Current Progress in Solution Precursor Plasma Spraying of Cermets: a Review

metals

Review Current Progress in Solution Precursor Plasma Spraying of Cermets: A Review

Romnick Unabia 1,2,*, Rolando Candidato Jr. 1 and Lech Pawłowski 2 ID

1 Physics Department, College of Science and Mathematics, MSU-Iligan Institute of Technology, Iligan City 9200, Philippines; [email protected] 2 IRCER UMR 7315 CNRS, University of Limoges, Limoges Cedex 87068, France; [email protected] * Correspondence: [email protected]; Tel.: +33-0-587-502-400

Received: 1 May 2018; Accepted: 1 June 2018; Published: 4 June 2018

Abstract: Ceramic and metal composites, known also as cermets, may considerably improve many material properties with regards to that of initial components. Hence, cermets are frequently applied in many technological fields. Among many processes which can be employed for cermet manufacturing, thermal spraying is one of the most frequently used. Conventional plasma spraying of powders is a popular and cost-effective manufacturing process. One of its most recent innovations, called solution precursor plasma spraying (SPPS), is an emerging coating deposition method which uses homogeneously mixed solution precursors as a feedstock. The technique enables a single-step deposition avoiding the powder preparation procedures. The nanostructured coatings developed by SPPS increasingly find a place in the field of surface engineering. The present review shows the recent progress in the fabrication of cermets using SPPS. The influence of starting solution precursors, such as their chemistry, concentration, and solvents used, to the micro-structural characteristics of cermet coatings is discussed. The effect of the operational plasma spray process parameters such as solution injection mode to the deposition process and coatings’ microstructure is also presented. Moreover, the advantages of the SPPS process and its drawbacks compared to the conventional powder plasma spraying process are discussed. Finally, some applications of SPPS cermet coatings are presented to understand the potential of the process.

Keywords: cermets; cermet applications; solution precursor plasma spraying; microstructure

1. Introduction Recent studies demonstrate that deposition of coatings composed of metal phase embedded in ceramic matrix have considerably improved properties, such as hardness, wear resistance, surface reactivity, etc. [1–10]. The composite system of ceramics and metallic phases are known as cermets. Atmospheric plasma spraying (APS) is a conventional thermal spray process which has been widely, and for many years, used for deposition of cermets used as protective coatings [1,3], thermal barrier coatings [4–6], and coatings in solid oxide fuel cells (SOFC) [7–9]. On the other hand, the production of coatings with fine microstructures (sub-micrometer/nanometer-scale) has been developed since the mid-1990s. These coatings improved its physical and mechanical properties, such as surface area, solubility, and many others [11–20]. One of the limitations of the APS process is the use of micrometer-sized (10–100 µm) powder feedstocks which makes it difficult to produce fine coating microstructures [17–20]. In addition, the resulting APS coatings have a characteristic lamellar structure due to successive impingement of splats, which reveals micro-defects such as unmelted particles, gas pores, and weak interconnection between splats that could deteriorate the coating’s functionality [4,5]. Therefore, the use of liquid feedstock in thermal spraying, such as the solution

Metals 2018, 8, 420; doi:10.3390/met8060420 www.mdpi.com/journal/metals Metals 2018, 8, 420 2 of 18 Metals 2018, 8, x FOR PEER REVIEW 2 of 19 precursor plasmaplasma sprayingspraying process process (SPPS), (SPPS), has has been been developed developed to overcometo overcome the limitationsthe limitations of the of APS the processAPS process [21,22 [21,22].]. SPPS offers a single-step deposition process of ﬁnely-structuredfinely-structured cermet coatings by employing molecularly-mixed liquid precursors directly as thethe feedstock, which enables avoiding the powder and suspension preparation as shown in Figure1 1[ [10,13,14,17–19].10,13,14,17–19]. The use of solution precursors for thermal spraying was initiated by KarthikeyanKarthikeyan etet al.al. [[21].21]. SPPS coating technology is industriallyindustrially advantageous due to its ease in feedstock preparation.

Figure 1. Feedstocks used for thermal spraying processesprocesses which enable to obtain nano-structured coatings: ( A)) the pre-processed nano-powder part particleicle used for the APS process; (B) thethe suspensionsuspension droplet used used for for the the SPS SPS process; process; and and (C)( Cthe) thehomogeneously-mixed homogeneously-mixed solution solution droplet droplet used usedfor SPPS for SPPSprocess. process.

Since the beginning of 21st cent century,ury, there has been rapid development in nanostructured coating deposition using the SPPS process as reflected reﬂected by a number of published ar articlesticles [15–22]. [15–22]. However, However, most of of the the accounted accounted papers papers focus focus on on the the depositi depositionon of ceramic of ceramic materials materials alone alone and only and onlya few a have few havebeen beenreported reported on SPPS on SPPS coating coating deposition deposition for for cermets cermets (ceramic-metal (ceramic-metal composite) composite) because because of of the oxidation potential of of the the metal-ions metal-ions in in the the solution solution precursor precursor [10,15]. [10,15 The]. The present present review review focuses focuses on onthe thecurrent current progress progress of ofsolution solution precursor precursor plasma plasma spray spray for for depositing depositing cermet cermet coatings coatings used in various applications. In particular, the review givesgives an overview of the various factors affecting the ﬁnalfinal coatingcoating material,material, such such as as solution solution precursor precursor preparations preparations and and physico-chemical physico-chemical interaction interaction of the of solutionthe solution precursor precursor with with the high-temperaturethe high-temperature plasma plasma jet. jet.

2. Solution Precursor Plasma Spraying of Cermets The properties of the starting solution precursorsprecursors and operational plasma spray process parameters greatlygreatly inﬂuencedinfluenced the the physical physical and and chemical chemical characteristics characteristics of of sprayed sprayed coatings. coatings. Thus, Thus, it isit veryis very important important toscreen to screen those those parameters parameters which wh inﬂuenceich influence mostof most the propertiesof the properties of cermet of coatings. cermet Somecoatings. of themSome are of them presented are presented in Figure in2 together Figure 2 withtogether the schematicwith the schematic representation representation of SPPS process.of SPPS Selectedprocess. Selected parameters parameters are discussed are discussed in the subsequent in the subsequent sections. sections.

Metals 2018, 8, 420 3 of 18 Metals 2018, 8, x FOR PEER REVIEW 3 of 19

Figure 2.2. Schematic diagram of the solution precursor plasma spraying process showing the many different and important operational spray parameters that could inﬂuenceinfluence the physical and chemical characteristics of cermetcermet coatings.coatings.

2.1. Solution Precursor Preparation 2.1. Solution Precursor Preparation Solution precursors are generally prepared by mixing salts of metals such as nitrates and acetates Solution precursors are generally prepared by mixing salts of metals such as nitrates and acetates in desired stoichiometry using various solvents (see Table 1). The chemistry of solution precursors, in desired stoichiometry using various solvents (see Table1). The chemistry of solution precursors, their concentration and solvent used, determine the characteristics of the resulting coatings [10,22– their concentration and solvent used, determine the characteristics of the resulting coatings [10,22–29]. 29]. The spraying process is accompanied by the decomposition of the precursor at the elevated The spraying process is accompanied by the decomposition of the precursor at the elevated temperature temperature and high heat transfer of the plasma jet [10,22–25]. and high heat transfer of the plasma jet [10,22–25].

Table 1. Solution precursor preparation for cermet coating fabrication via SPPS. Table 1. Solution precursor preparation for cermet coating fabrication via SPPS. Coating Final Coating Precursor Solvent Used Additive Reference Coating Final Coating Application Precursor Solvent Used Additive Reference Application Zinc nitrate [Zn(NO3)2·6H2O] Zinc nitrate [Zn(NO ) ·6H O] De-ionized Zinc ferrite, and ferric nitrate3 2 2 De-ionized 350 mM Dom, R. et Photocatalysis and ferric nitrate water 350 mM citric Dom, R. et al. Zinc ferrite, ZnFe2O4 Photocatalysis water ZnFe2O4 [Fe(NO[Fe(NO) ·9H3)3·9HO];2 ratioO]; acidcitric acid [10al.] [10] 3 3 2 EthyleneEthylene glycol glycol ratio[Zn/Fe [Zn/Fe = 1/2] = 1/2] ZirconiumZirconium acetate acetate [Zr(C2H4O2)4] [Zr(C H O ) ] and strontium Thermal and2 4strontium2 4 nitrate De-ionizedDe-ionized Li, X. et al. SrZrOSrZrO3 3 Thermal barrier nitrate tetrahydrate -- Li, X. et al. [12] barrier 3 2 2 water water [12] tetrahydrate[Sr(NO 3[Sr(NO)2·4H2O];) ·4H O] ; concentration = = 1.6 1.6 mol/L mol/L Zinc nitrate hexahydrate hexahydrate [Zn(NO ) ·6H O], calcium [Zn(NO332)2·6H2 2O], calcium Zinc-hydroxyapatite,Zinc- nitrate tetrahydrate De-ionized Candidato,Candidato, R. Biomedical nitrate tetrahydrate De-ionized - hydroxyapatite,Zn-HA Biomedical [Ca(NO3)2·4H2O], and water - et al.R. [13 et] al. tri-ethyl phosphite [P(OEt) ]; water Zn-HA [Ca(NO3)2·4H2O], and tri-ethyl3 [13] ratio [(Ca + Zn)/P = 1.67] phosphite [P(OEt)3]; ratio [(Ca + Biomedical, CobaltZn)/P nitrate= 1.67] Sanpo et al. Cobalt ferrite, biomagnetic, [Co.(NO3)2·6H2O] and ferric De-ionized Citric acid as [14]; 3 2 2 CoFe2O4 Biomedical,and Cobaltnitrate nitrate [Fe(NO [Co.(NO3)3·9H2O];) ratio·6H O] water chelatingCitric agent acidand SanpoSanpo et al. et Cobalt ferrite, biomagnetic,Antibacterial and[Co./Fe ferric = 1/2]nitrate De-ionized as [15al.] [14]; CoFe2O4 and ZrOCl ·6H O, Y(NO ) ·6H O water chelating and Sanpo Nickel- Yttria 2[Fe(NO2 3)3·9H32O];3 2 Solid oxide fuel and Ni(NO ) ·6H O; Wang, Y, and stabilized zirconia, Antibacterial ratio [Co./Fe3 3 = 1/2]2 Distilled water - agent et al. [15] cell (SOFC) concentration 60 vol. % of Coyle, T.W. [23] Ni-YSZ ZrOCl8%YSZ2·6H and2O, 40 Y(NO vol. %3 of)3· Ni6H2O Nickel- Yttria Solid oxide Wang, Y, stabilized fuel cell and Ni(NO3)3·6H2O; Distilled water - and Coyle, zirconia, Ni-YSZ (SOFC) concentration 60 vol. % of T.W. [23] 8%YSZ and 40 vol. % of Ni

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Strontium Table 1. Cont. doped La(NO3)3·6H2O, Sr(NO3)2 and Wang, X. SolidCoating oxide lanthanumFinal Coating Mn(NO3)2; Precursormolar ratio La:Sr:Mn SolventDistilled Used water Additive Referenceet al. [24] fuelApplication cell - manganite, = 0.8:0.2:1; total concentration 0.1 Ethanol Wang, X. Strontium doped (SOFC) La(NO ) ·6H O, Sr(NO ) Wang, X. et al. La0.8Sr0.2MnO3, 3 3 mol/L2 3 2 et al. [29] lanthanum Solid oxide fuel and Mn(NO ) ; molar ratio Distilled water [24] 3 2 - LSMmanganite, cell (SOFC) La:Sr:Mn = 0.8:0.2:1; total Ethanol Wang, X. et al. LaStrontiumSr MnO , LSM concentration 0.1 mol/L [29] 0.8 0.2 3 Sm(NO3)3·6H2O, Sr(NO3)2 and doped Solid oxide Sm(NO3)3·6H2O, Sr(NO3)2 Strontium doped Co.(NO3)3 6H2O; molar ratio Ethanol and Li, C.X. et samarium Solidfuel cell oxide fuel and Co.(NO· ) ·6H O; molar Ethanol and - Li, C.X. et al. samarium cobaltite, 3 3 2 distilled water - al. [25] cell (SOFC) Sm:Sr:Co.ratio Sm:Sr:Co. = 0.7:0.3:1; = 0.7:0.3:1; total distilled water [25] cobaltite,Sm Sr Co (SOFC) 0.7 0.3 3-δ total concentration 0.1 mol/L Sm0.7Sr0.3Co3-δ concentration 0.1 mol/L

Lay et al.al., [[26]26] reportedreported thethe influenceinfluence ofof thethe chemistrychemistry ofof thethe startingstarting precursorsprecursors onon nickel-basednickel-based SPPS cermet. They They found found that that nickel-samaria nickel-samaria dope dopedd ceria ceria (Ni-SDC) (Ni-SDC) has has larger quantity of finerfiner particles than nickel-yttria stabilized zirconia (Ni-YSZ)(Ni-YSZ) [[26].26]. Moreover, Wang etet al., [[24]24] reported that the utilizationutilization ofof strontium-dopedstrontium-doped lanthanumlanthanum manganite, manganite, (La (La0.80.8SrSr0.20.2MnO3)) as as SOFC SOFC cathode material shows promising characteristicscharacteristics i.e.,i.e., highhigh electrochemicalelectrochemical activityactivity andand thermalthermal stabilitystability duedue toto its high ◦ operating temperature (>800(>800 °C)C) compared to Sm0.5SrSr0.50.5CoOCoO3 3andand La La0.60.6SrSr0.4Co0.4Co0.2Fe0.20.8FeO0.83. O3. It was also reported thatthat thethe goodgood stoichiometricstoichiometric ratioratio ofof thethe ceramic-metallicceramic-metallic speciesspecies and/orand/or the addition of complexing/chelatingcomplexing/chelating agent agent such such as as citric citric acid acid in the solution precursor can modify the chemistry of the solution [[10,13–15].10,13–15]. Dom et al. [[10]10] showed that the stoichiometric ratio of Zn to Fe being aboutabout 1:2 1:2 could could thermodynamically thermodynamically favor favor the formation the formation of desired of cermetdesired zinc cermet ferrite zinc composite ferrite phasecomposite rather phase than rather their than corresponding their corresponding metal-oxide meta phasesl-oxide such phases as zinc such oxide, as zinc iron oxide, oxide, iron etc. oxide, [10]. Candidatoetc. [10]. Candidato et al. [13 ]et also al. [13] reported also reported that as thethat zinc as the ion zinc concentration ion concentration into the into hydroxyapatite the hydroxyapatite (HA) ceramic(HA) ceramic matrix matrix increases, increases, formation formation of HA of phases HA phases is hindered is hindered and favors and the favors formation the formation of calcium of zinccalcium phosphate zinc phosphate impurity impurity phase as phase shown as in shown Figure in3. Figure In addition, 3. In addition, Sanpo et Sanpo al. [ 15 et] presented al. [15] presented that the utilizationthat the utilization of citric acid of citric as chelating acid as agentchelating with ag 20%ent concentration with 20% concentration significantly significantly favors the formation favors the of moreformation than 90%of more of cobalt than ferrite90% of phase cobalt due ferrite to chelate phase effect due to attributed chelate effect to an increaseattributed in entropyto an increase resulting in toentropy a more resulting stable composite to a more material stable composite [15]. material [15].

Figure 3. X-rayX-ray diffraction diffraction diagram diagram of of the the Zn-doped Zn-doped HA HA cermet cermet coatings coatings showing showing the theevolution evolution of the of thecalcium calcium zinc zinc phosphate phosphate phase phase as zi asnc zincion concentration ion concentration increases. increases. Reproduced Reproduced with withpermission permission from from[13], Elsevier, [13], Elsevier, 2018. 2018.

The concentration of the solution precursors is another aspect to be considered as it significantly The concentration of the solution precursors is another aspect to be considered as it signiﬁcantly influences the viscosity and surface tension of solution but has small effect on the pyrolysis and inﬂuences the viscosity and surface tension of solution but has small effect on the pyrolysis and crystallization temperatures. High concentration solutions normally have higher viscosity and crystallization temperatures. High concentration solutions normally have higher viscosity and surface tension and need more injection pressure to be delivered to the core of the plasma jet. surface tension and need more injection pressure to be delivered to the core of the plasma jet. Solutions having low-concentration precursors generally experience surface precipitation leading to Solutions having low-concentration precursors generally experience surface precipitation leading the formation of shells, while a highly-concentrated solution experiences volumetric precipitation leading to fully-melted splat microstructures. In order to induce volumetric precipitation, solution

Metals 2018, 8, 420 5 of 18 to the formation of shells, while a highly-concentrated solution experiences volumetric precipitation leading to fully-melted splat microstructures. In order to induce volumetric precipitation, solution concentration must be near the equilibrium saturation concentration [17,27–30]. With these, high concentration solution precursors are used to deposit dense coatings. For example, Dom et al. [10] reported that a low concentration of zinc ferrite solutions yielded lower ﬁlm thickness build-up per pass and certain impurity phases were observed, while high concentration yields a major fraction of zinc ferrite phases. However, beyond the equilibrium saturation concentration, the presence of unreacted solution precursor species was observed due to incomplete pyrolysis, thereby obtaining impurity phases in the coating. The choice of solvent in the solution preparation is also important in the coating deposition as it relates to the ionic interactions at the molecular level due to macroscopic solvent properties, such as viscosity, surface tension, etc., and could modify the plasma jet’s thermodynamic and transport properties which directly affect the splat formation (see Table2)[10,24,28–30].

Table 2. Properties of two solvents commonly used in the SPPS process.

Surface Tension σs Speciﬁc Heat cp Latent Heat of Evaporation −2 Viscosity µs (Pa·s) −1 −1 Solvent (J·m ) (J·kg ·K ) (at Evaporation Lv Temperature (Room Temp.) −1 (Room Temp.) Room Temp.) (J·kg ) Tv (K) Water 72 × 10−3 10−3 4.18 × 103 2.26 × 106 373 Ethanol 22 × 10−3 1.06 × 10−3 2.44 × 103 0.84 × 106 351

Aqueous solution precursor feedstock is prone to produce larger droplet size due to its high surface tension affecting the droplet breakup. This phenomenon leads to the production of partially-pyrolyzed splats. A work by Wang et al. [24] on LSM cermet coatings reported porous sponge-like deposits and layered microstructure when the aqueous solution is used. These microstructures were attributed to the incomplete evaporation of solvent in-ﬂight producing partially-pyrolyzed splats and in situ pyrolysis further occurs on the substrate [24]. This phenomenon is observable in aqueous solution due to the cooling effect of water, resulting in a ca. 25% decrease of the zone of high temperature in the plasma jet at over 8000 K [30]. On the contrary, when ethanol was employed as solvent, LSM cermets had a porous morphology with a large number of agglomerated small particles (0.2 to 2 µm) [29]. Unlike the aqueous solution, microstructures with mostly-crystallized perovskite phases were observed in LSM coatings when using ethanol as the solvent [29]. In another work, the production of zinc ferrite coatings using organic solvent exhibited an impurity phase (Fe2O3) unlike using aqueous solution, as shown in Figure4[ 10]. This can be attributed to easier evaporation of ethanol compared to water and to increase of plasma enthalpy and thermal conductivity, leading to a complete physical and chemical change of the precursor droplets, i.e., evaporation, precipitation, and densiﬁcation inside the plasma jet region [24,28–30]. Chen et al. [28] investigated the phenomena related to the surface tension and evaporation. The authors reported that droplets having high surface tension and high boiling point solvent did not fully evaporate in the plasma while the solution droplets formed by the solvent having low surface tension and low boiling point could evaporate rapidly and could completely transform [28]. Lastly, appropriate solvents can be selected according to the application of the coatings, i.e., [27–30]:

• Aqueous solution precursor for the production of porous coatings (thermal barriers and biomedical ones); • Organic solvent resulting in fully-melted splats and a dense coating architecture for anti-wear applications. Metals 2018, 8, x FOR PEER REVIEW 5 of 19 concentration must be near the equilibrium saturation concentration [17,27–30]. With these, high concentration solution precursors are used to deposit dense coatings. For example, Dom et al. [10] reported that a low concentration of zinc ferrite solutions yielded lower film thickness build-up per pass and certain impurity phases were observed, while high concentration yields a major fraction of zinc ferrite phases. However, beyond the equilibrium saturation concentration, the presence of unreacted solution precursor species was observed due to incomplete pyrolysis, thereby obtaining impurity phases in the coating. The choice of solvent in the solution preparation is also important in the coating deposition as it relates to the ionic interactions at the molecular level due to macroscopic solvent properties, such as viscosity, surface tension, etc., and could modify the plasma jet’s thermodynamic and transport properties which directly affect the splat formation (see Table 2) [10,24,28–30].

Table 2. Properties of two solvents commonly used in the SPPS process.

Surface Tension σs Specific Heat cp Evaporation Viscosity µs (Pa·s) Latent Heat of Solvent (J·m−2) (J·kg−1·K−1) (at Temperature Tv (Room Temp.) Evaporation Lv (J·kg−1) (Room Temp.) Room Temp.) (K) Water 72 × 10−3 10−3 4.18 × 103 2.26 × 106 373 Ethanol 22 × 10−3 1.06 × 10−3 2.44 × 103 0.84 × 106 351

Aqueous solution precursor feedstock is prone to produce larger droplet size due to its high surface tension affecting the droplet breakup. This phenomenon leads to the production of partially- pyrolyzed splats. A work by Wang et al. [24] on LSM cermet coatings reported porous sponge-like deposits and layered microstructure when the aqueous solution is used. These microstructures were attributed to the incomplete evaporation of solvent in-flight producing partially-pyrolyzed splats and in situ pyrolysis further occurs on the substrate [24]. This phenomenon is observable in aqueous solution due to the cooling effect of water, resulting in a ca. 25% decrease of the zone of high temperature in the plasma jet at over 8000 K [30]. On the contrary, when ethanol was employed as solvent, LSM cermets had a porous morphology with a large number of agglomerated small particles (0.2 to 2 µm) [29]. Unlike the aqueous solution, microstructures with mostly-crystallized perovskite phases were observed in LSM coatings when using ethanol as the solvent [29]. In another work, the production of zinc ferrite coatings using organic solvent exhibited an impurity phase (Fe2O3) unlike using aqueous solution, as shown in Figure 4 [10]. This can be attributed to easier evaporation of ethanol compared to water and to increase of plasma enthalpy and thermal conductivity, leading to a complete physical and chemical change of the precursor droplets, i.e., evaporation, precipitation, and densification inside the plasma jet region [24,28–30]. Chen et al. [28] investigated the phenomena related to the surface tension and evaporation. The authors reported that droplets having high surface tension and high boiling point solvent did not fully evaporate in the plasma while the solution droplets formed by the solvent Metalshaving2018 low, 8, 420surface tension and low boiling point could evaporate rapidly and could completely6 of 18 transform [28]. Metals 2018, 8, x FOR PEER REVIEW 6 of 19

Figure 4. X-ray diffraction spectra of deposited zinc ferrite films using (a) ethylene glycol, and (b) water as precursor solvents. Reproduced with permission from [10], Elsevier, 2012.

Lastly, appropriate solvents can be selected according to the application of the coatings, i.e., [27– 30]: • Aqueous solution precursor for the production of porous coatings (thermal barriers and biomedical ones); • Organic solvent resulting in fully-melted splats and a dense coating architecture for anti-wear applications.

2.2. Injection of Solution Feedstock Figure 4. X-ray diffraction spectra of deposited zinc ferrite films using (a) ethylene glycol, and (b) The mechanism of solution injection into the plasma jet affects the heating of the droplets. The water as precursor solvents. Reproduced with permission from [10], Elsevier, 2012. solution inside a pressurized container can be fed using a pneumatic system or in an open container using2.2. Injection a peristaltic of Solution pump. Feedstock The flow rate can be easily adjusted by varying the pressure or rotation of the roller, respectively. In general, clogging is not a problem in SPPS, unlike in the APS process. However,The mechanism a flexible pipeline of solution must injection be used into and the fl plasmaushing the jet affectsinjector the with heating ethanol of theor droplets.water is Therecommended solution inside to avoid a pressurized gelling of containerthe solution can inside be fed the using injector. a pneumatic The types system of injection or in anthat open are frequentlycontainer using used ain peristaltic SPPS can pump.be categorized The flow depend rate caning be on easily the type adjusted of injector by varying used the(mechanical pressure or atomizingrotation of injector), the roller, the respectively. mode of injection In general, (internal clogging or external) is not aand, problem on the in choice SPPS, between unlike inradial the APSand process.axial injection However, setup a flexible[17,23,30,31]. pipeline The must solution be used inside and a flushingpressurized the injectorcontainer with can ethanol be fed orusing water a pneumaticis recommended system to or avoid in an gellingopen container of the solution using a insideperistaltic the injector.pump, as The shown types in ofFigure injection 5. that are frequentlyMechanical used ininjectors SPPS canmay be use categorized motor rollers depending to control on the the flow type rate of injector of the solution used (mechanical stream [22]. or Theatomizing formation injector), of droplets the mode from ofthis injection stream (internalis induced or by external) an aerodynamic and, on thebreakup choice through between constant radial applicationand axial injection of pressure, setup as [17 shown,23,30 in,31 Figure]. The solution5A [17]. The inside resulting a pressurized droplets container using a mechanical can be fed injector using a pneumaticare observed system to have or a in relatively an open containerwide distribution using a peristaltic of large droplets. pump, as shown in Figure5.

Figure 5. Schematic representation of the (A) mechanical injection and (B) spray atomization under Figure 5. Schematic representation of the (A) mechanical injection and (B) spray atomization under external injection mode. Reproduced with permission from [17], Elsevier, 2009. external injection mode. Reproduced with permission from [17], Elsevier, 2009. The large droplets may experience primary break-up before entering the plasma region. Upon penetratingMechanical at the injectors plasma region, may use these motor large rollers drople tots control undergo the secondary ﬂow rate break-up of the solution into smaller stream ones, [22]. Theas shown formation in Figure of droplets 6, influenced from this by the stream drag is force induced acting by on an the aerodynamic droplet and breakup its surface through tension, constant which leads to the formation of finer microstructures [32]. With the high momentum of the large droplets, it can penetrate into the plasma core, enabling the droplet to be heated effectively. However, the good

Metals 2018, 8, 420 7 of 18 application of pressure, as shown in Figure5A[ 17]. The resulting droplets using a mechanical injector are observed to have a relatively wide distribution of large droplets. The large droplets may experience primary break-up before entering the plasma region. Upon penetrating at the plasma region, these large droplets undergo secondary break-up into smaller ones, as shown in Figure6, inﬂuenced by the drag force acting on the droplet and its surface tension, which leads to the formation of ﬁner microstructures [32]. With the high momentum of the large droplets,Metals 2018, it 8,can x FOR penetrate PEER REVIEW into the plasma core, enabling the droplet to be heated effectively. However,7 of 19 the good penetration of the stream of solution requires that the dynamic pressure of the solution is penetration of the stream of solution requires that the dynamic pressure of the solution is greater than greater than the dynamic pressure of the plasma. This may be mathematically expressed as [17,22]: the dynamic pressure of the plasma. This may be mathematically expressed as [17,22]: 2 2 ρυρυsolution2solution >> ρυρυ2plasmaplasma, ,(1) (1)

Wang etet al.al. [[24],24], reportedreported thatthat mechanically-depositedmechanically-deposited strontium-dopedstrontium-doped lanthanum manganite (LSM) coatings present a partiallypartially crystallizedcrystallized perovskiteperovskite phase with a fraction of thethe amorphousamorphous structure which could be attributed to the penetrationpenetration and dwell time of the droplets in the plasma jet [[24].24].

Figure 6.6. AerodynamicAerodynamic interactions interactions between between the the liqui liquidd stream stream and and the the high-temperature jet. jet. Reproduced with permission from [[32].32].

Figure 55BB describesdescribes anan atomizingatomizing injectorinjector whichwhich utilizesutilizes anan atomizingatomizing gasgas toto provideprovide pressurepressure on the the solution solution precursor precursor in in order order to tobreak break it up it upinto into small small droplets. droplets. These These atomized atomized droplets, droplets, 2–100 2–100microns microns in size, in penetrate size, penetrate into intothe plasma the plasma jet core jet core inducing inducing an an immediate immediate thermal thermal reaction reaction of of the solute duedue to to fast fast evaporation evaporation of theof the solvent solvent [17,33 [17,]. The33]. directThe direct contact contact of the atomizedof the atomized solution solution droplets intodroplets the hotinto plasma the hot core plasma result core in theresult rapid in formationthe rapid formation of the LSM of coatings, the LSM indicating coatings, aindicating crystalline a crystalline La0.5Sr0.5MnO3 perovskite phase. The XRD spectra of the LSM coatings shows broadened La0.5Sr0.5MnO3 perovskite phase. The XRD spectra of the LSM coatings shows broadened peaks becausepeaks because of the lowof the peak-to-noise low peak-to-noise ratio of theratio deposits of the duedeposits to their due small to their crystallite small sizescrystallite associated sizes withassociated the breakup with the of breakup the solution of the precursor solution into precurso ﬁner dropletr into finer sizes droplet [24]. However, sizes [24]. the However, size distribution the size ofdistribution the droplets of inthe this droplets type of in injector this type still of depends injector in still the nozzledepends geometry, in the nozzle the gas geometry, velocity and theﬂow gas rate,velocity and and the flow property rate, ofand the the solvents property used. of the solvents used. The solutionsolution precursorprecursor can can be be delivered delivered into into the the plasma plasma jet byjet axialby axial or radial or radial direction direction [33]. In[33]. axial In injection,axial injection, the droplet the droplet enters enters directly directly into the into high-temperature the high-temperature plasma plasma core where core the where solvent the startssolvent to evaporatestarts to evaporate rapidly, that rapidly, results that in the results solute in concentration the solute concentration near the droplet near surface. the droplet This injection surface. mode This allowsinjection solute mode saturation allows solute in a short saturation period in of a time. short Due period to the of rapidtime. heatingDue to the process rapid in heating axial injection process the in axial injection the solution precursor droplet can undergo full pyrolization resulting in a dense coating formation [33]. Metcalfe et al., [31] studied the deposition of a Ni-YSZ cermet coating by axial injection with varying torch power. The authors reported a decrease of the nickel content in the composite coatings as the torch power decreased. This could result from the fast evaporation of nickel metal (boiling point at 2732.0 °C) [31]. With radial injection of the solution precursor the heating process is much slower compared to the axial one. The droplets interact first with the outer part of the plasma region, which is colder than the plasma core. This injection method is useful for large droplet sizes which may penetrate easier into the jet [17,18,33]. The study of Michaux [34] demonstrated the formation of Ni-based cermets with YSZ by SPPS using radial injection. It showed the homogeneous distribution of NiO in the YSZ

Metals 2018, 8, 420 8 of 18 solution precursor droplet can undergo full pyrolization resulting in a dense coating formation [33]. Metcalfe et al., [31] studied the deposition of a Ni-YSZ cermet coating by axial injection with varying torch power. The authors reported a decrease of the nickel content in the composite coatings as the torch power decreased. This could result from the fast evaporation of nickel metal (boiling point at 2732.0 ◦C) [31]. With radial injection of the solution precursor the heating process is much slower compared to the axial one. The droplets interact first with the outer part of the plasma region, which is colder than the plasma core. This injection method is useful for large droplet sizes which may penetrate easier into the jet [17,18,33]. The study of Michaux [34] demonstrated the formation of Ni-based cermets with YSZ by SPPS using radial injection. It showed the homogeneous distribution of NiO in the YSZ matrix having a fine microstructure that could be induced by efficient internal circulation of the solvent in the dropletsMetals and 2018, by 8, xrapid FOR PEER diffusion REVIEW of heat within the droplet. 8 of 19

2.3. Phenomenamatrix having in Flight a fine microstructure that could be induced by efficient internal circulation of the solvent in the droplets and by rapid diffusion of heat within the droplet. Different phenomena occur when the injected solution makes contact with the plasma jet, as discussed2.3. Phenomena in many in studies Flight [17,22,35–37], are presented in this section in a chronological way. Initially, the liquid stream or large droplets break-up into small droplets due to the strong shear stress of the Different phenomena occur when the injected solution makes contact with the plasma jet, as plasmadiscussed flow [36 in]. many After studies the break-up, [17,22,35–37], the are fragmented presented dropletsin this section experience in a chronological rapid solvent way. Initially, evaporation and thethe liquid increase stream ofsolute or large concentration droplets break-up follows. into sm Theall droplets precipitation due to the inside strong of liquidshear stress droplets of the leads to theplasma solid flow particle [36]. formationAfter the break-up, prior to the coating fragment deposition.ed droplets Theexperience droplets rapid may solvent undergo evaporation volumetric or surfaceand the precipitation. increase of solute Fine concentration droplets tend follows. to precipitate The precipitation volumetrically, inside of liquid which droplets is also leads known to as homogeneousthe solid particle nucleation, formation which prior results to coating in splat deposition. formation. The Ondroplets the other may hand,undergo larger volumetric droplets or tend to precipitatesurface precipitation. superficially Fine resulting droplets in tend shell to formation.precipitate volumetrically, If the solid shell which is impermeable, is also known internalas pressurizationhomogeneous takes nucleation, place leading which results to the in shell splat fracture. formation. This On maythe other lead hand, to the larger formation droplets of tend cracked to precipitate superficially resulting in shell formation. If the solid shell is impermeable, internal shells. The cracked shell may become molten and form spherical particles, as shown in Figure7. pressurization takes place leading to the shell fracture. This may lead to the formation of cracked The different particle morphologies formed by SPPS have been discussed in the work of Saha et al. [35]. shells. The cracked shell may become molten and form spherical particles, as shown in Figure 7. The Ozturkdifferent et al. [particle37] modeled morphologies the formation formed ofby the SPPS solid have crust been at discussed precipitation in the occurring work of Saha in a et single al. [35]. droplet. The solidOzturk particles et al. [37] undergo modeled heating the formation and melting of the depending solid crust on at theirprecipitation residence occurring time inside in a thesingle plasma jet. Thedroplet. evaporation The solid particles from the undergo molten heating solid mayand melting take place. depending The vaporson their mayresidence condense time inside and form the nanoparticles.the plasma jet. The evaporation from the molten solid may take place. The vapors may condense and Figureform the8 isnanoparticles. a high magnification micrograph of the surface of an SPPS Zn-HA coating showing the differentFigure particles 8 is a high formed magnification in-flight depositedmicrograph onto of the the surface substrate. of an SPPS The chemicalZn-HA coating phenomena showing of the solutionthe different occurring particles in-flight formed can in-flight be experimentally deposited on testedto the substrate. under equilibrium The chemical conditions phenomena with of the the use solution occurring in-flight can be experimentally tested under equilibrium conditions with the use of thermogravimetric-differential thermal analyzer (DTA-TGA). In general, the chemical reactions of thermogravimetric-differential thermal analyzer (DTA-TGA). In general, the chemical reactions start with the evaporation of solvent, pyrolysis of the precursor, and crystallization or oxidation of the start with the evaporation of solvent, pyrolysis of the precursor, and crystallization or oxidation of precursorthe precursor [27,28,38 [27,28,38].].

Figure 7. Possible route of particle formation in the SPPS process. Reproduced with permission from Figure 7. Possible route of particle formation in the SPPS process. Reproduced with permission [39], Elsevier, 2017. from [39], Elsevier, 2017.

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matrix having a fine microstructure that could be induced by efficient internal circulation of the solvent in the droplets and by rapid diffusion of heat within the droplet.

2.3. Phenomena in Flight Different phenomena occur when the injected solution makes contact with the plasma jet, as discussed in many studies [17,22,35–37], are presented in this section in a chronological way. Initially, the liquid stream or large droplets break-up into small droplets due to the strong shear stress of the plasma flow [36]. After the break-up, the fragmented droplets experience rapid solvent evaporation and the increase of solute concentration follows. The precipitation inside of liquid droplets leads to the solid particle formation prior to coating deposition. The droplets may undergo volumetric or surface precipitation. Fine droplets tend to precipitate volumetrically, which is also known as homogeneous nucleation, which results in splat formation. On the other hand, larger droplets tend to precipitate superficially resulting in shell formation. If the solid shell is impermeable, internal pressurization takes place leading to the shell fracture. This may lead to the formation of cracked shells. The cracked shell may become molten and form spherical particles, as shown in Figure 7. The different particle morphologies formed by SPPS have been discussed in the work of Saha et al. [35]. Ozturk et al. [37] modeled the formation of the solid crust at precipitation occurring in a single droplet. The solid particles undergo heating and melting depending on their residence time inside the plasma jet. The evaporation from the molten solid may take place. The vapors may condense and form the nanoparticles. Figure 8 is a high magnification micrograph of the surface of an SPPS Zn-HA coating showing the different particles formed in-flight deposited onto the substrate. The chemical phenomena of the solution occurring in-flight can be experimentally tested under equilibrium conditions with the use of thermogravimetric-differential thermal analyzer (DTA-TGA). In general, the chemical reactions start with the evaporation of solvent, pyrolysis of the precursor, and crystallization or oxidation of the precursor [27,28,38].

Metals 2018, 8, 420 9 of 18 Figure 7. Possible route of particle formation in the SPPS process. Reproduced with permission from [39], Elsevier, 2017.

Figure 8. Surface micrograph of Zn-HA coating showing the different particle morphologies present like ﬁne spheres, fragmented shells, and agglomerates. Reproduced with permission from [13], Elsevier, 2018.

Michaux et al. [34] identiﬁed the sequence of chemical reaction in the fabrication of NiO/8YSZ cermet: • The exothermic reaction occurs ﬁrst around 100 ◦C corresponding to ethanol solvent evaporation. Metals 2018, 8, x FOR PEER REVIEW 9 of 19 • The endothermic peak at 300 ◦C corresponded to decomposition of nitrate precursors. • Figure 8. Surface micrograph of Zn-HA◦ coating showing the different particle morphologies present The endothermiclike peak fine spheres, at around fragmented 400 shells,C and indicated agglomerates. the Reproduced oxidation with permission of precursors from [13], forming NiO doped with YSZ. Elsevier, 2018.

Michaux et al. [34] identified the sequence of chemical reaction in the fabrication of NiO/8YSZ 2.4. Cermet Coatingscermet: Formation • The exothermic reaction occurs first around 100 °C corresponding to ethanol solvent The microstructureevaporation. of the sprayed cermet depends on the in-ﬂight phenomena of the plasma jet discussed above.• TheThe particles endothermic arrivingpeak at 300 °C on corresponded the substrate to decomposition or previously-deposited of nitrate precursors. coating generally • The endothermic peak at around 400 °C indicated the oxidation of precursors forming NiO have sizes in the sub-micrometerdoped with YSZ. or nanometer range [40,41]. To understand the coating formation, the deposits obtained from a single torch scan, as shown in Figure9, can be studied [41]. 2.4. Cermet Coatings Formation µ Figure9a showsThe deposited microstructure melted of the sprayed or partially cermet depend melteds on the in-flight spherical phenomena particles of the plasma (~0.3 jet m in diameter), which resulted fromdiscussed the evaporationabove. The particles of arriving solvent on the at su thebstrate surface or previously-deposited of the small coating droplets generally [33 ,35,41]. The small have sizesµ in the sub-micrometer or nanometer range [40,41]. To understand the coating formation, sintered particlesthe (~1 depositsm) obtained are like from thea single melted torch scan, ones, as shown but in Figure deposited 9, can be studied before [41]. melting occurs, as shown in Figure9b [ 33,35,37Figure,41]. 9a These shows deposited small melted particles or partially could melted have spherical stuck particles together (~0.3 µm in forming diameter), agglomerates as which resulted from the evaporation of solvent at the surface of the small droplets [33,35,41]. The it reaches the substrate,small sintered as shownparticles (~1 in µm) Figure are like9 thec. melted Such ones, particles but deposited are thebefore major melting depositoccurs, as elements of the coating microstructureshown in which Figure 9b contributes [33,35,37,41]. These to the small small particles pores could inhave the stuck coating together [41forming]. The large pores in agglomerates as it reaches the substrate, as shown in Figure 9c. Such particles are the major deposit coatings due to theelements stacking of the coating of irregularly microstructure shaped which contributes agglomerates, to the small pores such in asthe dome-likecoating [41]. The ones, are shown in Figure 10[39]. large pores in coatings due to the stacking of irregularly shaped agglomerates, such as dome-like ones, are shown in Figure 10 [39].

Figure 9. Cont.

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Figure 9. Typical LSM deposits collected on the substrate at the distance 60 mm (a,b) and 70 mm (c–f) Figure 9. Typical LSMfrom the deposits plasma torch collected exit in the single on thescan substrateexperiments. Reproduced at the distance with permission 60 mm from( [41],a,b ) and 70 mm (c–f) Elsevier, 2013. Figurefrom the 9. Typical plasma LSM torch deposits exit in the collected single on scan the experiments. substrate at the Reproduced distance 60 with mm permission(a,b) and 70 frommm ( [c41–f],) fromElsevier, the 2013.plasma torch exit in the single scan experiments. Reproduced with permission from [41], Elsevier, 2013.

50µm 100µm

Figure 10. SEM images at the (a) surface and (b) cross-section of HA coating from low concentration solution showing a dome-like microstructure. Reproduced with permission from [39], Elsevier, 2017.

On the other hand, the wet agglomerates depicted in Figure 9d are some droplets entrained in the cold region of the plasma jet which experienced less heating and only partial pyrolization before impact. They may contribute in formation of a sponge-like structure, as reported by Wang et al. [24]. 50µm 100µm Figure 9e also shows the wet agglomerates formed with less amount of solvent. This type of deposit is formed through continuous evaporation of the solvent when vapors escaped by fine pores between small particles when impacting the substrate. Finally, the solution droplets that bounced away from the plasma core without experiencing effective heating are shown in Figure 9f. These wet droplets Figure 10. SEMcould images have beenat the generated (a) surface by larger and solution (b) cross-sectiondroplets which undergo of HA in coating situ pyrolization from lowafter concentration Figure 10. SEMimpacting images the at substrate’s the (a) surface surface [41]. and (b) cross-section of HA coating from low concentration solution showing a dome-like microstructure. ReproducedReproduced with permission fromfrom [[39],39], Elsevier, 2017.2017. 3. Microstructure of Cermet Coatings On the other hand,Microstructure the wet is agglomeratesan important factor depicteddetermining thein finalFigure properties 9d areof solution some plasma droplets entrained in On the othersprayed hand, coatings. the wet The microstructure agglomerates results depictedfrom impact of in the Figure different9 particlesd are on some the substrate droplets entrained in the cold region of the plasma jet which experienced less heating and only partial pyrolization before the cold region ofor the previously plasma deposited jet which coating, experiencednamely: (i) melted lessparticles; heating (ii) small and sintered only particles; partial (iii) dry pyrolization before impact. They mayagglomerates; contribute (iv) inwet formation agglomerates; and, of a(v) sponge wet droplets,-like as described structure, in many as papers reported [17,22,40]. by Wang et al. [24]. impact. They may contributeUnlike in conventional in formation plasma spraying of a sponge-like(APS), where large structure, splats are formed, asreported SPPS-prepared by Wang et al. [24]. Figure 9e also showscoatings the exhibit wet many agglomerates different microstructural formed features. with The less APS-deposited amount Ni-YSZ of solvent. (see Figure This 11) type of deposit Figure9e also shows the wet agglomerates formed with less amount of solvent. This type of deposit is is formed throughcermet continuous coating cross-section evaporation shows a of porous the lamesolventllar microstructure when vapors with voids escaped and unmelted by fine pores between formed through continuousparticles [42]. evaporation of the solvent when vapors escaped by ﬁne pores between small particles when impacting the substrate. Finally, the solution droplets that bounced away from small particles when impacting the substrate. Finally, the solution droplets that bounced away from the the plasma core without experiencing effective heating are shown in Figure 9f. These wet droplets plasma core without experiencing effective heating are shown in Figure9f. These wet droplets could could have been generated by larger solution droplets which undergo in situ pyrolization after have been generated by larger solution droplets which undergo in situ pyrolization after impacting impacting the substrate’s surface [41]. the substrate’s surface [41].

3. Microstructure Microstructure of Cermet Coatings Microstructure isis an an important important factor factor determining determin theing ﬁnalthe propertiesfinal properties of solution of solution plasma sprayedplasma sprayedcoatings. coatings. The microstructure The microstructure results from results impact from of the im differentpact of the particles different on theparticles substrate on the or previously substrate ordeposited previously coating, deposited namely: coating, (i) melted namely: particles; (i) melted (ii) smallparticles; sintered (ii) small particles; sintered (iii) dryparticles; agglomerates; (iii) dry agglomerates;(iv) wet agglomerates; (iv) wet and,agglomerates; (v) wet droplets, and, (v) as wet described droplets, in as many described papers in [ 17many,22,40 papers]. [17,22,40]. Unlike in conventional plasma spraying (APS), where large splats are formed, SPPS-prepared coatings exhibit many different microstructural fe features.atures. The The APS-deposited APS-deposited Ni-YSZ Ni-YSZ (see (see Figure Figure 11)11) cermet coating cross-section shows a porous lame lamellarllar microstructure with voids and unmelted particles [42]. [42].

Metals 2018, 8, 420 11 of 18 Metals 2018, 8, x FOR PEER REVIEW 11 of 19 Metals 2018, 8, x FOR PEER REVIEW 11 of 19

Figure 11. SEM cross-sectional microstructure of Ni-YSZ by APS. Reproduced with permission from Figure 11. SEM cross-sectional microstructure of Ni-YSZ by APS. Reproduced with permission [42],Figure Elsevier, 11. SEM 2008. cross-sectional microstructure of Ni-YSZ by APS. Reproduced with permission from from [42], Elsevier, 2008. [42], Elsevier, 2008. On the contrary, uniformly dense microstructures with fine-scale porosity characterize the SPPS- depositedOn the the Ni-YSZ contrary, contrary, cermet uniformly uniformly coatings dense dense as microstructuresreported microstructures by Michaux with with fine-scale et ﬁne-scaleal. [34]. porosity Wang porosity characterize and characterizeCoyle the[23] SPPS- also the SPPS-depositedreporteddeposited that Ni-YSZ Ni-YSZ Ni-YSZ cermet cermet cermet coatings coatings coatings as containing reported as reported byspherical byMichaux Michaux YSZ et particles etal. al.[34]. [34 of].Wang Wangabout and and0.5 µmCoyle Coyle in diameter[23] [23] also µ reportedwere held that together Ni-YSZ by cermet a continuous coatings containingNi matrix. sphericalThe strong YSZ bonding particles between of about particles 0.5 µmm indue diameter to the werecontinuous heldheld togethertogether Ni matrix byby could aa continuouscontinuous improve NiNithe matrix.matrix.mechanical The propertiesstrong bonding of the betweencoating. Consequently,particles due to there the continuousare no studies NiNi matrixmatrixyet on could couldthe investigation improve improve the the mechanicalof mechanical the mech propertiesanical properties properties of of the the coating. ofcoating. cermet Consequently, Consequently, coatings deposited there there are nousingare studiesno the studies SPPS yet on yetprocess. the on investigation the investigation of the mechanicalof the mech propertiesanical properties of cermet of coatings cermet depositedcoatings deposited using the SPPSusingCandidato process.the SPPS process.et al. [13], reported that microstructures of both HA and Zn-doped HA coatings depositedCandidato under et similar al. [[13],13 conditions], reported showed that microstructures the presence of ofagglomerated both HA and fine Zn-dopedZn-doped spherical HAparticles coatings and depositedmelted particles under at similar the surface, conditions which showed are characteristics the presence of of SPPS agglomerated coatings. However, ﬁnefine spherical the particlescross-section and meltedof the Zn-doped particles atHA the cermet surface, coating which seems are characteristics to be denser comparedof SPPS coatings. to the un-doped However, HA the (Figurecross-section 12). of the Zn-doped HA cermet coating seems toto bebe denserdenser comparedcompared toto thethe un-dopedun-doped HAHA (Figure(Figure 1212).).

2µm 50µm 2µm 50µm

Figure 12. SEM images at the (left) surface and (right) cross-section of the coatings of (0%-Zn) un- Figure 12. SEM images at the (left) surface and (right) cross-section of the coatings of (0%-Zn) un- Figuredoped HA 12. SEM and images(5%-Zn) at Zn-doped the (left) surfaceHA. Reproduced and (right) with cross-section permission of the from coatings [13], Elsevier, of (0%-Zn) 2018. un-doped doped HA and (5%-Zn) Zn-doped HA. Reproduced with permission from [13], Elsevier, 2018. HA and (5%-Zn) Zn-doped HA. Reproduced with permission from [13], Elsevier, 2018. Additionally, Nishida (2009) reported that the addition of nickel particles into samaria-doped ceria Additionally,modifies the microstructureNishida (2009) of reported coating thatfrom the including addition only of nickelspherical particles particles into into samaria-doped the one with nano-spheresceria modifies thewith microstructure embedded nanoparticles of coating from to includingbe used onlyin SOFC spherical applications particles into[43]. the A one porous with microstructurenano-spheres with with embeddedporosity in nanoparticles the form of intoterconnected be used in networks SOFC applications of voids was [43]. observed A porous for microstructure with porosity in the form of interconnected networks of voids was observed for

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Additionally, Nishida (2009) reported that the addition of nickel particles into samaria-doped ceria modiﬁes the microstructure of coating from including only spherical particles into the one with nano-spheres with embedded nanoparticles to be used in SOFC applications [43]. A porous microstructureMetals 2018 with, 8, x FOR porosity PEER REVIEW in the form of interconnected networks of voids was observed12 of for 19 cermet coatings produced from an aqueous solution precursor [29]. The void in the coating’s architecture cermet coatings produced from an aqueous solution precursor [29]. The void in the coating’s is a result from the decomposition of the partially pyrolyzed precursor which is submitted to in situ architectureMetals 2018, 8, x isFOR a PEERresult REVIEW from the decomposition of the partially pyrolyzed precursor which12 of is19 evaporationsubmitted of the to remainingin situ evaporation solvent of [ 44the]. remaining Figure 13 solvent displays [44]. the Figure surface 13 displays morphology the surface of the LSM cermetmorphology coatingcermet coatings deposited of the produced LSM using cermet from (Figure coating an 13aqueous depositeda) water solution us anding(Figure (Figureprecursor 13a)13 b)[29]. water ethanol The and void (Figure as solvents.in the13b) coating’s ethanol The aqueous SPPS coatingasarchitecture solvents. shows The is aaaqueous porousresult fromSPPS sponge-like coatingthe decomposition shows morphology a porous of thesponge-like with partially agglomerated morphologypyrolyzed solidprecursorwith agglomerated (~0.5 whichµm) particles.is On thesolid othersubmitted (~0.5 hand, µm)to ethanolin particles. situ evaporation SPPS On the coating other of hand,the shows remaining etha similarnol SPPS solvent porous coating [44]. shows morphology Figure similar 13 displays porous and agglomerated morphology the surface solid particles,andmorphology but agglomerated with theof the reﬁnement solid LSM particles, cermet of coating but particle with deposited the size refinement (~0.2 usingµ m)(Figure of whichparticle 13a) attributedsize water (~0.2 and µm) (Figure the which increase 13b) attributed ethanol of the triple the increase of the triple phase boundary length between the YSZ and LSM cathode, decreasing phase boundaryas solvents. length The aqueous between SPPS the coating YSZ shows and LSM a porous cathode, sponge-like decreasing morphology cathode with polarization. agglomerated cathodesolid (~0.5 polarization. µm) particles. On the other hand, ethanol SPPS coating shows similar porous morphology and agglomerated solid particles, but with the refinement of particle size (~0.2 µm) which attributed the increase of the triple phase boundary length between the YSZ and LSM cathode, decreasing cathode polarization.

Figure 13. Surface morphology of an LSM cermet cathode deposited by (a) aqueous solution Figure(reproduced 13. Surface with morphology permission from of [24], an LSMElsevier, cermet 2011) and cathode (b) ethanol deposited (reproduced by with (a) aqueous permission solution (reproducedfrom [29], with Elsevier, permission 2012) SPPS. from [24], Elsevier, 2011) and (b) ethanol (reproduced with permission from [29Figure], Elsevier, 13. Surface 2012) SPPS.morphology of an LSM cermet cathode deposited by (a) aqueous solution As(reproduced discussed with in permissionthe previous from sections, [24], Elsevier, the chemistry2011) and ( bof) ethanol the initial (reproduced precursors with permissioninfluences the microstructurefrom [29], Elsevier, of the final2012) cermetSPPS. coatings, as depicted in Figure 14, showing the SEM images of Asnickel-based discussed cermets in the previous(a) Ni-YSZ sections,and (b) Ni-SDC the chemistry. Nickel-samaria-doped of the initial ceria precursors (Ni-SDC) influencescoating the microstructuredisplaysAs discusseda of microstructure the final in the cermet previous compos coatings, edsections, of very as thedepicted finechemistry spherical in of Figure theparticles initial 14 ,precursorshaving showing micrometer influences the SEM and the images of nickel-basednanometermicrostructure cermets sizes (a)of similar the Ni-YSZ final to nickel-yttria-stabilizedcermet and (b) coatings, Ni-SDC. as Nickel-samaria-doped de pictedzirconia. in FigureThe morphology 14, showing ceria of (Ni-SDC) thenickel SEM particles images coating inof displays a microstructureSPPSnickel-based was already composed cermets reported (a) of Ni-YSZ veryto be very fine and fine, spherical(b) whichNi-SDC suggests particles. Nickel-samaria-doped that having its combination micrometer ceria with (Ni-SDC) andsamaria-doped nanometer coating sizes similarceriadisplays to nickel-yttria-stabilized (SDC) a doesmicrostructure not promote compos the zirconia. growthed of acceleration Thevery morphology fine ofspherical the particles of particles nickel [26]. particleshaving micrometer in SPPS was and already reportednanometer to be very sizes fine, similar which to suggestsnickel-yttria-stabilized that its combination zirconia. The with morphology samaria-doped of nickel ceria particles (SDC) in does not SPPS was already reported to be very fine, which suggests that its combination with samaria-doped promoteceria the (SDC) growth does acceleration not promote ofthe the growth particles acceleration [26]. of the particles [26].

Figure 14. Scanning electron micrographs of (a) Ni-YSZ and (b) Ni-SDC SPPS cermet coatings. Reproduced with permission from [26], Elsevier, 2012.

DueFigure to 14.the Scanningcomplex electronin-flight micrographs phenomena of and (a) theNi-YSZ unique and particle (b) Ni-SDC morphologies SPPS cermet formed coatings. during Figurethe process, 14.ReproducedScanning some with interesting electronpermission coatings’ micrographs from [26], microstruc Elsevier, of ( a2012.)tures Ni-YSZ and architectures and (b) Ni-SDC can be SPPS generated cermet in coatings.SPPS Reproduceddepending withon the permission set of parameters from [26 used], Elsevier, [26,33,34,42,45–47]. 2012. Lay et al. [45] reported on the deposition of Ni-SDCDue to using the complex different in-flight spray parameters phenomena (SP), and namely,the unique SP1, particle which morphologies has higher velocity, formed energy, during the process, some interesting coatings’ microstructures and architectures can be generated in SPPS

depending on the set of parameters used [26,33,34,42,45–47]. Lay et al. [45] reported on the deposition of Ni-SDC using different spray parameters (SP), namely, SP1, which has higher velocity, energy,

Metals 2018, 8, 420 13 of 18

Due to the complex in-flight phenomena and the unique particle morphologies formed during the process, some interesting coatings’ microstructures and architectures can be generated in SPPS dependingMetals 2018, 8, on x FOR the PEER set of REVIEW parameters used [26,33,34,42,45–47]. Lay et al. [45] reported on the deposition13 of 19 of Ni-SDC using different spray parameters (SP), namely, SP1, which has higher velocity, energy, and solutionand solution precursor precursor flow flow rate comparedrate compared to SP2. to SP2. The latterThe latter hasa has lower a lower solution solution concentration. concentration. Ni-SDC Ni- usingSDC using SP1 displays SP1 displays poorly-adhered poorly-adhered finer particles finer particles with greater with porosity greater comparedporosity compared to SP2 (see to Figure SP2 (see 15). TheFigure surface 15). morphologyThe surface ofmorphology Ni-SDC, obtained of Ni-SDC, using obtained SP1, shows using no SP1, splat-shaped shows no microstructural splat-shaped featuresmicrostructural which indicatesfeatures which that partialindicates pyrolization that partial ofpyrolization droplets takes of droplets place intakes the place plasma in the jet plasma due to lowjet due solution to low concentration solution concentration and higher plasmaand higher energy plasma than SP2energy with than high SP2 solution with concentration.high solution Thus,concentration. evaporation Thus, of evaporation most elements of most in the elements plasma, in and the subsequent plasma, and nucleation subsequent atthe nucleation surface ofat the substrate,surface of isthe highly substrate, possible is highly leading possible to the depositionleading to the of less-adheringdeposition of coatings.less-adhering coatings.

Figure 15. Surface morphologies of Ni-SDC cermet coatings using (a) SP1 and (b) SP2. Reproduced Figure 15. Surface morphologies of Ni-SDC cermet coatings using (a) SP1 and (b) SP2. Reproduced with permission from [45], ELECTROCHEMICAL SOCIETY, 2011. with permission from [45], ELECTROCHEMICAL SOCIETY, 2011. Moreover, the injection of solution precursor into the plasma jet may influence the microstructureMoreover, theof the injection cermet of coatings. solution The precursor droplets into obtained the plasma using jet a may mechanical influence injector the microstructure had initially oflarge the droplets cermet which coatings. could The have droplets undergone obtained secondar usingy break-up a mechanical to form injector finer droplets had initially as discussed large dropletsin Section which 2.2 [17,22,23,32]. could have Consequently, undergone secondary the resulting break-up microstructure to form finer of coatings droplets sprayed as discussed using inmechanically-injected Section 2.2[ 17,22,23 solution,32]. Consequently, shows a porous, the resultingagglomerated, microstructure dome-like ofstructure coatings with sprayed resolidified using mechanically-injectedparticles at the surface solution [23]. The shows injection a porous, by atomization agglomerated, produces dome-like about structure 2–100 micron with resolidified droplets, particleswhich may at undergo the surface rapid [23 ].evaporation. The injection Consequently, by atomization the complete produces pyrolysis about 2–100 of the micron solute droplets,particles whichmay occur may at undergo the interaction rapid evaporation. with the plasma Consequently, jet [17,33,34]. the completeThe injection pyrolysis by atomization of the solute leads particles to the mayformation occur of at irregular the interaction tower-like with ag theglomerates plasma jet of [ 17very,33 ,fine34]. particles The injection with bysmooth atomization surface leadsdeposits, to the as formationreported elsewhere of irregular [34]. tower-like agglomerates of very fine particles with smooth surface deposits, as reportedOn the elsewhereother hand, [34 the]. long interaction time between the solution precursor droplets and the plasmaOn jet the during other the hand, trajectory the long induces interaction the effects time of betweensintering and the solutionof local re-melting. precursor Michaux droplets et and al. the[34] plasmareported jet porous during architectures the trajectory with inducesvoids for the external/radially effects of sintering delivered and Ni-YSZ of local cermet re-melting. due to Michauxstacking of et al.partially-molten [34] reported poroussplats forming architectures caviti withes [34]. voids On forthe external/radiallyother hand, dense delivered uniform-layered Ni-YSZ cermetcoating duewith to fine-scale stacking porosity of partially-molten were observed splats for the forming solution cavities precursor [34]. introduced On the other within hand, the dense torch uniform-layeredconvergence tube, coating which with is fine-scaleattributed porosity to the werefaster observedheating forof the solutiondroplet precursorleading to introduced complete withinpyrolysis the (see torch Figure convergence 16) [31]. tube,Therefore, which the is attributedmicrostructure to the of faster the ceramic-metal heating of the composite droplet leading coatings to completecan be influenced pyrolysis by (see the Figure way of 16 injectio)[31]. Therefore,n depending the on microstructure the foreseen application. of the ceramic-metal composite coatings can be influenced by the way of injection depending on the foreseen application.

Metals 2018, 8, x FOR PEER REVIEW 13 of 19

and solution precursor flow rate compared to SP2. The latter has a lower solution concentration. Ni- SDC using SP1 displays poorly-adhered finer particles with greater porosity compared to SP2 (see Figure 15). The surface morphology of Ni-SDC, obtained using SP1, shows no splat-shaped microstructural features which indicates that partial pyrolization of droplets takes place in the plasma jet due to low solution concentration and higher plasma energy than SP2 with high solution concentration. Thus, evaporation of most elements in the plasma, and subsequent nucleation at the surface of the substrate, is highly possible leading to the deposition of less-adhering coatings.

Figure 15. Surface morphologies of Ni-SDC cermet coatings using (a) SP1 and (b) SP2. Reproduced with permission from [45], ELECTROCHEMICAL SOCIETY, 2011.

Moreover, the injection of solution precursor into the plasma jet may influence the microstructure of the cermet coatings. The droplets obtained using a mechanical injector had initially large droplets which could have undergone secondary break-up to form finer droplets as discussed in Section 2.2 [17,22,23,32]. Consequently, the resulting microstructure of coatings sprayed using mechanically-injected solution shows a porous, agglomerated, dome-like structure with resolidified particles at the surface [23]. The injection by atomization produces about 2–100 micron droplets, which may undergo rapid evaporation. Consequently, the complete pyrolysis of the solute particles may occur at the interaction with the plasma jet [17,33,34]. The injection by atomization leads to the formation of irregular tower-like agglomerates of very fine particles with smooth surface deposits, as reported elsewhere [34]. On the other hand, the long interaction time between the solution precursor droplets and the plasma jet during the trajectory induces the effects of sintering and of local re-melting. Michaux et al. [34] reported porous architectures with voids for external/radially delivered Ni-YSZ cermet due to stacking of partially-molten splats forming cavities [34]. On the other hand, dense uniform-layered coating with fine-scale porosity were observed for the solution precursor introduced within the torch convergence tube, which is attributed to the faster heating of the droplet leading to complete Metalspyrolysis2018, 8(see, 420 Figure 16) [31]. Therefore, the microstructure of the ceramic-metal composite coatings14 of 18 can be influenced by the way of injection depending on the foreseen application.

Figure 16. Cross-section of the Ni-YSZ anodes deposited via internal and axial injectors. Reproduced with permission from [31], Elsevier, 2014.

4. Applications of SPPS Cermet Coatings SPPS cermet coatings have a wide variety of potential and emerging applications. However, plasma spraying of ceramic-metal composite solution precursor is still in its development stage, this review presents an overview of some of the SPPS cermet applications as protective and performance-enhancing coatings.

4.1. Solid Oxide Fuel Cells (SOFC) Cermet coatings deposited using the SPPS process have primarily been used as electrodes in SOFC applications. Strontium-doped lanthanum manganite (LSM) as a cathode and nickel-Yttria stabilized zirconia (Ni-YSZ) as an anode are ones that have been extensively studied [23–26,29,31,34,41–43,45–48]. Wang et al. [24,29] investigated the polarization of LSM SOFC cathode material deposited using water and ethanol as solvent, respectively. The polarization of the cathode at 1000 ◦C gives the lowest specific surface resistance that reached 0.15 Ω·cm2 for an alcohol SPPS LSM cathode, which is half of the polarization of the aqueous SPPS LSM cathode, which is 0.3 Ω·cm2. This decrease in cathode polarization using ethanol is influenced by the increase of the triple phase boundary (TPB), i.e., associated with its catalytic property of the coating which is also related to the microstructure of the coating, as discussed in Section3[19,46,48]. Another study by Li et al. [25] reported a low polarization of SOFC cathode of 0.07 Ω·cm2 at 900 ◦C using strontium-doped samarium cobaltite perovskite sprayed using a stand-off of 130 mm. The long dwell time of droplets resulted in the complete reaction of the precursor and in fine microstructures of the perovskite phase. The Ni-YSZ composite anode studied by the SPPS process shows that the performance of this type of electrode is influenced by the spray parameters, such as the solution composition, plasma gas composition, and the stand-off distance [34]. Metcalfe et al., [31] reported one of the highest power densities of 0.45 W/cm2 at 0.7 V and an open circuit potential of 1.05 V at 750 ◦C thanks to the use of concentrated citric acid as an additive. Wang et al. [23] studied the electrical resistance of Ni-YSZ coatings at different temperatures which shows a decrease in the inverse of the electrical resistance with increasing temperature confirming the existence of continuous network of Ni in the coating [23]. The increase of spray distance and variation of plasma working gas mixtures modified the coatings microstructures [34]. Further works are still underway to better understand the electrochemical properties of SOFCs.

4.2. Catalyst Cermet photocatalysts by SPPS were rarely investigated. Dom et al. [10] demonstrates SPPS deposition of porous films of pure spinel zinc-ferrite phases with a band gap of ~1.9 eV which favors Metals 2018, 8, 420 15 of 18 the degradation of methylene blue pollutant under solar radiation. Figure 17 shows the substantial increase of photo-degradation efficiency η of SPPS zinc ferrite films with increasing film thickness from 25 µm to 50 µm. Although it is not comparable for the powder catalyst from solid state reaction routes, SPPS zinc ferrite films still demonstrate their potential in photocatalysis applications. Exploration of the SPPS process with appropriate controls on the critical spray parameters to produce new photocatalytic coatingsMetals 2018 has, 8, x beenFOR PEER taken REVIEW into account in order to have niche applications. 15 of 19

Figure 17. Methylene blue photo de-coloration efficiency versus ZnFe2O4 film thickness for films Figure 17. Methylene blue photo de-coloration efficiency versus ZnFe2O4 film thickness for films deposited at at precursor precursor pH pH = 8. = The 8. dotted The dotted horizontal horizontal line represents line represents the efficiency the efficiency of powder of ZnFe powder2O4 photocatalyst synthesized by a solid state reaction. Reproduced with permission from [10], Elsevier, ZnFe2O4 photocatalyst synthesized by a solid state reaction. Reproduced with permission from [10], Elsevier,2012. 2012.

4.3. Biomedical Coatings Biomedical coatings have been utilized to promote bone growth at the surface of implants Biomedical coatings have been utilized to promote bone growth at the surface of [13,15,39]. These coatings are deposited by an FDA-approved process, i.e., atmospheric plasma implants [13,15,39]. These coatings are deposited by an FDA-approved process, i.e., atmospheric spraying [20]. Generally, the bioceramic coatings have limited use because of their low mechanical plasma spraying [20]. Generally, the bioceramic coatings have limited use because of their low strength. Hence, by reinforcing the metal particles in the ceramic-based matrix forming the cermet mechanical strength. Hence, by reinforcing the metal particles in the ceramic-based matrix forming coating enhances the strength, i.e., the fracture toughness and the wear resistance of the coatings for the cermet coating enhances the strength, i.e., the fracture toughness and the wear resistance of the load-bearing applications [19,20]. Cermet coatings containing metal particles of zinc, silver, and coatings for load-bearing applications [19,20]. Cermet coatings containing metal particles of zinc, copper do not only improve the mechanical strength of the coatings, but may promote antibacterial silver, and copper do not only improve the mechanical strength of the coatings, but may promote properties [13]. A study of Candidato et al. [13] has successfully produced zinc-doped hydroxyapatite antibacterial properties [13]. A study of Candidato et al. [13] has successfully produced zinc-doped coatings using the SPPS process. The microstructure of the coatings transforms from a highly porous hydroxyapatite coatings using the SPPS process. The microstructure of the coatings transforms from coating to a dense one by adding zinc to the hydroxyapatite matrix (see Figure 12). It was also a highly porous coating to a dense one by adding zinc to the hydroxyapatite matrix (see Figure 12). reported that increasing the zinc concentration on the cermet coatings inhibits the formation of the It was also reported that increasing the zinc concentration on the cermet coatings inhibits the formation hydroxyapatite phase. The optimal Zn content for Zn-HA coatings is 5%. of the hydroxyapatite phase. The optimal Zn content for Zn-HA coatings is 5%. Sanpo et al. [15] reported the SPPS deposition of cobalt ferrite coatings for biomedical Sanpo et al. [15] reported the SPPS deposition of cobalt ferrite coatings for biomedical applications [15]. In their study, the phase content and the cermet coating microstructure were greatly applications [15]. In their study, the phase content and the cermet coating microstructure were influenced by the chelating agent used. As-sprayed cobalt ferrite coating shows incomplete pyrolysis greatly inﬂuenced by the chelating agent used. As-sprayed cobalt ferrite coating shows incomplete of the splats with a larger amount of other oxide phases. With the addition of citric acid as a chelating pyrolysis of the splats with a larger amount of other oxide phases. With the addition of citric acid as a agent, the formation of 90% cobalt ferrite phase was achieved and fully-melted splats were observed. chelating agent, the formation of 90% cobalt ferrite phase was achieved and fully-melted splats were In fact, the chelating agents could have helped in the homogeneous interaction of mineral ions observed. In fact, the chelating agents could have helped in the homogeneous interaction of mineral preventing the undesirable spontaneous condensation of cobalt and iron cations [14–16]. ions preventing the undesirable spontaneous condensation of cobalt and iron cations [14–16]. With these promising results, cermet coatings under the SPPS process are also a viable candidate With these promising results, cermet coatings under the SPPS process are also a viable candidate for biomedical application. However, evaluation on the biological properties of the cermet coatings for biomedical application. However, evaluation on the biological properties of the cermet coatings by by SPPS is yet to be done. SPPS is yet to be done.

Author Contributions:Contributions: L.P.L.P. initiated initiated this this review review article. article. R.C.J., R.C.J., R.U. R.U. and L.P.and conceptualized L.P. conceptualized the ﬂow the and flow contents and ofcontents this review. of this R.U. review. gathered R.U. thegathered necessary the literaturesnecessary forliteratures the article forand the wrotearticle theand body wrote of the reviewbody of paper. the review R.C.J. paper. R.C.J. helped on the abstract and subsections of the paper. L.P. contributes on correcting the technical aspect of the review article.

Acknowledgments: Romnick Unabia would like to acknowledge the Department of Science and Technology- Accelerated Science and Technology Human Resource Development Program (DOST-ASTHRDP) for the scholarship grant, and Department of Science and Technology- Philippine Council for Industry, Energy and Emerging Technology Research and Development (DOST-PCIEERD) for the financial grant.

Conflicts of Interest: The authors declare no conflict of interest.

Metals 2018, 8, 420 16 of 18 helped on the abstract and subsections of the paper. L.P. contributes on correcting the technical aspect of the review article. Acknowledgments: Romnick Unabia would like to acknowledge the Department of Science and Technology- Accelerated Science and Technology Human Resource Development Program (DOST-ASTHRDP) for the scholarship grant, and Department of Science and Technology- Philippine Council for Industry, Energy and Emerging Technology Research and Development (DOST-PCIEERD) for the financial grant. Conflicts of Interest: The authors declare no conflict of interest.

Symbol

ρ Density of the fluid −1 Lv Latent heat of vaporization, J·kg Ω Ohm η Photo-degradation efficiency, % −1 −1 cp Specific heat, J·kg ·K 2 σs Surface tension, J/m Tv Vaporization temperature, K 2 υ solution Velocity of the solution 2 υ plasma Velocity of the plasma gas µs Viscosity, Pa·s

Abbreviations

APS Atmospheric plasma spray CZP Calcium zinc phosphate DTA-TGA Thermogravimetric-differential thermal analyzer FDA Food and Drugs Administration HA Hydroxyapatite LSM Strontium-doped lanthanum manganite Ni-SDC Nickel samaria-doped ceria Ni-YSZ Nickel Yttria stabilized zirconia SOFC Solid oxide fuel cells SP Spray parameters SPPS Solution precursor plasma spray SPS Suspension plasma spray TCP Tri-calcium phosphate TPB Triple phase boundary Zn-HA Zinc-doped Hydroxyapatite

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