Plasma spraying with liquid feedstock offers an exciting opportunity to obtaincoatings with characteristics that are vastly different from those produced usingconventional spray-grade powders. The two extensively investigated variants of
this technique are suspension plasma spraying (SPS), which utilizes a suspensionof fine powders in an appropriate medium, and solution precursor plasmaspraying (SPPS), which involves use of a suitable solution precursor that canform the desired particles in situ. The advent of axial injection plasma spraysystems in recent times has also eliminated concerns regarding low depositionrates/efficiencies associated with liquid feedstock. The 10–100 μm size particlesthat constitute conventional spray powders lead to individual splats that are morethan an order of magnitude larger compared to those resulting from the fine(approximately 100 nm–2 μm in size) particles already present in suspensionsin SPS or formed in situ in SPPS. The distinct characteristics of the resultingcoatings are directly attributable to the above very dissimilar splats (“buildingblocks” for coatings) responsible for their formation. This chapter discusses thesalient features associated with SPS and SPPS processing, highlights their ver-satility for depositing a vast range of ceramic coatings with diverse functionalattributes, and discusses their utility, particularly for high-temperature applica-tions through some illustrative examples. A further extension of liquid feedstockplasma processing to enable use of hybrid powder-liquid combinations for plasmaspraying is also discussed. This presents a novel approach to explore new materialcombinations, create various function-dependent coating architectures withmulti-scale features, and enable convenient realization of layered, composite,and graded coatings as demonstrated through specific examples.
Thermal spraying has gained widespread industrial acceptance over the years fordepositing coatings to meet diverse functional requirements. The availability ofnumerous variants of the thermal spray technique, the diversity of available spraymaterials, and the continuing advances in spray equipment and associated controlsystems have together created new opportunities for various industrial stakeholders.The wide portfolio of thermal spray variants, such as flame spray, arc spray,atmospheric plasma spray (APS), high-velocity oxy-fuel (HVOF) spray, detonationspray, cold spray, etc., provides distinct gas temperature-gas velocity windows forparticle heat-up and acceleration, offering a range of quality and cost [1]. Theversatility of the thermal spray process is also unmatched by any other surfacemodification technology by virtue of an overwhelming choice of feedstock availablein the form of powders, wire, or rods [1, 2], enabling tailoring of surface propertiesfor any given coating application. Although the typically low deposition efficiencies(usually <60%) associated with thermal spraying have been a concern for long, the
2 S. Joshi et al.
recent availability of high-power, axial-feed plasma spray systems reflects theadvancements in thermal spray equipment aimed at addressing the above issue [3].
In recent times, there has been increasing interest in spraying nano- or submicron-sized powders to obtain refined microstructures that can translate into superiorproperties [4]. However, thermal spraying of fine-sized feedstock poses numerouschallenges, the most dominant being the difficulty in injecting nano- and submicron-sized particles into the core of a thermal spray plume/flame [5]. Powders comprisingfine particles do not possess good flowability, and their low momentum also seri-ously limits their ability to penetrate the high-velocity gas streams associated withmost popular thermal spray techniques, making it difficult to achieve controlledpowder feeding [6]. Spraying fine powder particles in a conventional manner canalso be an environmental issue and cause health problem for thermal spray operators.Although attempts to agglomerate the nano-/submicron-sized particles have beenmade, inhomogeneous melting characteristics of the agglomerated particles in theplume have often been noted [7, 8]. In view of the above, alternative approachesinvolving use of a liquid medium, either in the form of a suspension of fine particlesor as a solution precursor containing dissolved salts which can lead to particlegeneration in situ, have been conceived [3, 6, 9–12]. These suspension and solutionprecursor-based thermal spray processes constitute the central theme of this chapter.
Thermal Spraying Using Liquid Feedstock
As briefly mentioned above, liquid feedstock employed for thermal spraying can bebroadly classified into two types: (a) suspensions and (b) solution precursors. Theformer utilizes fine powders (comprising particles approximately 100 nm–2 μm insize) suspended in a suitable solvent, typically water or alcohol, to obviate theproblems associated with their feeding without the need for an additional agglom-eration step [4, 12, 13]. The injection of a suspension through a dedicated feedingdevice into a plasma/HVOF plume to deposit coatings has been the subject ofgrowing research in recent times. The process requires a stable suspension of thedesired powder, prepared in an aqueous or organic solvent. The mechanism leadingto coating formation is discussed in a subsequent section. It is pertinent to note thatplasma spraying has been the method of choice for a vast majority of suspension-based coating efforts, by virtue of the availability of a high-energy flux that is neededto eliminate the solvent [14]. Thus, this chapter will particularly focus on suspensionplasma spraying (SPS), rather than using suspensions with other thermal sprayvariants. The relatively recent development enabling axial injection of feedstock,and thereby permitting far more effective utilization of the energy available in theplasma plume, has been found to afford even greater versatility and is a potentialgame changer for liquid feedstock plasma spraying. Apart from eliminating concernsregarding low deposition rates/efficiencies associated with liquid feedstock, axialSPS has also been demonstrated to enable considerable microstructure controlthrough proper selection of processing conditions [15]. Relatively few but growingnumber of reports on suspension spraying employing HVOF torches (termed
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 3
s-HVOF) also represents additional possibilities of generating fine-structured coat-ings with interesting deposition characteristics using suspensions [16].
Although considerably more challenging, the utilization of solution precursorsrather than suspensions potentially stands to provide further added benefits. Forexample, the latter relies on pre-synthesis of nano- or submicron particles, which ismore expensive compared to conventional powder feedstock. Hence, a processinvolving single-step consolidation of in situ formed nanoparticles is more advan-tageous. Accordingly, solution precursor spraying is another variant of liquidfeedstock-based thermal spraying that is deemed interesting. In contrast to thegrowing academic and industrial interest in SPS, attention to use of solution pre-cursors as feedstock for thermal spraying has been rather subdued. This can beattributed to the even higher energy demands imposed by the solution precursorscompared to suspensions [12]. By and large, the higher energy demand results fromthe additional steps leading to pyrolysis and in situ powder formation that are neededwhen precursor salts are used [9, 12, 17, 18]. This correspondingly compounds thechallenges in achieving commercially interesting throughputs (e.g., coating thick-ness per pass) using conventional radial feed systems. Consequently, a majority ofprior efforts exploring the solution precursor approach have relied on plasmaspraying due to energy considerations [6, 12, 13, 19–28]. Thus, apart from SPS,solution precursor plasma spraying (SPPS) is the other area of focus in this chapter.In the context of SPPS, too, the newer axial plasma spray systems appear a muchmore promising alternative due to the improved thermal energy utilization that itenables.
In case of both SPS and SPPS, the process setup is nearly identical to thatemployed in a conventional APS process, wherein the powder feeder is replacedwith a liquid injection device comprising a pressurized precursor tank and anatomizer. A typical arrangement is schematically illustrated in Fig. 1 [9]. Althoughthe figure depicts radial injection of feedstock, it can also be readily adapted foraxial-feed plasma spray systems mentioned above. The fact that a suitable feeder todeliver the liquid feedstock is the only hardware that is additionally required apartfrom a routine APS setup makes induction of the SPS/SPPS into a regular plasmaspray line quite straightforward. Notwithstanding the fact that availability of signif-icantly higher thermal energy is conducive for coating formation using precursorsalts, some studies on HVOF spraying of solution precursors have also beenreported [29].
An extension of liquid feedstock thermal spraying, by combining it with conven-tional powder processing, has also been demonstrated to present a novel method fordepositing coatings with unusual microstructures. The above hybrid approachinvolves either sequential or simultaneous feeding of powder feedstock and asuspension/precursor solution with independent control of their feed rates, to con-veniently realize diverse coating architectures [30, 31]. Depending upon the strategyadopted for feeding powder and liquid feedstocks, layered (sequential feeding),composite (simultaneous feeding with constant feed rates), as well as functionallygraded coatings (simultaneous feeding with progressively varying relative feed
4 S. Joshi et al.
rates) can be achieved, opening up exciting avenues for realizing function-specificarchitectures [21].
The application potential of the above liquid feedstock and powder-liquid hybridfeedstock approaches is immense. Accordingly, the following sections discuss thesalient features of the SPS, SPPS, and hybrid techniques, highlighting their versa-tility for depositing a vast range of ceramic coatings with diverse functional attri-butes. Their utility, particularly for high-temperature applications, is also discussedthrough some illustrative examples.
Suspension Plasma Spraying
Among all the liquid feedstock thermal spray approaches, the SPS technique hasbeen the most widely investigated. Although most early efforts were motivated bythe desire to seek pathways to address issues associated with utilizing fine powderfeedstock (20 nm–5 μm) for coating deposition [6], the versatility of the approachand its inherent ability to yield unique microstructures was soon realized. Forexample, thermal barrier coatings (TBCs) produced by conventional APS havebeen widely used in the gas turbine industry for several decades, and, more recently,coatings deposited by electron beam physical vapor deposition (EBPVD) haveemerged as an alternative due to their inherent strain-tolerant columnar structure.
Fig. 1 Schematic illustration of a typical liquid feedstock delivery arrangement that can be utilizedfor SPS and SPPS [9]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 5
The SPS technique has been shown to be capable of cost-effectively yielding suchcolumnar microstructures. Thus, the attractive TBC applications have predominantlyfueled the widespread initial interest in SPS deposition of yttria-stabilized zirconia(YSZ), which has now gradually begun to be extended to other functional applica-tions also.
Processing
In SPS, fine powder particles (solute) are mixed with a liquid (solvent) to forma suspension, which is injected into the plasma plume as the feedstock material[32–34]. The most commonly used solvents are water, alcohol (typically ethanol), ortheir suitable mixtures. While both water and alcohol have distinct advantages andshortcomings, it is acknowledged that the type of solvent plays a key role indetermining the coating microstructure. The heat required to vaporize ethanol isone-third of that necessary for water, which leads to higher power being needed forspraying aqueous suspensions [35].
Consequently, the deposition efficiency is also typically higher in case of theformer with, for example, the efficiency being nearly doubled when switching from awater-based YSZ suspension to an ethanol-based YSZ suspension, all other param-eters being identical [36]. The solute is usually a ceramic powder, with particle sizeranging from nano-metric to a few microns. In practice, 25 wt.% solid loading(powder quantity) in the suspension is commonly used, although there is a constantdesire to increase the solids content in a suspension in an effort to enhance through-put. The solid loading is also occasionally dictated by the type of solute and solventas well as by the type of the microstructure being targeted, since it influences theviscosity and surface tension of the suspension which crucially govern SPS coatingformation.
An illustration of the various stages involved in coating formation in case of SPS,from the time the suspension is injected to the eventual deposition of thecorresponding coating on the substrate, is depicted in Fig. 2 [37]. Powders ofdifferent sizes/morphology used in the suspension, and their state just prior to impactwith the substrate, are also shown; these significantly influence the microstructure ofthe resulting coating. It is important to have a good understanding of each of therepresented stages in order to develop an ability to control the process and influencethe resulting coating microstructure.
The first stage corresponds to injection of the suspension. The suspension can beinjected into the plasma plume either radially or axially, as illustrated in Fig. 3[37]. In case of radial injection, the size and velocity of the suspension droplets atinjection play a particularly crucial role since the momentum of droplets cansignificantly influence their trajectories in-flight as they traverse through the plasmaplume. For example, very small droplets may not be able to penetrate the plasma atall and “bounce off,” whereas large droplets may completely pass through theplasma plume as illustrated in Fig. 3a, reaching the substrate in unmolten conditionin both cases due to their inability to fully utilize the energy available in the plasma
6 S. Joshi et al.
plume. Moreover, suspension droplets of varying size entering different zones of theplasma plume experience different dwell times (the total time a droplet spends in theplasma plume), and this leads to a variable heat-up and acceleration of the droplets.The net outcome is often a heterogeneous microstructure and/or overspray. Theabove problems are significantly mitigated when axial injection is resorted to, as thetrajectory of all particles is predominantly coaxial with the plasma plume andvariations in particle heat-up and acceleration are minimized. Additionally, intro-duction of the suspension in such a manner permits more intimate droplet-plasmacontact for effective utilization of the plasma energy, thereby also enabling higherthroughputs. Thus, the relatively recent advent of high-energy axial-feed plasmasystems is a game changer for energy-intensive liquid feedstock spraying.
Fig. 2 Schematic of various stages of a suspension droplet having varied constituent particle sizes,i.e., medium (I and II), very fine (III), and coarse (IV), in-flight during plasma spraying, frominjection of the suspension to the final deposition on the substrate [37]
Radial injection ofparticles/droplets
a bAxial injection ofparticles/droplets
Dispersion ofparticles/droplets
Plasma Jet Plasma Jet
Sp
ray
Gu
n
Sp
ray
Gu
n
Fig. 3 Schematic of radial injection (a) versus axial injection (b) of liquid feedstock [37]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 7
In the second stage, the injected suspension undergoes atomization to form finedroplets that follow the plasma stream. Atomization is driven by a balance betweentwo competing driving forces: while the drag from the plasma generates a shear forceon the droplet that tends to disintegrate the droplet further, the surface tension resistsits further breakup. Fazilleau et al. [38] modelled droplet breakup during SPSprocessing and derived a relation between the various suspension parameters andthe atomized suspension droplet size, as shown in Eq. 1 [39]:
D ¼ 8 σCD ρ u2
ð1Þ
In the above expression, D (m) is the droplet diameter following atomization,σ (N/m) is the surface tension of the suspension, CD is the dimensionless dragcoefficient, ρ (kg/m3) is the density, and u (m/s) is the plasma stream velocity.From the above equation, it can be seen that formation of a smaller droplet isfacilitated by a lower surface tension and a higher plasma drag force, which dependson plasma density and velocity. The viscosity of the suspension, which is notdiscussed in the above equation, also influences atomization with a lower viscosityfavoring greater atomization of the suspension [35].
After the suspension droplets are formed, they are rapidly heated up by theplasma stream having temperatures in excess of 15,000 K in the core, resulting inevaporation of the solvent. The vaporization step consumes a significant amount ofthermal energy from the plasma. Since a “colder” plasma may result in insufficientmelting of the remnant solid particles following vaporization of the solvent, morepowerful plasma guns capable of providing higher thermal energy are needed forsuspension plasma spraying as compared to conventional powder spraying. Thedroplets are also concurrently accelerated during the vaporization stage. After thesolvent evaporates, the fine remaining particles also tend to undergo agglomeration.The extent of agglomeration is governed by the size of the particles as has beenattempted to be illustrated in Fig. 2. For example, in case IV where there are coarsepowder particles constituting the original suspension, the agglomeration stage is notvalid; however, when the constituent powder particle size is extremely fine as in caseIII, the agglomeration can be notable. The particles may also often be sintered orfused into larger particles, and all of the above can potentially alter the initialcharacteristics (such as mean particle size) of the solute particles originallysuspended in the solvent.
Complete vaporization of the solvent brings the fine solid powder particles,agglomerated or otherwise, in direct contact with the plasma whereupon they areexpected to undergo melting. The extent of melting of the particles is governed bythe particle size as well as by the operating parameters such as power employedduring the process, composition of plasma gases (and thereby enthalpy), dwell time,and feed rate of the suspension [39–41].
In the final stages of coating formation, the molten or semi-molten particles areaccelerated toward the substrate where they form fine “splats” on impact. Eachincoming molten particle solidifies on a previously formed splat leading to coating
8 S. Joshi et al.
deposition. Overlapping of the splats as a consequence of relative gun-substratetraverse leads to desired area coverage, while stacking up of the solidified splatsleads to the desired coating thickness These “splats” are essentially the buildingblocks for coating formation, and their shape, as well as other microstructuralfeatures in the coating, depends greatly on the extent of melting of the particlesdictated by the spraying conditions and suspension characteristics. The abovecoating buildup during the SPS process is quite distinct from that in conventionalpowder-derived APS coatings and is intimately related to the generation of suspen-sion droplets and their in-flight history prior to impact. The trajectory of the fineparticles constituting the suspensions, predominantly smaller than 5 μm, can beconsiderably affected by the plasma stream in the boundary layer close to thesubstrate. This is because of the low momentum of these smaller particles, whichcan be influenced by the drag of the plasma stream in the boundary layer close to thesubstrate. Since the particle size in SPS is dimensionally of a similar order and evenfiner than the surface asperities, particle deposition can take place at shallow angleson surface asperities due to the above, leading to shadowing effect [42] as depictedschematically in Fig. 4 [37].
The influence of low particle momentum on their trajectory can be more clearlyseen in Fig. 5 which was adapted [37] from Berghaus et al. [42], who had previouslysimulated the influence of particle velocity and size on the deviation of the particletrajectory from the central axis of the plasma plume. Bigger droplets, with highermomentum, do not follow the plasma stream and maintain a perpendicular trajectorytoward the substrate. This may result in a lamellar coating structure, similar to that ina conventional APS process. In such a lamellar structure, the presence of microcracks can also result in formation of vertical cracks as the splats overlap duringcoating growth. Vassen et al. [34], however, have shown that driving force forvertical crack growth in SPS coating is the higher tensile stresses present in thecoating. The increased tensile stresses in the SPS coating than in APS are due to thepresence of fewer micro cracks, which have the ability to accommodate stresses, inthe former.
Asperity
SUBSTRATE
VNormal = 0
VParallel = 0
Plasma flowFig. 4 Schematic showingthe shadowing effect due toshallow angle deposition offine particles on the substrateasperities during SPS [37]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 9
Microstructure
Due to the much smaller particle size used in SPS than in conventional dry powderspraying, i.e., nano-metric or sub-micrometric, compared to tens of microns, thecoating formation mechanism in SPS is complex. However, the considerable influ-ence of particle momentum on its trajectory in SPS as discussed above, combinedwith the comparable magnitude of particle size and surface asperities, provides thepossibility of tailoring different types of microstructures. This, in turn, leads todifferent properties of the coatings. As the very fine particles follow the trajectoryof the plasma stream, in close proximity to the substrate, they travel in a nearlytangential direction to the substrate and impact the substrate’s asperities at shallowangles. When they “stick” on the asperities at such shallow angles as depicted inFig. 4, both lateral and vertical growths are enabled during subsequent successiveimpact of the molten particles, which leads to formation of columns and their growthduring the entire spraying process. A typical columnar microstructure of a SPS TBCis presented in Fig. 6 [43].
Based on the above concepts, VanEvery et al. [44] have proposed three distincttypes of coating microstructures in SPS. If the droplet size after the fragmentation isextremely small (<1 μm), the shadowing effect is larger, which can generate acolumnar-type structure. Droplets having a relatively higher momentum (>1 μm)but still small enough to be affected by the plasma flow (<5 μm) can form a structurewith some porosity bands within the columns, which can be termed as a featherycolumnar structure. Evolution of such columns is illustrated in Fig. 7 [37] where itcan be clearly seen how columns build up after 1, 2, 3, and 60 layers deposition(passes) on a bond coat asperity due to the shadowing effect. After only 2–3 passes,
0.10
50
100
Vel
ocity
nor
mal
to s
ubst
rate
(m
/s)
150
200
250
300
350
400
0.2 0.3 0.4 0.5 0.6
Distance above substrate (cm)
Droplet/ZrO2 particlesPlasma gas
Plasma gun
0.7 0.8 0.9 1.0
2 µm
3 µm
5 µm
40 µm
1 µm
Fig. 5 Schematic showing the effect of plasma flow on trajectory of droplets of varying momentumprior to final impact with the substrate [37]
10 S. Joshi et al.
it can be clearly seen that columns and intercolumnar gaps start developing contourson the substrate’s rough surface. Further larger droplets (>5 μm) have an even highermomentum and impact the substrate almost perpendicularly. This can result in adense coating (with no columnar structure) or a vertically cracked coating. Theabove three types of coatings are exemplified in Fig. 8 which illustrates the micro-structural tailoring that is possible in case SPS [45]. Depending on a specificcombination of numerous factors such as feedstock material, spray technique,spray parameters, etc., any of a wide range of structures similar to these can beobtained.
Apart from the typical microstructural features present in standard APS TBCs, theSPS coatings are characterized by some additional ones such as spacing betweencolumns (intercolumnar spacing) or very fine (sub-micrometric or nano-metric)pores, which may be either interconnected or isolated. All these different featuresare marked in Figs. 9 and 10, which show a typical cross section and top view,respectively, of a SPS TBC.
Versatility: Materials and Functionality
All microstructural features shown in Figs. 8, 9, and 10 can significantly affect boththe thermally insulating nature as well as the lifetime of the SPS TBCs. Verticalcracks or intercolumnar spacing are through the thickness of the TBC, and theyconfer strain tolerance to coatings [46, 47]. Branching cracks and inter-pass porositybands are perpendicular to the direction of heat flow within the coating and, hence,can act as significant thermal barriers within the coating [46, 48]. Other fine featuressuch as cracks and pores also help in decreasing the overall thermal conductivity ofthe coating.
Fig. 6 Typical columnar microstructure of a SPS TBC [43]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 11
Fig.7
Exp
erim
entaldemon
stratio
nof
columnbu
ildup
pass-by-pass
onan
as-sprayed
bond
coat[37]
12 S. Joshi et al.
Since the microstructure of an SPS TBC can vary significantly as presented inFig. 8 and also plays a central role in determining the functional performance of thecoatings, it is important to have a good understanding of factors governing coatingformation. For example, Ganvir et al. have revealed the close correlation betweenspray conditions and the in-flight characteristics of the particles which, in turn,influence coating formation and the resulting microstructures [49]. Using an axialsuspension feeding capable plasma spray gun (Mettech III) and a feedstockconsisting of ethanol and 25 wt.% 8YSZ powder (D50 ¼ 492 nm), five differentsets of process parameters (Table 1) were employed to produce top coats on identicalsubstrate specimens [49]. As can be observed in Fig. 11a, b, the total porosity in thetop coat as well as the content of both fine pores (<1 μm2) and coarse pores(>1 μm2) can vary significantly with spray conditions [49]. Microstructural featurescan also undergo changes when the coatings are exposed to cyclic thermal loads, asreflected in considerable variation in the coarse-fine porosity distribution inas-sprayed and heat-treated conditions (Fig. 11). Upon long time exposure toelevated temperatures, sintering can occur in coatings. A commonly observed effectof sintering on the exposed coatings is that the smaller microstructural features (fine
Fig. 8 SEM micrographs of cross sections of SPS-derived coatings, showing the variety ofmicrostructures of TBCs which can be produced by suspension plasma spraying. (Verticallycracked (a), highly porous (b), and columnar (c)) [45]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 13
pores and cracks) tend to coalesce depending on their size and shape, as well as uponthe time and temperature of exposure. Healing of the small pores/cracks leads toreduction in fine porosity content after heat treatment (Fig. 11b) but is also accom-panied by a volume contraction of the columns that results in increasing theintercolumnar spacing and globally to increasing coarse porosity content(Fig. 11a). This mechanism seems to be valid for most of the investigated coatingspresented in Fig. 11, excepting sample Exp4 that shows an inverse trend. As theauthors of the study observed, this particular sample had a relatively high totalporosity content with a mean pore size higher than in the rest of the coatings.Thus, the sintering effect manifested more in the form of partial healing of the fineand coarse pores within the columns, while the intercolumnar spacing did not change
Fig. 9 SEM micrographs of a cross section of a SPS TBC showing various characteristicmicrostructural features
14 S. Joshi et al.
significantly. Thus, both the fine and coarse porosities were found to decrease uponhigh-temperature exposure.
The thermal properties of the coatings are also affected by the above microstruc-tural variations in terms of pore volume content as well as size and shape of pores.
Fig. 10 SEM micrographs of a top view of a SPS TBCs showing various characteristic micro-structural features
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 15
Figure 12 presents the thermal conductivity of the samples presented in Fig. 11,measured by laser flash techniques [49]. As expected, the lower thermal conductivityvalues are observed in coatings with higher porosity. It was also found that thecoatings respond differently to heat treatment, in terms of variations in thermalconductivity (Fig. 12). No specific trend in thermal conductivity with heat treatment
Table 1 Process conditions employed for depositing coatings presented in Figs. 10 and 11 [49]
Process parameter
Specimen nomenclature
Exp1 Exp2 Exp3 Exp4 Exp5
Spray distance (mm) 75 50 100 100 100
Surface speed (cm/s) 145.5 75 75 216 216
Suspension feed rate (mL/min) 70 45 45 100 45
Total gas flow rate (L/min) 250 200 300 300 200
Total power during spray (kW) 125 101 124 124 116
Total enthalpy during spray (kJ) 13 11.2 12.5 12.5 11.2
Exp-1 Exp-2 Exp-3 Exp-4 Exp-5
Exp-1 Exp-2 Exp-3 Exp-4 Exp-5
a
b
20
15
10
5
0
20
25
30
15
10
5
0
Coa
rse
poro
sity
(%
)F
ine
poro
sity
(%
)
Before Heat Treatment After Heat Treatment
Before Heat Treatment After Heat Treatment
Fig. 11 (a) Coarse porosity and (b) fine porosity values before and after isothermal heat treatment(argon at 1150 �C for 200 h) determined by image analysis technique [49]
16 S. Joshi et al.
was discerned: the heat treatment was found to either increase or decrease thethermal conductivity, and, in some samples, it even remained nearly unchanged.Among the samples with significant variation in thermal conductivity after heattreatment was Exp3. Although the exact reason for the significant reduction ofthermal conductivity in this particular case is not well understood, the authors ofthe study [49] have explained this based on a combination of three types ofmicrostructural changes observed after heat treatment, namely, sintering, pore coars-ening, and crystallite size growth. It is known that the scattering interfaces in TBCsaffect the thermal conductivity of the coatings due to phonon scattering. Sinteringdecreases the porosity by healing the pores/cracks (reducing the scattering inter-faces) leading to a thermal conductivity increase. Pore coarsening, on the other hand,increases the overall porosity, thus causing thermal conductivity decrease. Growth incrystallite size decreases the grain boundary interfaces and can increase the thermalconductivity, similar to sintering.
As in case of conventional powder-derived TBCs, 8 wt.% partial stabilizedzirconia (8YSZ) is the most widely used material for suspension plasma sprayingof TBCs. With its continuous deployment in the gas turbine industry for more thanthree decades, 8YSZ has been one of the materials with highest longevity in thermalspraying [50]. A unique combination of low thermal conductivity, high thermalexpansion, and good mechanical properties make 8YSZ the material of choice fordemanding high-temperature insulating applications. The main drawback of 8YSZ,however, is that its working temperature is limited to 1200 �C. Beyond this temper-ature, apart from significant sintering, irreversible phase transformations occurwhich can contribute to premature coating failure. With increasing demand forimproved efficiency of gas turbine engines, higher inlet temperature of the engineis one of the envisaged approaches. Therefore, intensive work has been dedicated toexploring new TBC materials capable of performing at higher operating
Exp-1 Exp-2 Exp-3 Exp-4 Exp-5
1.5
0
0.3
0.6
0.9
1.2
The
rmal
Con
duct
ivity
(W
/(m
*K))
Before Heat Treatment After Heat Treatment
Fig. 12 Thermal conductivity of various SPS TBCs before and after heat treatment as determinedby laser flash technique [49]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 17
temperatures without compromising important requirements such as sintering resis-tance, phase stability, thermal conductivity, oxidation resistance, and resistance toCMAS -(calcium-magnesium-aluminosilicate).
In response to the above, new ceramic materials such as pyrochlores, perovskites,hexaaluminates, etc. have been proposed as alternatives to YSZ for high-temperatureapplications [51–53]. As far as powder plasma spraying is concerned, significantimprovement in the functional performance (thermal conductivity, thermal cycliclife) of the TBCs has been reported using perovskite, pyrochlore, and garnetcompositions compared to YSZ [54–57]. However, these new TBC materials haveso far been mostly studied as coatings deposited by APS or EB-PVD, and relativelyfew SPS efforts focusing on these materials have been reported. In one such study,the family of pyrochlores of rare earth zirconates has been shown to be verypromising as they offer the lowest thermal conductivity over many other ceramicmaterials including YSZ as well as a high thermal expansion coefficient (CTE)almost similar to YSZ [58]. Two of the most studied pyrochlores are gadoliniumzirconate (Gd2Zr2O7, henceforth referred to as GZ) and lanthanum zirconate(La2Zr2O7, henceforth referred to as LZ). The main drawback of LZ is the difficultyin preserving the desired stoichiometry of the coating material due to the propensityof La2O3 to evaporate during spraying [59–61]. On the other hand, GZ shows goodstability at temperatures over 1200 �C [62], besides exhibiting very good resistanceto sintering and CMAS attack [63, 64]. Yet, a significant limitation of GZ is its lowerfracture toughness compared to YSZ [65, 66], which seriously restricts its resistanceto thermal shock. It also has a tendency to react with the alumina from the thermallygrown oxide (TGO) layer that develops at the top coat/bond coat interface, thuslimiting the capacity of the TBC to withstand long thermal exposure [53].
A possible solution to overcome the abovementioned limitations while preservingthe outstanding high-temperature properties of GZ is to spray it as a multilayeredcoating system [67, 68]. The thermal properties and functional performance ofdouble-layered GZ/YSZ and triple-layered GZ/GZ/YSZ systems have been com-pared to standard single layer YSZ by Mahade et al. [39, 62, 69–72]. The reason foradding the third GZ layer was to improve the erosion resistance and CMASpenetration by adding a dense and thin (approx. 30 μm) layer on the top. Themicrostructural SEM images of the above double- and triple-layered GZ-YSZcoating systems, along with a conventional YSZ-based TBC, sprayed on identicalsubstrates and bond coats, are shown in Fig. 13 [71].
Thermal conductivity measurements by laser flash technique have revealedsuperior properties of GZ-based top coats as compared to a YSZ TBC. Thermalconductivity of the GZ TBCs was reported to be lower at both room temperature andover the entire temperature range of interest between RT and 1000 �C [62, 69]. TheGZ-based multilayered TBCs also exhibited superior performance during lifetimeassessment tests, i.e., thermal cyclic fatigue at 1100 �C and 1200 �C and cyclicthermal shock at 1300 �C. Given the inferior mechanical properties of bulk GZ, thedouble-layered GZ TBC had the lowest erosion resistance followed by the triple-layered GZ, while the single-layer YSZ had the best erosion performance [72]. Dueto the columnar structure of the SPS TBCs, the erosion performance of the YSZ
18 S. Joshi et al.
coatings is better than that of corresponding APS (dry powder) TBCs and compa-rable to EB-PVD TBCs [71]. It is pertinent to mention that the erosion results ofGZ-based multilayered TBC are not very dissimilar to those of the YSZ TBCsprayed by APS [71].
Although the main focus of SPS research has hitherto been on TBCs, there is nowa growing emphasis on its use for realizing other functional coatings too. By virtueof the possibility of spraying nano- and submicron-sized particles suspended indifferent solvents, the SPS coatings can be tailored to produce a variety of micro-structures such as porous columnar, dense vertically cracked, etc. [73–75]. Thepossibility to realize refined Al2O3 microstructures more appropriate for wearresistance applications than APS coatings has also been demonstrated by Goelet al. [76]. Preliminary work in the authors’ group exploring the potential of SPSCr2O3, Cr3C2, and TiC coatings has also shown promise of exploiting SPS fordeveloping a new generation of wear resistance coatings [10, 77]. A typicallydense SPS-derived layer of Cr2O3 with fine porosity is shown in Fig. 14[77]. Mubarok et al. have reported SPS processing of SiC coatings [78], whileBerghaus et al. have shown the capability of SPS to minimize in-flight oxidationof WC-Co feedstock [79]. In recent times, an increasing number of other functional
Fig. 13 SEM micrographs of cross sections of different TBC systems: (a) single layer YSZ;(b) double layer GZ/YSZ; (c) triple layer GZ/GZ/YSZ TBC [71]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 19
applications employing the SPS route have also been reported. These includecoatings for superhydrophobic and icephobic [80], photocatalytic [81], and biomed-ical [82] applications.
Solution Precursor Plasma Spraying
SPPS is an exciting method to produce a wide variety of functional oxide ceramiccoatings, starting with appropriate precursor salts dissolved in a suitable solvent, incontrast to either a conventional “coarse” powder feedstock or a suspension involv-ing already formed “fine” powders suspended in a solvent. The technique utilizesaqueous/organic chemical precursor solutions fed into the high-temperature plasmaplume employing a dedicated delivery device as shown previously in Fig. 1. TheSPPS process opens up new avenues for developing compositionally complexfunctional oxide coatings. Some of the main potential benefits are as follows:
• Ability to create nanosized microstructures without any feeding problems nor-mally associated with powder-based systems
• Flexible, rapid exploration of novel precursor compositions and theircombinations
• Circumvention of expensive powder feedstock• Better control over chemistry of the deposit
Processing
The idea of utilizing solution precursors for synthesizing ultrafine particles using acombustion flame dates well back to the 1960s. Such salts were decomposed in thepresence of natural gas and air for producing metal oxide (TiO2, SiO2, etc.) powdersfrom corresponding metal chlorides in hydrocarbon flames [83]. Subsequently, many
Fig. 14 SEM image of cross section of SPS Cr2O3 showing (a) low-magnification cross-sectionaloverview and (b) high-magnification microstructure with fine distributed porosity [77]
20 S. Joshi et al.
variants of this process have emerged for synthesizing powders as well as filmsthrough spray pyrolysis-based routes, and SPPS is one such attractive variant. Todevelop thick ceramic oxide films/coatings, solutions of metal salt precursors likenitrates, acetates, isopropoxides, butoxides, etc., dissolved in a suitable solvent likewater, methanol, isopropanol, etc., have been employed [18, 23–25]. A number ofsteps, including droplet breakup, solvent evaporation, solute precipitation, gelation,pyrolysis, sintering, and melting, have been proposed to be involved during thetransformation of an injected precursor droplet into a coating [20, 84–87]. A plau-sible sequence of event proposed in the literature is schematically shown in Fig. 15[85]. Moreover, all these steps need to occur in a relatively short time scale that istypical of plasma spraying, usually of the order of milliseconds. Thus, a high-energysource is preferred, and this is the reason why high-energy plasma spray is widelyused for precursor-based deposition than any of the other thermal spray variants [18–28, 51, 88–94]. A prominent feature of SPPS is the possibility of single-stepconsolidation, which avoids the need for any secondary treatment processes thatare typically adopted in spray pyrolysis routes [95–97].
In case of SPPS, the high-intensity plasma source can also be used to preheat thesubstrate which, in turn, not only enhances the bonding of the impacting splats butalso leads to enhanced diffusion and surface mobility of the adsorbed atoms on thesubstrate surface during deposition [84]. Among the various precursor-based depo-sition processes, the rapid reaction rates and high deposition rates are specificadvantages of the SPPS process. As may be expected, the process parameters playa crucial role and need to be optimized to achieve the desired coating characteristics.These parameters include both the spray process variables (like plasma power, gas
Fig. 15 Schematic illustration of steps typically associated with coating formation by the SPPSroute [85]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 21
flow rates, nozzle type, standoff distance, substrate temperature, etc.) and theprecursor-related variables (such as chemistry, concentration, and flow rate).
A vast majority of early SPPS studies were primarily focused on YSZ coatings,due to the immediate industrial applications of TBCs in land-based and aero gasturbines, SOFCs, etc. The first step in deposition of SPPS YSZ coatings involves theformulation of appropriate precursors to achieve coatings with desired characteris-tics. It is pertinent to point out that the choice of zirconia-forming precursors usuallyincludes zirconium alkoxides, zirconium oxychloride, or zirconium acetate. Ingeneral, alkoxides of yttrium and zirconium are highly moisture sensitive and readilyform precipitates upon exposure to environment, while chlorides are known toproduce harmful fumes, and, thus, their use is avoidable [98]. Zirconium acetatehas the ability to dissolve in water to form an aqueous solution, and the exothermicreaction resulting from the carbonaceous species can be an added benefit duringspraying of energy-intensive solution precursors. Similarly, yttrium acetate, yttriumisopropoxide, and yttrium nitrate have been used as yttria-forming precursors inprior studies [99]. Apart from the relatively lower cost of yttrium nitrate compared toyttrium acetate, one additional factor that makes the former an attractive choice is thefact that Y2O3 forms at a relatively lower temperature of 450 �C when yttrium nitrateis used, whereas complete transformation from yttrium acetate to Y2O3 occurs onlyat 650 �C [99]. Such considerations usually dictate the selection of an appropriateprecursor for deposition of a targeted chemistry by SPPS.
Notwithstanding a reasonably large number of studies involving SPPS, one of thereasons why the mechanism of coating formation during SPPS processing was notwell understood for long was because the in-flight diagnostic tools successfully usedwith various conventional thermal spray processes involving coarser powder parti-cles could not be used with SPPS. The typically fine size (~1 μm) of droplets/particles associated with SPPS was a major hurdle, and, hence, development of agood process understanding has relied on investigation of particles collected in-flightand a study of the splat morphologies [4, 85, 100]. The latter approach has beenfound to be extremely useful in gaining valuable insights into in-flight particlegeneration and splat formation leading to coating deposition [9].
Figure 16 reveals an illustrative example of how different processing conditionscan lead to vastly varying particle morphologies during SPPS of an aluminum nitrateprecursor for deposition of alumina coatings [101]. For example, use of a lowerplasma power results in an irregularly shaped particle which corresponds to thepresence of a mainly unpyrolyzed precursor. On the other hand, the hollow particlesseen in the figure are an outcome of precursor droplets experiencing high surfaceevaporation rates when exposed to high plasma power. The precursor concentrationand the droplet trajectory within the plasma plume also significantly influence thepyrolysis action and, if appropriate conditions exist, lead to formation of solidparticles as desired.
Similar to studies on in-flight particle generation, investigation of splat morphol-ogies can also be extremely revealing, as clearly illustrated in Fig. 17 [101]. Theremnant gel-like unpyrolyzed mass at low plasma power levels does not lead toformation of well-defined splats. Significant amount of fine spherical particles can
22 S. Joshi et al.
result from secondary pyrolysis of such unpyrolyzed mass upon impacting thepreheated substrate. At higher plasma power levels, molten splats can be observed.The precursor chemistry, too, plays a crucial role. For example, splats observed withuse of a modified aluminum acetate precursor yielded splats with greater degree ofmelting even at low plasma power levels compared to an aluminum nitrate precursorduring alumina coating deposition. Use of organic solvents increases specificenthalpy, and their lower surface tension presumably also assists defragmentationof droplets and facilitates transformation and/or melting of particles prior to impactwith the substrate [101].
Microstructure
A primary reason, aside from the considerable application potential, why develop-ment of TBCs employing SPPS technique had greatly attracted the spray communityinitially is the inherent advantages that this route offers over conventional powder-based atmospheric plasma-sprayed coatings [18, 23–25]. In terms of coating char-acteristics, SPPS-deposited YSZ coatings have been shown to possess interesting
Fig. 16 Summary of different in-flight-formed particle morphologies during SPPS deposition ofalumina coatings at varied processing conditions [101]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 23
intrinsic features like fine grains, vertical cracks, fine-distributed porosity, reducedinter-splat boundary sizes between the lamellae, etc. The superior reported durabilityof SPPS TBCs over conventional APS TBCs is attributed to the unique microstructurein case of the former, which provides better strain tolerance and toughness [23–25].
Based on the improved understanding of coating formation through studies onin-flight particle generation and subsequent splat formation on impacting the sub-strate as discussed above, the role of process parameters like plasma current, standoffdistance, primary/secondary gas flow rates, substrate preheat temperature, precursorfeed rate, etc. is now better appreciated. Of course, the splat formation also has adirect bearing on the resulting microstructure. The microstructures of SPPS YSZcoatings generated at varied plasma powers and substrate preheat temperatures areshown in Fig. 18, and these clearly correlate with the in-flight particle formation andsplatting characteristics [9]. The importance of thermal energy input in promotingprecursor pyrolysis and subsequent melting of particles formed in situ, and addi-tionally of the substrate temperature in subsequently forming disk-shaped splats, canbe discerned from the coating microstructures shown in Fig. 18 [9]. Both thecombinations, low plasma power with substrate preheat and high plasma powerwith no preheat, lead to relatively higher porosity and inferior deposition ratesthereby correlating well with earlier findings of incomplete YSZ particle formationunder these conditions. On the other hand, higher plasma powers with substrate
Fig. 17 Summary of splat formation during SPPS deposition of alumina coatings at variedprocessing conditions [101]
24 S. Joshi et al.
preheat yielded relatively denser coatings because of complete pyrolysis due toenhanced thermal energy. Very higher plasma power leads to substantially denseregions in the microstructure with accompanying cracks and pores, the latter beingplausibly due to significant splashing or disintegration of splats shown previously.
Although the SPPS YSZ coatings meet some of the key guidelines on micro-structure and characteristics for TBCs [102] in terms of finer splats, smaller pores,absence of horizontal cracks, etc., the superior thermal cycling performance of thesecoatings is derived from the presence of vertical cracks. It has been shown that theSPPS YSZ microstructures can be engineered to make the coatings more straintolerant and contribute to thermal cycling life enhancement. Following the approachof investigating in-flight particle generation and splat formation as previouslydiscussed at different precursor flow rates, it was observed that varied amounts of
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 25
unpyrolyzed mass, secondary particles resulting from pyrolysis on the preheatedsubstrate surface or on previously deposited splats, pores/voids, etc., can get incor-porated during coating buildup. YSZ coatings generated at constant plasma sprayand liquid injection parameters, but with varying precursor flow rates, have beenfound to result in distinct microstructures [103]. Depending on the employed processconditions, fully molten regions, unmelted particles, pores, and even incorporatedunpyrolyzed precursor present in varying extent constitute the coating microstruc-ture during early stages of coating formation. The unpyrolyzed mass is reported to beresponsible for producing pyrolytic stress, attributable to volumetric changes asso-ciated with release of acetates, nitrates, and water molecules that constitute thesolution precursors. Beyond a certain unpyrolyzed precursor content incorporatedduring coating buildup, this stress can even exceed the tensile strength of YSZcoatings and manifest in the form of vertical cracks in the microstructure [86]. Themechanism of vertical crack formation has been discussed in considerable detailelsewhere [103] and schematically illustrated in Fig. 19. Such an understanding ofvertical crack formation in a TBC microstructure can potentially enable designingTBCs with alternate promising chemistries such as GZ, LZ, CeO2-ZrO2-Y2O3, etc.
It is pertinent to note that the approximate stress reduction through introduction ofvertical cracks in brittle ceramics can be more than 50%, thereby bearing promise tosubstantially delay onset of spallation as compared to conventional APS TBCswithout vertical cracks [104]. Although the crack density in SPPS YSZ microstruc-tures is significantly lower than in case of electron beam physical vapor deposition(EBPVD) YSZ, it is comparable with reported values for the segmented densevertically cracked YSZ microstructures realized by APS [103].
Thermal shock resistance is an important performance requirement for any TBC.Prior work has shown the thermal cycling behavior of SPPS YSZ to be significantlybetter than the APS YSZ coatings [30]. Some reports have also indicated that itsdurability could be even better than that of the dense vertically cracked (DVC) andEB-PVD YSZ. The superior durability of SPPS TBCs over that of other coatingsis attributed to the improved strain tolerance and toughness of the SPPS TBCs [18,23–25].
As previously discussed, development of coatings resistant to CMAS attack is yetanother growing requirement. Due to the versatility of SPPS, the process can beconveniently adapted to deposit multilayered coatings or to even modify the chem-ical composition of the top layer. A typical two-layered architecture having, forexample, a Gd2O3-doped La2Ce2O7 over a conventional YSZ/MCrAlY can providemultiple functionalities in terms of low thermal conductivity, hot corrosion resis-tance, and the ability to crystallize into the Ca2(LaxCe1 � x)8(SiO4)6O6 � x apatitephase to arrest infiltration of CMAS [105, 106]. An alternative formulation incor-porating Al2O3 and TiO2 formers into a YSZ-forming precursor also has potential tocreate the necessary coating chemistry that can mitigate CMAS attack [51]. TheSPPS route provides a very convenient pathway to modify the TBC chemistry asdetermined by the end applications.
26 S. Joshi et al.
Versatility: Materials and Functionality
Over the years, plasma spraying of solution precursors has been demonstrated to bean exciting method to produce a wide variety of functional oxide ceramic coatings,
Fig. 19 Schematic representation of vertical formation mechanism in solution precursor plasma-sprayed YSZ coatings. (Nomenclature: UM unpyrolyzed mass, HP hollow particles, SS sphericalparticles from secondary pyrolysis, HS hollow splats, MS molten splats, VC vertical cracks, UM-Tunpyrolyzed mass after transformation, BC bond coat) [103]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 27
starting with appropriate precursor chemistries, and provides a convenient pathwayto deposit nanostructured coatings. The earlier limitations in understanding the SPPSprocess mainly due to lack of relevant particle diagnostic tools have been addressedby various meticulously designed studies [9, 12] including investigation of in-flight-formed particles and subsequent splat formation as discussed above. These studieshave been educative and can form the basis for design and development of novelcoating systems. Research efforts by various groups on SPPS coatings have dem-onstrated that the process is versatile and allows convenient deposition of manydiverse oxides to yield exciting properties [12, 19, 24, 26–28, 88, 93].
One promising class of SPPS coatings is the TiO2-based photocatalytic coatingsknown for their attractive semiconductor characteristics like large bandgap, chemicalstability, nontoxicity, and low cost. However, its relative inactivity under visible-light illumination is a limitation which can be overcome by Cu2+-modified Ti3+ self-doping. The SPPS technique has been shown to be well-suited to achieve coatingswith such complex chemistries via a one-step process [107]. SPPS-deposited TiO2-based coatings have also been found to exhibit good antibacterial activity forinactivation of Escherichia coli pathogens under blue LED illumination[107]. ZnFe2O4 [26, 108], CdS [88], and ZnO coatings [109, 110] for photocatalyticapplications by SPPS have been reported. The SPPS technique has also been used todeposit a catalytic CuO/ZnO/Al2O3 layer on micro-channelized surfaces of protonexchange membrane fuel cells (PEMFCs) [111]. Compared to other conventionalmultistep deposition cum-sintering approaches, the possibility of achieving highsurface area and fine pores with a single-step SPPS method has great implicationsfor industrialization. Apart from the above, anode and cathode coatings of Ni-YSZ[19] and LaSrMnO3 [93] for SOFC applications have been successfully demon-strated using SPPS. Coatings of LiFePO4, LiCoO2, Co3O4 [112], V2O5 [113], etc.have also been studied for Li-ion battery applications and exhibited good charge-discharge characteristics. Nanostructured ceria coatings deposited for high-temperature corrosion and oxidation resistance [90], Eu:Y2O3 phosphor coatingfor photoluminescence [114], and orthorhombic α-MoO3 for super capacitor elec-trode applications [115] also reflect the wide-ranging potential of SPPS. Yttrium irongarnet (Y3Fe5O12), Ni0.5Zn0.5Fe2O4, and Ni0.5Zn0.5Fe2O4/SiO2 coatings [28, 116]produced by SPPS have shown single phase of spinel ferrite with nanocrystallinefeatures and are a good example of how coatings with extremely complex chemis-tries can be realized by virtue of the atomic-level mixing possible in SPPS. A partiallist of coatings already reported using plasma spraying or flame spraying of solutionprecursors is provided in Table 2.
Although the concept of solution precursor-based spraying was first introduced inthe 1990s and its versatility as well as benefits has since been demonstrated throughseveral studies, no significant headway has been made as far as its commercializationis concerned [117]. A primary reason can be attributed to the low deposition ratesreported in case of solution precursor spraying processes that demand high-energyconsumption [118], especially with radial feed plasma torches that have typicallybeen employed in such studies. Therefore, augmentation of deposition rates ofsolution-based thermal spray approaches to levels acceptable for their industrial
28 S. Joshi et al.
Table 2 Partial list of coatings thermal sprayed using solution precursors
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 29
acceptance is desirable, as indeed is identification and validation of niche applica-tions. Prior work has already shown that the role of plasma power on deposition rateis immense [119] and, therefore, the recent advent of high-power axial-feed plasmaspray systems could be potential game changers for SPPS.
Hybrid Powder-Liquid Feedstock Processing
As discussed above, both SPS and SPPS are already two extensively investigatedvariants of liquid feedstock-based thermal spray processing with demonstratedability to deposit varied coatings with refined microstructural features. Thepresent-day availability of spray systems that permits improved thermal exchangebetween the energy-demanding liquid feedstock and the plasma plume by enablingaxial injection also makes this an opportune time to further explore possibilities withsuspensions and solution precursors. In this context, hybrid powder-liquid feedstockspraying provides an opportunity to conveniently combine constituents at vastlydifferent relative length scales, by using them in the form of conventional “coarse”spray-grade powders (usually 10–100 μm in size) or as fine particles (approximately100 nm–2 μm) already suspended in a suitable medium or generated in situ from asolution precursor [120, 121]. The resulting splats of constituent molten particles,which are the building blocks for thermal-sprayed coatings, also differ by more thanan order of magnitude, and the consequent combination of coarse-fine features canpotentially yield unique coating microstructures.
Processing
A key aspect of realizing the powder-liquid hybrid coatings is the independentlycontrollable introduction of conventional micron-sized powder feedstock and liquidfeedstock in the form of solution or suspension into the plasma plume to form thedesired coatings [120, 121]. One such experimental arrangement is shown in Fig. 20and utilizes the “standard” coaxial liquid feedstock injector tube, with the suspensionfed into the inner tube as usual but with the atomizing gas being replaced by apowder-entrained carrier gas [121]. The carrier gas also serves to atomize the liquiddroplets at the end of the coaxial tube, as depicted in Fig. 5. Individual control of theamounts of powder, liquid, and atomizing/carrier gas fed is a salient feature of thisarrangement, with ability to completely turn off either of the feedlines. As is apparentfrom the figure, different variations of the feeding arrangement to allow bothconstituents to be fed radially or one radially and the other axially are convenientlypossible.
30 S. Joshi et al.
Possible Coating Architectures
The above arrangement can be readily utilized to yield different unique coatingarchitectures that are function-specific and could be relevant for varied applications[21, 30, 31, 120–124]. For example, sequential injection of powder and liquidfeedstock can yield a layered coating comprising powder-derived and solutionprecursor-/suspension-derived layers in either order, with each involving featuresof very distinct length scales. Similarly, simultaneous powder-liquid feeding canyield composite coatings with a distributed fine second phase in a coarser matrix orvice versa. As a specific case of the simultaneous feeding, the relative feed rates ofthe two constituents can also be continuously varied to achieve functionally gradedcoatings. Such on-demand architectures, schematically illustrated in Fig. 21 [121],are not convenient to realize by conventional thermal spray methods.
Microstructures
The authors have demonstrated realization of each of the above architectures utiliz-ing both solution precursors and suspensions in tandem with powder feedstock,employing different material systems in separate studies [21, 30, 31, 120–123]. Thechoice of the materials was dictated by relevant practical applications and illustratedthe generic applicability of the powder-liquid hybrid approach, with virtually noconstraints imposed by the choice of powder and/or liquid feedstock. For example,
Powder feeder
Powder feeder
Plasma torch
Plasma torch
Carrier gas
Carrier gas
Atomizing gas Plasma torch
Powder
Suspension
d
e
c
b
aAxial feed of powder
Axial feed of suspension with coaxialfeed of atomizing gas
Axial feed of suspension with coaxialfeed of powder withcarrier gas
Suspension feeder
Suspension feeder
Fig. 20 Schematic illustration of an axial-feed plasma spray system depicting (a) usual powderdelivery mechanism, (b) usual suspension delivery mechanism, and (c) proposed arrangement forhybrid powder-suspension delivery, with blowups (d), and (e) showing the coaxial feedstockinjector tube and its effective utilization [121]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 31
Fig. 22 [121] depicts the surface micrographs of different powder-suspension com-posite coatings, clearly showing the very different sizes of splats ensuing from thepowder and suspension constituents. Similarly, Fig. 23 [120] shows examples oftypical layered, composite, and graded coating architectures developed using threedifferent combinations of powder and solution precursor feedstocks.
Versatility: Materials and Functionality
The versatility of the powder-liquid hybrid spraying approach is already amplyevident from the discussion in the preceding section. It is apparent that the methodcan be harnessed to spray virtually any material combination, permitting greatflexibility in deposition of complex coating systems. The wide range of suspensionsand solution precursors that have been employed to deposit coatings already,combined with the overwhelmingly large portfolio of spray-grade powders that arecommercially available, opens new vistas for producing superior coatings withunique combinations of materials. The abundant choice of materials is furthercomplemented by the ability to realize tailored architectures leading to numerouspossibilities. Only a couple of examples are illustrated herein based on the authors’prior work to highlight the performance benefits of the composite hybrid coatings.For example, the unique properties achieved through hybrid processing of
Layered(sequential injection of powder
and suspension feedstock)
Composite(Simultaneous injection of powder
and suspension feedstock)
Functionally graded(dynamically changing relativefeed rates of hybrid feedstock)
Substrate
From powder
a b c
From suspension
Substrate Substrate
Fig. 21 Schematic illustration of (top) different coating architectures achievable using hybridpowder-liquid plasma spraying and (bottom) surface morphologies depicting (a) “coarse” powder-derived splats, (b) “fine” splats originating from suspension/solution precursor, and (c) the mixedcoarse-fine splats formed in case of a composite coating [121]
32 S. Joshi et al.
“composite” YSZ (powder) – YSZ (solution precursor) coatings were found toexhibit enhanced erosion resistance compared to the stand-alone SPPS and APSYSZ coatings as shown in Fig. 24 [120]. The role of in situ formed submicron-sizedsplats within the large-sized powder-based splats plausibly improves toughness,leading to better erosion resistance as well as better thermal cyclic life. This canfind relevance in TBCs deployed in dust-laden environments. In yet another studyinvolving such composite YSZ coatings deposited by the powder-solution precursorhybrid plasma spraying technique, the hybrid YSZ TBCs were found to performbetter than the APS and SPPS YSZ coatings under hot corrosion conditions at900 �C in both chloride and vanadate salt environments [123].
In another example of nanocomposite coatings deposited using hybrid powder-solution precursor plasma spraying, a molybdenum-based alloy powder(Mo + NiCrBSiFe+NiAlMo) and a YSZ-forming precursor were simultaneouslysprayed to achieve enhanced scuffing wear resistance [21] for potential piston ringapplications. The incorporation of fine-sized YSZ was observed to enhance wear
Fig. 22 Surface morphology of composite coatings deposited by simultaneous spraying of apowder and a suspension showing multi-scale splat structure: (a) Al2O3 powder-YSZ suspension,low magnification; (b) Al2O3 powder-YSZ suspension, high magnification; and (c) T400 powder-Cr3C2 suspension, high magnification [121]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 33
behavior under dry as well as lubricated sliding conditions as depicted inFig. 25 [21].
Conclusions
Liquid feedstock thermal spraying represents an exciting approach to obtain coatingswith vastly different properties compared to the conventional powder-derived coat-ings. Since use of a liquid feedstock in the form of either a suspension or a solutionprecursor demands more energy because of the need to evaporate the solvent, SPSand SPPS have been the two more widely investigated variants of the aboveapproach by virtue of the high thermal energy offered by a plasma plume. Both ofthe above have matured considerably in recent years, as a result of numerousresearch efforts that have contributed handsomely to enhancing the knowledgebase by providing crucial insights into mechanisms responsible for coating forma-tion. The recent availability of axial feed capable plasma spray systems has alsoaddressed concerns regarding low deposition rates/efficiencies previously associatedwith use of liquid feedstock by virtue of the considerably improved thermal
Fig. 23 Illustrative examples of typical coating architectures developed through hybrid pro-cessing: (a) layered coatings by sequential feeding of YSZ powder (APS) and YSZ-formingprecursor (SPPS), (b) composite coating by simultaneous feeding of a Mo alloy powder (APS)and YSZ-forming precursor (SPPS), (c) graded microstructure by simultaneous feeding ofNiCoCrAlY powder (APS) and YSZ-forming precursor (SPPS) with controlled variation in theirrelative feed rates [120]
34 S. Joshi et al.
exchange between the plasma plume and the injected feedstock compared to con-ventional radial feeding. The process parameter impact on characteristics as well asperformance of resulting coatings is also now well understood and has laid thefoundation for increasing versatility of the above techniques, leading to a remarkablewidening of the portfolio of coating materials that can be deposited.
Fig. 24 Thermal cycling and erosive wear performance of composite APS-SPPS YSZ coatings incomparison with stand-alone SPPS and APS YSZ coatings [120]
2.0
1.8
1.6
1.4
1.2
1.0
0.8
Wei
ght L
oss
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ght L
oss
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Mo alloy blend powder
Hybrid Mo alloy + YSZ
Distance (km)Lubricated test conditions
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14 16 18 20 22 1.00
2
4
6
8
10
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14
1.5 2.0 2.5 3.0 3.5 4.0
a bMo alloy blend powder
Hybrid Mo alloy + YSZ
Fig. 25 Wear test results for powder-based APS and hybrid APS + SPPS powder + solutionprecursor-based coatings tested under (a) lubricated conditions and (b) dry conditions [21]
New Generation Ceramic Coatings for High-Temperature Applications by Liquid. . . 35
The above versatility has led to realization of diverse functionalities that can beharnessed for a wide range of industrially relevant applications, e.g., advancedthermal barriers, wear resistance, corrosion-oxidation protection, etc. The ability ofSPS and SPPS routes to uniformly deposit coatings much thinner than conventionalAPS coatings also opens possibilities for some niche applications to be explored inthe future. Extension of SPS and SPPS to enable use of hybrid powder-liquidfeedstock for plasma spraying presents a novel approach that promises to furtherexpand the horizon. The potential of hybrid processing to explore new materialcombinations and create tailored function-dependent coating architectures, such aslayered, composite, and graded coatings with multi-scale features, has already beendemonstrated and appears to be an exciting pathway to achieve superior properties.This method enables spraying of virtually any material combination, allowingtremendous flexibility in fabrication of complex coating systems.
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