Quantification of void networks of as-sprayed and annealed nanostructured yttria-stabilized zirconia (YSZ) deposits manufactured by suspension plasma spraying

Surface & Coatings Technology 205 (2010) 683–689

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

Quantification of void networks of as-sprayed and annealed nanostructured yttria-stabilized zirconia (YSZ) deposits manufactured by suspension plasma spraying Antoine Bacciochini a,⁎, Fadhel Ben-Ettouil a, Elodie Brousse a, Jan Ilavsky b, Ghislain Montavon a,⁎, Alain Denoirjean a, Stéphane Valette a, Pierre Fauchais a a b

SPCTS, UMR CNRS n°6638, Faculté des Sciences et Techniques, Université de Limoges, 123 Avenue Albert Thomas, 87060 Limoges cedex, France Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA

a r t i c l e

i n f o

Article history: Received 20 January 2010 Accepted in revised form 6 June 2010 Available online 11 June 2010 Keywords: Suspension plasma spraying YSZ Void network Nanovoid Annealing USAXS

a b s t r a c t Suspension plasma spraying (SPS) allows processing a stabilized suspension of nanometer-sized feedstock particles to form thick (from 20 to 100 μm, average values) deposits. The void content and porous network of such deposits are difficult to quantify (in terms of void and size distributions, anisotropy, etc.) using conventional techniques due to their low resolution. The combination of ultra-small-angle X-ray scattering (USAXS) and helium pycnometry permits to address some of the characteristics of this void network. Deposits of yttria-partially stabilized zirconia (YSZ) were manufactured by plasma spraying a suspension made of solid sub-micrometer-sized particles (50 and 400 nm) with several sets of spray operating parameters. Results indicate that the average void size exhibits the same scale as the solid structure; i.e., nanometer sizes and multimodal size distribution which varies with spray operating parameters. About 90% of voids (by number) exhibit characteristic dimensions smaller than 40 nm. The cumulative void volume fraction of such as-sprayed deposits varies between about 13 and 20%, depending upon operating parameters. The void network architecture evolves also with annealing conditions: the void size distribution evolves toward higher void characteristic dimensions as a result of sintering of smallest voids but the cumulative void content does not decrease significantly. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Among the different possible routes to produce finely (i.e., submicrometer to nanometer scale) structured deposits, suspension plasma spraying (SPS) appears as one of the most flexible processes [1]. Indeed, SPS is an alternative to conventional atmospheric plasma spraying (APS) to produce thinner deposits (i.e., 10 to 100 μm) while using fine feedstock particles — from few tens of nanometers to about one micrometer. SPS spraying involves the injection within the thermal plasma flow of a suspension made of the feedstock particles, a liquid phase and a dispersant. This suspension can be injected either as a continuous liquid stream or as drops of a few hundreds of micrometers. Upon penetration within the plasma flow, the liquid stream or drops encounter two mechanisms, fragmentation and

⁎ Corresponding authors. Bacciochini is to be contacted at Tel.: + 33 5 55 45 75 40. Montavon, LERMPS – EA3316, Université de Technologie de Belfort-Montbéliard, site de Sévenans, 90010 Belfort cedex, France. Tel.: + 33 3 84 58 31 61. E-mail addresses: [email protected] (A. Bacciochini), [email protected] (G. Montavon). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.06.013

vaporization [2]. In a first approximation and whatever the suspension stream/drop characteristic dimension (from a few micrometers to a few hundreds of micrometers in diameter), the fragmentation duration is at least two orders of magnitude shorter than the liquid phase vaporization [3]. Droplets resulting from fragmentation encounter then liquid phase evaporation that leads to the formation of a single particle or aggregates of a few particles, depending upon the route solid particles have been manufactured. Then, these particles melt and form liquid particles that impact, spread and solidify to form flattened lamellae of equivalent diameters ranging from a few hundred nanometers to a few micrometers with average flattening ratio varying from 1 to 2 [4]. The deposits result from the stacking of such lamellae. Their architecture evolves from relatively dense to very porous, with a smooth (homogeneous) or irregular (heterogeneous) surface morphology, according to the operating parameters among which plasma power parameters (plasma torch operating mode, plasma flow mass enthalpy, spray distance, etc.), suspension properties (particle size distribution, powder mass percentage, viscosity, surface tension, etc.), and substrate characteristics (topology, temperature, etc.) play relevant roles [5–7].

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Quantification of the void network of such deposits is fundamental since it controls most of their thermomechanical properties (i.e., apparent Young modulus, tightness, apparent thermal conductivity, etc.). Void network quantification usually includes: i) the void total content, often typified as porosity, ii) the void size distribution, iii) the discrimination by void shape (globular, cracks, delaminations) and iv) the void connectivity, to the surface on the one hand (open void) and to the substrate on the other hand (connected void). Several methods are commonly implemented to quantify voids at the micrometer scale in a porous medium such as thermal spray coating [8]:

700 nm for feedstock identified here as UC02. Average diameters (d50) were 50 and 400 nm for UC01 and UC02, respectively. These feedstock powders were manufactured by cryomilling and particles exhibit typical angular shapes. An electrosteric dispersant developed in house was used to reach the optimum suspension dispersion corresponding to the lowest suspension viscosity [2]—around 2 mPa s at room temperature. After mixing powder with liquid phase and dispersant, the resulting suspension was sonicated (250 W, average power) for several minutes in order to disperse the remaining agglomerates.

i) physical methods such as Archimedean [9] and mercury intrusion [10,11] porosimetries and helium pycnometry [12]. The main drawbacks of these techniques are that measurement is not direct and that the impregnation of the whole network is not guaranteed. Two main limitations impede the accuracy/ reliability of such methodologies, on the one hand the high pressure applied to liquid for percolating through the deposit when considering MIP which can induce material failure and on the other hand the minimum sample volume to be considered for accurate analyses; ii) stereological protocols allowing quantifying deposit void content vs. void size distribution [13,14], regardless of the network connectivity. An appropriate magnification must be determined to include in analyses as fine as possible details (i.e., microcracks) using adequate image resolution to relate to the representative elementary volume (REV) of the structure [15]. Two main limitations impede the accuracy/reliability of these methods, the limited resolution on the one hand which makes almost impossible to take into account features smaller than 0.1 μm, average value, and the artifacts (i.e., pull-outs, scratches, etc.) on the other one resulting from cutting and polishing steps [16–18]; iii) electrochemical methods in particular electrochemical impedance spectroscopy (EIS) technique consisting in measuring the impedance of the electrochemical cell with the immersed coating/substrate interface behaving as the working electrode [19–21]. The simplistic test of de-ionized water droplet permits to determine the smallest open void diameter into which the water is able to percolate, merely using Washburn's equation [22]. For example, contact angle, θ, between zirconia and deionized water was measured to be about 59 deg. Surface energy, γ, of de-ionized water is 72.8 mN m−1 at room temperature. At atmospheric pressure (about 105 Pa), pure water percolates consequently into open voids of equivalent diameter equal or larger than 1.5 μm. Consequently, EIS analysis is not hence suitable to quantify the connectivity of SPS deposits since the solution cannot percolate through the coating void network.

2.1.2. Spray parameters So-called mechanical injection used in this study consists in a pressurized container in which the liquid is stored and forced through an injector of d0 = 150 μm internal diameter. At the injector exit, a liquid jet of diameter 1.9 × d0 is generated [24]. After a certain distance corresponding to about 160 times the liquid stream average diameter, Rayleigh–Plateau type instabilities develop and drops are generated by primary atomization [25]. The injector was fixed on the torch nozzle in such a way that the suspension liquid stream penetrates counter-flow the plasma jet prior to its fragmentation by primary atomization. The pressure applied to the suspension container was about 0.5 MPa leading to a liquid stream velocity of about 22 m s−1 at injector exit [25]. An F4-MB torch (Sulzer-Metco, Wohlen, Switzerland) equipped with a 6 mm internal diameter anode nozzle was used to process the suspension and manufacture the coatings. Power and injection parameters were optimized in previous works [26], Table 1. No specific sample-cooling device was implemented during spraying. Substrates were fixed on rotating sample holder. The average thickness deposited per pass was about 2 μm.

Limits of above listed conventional protocols do not allow all of them to be applied successfully to study the void content of thick SPS YSZ deposits manufactured from nanometer-sized (50 nm, average value) feedstock [23]. The objectives of this work is to address the void content of such assprayed and annealed deposits by a combination of two other techniques, ultra-small-angle X-ray scattering (USAXS) and He pycnometry. 2. Experimental set-ups 2.1. Coating deposition and annealing 2.1.1. Suspensions The suspension was made of ethanol (95% purity) with 20 wt.% of YSZ (13 wt.% of Y2O3) from Unitec Ceramic (Stafford, UK). Two different particle size distributions (d10–d90) were used: i) from 30 to 290 nm for feedstock identified here as UC01 and ii) from 250 to

2.1.3. Substrates Low carbon steel button-type substrates 25 mm in diameter and 20 mm in thickness were used for structural characterizations. Mechanical anchorage is assumed to be the adhesion mechanism of coatings onto substrates. Mechanical anchorage requires substrate average roughness to be about the same order of magnitude than flattened lamellae thickness. This is why with the considered particle sizes, substrates were pre-polished using SiC papers of various grades and polished using diamond suspensions of various average diameters until specular finishing was reached (this means that the sample average roughnesses were lower than 0.1 μm, average value). 2.1.4. Annealing Free-standing deposits were subjected to annealing in air at atmospheric pressure (they were at first removed from the substrate by substrate acid pickling in HCl–HNO3 mixture (50-50% in volume)). Samples sprayed with 30 mm spray distance for both studied plasma gas mixtures (Ar–He and Ar–H2) were selected for annealing experiments. Samples were annealed at 800 or 1100 °C and times of 10 or 100 h. Table 1 Optimized SPS spray operating parameters. Type

Sulzer Metco F4-MB

Anode internal diameter at plasma torch exit (mm) Plasma gas mixture Plasma gas flow rate (L min−1) Arc current intensity (A) Plasma flow average mass enthalpy (MJ kg−1) Spray distance (mm) Spray velocity (m s−1) Scanning step (mm per pass)

6 Ar–He 40–20 500 12 30 to 50 1 10

Ar–H2 55–5 15

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2.2. Microstructure characterization 2.2.1. Scanning electron microscopy Jeol JSM-7400F FESEM (Field Emission Scanning Electron Microscope) was used for fractography facieses analyses of free-standing manually fractured YSZ deposits. 2.2.2. Helium pycnometry Helium pycnometry allows measuring the true volume of samples. Indeed gas atoms form a single layer on the total exposed surface area of material, including open and connected voids. This technique has already been successfully employed to characterize plasma coatings apparent densities [27]. An AccuPyc 1330 pycnometer (Micromeritics, Norcross, GA, USA) was used to carry out those measurements. 2.2.3. Thermal diffusivity measurements Thermal diffusivity measurements in the 20–250 °C range were carried out with a LFA-447 nano-Flash device (NETZSCH, Selb, Germany), using a Xe flash lamp of 10 J per pulse of 18 ms duration operating in the 25–2000 nm wavelength range. In this device, thermal flux transmitted through the system composed of first the substrate and then the YSZ deposit is collected and measured by an IR sensor (Indium antimonide, InSb) cooled by liquid nitrogen as a function of sample temperature measured with a K-type thermocouple. A Cowan-type model was used to calculate the resulting thermal diffusivity [28]. Mean values and associated standard deviations result from the average of 15 measurements at each studied temperature. 2.2.4. Ultra-small-angle x-ray scattering High energy photon beams allow measuring the coatings' void content and size distribution by scattering of incident X-ray beam. A wide range of techniques was developed for material characterization using small-angle scattering, varying in instrumentation and/or methods used [29–31]. Ultra-small-angle X-ray scattering (USAXS) measurements were conducted on beamline 32-ID at Advanced Photon Source (Argonne National Laboratory, Argonne, IL, USA) [32]. At this beamline, a Bonse–Hart double crystal diffraction optics allows recording USAXS scattering curves (SC) with an angular resolution of 0.0001 Å−1 at a q (scattering vector) range from 10−4 to 1 Å−1 [33]. The 32-ID beamline includes undulator A, a double crystal monochromator (using Si 111 crystals) for photon energy selection and two mirrors for harmonic rejection. This set-up delivers approximately 1013 photons per second in a 1 mm2 area at the sample position, over wide range of energies. For current experiment, the selected energy of incident photons was 16.9 keV, just below the absorption edge of yttria. USAXS data are fully corrected for all instrument effects and analyzed using Igor Pro1 software from WaveMetrics Inc. (Oswego, OR, USA) coupled to Irena1 package for analyzing small-angle scattering data. 3. Results and discussion 3.1. As-sprayed coatings architecture The stacking of those non-elongated particles generates defects as in the case of atmospheric plasma spraying but at a reduced scale and with fewer classes of defects. Indeed, deposit seems to be inter- and intra-lamellar crack-free through-thickness. Fig. 1 depicts the specific structures of SPS coatings manufactured at 50 mm (Fig. 1A) and 30 mm (Fig. 1B) spray distances. Void size distribution can be obtained as a result from USAXS data by a relatively straightforward analysis using commonly accepted methods [34], in particular maximum entropy method onto which 1 These software packages can be accessed at http://www.wavemetrics.com/ and http://www.usaxs.xor.aps.anl.gov/staff/ilavsky/irena.html, respectively.

Fig. 1. Fractography of SPS coatings manufactured with a F4-MB plasma torch operated at 23 kW with Ar 66% wt. He plasma and with UC02 powder; A) 50 mm and B) 30 mm spray distances.

"Sizes" tool in Irena package relies. Void network is then quantified by the total void content [%], the void mode diameter [nm] which represents the void diameter range that includes the highest number of voids and finally the void median diameter [nm] which depicts asymmetric distribution better than the mean value. Fig. 2A displays a typical scattering curve fitted and analyzed of as-sprayed YSZ SPS deposit. Over the considered operating window (see Table 1), the total void content is between 12.9% and 20.6%, with median diameters ranging from 270 to 400 nm and most of the voids (in number) of characteristic dimension (i.e., mode diameter) smaller than 20 nm (Table 2), contrary to micrometer-sized plasma sprayed coatings which do not contain significant contribution of nanovoids smaller than 20 nm, as shown in 1997 by Ilavsky et al. on alumina coatings [35]. Other data can be extracted by deconvolution of the size distributions by Gaussian functions: they correspond to void size classes. Whatever the considered operating spray parameters and the feedstock size distribution, four void size classes can be systematically identified: [1–10], [10–20], [20–40] and [40–100] nm (cf. Fig. 2B). Note that the voids of average diameters smaller than 40 nm account for 90% of the voids by number, but their cumulative volume represents only 10% of the total void content by volume. The cubic relationship between the diameter and the volume of voids explains such a figure of course.

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Fig. 2. A) Typical scattering curve — deposits' void size distribution and B) deconvolution of void size distribution of SPS deposit manufactured with an F4-MB plasma torch operated at 23 kW with Ar 66% wt. He plasma at 30 mm spray distance with UC01 powder.

Those results permit to identify in particular the spray distance as a key parameter in tailoring deposit total void content. Indeed, the shorter the spray distance, the denser the coating (with a decrease of 1/5th to 1/3rd in void total content when decreasing the spray distance from 50 to 30 mm, whatever the other spray parameters). This trend can be fully explained by the high deceleration and low

Table 2 Characteristic void size resulting from USAXS measurements.

Powder average diameter [nm] Spray distance [mm] Total void content [%] Median diameter [nm] Mode diameter [nm]

Ar–He (12 MJ kg−1)

Ar–H2 (15 MJ kg−1)

50 (UC01)

390 (UC02)

50 (UC01)

390 (UC02)

50 19.5 343 7

50 18.4 395 5

50 20.6 345 7

50 19.3 370 7

30 15.5 297 6

30 14.3 268 6

30 16.1 362 6

30 12.9 344 6

thermal inertia of considered particles: increasing the spray distance leads to a strong decrease in particle momentum and temperature upon impact [36]. Surprisingly, no clear effect of feedstock average size on void content has been identified. Increasing the feedstock average size from 50 to 400 nm does not change significantly the coating total void content which evolves for example from 20.6% to 19.3% for an Ar–H2 plasma gas mixture (the average size of particles exhibits one order of magnitude difference). This phenomenon could be due to the fact that agglomerates of similar sizes are formed in suspension resulting in molten droplets of identical diameters upon impact. This, nevertheless, has to be further investigated. The effect of the plasma gas mixture on the deposit total void content is not significant in a first approximation since the relative difference is in the order of 1/10th when considering Ar–H2 and Ar– He plasma gas mixtures (absolute difference around 1%), respectively, other operating spray parameters remaining constant.

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Closed void content as measured by He pycnometry varies between 1 and 6% (absolute values) according to spray operating parameters. By subtracting the closed void content as measured by He pycnometry from the total void content as measured by USAXS, the open void content can be estimated and expressed in term of relative fraction. SPS nanometer-sized YSZ coatings exhibit the same fraction of open voids than APS micrometer-sized ones, between 0.6 and 0.8 of the entire volume of porosity. Nevertheless, the networks are different, since cracks have not been detected in SPS deposits whereas cracks (perpendicular to substrate surface) and delaminations (parallel to substrate surface) constitute most of the connected network in APS ceramic coatings. Considering the relative open void contents, Ar–He plasma gas mixture leads to values comparable to those obtained with Ar–H2 mixture. One can assume that the lower plasma flow enthalpy resulting from the Ar–He plasma gas mixture together with a lower coefficient of thermal transfer lead to lower particle average temperatures upon impact which induces in turn higher density of stacking defects. The apparent thermal diffusivity has been routinely studied on zirconia-based materials when used as thermal barrier coatings (TBCs) [37]. In this study, the measured thermal diffusivity between 50 and 250 °C (upper temperature limit of the implemented instrument) of nanometer-sized YSZ 67 μm thick deposits evolved from (1.50 ± 0.12) to (2.50 ± 0.22) × 10−7 m2 s−1 (average values and associated standard deviations from 15 measurements). One of the lowest thermal diffusivity values of YSZ plasma sprayed coating identified by authors is around 1.80 × 10−7 m2 s−1 at 1000 °C [38] and results from the processing of nanometer-sized agglomerated particles at the micrometer scale. In such a case, the strategy is to adjust plasma spray operating parameters in such a way that only the particles' outer shells are molten. Upon solidification, those molten shells form micrometer-sized area within the deposit to ensure its cohesion while the unmelted particles' inner cores will keep their nanometer-sized structure. The relatively low thermal diffusivity values measured for SPS coatings could be the consequence of the large number of nanometersized voids acting as thermal resistances in the coating. Due to the size of voids, rarefaction effect, known as Knudsen effect, also takes place and increases even further the void thermal resistances [39]. However, other mechanisms such as phonons scattering (due to nanometer-sized particles and oxygen vacancies within particles) [40] and thermal resistance at interfaces between grains play also very likely a relevant role in such low thermal diffusivity values. The relative contributions of each mechanism are nevertheless not yet known and have to be studied. 3.2. Annealed coatings' void networks After annealing at 800 °C for 100 h, deposit structure appears almost identical to the as-sprayed one (Fig. 3A), with the exception of some noticeable grain growth where particles were in perfect contact (i.e., without inter-lamellar voids) (Fig. 3B). A coarsening of the columnar grains extending across several lamellae is also noticeable. Nevertheless, the granular structure remains unchanged. After annealing at 1100 °C for 100 h, the structure change is more pronounced with radical grain growths and diffusion effects at grain boundaries between spherical and flattened particles (Fig. 3C). The columnar structure inside lamellae has totally disappeared to be replaced by nearly equiaxed grains. Nevertheless, small voids (b100 nm) are still embedded inside grains and at their boundaries. Annealed free-standing deposits' void contents were also analyzed by USAXS. The voids volume distributions are affected by annealing and noticeable changes are found for annealing temperatures as low as 800 °C and ageing times of 100 h (Fig. 4). Actually, it is not possible anymore to discriminate several size classes for void sizes between 10 and 100 nm. Nevertheless, sub-micrometer-sized void contents do

Fig. 3. YSZ coating structure (FESEM fractographies) for several layer states: A) assprayed; B) after ageing at 800 °C for 100 h; C) after ageing at 1100 °C for 100 h (Ar 66% wt. He plasma with UC01 feedstock at a 30 mm spray distance).

not evolve significantly. Indeed, deposit void total content does not change significantly after annealing (Table 3): the relative influence does not exceed 1/10th between as-sprayed and annealing at 1100 °C for 100 h since the volume fraction of nanometer-sized voids accounts for only 10% of the total volume void content, Fig. 5. Surprisingly, after annealing treatment for 100 h at 1100 °C, the peak corresponding to the smallest voids (i.e. around 5 nm) did not significantly decrease.

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Fig. 5. Deposits' void content evolutions per size classes for various annealing for SPS coating made with Ar 66% wt. He plasma at 30 mm spray distance with UC01 feedstock. Fig. 4. Volume distribution of voids of SPS coating manufactured with Ar 66% wt. He plasma at 30 mm spray distance with UC02 feedstock for different annealing temperatures.

This may result to the shrinking of larger closed voids during annealing. Coatings' non-connected void content evolves with annealing (Table 3). For a treatment of 100 h at 800 °C, it decreases below 1% due to the sintering process; (0.5 ± 0.1)% for Ar–He and (0.7 ± 0.2)% for Ar–H2. For a treatment of 100 h at 1100 °C, the rate slightly increases; (1.1 ± 0.3)% for Ar–He and (1.4 ± 0.1)% for Ar–H2. However, considering standard deviations of measurements, the closed void content seems to be likely unchanged between 800 °C and 1100 °C. Nevertheless, open void content decreases (from 12% to 10%) with temperature increases: this demonstrates that sintering mechanism takes place at the same time than elastic strain relaxation and that open voids are rearranged into closed voids.

4. Conclusion Combination of helium pycnometry and ultra-small-angles X-ray scattering analyses proved to be suitable for quantifying SPS deposits' void contents. Large populations of nanometer-sized voids have been detected. Their characteristic dimensions cover 4 orders of magnitude (i.e., from 1 to 10,000 nm). Moreover, about 90% of voids in the SPS deposits exhibit characteristic dimensions smaller than 40 nm. This is very likely one of the reasons, together with phonon scattering and thermal resistances at interfaces between particles, why very low thermal diffusivity values have been measured between room temperature and 250 °C (about 10 times smaller than typical values for micrometer-sized conventional APS coatings). Thermal ageing induces structural modifications with no noticeable evolution in deposits' void contents but rather with very noticeable evolutions in void size distributions. Nevertheless, the void sizes in annealed coatings remain in the same limits than in assprayed ones, than is to say smaller than 1 μm, average limit. Nonreactive natural sintering in solid state is very likely the involved mechanism but additional experiments are required to address this point.

Table 3 Evolution of void contents of deposits manufactured with UC01 feedstock for different annealing.

Ar–He 30 mm Ar–H2 30 mm

As-sprayed

800 °C 100 h

1100 °C 100 h

14.3 1.9 12.9 3.3

11.8 0.5 11.8 0.7

11.5 1.1 11.5 1.4

Total void content [%] Non-connected void content [%] Total void content [%] Non-connected void content [%]

Acknowledgments Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DEAC02- 06CH11357.

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