Micro-chemical and -morphological features of heat treated plasma sprayed zirconia-based thermal barrier coatings

TSF-32253; No of Pages 9 Thin Solid Films xxx (2013) xxx–xxx

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Micro-chemical and -morphological features of heat treated plasma sprayed zirconiabased thermal barrier coatings Barbara Cortese ⁎, Daniela Caschera, Tilde de Caro, Gabriel Maria Ingo Istituto per lo Studio dei Materiali Nanostrutturati, Consiglio Nazionale delle Ricerche (ISMN-CNR), via Salaria km 29.5, 00015 Monterotondo, Rome, Italy

a r t i c l e

i n f o

Available online xxxx Keywords: Ceria–yttria stabilized zirconia Yttria stabilized zirconia Thermal barrier coatings Heat treatment Phase composition

a b s t r a c t Zirconia-based plasma-sprayed coatings are extensively used in jet and land-based engines as thermal barrier coatings (TBCs) for protecting and insulating gas turbine metal components from the extreme temperature in the hot gas extending the engine life capabilities and service performances as well as reducing fuel consumption. Zirconia-based thermal barrier coatings stabilized with yttria and ceria were prepared by means of atmospheric plasma spray (APS) and thermal treated at different temperatures. The resulting fractured heated surfaces have been studied by means of X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM) combined with energy dispersive spectrometry (EDS) and secondary ion mass spectrometry (SIMS) in order to study the surface micro-chemical composition and morphology. The results disclose the variation of the stabilizing oxide amount, the occurrence of valence state modifications of cerium, impurity segregation phenomena and sintering. High temperature sintering influenced the porous microstructure leading to structural changes of the surface. This information confirmed that chemical and morphological aspects in plasma sprayed TBCs must be known in order to understand and predict relationships between the parameters of plasma spray process and TBC features, properties and performances for a better design of reliable TBCs. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Thermal barrier coatings have proved to be a key technology in thermal stability and protective coatings within severe environments as aerospace gas turbines which along with engine life and service performance operate at high implementation temperatures [1–8]. Current common strategies to produce thermal barrier coatings for performance improvement are centered on the use of zirconia based thermal spray techniques, as atmospheric plasma spraying (APS) [9]. APS deposition is a cost-effective technique with high deposition rate, up to the magnitude of μm/s, determined by parameters such as powder injection rate and plasma gas temperature. Deposition morphology of APS coatings is characteristically a high-porous lamellar microstructure with micro-pores and micro-cracks present in the coatings [10]. Common zirconia based TBCs are Y2O3 stabilized. Fewer studies have started investigating the use of a different stabilizer such as CeO2 [11,12], showing some advantages over Y2O3-stabilized ZrO2. In fact, zirconia doped with oxides such as Y2O3, CeO2, MgO, or CaO shows improved thermal and mechanical properties by combining a low thermal conductivity and phase stability up to high operating temperatures [13]. In particular, considering that TBCs, during operating lifetime, are subjected to thermal and mechanical stresses that may alter their microstructure, ensuring greater structural stability would be strategic to improve their performance during long-term service. ⁎ Corresponding author. Tel.: +39 069062409. E-mail address: [email protected] (B. Cortese).

Well-known zirconia is characterized by three crystallographic phases: cubic, tetragonal and monoclinic. Phase transformations of the structure during heating and cooling processes are closely related to stress between metal substrate and ceramic coating. Moreover, a lower thermal expansion of the ceramic material can generate thermal stresses in the ceramic coating therefore inducing a lower durability. High temperature operating conditions cause two kind of structural changes in ZrO2-based systems such as sintering and grain growth effects and phase transformations that are reciprocally dependent. Such changes influence severely the mechanical properties of the TBCs. Annealed or thermally shocked coatings may lead to monoclinic phase transformations to the tetragonal ones [14,15]. This is because the high temperature accelerates thermally activated processes, as the tetragonal to monoclinic martensitic phase transformation of zirconia based solid solutions, which results in a volume expansion that can cause spallation. Therefore to improve thermal efficiency it is significant to improve the phase stability of the thermal barrier coating. Stabilizing zirconia with a six to eight weight percent yttria (6–8% YSZ) contributes to a more stable tetragonal phase and minimization of the disadvantageous monoclinic phase at room temperature. Though thermal conductivity of YSZ decreases with the increasing of the yttria content, the industrial standard is typically zirconia partially stabilized with 6–8% yttria as it would be more adherent and spall-resistant to high temperature thermal cycling than YSZ TBC containing greater and lesser amounts of yttria. Besides, the microstructural assets can locally vary within each coating, due to different concentrations of yttria from grain to grain or within a grain and by the presence of relatively small

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impurities within the coating. Impurities can in fact enhance the diffusion rates for grain boundaries, lattice and surface diffusion [16,17]. For example, as-sprayed yttria partially stabilized zirconia (YSZ) coatings are predominantly composed of a non-transformable tetragonal metastable t' zirconia with smaller amounts of cubic and monoclinic phases. Performances of YSZ coatings can be severely compromised due to exposure at high temperatures (e.g. 1300 °C), which transform the t' zirconia phase to a high-yttria cubic phase and a low-yttria tetragonal phase [18]. Subsequent slow cooling to room temperature causes the low-yttria tetragonal phase to transform to the monoclinic phase, whereas the high-yttria cubic phase may be retained or it may transform to a high-yttria tetragonal t' phase [19]. This transition is associated to volume change which can support formation of cracks, leading to spallation. Moreover, high temperature sintering of the porous microstructure may also reduce its strain tolerance and the volume of the pores. Hence, the addition of rare earth oxides with fluorite structure, as CeO2, to the YSZ would stabilize the zirconia's tetragonal and cubic phase, leading to high temperature phase stability and thermal shock resistance [20,21] as well as hot corrosion resistance [22]. Few reports of ceria-stabilized zirconia coatings containing different mol% CeO2 are reported in literature and thermal properties are not fully understood. Cerium is known to improve tetragonal stability at low concentrations, such that the addition is augmented to contribute to stabilizing mechanisms that are active in the ZrO2 lattice, namely oxygen vacancies and lattice distortions and such that t → m transition temperature is decreased with increasing additions of CeO2 [23,24]. Addition of CeO2 decreases thermal diffusivity mainly due to the occurrence of substitutional lattice defects and grain-boundary scattering. Although an increase of the dopant molar (above 10%) decreases the cyclic life of the coating, a 25 wt.% CeO2–ZrO2 coating acts to corroborate higher temperature phase stability [25]. As pointed out, though, there is still a poor understanding placed in identifying the interaction between the plasma gas and ceramic particles on the morphological and chemical properties of TBCs [15], a clearer perception of these aspects would be of a key role of significant importance for TBC designers. The purpose of the present work is to investigate the combined chemical aspects in plasma spraying the chemistry of 8 wt.% Y2O3– ZrO2 (YSZ) and 25.5 wt.% CeO2–2.5 Y2O3–ZrO2 (Ce-YSZ). Coatings were deposited using atmospheric plasma spraying technique (APS). Investigations dealing with the question of the long-term behavior of CeO2-stabilized TBCs, especially after heat treatments at typical operating temperatures on the structural and thermo-physical properties of the coatings have been carried out. Scanning electron microscopy (FE-SEM) and XPS were performed to investigate changes in phase composition, microstructure, and morphological properties of the TBCs with different hydrothermal aging. 2. Materials and methods 2.1. Materials Molar mixtures were selected so that the total amounts were in the range of industrially available products. This led to YSZ consisting of 8 wt.% Y2O3–ZrO2 and Ce-YSZ consisting of 25.5 wt.% CeO2–2.5 Y2O3– ZrO2. The chemical composition of 8 wt.% Y2O3–ZrO2 and 25.5 wt.% CeO2–2.5 Y2O3–ZrO2 materials is reported in Table 1. TBCs 0.35 mm thick were deposited onto a substrate of AISI 316 stainless steel using a commercial air plasma spray (APS) equipment [26]. During the APS deposition the ZrO2 based TBCs were air-cooled below 170 °C by

employing an air-jet blown continuously on the front face of the coating. 2.2. XPS The small-area XPS and X-ray-induced Auger electron spectroscopy (AES) studies were carried out on an Escalab Mk II spectrometer using both AlKa1,2 and MgKa1,2 radiation as excitation sources (hν = 1486.6 and 1253.6 eV, respectively). The SiKL2,3L2,3 and AlKL2,3L2,3 Auger peaks were excited by the Bremsstrahlung continuum above the characteristic AlKα1,2 and MgKα1,2 radiation. The hemispherical electron analyzer was operated in fixed analysis transmission (FAT) mode by selecting a constant pass energy of 20 eV; under these operating conditions, the measured full width at half maximum (FWHM) of the Ag03d5/2 line recorded from the argon ion cleaned Ag, was 1.0 eV. The analyzed area was about 0.8 cm2. The sampled area varied from 1 mm to 0.30 mm according to the size of the fracture surface. All measurements were performed at pressures lower than 5 × 10− 10 mbar. The binding energies (BEs) were referenced to the Fermi level of the electron analyzer and the confidence in the linearity of BE scale was based upon setting the position of Au4f7/2, CuL3M4,5M4,5 and Cu2p3/2 peaks at 84.0, 334.9 and 932.7 eV, respectively. Corrections to the energy shift, due to the steady-state charging effect, were accomplished by assuming the C1s line, resulting from the ubiquitous surface layer of adsorbed hydrocarbons, lies at 284.6 eV, and that the Zr3d5/2 line in ZrO2 lies at 182.4 eV. The mean value of 182.4 ± 0.1 eV was obtained with reference to the Au4f7/2 line from evaporated Au on stoichiometric ZrO2, and this value is in good agreement with literature data [2,26]. Reproducibility of the results was within ±0.15 eV. Quantification and elemental atomic ratios were determined for each sample in terms of the most intense core level peak areas after smoothing (polynomial cubic function), performed over five points, subtraction of the X-ray satellite structure and “S” type integral background profile correction for sensitivity factors whose validity had already been demonstrated in previous work [26]. Due to the Si 2s and Y3d overlap, the effective area of this latter photoemission signal has been extracted from the envelope of the peaks by curve fitting. The XPS spectra were acquired and processed using a computer and a dedicated data-handling system. 2.3. SEM FESEM-EDS characterization was carried out by a high-brilliance LEO 1530 field emission scanning electron microscope equipped with an energy dispersive X-ray spectrometer (EDS) INCA 450 and a four sector backscattered-electron detector (BSD), the FESEM images were recorded both in the secondary electron image (SEI) and backscattered image (BSD) mode at an acceleration voltage ranging from 2 to 20 kV. SEM-EDS characterizations were carried out using a Cambridge 360 SEM microscope equipped with a LaB6 filament, an energy dispersive X-ray spectrometer (EDS) INCA 250 and a four sector backscatteredelectron detector (BSD). SEM images were recorded both in the secondary electron image (SEI) and backscattered image (BSD) mode at an acceleration voltage of 20 kV. The TBC fractured samples were coated with a thin layer of carbon or chromium in order to observe the samples without charging effects. Carbon coating was deposited using an Emitech sputter coater K550 unit, a K 250 carbon coating attachment and a carbon cord at a pressure of 1 × 10−2 mbar in order to produce a carbon film with a constant thickness of about 3.0 nm. The chromium

Table 1 Chemical composition of YSZ and Ce-YSZ starting powders expressed as weight percent (wt %). Material

CeO2

Y2O3

ZrO2

HfO2

CaO

TiO2

Fe

Al

Na

Si

YSZ Ce YSZ

– 25.10

7.98 2.54

90.34 71.00

1.09 0.98

0.20 0.08

0.17 0.02

0.12 0.11

655 ppm 586 ppm

1067 ppm 986 ppm

548 ppm 753 ppm

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coating was deposited by using a Bal-Tech SCD 500 equipped with turbo pumping for ultra-clean preparations at a pressure of 5 × 10−3 mbar in order to produce a chromium film with a constant thickness of about 0.5 nm. 3. Results and discussion TBCs formed by plasma-spraying of yttria-stabilized zirconia (YSZ) result in a complex microstructure of multiple stacked discs termed lamellae. Nano-sized powder compositions of YSZ and Ce-YSZ as reported in Table 1, were plasma sprayed on stainless steel substrates. The XPS spectra for both as-sprayed APS TBCs samples and annealed coatings were investigated. Previous XPS investigations on YSZ have shown an enrichment of a surface impurity phase dependent on the thermal treatment [26]. Atomic concentration ratios of coatings deposited at different conditions are reported in Table 2. Concerning the as-sprayed sample, a complex microchemistry of the structure was detected. From the results shown in Table 2, the main constituents of the as-deposited film were Zr and Y. Also, presence of oxygen was detected (results not shown) which may result from the inherent oxygen contamination of the atmospheric pressure deposition and the highly porous structures. Mostly significant was the presence of a surface impurity phase whose enrichment was dependent on the treatment temperature, as shown in Table 2. Segregation of silicon, sodium and aluminum on the surface phase was observed on the surface after heat treatment at 1200 °C, which evidently came from impurities of source materials and external contamination. Moreover yttrium enrichment with increasing of temperature was observed while the segregate phase of silicon decreased. This chemical composition change was attributed to a scavenging effect of the sodium silicate that induced a detrimental reduction of the stabilizing yttrium oxide from zirconia grains. High temperature of the YSZ affected yttrium chemical bonds yielding to the presence of two bonds at the segregated regions attributed to the yttrium oxide (152 eV and 159,1 eV) and to the yttrium silicate (157,8 eV and 159,7 eV). Quite the opposite is the behavior of zirconium which seems to not participate to the formation of the segregated phases. A more detailed discussion has been reported elsewhere [27]. The effects induced by different annealing process on the chemical composition and the valence states of Ce-YSZ-TBC are evaluated by XPS analysis. The stabilization effect exerted by the incorporated ceria can be related to the crystallization process of zirconia. Annealed or thermally shocked coatings led to slight changes of the chemical state of Ce2O3 as exposure to the ambient oxygen to reoxidation to CeO2 [13]. Also the chemical state variation of cerium oxide is influenced by the TBC bulk and not limited to the first layers. Moreover a surface impurity phase enrichment of Na+ and Si4+ was observed. The XPS spectra of Ce3d3/2 and 3d5/2 states for 25.5 wt.% CeO2–2.5 Y2O3–ZrO2 TBCs of as-sprayed APS samples and annealed coatings are shown in Fig. 1a,b. These spectra are consistent with those reported in literature for Ce+4 [27,28]. The Ce3d photoelectron spectra are complex and interpretation is complicated. In Fig. 1b, six peaks referring to three pairs of spin–orbit doublets can be identified and they are characteristic of Ce4+ 3d final states. In particular, the doublet at 882.4 and 900.8 eV is corresponding

Table 2 Atomic ratios of XPS measurements on fractured surfaces of YSZ thermally treated TBCs. Atomic ratios of YSZ thermally treated TBCs Sample

Si/Y

Si/Al

Y/Zr

Si/Zr

Na/Y

As sprayed 1200 °C 1450 °C

n.da 0.95 0.77

n.d 1.6 0.9

0.085 0.13 0.15

n.d 0.125 0.12

n.d 0.95 0.63

a

n.d. Not detected.

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to Ce(IV)3d94f1O2p4 final state; the doublet at 888.8 and 907.4 eV is assigned to the final state of Ce(IV)3d94f1O2p5. Finally, the high binding energy doublet at 898.5 and 916.7 eV is attributed to the final state of Ce(IV)3d94f0O2p6 [29,30], while the spin–orbit doublet at 885.3 and 903.9 eV corresponds to the Ce3+3d final state Ce(III)3d94f2O2p5 [31]. This indicated the presence in the coatings of both Ce4+ and Ce3+ and that the plasma-spraying process could promote the reduction of CeO2 to Ce2O3 according to the following reaction: CeO2 ↔ CeO2−x þ ðx=2Þ O2 ð0 ≤ x ≤ 0:5Þ: This result can be explained by considering that a TBC is produced by successive impingement of several thousand individual particles (about 104 particles mm−3) injected in the hot plasma jet which has been subjected to a reducing atmosphere. Due to the complexity of the Ce3d and Ce4d spectra, no precise quantification was attempted as to the partitioning of the Ce+3 and Ce+4 species. A qualitative analysis of the spectra showed that the annealed or thermally shocked coatings led to slight changes of the chemical state of Ce2O3. Conventionally Ce4+ would not affect O2− vacancies due to its equal valence to Zr4+ whereas the presence of Ce3+ creates oxygen vacancies that will induce more inward diffusion of oxygen. High-temperature processes generally promote changes in valence state, i.e. the reduction of Ce4+ ions to Ce3+ ions leading to an increase in the concentration of oxygen vacancy defects. In fact, at high temperatures a trivalent behavior is evidenced and O2− vacancies are produced that may or may not remain depending on the cooling rate as Ce4+ tends to reduce to Ce3+, causing a stoichiometric shift from CeO2 to Ce2O3. This shift to 3+ valence at high temperatures provides additional vacancies which then become unstable at low temperatures. Upon cooling a shift back to Ce4+ and CeO2 stoichiometry is induced with annihilation of the vacancies produced at high temperatures. A rapid cooling inhibits most O2− ingress meanwhile a slower cooling would allow much more O2− ingress which annihilated the oxygen vacancies produced by cerium. The cerium ion presenting in the trivalent state gives rise to high oxygen vacancy concentrations with an increased stability effect. In particular, the presence of Ce2O3 tends to stabilize a cubic zirconia with pyrochlore structure [32]. The thermal treatment causes partially oxidation of Ce 3+ to Ce 4+ transformation, as evidenced by XPS spectra. Also, peaks related to Al2s, Fe3d and Si2p, respectively at 74.3 eV, 90 eV and 102.3 eV could be observed in Fig. 1. These features depended on the presence of surface impurity phase whose enrichment seemed dependent on the treatments temperature. The presence of silicon, in particular, could be due to the segregation phenomenon of this element which occurred during the interaction between the ceramic powder particles and the plasma-spraying atmosphere. Silicon may originate from the starting raw material used to produce ZrO2, therefore its presence was not completely removed. The Zr 3d5/2 peaks for all three samples have binding energies around 182.5 eV (Fig. 2), in good agreement with those of Zr4+ in ZrO2 [26]. No suboxidation states of Zr (Zr3+, Zr2+, Zr+) or metallic Zr (Zr0) were observed in the Zr 3d spectrum, formed during the oxidation of Zr or due to the reduction of ZrO2 as a consequence of the hightemperature treatment in reducing atmosphere during the plasma spray process. Presence of Zr+3 is formerly observed by XPS and ESR measurements in yttria–zirconia materials subjected to reducing treatments [26]. Y3d binding energies are assigned to the presence of Y3+ as Y2O3 (Fig. 2). The peak broadening of the Y 3d5/2-3/2 doublet in the as-sprayed YSZ suggested the presence of Y2O3-x species. In Table 3, atomic ratios of different elements in Ce-YSZ, as sprayed and thermally treated are reported. The results indicate remarkable qualitative and quantitative differences compared with the starting material. At higher temperatures the data indicate noticeable changes in the chemical composition. In fact, in the annealed samples it is evident in an enrichment silicon, sodium and yttrium, and increasing with temperature. Starting from 1200 °C, an aluminum surface segregation becomes considerable, and at the highest temperature sodium

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Fig. 1. XPS spectra of the Ce4d and Ce3d energy region for Ce-YSZ TBCs, as-sprayed and at different annealing temperatures. On the left, peaks related to Al2s, Fe3d and Si2p, respectively at 74.3 eV, 90 eV and 102.3 eV are clearly visible. On the right doublet peaks related to Ce 3+ and Ce 4+ can be observed.

depletion, probably due to evaporation phenomena, is evident. On the contrary, the continuous decrease of the amount of cerium clearly indicates that cerium does not migrate to form the segregated phase, but rather is covered by the other elements [33]. In order to better understand the chemical composition of the segregated phase, SIMS characterizations have been performed using a microbeam cesium ion source. After heat treatments a strong segregation of impurities was generally observed. In the case of YSZ, the main contaminants were the oxides of Na, Si, Al, Ca, Fe, and Hf. These impurities were inclined to segregate to the surface at similar temperatures to that of Y2O3. Scanning ion images of the YSZ have been previously discussed [26]. Briefly results evinced that enrichment of aluminum and yttrium on fracture surfaces was due to different driving forces as both segregated either separately and on the silicate segregated regions, thus influencing the microstructure of the surface.

Regarding the Ce-YSZ coatings, scanning ion images of Al, Si, Na, Y, Zr and Ce, Ce-YSZ have been reported in Fig. 3. These results reveal that aluminum segregates both at the silicate segregated regions as well as separately and therefore, the enrichment of aluminum on fracture surfaces is probably due to different driving forces. The XPS and SIMS results have evidenced segregation phenomena of bulk dissolved impurities such as Si, Na, Al and Fe as well as of Y2O3 stabilizing oxide. This segregated phase forms on the easy fracture regions an infinite chain silicate of sodium and yttrium, with eventually the presence of aluminum. Furthermore, SIMS ion images have shown that this latter segregates both at the silicate enriched regions as well as separately. From a technological point of view, XPS and SIMS results have evidenced that the presence of a brittle silicate induces a decrease of strength of the TBC and also influences the grain growth. Indeed, internal surfaces coated with a silicate segregated film, provide sites at

Fig. 2. XPS spectra of the Y3d (left) and Zr3d (right) for Ce-YSZ TBC at different annealing temperatures.

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B. Cortese et al. / Thin Solid Films xxx (2013) xxx–xxx Table 3 XPS quantitative chemical composition of Ce-YSZ thermally treated starting powders. Atomic ratios of 25 wt.% CeO2–2.5 Y2O3–ZrO2 (Ce-YSZ) thermally treated TBCs Sample

Ce/Zr

Si/Y

Si/Al

Y/Zr

Si/Zr

Na/Y

As sprayed 1200 °C 1450 °C

0.3 0.21 0.13

5.5 8.5 8

11 5.7 8

0.01 0.02 0.03

0.05 0.17 0.3

2 1.5 1.3

which a fracture can originate and this is surely a strength limiting factor that decreases the TBC high temperature reliability. Furthermore, the presence of a silicate-segregated phase also affects the sintering rate and hence the grain size, which is a critical parameter in determining the high-temperature mechanical properties of these materials.

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The development of thermal stresses within the TBC is strongly related to high-temperature phase changes, grain growth and sintering of the porous microstructure and therefore can severely influence its service life. Even moderately short heat treatments at typical operation temperatures can lead to the formation of sintering necks between the single lamellae of porous TBCs and, therefore a reduced strain tolerance. Operating temperatures of TBCs are normally in the range between 900 °C and 1300 °C with a temperature gradient across the TBC thickness of around 200 μm [34]. FE SEM micrographs of the fracture cross sections for APS sprayed and annealed coatings for the YSZ and the Ce-YSZ showed that the coating differs in the microstructure. For both the as sprayed TBCs, a rapid and directional solidification due the large thermal gradient between the relatively cool substrate and the newly sprayed lamellae was seen (Fig. 4). Columnar grains were formed following an oriented structure perpendicular to the substrate, within

Fig. 3. SIMS ion images of aluminum, silicon, sodium, yttrium, zirconia and ceria obtained on a fracture surface of a Ce-YSZ TBC subjected to air thermal treatment at 1450 °C. The maximum value for the color scale corresponds to the most intense pixel in the ion images.

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Fig. 4. FE-SEM micrographs of YSZ (a, b, c, d) and Ce-YSZ (e, f, g, h) of the as sprayed APS TBCs. The tetragonal phase for the 8 wt.% Y2O3–ZrO2 is clearly visible at the surface region. Scale bar 200 nm.

each lamella, with a thickness in the range from about 100 to 300 nm. Also, the microstructure of the YSZ evidently shows the tetragonal crystalline phases of ZrO2, while there is no apparent presence on the Ce-YSZ TBCs surface, as clearly shown in Fig. 4e, f, g, h. The fractured surface reveals that a higher thickness of the lamellae is approximately between 300 nm and 0.5 μm. The average size of the grains is between 60 and 100 nm. Nonetheless, the Ce-YSZ coating compared to the YSZ coating has finer grain size and more complicated microstructures. Annealing at 1200 °C led to a change of the microstructures showing a tendency to undergo sintering. Changes in the microstructure for the YSZ in length and the thickness of each lamella were clearly visible as shown in Fig. 5, and clearly asymmetrical. In fact the grain orientation showed changes of behavior as growth and replacement of equiaxed grains on the columnar structure tends to disappear, as clearly evident in Fig. 5. The grain surface gradually became spherical like caps as grooves were formed along the boundaries of the columnar grains. Non-uniform grooving and asymmetric grain-boundary were associated

to variations of the surface diffusion coefficient with orientation [35]. Surface diffusivity anisotropy was probably caused by crystallographic faceting from surface energy anisotropy of the plasma-sprayed coating [36] or a heterogeneous morphology across the surface of the coating [37]. On prolonged exposure at elevated temperatures, the metastable t' phase decomposes into high yttria and low yttria phases. The latter transforms on cooling to the monoclinic phase with an associated large volume increase, which eventually can result as a disadvantage and consequently failure of the TBC. The surface of the grain displayed extensive surface roughening assuming a feather-like aspect. The annealing process visibly affected effectively also the morphological surface of Ce-YSZ. The columnar structure collapsed giving rise to smaller grains (roughly 100–200 nm in width) observed on the columns of the lamellae coatings after high-temperature exposure. With further annealing at temperatures from 1200 to 1450 °C, a disappearance of the intralamellar columns fused into a unique

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Fig. 5. FE-SEM micrographs of YSZ (a, b) and Ce-YSZ (c, d) APS TBCs heat-treated at: 1200 °C. The surface of the grains clearly shows an increase of the diameter of the columnar grain to about 1 μm with the higher temperature. Scale bar 1 μm.

structure of the YSZ coatings was observed, as shown in Fig. 6. The surfaces of adjacent grains sufficiently close caused the bridging and disappearing of the grain. As for the Ce-YSZ sample, the structure of the columnar grains coarsened [38] as the existing grains enlarged at the expense of smaller grains during each heat treatment. Microsized fragments were present as 1–5 μm spherical or quasi-spherical clusters as clearly visible in Fig. 6. The high temperature heattreatments (1450 °C) resulted in an even higher tetragonality in the t-ZrO2 phase, and is consistent with the higher concentrations of precipitated. The coating at higher deposition temperature (1450 ºC) is relatively denser and has worse uniformity causing sintering across the interlamellar pores which caused the pores to become more spheroidal and segregated. This densification resulted from grain grooving directed across the pore at depths below the exposed surface grains with surface grains merging with the grain beneath, which was responsible for closing the pore. To investigate the effects of the sintering process on the morphology of the TBCs, a prolonged exposition to 1450 °C for 50 h was carried out. It is evident from Fig. 7, that an irregular distribution of extinction fringes in the grain interior was observed which means that the stresses would be released during annealing process. After 50 h at 1450 °C, sintering was observed to cause spheroidal and segregated agglomerates. This densification resulted from grain grooving directed across the pore at depths below the exposed surface grains and strain energy minimization. At the surface of some grains (Fig. 7d), a terraced structure consisting of a series of layered circular plates was observed. Such terraced structure may have been caused by the faceting process via two-dimensional nucleation. When a layer grows larger than a critical size, it can act as a substrate for the nucleation of new grains. Thus an outward growth in the normal direction takes place together with the lateral growth of each layer, with consequently the yield of a series of terraces. The grains displayed surface roughening of the interfaces enclosed approximately in pyramidal (conical)-shaped monoclinic regions. Observation of nucleation and growth of monoclinic regions on the surface of the tetragonal grain, suggested that during exposure to high temperature, diffusion occurred also along the monoclinic/ tetragonal interfaces. Also it was suggested that formation of ZrO2 phases on the grains was due to segregation of excess of oxygen atoms from interstitial sites to grain boundaries which promoted the

formation of the tetragonal and the monoclinic phases of zirconium oxides as a consequence of heat treatment [39]. The results of this present work enable a closer look at the phenomena behind exposure to typical service temperatures, showing the grounds leading to decreased durability and performance of TBCs. High strength and high resistance to fracture of TBCs are strongly related to the tetragonal phase of doped zirconia. Grain size and interfacial characteristics (e.g. defects) should be important as far as the properties of microcrystal materials were considered. Rumpling of the coatings can introduce out-of-plane stress and initiate damages at the interface. The presence of rounded grains can act as stress concentrators. A reduction of the stabilizing oxide would cause a decrease of the stability of the tetragonal zirconia grains leading more rapidly to a transformation in a monoclinic phase. Impurity segregation would therefore act as a strength limiting factor at high temperature, by increasing the susceptibility to fracture following thermal shock because of their influence on the chemical composition of the grain boundaries, sintering rate and grain size. 4. Conclusion The application requirement of TBCs and their demanding operation conditions are attracting more and more support and motivations for investigation of materials and methods for TBC depositions to broaden their potential. However topography and microstructural aspects of ZrO2-based plasma-sprayed TBCs are significantly affected by relatively short heat treatments at typical service temperatures. This is because high temperature operating conditions accelerate thermally activated processes, leading to volume expansion of the coating and therefore ruptures of the same. Segregation behavior is still very controversial. Besides, the morphology of the deposited microstructure also plays an important role in the enhancement of fracture toughness. To this purpose, 25.5 wt.% CeO2–2.5 Y2O3–ZrO2 (Ce-YSZ) and 8 wt.% Y2O3– ZrO2 (YSZ) coatings prepared via atmospheric plasma spray technique (APS) were investigated. The chemical composition of the coatings was investigated through XPS and SIMS techniques. It was found that thermal treatment strongly affected the balance of the composition of the coatings. FE-SEM investigations revealed deposition morphology with lamellar porosity. Lamellar crystal size and morphology depended

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B. Cortese et al. / Thin Solid Films xxx (2013) xxx–xxx

Fig. 6. Schematization showing the change underwent of the grain after sintering at 1450 °C (a). FE-SEM micrographs of YSZ (b, c) and Ce-YSZ (d, e) APS TBCs heat-treated at: 1450 °C. Scale bar 1 μm.

on the nucleation and growth rates of crystals, on the cooling rate and on the maximum temperature of the substrate which may lead to significant inhomogeneities in the microstructure following reduced performance reliability. With increased annealing temperatures and times, the microstructure of the plasma-sprayed coatings was observed

to coarsen and sinter. Diffusion processes during thermal treatment, caused material redistribution and surface roughening. At annealing temperatures of 1200 °C and above, surrounding columnar grains were able to bridge the widths of the voids and come into contact, sintering with other bridging grains. Interlamellar pores were closed

Fig. 7. FE-SEM micrographs of 8 YSZ (a, b) and Ce-YSZ (c, d) APS TBCs heat-treated at: 1450 °C for 50 h. Scale bar 1 μm.

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B. Cortese et al. / Thin Solid Films xxx (2013) xxx–xxx

at annealing temperatures of 1450 °C. Surrounding columnar grains were able to bridge the widths of the voids and come into contact, sintering with other bridging grains. The presence of a coarser surface as evident is advantageous as it can absorb eventually crack energy and limit further movement within the microstructure. During isothermal aging, phase separation occurred due to the diffusion of stabilizing elements. It was observed that Ce-YSZ showed higher phase stability than YSZs. In order to develop the full potential of TBCs, future research must focus on understanding the interplay among the processing parameters, service conditions, and the properties of TBCs to provide reliable data for the design of a TBC of improved lifetime. Acknowledgment This work was financially supported by the FIRB RBPR05JH2P ITALNANONET (Research Unit Padeletti ISMN-CNR) project funded by the Italian MIUR. The authors would like to thank Mr. Fulvio Federici for the assistance and useful technical support. References [1] C.C. Berndt, W. Brindley, A.N. Goland, H. Herman, D.L. Houck, K. Jones, R.A. Miller, R. Neiser, W. Riggs, S. Sampath, M. Smith, P. Spanne, J. Therm. Sci. Technol. 1 (1992) 1. [2] D.R. Clarke, S.R. Phillpot, Mater. Today 8 (2005) 22. [3] R. Rishi, J. Am. Ceram. Soc. 76 (1993) 2147. [4] H.D. Steffens, U. Fisher, Surf. Coat. Technol. 32 (1987) 327. [5] V. Lughi, D.R. Clarke, Surf. Coat. Technol. 200 (2005) 1287. [6] M.R. Winter, D.R. Clarke, Acta Mater. 54 (2006) 5051. [7] M.D. Chambers, D.R. Clarke, Surf. Coat. Technol. 201 (2006) 3942. [8] D.E. Mack, S.M. Gross, R. Vassen, D. Sover, J. Therm. Spray Technol. 15 (2006) 652. [9] R.B. Heimann, Plasma-Spray Coating: Principles and Applications, VCH, Weinheim, Germany, 1996. 164 , (209–23). [10] C.C. Berndt, P. Michlik, O. Racek, Proc. Intl. Thermal Spray Conf., Osaka, Japan, 2004, p. 1110. [11] W. Holmes, B.H. Pilsner, in: D.L. Houck (Ed.), Thermal Spray: Advances in Coatings Technology, ASM International, Materials Park, OH, 1998, p. 259.

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