface of the coating layer for next processing steps. Figure7 represents a schematic of an example of a sol–gel coating process.

Figure 7. 7. SchematicSchematic sol–gel sol–gel coating coating process process from soluti fromon solution preparation preparation to the final to thesolid final structure solid structureformation. formation.

Advantages of the sol–gel process include high adhesionadhesion of the coating layer, ability to coat complex geometries, flexibility flexibility in the composition of the coating layer, and lower cost than other similar coating processes. Additiona Additionally,lly, there is no need to have a conductive material as a substrate as there is no extremeextreme heatingheating or vacuumvacuum applied to the parts meaning that the substrate will be virtually untouched during during the the coating coating process. process. Sol– Sol–gelgel coating coating is isdone done in indifferent different forms forms such such as asdip-coating, dip-coating, spra spraying,ying, and andspinning spinning [63,64]. [63 ,Figure64]. Figure 8 shows8 shows a sol–gel a sol–gel deposited deposited coating coating layer with layer a withrough a and rough porous and porousmicrostructure. microstructure. One disadvantage One disadvantage of this process of this could process be could that a beconstant that a constantspeed of speeddipping of dippingand withdrawing and withdrawing is needed is neededto maintain to maintain a uniform a uniform thickness thickness of coating of coating throughout throughout the thesubstrate substrate surface. surface. There There is also is alsoalways always a possibility a possibility of coating of coating failure failure during during heat heat treatment treatment on onmultilayered multilayered coating coating structures. structures. Sol–gel Sol–gel coating coating for for industrial industrial applications applications is is considered considered a a slow process and is not cost effective in high production ratesrates [[65].65]. All these being said, the sol–gel process performs well when it comes to protecting a substratesubstrate against corrosion and decreasing ion release, as reportedreported inin many many scientific scientific studies studies [66 [66,67].,67]. Moreover, Moreover, in ain study a study by Faustini,by Faustini, et al. et [68 al.], models[68], models were proposedwere proposed in order in to explainorder andto predictexplain sol–geland predict behavior sol–gel and the effectbehavior of dipping/withdrawingand the effect of stepsdipping/withdrawing on the final quality steps of the on coating the final layer. quality Likewise, of the many coating other layer. studies Likewise, investigated many andother proposed studies continuum-based/numericalinvestigated and proposed continuum-based/numeri models that could be implementedcal models in that predicting could sol–gelbe implemented mechanisms in duringpredicting the sol–gel process mechanisms and characteristics during ofthe the process coating and layers characteristics [69–73]. Hybrid of the network coating materialslayers [69–73]. were generatedHybrid network using sol–gelmaterials process were whengenerated either using organic sol–gel moieties process or polymericwhen either categories organic moieties chemically or bondedpolymeric to categories an inorganic chemically component. bonded The to chemical an inorganic bond betweencomponent. the The organic chemical and inorganicbond between network the

J. Manuf. Mater. Process. 2019, 3, 28x FOR PEER REVIEW 99 of of 2122 organic and inorganic network can be addressed through introducing functional groups into the canpolymeric be addressed part by throughsilane, silanol, introducing etc., using functional pre-introduced groups into fu thenctional polymeric groups part in the by silane,polymer, silanol, and etc.,exploiting using pre-introducedalkoxysilanes precursors. functional Poly(dimethylsiloxane) groups in the polymer, and(PDMS), exploiting poly(ether alkoxysilanes ketone) (PEK) precursors. , and Poly(dimethylsiloxane)polycarbonate are among (PDMS), numerous poly(ether polymeric ketone) material (PEK),s that and have polycarbonate been used in are the among sol–gel numerous process polymeric[74–77]. materials that have been used in the sol–gel process [74–77].

Figure 8. SEMSEM ofof sol–gel-depositedsol–gel-deposited CaP CaP microstructure microstructure in differentin different magnifications magnifications of the of samethe same area [area78]. [78]. 2.6. Thermal Spray Coating 2.6. ThermalThermal Spray spray Coating coating is a general term for a series of processes that utilize a plasma, electric, or chemicalThermal combustion spray coating heat is a source general to term melt for a set a series of designed of processes materials that utilize and spray a plasma, the melt electric, on the or surfacechemical in combustion order to produce heat source a protective to melt layer. a set Theseof designed are reliable materials types and of corrosion-spray the melt and wear-resistanton the surface coatings.in order to In thisproduce process, a protective a heat source, layer. which These is mostlyare reliable provided types by of chemical corrosion- combustion and wear-resistant or plasma discharge,coatings. In heats this process, up the materials a heat source, to a molten which is or mostly semi-solid provided phase by and chemical sprays combustion them on the or substrate plasma withdischarge, a high heats speed up of the a jet. materials The thickness to a molten achieved or semi-solid in thermal phase spray coatingand sprays techniques them on can the range substrate from 20withµm a highto several speed millimeters of a jet. The which thickness is significantly achieved in higher thermal than spray the thicknesscoating techniques offered by can electroplating, range from CVD,20 μm or to PVD several processes millimeters [79]. In which addition, is thesignificantly materials thathigher can than be used the as thickness feedstock offered of thermal by sprayelectroplating, coatings rangeCVD, fromor PVD refractory processes metals [79]. andIn addition, metallic alloysthe material to ceramics,s that can plastics, be used and as composites feedstock andof thermal can easily spray cover coatings a relatively range from high refractory surface area meta oflsa and substrate metallic [79 alloys]. Thermal to ceramics, spray plastics, coatings and are categorizedcomposites and into can different easily types cover based a relatively on their high characteristics surface area and of processa substrate specifications. [79]. Thermal The spray most popularcoatings categoriesare categorized are plasma, into different detonation, types warm/cold, based on their high-velocity characteristics air fueland (HVAF),process specifications. high-velocity oxyfuelThe most (HVOF), popular flame, categories and wireare plasma, arc spraying detonation [80,81, ].warm/cold, high-velocity air fuel (HVAF), high- velocity oxyfuel (HVOF), flame, and wire arc spraying [80,81]. 2.6.1. High-Velocity Oxy-Fuel Coating (HVOF) 2.6.1.Figure High-Velocity9a represents Oxy-Fuel an HVOF Coating coating (HVOF) process in a schematic format. A mix of fuel, such as acetylene, propane, methane, hydrogen, or natural gas, and oxygen in gas or liquid phase undergo Figure 9a represents an HVOF coating process in a schematic format. A mix of fuel, such as continuous combustion in a designed combustion chamber to provide a high-pressure steam of hot acetylene, propane, methane, hydrogen, or natural gas, and oxygen in gas or liquid phase undergo gas. The combustion chamber releases the combustion products into a nozzle to create a spray with a continuous combustion in a designed combustion chamber to provide a high-pressure steam of hot speed of more than 1000 m/s [82]. After combustion, coating materials in powder form are injected gas. The combustion chamber releases the combustion products into a nozzle to create a spray with inside this hot jet stream to get partially melted accelerated while they are leaving the nozzle tip. a speed of more than 1000 m/s [82]. After combustion, coating materials in powder form are injected The hot jet pushes the semisolid particles against the substrate and creates a coating layer with varying inside this hot jet stream to get partially melted accelerated while they are leaving the nozzle tip. The thicknesses up to several millimeters. The advantage of this process is that the coating layer has a hot jet pushes the semisolid particles against the substrate and creates a coating layer with varying high density and adheres to the substrate well, while it is able to utilize coating materials such as thicknesses up to several millimeters. The advantage of this process is that the coating layer has a hydroxyapatite (HA), W, Cr, Al, Zr, and their oxides/carbides or polymeric materials such as nylon high density and adheres to the substrate well, while it is able to utilize coating materials such as 11/silica nanocomposites to deposit corrosion- and wear-resistant layers [83,84]. Figure9b represents hydroxyapatite (HA), W, Cr, Al, Zr, and their oxides/carbides or polymeric materials such as nylon a multilayer coating provided by HVOF. The coating layer could be performed on non-conductive 11/silica nanocomposites to deposit corrosion- and wear-resistant layers [83,84]. Figure 9b represents materials such as polymers and ceramics that are able to undergo high velocity and temperature of a multilayer coating provided by HVOF. The coating layer could be performed on non-conductive the jet stream and the particle [85]. To date, many researchers have investigated corrosion and wear materials such as polymers and ceramics that are able to undergo high velocity and temperature of resistance of HVOF coatings in various applications and corrosive environments. Based on these the jet stream and the particle [85]. To date, many researchers have investigated corrosion and wear studies, coating layers made by this technique served well and improved corrosion-wear properties of resistance of HVOF coatings in various applications and corrosive environments. Based on these substrates [86–88]. A summary of these studies is listed in Table2. studies, coating layers made by this technique served well and improved corrosion-wear properties of substrates [86–88]. A summary of these studies is listed in Table 2.

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(a) (b)

FigureFigure 9. 9. (a)( aSchematic) Schematic setup setup of a of high-velocity a high-velocity oxy-fuel oxy-fuel (HVOF) (HVOF) coating coating system system [89] and [89 (]b) and cross- (b) sectioncross-section SEM micrograph SEM micrograph of HVOF-sprayed of HVOF-sprayed multilayer multilayer coating coating on Al7075-T7351 on Al7075-T7351 plates plates[87]. [87].

Table 2. Electrochemical corrosion measurements of different coating composition provided by Table 2. Electrochemical corrosion measurements of different coating composition provided by HVOF [90]. HVOF [90]. Coating Composition Corrosion Rate (mm/y) Coating Composition Corrosion Rate (mm/y) 0.1M NaOH 0.1M H2SO4 Sea Water 0.1M NaOH 0.1M H2SO4 Sea water WC−Cr3C2−Ni 0.38 0.15 − WC−Cr3C2−Ni 0.38 0.15 − Cr3C2−NiCr 0.17 0.077 − WC−Co Cr3C2−NiCr −0.17 0.077 − − 0.76 WC−Co−Cr WC−Co −− −− 0.76 0.32 −5 −5 Cr2O3−Al2O3−WCTiO2−Co−Cr 3.2 ×10 − 3.6 ×−10 0.32 − −3 −3 Cr2O3 7.6 ×10 1.5 ×10 − Cr2O3−Al2O3−TiO2 3.2 × 10 3.6 × 10 − Cr2O3 7.6 × 10 1.5 × 10 − 2.6.2. Plasma Spray Coating 2.6.2. FigurePlasma 10 Spray illustrates Coating a schematic view of a plasma spray coating setup. This process can be done underFigure vacuum 10 illustrates or atmospheric a schematic conditions. view of In a this plasma process, spray a plasma coating gun setup. provides This process a high-temperature can be done underDC/induction vacuum plasmaor atmospheric (up to 10000 conditions. K), which In canthis easilyprocess, melt a plasma refractory gun metals, provides ceramics, a high-temperature and polymers. DC/inductionThe materials usedplasma in the(up stabilization to 10000 K), of plasmawhich can beeasily gas, water,melt refractory or a mixture metals, of these ceramics, two, known and polymers.as hybrid The plasma. materials The materialsused in the to stabilization be deposited of are plasma fed into can this be gas, hot plasmawater, or stream a mixture and theof these high two,temperature known as melts hybrid the feedstock.plasma. The Due materials to the high to be speed deposited of plasma are fed at the into tip this of ahot converging plasma stream nozzle, and the themolten high droplets temperature are deposited melts the instantly feedstock. on theDue substrate to the high against speed the of coating plasma setup. at the The tip of flexibility a converging of this nozzle,process the facilitates molten thedroplets utilization are deposited of different instantl typesy of on feedstock the substrate such asagainst powder, the slurry,coating suspensions, setup. The flexibilityand liquids of [this91]. Theprocess resulting facilitates coating the layerutilization has a highof different corrosion types and of wear feedstock resistance such and as powder, it is able slurry,to adhere suspensions, to the substrate and liquids due to surface[91]. The tension result anding highcoating temperature. layer has a A high significant corrosion corrosion- and wear and resistancewear-resistance and it enhancement is able to adhere was reportedto the substrate in many due studies to surface on different tension materials and high such temperature. as chromium A significantoxide and corrosion- NiCr alloys and [92 wear-resistance,93]. Plasma sprayed enhancem coatingent was of polymers,reported in especially many studies PEEK, on have different been materialsimplemented such for as corrosionchromium protection oxide and of metalNiCr alloys substrates [92,93]. (nylon, Plasma PVDF), sprayed antistick coating coatings of polymers, of papers especiallyand rollers, PEEK, plastic have moldings, been implem wear-resistantented for corrosion coatings, protection moisture protection of metal substrates materials, (nylon, and electrical PVDF), antistickbarrier coatings coatings [94 of]. papers On the and other rollers, hand, plastic vacuum moldings, plasma wear-resistant spraying is a low-temperature coatings, moisture process protection and is materials,mostly used and for electrical materials barrier that cannot coatings perform [94]. On reactions the other in atmospheric hand, vacuum pressure plasma to modifyspraying the is surfacea low- temperatureof the substrate. process The mostand is popular mostly applicationused for materi of vacuumals that plasma cannot spraying perform isreactions the surface in atmospheric modification pressureof engineering to modify polymers the surface and plastics, of the rubbers,substrate. metals, The most and fiberspopular [95 ].application In this process, of vacuum a material plasma can sprayinggo through is the cross-linking, surface modification friction decrease, of engineerin adherenceg polymers increase, and etc. plastics, [95–97 ru]. bbers, metals, and fibers [95]. In this process, a material can go through cross-linking, friction decrease, adherence increase, etc. [95–97].

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Figure 10. SchematicSchematic plasma spray coating setup and its parts [98] [98].. Figure 10. Schematic plasma spray coating setup and its parts [98]. 2.6.3. Cold Spray Coating 2.6.3. Cold Spray Coating Cold spray coating is a technique that relies on impact and solid mechanics of particles. Unlike Cold spray coating is a technique that relies on impact and solid mechanics of particles. Unlike HVOF andand plasmaplasma spray spray coating coating methods, methods, this this process process does does not utilizenot utilize a heat a source heat source to perform to perform coating HVOF and plasma spray coating methods, this process does not utilize a heat source to perform coatingon substrates. on substrates. The general The general working working mechanism mechanism of cold sprayof cold coating spray dependscoating depends on particle on size,particle the coating on substrates. The general working mechanism of cold spray coating depends on particle size,temperature the temperature of the target, of the material target, material properties properties of coating ofparticles, coating particles, and a critical and a velocity critical velocity [99]. Powder [99]. size, the temperature of the target, material properties of coating particles, and a critical velocity [99]. Powdermaterials materials are fed to are a streamfed to a of stream high-velocity of high-vel mediumsocity mediums (helium (helium and nitrogen) and nitrogen) to achieve to achieve the desired the Powder materials are fed to a stream of high-velocity mediums (helium and nitrogen) to achieve the desiredkinetic energy.kinetic Afterenergy. particle-substrate After particle-substrate impacts takeimpacts place, take this place, energy this deforms energy thedeforms particles the and particles bond desired kinetic energy. After particle-substrate impacts take place, this energy deforms the particles andthem bond to the them substrate. to the substrate. Another mechanism Another mechanism of this process of this can process be penetration can be penetration of the particles of the inside particles the and bond them to the substrate. Another mechanism of this process can be penetration of the particles insidesubstrate. the substrate. Using a high Using flow a high rate offlow accelerated rate of acce particles,lerated theparticles, surface the is coatedsurface withis coated desired with materials. desired inside the substrate. Using a high flow rate of accelerated particles, the surface is coated with desired Thematerials. most-used The most-used powder materials powder consistmaterials of aconsis widetrange of a wide of plastics, range metals,of plastics, ceramics, metals, composites, ceramics, materials. The most-used powder materials consist of a wide range of plastics, metals, ceramics, composites,and metallic and alloys metallic [100,101 alloys]. In [100,101]. addition, In the addi most-studiedtion, the most-studied substrate materials substrate include materials soft include metals composites, and metallic alloys [100,101]. In addition, the most-studied substrate materials include softsuch metals as Al andsuch Cu, as Al while and in Cu, literature, while in the literature coating, the of some coating hard of materialssome hard such materials as W and such Ti as has W been and soft metals such as Al and Cu, while in literature, the coating of some hard materials such as W and Tireported has been [102 reported,103]. In [102,103]. some studies, In some the studies, temperature the temperature of the accelerating of the accelerating medium has medium been increased has been in Ti has been reported [102,103]. In some studies, the temperature of the accelerating medium has been increasedorder to enhance in order process to enhance efficiency process [104 ].efficiency Although [1 this04]. processAlthough is simplethis process and cheap is simple compared and cheap to the increased in order to enhance process efficiency [104]. Although this process is simple and cheap comparedother thermal to the spray other methods, thermal the spray operation methods, range the is op veryeration limited. range Figure is very 11 lishowsmited. SEM Figure micrographs 11 shows compared to the other thermal spray methods, the operation range is very limited. Figure 11 shows SEMof cold micrographs spray-treated of cold surfaces spray-treated and a schematic surfaces view and of a coldschematic spray view coating of cold process. spray coating process. SEM micrographs of cold spray-treated surfaces and a schematic view of cold spray coating process.

Figure 11. Top: SEM micrographs of cold spray-coated AA6061 substrate by (a) as-received and (b) Figure 11. Top:Top: SEM SEM micrographs micrographs of of cold cold spray-coated spray-coated AA6061 AA6061 substrate substrate by by(a) (as-receiveda) as-received and and (b) heat-treated AA7075 particles with a mechanical interlocking between the coating and the substrate (heat-treatedb) heat-treated AA7075 AA7075 particles particles with a with mechanical a mechanical interlocking interlocking between between the coating the and coating the substrate and the [105]. Bottom figure: a schematic view of cold spray coating process [106]. substrate[105]. Bottom [105 ].figure: Bottom a schematic figure: a schematic view of cold view spray of cold coating spray process coating [106] process. [106]. 2.6.4. Warm Spray Coating 2.6.4. Warm Spray Coating Figure 12 (left) shows a schematic representation of the warm spray coating method. As stated Figure 12 (left) shows a schematic representation of the warm spray coating method. As stated about cold spray coating, low working temperature decreases efficiency and reliability of thermal about cold spray coating, low working temperature decreases efficiency and reliability of thermal spray coating methods. However, high temperatures melt feedstock and introduce new chemical spray coating methods. However, high temperatures melt feedstock and introduce new chemical reactions, which may cause oxidation or change of properties due to extreme heating of particles and reactions, which may cause oxidation or change of properties due to extreme heating of particles and substrates. In order to overcome this problem, a new technique was introduced as a warm spray substrates. In order to overcome this problem, a new technique was introduced as a warm spray coating. This is a modification of HVOF coating that enjoys reduced temperature in the combustion coating. This is a modification of HVOF coating that enjoys reduced temperature in the combustion chamber by introducing nitrogen to the fluid mixture. As a result, this method is categorized chamber by introducing nitrogen to the fluid mixture. As a result, this method is categorized

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2.6.4. Warm Spray Coating Figure 12 (left) shows a schematic representation of the warm spray coating method. As stated about cold spray coating, low working temperature decreases efficiency and reliability of thermal spray coating methods. However, high temperatures melt feedstock and introduce new chemical reactions, which may cause oxidation or change of properties due to extreme heating of particles and substrates. In order to overcome this problem, a new technique was introduced as a warm spray coating. This is a modification of HVOF coating that enjoys reduced temperature in the combustion chamber by introducing nitrogen to the fluid mixture. As a result, this method is categorized somewhere between cold spray coating and HVOF coating and provides a high efficiency of the coating process [107]. However, as reported in the literature, the achieved coating layer contains many impurities relative to the other two processes due to low temperature and existence of oxygen in the accelerating stream. These porosities and oxide phases are illustrated in Figure 12 (right). Advantages of using this process rise in the coating of materials, which are sensitive to oxidization in high temperatures or the materials that cannot withstand high working temperatures such as bio-metal-glasses, Ti and its alloys, engineering plastics, and polymers such as PEEK [108,109]. Cold/warm spray coatings are not used for extremely harsh environments, however many research studies investigated corrosion properties of this type of coating on different materials such as Ti, bio-metal-glasses, WC-Co cermet, etc., under different corrosion conditions and they claimed an enhancement in corrosion resistance of substrates [110,111].

2.6.5. Arc Wire Spray Coating Another type of thermal spray coating technique is called arc wire spray coating (Figure 13a). In this process, two consumable metallic wires, which are charged with a DC supply, generate an arc between them resulting in a melting process of the feeding wires. The products of this melting process are then pumped out of a converging nozzle tip toward the target with the supplied pressure of compressed gas. Although the flexibility of this process allows for the use of many metallic alloys as coating layers, this process is limited to conductive wires and materials. In order to solve this issue, a modified version of arc wire plasma was introduced having one consumable wire, which makes an arc with a non-consumable metallic cathode [112]. The remaining steps of the process are the same as the original version. This method is well-known for applications of an internal surface coating such as engine blocks, etc., that offers a lighter metal as the whole block while the internal surfaces are coated with a wear- and corrosion-resistant metallic alloy. This flexibility can significantly reduce production cost. Almost all of the conductive materials such as Al, Zn, Mo, Ni, and other metallic alloys such as Ni and Ti alloys can be used as feedstock in this process [79,113]. In addition, utilization of cored wires is reported in the literature [114]. Figure 13b represents the microstructure of an arc wire spray-coated substrate. To this point, many of the popular thermal spray coating techniques have been introduced. However, there is no doubt that other processes could be investigated regarding their working mechanisms, coating efficiency, coating quality, speed of process, and ease of applications. J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 12 of 21

somewhere between cold spray coating and HVOF coating and provides a high efficiency of the coating process [107]. However, as reported in the literature, the achieved coating layer contains many impurities relative to the other two processes due to low temperature and existence of oxygen in the accelerating stream. These porosities and oxide phases are illustrated in Figure 12 (right). Advantages of using this process rise in the coating of materials, which are sensitive to oxidization in high temperatures or the materials that cannot withstand high working temperatures such as bio- metal-glasses, Ti and its alloys, engineering plastics, and polymers such as PEEK [108,109]. Cold/warm spray coatings are not used for extremely harsh environments, however many research studies investigated corrosion properties of this type of coating on different materials such as Ti, bio- metal-glasses, WC-Co cermet, etc., under different corrosion conditions and they claimed an enhancement in corrosion resistance of substrates [110,111].

2.6.5. Arc Wire Spray Coating Another type of thermal spray coating technique is called arc wire spray coating (Figure 13a). In this process, two consumable metallic wires, which are charged with a DC supply, generate an arc between them resulting in a melting process of the feeding wires. The products of this melting process are then pumped out of a converging nozzle tip toward the target with the supplied pressure of compressed gas. Although the flexibility of this process allows for the use of many metallic alloys as coating layers, this process is limited to conductive wires and materials. In order to solve this issue, a modified version of arc wire plasma was introduced having one consumable wire, which makes an arc with a non-consumable metallic cathode [112]. The remaining steps of the process are the same as the original version. This method is well-known for applications of an internal surface coating such as engine blocks, etc., that offers a lighter metal as the whole block while the internal surfaces are coated with a wear- and corrosion-resistant metallic alloy. This flexibility can significantly reduce production cost. Almost all of the conductive materials such as Al, Zn, Mo, Ni, and other metallic alloys such as Ni and Ti alloys can be used as feedstock in this process [79,113]. In addition, utilization of cored wires is reported in the literature [114]. Figure 13b represents the microstructure of an arc wire spray-coated substrate. To this point, many of the popular thermal spray coating techniques have been introduced. However, there is no doubt that other processes could be investigated J. Manuf. Mater. Process. 3 regarding their working2019, , 28mechanisms, coating efficiency, coating quality, speed of process, and13 ofease 22 of applications.

J. Manuf.Figure Mater. 12. Process.Left: 2019 A schematic, 3, x FOR PEER of warm REVIEW spray coating technique and its setup [115]. Right: Cross-section 13 of 21 Figure 12. Left: A schematic of warm spray coating technique and its setup [115]. Right: Cross-section BSE micrographs of warm spray-deposited Ti-6Al-4V on low-carbon steel substrate processes spray BSE micrographs of warm spray-deposited Ti-6Al-4V on low-carbon steel substrate processes spray 1.5pressure m3/min, and (d nitrogen) 4 MPa flowand 0.5 rates m3/min, of (a) 1 ( MPae) 4 MPa and 0.5and m3/min, 1 m3/min, (b) and 1 MPa (f) and4 MPa 1 m3/min, and 1.5 (m3/min,c) 1 MPa pressure and nitrogen flow rates of (a) 1 MPa and 0.5 m3/min, (b) 1 MPa and 1 m3/min, (c) 1 MPa and respectivelyand 1.5 m3/min, [116]. (d) 4 MPa and 0.5 m3/min, (e) 4 MPa and 1 m3/min, and (f) 4 MPa and 1.5 m3/min, respectively [116].

FigureFigure 13. 13. (a(a) )Schematic Schematic arc arc wire wire spray spray coating coating setu setupp and and mechanism mechanism of of operation operation [117]. [117]. (b (b) )SEM SEM micrographmicrograph of arcarc wire wire spray-coated spray-coated surface surface by Tafa’sby Tafa's steel (95MXC)steel (95MXC) cored wirecored feedstock. wire feedstock. The substrate The substratematerial ismaterial not mentioned is not mentioned in Reference in Reference [114]. [114]. 3. Summary 3. Summary In order to have a successful coating deposition on a substrate, there are several affecting In order to have a successful coating deposition on a substrate, there are several affecting parameters including deposition materials, substrate materials, feedstock form (powder, wire, rods, parameters including deposition materials, substrate materials, feedstock form (powder, wire, rods, precursors, etc.), and deposition methods. However, the deposition processes are the most important precursors, etc.), and deposition methods. However, the deposition processes are the most important ones as they deal with chemical alteration of materials and alloying of composition elements in the ones as they deal with chemical alteration of materials and alloying of composition elements in the coating layer. In addition, based on the characteristics of different feedstock and substrate materials, coating layer. In addition, based on the characteristics of different feedstock and substrate materials, one can easily choose the best option for deposition. The processes that are the most successful and the one can easily choose the best option for deposition. The processes that are the most successful and most investigated deposition means are physical/chemical vapor deposition (PVD/CVD), micro-arc the most investigated deposition means are physical/chemical vapor deposition (PVD/CVD), micro- oxidation (MAO), electrodeposition (i.e., electrolytic deposition (ELD) and electrophoretic deposition arc oxidation (MAO), electrodeposition (i.e., electrolytic deposition (ELD) and electrophoretic (EPD)), sol–gel, and different types of thermal spraying processes (i.e., HVOF, plasma, cold, warm, and deposition (EPD)), sol–gel, and different types of thermal spraying processes (i.e., HVOF, plasma, arc wire spraying). The mentioned processes utilize different mechanisms in order to deposit specific cold, warm, and arc wire spraying). The mentioned processes utilize different mechanisms in order types of materials on substrates making the material selection important in order to have the highest to deposit specific types of materials on substrates making the material selection important in order efficiency of the coating. Some of the processes use thermal sources to change the state of feedstock to have the highest efficiency of the coating. Some of the processes use thermal sources to change the to liquids and semisolids in forms of particles, droplets, and clusters. Some others use the difference state of feedstock to liquids and semisolids in forms of particles, droplets, and clusters. Some others between electrochemical charges between poles and some deposit materials without chemical change use the difference between electrochemical charges between poles and some deposit materials of state. Depending on the substrate materials, feedstock, and means of deposition, the coating layers without chemical change of state. Depending on the substrate materials, feedstock, and means of deposition, the coating layers are different in thickness, microstructure, and functionality. In addition, some processes are specific to metallic feedstocks, which are conductive, while the rest can deposit polymers, ceramics, and metallic alloys regardless of their physical properties. In summary, the thermal processes, such as various types of thermal spray coating, implement high temperatures and high speed of plasma jets in order to have a higher material deposition rate. In these methods, the high temperatures and high-speed jets allow feedstock deposition and eliminate the adverse effects of high melting point on ceramics and superalloys. The obtained coating thicknesses, in these cases, are high (up to several hundred microns). However, the coating microstructure consists of oxide and carbide inclusions and provides porosity, depending on process parameters. In the vaporization-based processes, the deposited coating layer is a thin film with high corrosion/wear-resistance mostly used in tool coating and protection of sliding components of machines. The coating achieved in these types is a thin solid film with low porosity and high adherence to the substrate. Micro-arc oxidation is a high-voltage version of anodization, which is mostly implemented on biomaterials for bone implant and biomedical device applications. In addition, valve metals (Al, Ti, Zr, Hf, V, Nb) have been extensively used as substrates as well. The coating structured by MAO offers high substrate/coating adhesion with a porous structure that is crucial for biomaterial coating as the bone ingrowth increases. The porous microstructure also enhances the corrosion resistance and facilitates the engineering calculations on the lifetime of the implants, either degrading fast or staying for a longer time. Sol–gel is another type of material deposition that is significantly flexible in feedstock composition, and as the process utilizes aqueous

J. Manuf. Mater. Process. 2019, 3, 28 14 of 22 are different in thickness, microstructure, and functionality. In addition, some processes are specific to metallic feedstocks, which are conductive, while the rest can deposit polymers, ceramics, and metallic alloys regardless of their physical properties. In summary, the thermal processes, such as various types of thermal spray coating, implement high temperatures and high speed of plasma jets in order to have a higher material deposition rate. In these methods, the high temperatures and high-speed jets allow feedstock deposition and eliminate the adverse effects of high melting point on ceramics and superalloys. The obtained coating thicknesses, in these cases, are high (up to several hundred microns). However, the coating microstructure consists of oxide and carbide inclusions and provides porosity, depending on process parameters. In the vaporization-based processes, the deposited coating layer is a thin film with high corrosion/wear-resistance mostly used in tool coating and protection of sliding components of machines. The coating achieved in these types is a thin solid film with low porosity and high adherence to the substrate. Micro-arc oxidation is a high-voltage version of anodization, which is mostly implemented on biomaterials for bone implant and biomedical device applications. In addition, valve metals (Al, Ti, Zr, Hf, V, Nb) have been extensively used as substrates as well. The coating structured by MAO offers high substrate/coating adhesion with a porous structure that is crucial for biomaterial coating as the bone ingrowth increases. The porous microstructure also enhances the corrosion resistance and facilitates the engineering calculations on the lifetime of the implants, either degrading fast or staying for a longer time. Sol–gel is another type of material deposition that is significantly flexible in feedstock composition, and as the process utilizes aqueous solutions as particle carriers, the complexity of geometry does not affect the coating quality. In addition, sol–gel-deposited layers are a reliable sealant for porous substrates and coatings in order to increase their corrosion resistance. Although sol–gel offers high flexibility and capability in coating purposes, the process is relatively slow and increases the production time. Electrochemical processes are another type of aqueous deposition methods utilizing the difference between electrochemical charges of anode and cathode poles of a chemical unit cell. The flexibility in the coating composition and a wide variety of substrates used in this method make it a reliable deposition process. However, this process suffers from depending on conductivity properties of substrates (poles) for material deposition as the charges need to move freely in the circuit. Although these processes are reliable means of material deposition and surface protection, there are disadvantages to all of them in different applications. Thus, there have been studies on combining these techniques in order to benefit from an advantageous point of each process and minimize the negative effect of each method. The most applicable way of deposition modification and protection enhancement is known to be multilayered coating deposition. The different layers deposited on top of the previous ones can have different thicknesses, compositions, and physical/chemical properties. This consideration has more importance while porous structures or thin films are deposited. As an instance, a PVD-coated substrate has a thin film with notable corrosion/wear resistance, but a higher layer thickness can increase the functional lifetime of the coated component. As another example, the porous microstructure of a MAO-treated surface can be sealed with several layers of sol–gel deposition in order to decrease the corrosive medium penetration to the substrate and increase its corrosion resistance while maintaining the porous structure. All being said, in order to have the highest efficiency and functionality of a protective layer, different aspects need to be considered carefully. Layer thickness, coating composition, suitability of the deposition technique regarding feedstock and substrate materials, and physical/chemical properties of the deposited layers are the key features changing the final protection quality. Table3 presents a brief overview of the discussed processes and their features. In addition, pros and cons of covered coating techniques in this review are summarized in Table4. J. Manuf. Mater. Process. 2019, 3, 28 15 of 22

Table 3. Summary of coating processes and their specific feature.

Deposition Process Source Feedstock Material Substrate Material Coating Thickness (µm) Reference AISI M2 steel, SS, glass, Si, TiCxOy-ZrCxOy, TiN, PE, PVDF, potassium PVD Physical 1.2–6.3, 5, 0.2, 0.2, 0.1, 0.1 [10,11,17,18,118] PTh bromide(KBr)-carbon-Au-Al, Ag-Au-Cu-Al Niobium oxide(Nb2O5), W-TiN-WSi -Ta O -Cu-SiO , 0.05–0.2, -, 0.2–0.6, CVD Chemical 2 2 5 2 Glass, Si, Si, Kleenex, Ni-Co-Fe [24,25,119–121] polycrystalline Si- Si3N4-SiO2, 0.04–0.1–16 PTFE, Ni3Ti Hydroxyapatite (HA)/TiO , PCL MAO Electrochemical 2 Ti-6Al-4V, Mg, NiT 10–20, 2–3, 7 [32,34,37] duplex, HA- HA/ZrO2 Ni-Co-Al O , Ni-Co-SiC, ELD Electrochemical 2 3 Steel, carbon steel, mild steel, Cu 50–200, 10–70, -, - [48–51] Zn-Ni-Al2O3, PPy AISI 316L SS, AISI 304 SS, AISI bioactive glass (45S5 316 L SS, Ti-6Al-4V alloy (TC4), EPD Electrochemical BG)-Cu-doped BG, SiO , -, 7, 1–6, -, - [56–59,122] 2 Aramid-carbon-cellulose fibers chitosan, grapheme, SiC composite TiCl4-(tetraethyl orthosilicate) TEOS- (methyltriethyl Sol–gel Physical Si, NiTi, stainless steel 0.01–1, 1–4 [62,68,74,123], orthosilicate)MTEOS, HA, PDMS, Polycarbonate Ti-6Al-4V, Inconel 738 metal, AISI HVOF Thermal HA, CoNiCrAlY, WC 70, -, 100 [83,84,124] 4340 SS Al2O3-ZrO2- yttria stabilized zirconia (YSZ), Metco 447- Plasma spray Thermal Alumina/Titania 87/13- Nicrome SS, steel, AISI 4140 steel -, 0.5–1, - [91,92,96] 80/20- Hastalloy G30, TiC-NiCrBSi HA, AA7075, Ni/Al, mixed Cold spray Physical Ti-6Al-4V, Al 6061-T6, Al 6061 100–1000, 40–300, - [103,105,106] Ni/Al/MoO3, and Ni-clad Al Zr-Cu Ni -Al , Ti, Ti, Warm spray Physical 12.3 7.6 3.5 316L SS, steel, steel, carbon steel 400–1000, -, 400, 300 [108,110,111,125] WC-Co Arc wire spray Thermal MoS2-TiC-Fe, Ti/Al Carbon steel, SUS 304 1000, - [112,113] J. Manuf. Mater. Process. 2019, 3, 28 16 of 22

Table 4. Advantages and disadvantages of reliable coating processes.

Deposition Process Advantages Disadvantages Reference Corrosion and wear resistance/thin film deposition is Requires a high vacuum/corrosion resistance is affected PVD possible/adjustable mechanical, corrosion and aesthetic by abrasion/degradation control is challenging for [9,17,18] properties polymer deposition applications Corrosion and wear resistance/deposition of various Requires ultra-high vacuum/requires heat resistant CVD types of materials with different microstructures/works [24,25,30,31] substrates/small amount of coating materials waste with low and atmospheric pressures High corrosion resistance and hardness/porous MAO structure for biomedical applications/different scales of Mostly applicable to valve metals [33,35] porosity through the thickness/ Decorative and low-corrosion/wear ELD Works for conductive substrates [46,47] applications/high-temperature applications Various kinds of selective, graded material, and porous EPD structure depositions/biomedical applications/wear Works for conductive substrates [53,54,59] resistant Cost effective/biomedical applications/providing corrosion and ion release protection/multilayered Thickness control/slow rate of coating cycle/possibility Sol–gel (thick) coating/high adhesion/ability to coat complex of coating failure during heat treatment on multilayered [66,67] geometries/flexibility in the composition/no need of coating structures conductive substrates High density of coating layer and well substrate Requires a small range of powder size (5–60 µm) with a HVOF adherence/works for non-conductive narrow size distribution/numerous process variable to [83–88] substrates/corrosion and wear resistance change the coating structure/requires a heat source High corrosion and wear resistance/high substrate A low-temperature process that is mostly used for adherence/surface modification of engineering materials that cannot perform reactions in atmospheric Plasma spray [92,93] polymers, rubbers, metals, and fibers/anti-stick pressure to modify the surface of the substrate/requires coatings a heat source Limited operation range/mostly used for soft and hard Simple and cheap method compared to the other metal substrates/low efficiency and reliability due to Cold spray [102,103] thermal spray methods low temperatures/not useful extremely harsh environments Applicable to materials with sensitivity to oxidization at Impurity complications/not useful extremely harsh Warm spray [108,109] high temperatures or heat sensitive materials environments Internal surface coatings such as engine blocks/wear Limited to conductive wires and materials as the Arc wire spray [79,113] and corrosion resistant coating layer J. Manuf. Mater. Process. 2019, 3, 28 17 of 22

Funding: This research received no external funding. Conflicts of Interest: The authors of this work ensure that there is no conflict of interest by any means and the study is not funded by any organization.

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