Lanthanum zirconate thermal barrier coatings deposited by spray pyrolysis

https://doi.org/10.1016/j.surfcoat.2012.09.019Get rights and content

Abstract

The fabrication of thick nanostructured lanthanum zirconate thermal barrier coatings deposited by spray pyrolysis from aqueous nitrate based precursor solutions is presented. After deposition, the green coatings are decomposed to introduce vertical cracks which are beneficial to enhance the thermo-mechanical compliance of the coatings. The cracks were stable after crystallization of the coatings to the cubic pyrochlore structure by further heat-treatment. Nanocrystalline mono- and multi-layered coatings with contrasting crack patterns were produced based on the knowledge of the influence of the different parameters. Multilayered coatings with small crack spacing and crack opening exhibited a higher density, a lower thermal diffusivity, and a higher thermal conductivity of ~ 0.34 W/(m·K) at elevated temperatures compared to monolayered coatings of similar thickness with larger crack spacing and crack opening. The low thermal properties of the crystalline coatings were attributed to the nanostructure of the coatings.

Highlights

► Spray pyrolysis of La2Zr2O7 coatings from aqueous nitrate based solutions is reported. ► Thick, porous and crystalline coatings were obtained after heat treatment. ► Mono- and multi-layered coatings were produced with different crack patterns. ► Low thermal diffusivity of multilayered coatings is promising for long-term applications.

Introduction

Thermal barrier coatings (TBCs) are multilayered material systems deposited on modern gas-turbine engines to thermally insulate them and to protect them against the hot and corrosive gas stream. State-of-the-art TBCs consists of a NiCoCrAlY bond-coat with a thickness of 75–150 μm applied on a nickel based superalloy and an yttria-stabilized zirconia top layer with a typical thickness of 100–400 μm deposited by air plasma spraying (APS) or electron-beam physical-vapor deposition methods (EBPVD) [1]. Further improvements of TBCs involve the investigation of new material candidates, such as the promising lanthanum zirconate [2], [3], the deposition of nanostructured coatings [4], [5], which are beneficial to reduce the thermal conductivity of the top layer, and the development of multilayered or graded coatings of different compositions [6]. In addition, the use of liquid precursor feedstock to achieve better properties at lower cost [8], and the optimization of the microstructure with dense vertical cracks to increase the thermo-mechanical compliance [7] are two recent routes considered with great potential. Nevertheless, whereas the importance of vertical cracks has been demonstrated to improve the lifetime of the TBCs [9], the generation of vertical cracks in a suitable microstructure has only been possible using liquid precursor feedstock [10]. In addition, to the authors' knowledge, no work has been carried out to design such crack patterns to further enhance the properties.

Accordingly, spray pyrolysis is a wet chemistry method with a great potential which aims at atomizing a liquid precursor towards a heated substrate to produce thin or thick, dense or porous, cracked or crack-free films or coatings [11]. The deposition of thick porous coatings, as well as thin dense films, corresponds to the deposition of ionic salt precipitates [12], [13], and crystalline oxide layers are obtained after a final heat-treatment step [14]. Thus, this study aims at investigating the feasibility of depositing nanostructured TBCs by spray pyrolysis from environmentally friendly aqueous nitrate based precursor solutions. Emphasis was placed on the ability of depositing thick coatings, the capability of producing coatings with various crack patterns and the resulting effect on the thermal properties.

Section snippets

Preparation of the coatings

Zirconyl (IV) oxynitrate hydrate (ZrO(NO3)2·xH2O; Sigma-Aldrich Gmbh, 99%) and lanthanum (III) nitrate hexahydrate (La(NO3)3·6H2O; Alfa Aesar Gmbh & Co., 99.9%) were dissolved in deionized water and further mixed in a molar ratio of 1:1 of La to Zr after standardization of the solutions by the thermogravimetric method. The final concentration of the precursor solution was ~ 0.25 mol/L.

The precursor solution was deposited by a home-made air-blast spray pyrolysis apparatus described elsewhere [12]

Producing the crack pattern

The influence of the spray pyrolysis parameters on the crack pattern of decomposed lanthanum zirconate coatings was studied. The deposition and decomposition temperatures did not exhibit a large effect on the microstructure due to a too small variation in composition of the coatings within the investigated temperature intervals. Nevertheless, the crack spacing increases linearly with increasing thickness of the decomposed lanthanum zirconate coatings as illustrated in Fig. 1.

Accordingly, mono-

Discussion

Green coatings deposited by spray pyrolysis are crack-free after deposition and cracking occurs during the decomposition of the deposited nitrate or oxynitrate species due to the associated volume change [12]. Assuming a simple model for the decomposition of the coatings, the estimated volume change from a green metal salt based coating deposited at 280 °C at a distance of 20 cm to the porous oxide coating is around 54%. Furthermore, the acquired knowledge about film cracking has barely been used

Conclusion

Nanostructured mono- and multi-layered lanthanum zirconate coatings with vertical crack patterns were deposited by spray pyrolysis of aqueous nitrate based precursor solutions according to the influence of the deposition parameters. The difference in the crack patterns is due to the deposition and decomposition of layers with different thicknesses. The resulting mono- and multi-layered coatings with a thickness of ~ 200 μm were porous and crystalline after heat treatment with nanoparticles of,

Acknowledgments

Financial support from NTNU (Strategic area of Materials) is acknowledged.

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