Influence of Microstructure and Composition on Optical Properties of

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Influence of Microstructure and Composition on Optical Properties of Plasma Sprayed Al/AlO Cermets 2

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Denis Toru, Aurélie Quet, Domingos De Sousa Meneses, Leire del Campo, and Patrick Echegut J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5113137 • Publication Date (Web): 14 Jan 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Influence of Microstructure and Composition on Optical Properties of Plasma Sprayed Al/Al2O3 Cermets Denis Torua, Aurélie Quet*a, Domingos De Sousa Menesesb, Leire del Campob, Patrick Echegutb a

CEA DAM Le Ripault – BP16 – 37 260 Monts – France CNRS, CEMHTI UPR 3079 – 45 071 Orléans – France

b

Corresponding author: E-mail: [email protected] Phone: +33 2 47 34 40 00 Fax: +33 2 47 34 51 08

1 ACS Paragon Plus Environment


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Influence of Microstructure and Composition on Optical Properties of Plasma Sprayed Al/Al2O3 Cermets Abstract Normal hemispherical spectral reflectances of plasma sprayed Al2O3/Al cermets were acquired from visible to infrared wavelengths for several metal concentrations. Since optical response of these coatings is highly dependent on volumetric and/or surface light scattering, a special attention was paid to characterization of composition, microstructure and surface roughness of such coatings. The roles of volumetric and surface effects are highlighted by a comparison between the spectral data measured on as-sprayed coatings and on polished coatings. As expected, reflectance rises with the metal addition, but in the transparent region of alumina, a noteworthy behavior is noticed for low aluminum concentrations. Indeed, the reflectance presents strong composition dependence and remains lower than that of the pure alumina sample. The results show that the optical properties of cermets produced by plasma spraying are controllable and therefore open the door to applications in the areas of solar energy or aeronautics.


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1. Introduction Atmospheric plasma spraying (APS) is a process where particles are deposited on a substrate in a molten or semi-molten state. Indeed, particles are heated in the plasma jet at very high temperature and propelled at high velocity to impact the substrate, where they flatten and solidify.1 The main advantage of this process is its ability to spray a wide range of materials, from metals to ceramics, on a large variety of geometric shapes and sizes of substrates. In addition, it is a rather inexpensive technique which enables to produce at industrial scale thick coatings, with thicknesses ranging from 50 µm to a few millimeters. To date, plasma sprayed coatings confer industrial solutions for heat and oxidation protection, wear and erosion resistance, but very few studies have so far been carried on optical properties brought by such coatings.2-6 Optical coatings are indeed usually obtained by thin film processes such as PVD, CVD or sol-gel, giving homogeneous materials. On the contrary, plasma sprayed coatings have heterogeneous microstructure, they include a multi-scale, open and closed porosity (5 to 20%), and have rough surfaces. Such characteristics influence radiative and optical properties. For example, studies on optical behavior of transparent and heterogeneous materials showed that reflectance is not only linked to the complex optical index of the matrix, but also to heterogeneities such as porosity, roughness, grain boundaries.2-10 Actually, optical properties are mainly affected by roughness and porosity rather than grain boundary effects, which can be neglected when the porosity rate reaches a certain level.8,9 As a consequence, optical properties of plasma sprayed coatings are different compared to optical properties of thin film coatings. Plasma spraying could consequently provide original properties and could constitute an alternative process to adjust reflectance or emittance, by capitalizing on its abilities and its high versatility. Low or high-emissivity coatings are of great interest in 3 ACS Paragon Plus Environment


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applications such as solar collectors, automotive or aeronautical components, and plasma sprayed coatings could provide alternative solutions in these fields. For example, increasing the reflectance in the near infrared is of great interest for thermal barrier coatings. This paper aims to improve the understanding of optical properties of porous materials, such as plasma sprayed coatings. It focuses particularly on the optical behavior of oxide ceramics and cermets (ceramic– metal materials), the latter having never been reported in the previous literature. Cermets possess properties of both ceramics and metals. By varying the amount of either material, cermets can be designed to obtain desired mechanical, thermal or electrical properties.11,12 Similarly, reflectivity may be adjusted through the use of cermet coatings. In the visible, near and mid-infrared (NIR and MIR), metals and dielectric oxides have very contrasted optical properties. Taking into account the investigated material thicknesses, metals are opaque and highly reflective. Ceramics are transparent from the visible to NIR, and opaque in the phonon spectral range (MIR). Furthermore, microstructure (porosity and roughness) strongly influences optical behavior. Here, aluminum and alumina were selected as demonstrative materials. By modifying the metallic charge contained in cermets, the evolution of the reflectance was studied. To explain the optical behavior, a fine characterization of aluminum concentration and of porosity, through several techniques like image analysis or XRD analysis was mandatory. A comparison of as-sprayed and polished coating reflectances emphasized the roles of volumetric and surface effects, depending on the wavelength range. 2. Material and methods In order to avoid transparency, 2-millimeter thick samples were deposited on aluminum plates (50 x 50 x 2 mm3). Aluminum powder (HC Starck, 22-45 µm, Amperit 740.1) and alumina powder (Sulzer Metco, 22-45 µm, 54NS-1) were used. Prior to deposition, the surface of the 4 ACS Paragon Plus Environment

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substrates was grit-blasted and degreased by heating. Plasma spraying was performed with a F4VB torch (Sulzer Metco) working under atmospheric pressure. Metal and ceramic powders were fed radially and separately into the plasma jet, using argon as carrier gas. They were heated and accelerated by the plasma jet towards the substrate. Aluminum was injected farther into the plasma jet than the ceramic due to their very different thermal properties. Selected plasma parameters are shown in Table 1.

Table 1. Spray parameters for plasma spraying. Arc current intensity, A Argon flow rate (%) Helium flow rate (%) Hydrogen flow rate (%) Injector diameter, mm Spray distance, mm Powder deviation angle, °

500 30 54 16 1.5 140 3.5

Mixtures of Ar-He-H2 were used as plasma gas. Resulting particle velocity and temperature, measured by a DPV 2000 diagnostic system (Tecnar Automation, QC, Canada) at spraying distance, were about 220 ms-1 and 2350 K. Quantity of injected metal was adjusted from 0 wt.% to 100 wt.%. Porosity level was measured by the Archimedes method. Results were compared with those given by image analysis, performed thanks to ImageJ software, on ten cross sections of coatings (resolution 0.09 µm/pixel) obtained by scanning electron microscopy (SEM, LEO 440). Pore size distribution was obtained by analyzing SEM cross sections. Roughness was measured with a Perthometer S2. Crystalline structure was assessed with a D5000 X-ray diffractometer (XRD, Siemens A.G.) using Cu Kα radiation. In order to establish a correlation between composition and optical behavior, the accurate quantification of aluminum rate in cermets was needed.



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Therefore, four methods, based on image analysis (SEM and optical microscopy (OM)), Archimedes method (AM) and XRD measurements, were used to evaluate the quantities of metal contained in the coatings. AM method consists in determining the real density of the sample from the measured weights and estimating the aluminum rate with theoretical densities of Al and Al2O3. XRD method consists in adding increasing quantities of aluminum to the crushed plasma sprayed deposit. Then the evolution of the ratio between the alumina XRD main peak and the aluminum XRD main peak leads to quantification of the alumina and aluminum phases in the coating. Unfortunately, the alumina signal becomes too weak to use this method when aluminum rate is higher than 25 wt.%. Total and scattered reflectances were measured from the visible wavelengths up to 16 µm with Cary 5000 and Bruker IFS66 spectrometers. A 15.1cm diameter integrating sphere with gold inner coating was used in order to collect hemispherical radiation. The scattered part of the radiation was collected by removing the gold sphere stopper, to enable the specular part to escape. 3. Results and discussion 3.1 Composition and microstructure The coatings exhibit the traditional characteristics of plasma sprayed coatings13 such as the presence of unmelted particles and globular, interlamellar or intralamellar porosity (Figure 1).


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Figure 1. SEM image of a polished cross-section of the Al2O3 coating.

The multi-scale and multi-shape porosities, ranging from hundred nanometers to about ten micrometers, make difficult a fine characterization of the porosity. Total porosity rate was estimated at 10% for alumina coatings according to Archimedes method. This level decreases with aluminum addition up to 3%. Image analysis confirms this trend (Figure 2).

Figure 2. Cermet porosity depending on the aluminum rate estimated by SEM image analysis.

Figure 3 shows the relative area of the porosity which is the ratio of the area of a given pore class over the total pore surface.



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Figure 3. Relative pore surface as a function of the circularity (1= globular, 0= crack).

A representative set of images was acquired with a 3000 magnification that gives a sharp vision of interlamellar porosity and cracks. The main part of the pore equivalent diameters is smaller than 10 µm and 80% of pores have an equivalent diameter lower than 1 µm (Figure 3). The pore equivalent diameter is defined as the diameter of the pore if it were a disc, keeping the same area. It was noted that the 2 µm pore class has the main contribution to the surface porosity rate. As regards shape, small pores are more globular (circularity close to 1) and higher diameters with a lower circularity correspond, in most cases, to interlamellar porosity. Diffractograms evidence the presence of both γ and α-Al2O3 phases in the coatings, as shown in Figure 4.


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Figure 4. XRD pattern of a plasma sprayed Al2O3 coating indicating the presence of α and γ phases.

The alumina initial powder was composed of α-alumina but liquefaction and then quick solidification of molten particles lead to the formation of the γ-phase since the latter requires less energy to be formed.14 Quantification of the amount of each crystalline phase, by using XRD, showed that 96% of the coating is composed of γ-alumina and the remaining 4% is due to αalumina unmelted particles.15 The quantification of aluminum rate in cermets is summarized in Figure 5. Results are detailed in Table 2.



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Figure 5. Mean aluminum rate of cermets versus target rate, estimated with the plasma spraying efficiency.

Table 2. Determination of the aluminum rate. Target aluminum rate (%)

XRD (%)

SEM (%)

OM (%)

AM (%)

75 50 33 20 10 5

/ / / 12.6 9.3 4.8

72.3 38.9 31.7 11.6 8.8 5.8

74.3 50.0 30.4 17.5 8.8 4.9

88.9 54.1 36.3 17.1 7.0 4.3

Mean Standard calculated deviation aluminum (%) rate (%) 9.1 78.5 7.7 47.7 3.1 32.8 3.0 14.7 1.0 8.5 0.6 4.3

Figure 6 illustrates the microstructure of an as-sprayed cermet coating containing 33 wt.% aluminum; the metallic phase is spread and randomly distributed on the coating.

Figure 6. SEM image of an as-sprayed cermet surface with 33 wt.% Al (light grey: Al, dark grey: Al2O3, dark: porosity/roughness).


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On the surface, some areas can be considered as homogeneous aluminum or alumina areas, with an average diameter higher than 100 µm. Statistical data on aluminum splat diameters are available in Figure 7.

Figure 7. Relative surface of plasma sprayed aluminum splats.

Diameters smaller than 20 µm are not taken into account in Figure 7, due to roughness that restricts quality of SEM pictures. Different spreading degrees of each material lead to a roughness of as-sprayed coatings ranging between 5 and 10 µm. That of polished coatings is lower than 1 µm. Nevertheless, it can be seen that a splat diameter is around 140 µm. Other areas can be considered as heterogeneous, composed of porosity, alumina and aluminum. Unfortunately, the coating topography complicates the contrast adjustment and the determination of the aluminum particle sizes on heterogeneous areas. Figure 8 shows the surface of a polished coating containing 50 wt.% aluminum.

Figure 8. SEM image of a polished cermet surface with 50 wt.% Al (light grey: Al, dark grey: Al2O3, dark: porosity/roughness).



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Thanks to a better contrast induced by planarity, alumina, aluminum and porosity were easier to identify. There are still some homogeneous areas of alumina and aluminum, with a diameter ranged from 40 to more than 100 µm, but there are also more heterogeneous areas. They are composed of alumina with small aluminum inclusions, from about a hundred of nanometers to several micrometers, and of porosity. After careful analysis of SEM pictures, these heterogeneous areas appear to be larger with increasing aluminum rates, because homogeneous alumina areas become dispersed due to plasma sprayed aluminum splats becoming plentiful. Polished coatings exhibit more mixed area than as-sprayed coatings, because the aluminum splat spreading follows roughness and not polishing plan.

3.2 Optical properties Optical properties are studied through the average reflectance values obtained for the following three spectral domains wavelengths λ ranging from (a) 0.4 to 6 µm, (b) 6 to 10 µm and (c) 10 to 16 µm. These intervals correspond to specific absorption regimes of alumina as set forth in Figure 9, i.e. regions of low, intermediate and high absorption coefficients, respectively.


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Figure 9.Reflectance of a plasma sprayed alumina coating and reflectance of single crystal of αalumina (left axis). Absorption coefficient of α-alumina computed from extinction index16 (right axis).

The composition dependence of the average normal hemispherical reflectance of as-sprayed coatings is presented in Figure 10.

Figure 10. Composition dependence of total and scattered reflectances of as-sprayed cermets for three wavelength ranges.

The multi-scale microstructure and multiphase nature of the cermets lead to a complex optical response. Reflectance shows strong and non-monotonous composition dependence. Furthermore, its behavior is different in the three spectral domains. In order to understand the physical origins 13 ACS Paragon Plus Environment


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of the highly variable optical response and to evaluate the effects of surface and volumetric light scattering, another set of measurements were performed on the cermets after polishing (Figure 11).

Figure 11. Composition dependence of total and scattered reflectances of polished cermets for three wavelength ranges.

As explained in the following, the surface of the cermets can be seen as a patchwork. This patchwork includes not only aluminum splats and nearly pure alumina regions with characteristic sizes much larger than the radiation wavelengths, but also large mixed domains including small aluminum and alumina particles. Reflectance of the mixed domains including objects smaller than the wavelengths studied here, can be approximately reproduced by using homogenized optical indexes computed with an effective medium theory. The full optical response results from 14 ACS Paragon Plus Environment

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the weighted response of the elements constituting the patchwork and from variable contributions of light scattering due to surface roughness and volumetric heterogeneity, such as porosity. 3.2.1 Wavelengths from 10 to 16 µm In this spectral range, both alumina and aluminum have strong absorption coefficients16,17 leading to a very small penetration depth of the electromagnetic radiation inside the coatings. The optical response results essentially from the microstructure and composition of the sample surface. 3.2.1.1 Single material coatings Plasma sprayed alumina does not only absorb, but also reflects a significant part of the radiation. By comparison with a single crystal of α-alumina that reflects about 80% of radiation in this spectral range18, the alumina plasma sprayed coating only exhibits 20% of reflectance. This is due to several factors. First, the coating is mainly composed of γ-alumina that is less reflective than α-alumina. An annealing of the coating highlights this observation since the return to the αphase leads to a significant increase of reflectance of the sample (between 10 and 20% depending on the wavelength). Roughness also leads to absorption (between 10 and 20%, depending on the wavelength). With an arithmetic roughness of about 6 µm, a large part of the radiation is absorbed due to trapping and multiple reflections of photons on the heterogeneities of the surface. Increase of reflectance after polishing highlights this effect. The same trend is observed as regards the optical behavior of aluminum, which is more reflective when polished. 3.2.1.2 Cermet coatings As expected, the optical response of the cermet coatings is somewhere between those of aluminum and of alumina. In this spectral domain, the difference of optical response between polished and as-sprayed coatings is due to scattering by surface roughness. But there is also a 15 ACS Paragon Plus Environment


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specific behavior around the 50 wt.% composition that must be understood (Figures 10 and 11). In order to explain the origin of the slight bowing of reflectance that occurs around 50 wt.% Al for both as-sprayed and polished coatings, we attempted to reproduce the behavior of polished cermets with simplified models. The simpler one consists in assimilating the cermet surface to a patchwork of pure aluminum and alumina areas with characteristic sizes far bigger than the wavelengths of this spectral domain. Under these conditions, the reflectance of the cermets R can be evaluated by using the following biphasic mosaic law: R = x . RAl + (1-x) . RAl2O3

(Eq 1)

x designates the aluminum content, RAl the reflectance of the aluminum coating and RAl2O3 the reflectance of alumina. The reflectance defined by equation 1 is compared to experimental data in Figure 12.

Figure 12. Comparison between reflectances evaluated using equations 1 and 2 and average experimental reflectance measured from 10 to 16 µm.

As can be seen in such figure, the model is good enough to reproduce response of aluminum rich and alumina rich cermets, but totally fails to reproduce the bowing observed for intermediate compositions which indicates that something is missing in the model. As reported in the 16 ACS Paragon Plus Environment

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microstructure section and evidences in Figure 8, cermets exhibit a multi-scale texture that can be approximated by a triphasic model including, in addition to pure alumina and aluminum regions, mixed aluminum/alumina areas. Making a supplementary hypothesis taking into account the optical response of an effective medium, it is possible to write a more sophisticated model. Since the characteristic sizes of the mixed area are well below the radiation wavelength in this domain, it is possible to estimate its optical response by using the Bruggeman effective medium theory19. Figure 13 reports the computed reflectance for effective Bruggeman media versus aluminum content.

Figure 13. Composition dependence of reflectance of mixed aluminum/alumina medium evaluated with the Bruggeman effective medium approximation for three wavelength ranges.

Whatever the aluminum content of the cermet is, we assume that the aluminum rate of the mixed domains remains roughly the same, even if it is extremely difficult to highlight this hypothesis regarding SEM images. Statistically, the aluminum rate of the effective medium ranges between 15 and 30 wt.%. For such aluminum rates, reflectance of the corresponding Bruggeman effective medium is quite low (Figure 13). In order to simplify our triphasic model, the aluminum rate in



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mixed domains is considered to be constant and fixed at 20 wt.%. Within this framework, the mosaic law can be rewritten as follows: R =P(x).RBrugg + (1-P(x)). x . RAl + (1-P(x)). (1-x) . RAl2O3

(Eq 2)

In this equation, P(x) represents the percentage of the surface corresponding to mixed domains, and RBrugg designates reflectance of mixed domains evaluated with the Bruggeman effective medium theory. In order to evaluate reflectance using equation 2, the assumption that P(x) can be approximated by a Gaussian profile centered at an aluminum rate of 50 wt.% was made. The results reported in Figure 12 were obtained with a 0.4 amplitude for the Gaussian profile, meaning that the effective medium cannot represent more than 40% of the cermet surface. The existence of a bowing in the reflectance for intermediate compositions is well reproduced by the triphasic model and the results are close to experimental data.

3.2.2 Wavelengths from 0.4 to 6 µm In this spectral range, aluminum still has a strong absorption coefficient16, but alumina exhibits a very low absorption coefficient17, leading to penetration of radiation inside the coating. As a consequence, the optical response results from the sample surface, but also from its volume.

3.2.2.1 Single material coatings At short wavelengths, alumina-sprayed reflectance is very high (about 80%) compared to a single crystal of α-alumina, that reflects about 10% of radiation in this spectral range.18 This is due to interactions between electromagnetic radiation and material microstructure.2,20 Despite the very low absorption coefficient of alumina, no transparency was observed, because of the 2-mm-thick


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coating and the scattering induced by porosities. Volume scattering includes three types of phenomena21: (a) diffraction, which results in a modified direction of light propagation around heterogeneity; (b) refraction, which involves penetration of light in heterogeneity, as well as modification of the emerging direction; and (c) multiple reflections at the interface between heterogeneity and the matrix medium. These interactions between radiation and alumina matrix are called “volumetric scattering”. According to Kirchhoff law, light may be partially absorbed (20%) by impurities or structural defects. The absorption peak noticed at 3 µm corresponds to the O-H vibration, due to hydroxyl groups or moisture adsorbed by alumina. Reflectance slowly decreases between 1 and 6 µm because the extinction coefficient rises with the wavelength and backscattering decreases.2,5 When wavelength rises, in relation to the pore size distribution (Figure 3), the size parameter becomes considerably lower than 1. As a consequence, the scattering orientation tends to turn toward the front of the sample, which enhances absorption. Annealing of the coating has no effect on optical behavior, meaning that optical indexes of γ and α-alumina are very close in this wavelength range and that substoichiometry in oxygen did not occur. As-sprayed and polished alumina total reflectances are identical, pointing to a low effect of the sample surface in this spectral range.

Reflectance of the aluminum coating is high (about 80%) on the entire considered wavelength range but lower than that of homogeneous metal because of the 10 µm arithmetic roughness. After polishing, the increase of the specular part does not contribute to reflectance of the sample; the strengthening of the specular part is made only to the detriment of the scattered one. This is due to the fact that quality of the polishing is not good enough for short wavelengths. Absorption due to surface scattering remains at an important level in this spectral domain.



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3.2.2.2 Cermet coatings Where level of aluminum is low, reflectance decreases compared to that of alumina. This composition-dependent trend originates from the scattering and absorption by a low density of aluminum splats in the alumina matrix (Figure 14).

Figure 14. Illustration of scattering mechanisms occurring in cermets in the transparent region. (1) Refraction between air and alumina and multiple reflections on aluminum splats; (2) Pore scattering; (3) Surface reflections on aluminum splats.

The aluminum lamella dispersed within the sample drastically modifies the photon travels, leading to a higher absorption and to a significant reduction of reflectance. Indeed, in this composition range, there are not enough metallic lamellae at the surface vicinity of the coating to favor surface reflectance. Volumetric effects prevail over surface effects. At some point, when the aluminum rate becomes high enough, the opposite behavior is observed. Surface effects prevail on volumetric effects, so reflectance rises to the 100 wt.% aluminum coating level.


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Moreover, this transition happens also due to the decrease of the porosity level, which decreases when aluminum rate increases (Figure 2). Some additional information about transition from volumetric to surface effects is obtained by comparing optical behavior of as-sprayed coatings with optical behavior of polished coatings (Figures 10 and 11). In Figure 10, the scattered part of reflectance includes volumetric scattering plus surface scattering, while in Figure 11, it is mainly the volumetric scattering. Therefore, the difference between the two curves gives an indication of the surface contribution to the reflectance. Surface effects start to become predominant from 15 wt.% of aluminum because the scattered part of the reflectance starts to decrease for polished coatings whereas it continues to grow up for as-sprayed coatings. As a consequence, cermet reflectance results from the addition of surface and volumetric contributions. Whereas volumetric effects prevail between 0 wt.% and 15 wt.%, reflectance decreases with increasing aluminum rate and as-sprayed cermets have a higher reflectance than corresponding polished ones. That behavior could be explained by the probability for the radiation to penetrate deeper in the polished material by comparison with the rough coating. Photons interact with the surface at normal incidence as far as polished interface is concerned, but as regards as-sprayed interface, roughness induces a large distribution of penetration angles. Penetration depth is maximal when the angle corresponds to normal incidence (0°) and diminishes towards the grazing angle of incidence (90°). As a consequence, a polished cermet allows deeper penetration than a rough as-sprayed cermet. Therefore, a higher absorption is observed for polished coatings involved by longer photon paths, and more interactions with aluminum splats. When the aluminum rate ranges from 15 wt.% to 50 wt.%, reflectance keeps decreasing in polished coatings, despite surface effects becoming predominant. Therefore, the above explanation about photon penetration into the volume cannot totally explain the whole behavior. 21 ACS Paragon Plus Environment


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As in the 10 to 16 µm spectral domain, the influence of mixed aluminum alumina areas is also present and equation 2 is still valid to explain surface contribution to reflectance. The mixed area percentage is reduced in this spectral range since only aluminum inclusions smaller than few hundred nanometers can be treated as an effective medium. Nevertheless, as shown in Figure 13, when reflectance of the mixed areas is smaller than in the 10 to 16 µm spectral range, the global impact on the optical response remains approximately the same in the two spectral domains. The model described by equation 2 can be used to simulate the measured reflectance, but with an additional term that takes into account volume scattering. As evidenced in Figure 11, the volumetric contribution behaves like an exponentially decreasing function. When aluminum splats are dispersed in a nearly nonabsorbent matrix, apparent optical thickness of the cermet is nearly proportional to the aluminum content. This leads to a contribution to the global reflectance that exponentially decreases with the aluminum rate. The modified model leads to reproduction of the main optical trends visible in this wavelength range (Figure 15).

Figure 15. Predicted optical behavior from 0.4 to 6 µm with surface and volume contributions.

The reported data were computed with the optical indexes of aluminum and alumina found in literature.16,17 The percentage of effective medium areas follows the same law than in the 10 to 16 22 ACS Paragon Plus Environment

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µm spectral range. An interesting conclusion of this section and for this spectral range is that this kind of effective medium is rather absorbent when aluminum rate is low (Figure 13). The main departure of the simulated behavior from the measured one concerns high aluminum rates. This is not surprising since the model considers a perfect cermet surface with only specular reflection which is clearly not the case in this measurement. Polishing of the sample was not good enough for short wavelengths, and surface scattering remains important in this spectral region.

3.2.3 Wavelengths from 6 to 10 µm In this wavelength range, both aluminum and alumina are opaque with respectively a high and an intermediate absorption coefficient compared with the two previous spectral ranges. 3.2.3.1 Single material coatings In this spectral range, the alumina absorption coefficient rises abruptly16 (Figure 9), so it becomes opaque and absorbent. As a result, reflectance decreases to zero. Penetration depth of the radiation decreases from hundreds of micrometers to 20 µm with the wavelength.16 At 10 µm, which is the Christiansen wavelength for alumina8,18, reflectivity is close to zero, as predicted by the Fresnel law. Indeed, the refractive index equals 1 and the extinction coefficient is negligible. As regards the polished coatings in this domain, roughness has no influence on total reflectance, because of the absorbing capacity of alumina. The aluminum behavior is the same than in the 10 to 16 µm domain. 3.2.3.2 Cermet coatings The optical behavior is roughly the same in this spectral range as in the 0.4-6 µm spectral range, except that the volumetric effects do not act in the same way due to the higher absorption 23 ACS Paragon Plus Environment


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coefficient of alumina, inducing a lower reflectance in this region (Figure 9). Actually, volumetric effect can be observed thanks to aluminum splats and thanks to the penetration depth of the radiation into alumina. For both as-sprayed and polished samples, reflectance appears to be directly linked to the amount of aluminum: the higher the aluminum quantity is, the higher the reflected radiation, even if there is no proportionality when results are proposed in wt.% and not in vol.%. Below 15 wt.% of aluminum content, there is a strong increase of reflectance, due to the backscattering of electromagnetic radiation of aluminum splats located at the vicinity of the sample surface. This is the reflective character of the splats which tends to diminish the photon mean travel inside the absorptive matrix, and which therefore explains the observed trend. When surface effects start to prevail over volumetric ones, the increase is less pronounced. In this domain, the effective medium contribution concerns mixed areas that involve aluminum inclusions including intermediate sizes (typically below 1 µm). The induced absorption has the same trend than the one defined in the 0.4-6 µm spectral range, as shown in Figure 13. By taking into account the new kind of contribution due to volumetric effects imposed by the absorbent character of alumina in this region, it is possible to reproduce the main optical trends observed in the experimental data (Figure 16).

Figure 16. Predicted optical behavior from 6 to 10 µm with surface and volume contributions. 24 ACS Paragon Plus Environment

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4. Conclusion Several Al2O3/Al cermets were plasma sprayed, adjusting their aluminum rate, in order to study influence of the latter on optical properties. Three characteristic wavelength ranges of alumina spectrum were selected in order to understand optical behavior. For 0.4-6 µm wavelengths where alumina has a low absorption coefficient, volumetric effects occur, and a cermet is less reflective than an alumina coating where the aluminum rate is low. Otherwise, reflectance is proportional to the aluminum rate, driven by surface effects. The influence of roughness was investigated and correlated to the penetration depth to explain the optical behavior when alumina has a low absorption coefficient. As regards surface effects, it is possible to define some areas on cermet coatings which can be assimilated to an effective medium composed of alumina with small aluminum inclusions in comparison with the wavelength. This induces behaviors that favor absorption or reflection, depending on the aluminum rate of the effective medium and the wavelength domain. Depending on the final application, it becomes therefore possible to adjust the optical properties of a plasma sprayed coating. If on one hand the aim is to increase emittance, then a low amount of aluminum is advised to promote volumetric effects. If on the other hand the aim is to favor reflectance, an alumina coating leads to a high scattered reflectance, whereas an aluminum coating leads to a high specular reflectance. The reflectance level could be controlled with the aluminum concentration. In the future, efforts will be focused on the modeling of optical behavior of plasma-sprayed cermet in the 0.4 to 10 µm spectral range, based on Monte Carlo ray tracing approach. Numerical materials will be generated from SEM and X-ray microtomography statistical data.



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Influence of Microstructure and Composition on Optical Properties of

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