Current Progress in Solution Precursor Plasma ...

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 1 2

*

ID

Physics Department, College of Science and Mathematics, MSU-Iligan Institute of Technology, Iligan City 9200, Philippines; [email protected] 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

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precursor has been developed to overcome the limitations of theofAPS precursor plasma plasmaspraying sprayingprocess process(SPPS), (SPPS), has been developed to overcome the limitations the process [21,22]. APS process [21,22]. SPPS SPPS offers offers aa single-step single-step deposition deposition process process of of finely-structured finely-structured cermet cermet coatings coatings by by employing employing molecularly-mixed liquid precursors directly as the feedstock, which enables avoiding the molecularly-mixed liquid precursors directly as the feedstock, which enables avoiding the powder powder and The use use of of solution solution precursors precursors for and suspension suspension preparation preparation as as shown shown in in Figure Figure 11 [10,13,14,17–19]. [10,13,14,17–19]. The for thermal thermal spraying spraying was was initiated initiated by by Karthikeyan Karthikeyan et et al. al. [21]. [21]. SPPS SPPS coating coating technology technology is is industrially industrially advantageous advantageous due due to to its its ease ease in in feedstock feedstock preparation. preparation.

Figure 1. Feedstocks Feedstocks used for thermal spraying processes which enable to obtain nano-structured nano-structured coatings: (A) the pre-processed nano-powder nano-powder particle particle used for the APS process; (B) the suspension suspension droplet used used for forthe theSPS SPSprocess; process; and (C) the homogeneously-mixed solution droplet for and (C) the homogeneously-mixed solution droplet used used for SPPS SPPS process. process.

Since the the beginning there has has been Since beginning of of 21st 21st century, century, there been rapid rapid development development in in nanostructured nanostructured coating coating deposition using the SPPS process as reflected by a number of published articles deposition using the SPPS process as reflected by a number of published articles [15–22]. [15–22]. However, However, most of onon thethe deposition of ceramic materials alone and only a fewahave most of the theaccounted accountedpapers papersfocus focus deposition of ceramic materials alone and only few been reported on SPPS coating deposition for cermets (ceramic-metal composite) because of the have been reported on SPPS coating deposition for cermets (ceramic-metal composite) because of the oxidation potential [10,15]. The present review focuses on oxidation potential of ofthe themetal-ions metal-ionsininthe thesolution solutionprecursor precursor [10,15]. The present review focuses the current progress of solution precursor plasma spray for depositing cermet coatings used in on the current progress of solution precursor plasma spray for depositing cermet coatings used in various applications. In particular, the review gives an overview of the various factors affecting the various applications. In particular, the review gives an overview of the various factors affecting the final coating coating material, material,such suchasassolution solutionprecursor precursor preparations and physico-chemical interaction of final preparations and physico-chemical interaction of the the solution precursor with the high-temperature plasma jet. solution precursor with the high-temperature plasma jet. 2. Solution Precursor Plasma Spraying of Cermets The properties of the starting solution precursors and operational plasma spray process parameters greatly greatlyinfluenced influencedthe thephysical physicaland andchemical chemicalcharacteristics characteristics sprayed coatings. Thus, it parameters ofof sprayed coatings. Thus, it is is very important to screen parameters influence of the properties cermet very important to screen thosethose parameters whichwhich influence most ofmost the properties of cermetofcoatings. coatings. Someare of them are presented Figure 2 together the schematic representation of SPPS Some of them presented in Figurein 2 together with the with schematic representation of SPPS process. process. Selected parameters are discussed in the subsequent Selected parameters are discussed in the subsequent sections. sections.

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Figure 2. 2. Schematic Schematic diagram diagram of of the the solution solution precursor precursor plasma plasma spraying spraying process process showing showing the the many many Figure different and and important important operational operational spray spray parameters parameters that that could could influence influence the the physical physical and and chemical chemical different characteristics of cermet coatings. characteristics of cermet 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 Table 1). 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. Final Coating Final Coating

Coating Coating Application Application

Zinc ferrite, Photocatalysis Zinc ferrite, Photocatalysis 2O4 ZnFe2 O4 ZnFe

SrZrO SrZrO 3 3

Zn-HA

Cobalt ferrite, CoFe2O4

Biomedical, biomagnetic, Biomedical, and Antibacterial biomagnetic,

and

Nickel- Yttria Solid oxide fuel stabilized zirconia, Antibacterial cell (SOFC) Ni-YSZ

Nickel- Yttria stabilized zirconia, Ni-YSZ

Solvent Used

Zinc nitrate [Zn(NO3)2·6H2O] Zinc nitrate [Zn(NO3 )2 ·6H2 O] and ferric nitrate and ferric nitrate [Fe(NO 3)32 ·9H O]; [Fe(NO O];2ratio 3 )3 ·9H [Zn/Fe = 1/2] ratio [Zn/Fe = 1/2]

Solvent Used

Solid oxide fuel cell (SOFC)

Additive

Additive

Reference Reference

De-ionized

350 mM Dom, R. et De-ionized Dom, R. et al. water 350 mM citric water citric acid al. [10] acid [10] Ethylene Ethylene glycol glycol

Zirconium acetate Zirconium acetate [Zr(C2H4O2)4] [Zr(C O2 )4 ] and strontium 2 H4strontium and nitrate Thermal De-ionized De-ionized Thermal barrier nitrate tetrahydrate water water barrier tetrahydrate [Sr(NO 3 ) 2 · 4H 2 O] ; [Sr(NO3 )2 ·4H2 O]; concentration mol/L concentration= =1.61.6 mol/L Zinc nitratehexahydrate hexahydrate Zinc nitrate

Zinc-hydroxyapatite, ZincBiomedical Zn-HA hydroxyapatite, Biomedical

Cobalt ferrite, CoFe2 O4

Precursor Precursor

[Zn(NO3 )2 ·6H2 O], calcium [Zn(NO3)2·6H2O], calcium nitrate tetrahydrate De-ionized De-ionized nitrate3 )tetrahydrate [Ca(NO water 2 ·4H2 O], and water tri-ethyl3)phosphite [P(OEt) 3 ]; [Ca(NO 2·4H2O], and tri-ethyl ratio [(Ca + Zn)/P = 1.67]

-

-

-

-

Li, X. et al. [12]

Li, X. et al. [12]

Candidato, R. Candidato, et al.R. [13] et al.

[13]

phosphite [P(OEt)3]; ratio [(Ca + Cobalt Zn)/Pnitrate = 1.67]

Sanpo et al. [Co.(NO3 )2 ·6H2 O] and ferric De-ionized Citric acid as [14]; 3)2·6H2O] Cobalt Sanpo et nitratenitrate [Fe(NO[Co.(NO water chelatingCitric agent acid and Sanpo et al. 3 )3 ·9H2 O]; ratio [Co./Fe = 1/2] [15] De-ionized and ferric nitrate as al. [14];

water ZrOCl2[Fe(NO ·6H2 O, 3Y(NO )3·9H32)O]; 3 ·6H2 O and Ni(NO3 )3 ·6H2 O; ratio [Co./Fe = 1/2] Distilled water concentration 60 vol. % of 2·6H )3·Ni 6H2O ZrOCl 8%YSZ and2O, 40 Y(NO vol. %3of and Ni(NO3)3·6H2O; concentration 60 vol. % of 8%YSZ and 40 vol. % of Ni

Distilled water

chelating and Sanpo Wang,etY,al. and agent [15] Coyle, T.W. [23]

-

Wang, Y, and Coyle, T.W. [23]

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Table 1. Cont. Strontium doped La(NO3)3·6H2O, Sr(NO3)2 and SolidCoating oxide Distilled lanthanum molar ratio La:Sr:MnSolvent Mn(NO3)2;Precursor Used waterAdditive Final Coating fuel cell Application Ethanol manganite, = 0.8:0.2:1; total concentration 0.1 (SOFC) doped La(NO3 )3 ·6H2 O, Sr(NO3 )2 0.8Sr0.2MnO3, LaStrontium mol/L lanthanum Solid oxide fuel and Mn(NO3 )2 ; molar ratio Distilled water LSM manganite, cell (SOFC) La:Sr:Mn = 0.8:0.2:1; total Ethanol concentration 0.1 mol/L LaStrontium Sr MnO , LSM 0.8 0.2 3 Sm(NO3)3·6H2O, Sr(NO3)2 and doped Solid oxide Sm(NO ) ·6H O, Sr(NO )2 Ethanol and Strontium doped Co.(NO33)33·6H22O; molar3 ratio samarium fuel cell Solid oxide fuel and Co.(NO3 )3 ·6H2 O; molar Ethanol and distilled water samarium cobaltite, Sm:Sr:Co. = 0.7:0.3:1; total cell (SOFC) ratio Sm:Sr:Co. = 0.7:0.3:1; distilled water cobaltite, (SOFC) Sm0.7 Sr0.3 Co3-δ total concentration0.1 0.1 mol/L mol/L concentration Sm0.7Sr0.3Co3-δ

Wang, X. et al. [24] Reference Wang, X. Wang,etX.al. et [29] al.

[24] Wang, X. et al. [29]

Li, C.X. et

Li, C.X. et al. al. [25] [25]

al.,[26] [26]reported reportedthe theinfluence influenceof of the the chemistry chemistry of of the the starting starting precursors precursors on nickel-based Lay et al. SPPS cermet. They They found found that that nickel-samaria nickel-samaria doped doped ceria ceria (Ni-SDC) (Ni-SDC) has has larger quantity of finer particles than nickel-yttria stabilized zirconia (Ni-YSZ) [26]. Moreover, Wang Wang et et al., [24] reported that particles utilization of of strontium-doped strontium-dopedlanthanum lanthanummanganite, manganite,(La (La0.80.8Sr Sr0.2 0.2MnO3 3)) as the utilization as SOFC cathode material shows promising promising characteristics characteristics i.e., i.e., high high electrochemical electrochemicalactivity activity and and thermal thermal stability stability due due to to its high shows temperature (>800 (>800 ◦°C) 0.5Sr CoO 3 and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 . operating temperature C) compared to Sm0.5 Sr0.5 CoO and La Sr Co Fe O . 0.5 3 0.6 0.4 0.2 0.8 3 reported that that the the good good stoichiometric stoichiometric ratio ratioof of the the ceramic-metallic ceramic-metallicspecies speciesand/or and/or the It was also reported complexing/chelating agent addition of complexing/chelating agentsuch such as as citric citric acid acid in in the solution precursor can modify the chemistry of the solution [10,13–15]. Dom et al. [10] showed that the stoichiometric ratio of Zn to Fe being about about1:21:2 could thermodynamically the formation of cermet desiredzinc cermet ferrite being could thermodynamically favorfavor the formation of desired ferritezinc composite composite phase their corresponding metal-oxide phases such as zinc oxide, iron oxide, phase rather thanrather their than corresponding metal-oxide phases such as zinc oxide, iron oxide, etc. [10]. etc. [10]. Candidato et al. [13] also reported that as the zinc ion concentration into the hydroxyapatite Candidato et al. [13] also reported that as the zinc ion concentration into the hydroxyapatite (HA) (HA) ceramic increases, formation HA phases is hindered andthe favors the formation of ceramic matrixmatrix increases, formation of HA of phases is hindered and favors formation of calcium calcium zinc phosphate 3. In addition, al. [15] presented zinc phosphate impurityimpurity phase asphase shownasinshown Figurein3.Figure In addition, Sanpo etSanpo al. [15]etpresented that the that the utilization of citric acid as agent chelating with 20% concentration favors the utilization of citric acid as chelating with agent 20% concentration significantlysignificantly favors the formation of formation of more than ferrite 90% ofphase cobalt ferrite phaseeffect due to chelate effect attributed to an increase in more than 90% of cobalt due to chelate attributed to an increase in entropy resulting entropy to a morematerial stable composite material [15]. to a moreresulting stable composite [15].

Figure 3. 3. X-ray HA cermet coatings showing thethe evolution of the Figure X-raydiffraction diffractiondiagram diagramofofthe theZn-doped Zn-doped HA cermet coatings showing evolution of calcium zinc phosphate phase as zinc ion concentration increases. Reproduced with permission from the calcium zinc phosphate phase as zinc ion concentration increases. Reproduced with permission [13], Elsevier, 2018.2018. from [13], Elsevier,

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

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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 Table 2) [10,24,28–30]. Table 2. Properties of two solvents commonly used in the SPPS process.

Solvent

Surface Tension σs (J·m−2 ) (Room Temp.)

Viscosity µs (Pa·s) (Room Temp.)

Specific Heat cp (J·kg−1 ·K−1 ) (at Room Temp.)

Latent Heat of Evaporation Lv (J·kg−1 )

Evaporation Temperature T v (K)

Water Ethanol

72 × 10−3 22 × 10−3

10−3 1.06 × 10−3

4.18 × 103 2.44 × 103

2.26 × 106 0.84 × 106

373 351

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 (Fe2 O3 ) 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 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.

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 Metals 2018, 8, 420 6 of 18 transform [28].

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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 solution injection with into permission the plasmafrom jet affects the heating water as precursorof solvents. Reproduced [10], Elsevier, 2012. of the droplets. The solution inside a pressurized container can be fed using a pneumatic system or in an open container using a peristaltic pump. The flow rate can be easily adjusted by varying the pressure or rotation of 2.2. Injection of Solution Feedstock the roller, respectively. In general, clogging is not a problem in SPPS, unlike in the APS process. The mechanism of solution injection into theflushing plasma the jet affects heating of the However, a flexible pipeline must be used and injectorthewith ethanol or droplets. water is The solution inside a pressurized container can be fed using a pneumatic system or in recommended to avoid gelling of the solution inside the injector. The types of injection an thatopen are container using a peristaltic pump. The flow rate can be easily adjusted by varying the pressure or frequently used in SPPS can be categorized depending on the type of injector used (mechanical or rotation of the roller, respectively. In general, clogging is not a problem in SPPS, unlike in the APS atomizing injector), the mode of injection (internal or external) and, on the choice between radial and process. However, pipeline be used andaflushing the injector with water axial injection setupa flexible [17,23,30,31]. Themust solution inside pressurized container canethanol be fedor using a is recommended to avoid gelling of the solution inside the injector. The types of injection that are pneumatic system or in an open container using a peristaltic pump, as shown in Figure 5. frequently used in SPPS can categorized depending on the the flow type rate of injector used (mechanical or Mechanical injectors maybeuse motor rollers to control of the solution stream [22]. atomizing injector), the mode injection external) and, on the choice between radial The formation of droplets fromof this stream (internal is inducedorby an aerodynamic breakup through constant and axial injection setup [17,23,30,31]. The solution inside a pressurized container can be fed using application of pressure, as shown in Figure 5A [17]. The resulting droplets using a mechanical injectora pneumatic system or ainrelatively an open container using a peristaltic pump, as shown in Figure 5. are observed to have wide distribution of large droplets.

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 Mechanical may use motor rollers to control thesecondary flow rate break-up of the solution streamones, [22]. penetrating at theinjectors plasma region, these large droplets undergo into smaller The formation of droplets from this stream is induced by an aerodynamic breakup through constant as shown in Figure 6, 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, it can penetrate into the plasma core, enabling the droplet to be heated effectively. However, the good

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application of pressure, as shown in Figure 5A [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 Figure 6, 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 Metals 2018,it8,can x FOR PEER REVIEW 7 of 19 droplets, penetrate into the plasma core, enabling the droplet to be heated effectively. However, 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 ρυρυ > ρυ ρυ22plasma 2solution > solution plasma , ,

(1) (1)

Wang et et al. al. [24], [24], reported reported that that mechanically-deposited mechanically-deposited strontium-doped strontium-doped lanthanum lanthanum manganite manganite Wang (LSM) coatings present a partially crystallized perovskite phase with a fraction of the amorphous (LSM) coatings present a partially crystallized perovskite phase with a fraction of the amorphous structure which which could could be be attributed attributed to the penetration penetration and the droplets droplets in in the the plasma plasma structure to the and dwell dwell time time of of the jet [24]. [24]. jet

6. Aerodynamic Figure 6. Aerodynamic interactions interactions between between the the liquid liquid stream stream and and the the high-temperature high-temperature jet. jet. from [32]. [32]. Reproduced with permission from

Figure 5B 5B describes describes an an atomizing atomizing injector injector which which utilizes utilizes an an atomizing atomizing gas to provide provide pressure pressure Figure gas to on the solution precursor in order to break it up into small droplets. These atomized droplets, 2–100 on the solution precursor in order to break it up into small droplets. These atomized droplets, microns in size, penetrate into the plasma jet core inducing an immediate thermal reaction of 2–100 microns in size, penetrate into the plasma jet core inducing an immediate thermal reaction of the the solute due to fast evaporation of the solvent [17,33]. The direct contact of the atomized solution solute due to fast evaporation of the solvent [17,33]. The direct contact of the atomized solution droplets droplets intoplasma the hotcore plasma result in formation the rapid formation the LSMindicating coatings, aindicating into the hot resultcore in the rapid of the LSMofcoatings, crystallinea crystalline La 0.5Sr0.5MnO3 perovskite phase. The XRD spectra of the LSM coatings shows broadened La0.5 Sr0.5 MnO3 perovskite phase. The XRD spectra of the LSM coatings shows broadened peaks peaks because of the low peak-to-noise ratio of the due deposits duesmall to their small sizes crystallite sizes because of the low peak-to-noise ratio of the deposits to their crystallite associated associated with the breakup of the solution precursor into finer droplet sizes [24]. However, the size with the breakup of the solution precursor into finer droplet sizes [24]. However, the size distribution distribution of in the droplets this type injectorinstill depends in the nozzle theflow gas of the droplets this type ofin injector still of depends the nozzle geometry, the gasgeometry, velocity and velocity and flow rate, and the property of the solvents used. rate, and the property of the solvents used. The solution solutionprecursor precursorcan canbebedelivered delivered into plasma jet axial by axial or radial direction [33]. In The into thethe plasma jet by or radial direction [33]. In axial axial injection, the droplet the high-temperature corethe where the starts solvent injection, the droplet enters enters directlydirectly into theinto high-temperature plasma plasma core where solvent to starts to evaporate rapidly, that results in the solute concentration near the droplet surface. This evaporate rapidly, that results in the solute concentration near the droplet surface. This injection mode injection mode allows solute saturation shortDue period of rapid time. heating Due to the rapidinheating processthe in allows solute saturation in a short periodin ofatime. to the process axial injection 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].

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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 droplets diffusion Metalsand 2018,by 8, xrapid FOR PEER REVIEW of heat within the droplet. 8 of 19 matrix having a fine microstructure that could be induced by efficient internal circulation of the 2.3. Phenomena in Flight 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 discussed in many in studies 2.3. Phenomena 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 plasma flow [36]. After the break-up, the are fragmented experience rapid solvent evaporation discussed in many studies [17,22,35–37], presented droplets in this section in a chronological way. Initially, and the increase of solute concentration follows. The precipitation inside of liquid droplets the liquid stream or large droplets break-up into small droplets due to the strong shear stress of theleads to theplasma solid flow particle prior tothe coating deposition. droplets may undergo volumetric [36]. formation After the break-up, fragmented dropletsThe experience rapid solvent evaporation or surface Fine droplets tend to precipitate volumetrically, which is also known and theprecipitation. increase of solute concentration follows. The precipitation inside of liquid droplets leads to as the solid particle formation prior to coating deposition. TheOn droplets may hand, undergo volumetric or tend homogeneous nucleation, which results in splat formation. the other larger droplets surface precipitation. Fine dropletsintend precipitate volumetrically, which is also knowninternal as to precipitate superficially resulting shelltoformation. If the solid shell is impermeable, homogeneous nucleation, which results in splat formation. On the other hand, larger droplets tend pressurization takes place leading to the shell fracture. This may lead to the formation of 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 Figure 7. 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 Ozturk et al. [37] modeled the formation solid crust precipitation in a et single droplet. different particle morphologies formedof bythe SPPS have beenatdiscussed in theoccurring work of Saha al. [35]. The solid particles undergo heating and melting depending residence time inside plasma Ozturk et al. [37] modeled the formation of the solid cruston attheir precipitation occurring in a the single jet. The evaporation from theundergo moltenheating solid may take place. The vapors condense and form droplet. The solid particles and melting depending on theirmay residence time inside the nanoparticles. the plasma jet. The evaporation from the molten solid may take place. The vapors may condense and form the8 nanoparticles. Figure is a high magnification micrograph of the surface of an SPPS Zn-HA coating showing Figure 8 is a high magnification micrographonto of thethe surface of an SPPS Zn-HA coating showingof the the different particles formed in-flight deposited substrate. The chemical phenomena 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 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 precursor [27,28,38]. the precursor [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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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. 8. Surface micrograph of Zn-HA showing the different particle morphologies present The endothermicFigure peak at around 400 ◦ C coating indicated the oxidation of precursors forming NiO doped like fine spheres, fragmented shells, and agglomerates. Reproduced with permission from [13], Elsevier, 2018. with YSZ. Michaux et al. [34] identified the sequence of chemical reaction in the fabrication of NiO/8YSZ

2.4. Cermet Coatings Formation cermet: •

The exothermic reaction occurs first around 100 °C corresponding to ethanol solvent

The microstructure of the sprayed cermet depends on the in-flight phenomena of the plasma jet evaporation. Theparticles endothermic arriving peak at 300 °Con corresponded to decomposition of nitrate precursors. discussed above. •The the substrate or previously-deposited coating generally • The endothermic peak at around 400 °C indicated the oxidation of precursors forming NiO have sizes in the sub-micrometer or nanometer range [40,41]. To understand the coating formation, doped with YSZ. the deposits obtained from a single torch scan, as shown in Figure 9, can be studied [41]. 2.4. Cermet Coatings Formation Figure 9a showsThe deposited melted or partially melted spherical particles (~0.3 µm in diameter), microstructure of the sprayed cermet depends on the in-flight phenomena of the plasma jet discussed above. The particles on the or previously-deposited generally which resulted from the evaporation ofarriving solvent at substrate the surface of the small coating droplets [33,35,41]. The small have sizes in the sub-micrometer or nanometer range [40,41]. To understand the coating formation, sintered particlesthe (~1 µm) are like the melted ones, but deposited before melting occurs, as shown deposits obtained from a single torch scan, as shown in Figure 9, can be studied [41]. Figure 9aThese shows deposited or partially melted spherical particles (~0.3 µm informing diameter), agglomerates as in Figure 9b [33,35,37,41]. smallmelted particles could have stuck together which resulted from the evaporation of solvent at the surface of the small droplets [33,35,41]. The it reaches the substrate, as shown inµm) Figure Suchones, particles are before the major deposit small sintered particles (~1 are like 9c. the melted but deposited melting occurs, as elements of the shown inwhich Figure 9b [33,35,37,41]. These small particles could in have together[41]. forming coating microstructure contributes to the small pores thestuck coating The large pores in agglomerates as it reaches the substrate, as shown in Figure 9c. Such particles are the major deposit coatings due to the stacking of irregularly shaped agglomerates, such as dome-like elements of the coating microstructure which contributes to the small pores in the coating [41]. Theones, 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 LSM collected on the at the distance 60 mm from thedeposits plasma torch exit in the single scan substrate experiments. Reproduced with permission from(a,b) [41], and 70 mm (c–f) Elsevier, 2013. from the plasma torch exit in the single scan experiments. Reproduced with permission [41], Figure 9. Typical LSM deposits collected on the substrate at the distance 60 mm (a,b) and 70from mm (c–f) Elsevier, from the 2013. plasma torch exit in the single scan experiments. Reproduced with permission from [41],

Elsevier, 2013.

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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 could have beenthe generated by larger solution droplets which undergo in situ pyrolization after SEM images SEM images at at the (a) (a) surface surface and and (b) (b) cross-section cross-section of of HA HA coating coating from from low low concentration concentration impacting the substrate’s surface [41].

Figure Figure 10. 10. solution Reproduced with with permission permission from from [39], [39], Elsevier, Elsevier, 2017. 2017. solution showing showing aa dome-like dome-like microstructure. microstructure. Reproduced 3. Microstructure of Cermet Coatings

Microstructure an important factor depicted determining the properties of solution On the other hand, the wetis agglomerates in final Figure 9d are someplasma droplets entrained in The microstructure results depicted from impact ofin theFigure different particles the substrate On the othersprayed hand,coatings. the wet agglomerates 9d areonsome droplets entrained in the cold region oforthe plasma jet which heating only partial pyrolization before previously deposited coating, experienced namely: (i) melted less particles; (ii) smalland sintered particles; (iii) dry the cold region of the plasma jet agglomerates; which experienced less heating and only partial pyrolization before (iv) in wetformation and, wet droplets, as described in many [17,22,40]. impact. They mayagglomerates; contribute of a(v)sponge-like structure, aspapers reported by Wang et al. [24]. Unlike in conventional plasma spraying (APS), where largestructure, splats are formed, SPPS-prepared impact. They may contribute in formation of a sponge-like as reported by Wang et al. [24]. Figure 9e also shows the wetmany agglomerates formed with amountNi-YSZ of solvent. type of deposit coatings exhibit different microstructural features. Theless APS-deposited (see FigureThis 11) Figure 9e also shows the wet agglomerates formed with less amount of solvent. This coating cross-section shows a porous lamellar microstructure with voids and unmelted type of deposit is is formed throughcermet continuous evaporation of the solvent when vapors escaped by fine pores between particles [42]. 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 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 Figure 9f. 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. 3. Microstructure Microstructure of of Cermet Cermet Coatings Coatings Microstructure important factor determining the properties final properties of solution plasma Microstructure isisananimportant factor determining the final of solution plasma sprayed sprayed coatings. The microstructure results from impact of the different particles on the substrate coatings. The microstructure results from impact of the different particles on the substrate or previously or previously deposited coating, namely: (i) melted particles; (ii) small sintered particles; (iii) dry deposited coating, namely: (i) melted particles; (ii) small sintered particles; (iii) dry agglomerates; agglomerates; (iv) wet agglomerates; and, (v) wet droplets, as described in many papers [17,22,40]. (iv) wet agglomerates; and, (v) wet droplets, as described in many papers [17,22,40]. Unlike formed, SPPS-prepared SPPS-prepared Unlike in in conventional conventional plasma plasma spraying spraying (APS), (APS), where where large large splats splats are are formed, coatings coatings exhibit exhibit many many different different microstructural microstructural features. features. The The APS-deposited APS-deposited Ni-YSZ Ni-YSZ (see (see Figure Figure 11) 11) cermet coating cross-section shows a porous lamellar microstructure with voids and unmelted cermet coating cross-section shows a porous lamellar microstructure with voids and unmelted particles particles [42]. [42].

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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 Figure 11. SEM cross-sectional microstructure of Ni-YSZ by APS. Reproduced with permission from [42], Elsevier, 2008. from [42], Elsevier, 2008. [42], Elsevier, 2008.

On the contrary, uniformly dense microstructures with fine-scale porosity characterize the SPPSOn the contrary, uniformly dense microstructures porosity the On the contrary, uniformly dense withwith fine-scale porosity characterize deposited Ni-YSZ cermet coatings asmicrostructures reported by Michaux et fine-scale al. [34]. Wang andcharacterize Coyle the [23]SPPSalso SPPS-deposited Ni-YSZ cermet coatings as reported by Michaux et al. [34]. Wang and Coyle [23] also deposited Ni-YSZ cermet coatings as reported by Michaux et al. [34]. Wang and Coyle [23] also reported that Ni-YSZ cermet coatings containing spherical YSZ particles of about 0.5 µm in diameter reported that Ni-YSZ cermet coatings YSZ particles of 0.5 µm in diameter reported that Ni-YSZby cermet coatings containing containing spherical YSZ particles of about aboutparticles 0.5 µm in diameter were held together a continuous Ni matrix. spherical The strong bonding between due to the were held together by a continuous Ni matrix. The strong bonding between particles due the were held together by a continuous Ni matrix. The strong bonding between particles due tothere the continuous Ni matrix could improve the mechanical properties of the coating. Consequently,to continuous Ni matrix could improve the mechanical properties of the coating. Consequently, there are continuous Ni matrix improve theofmechanical properties of theof coating. there are no studies yet on could the investigation the mechanical properties cermetConsequently, coatings deposited no studies yet on the investigation of the mechanical properties of cermet coatings deposited using the are nothe studies on the investigation of the mechanical properties of cermet coatings deposited using SPPS yet process. SPPS process. usingCandidato the SPPS process. et al. [13], reported that microstructures of both HA and Zn-doped HA coatings Candidato et al. reported that both Zn-doped coatings Candidato al. [13], [13], reportedshowed that microstructures microstructures of both HA HA and and Zn-doped HA coatings deposited underet similar conditions the presence ofof agglomerated fine spherical HA particles and deposited under similar conditions showed the presence of agglomerated fine spherical particles and melted particles at the surface, which are characteristics of SPPS coatings. However, the cross-section melted particles at thecermet surface, whichseems are characteristics of SPPS coatings. However,HA the (Figure cross-section of the Zn-doped HA coating to be denser compared to the un-doped 12). of the Zn-doped HA cermet coating seems to be denser compared to the un-doped HA (Figure coating seems to be denser compared to the un-doped HA (Figure 12). 12).

2µm

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2µm

50µm

2µm

50µm

2µm

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Figure 12. SEM images at the (left) surface and (right) cross-section of the coatings of (0%-Zn) unFigure HA 12. SEM images at the (left)HA. surface and (right) cross-section of the[13], coatings of (0%-Zn) undoped and images (5%-Zn) Reproduced permission from 2018. Figure 12. SEM atZn-doped the (left) surface and (right) with cross-section of the coatings Elsevier, of (0%-Zn) 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 Nishida (2009)of reported addition of nickel particles intointo samaria-doped ceriaAdditionally, modifies the microstructure coating that fromthe including only spherical particles the one with ceria modifies the of coating from spherical particles into with nano-spheres withmicrostructure embedded nanoparticles toincluding be used only in SOFC applications [43].theAone porous nano-spheres with embedded nanoparticles to be used in SOFC applications [43]. A porous microstructure with porosity in the form of interconnected networks of voids was observed 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 microstructure porosity in the form of interconnected networks of voids was observed Metals 2018,with 8, x FOR PEER REVIEW 12 offor 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 architecture is a result from the decomposition of the partially pyrolyzed precursor which is Metals 2018, 8, x FOR PEER REVIEW 12 of 19 evaporation of the solventof[44]. Figure 13solvent displays surface morphology of the LSM submitted to remaining in situ evaporation the remaining [44].the Figure 13 displays the surface cermet morphology coating deposited using (Figure water and (Figure 13b) ethanol as solvents. The aqueous of theproduced LSM cermet coating deposited using (Figure 13a) water andvoid (Figure 13b)coating’s ethanol cermet coatings from an 13a) aqueous solution precursor [29]. The in the as solvents. The SPPS coating shows a porous sponge-like morphology with agglomerated SPPS coating shows porous sponge-like morphology with agglomerated (~0.5which µm) particles. architecture is aaaqueous result from the decomposition of the partially pyrolyzed solid precursor is (~0.5 µm) particles. On thecoating other ethanol SPPS coating shows similar porous morphology submitted to ethanol in situ evaporation of hand, the remaining solvent [44]. Figure 13 displays the surface solid On the solid other hand, SPPS shows similar porous morphology and agglomerated and agglomerated solid but with the of which particle size (~0.2 attributed morphology of the LSMparticles, cermetofcoating deposited using (Figure 13a)attributed water andµm) (Figure 13b) ethanol particles, but with the refinement particle sizerefinement (~0.2 µm) thewhich increase of the triple the increase The of the triple SPPS phasecoating boundary length between the YSZmorphology and LSM cathode, decreasing as solvents. aqueous shows a porous sponge-like with agglomerated phase boundary length between the YSZ and LSM cathode, decreasing cathode polarization. cathode polarization. solid (~0.5 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 morphology of [24], an LSM cermet cathode deposited by with (a) aqueous with permission from Elsevier, 2011) and (b) ethanol (reproduced permissionsolution (reproduced with permission from [24], Elsevier, 2011) and (b) ethanol (reproduced with permission from [29], Elsevier, 2012) SPPS. from [29], Elsevier, 2012) SPPS. Figure 13. Surface morphology of an LSM cermet cathode deposited by (a) aqueous solution

As discussed in permission the previous the chemistry ofethanol the initial precursors influences the (reproduced with fromsections, [24], Elsevier, 2011) and (b) (reproduced with permission microstructure of the final cermet coatings, as depicted in Figure 14, showing the SEM images of from [29], Elsevier, 2012) SPPS. Asnickel-based discussed in the previous the chemistry of the initialceria precursors the cermets (a) Ni-YSZsections, and (b) Ni-SDC. Nickel-samaria-doped (Ni-SDC) influences coating microstructure of the final cermet coatings, as depicted in Figure 14, showing the SEM images of displays a microstructure composed of very spherical particles having micrometer As discussed in the previous sections, the fine chemistry of the initial precursors influencesand the nanometer sizes(a) to nickel-yttria-stabilized zirconia. The morphology nickel nickel-based cermets Ni-YSZ and (b)coatings, Ni-SDC.asNickel-samaria-doped ceria of (Ni-SDC) coatingin microstructure ofsimilar the final cermet depicted in Figure 14, showing the SEMparticles images ofdisplays SPPS was already reported to be very suggests that its combination with samaria-doped nickel-based cermets (a)ofNi-YSZ andfine, (b) which Ni-SDC. Nickel-samaria-doped ceria (Ni-SDC) coating sizes a microstructure composed very fine spherical particles having micrometer and nanometer ceria (SDC) not promotecomposed the growthofacceleration the particles [26]. having micrometer and displays a does microstructure very fine of spherical particles

similar to nickel-yttria-stabilized zirconia. The morphology of nickel particles in SPPS was already sizes similar to nickel-yttria-stabilized zirconia. The morphology of nickel particles in reportednanometer to be very fine, which suggests that its combination with samaria-doped ceria (SDC) does not SPPS was already reported to be very fine, which suggests that its combination with samaria-doped promoteceria the(SDC) growth acceleration of the particles [26]. does not promote the growth acceleration 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.

Due to 14. the Scanning complex electron in-flightmicrographs phenomenaofand unique morphologies formed during Figure (a) the Ni-YSZ andparticle (b) Ni-SDC SPPS cermet coatings.

Figure 14. Scanning electron micrographs of (a) Ni-YSZ and (b) Ni-SDC SPPS cermet coatings. the process, some interesting coatings’ Reproduced with permission from [26],microstructures Elsevier, 2012. and architectures can be generated in SPPS dependingwith on the set of parameters usedElsevier, [26,33,34,42,45–47]. Lay et al. [45] reported on the deposition Reproduced permission from [26], 2012. of Ni-SDC differentin-flight spray parameters SP1, whichmorphologies has higher velocity, Due tousing the complex phenomena(SP), and namely, the unique particle formedenergy, 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,

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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 depending the PEER set ofREVIEW parameters used [26,33,34,42,45–47]. Lay et al. [45] reported on the deposition Metals 2018, 8,on x FOR 13 of 19 of Ni-SDC using different spray parameters (SP), namely, SP1, which has higher velocity, energy, and solution precursor flow flow rate compared to SP2. The latter has ahas lower solution concentration. Ni-SDC and solution precursor rate compared to SP2. The latter a lower solution concentration. Niusing SP1 displays poorly-adhered finer particles with greater to SP2 (seetoFigure 15). SDC using SP1 displays poorly-adhered finer particles withporosity greater compared porosity compared SP2 (see The surface morphology of Ni-SDC, obtained using SP1, shows no splat-shaped microstructural Figure 15). The surface morphology of Ni-SDC, obtained using SP1, shows no splat-shaped features which indicates that partial pyrolization droplets takes place in theplace plasma jet plasma due to microstructural features which indicates that partialof pyrolization of droplets takes in the low solution concentration and higher plasma energy than SP2 withthan highSP2 solution jet due to low solution concentration and higher plasma energy with concentration. high solution Thus, evaporation of evaporation most elements in theelements plasma, in and nucleation at the surface of concentration. Thus, of most thesubsequent plasma, and subsequent nucleation at the substrate, highly possible leading to the deposition of less-adhering surface of is the substrate, is highly possible leading to the deposition 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 Moreover, the injection solutionThe precursor into the plasma jetamay influence the microstructure microstructure of the cermetofcoatings. droplets obtained using mechanical injector had initially of the cermet coatings. The droplets obtained using a mechanical injector had initially large large droplets which could have undergone secondary break-up to form finer droplets as discussed 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 in Section 2.2 [17,22,23,32]. Consequently, the resulting microstructure coatingswith sprayed using mechanically-injected solution shows a porous, agglomerated, dome-like of structure resolidified mechanically-injected shows a porous, agglomerated, dome-like structure with resolidified particles at the surfacesolution [23]. The injection by atomization produces about 2–100 micron droplets, particles at the surface [23]. The injection by atomization produces about 2–100 micron which may undergo rapid evaporation. Consequently, the complete pyrolysis of the solute droplets, particles which may at undergo rapid evaporation. Consequently, the complete pyrolysis of the solute particles may occur the interaction with the plasma jet [17,33,34]. The injection by atomization leads to the may occur at the interaction with the plasma jet [17,33,34]. The injection by atomization leads to formation of irregular tower-like agglomerates of very fine particles with smooth surface deposits,the as formation of irregular tower-like agglomerates of very fine particles with smooth surface deposits, reported elsewhere [34]. as reported [34]. On theelsewhere other hand, the long interaction time between the solution precursor droplets and the On the other hand, the long interaction time the solution precursor Michaux droplets etand plasma jet during the trajectory induces the effects of between sintering and of local re-melting. al. the plasma jet during the trajectory induces the effects of sintering and of local re-melting. [34] reported porous architectures with voids for external/radially delivered Ni-YSZ cermet due to Michaux et al. [34] reported porous architectures with voids Ni-YSZ stacking of partially-molten splats forming cavities [34]. On for the external/radially other hand, densedelivered uniform-layered cermet due to stacking of partially-molten splats forming cavities [34]. On the other hand, coating with fine-scale porosity were observed for the solution precursor introduced within thedense torch uniform-layered coating with the solution convergence tube, which is fine-scale attributedporosity to the were fasterobserved heating for of the droplet precursor leading tointroduced complete within the(see torch convergence whichthe is attributed to theoffaster heating of thecomposite droplet leading to pyrolysis Figure 16) [31]. tube, Therefore, microstructure the ceramic-metal coatings complete pyrolysis (see Figure 16) [31]. Therefore, the microstructure of the ceramic-metal composite can be influenced by the way of injection depending on the foreseen application. coatings can be influenced by the way of injection depending on the foreseen application.

[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 pyrolysis (see Metals 2018, 8, 420Figure 16) [31]. Therefore, the microstructure of the ceramic-metal composite coatings 14 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 Section 3 [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

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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 coatings taken into account in order to have niche applications. Metals 2018,has 8, xbeen FOR PEER REVIEW 15 of 19

Figure 17. 17. Methylene photo de-coloration de-coloration efficiency efficiency versus versus ZnFe ZnFe2O 4 film thickness for films Figure Methylene blue blue photo 2 O4 film thickness for films deposited at = 8.= The horizontal line represents the efficiency of powder 2O4 deposited atprecursor precursorpH pH 8. dotted The dotted horizontal line represents the efficiency of ZnFe powder photocatalyst synthesized by a solid state reaction. Reproduced with permission from [10], Elsevier, ZnFe2 O4 photocatalyst synthesized by a solid state reaction. Reproduced with permission from [10], 2012. Elsevier, 2012.

4.3. Biomedical Coatings 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 review article. R.C.J., andconceptualized L.P. conceptualized and Author thisthis review article. R.C.J., R.U. R.U. and L.P. the flowthe andflow contents contents of thisR.U. review. R.U.the gathered the literatures necessary for literatures forand the wrote articlethe andbody wrote body ofpaper. the review of this review. gathered necessary the article of 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 TechnologyAccelerated 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.

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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 TechnologyAccelerated 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 Lv Ω η cp σs Tv υ2 solution υ2 plasma µs

Density of the fluid Latent heat of vaporization, J·kg−1 Ohm Photo-degradation efficiency, % Specific heat, J·kg−1 ·K−1 Surface tension, J/m2 Vaporization temperature, K Velocity of the solution Velocity of the plasma gas Viscosity, Pa·s

Abbreviations APS CZP DTA-TGA FDA HA LSM Ni-SDC Ni-YSZ SOFC SP SPPS SPS TCP TPB Zn-HA

Atmospheric plasma spray Calcium zinc phosphate Thermogravimetric-differential thermal analyzer Food and Drugs Administration Hydroxyapatite Strontium-doped lanthanum manganite Nickel samaria-doped ceria Nickel Yttria stabilized zirconia Solid oxide fuel cells Spray parameters Solution precursor plasma spray Suspension plasma spray Tri-calcium phosphate Triple phase boundary Zinc-doped Hydroxyapatite

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