Microstructural, mechanical and thermal characteristics of zirconia-based thermal barrier coatings deposited by plasma spraying

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Microstructural, mechanical and thermal characteristics of zirconia-based thermal barrier coatings deposited by plasma spraying G. Di Girolamo, C. Blasi, A. Brentari, M. Schioppa

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Cite this article as: G. Di Girolamo, C. Blasi, A. Brentari, M. Schioppa, Microstructural, mechanical and thermal characteristics of zirconia-based thermal barrier coatings deposited by plasma spraying, Ceramics International, http://dx.doi.org/10.1016/j. ceramint.2015.05.145 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Microstructural, mechanical and thermal characteristics of zirconia-based thermal barrier coatings deposited by plasma spraying G. Di Girolamoa,*, C. Blasib, A. Brentaric, M. Schioppab a

ENEA, Materials Technology Unit, Casaccia Research Center, Rome, Italy

b

ENEA, Materials Technology Unit, Brindisi Research Center, Brindisi, Italy

c

ENEA, Materials Technology Unit, Faenza Research Center, Faenza, Italy

* Corresponding author: [email protected]

Abstract Ceramic thermal barrier coatings (TBCs) are potential tools to increase the durability of metal parts operating in turbine engines. In this work, partially yttria stabilized zirconia (YSZ) and lanthanum zirconate (LZ) coatings were deposited by plasma spraying on stainless steel substrates. X-Ray Diffraction (XRD) analysis revealed that plasma spraying promoted the formation of metastable phases in both the cases. Both YSZ and LZ coatings exhibited porous microstructure with a network of embedded pores and microcracks, that resulted in similar porosity values. The average microhardness of LZ coating was about 92 % of that of YSZ one (5.4 against 5.9 GPa), whereas the thermal expansion coefficient (CTE) of LZ coating was about 86 % of that of YSZ one. Based on the results herein discussed the development of multilayered microstructures represents a promising solution in order to develop TBCs with enhanced performance. Keywords: A. plasma spraying; C. microstructure; thermal expansion coefficient; D. zirconia; E. thermal applications

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1 Introduction Several hot-section metallic components of turbine engines, such as blades, vanes and transition pieces, are notoriously subjected to fast degradation at typical service conditions, because of the high temperature they experience as well as of the severe attack promoted by oxygen and molten salts [1,2]. The application of overlay metallic coatings allows to improve their lifetime, by protecting their surface against oxidation and hot corrosion [3]. However, the operating temperatures of next-generation turbines are substantially near the temperature capability of the MCrAlY alloys (M = Ni and/or Co). In addition, air cooling is often required to reduce the temperature of the component surface. Therefore, the constant demand for more efficient engines suggests the development of materials and systems with better performance. It is well recognized that ceramic thermal barrier coatings (TBCs) with low thermal conductivity are able to reduce the heat transfer to the metallic surface as well as the damaging promoted by hot corrosion and erosion [4,5]. Plasma spraying is a cost-effective technology for fabrication of porous and strain tolerant ceramic TBCs. Partially yttria stabilized zirconia (YSZ) is the state-of-the-art TBC material, owing to its good hightemperature properties as well as to the established knowledge about the related performance in turbine service environment [6]. However, the attention of the researchers is actually focused on materials with better temperature capability because, when exposed at very high temperature (> 1200 °C), YSZ TBCs are commonly affected by phase changes and accelerated sintering, this last involving reduction of strain tolerance and increase of thermal conductivity [7,8]. In the last decade a great variety of TBC materials has been proposed, including zirconates, perovskites and hexa aluminates [9,10,11,12,13]. Among them rare-earth zirconates seem particularly promising because of their high-temperature stability, low thermal conductivity and low ionic conductivity, which is expected to reduce the oxygen penetration and the 2

following formation of thermally grown oxide (TGO) at bond coat/top coat interface, that is recognized as the main factor assisting TBC spallation [14]. However, some technical restrictions have been remarked for single lanthanum zirconate (LZ) TBCs: their relatively low thermal expansion coefficient (CTE) and fracture toughness can assist cracking and delamination when the substrate expands at high temperature [15,16]. Therefore, further investigations are needed in order to explain some aspects about the mechanical and thermal properties of these coatings, and to enhance their characteristics, by producing optimized microstructures, in terms of total porosity, microcrack network distribution and phase composition. Particles with various morphologies can be processed to this aim. A previous study, focused on LZ TBCs deposited using spray dried spherical particles, demonstrated that LZ coatings possess good resistance to sintering and promising high-temperature mechanical properties [17]. In this work YSZ and LZ coatings were deposited by plasma spraying. Their phase composition, microstructure, microhardness and thermal expansion were investigated. The study was then addressed to the development of multilayered TBCs.

2 Experimental All the coatings were deposited using an atmospheric plasma spraying (APS) system equipped with F4-MB plasma torch with 6 mm internal diameter nozzle (Sulzer Metco, Wolhen, Switzerland). Stainless steel substrates (Aisi 310S, 25x25 mm) were sand blasted with alumina abrasive particles to increase their surface roughness and the mechanical interlocking between coating and substrate. The measured roughness of substrate surface was of about 7 µm. The substrates were ultrasonically cleaned in ethanol, placed on a rotating sample holder and coated with 160 µm thick NiCoCrAlYRe bond coat, using the processing 3

parameters reported elsewhere [18]. Two different ceramic powder feedstocks were used for TBC deposition, herein named as YSZ (ZrO2-8Y2O3, Metco 204NS, Sulzer Metco, Westbury, USA) and LZ (La2Zr2O7, Treibacher Industrie AG, Althofen, Austria). The total thickness of ceramic TBCs, single or double layer, was of about 600 µm. The thickness of the samples addressed to thermal characterization was increased to 2 mm. The plasma spraying parameters employed in this work are summarized in Table I. During deposition the substrates were cooled using two air jets at 5 bar pressure. The deposition efficiency was calculated from the ratio between the coating mass and the total feedstock mass delivered to the plasma torch, which was determined from the powder feed rate and the spraying time on the substrate. The average deposition rate, i.e. the thickness per torch pass, measured by a digital multimeter with resolution of 1 µm, was 12.3 ± 1.2 µm for YSZ coating and 9.3 ± 0.6 µm for LZ coating. The deposition efficiencies of YSZ and LZ powder feedstocks were 48 ± 5 % and 73 ± 4 %, respectively. The phase composition of powder feedstocks and as-sprayed coatings was analyzed by X-Ray Diffractometer (PW1880, Philips, Almelo, Netherlands) operating with CuKα radiation, produced at 40 kV and 40 mA. The θ-2θ scan was performed between 20 and 80°, by step width of 0.02° and 5 s time per step. Coating samples were cut by low-speed diamond saw, cold mounted in vacuum in polymer and polished to 0.25 µm. The polished cross sections were then analyzed by Scanning Electron Microscopy (SEM-LEO 438 VP, Carl Zeiss AG, Oberkochen, Germany). Cross sectional SEM images were processed by image analysis software to measure the average porosity of as-sprayed YSZ and LZ coatings. For each sample ten micrographs at two different magnifications, i.e. 1000 and 2000 x, were analyzed. In order to minimize the uncertainty in porosity measurements, all the images were acquired using the same setting values and then transformed in binary images by auto-tresholding. Image analysis is very

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sensitive to the parameters used by the operator, but it can be easily employed to investigate the detailed microstructure of plasma sprayed coatings. For example, it has been successfully adopted to quantify the effects of high-temperature exposure on zirconia-based TBCs, such as the formation of sintering necks at splat boundary and the partial closure of fine microcracks [19]. Microhardness measurements were carried out at room temperature on the polished cross sections of the coatings using a Vickers microindenter (VMHT MOT, Leica Microsystems, Weztlar, Germany). The indentations were performed at 300 gf for a dwell time of 15 s. The spacing between the indentations was kept at least three times the diagonal to avoid stresses produced by the interaction between consecutive indentations. The microhardness data were then analyzed by Weibull statistics. Free standing samples were employed to measure the thermal expansion coefficient of plasma sprayed YSZ and LZ coatings. The coated samples were cut and stripped from the underlying metal by chemical etching, by using a 50/50 (vol.%) HCl-H2O solution, then cleaned and dried. The linear thermal expansion was measured using a Netzsch dilatometer (TMA 402, Netzsch- Geratebäu GmbH, Selb, Germany) in static air atmosphere. The model herein employed is a vertically quartz pushrod type using a linear variable displacement transducer (LVDT) with thermo-stated housing. The thermal expansion measurements were carried out on 7 mm long sample bars, from room temperature up to 900 °C, at heating rate of 10 °Cmin1

. A certified synthetic sapphire sample (NS SRM 732) was employed for instrument

calibration. Three consecutive scans were collected for each type of sample.

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3 Results and Discussion 3.1 Phase composition Figure 1a shows the XRD spectra of YSZ powder and as-sprayed coating, while Fig. 1b shows the XRD spectra related to LZ powder and coating. High-temperature plasma spraying, characterized by melting of micronsized powder particles followed by quenching at the substrate surface, promoted the formation of metastable phases. Based on the Joint Committee in Powder Diffraction Standards (JCPDS), available at International Centre for Diffraction Data, the YSZ powder was composed of a mixture of tetragonal (card No. 81-1544) and monoclinic (card No. 37-1484) zirconia phases. In turn, plasma sprayed YSZ coating was mainly composed of metastable tetragonal t’ phase with small amounts of cubic (card No. 491642) and monoclinic zirconia phases, derived from unmelted particles embedded in the coating microstructure. As reported in a previous work, during prolonged exposure at temperature higher than 1300 °C followed by cooling to room temperature the partial decompositon of the metastable t’ zirconia phase leads to the formation of stable tetragonal zirconia phase, gradually increasing the amount of the monoclinic phase in the coating [8]. The formation of monoclinic phase is notoriously accompanied by volume expansion and significant thermal stresses which can affect the durability of the TBC system. The LZ powder feedstock was composed of pyrochlore cubic structure (card No. 71-2623) with a small amount of La2O3. Based on the La2O3-ZrO2 phase diagram this pyrochlore structure is typically formed by doping zirconia with >35 mol % La2O3 and it is stable up to the melting point (~2300 °C) [20]. As shown in Fig. 1b, plasma spraying of LZ particles promoted the formation of defective fluorite structure. Indeed, during the flight of melted sprayed particles and their quenching at the substrate surface a potential loss of lanthanum occurs, producing locally varying stoichiometry. On the contrary, the particles which were 6

solidified in the plasma jet were deposited as pyrochlore structure [21]. This is confirmed by the missing peaks in the related XRD pattern, i.e. the peaks indexed as (311), (331), (511) and (531) in the spectrum referred to LZ powder feedstock. Peak broadening suggests small crystallite size and crystal disorder. It has been demonstrated that high-temperature exposure of LZ TBCs at 1350 °C promotes crystallite growth and transformation of fluorite-type structure to ordered pyrochlore phase, without formation of monoclinic zirconia phase [17]. The high phase stability of LZ coatings at elevated temperature represents a significant improvement with respect to conventional YSZ coatings. In addition, as reported in [22], the oxygen defects in conventional yttria stabilized zirconia (YSZ) structure are very mobile and can contribute to the sintering of the microstructure, whereas in the pyrochlore structure the oxygen defects are ordered and, hence, are more resistant to high-temperature sintering. Figure 1c shows the XRD patterns of NiCoCrAlYRe powder and coating, which are mainly composed of β-NiAl intermetallic phase. This suggests that no significant Al depletion occurred at the surface of the melted metallic particles during spraying, i.e. their in-flight oxidation was restricted. The small peaks of γ-(Ni,Cr,Co) phase tend to disappear in assprayed coating, where the peak broadening is representative of grain refinement during processing. It is well known that the Al-rich β phase acts as an Al reservoir and assists the formation of a protective Al2O3 scale during next selective oxidation.

3.2 Microstructure The micrographs reported in Fig. 2 show the SEM morphology of YSZ and LZ powder particles. The YSZ powder feedstock is composed of spherical hollow particles with diameter between 10 and 125 µm. Figure 2b shows the detailed morphology of nanosized YSZ grains which are well detectable within the micronsized agglomerates produced by spray drying. As shown in Fig. 2c, the LZ powder feedstock is composed of fused and crushed particles with 7

typical blocky or angular morphology and narrower size distribution in comparison with YSZ particles. Figures 3a and 3b present the microstructure of as-sprayed YSZ and LZ coatings. Their porous lamellar microstructure is characterized by the presence of some typical defects, such as pores, splat boundaries and vertical microcracks. The pores are associated to filling defects and to the gas entrapped between the molten splats during coating build-up. The splat boundaries were produced by weak bonding between the overlapped splats, owing to the temperature gradient along the growing thickness, whereas the vertical microcracks derived from the relaxation of thermal stresses during cooling to room temperature. The splat boundaries typically affect the value of thermal conductivity, whereas the vertical microcracks influence high-temperature strain tolerance of the TBC. As well observable in Fig. 3a, the as-sprayed YSZ coating exhibits a splat-like microstructure with lamellae separated by splat boundaries and whose thickness is approximately in the range between 1 and 7 µm, depending on the size as well as on the flattening degree of the impacting molten particles. The lamellar microstructure is less observable in plasma sprayed LZ coating, because of the irregular morphology of the starting particles employed. A finer and more uniform distribution of voids and crack networks is observable with respect to the YSZ coating, where the retention of partially melted particles produced isolated pull-out effects during polishing step. As shown in Fig. 4a, the interface between NiCoCrAlYRe bond coat and LZ coating is characterized by qualitatively good adhesion. The average roughness measured on the NiCoCrAlYRe bond coat surface before TBC deposition was 13.5 µm, two times higher than that of the underlying substrate surface. As detectable in Figs.4a and 4b, the NiCoCrAlYRe coating exhibits two-phase microstructure, where darker β-NiAl rich precipitates are embedded in γ-NiCrCoAlYRe matrix and are well dispersed in coating microstructure. They 8

represent an aluminum reservoir during selective oxidation [18]. Figures 4c and 4d show some examples of local EDS spectra from γ phase (spot 1 in Fig. 4b) and β-rich phase (spot 2 in Fig. 4b), respectively. Dark gray β precipitates are more rich of Al in comparison with the surrounding matrix. They also exhibit higher content of Ni as well as lower content of Cr, Co and Re. As previously reported, after processing no depletion of Al was noticed by EDS analysis in the NiCoCrAlYRe coating with respect to the powder feedstock [18]. Splat boundaries and pores originated by weak bonding between overlapped splats during processing and pull-out effects during polishing, respectively, are also observable in the microstructure displayed in Fig. 4b. Figure 5 shows the top-surface of La2Zr2O7 coating. Some networks of small cracks are well detectable within the smooth molten splats (denoted by white arrows in the picture), as a result of stress relaxation during solidification and quenching at the substrate surface. The morphology of the coating top-surface is associated to the degree of melting of the sprayed particles, i.e. to their thermal history in the plasma jet, as well as to their kinetic energy upon impact, that affects their flattening degree. Energy Dispersive Spectroscopy (EDS) analysis demonstrated that plasma spray processing produced La2O3 loss, since the average wt.% of La2O3 measured in the as-sprayed LZ coating was lower than that found in the starting powder particles (54.3 % against 57.5 %). The porosity level, measured by image analysis, was 12.2 ± 1.6 % for as-sprayed YSZ coating and 12.2 ± 2.4 % for as-sprayed LZ coating. It is worth noting that plasma sprayed lanthanum zirconate coatings deposited using spray dried spherical particles at constant processing parameters had exhibited higher porosity value (~ 15 %) and good resistance to hightemperature sintering [17].

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3.3 Mechanical and thermal properties The mechanical tests were performed on the cross section of the coatings. The indentation size was significantly larger than the size of all the typical microstructural features, so that the scatter found in the experimental data allowed to evaluate the effect of the local porous microstructure on the resulting mechanical properties. Figure 6 shows the Weibull distribution of microhardness data measured on as-sprayed YSZ and LZ coatings. The data measured on stainless steel substrate and NiCoCrAlYRe bond coat are also reported. The values of Weibull modulus are herein indicated; the modulus reflects the scatter in the experimental data, related to the presence of microstructural defects in the coatings. The mean values of microhardness data are reported in Table II. The average microhardness of as-sprayed YSZ coating was 5.9 GPa, while the average microhardness of LZ coating was found to be 5.4 GPa, similar to the value reported in [23] for plasma sprayed LZ coating with lower porosity (~7.5 %). Therefore, the microhardness of LZ coating was 92 % of that of YSZ coating, even if it should be highlighted that the scatter in the data distribution of LZ coating was smaller, because of the finer distribution and size of voids and microcracks. As remarked in the previous section, the shape and the size of these microstructural features strongly depend on the morphology and granulometry of the starting powder particles. As general remark, the application of a ceramic TBC on the bond coated substrate allows to improve the mechanical properties and the resistance to wear and erosion phenomena. Figure 7 shows the averaged thermal expansion curves for as-sprayed YSZ and LZ coatings. The thermal expansion approximately increases linearly over the temperature range and the shrinkage was 9.3x10-3 for YSZ sample and 7.9x10-3 for LZ sample. The averaged CTE values of as-sprayed YSZ and LZ coatings, measured between 50 and 900 °C, are summarized in Table II. The CTE value of lanthanum zirconate coating was 86 % of that of YSZ one. It is 10

worth noting that the values herein obtained are higher than those previously reported for similar coatings [24,25]. In particular, the reason for the improvement of CTE value can be found in the microstructure of the LZ coating, herein characterized by more uniform distribution of crack networks as well as by lower stress state in planar direction after the deposition of the melted particles at the substrate surface. To this purpose, it is acceptable that the distribution of the microstructural features within the coating can produce some anisotropic effects, since the interfaces are highly curvy and are microscopic regions of structural mismatch [26]. Previous studies focused on the CTE evolution of YSZ coatings after high-temperature exposure have highlighted the dependence of the CTE on various factors, such as microstructure, thermal stress relaxation and local structures, i.e. areas with different stoichiometry or oriented crystallites at boundary layers [8,26]. It is clear that the combined interaction between these factors cannot be quantified. Moreover, it was found that the CTE of YSZ coating tended to increase with increasing the testing temperature, from 9.2 x 10-6 K-1 at 100 °C to 14.3 x 10-6 K-1 at 900 °C. Otherwise, the CTE of LZ coating showed a more complex trend as the temperature increased. It was close to 10 x 10-6 K-1 at 100 °C. Between 200 °C and 400 °C rapidly dropped to values lower than 8 x 10-6 K-1, and then increased up to 900 °C. These fluctuations could be probably related to the relaxation of internal thermal stresses during heating cycle. A similar behavior has been found for LZ coatings produced using spray dried spherical particles [17]. It should be noted that the thermal expansion mismatch between the ceramic TBC and the underlying metal can produce high stresses at the interface during high-temperature cycling, thus assisting delamination and TBC spallation. To this purpose previous experiments have demonstrated that single lanthanum zirconate TBCs failed by spallation at very low number of cycles, while YSZ TBCs survived several hundreds of thermal cycles at peak temperatures higher than 950 °C [27,28]. The results

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herein discussed are promising in order to develop TBCs with tailored multilayered microstructure (YSZ+LZ). The application of an intermediate YSZ layer is able to compensate for the relatively low CTE and toughness of LZ TBC, thus reducing the stresses at the interface and improving the adhesive strength of the same TBC system. It has been reported that, when exposed at 1300 °C for 50 h, single LZ TBC is typically characterized by spallation from the TGO, because the relatively low toughness of LZ coating assists lateral crack formation and propagation at the interface, so that the lifetime is lower than that of YSZ one. Otherwise, in the case of multilayered TBC the formation of an adhesive and homogeneous TGO prevents delamination and spallation, thus providing better oxidation resistance and durability [29]. Lanthanum zirconate exhibits lower oxygen ionic diffusivity than YSZ (9.2±0.3 x 10-4 Ω-1cm1

against 0.1 Ω-1cm-1) at 1000 °C [30], so that can restrict the oxygen diffusion mechanism

through the crystalline structure, whereas the most noticeable gas penetration mechanism is associated to the distribution of pores and microcracks within the microstructure. By reducing the oxygen partial pressure and the oxygen activity at the interface between ceramic TBC and metal bond coat the growth rate of TGO is reduced, thus positively affecting the stress state at the interface and the adhesive strength. The formation of a dense and continuous Al2O3-rich TGO is promoted, because the formation of spinels commonly requires higher oxygen activity and generally occurs because of Al depletion and reaction of Ni and Cr with oxygen. Further improvements are expected in terms of hot corrosion resistance of multilayered TBCs. It is well known that during fuel combustion, some detrimental compounds, such as vanadates and sulfates, are formed. It has been reported that lanthanum zirconate TBC was more resistant than YSZ one at the attack of V2O5 at 1000 °C: no spallation and no meaningful changes in microstructure occurred. Otherwise, YSZ TBC failed by cracking and spallation

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[31]. However, when LZ TBC was attached by sulfates at 900 °C, it was rapidly degraded after a few hours, whereas after 360 h YSZ TBC preserved structural integrity and adhesion to the substrate. Moreover, the application of an upper LZ coating on the surface of zirconia-based TBC improves the temperature capability, reduces the thermal conductivity of the TBC system and improves the related high-temperature sintering resistance [17,29]. Multilayered TBCs can better tolerate the thermal stresses arising from thermal expansion mismatch during thermal cycling. In addition, they can retard the formation of the thermally grown oxide at the interface between top coat and bond coat and reduce the related thickness, thus improving the component lifetime at temperature higher than 1350 °C [30]. The current researches are then focused on the development of multilayered TBCs by properly combining the basic characteristics of different zirconia-based materials: an example is reported in Fig. 8a. The system herein displayed is composed of a ceria-yttria co-stabilized zirconia (CYSZ) coating, deposited on the bond coat surface and followed by an upper LZ layer. The porosity of the CYSZ layer is close to 10 %. As reported elsewhere, CYSZ is a good TBC candidate in substitution of YSZ, because it exhibits higher CTE (12.6 x 10-6 against 10.7 x 10-6 K-1), higher hardness (6.5 against 5.9 GPa), higher phase stability and good resistance to high-temperature sintering [19,32]. The CTE of CYSZ coating is more close to that of the bond coat material and not quite different from that of LZ coating herein manufactured. Figure 8b shows a detail of the interface between plasma sprayed CYSZ and LZ layers. The application of an alumina coating on the surface of zirconia-based TBC is a further technological solution to increase the resistance to erosion, corrosion and oxygen infiltration [33,34]. Indeed, it acts as diffusion barrier to suppress the formation of spinels at the interface during thermal exposure at 1000-1100 °C. Alumina is characterized by low oxygen ionic 13

diffusivity, so that it is able to restrict the diffusion channels for oxygen and reduce the value of oxygen partial pressure at the interface between zirconia TBC and metal bond coat, thus promoting the formation of a dense and continuous Al2O3-rich TGO and suppressing the growth of detrimental spinel-type oxides [35]. By reducing the permeation of oxygen towards the bond coat, the growth rate of the TGO decreases and lesser tensile stresses arise at the interface. It is well known that high stress state can assist horizontal cracking and TBC spallation: spallation typically occurs when the length of lateral cracks reaches the length of the critical crack. The formation of a dense and continuous Al2O3-rich TGO improves both the oxidation resistance and the adhesive strength, thus prolonging TBC lifetime [36,37]. At high temperature oxygen can propagate through the YSZ layer via ionic diffusion mechanism through the crystalline structure and via gas penetration mechanism through pores and microcracks. Oxygen penetration through the crystalline structure of Al2O3 and LZ is much lower than that in YSZ. Nanostructured Al2O3 coatings exhibits lower amount of pores and microcracks than their conventional counterparts and are more suitable to reduce oxygen permeation because of their higher density. Lower TGO growth rate and reduced formation of spinels are then noticed [38,39]. The use of nanostructured materials can produce further improvements. To this purpose nanostructured bond coat is more prone to assist formation of a continuous and dense alumina TGO layer, reducing the oxygen partial pressure at the interface and suppressing the fast growth of spinel-type brittle oxides which are notoriously affected by cracking and delamination.

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At the same time the application of an Al2O3 top layer on zirconia-based TBC is able to reduce the infiltration of molten salts (V2O5+Na2SO4) at 1050 °C, the formation of monoclinic zirconia and YVO4 crystals and the delamination, thus improving both the hot corrosion resistance and the lifetime of the TBC system [37,40,41].

4 Conclusions Yttria stabilized zirconia (YSZ) and lanthanum zirconate (LZ) thermal barrier coatings were deposited by plasma spraying on metal substrates, previously coated with NiCoCrAlYRe bond coat. Plasma spray processing promoted the formation of metastable phases, i.e. tetragonal t’ phase in YSZ coating and fluorite defective structure in LZ coating. According to SEM analyses both the coatings exhibited porous microstructure with lamellae embedded in a network of pores, splat boundaries and microcracks. YSZ and LZ coatings showed similar values of porosity: a finer distribution of voids and microcracks was noticed in LZ coating. LZ coating exhibited values of microhardness and CTE not quite different from those of YSZ one. More uniform distribution of microcrack networks and lower stress state after processing increased the CTE of LZ coatings with respect to similar coatings produced in previous works. Based on the results herein discussed multilayered TBCs seem particularly promising to increase the performance of single-layer TBC systems. YSZ can be used as intermediate coating or replaced by zirconia doped with different stabilizers. For example, ceria-yttria costabilized zirconia notoriously exhibits a CTE value more close to that of the underlying bond coat as well as enhanced high-temperature properties. Lanthanum zirconate or alumina top layers can be successfully applied on the surface of zirconia-based TBC: these multilayered systems improve hot corrosion resistance, reduce the thermal stress state due to CTE mismatch between overlapped layers and restrict the growth rate of the TGO at the interface, 15

preventing spallation and prolonging the lifetime. The reduction of oxygen infiltration towards the bond coat through TBC microstructure is able to reduce the oxygen partial pressure at the interface, promoting the formation of a dense and continuous Al2O3-rich TGO layer. In conclusion, multilayered TBCs with enhanced durability can be potentially engineered by proper matching the microstructural, mechanical and thermal properties of various ceramic materials.

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19

YSZ

LZ

Current [A]

600

500

Voltage [V]

64

61

Primary gas flow rate [slpm]

33

35

Secondary gas flow rate [slpm]

10

8

Carrier gas flow rate [slpm]

2.6

3

Powder feed rate [gmin-1]

42.6

19

Injector diameter [mm]

1.8

1.8

Torch-substrate distance [mm]

100

100

Turntable tangential speed [mms-1]

2083

2083

Table I – Plasma spraying parameters used in this work.

20

Substrate

NiCoCrAlYRe

YSZ

LZ

Microhardness [GPa]

3.1 ± 0.3

5.0 ± 0.7

5.9 ± 1.0

5.4 ± 0.5

CTE [10-6 K-1]

-

-

10.7

9.2

Table II – Microhardness data and CTE values of as-sprayed YSZ and LZ coatings. The microhardness values of substrate and bond coat are also reported.

21

Figure Captions Figure 1 – XRD spectra of a) YSZ, b) LZ and c) NiCoCrAlYRe powders and coatings. Figure 2 – SEM pictures of a) YSZ hollow spherical particles, b) YSZ nanograins in micronsized agglomerated particles, c) distribution of LZ powder particles and d) LZ particles with blocky and angular morphology. Figure 3 – Cross sectional SEM pictures showing the lamellar microstructures of a) YSZ and b) LZ coatings. Figure 4 –SEM pictures showing a) the interface between the LZ coating and the underlying NiCoCrAlYRe bond coat and b) the cross sectional microstructure of NiCoCrAlYRe coating showing β precipitates embedded in γ matrix. Figures 4c and 4d show spot EDS spectra from γ phase and β-rich phase, respectively. Figure 5 – SEM morphology of LZ coating top-surface, showing the presence of fine microcracks embedded in the molten splats. Figure 6 – Weibull distribution of microhardness data for as-sprayed YSZ and LZ coatings. Figure 7 – Thermal expansion curves for YSZ and LZ samples. Figure 8 – Cross sectional SEM microstructure showing a) multilayered TBC system, composed of NiCoCrAlYRe bond coat, ceria-yttria co-stabilized zirconia (CYSZ) intermediate coating and external LZ coating, and b) the interface between plasma sprayed CYSZ and LZ coatings.

22

Figure 1a

Figure 1b

Figure 1c

Figure 2a

Figure 2b

Figure 2c

Figure 2d

Figure 3a

Figure 3b

Figure 4a

Figure 4b

Figure 4c

Figure 4d

Figure 5

Figure 6

Figure 7

Figure 8a

Figure 8b