Operating parameters for suspension and solution plasma-spray coatings

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Surface & Coatings Technology 202 (2008) 4309 – 4317 www.elsevier.com/locate/surfcoat

Operating parameters for suspension and solution plasma-spray coatings P. Fauchais, V. Rat, J.-F. Coudert, R. Etchart-Salas, G. Montavon ⁎ SPCTS — UMR CNRS 6638, Faculty of Sciences, University of Limoges, 123 avenue Albert Thomas, Limoges, France Available online 12 April 2008

Abstract The interest to manufacture on large surfaces thick (i.e., 10 to 20 μm, average thickness) finely structured or nano-structured layers is increasingly growing since about 10 years. This explains the interest for suspension plasma spraying (SPS) and solution precursor plasma spraying (SPPS), both allowing manufacturing finely structured layers of thicknesses varying between a few micrometers up to a few hundred of micrometers. SPS aims at processing a suspension of sub-micrometric-sized or even nano-metric-sized solid particles dispersed in a solvent. The liquid solvent permits to inject particles in the thermal flow (i.e., due to their size, a carrier gas cannot play this role). SPTS aims at processing a solution of precursors under the same conditions. Upon evaporation of the liquid, the precursor concentration increases until precipitation, pyrolysis and melting of small droplets. Compared to conventional plasma spraying, SPS and SPPS are by far more complex because fragmentation and vaporization of the liquid control the coating build-up mechanisms. Numerous studies are still necessary to reach a better understanding of involved phenomena and to further develop the technology, among which injection systems, suspension and solution optimizations, spray kinematics, etc. This review presents some recent developments in this field. © 2008 Elsevier B.V. All rights reserved. Keywords: Suspension plasma spraying; Solution precursor plasma spraying; Nano-sized feedstock; Finely structured layers; Process parameters; Coating architecture; Porosity network

1. Introduction Nanostructured materials offer significant improvement in engineering properties due to the reduction of grain sizes by a factor of almost two orders of magnitude over conventional coatings [1]. Since the beginning of the mid-1990s, works have started to use the spray technology for the deposition of finely or nano-structured coatings [2,3]. To produce finely structured coatings by thermal spray techniques, four routes have been suggested: ■ to spray complex alloys containing multiple elements, such as for example the SAM2X5 (Fe–Cr–Mo–W–Mn–B–C–Si) [4] which exhibits a glass forming capability when cooled-down. After deposition, upon subsequent heating in the 500–750 °C range, the metallic glass precursor transforms into multiple crystalline phases and the resulting structures are nanoscaled [5]; ■ to spray conventional micrometric particles (in the 30–90 μm range) made of agglomerated nanoparticles. Nevertheless, the operating parameter window for which particles are only ⁎ Corresponding author. Tel.: +33 55 45 75 55. E-mail address: [email protected] (G. Montavon). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.003

partially melted (i.e., the molten part acts as a “cement” bonding unmolten nanograins) is narrow (see the review of Lima and Marple [6]); ■ to spray sub-micron-sized or nano-sized particles via a suspension (suspension thermal spraying, STS). To circumvent the too high gas flow rates to spray particles below 5 μm [7], a liquid carrier is used instead of a gas one [8–10]. Once it has been fragmented and vaporized by the plasma flow or the HVOF flame, particles contained in the droplets are heated, accelerated and sprayed onto the substrate; ■ spray solutions of final material precursor (solution precursor thermal spraying, SPTS). As with suspension, the liquid undergoes rapid fragmentation and evaporation once injected in the SPTS jet. This is followed by precipitation or gelation, pyrolysis and melting to result finally in the impact of molten liquid droplets with average diameters ranging from 0.1 to a few micrometers [11–13]. In this paper, only the two last routes; i.e., suspension plasma spraying (SPS) and solution precursor plasma spraying (SPPS) will be developed. Only d.c. plasma jets will be considered to process feedstock. Compared to conventional coatings, those manufactured from solutions or suspensions exhibit quite

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interesting features such as the absence of lamella boundaries and cracks and porous microstructures with nanometer-sized grains. However, liquid injection within plasma jets is quite different from particle injection by a carrier gas in particular the fragmentation and vaporization mechanisms taking place, as well as the manner liquid droplets or liquid jets are produced. Indeed, mechanisms encountered in SPS or SPPS are by far more sensitive to arc root fluctuations than those encountered with conventional spraying. In the following will be successively presented: i. the liquid injection with the parameters quantifying fragmentation and vaporization, ii. the way coatings are produced considering SPS and SPPS. 2. Liquid injection

Fig. 2. Blast atomizer droplet interaction with an Ar–H2 d.c. plasma jet, from [15].

2.1. Atomization

2.2. Mechanical injection

Four different atomizer technologies were used for example by Jordan et al. [14]:

Mechanical injection is achieved by using a pressurized container in which the liquid is stored and forced through a nozzle of specified internal diameter, dn, varying from 50 to 300 μm. At the injector exit, a liquid jet is generated. Its diameter is about 1.9 × dn [10,17,18] (Fig. 3a). After a certain distance, Rayleigh– Taylor type instabilities develop and droplets are generated by primary atomization (Fig. 3b). It is thus possible to inject within the plasma flow either a liquid jet or calibrated drops which perturbation by the plasma flow is far less drastic than with secondary atomized drops (Fig. 3c to be compared with Fig. 2). The mass flow rate ml0, expressed regardless of the pressure loss in the injector, is indeed expressed as follows [10]:

■ home made capillary atomizer (proprietary system), ■ fan nozzle: a narrow angle atomizing fan nozzle (mechanical secondary atomization), ■ air cap transverse air blast atomizing nozzle, typified as air blast atomizer, ■ nebulizer (transverse jet secondary atomization). Fig. 1 displays droplet size distributions measured at the systems exit implementing phase Doppler anemometry (PDA) for the different considered technologies. Fig. 2 shows that implementing air blast atomizer, the plasma flow is nonuniformly perturbed and a fraction of droplets by-pass it [15]. Air blast atomization has been studied also by Rampon et al. [16] who have demonstrated that for suspensions, the droplet size distribution depends upon the atomizing gas flow rate/ suspension feed rate ratio, RSG.

m0l ¼ ql d vl d

pdn2 4

ð1Þ

where ρl and vl represent the liquid specific mass and the liquid average velocity at the injector exit, respectively, and dn the injector internal diameter at exit. The gas pressure difference, Δp, between the pressure in the container and the surrounding atmosphere is depicted as follows: 1 Dp ¼ d f d ql d v2l 2

ð2Þ

where f represents the friction coefficient of the liquid in the injection nozzle. For example, for an identical mass flow rate of 0.47 cm3 s− 1 and considering an internal diameter of 150 μm, the pressure difference is approximately 0.5 MPa while it goes up to 41 MPa when reducing the internal diameter to 50 μm leading to technological constraints. To overcome this problem, magnetostrictive rods can be used [19]. 2.3. Solution and suspension preparation

Fig. 1. Droplet size distributions for different atomizers: A) capillary atomizer, B) fan nozzle, C) air blast atomizer and D) nebulizer, from [14].

Either for solutions or suspensions, the main constituent is the solvent that is either water or organic, the most used one being ethanol. As it can be expected, fragmentation of the liquid will depend upon its surface tension, σs, and viscosity, μs, and

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2.3.2. Suspensions The liquid feedstock is prepared by adding a dispersant (electrostatic, stearic or electro-stearic one) to stabilize the ceramic powder within the liquid solvent. The optimum fraction of dispersant has to be optimized by measuring the suspension apparent viscosity versus the dispersant fraction that must be the lowest [23]. The optimal fraction of dispersant is also closely linked to the specific surface of the dispersed powder. The suspension viscosity also depends upon the particle mass load with which it increases. Surface tension, on the opposite, is not related to the mass load. It must be also underlined here that the behavior of such a suspension is a shear-thinning non-Newtonian one. 3. Plasma–liquid interaction

Fig. 3. Liquid flow exiting an injector: a) general view, b) detail of view a) 25 mm downstream of the nozzle exit, c) primary atomization of the flow at a distance from injector exit higher than 27 mm, from [10,18].

its vaporization upon its thermal properties, among which its specific and latent heats and its vaporization temperature at atmospheric pressure, c, Lv and Tv respectively (Table 1). One can deduce from such values that in a first approximation water requires more energy than ethanol to get fragmented and more energy to get vaporized. Nevertheless, ethanol contains carbon that might pollute the coating meanwhile no clear proof of that was ever given from the author's knowledge. Another important point to take into consideration is that for an easier liquid injection, the viscosity of the suspension or the solution must remain as close as possible as the one of the solvent. 2.3.1. Solutions The liquid feedstock is prepared by dissolving the precursor chemicals in either water (aqueous suspension) or ethanol (organic suspension). Aqueous solutions allow higher precursor concentration than organic ones, are less expensive to produce and are safer to store and to handle [21]. The equilibrium saturation concentration is determined by concentrating the precursor in an evaporator at room temperature until precipitation occurs [22]. This permits then to prepare several solutions with different concentrations. It results in different viscosity, surface tension and specific mass: viscosity increases (in a ratio of 5) and surface tension decreases (in a ratio of 1.23) when increasing solution concentration in a ratio of 4. It must be moreover underlined that solutions behave as the solvent; i.e., as a Newtonian fluid.

According to the complexity of involved phenomena, a quasi-stationary plasma flow will be considered at first to depict them before considering a fluctuating flow. This is the case of an Ar–He plasma gas mixture for which voltage fluctuations can be limited to ΔV/Vm = 0.3 (Table 2) (corresponding to the takeover plasma torch operating mode) when the torch is operated under an arc current intensity of 700 A (and an anode internal diameter of 6 mm). Fluctuating plasma jet will be characteristic of an Ar–H2 plasma gas mixture for which voltage fluctuations can be higher than ΔV/Vm = 1 (corresponding to the restrike plasma torch operating mode). Three major zones can be identified within the plasma flow: ■ the plasma jet core (T N 8000 K) where the liquid can encounter the highest heat and momentum transfers, ■ the plasma plume (3000 N T N 6000 K) where the heat and momentum capabilities from the plasma are drastically reduced compared to the ones in the plasma core, ■ the plasma fringe (around the plasma core) where the momentum might be high enough to fragment the liquid stream but where the droplet heat treatment will be by far insufficient. It will be thus of prime importance to inject the liquid as close as possible to the torch nozzle exit and have it penetrating in the plasma core without being fragmented in its fringes. 3.1. Stationary plasma 3.1.1. Suspensions If the liquid stream or drops would behave as particles, its optimal trajectory should correspond to trajectory within the plasma core [14]. However, when it is mechanically injected,

Table 1 Some properties of the most encountered solvents

Water Ethanol

Surface tension σs [J m− 2] (room temp.)

Viscosity μs [Pa s] (room temp.)

Specific heat cp [J kg− 1 K− 1] (room temp.)

Latent heat of vaporization Lv [J kg− 1]

Vaporization temperature Tv [K]

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

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Table 2 Typical values of voltage, intensity and electric power relative fluctuations for several gas mixtures corresponding to several operating modes Plasma forming gas mixture

Plasma torch operating mode

Voltage a Typical relative fluctuation

Arc current intensity b Typical frequency

Typical relative fluctuation

Restrike

1.2–2 0.8–1.6

Takeover

0.3–0.8

Typical relative fluctuation

600 Hz

Same as voltage in a first approximation

ΔV/Im

ΔV/Vm N2–H2 Ar–H2 Ar–H2–He Ar–He

Electric power Typical frequency

3000–5000 Hz

0.02–0.06

a

Voltage fluctuations are related to arc root fluctuations which depend upon the boundary layer thickness developing at the anode wall and which depend itself upon the plasma forming gas mixture. Voltage fluctuations define the plasma torch operating mode. b Arc current intensity fluctuations are related to electric power source type, that is to say the technological way used to convert alternative current to direct current (i.e., thyristorized or transistorized sources). Those fluctuations are independent from the voltage fluctuations.

the suspension stream is fragmented at the “neck” of the instabilities by the shear stresses (Fig. 4, left part). Under such conditions, several clouds of materials (liquid and/or solid) can be identified in the plasma flow. They are composed of a compact “head” of suspension coupled to a “tail” made of small droplets or solid particles. The clouds are equally spaced between themselves, the average distance between them corresponding to the wavelength of the jet instabilities before entering the plasma. This signifies that the initial velocity of the suspension is kept along the injection axis after its penetration into the plasma jet [18,20]. It should be also noted in Fig. 4. that only very few fragmentation occurs at the jet fringes. When considering liquid, one has to compare the liquid momentum density ρlv12 to the one of the plasma jet ρv2 [9,10,18,20]. In general, an “appropriate” liquid penetration is achieved for: ql v 2l N qv 2 ðat least 10 times greaterÞ

ð3Þ

For example [20], an injection momentum of 0.96 MPa (vl = 33.5 m s− 1) correspond to a “good” injection for a plasma flow typified with a momentum density of 0.03 MPa. This is due to the fact that upon fragmentation droplets are increasingly smaller and thus their velocity has to be increasingly higher. Fragmentation and vaporization times have been calculated for an Ar–H2 plasma jet (the jet was supposed stationary: that is to say with no arc root fluctuation) versus the drop average diameter [9].

Fig. 4. Interaction of liquid drops with an Ar–H2 plasma jet at the minimum voltage value (40 V) of the plasma torch, from [18].

Results are presented in Fig. 5. It can be seen that fragmentation is about two orders of magnitude faster than vaporization. Vaporization occurs only when liquid drops or jet (in the presented case of about 300 μm in diameter) are fragmented into droplets of a few micrometers in diameter. The whole mechanisms (fragmentation + vaporization) is achieved about 10 to 15 mm downstream the injection location, as shown by spectroscopic measurements considering water injected into an Ar–H2 plasma flow [9,15]. The solvent vaporization cools down of course rapidly the plasma jet and the spray distance has to be reduced accordingly to about 40–50 mm instead of 100–120 mm usually considered in conventional plasma spraying. It is fairly difficult to extrapolate from the literature the interactions between a liquid and a plasma flow since literature usually considers a cold gas flow and isothermal phenomena. Nevertheless, the literature correlates fragmentation to the dimensionless Weber [24] and Ohnersorge [25] numbers of the flow as follows: Weber

We ¼

Ohnersorge

qDv 2 dl rl

Al ffi Oh ¼ pffiffiffiffiffiffiffiffiffiffiffi ql dl rl

ð4Þ ð5Þ

where ρ and ρl are the gas and liquid specific masses, respectively, Δv = v − vl where v represents the gas velocity

Fig. 5. Evolution of the fragmentation and the vaporization (without or with correction taking into account the buffer effect of the vapor cloud) characteristic times versus the drop diameter in a stationary plasma jet (Ar: 45 SLPM, H2: 15 SLPM, I: 500 A, V: 65 V, h: 17.9 MJ kg− 1) [9].

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It is however important to underline that, even with optimized injection conditions such as the ones previously depicted, many spheroidized particles are collected at the periphery of flattened particle pattern when the spray pattern had interacted with a substrate (Fig. 6). This might be due to the thermophoresis force [29] acting on smallest particles in high temperature gradient zone which drag them towards the coldest zones within the plasma flow, reducing drastically their thermal treatment.

Fig. 6. Scheme of lamellae and poorly treated particles collected on a fixed substrate [18].

and vl the liquid velocity, respectively, dl the liquid diameter (drop or jet) and μl and σl its viscosity and surface tension, respectively. Fragmentation starts for a critical We N 14 and an increase in Oh leads to an increase of the critical We, delaying hence the fragmentation. Measurements carried-out by EtchartSalas et al. [18,20] confirmed this tendency: ■ when the injection velocity increases, fragmentation is delayed in the jet fringes, ■ the liquid penetrates deeper in the plasma when vl increases (without any modification in the dispersion and deviation angles, ■ the powder mass load increase corresponds to a higher liquid viscosity that stabilizes the suspension and hence decreases the dispersion angle (from 33 ± 3° for a mass load of 7% down to 16 ± 3° for a mass load of 40%).

3.1.2. Solutions No measurement by laser flash has ever been performed for SPPS. The different mechanisms occurring are summarized in Fig. 7 [13]. The droplets resulting from fragmentation are heated resulting in the precipitation of the solute as a shell (case A in Fig. 7). Depending on the nature of the solution itself, the shell is more or less porous. For small droplets (dl b 5 mm), precipitation encompasses the whole droplet. For larger droplets, shell precipitation occurs and the particle can later fragment depending on the pressure inside the shell and its mechanical properties. For small and dense particles, pyrolysis and sintering occur (case B in Fig. 8). For particles traveling in the warm core of the plasma flow, mechanisms A, B and C occur. In the jet fringes, mechanisms A and B are the most probable. At last, if the melted droplet keeps traveling in the plasma plume, solidification and crystallization occur (case D in Fig. 7). Moreover, precursors with low concentrations generally experience surface precipitation and resulting coatings consist in semi-pyrolyzed material stacking and exhibit soft porous architecture [22]. On the opposite, precursors with high concentration experience volume precipitation and resulting coatings consist in fully molten lamella stacking and exhibit dense architecture. The manner the atomization is performed is also important. If a broad drop jet penetrates the plasma flow, many particles by-pass the warm core and travel in the jet fringes (on the opposite, a narrow drop jet penetrate into the

Fig. 7. Scheme of SPPS process with successively: A) droplet evaporation and solution precipitation, B) pyrolysis and sintering, C) melting, D) crystallization [13].

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4. Coating generation 4.1. General remark Both for SPS and SPPS, three types of particles can be collected on the substrate [26]:

Fig. 8. Evolution, according to the torch voltage level, of the fragmented droplet diameter along their radial penetration into the plasma jet (under operation, the voltage fluctuates continuously due to arc root fluctuations, the amplitude of the voltage fluctuations being function of the plasma torch operating mode; i.e., restrike or takeover) [18].

warm core of the plasma jet). It is hence not surprising that the edges of a deposited bead are formed of a high density of unpyrolyzed particles. At last, according to Eq. (4), to delay the fragmentation and avoid it occurs in the jet fringes, different strategies can be envisaged: ■ to decrease the droplet size when using ethanol as solvent (droplets of 10 to 20 μm in diameter are not fragmented in the plasma fringes), ■ to increase the solution (or suspension solvent) surface tension by using water instead of ethanol or even by adding surfactants such as glycol. Nevertheless, in most of the cases, liquids with higher surface tension require a higher energy to get vaporized. 3.2. Fluctuating plasma For example, with a PT-F4 (Sulzer-Metco, Wohlen, Switzerland) working with an Ar–H2 plasma forming gas mixture under conditions depicted in Fig. 5 caption, the transient voltage varies between 35 and 95 V for Vm = 65 V, the fragmentation of the suspension varies drastically with the voltage, as shown in Fig. 8 [18] which depicts the minimum droplet diameter versus the radial distance to the torch centerline axis when the liquid stream (or drop) penetrate in the plasma jet. Indeed, fragmentation will be drastically different at 35 V compared to the one occurring at 95 V [18,20]. With an Ar–H2 plasma jet (of characteristics displayed in Fig. 6 caption), the deviation angle of the droplet flow pattern is 58° with vl = 26.6 m s− 1 to be compared to 63.5° obtained with an Ar–He plasma (same vl). The dispersion angle is highly dependent upon the plasma jet nature (i.e., so dependent upon fluctuations) as it varies from 67° (large dispersion of droplets in the jet) for an Ar–H2 plasma gas mixture to be compared to 15° for an Ar–He one. The effects of the injection parameters (injection velocity and mass load) provide the same trends than with the Ar–He plasma jet. At last, the fragmentation of the liquid stream starts as soon as it penetrates in the jet fringes.

■ those which have been well treated (i.e., fully molten) and which form lamellae upon impact and spreading, ■ those which have traveled in the jet fringes and are either only pyrolyzed (for the solution) or in a powdered state (for the suspension when fragmentation starts in the jet fringes resulting in poorly treated droplets), ■ those which have been ejected from the plasma jet warm core by thermophoresis force (in the high temperature gradient zone). Anyhow, apart the case of highly porous coating manufacturing, the inclusion of poorly treated particles in the spray bead periphery is undesirable. Thus, the spray pattern will play a key role in the coating formation mechanisms [8,9,18]. The impact of fully molten particles with diameters in the 0.06–0.5 μm range will result in lamellae with average diameters smaller than 1–2 μm (Fig. 9). It can be seen that with impacting a preheated substrate (since the spray distance is shorter compared to conventional plasma spraying), no peripheral splashing occurs at the periphery of the lamellae. Moreover, the residual quenching stress level within each lamella is lower than the material intrinsic resistance resulting in no intra-lamellar micro-cracking, even if coatings exhibit adhesion higher than 40 MPa. At last, the fact that the smallest particles still impact the substrate surface without following the gas flow implies that the dimensionless Stockes number, St, is higher than unity [27]. St is expressed as follows: St ¼

qp d p2 vp Ag e BL

ð6Þ

where ρp is the particle specific mass, dp its average diameter, vp its velocity at the considered location (near the substrate surface), μg the gas (plasma) viscosity and eBL the thickness of the gas (plasma)–substrate boundary layer (in the order of the tenth of

Fig. 9. Y-PSZ (8Y from Tosoh) flattened lamellae sprayed under SPS conditions (Ar–He d.c. plasma jet) collected onto a polished (Ra b 0.05 μm) stainless steel substrate [18].

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millimeter). The impact velocity vp plays a key role in the Stokes number. A high Stokes number is hence preferred compared to a lower one. For example, typical velocities of zirconia particles (29 to 68 nm) injected in an Ar–H2 plasma forming gas mixture jet issued from a Axial II Mettech d.c. plasma torch range from 260 to 470 m s− 1 [28] while WC–Co particles (20 to 250 nm) reach velocities higher than 700 m s− 1 [29]. Assuming a boundary layer thickness of 10− 4 m at the substrate and according to particle velocities of about 300 m s− 1 at a spray distance of 40 to 50 mm, the dimensionless Stokes number equals 100 for a 1 μm and goes down to 0.1 for a 0.1 μm particle traveling in the same conditions. Nevertheless, collecting under those conditions lamellae of 0.08 μm in average diameter and assuming a flattening degree of 2 would correspond to impacting particles of 0.04 μm in average diameter, the smaller ones being largely deviate near the substrate surface. At last, due to their very low inertia, particles smaller than 0.1 mm decelerate and cool down rapidly [26], implying spray distances of 40 to 50 mm, at the maximum. Particle collected at the periphery of the spray pattern exhibit spherical morphologies (indicating they well melted in flight) but already in a plastic state at that distance [26]. The consequence of such short spray distances is that the heat flux imparted by the plasma jet to the substrate and already deposited layers is high, for example 21.0 MW m− 2 considering an Ar–H2 plasma flow (Ar: 45 SLPM, H2: 15 SLPM, I: 500 A, h: 17.9 MJ kg− 1) and 16.0 MW m− 2 considering an Ar–He plasma flow (such as the one depicted in Fig. 9; i.e., Ar: 30 SLPM, H2: 30 SLPM, I: 700 A, h: 17.9 MJ kg− 1). It has to be reminded that the heat flux is about 2 MW m− 2 when considering the same Ar–H2 plasma flow at a spray distance of 100 mm. These high heat fluxes of course have consequences on coating morphology. In the following, SPS and SPPS of zirconia will be considered in order to depict coating structural differences between these two techniques. 4.2. Case of zirconia layers 4.2.1. SPPS as manufacturing route Sprayed zirconia layers with a precursor aqueous solution exhibit typical morphologies as the ones depicted in Fig. 10a

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[11,13,14,30,31]. They are made of micron-sized lamellae, have grain sizes smaller than 30 nm, exhibit a porous architecture at two scales (nano- and micro-scales) made of voids and throughthickness stress relieving intra-lamellar cracks. The unique intra-lamellar cracks could result from pyrolyzation shrinkage but stronger evidences still have to be given to support this assertion. Indeed, as explained in Section 3.1.2, unpyrolyzed particles (mainly those traveling in the jet fringes) are embedded within the layers during manufacturing. The embedded precursor decomposes when the coating temperature becomes higher than the precursor decomposition temperature. This is achieved directly during deposition of upper coating layers or during post-treatment. This decomposition generates tensile stresses within the structure and which lead to the formation of cracks developing in this case from the top to the bottom of the coating. Porosities as the ones shown in Fig. 10b result also from the entrapment of poorly treated droplets in the coating. The fraction of unpyrolyzed material has hence to be carefully controlled through the drop atomization and the spray pattern. To manufacture on the opposite a dense (and hard) coating under the same operating conditions, the fraction of poorly treated material in the jet fringes needs to be significantly reduced, if not suppressed. In this case, coating architectures as the ones presented in Fig. 11 are obtained. Those layers are reasonably dense (88% total pore level), hard (1023 VHN, average value) and exhibit no apparent intra-lamellar cracking. 4.2.2. SPS as manufacturing route An important point, already underlined [26], is the choice of the powder that must have a fairly narrow particle size distribution, whatever its average diameter (d50). Moreover, the powder must not form agglomerates (or at least as less as possible). The SPS route will be illustrated considering two powders processed with highly fluctuating (“restrike” mode) plasma jets: an Ar–H2 (ΔV/Vm = 1.4) plasma flow already depicted and a ternary Ar–H2–He (ΔV/Vm = 0.7) plasma flow (Ar: 40 SLPM, H2: 10 SLPM, He: 50 SLPM, I: 500 A, h: 22.3 MJ kg− 1). The powder considered here is a TZ-8Y from Tosoh of particle size distribution ranging from 30 to 80 nm

Fig. 10. Intra-lamellar cracked zirconia coatings deposited by SPPS with an Ar–H2 plasma produced by a 9-MB Sulzer-Metco (Wohlen, Switzerland) d.c. plasma torch [14] (a) general view, b) detail).

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Fig. 11. Dense and hard zirconia coatings deposited by SPPS with an Ar–H2 plasma using a capillary atomizer [14].

(supplier data). The particles have the tendency to agglomerate or aggregate, modifying the apparent particle size distribution (Fig. 12a) which ranges from 50 nm to 3 μm (laser particle size analysis). The lamella average diameter distribution displayed also in Fig. 12 evolves from 0.3 and 2 μm. Considering a flattening ratio between 1.5 and 2.5 [26], this signifies that all

Fig. 13. Coating architecture manufactured with an Ar–He plasma jet (powder: UC001h from Unitec Ceramic — plasma jet: Ar: 30 SLPM, He: 30 SLPM, I: 700 A, anode nozzle internal diameter at exit: 6 mm, liquid injection velocity: 26 m s− 1 average voltage Vm: 40 V, relative voltage variation ΔV/Vm: 0.3) [20].

particles smaller than 0.2 μm, average value, have either not been treated (melted) and have rebounded upon impact on the substrate or have been vaporized. However, spheres with characteristic sizes ranging from 0.1 to 0.7 μm (average size of 0.3 μm) are collected onto the substrate. Those spheres very likely result from the recondensation of vaporized small particles. The coating architecture displayed in Fig. 12b exhibits a high level of porosity mostly defined by columnar structures developing through the layer thickness, each column presenting 10 to 20 μm characteristic size. In the porous zones of 1 μm characteristic dimension, many small spherical particles of 0.2 to 0.3 μm characteristic size can be detected. Once again, such a typical structure is very likely linked to the vaporization of particles of 0.2 to 0.3 μm, average sizes, within the plasma warm core and to untreated particles traveling in the jet fringes and being poorly treated (this poor treatment being itself emphasized by the plasma jet fluctuations). Moreover, since no lamella of size larger than 2 μm is detected within the structure, this signifies that large agglomerates and aggregates have “exploded” with the flow upon heating. Finally, Fig. 13 displays the coating architecture manufactured with an Ar–He plasma jet operated under operating conditions listed in Fig. 5 caption. In the present case, the considered powder (UC001h from Unitec Ceramic, Manchester, United Kingdom) exhibits a particle size distribution ranging from 0.03 to 0.29 μm (with a few agglomerates). The suspension was injected into the plasma flow at a velocity of 33.5 m s− 1 making it possible to penetrate in plasma torch center. Under such operating parameters, the coating pore level is about 3.9% and no evidence of connected porosity was detected. 5. Conclusions

Fig. 12. a) Distribution by number of the Y-PSZ TZ-8Y Tosoh powder particles in the suspension to be compared with the distribution by number of lamellae and spherical particles collected onto a polished substrate at a spray distance of 40 mm, b) resulting coating architecture manufactured with an Ar–H2 plasma (h: 17.9 MJ kg− 1) [26].

Since about a decade, the interest to manufacture on large surfaces “thick” finely structured or nano-structured layers has been increasingly growing. If nano-structured architectures can be manufactured by gas condensation routes (CVD, PE-CVD, PVD, EB-PVD, etc.), their thicknesses can hardly be higher than a few micrometers. On the contrary, plasma-spray coating

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thicknesses between 50 and a few millimeters are easily achieved but with no nano-structured architectures after particle melting. This explains the interest for suspension plasma spraying (SPS) and solution precursor plasma spraying (SPPS), both allowing achieving finely structured layers of thicknesses varying between a few micrometers up to a few hundred of micrometers. As the deposition rate can reach one fifth to one fourth of the one encountered in conventional plasma spraying, these techniques seem promising to manufacture dense or porosity controlled coatings and functionally graded layers. Nevertheless, compared to conventional plasma spraying, SPS and SPPS are by far more complex because fragmentation and vaporization of the liquid control the processes. Numerous studies are still necessary to reach a better understanding of involved phenomena and for that, the development of diagnostic techniques to quantify either the droplets or particles in flight temperature and velocity or to visualize the droplets/ plasma interactions must be improved or newly developed. References [1] C.C. Koch, Nanostructured material-processing, properties and applications, Noyes Publications, William Andrew Publisher, Norwich, NY, USA (2002). [2] J. Karthikeyan, C.C. Berndt, J. Tikkanen, S. Reddy, H. Herman, Mater. Sci. Eng. A 238 (2) (1997) 275–286. [3] M. Gell, Mater. Sci. Eng. A 204 (1–2) (1995) 246–251. [4] D.J. Branagan, M.C. Marshall, B.E. Meacham, L.F. Apriliano, R. Bayler, E.J. Lemieux, T. Newbauer, F.J. Martin, J.C. Farmer, J.J. Haslan, S.D. Day, in: B.R. Marple, M.M. Hyland, Y.C. Lau, R.S. Lima, J. Voyer (Eds.), Building on 100 Years of Success: Proceedings of the 2006 International Thermal Spray Conference, CD-Rom, Pub. ASM International, Materials Park, OH, USA, 2006, ISBN: 0-87170-809-4. [5] D.J. Branagan, M.C. Marshall, B.E. Meacham, in: E. Lugscheider (Ed.), Thermal Spray Connects: Explore Its Surfacing Potential! Pub. DVS-Verlag GmbH, 40223 Düsseldorf, Germany, 2005, ISBN: 3-87155-793-5. [6] R.S. Lima, B.R. Marple, J. of Therm. Spray Tech. 16 (1) (2007) 40–63. [7] M. Vardelle, A. Vardelle, P. Fauchais, K.-I. Li, B. Dussoubs, N.J. Themelis, J. of Therm. Spray Tech. 10 (1) (2001) 267–286. [8] P. Fauchais, V. Rat, C. Delbos, J.-F. Coudert, T. Chartier, L. Bianchi, IEEE Trans. On Plasma Sci. 33 (2) (2005) 920–930. [9] J. Fazilleau, C. Delbos, V. Rat, J.-F. Coudert, P. Fauchais, B. Pateyron, Plasma Chem. Plasma Proc. 26 (4) (2006) 371–391. [10] P. Fauchais, R. Etchart-Salas, V. Rat, J.-F. Coudert, N. Branland, K. Wittmann-Teneze, J. of Therm. Spray Tech. 17 (1) (2008) 31–59. [11] T. Bhatia, A. Ozturk, L.D. Xie, E.H. Jordan, B.M. Cetegen, M. Gell, X.C. Ma, N.P. Padture, J. of Mat. Res. 17 (9) (2002) 2363–2372.

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