Surface & Coatings Technology 220 (2013) 219–224

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Surface & Coatings Technology

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Process development and coating characteristics of plasma spray-PVD

Georg Mauer ⁎, Andreas Hospach, Robert VaßenForschungszentrum Jülich GmbH, Institute of Energy and Climate Research, IEK-1: Materials Synthesis and Processing, 52425 Jülich, Germany

⁎ Corresponding author. Tel.: +49 2461 61 5671; faxE-mail address: g.mauer@fz-juelich.de (G. Mauer).

0257-8972/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.surfcoat.2012.08.067

a b s t r a c t

a r t i c l e i n f o

Available online 4 September 2012

Keywords:Low pressure plasma sprayingPS-PVDOptical emission SpectroscopyVapor deposition

Plasma spray physical vapor deposition (PS-PVD)wasdevelopedwith the aimof depositing uniformand relative-ly thin coatings with large area coverage. At high power input (~150 kW) and very low pressure (~100 Pa) theplasma jet properties change considerably compared to conventional plasma spraying and it is even possible toevaporate the powder feedstock material enabling advanced microstructures of the deposits. This relativelynew technique bridges the gap between conventional plasma spraying and physical vapor deposition (PVD).Moreover, the resulting microstructures are unique and can hardly be obtained by other processes.In this paper, plasma characteristics of different gas mixtures are investigated. The measurements and calcu-lations provide indications of the growth modes and help to explain the resulting microstructures and coat-ing chemistries. Coatings sprayed from different ceramic powders are discussed.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Plasma spraying (PS) has been well established in industry overthe last decades. In this process, the coating material is injected intoa hot plasma in the form of a powderous feedstock, then meltedand accelerated. When these particles impact the substrate, the mate-rial solidifies and builds up into a typical lamellar structure. Suchcoatings usually do not need any post-treatment after spraying.These techniques are continuously enhanced. A new spray processcalled Plasma Spray-Physical Vapor Deposition (PS-PVD) [1] was de-veloped as an extension of the low pressure plasma spray process(LPPS, also known as vacuum plasma spraying, VPS). Due to the lowchamber pressure at LPPS, the deposition profile is broadened andmore homogeneous so that the coating area is enlarged and relativelythin and dense ceramic coatings can be obtained for special applica-tions like solid oxide fuel cells [2], gas separation membranes [3]and wear protection [4]. Beyond that, by increasing the plasmapower, the feedstock powder can be even evaporated so as to depositfrom gas instead of liquid phase.

PS-PVD is intended to combine the advantages of thermal spraying(high deposition rates and cost-efficiency) with the special featureof PVD-type processes as to deposit columnar structured coatings[5] or dense, gas-tight coatings as well. This opens new applicationfields in which neither plasma spraying nor PVD processes were ableto become accepted up to now.

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2. Experimental

All coating experiments were carried out on a Multicoat system(Sulzer Metco, Wohlen, Switzerland), which is in our case anupgraded LPPS facility. To achieve the low working pressure of100 Pa and a power input of 150 kW, the vacuum pump system andthe power supply had to be replaced completely. Since the standardF4-VB plasma torch cannot be operated at such higher power, thetorch was changed to the O3CP. The samples presented here weresprayed with a power input of 110–140 kW and a plasma gas flowof 120–140 slpm (using helium and argon as well as argon and hydro-gen) at 200 Pa chamber pressure. Applying these parameters the life-time of the O3CP torch reaches between 40 and 100 operating hours,depending on the numbers of ignitions.

Two spraying powders were used for the coatings discussed here. Atitania powder fromH.C. Starck (Goslar, Germany)with a d10 and d90 of5 and 20 μm, respectively, as well as an agglomerated zirconia powderwith 7 wt.% yttria from Sulzer Metco (Switzerland) with a d10 and d90of 2 and 18 μm, respectively. The plasma plume is used to heat thesubstrate; the temperature is controlled by sweep movements of theplasma jet over the substrate or the plasma parameters. Because ofthe largeness of the plasma plume at 200 Pa no meander movementof the torch was necessary for sample dimensions below 100 mm.

For plasma characterization, two optical emission spectrometerswere used. One was the MCS 601 UV-NIR spectrometer (Carl ZeissMicroImaging, Göttingen, Germany) in the 190 nm to 1020 nmwavelength range with a spectral pixel dispersion of approximately0.8 nm per diode. Between 220 nm and 1000 nm the spectral resolu-tion was 3 nm. This is sufficient to identify qualitatively the emittinggas species. As the spectra had to be evaluated also quantitatively

Table 1Estimation of the plasma temperatures of different plasma gas compositions based onspecific enthalpies at the same plasma net power.

Plasma gas flow, slpm 100 Ar, 10 H2 35 Ar, 60 He 35 Ar, 60 He, 10 H2

Molar flow, mol min−1 4908 4291 4685Net power, kW 60 60 60Enthalpy, J mol−1 733,496 838,965 768,410Temperature, K ~12,860 ~15,550 ~14,240

Fig. 2. Specific enthalpy vs. temperature of different plasma compositions; the X-markedpoints indicate the enthalpies and temperatures of the investigatedplasmagas compositions.

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to establish Boltzman plots, the second applied spectrometer Spectrelle20000 (GWU-Lasertechnik Vertriebsges. mbH, Erftstadt, Germany)had an enhanced spectral resolution of 0.017 nm. It was equippedwith a 1024×1024 element CCD array and covered the wavelengthrange from 365 nm to 705 nm.

3. Results and discussion

3.1. Plasma gas composition

Fig. 1 shows photographs of PS-PVD plasma jets using differentgas compositions; on the left without any powder, on the right withYSZ powder injection. In all cases, the chamber pressure and netpower input were kept constant at 200 Pa and 60 kW, respectively.It is evident that the supplement of hydrogen affects in particularthe radial temperature distribution in the jet as indicated by the dis-tribution of radiation intensity. The addition of helium concentratesthe jet due to its high viscosity [6]. The images on the right demonstratethe influence of the gas mixture on the particle trajectories which canbe identified by the bluish radiation characteristic for zirconium. It isobvious that helium is important not only to concentrate the plasmagas flow but also the particle plume. As hydrogen broadens the temper-ature distribution its addition affects the particle heating.

The significant influence of theplasmagas composition in the thermalhistory of the particles is confirmed by the large variety of microstruc-tures which can be deposited. Dense and gas-tight coatings, columnarstructured coatings from nano-sized clusters and vapor phase are possi-ble. In certain cases, combined splat deposition is observed aswell as em-bedded particles having already solidified in-flight. Examples are givenin a previous work [7].

Columnar microstructures as intended for thermal barrier coatingswere deposited from nano-sized clusters as well as from evaporated

Fig. 1. Photographs of PS-PVD plasma jets using different gas compositions; on the left witpower input were kept constant at 200 Pa and 60 kW in all cases; the numbers denote the

feedstock using an Ar/He gas mixture, while adding H2 instead ofHe the amount of non-vaporized droplets (= cluster, splats) in-creased [8]. There are two explanations for this effect. On one hand,

hout any powder, on the right with YSZ powder injection. Chamber pressure and netargon, helium and hydrogen flows (slpm) as well as the plasma currents (A).

Fig. 3. Radial cross sections of specific line emission through the plasma jet in 1 m spray distance obtained by optical emission spectroscopy; plasma gas composition A) 100 slpmAr, B) 100slpm Ar plus 10 slpm H2, C) 35 slpm Ar plus 60 slpm He, D) 35 slpm Ar plus 60 slpm He plus 10 slpm H2. Emission lines are at 811.6 nm (Ar), 587.6 nm (He) and 656.3 nm (H).

Fig. 4. Radial cross sections of specific line emissions through the plasma jet in 1 m spray distance obtained by optical emission spectroscopy; same compositions and sprayingdistance as for Fig. 3 but with injection of YSZ powder (2×10 g min−1). Emission lines are at 811.6 nm (Ar), 587.6 nm (He), 656.3 nm (H) and 423.1 nm (Zr).

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Fig. 5. Optical emission intensities of one powder emission line (Zr 360.1 nm) for avariation of the powder feed rate resulting in different deposition modes.

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based on the specific enthalpies of the investigated plasma gas compo-sitions, the plasma temperature of the Ar/H2 plasma can be estimatedto be considerably lower than the one of the Ar/He plasma, Table 1and Fig. 2. These temperatures correspond to the location of powderinjection inside the nozzle where the feedstock is suggested to bemain-ly heated.

The second explanation why the Ar/H2 hydrogen plasma showsa lower vaporization degree of the powder was already mentionedin the context of Fig. 1. Obviously, the Ar/H2 plasma jet is visuallybroader than for Ar/He, particularly close to the nozzle exit. Further-more, demixing of the constituents takes place due to frictional forcesand thermal diffusion [9]. This can be elucidated by specific line emis-sion measurements obtained by optical emission spectroscopy forthe investigated plasma gas compositions, Fig. 3. They were recorded

Ar-He 7YSZ

20 g/min

20 g/min

2 g/min

Fig. 6. Overview (SEM images) on PS-PVD manufactured coatings with different microstru

along radial cross sections through the plasma jet at 1 m distancefrom the nozzle exit. They were not Abel inverted.

The profiles show slightly asymmetric shapes and are less pro-nounced for the plain Ar plasma. The addition of H2 results in a con-centration of Ar towards the jet axis. When adding He, the otherspecies are displaced to the edge of the jet while He is focused inthe center. It is obvious, that there is some kind of separation of theplasma constituents. In Fig. 4, the analogue profiles are shown, how-ever with YSZ powder injection. Besides injection conditions, thestrong influence of the plasma gas composition on the feedstock dis-tribution and thus on feedstock transport and treatment is evident.

These figures show one representative emission line for eachconstituent. Previously, a lot of work was done to find representativelines which show the same tendency like the sum of them. Many otherlines showed different or contrary trends due to absorption effects.

3.2. Powder feed rate

The variation of the powder feed rate given in Fig. 5 shows thatthe use of more powder results in more vaporized feedstock (higheremission intensity) until a certain amount is reached. Injecting morethan 2×10 g min−1, the powder absorbs locally so much energy andcools down the plasma that such high powder feed is only insufficientlymelted and vaporized. Here, the amount of vapor decreases with furtherincrease of the powder feed rate (= load effect). This it is not an effectdue to shadowing the plasma by the powder itself, which was cross-checked by using a higher plasma power. In this case the maximum in-tensity was shifted to higher powder feed rates (However, the perma-nent use of higher plasma power was not favorable because thelifetime of the torch electrodes is shortened considerably). If all thepowder would be vaporized also at higher feed rates like this, a linearslope should be the result which is not the case. Modifications of the

Splat/cluster/vapor deposition

Cluster/vapor deposition

Vapor deposition

ctures on stainless steel substrates having specific advantages for various applications.

Fig. 8. Columnar, tetragonal YSZ coating (SEM image) deposited by vapor deposition onthe front side of the sample; parameter 35 slpm Ar, 60 slpm He, current 2600 A, powderfeed rate 2×0.5 g min−1, spray distance 1000 mm, substrate temperature ~1500 °C.

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injector geometry to obtain a more efficient heating in the nozzlewere not tried within the present work.

The evaluation of the same line emission intensity at differentcarrier gas flows advised the use of a lower carrier gas as the maxi-mum intensity was found for 2×12 slpm instead of 2×16 slpm. Butusing 2×12 slpm, partly unsteady powder flow was visible resultingin a higher amount of non-vaporized powder. Thus it was decidedto apply the 2×16 slpm again and to accept the slight over-injection.

3.3. Coating microstructures

Based on the new and systematic investigation of the processconditions presented above, the vaporization degree of the powdercan be controlled immediately by setting the plasma gas compositionand the powder feed rate. This enables to develop a broad spectrumof different microstructures with specific advantages for various ap-plications, Fig. 6.

• A very low vaporization degree can be achieved using a cold andbroadened plasma (Ar/H2) combined with a high powder feed rate(40–80 g min−1), resulting in gas-tight splat deposition (splat sizein the μm range). Such coatings may be applied as insulation layersor gas-tight electrolytes or gas separation membranes since theycan be deposited also on porous substrates. Measured He-leakagerates of coating samples as given in Fig. 7 were smaller than10−3 mbar ℓ s−1 cm−2.

• A medium vaporization degree can be obtained using a cold andbroadened plasma (Ar/H2) combined with a medium powder feedrate (20 g min−1), resulting in a mixed splat and vapor deposition.

• Ahigh vaporization degree is attained by a high-enthalpy and concen-trated plasma (Ar/He) combined with a medium powder feed rate(20 g min−1), resulting in new and unique gas tight or columnar de-position of nanosized solid clusters (= quasi PVD)with very high de-position rates (~100 μm min−1) and out-of-sight-deposition.

• A very high vaporization degree can be realized by a hot and focusedplasma (Ar/He) combined with a low powder feed rate (~2 g min−1),resulting in vapor deposition (= real PVD) with comparably highgrowth rates (~10 μm min−1) and non-line-of-sight-deposition. Incontrast to splat or cluster structures such coatings show pronouncedtextured growth as revealed by XRD. Fig. 8 shows an example of avapor deposited, columnar structured YSZ coating (tetragonal phase).

3.4. Coating chemistry

Applying very low powder feed rates, not only the growth moduschanges from cluster to vapor deposition, but also the chemical com-position of the coatings can change. Without addition of oxygen intothe plasma chamber, a strong oxygen loss was noticed in the deposited

Fig. 7. Gas-tight, dense YSZ layer (laser-microscopic image) deposited on a porous metalsubstrate (Plansee ITM14); parameter 100 slpm Ar, 12 slpm H2, current 2400 A, powderfeed rate 2×20 g min−1, spray distance 300 mm, torch sweep angle±24°, substrate tem-perature ~800 °C.

coatings (well known for PVD processes). We found that the oxygencontent of coatings sprayed from oxide powder (e. g. YSZ) decreasedwith shorter spraying distances. Spraying YSZ at extreme conditionsvery close to the nozzle (0.3 m, 35 slpm Ar, 60 slpm He, 2×1 g/min)on graphite substrates (substrate temperature ~2100 °C), the coatingeven showed a metallic gloss. XRD results showed ZrC and minor Zr re-flexes. To make sure that this reduction was not the effect of the graph-ite substrate, we sprayed YSZ on tungsten substrates and addednitrogen in a further test. Examples are given in the upper part of Fig. 9.

The XRD results revealed that besides diffraction patterns of thetungsten substrate, also ZrW2 and Zr peaks are visible indicatingthat zirconium reacted with the tungsten substrate. Adding nitrogento the plasma gas (5 slpm), the coating consisted of ZrN (also minortungsten reflexes were visible originating from the substrate). Thissupports the assumption that there must have been free metallic Zrpresent in theplasma; else ZrNwould not have been formed. Dependingon the availability, the Zr atoms reacted to ZrC, ZrW2 or ZrN. Similarreactions are known from carbonitriding treatment of zircon [10].

A comparable effect was found when spraying very small amountsof titania powder on graphite substrates at a long spraying distanceof 1.5 m (35 slpm Ar, 60 slpmHe, 2×1 g/min). The substrate tempera-ture was approximately 1250 °C. Two samples are shown in the lowerpart of Fig. 9. Introducing 8 slpm oxygen to the vacuum chamber, arutile coating was deposited. With addition of 5 slpm nitrogen to theplasma gas, the XRD investigation revealed TiN reflexes. No TiC was

Fig. 9. Above, deposition of ZrO2−x (spray distance 1 m) and Zr+ZrC (spray distance0.3 m), coating thicknesses ~50 μm; below, deposition of TiO2 (with 8 slpm O2 additioninto chamber), TiO2−x (without any addition) and TiN (5 slpm N2 addition to plasmagas) on graphite substrates, coating thicknesses ~10 μm(spray distance1.5 m ineach case).

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created in contact with the graphite substrate because the titaniumobviously oxidized already during flight. Adding neither oxygen nor ni-trogen, suboxides of titania were identified, confirming that part of theoxygen content in the feedstock gets lost during spraying.

4. Conclusion

Based on previous work on PS-PVD, plasma conditions and feed-stock evaporationwere investigated systematically. In the experiments,

optical emission spectroscopy turned out to be an effective tool to char-acterize the plasma jet as well as to tune process parameters like pow-der feed rate and carrier gas flow.

Examples of sprayed coatings show the variability of PS-PVD regard-ing their microstructure and chemistry. On the one hand, dense,gas-tight coatings open up new applications like electrolyte layers orgas separation membranes. On the other hand, columnar structured,strain tolerant thermal barrier coatings can be obtained. Non-line ofsight deposition enables also complex shaped parts to be coated. Incase of vapor deposition, textured growth of columns is achieved show-ing similar characteristics like those being deposited by electron beamevaporation (EB-PVD), however at considerably higher deposition rates.

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