Interaction between ceramic powder and molten calcia-magnesia-alumino-silicate (CMAS) glass, and its implication on CMAS-resistant thermal barrier coatings

doi:10.1016/j.scriptamat.2015.09.027

Scripta Materialia

Volume 112, February 2016, Pages 118-122

https://doi.org/10.1016/j.scriptamat.2015.09.027 Get rights and content

Abstract

Calcia-magnesia-alumino-silicate (CMAS)-attack mitigation mechanisms of 2ZrO₂·Y₂O₃ composition TBCs are investigated through a ‘model’ study where 2ZrO₂·Y₂O₃ ceramic powders are immersed in molten CMAS glass for various durations at 1300 °C. The key insight gained from this study is that the formation of the CMAS-blocking apatite phase requires high concentration of Y^3 + within a confined volume of CMAS, whereas the presence of a large-volume CMAS sink for Y^3 + precludes the formation of apatite. It is also demonstrated that the ceramic microstructure has a profound effect on its dissolution and penetration by CMAS, and on the concomitant crystallization of apatite.

Graphical abstract

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In this study, the CMAS corrosion behavior of two high-yttria-stabilized zirconia thermal barrier coatings (38YSZ and 55YSZ) deposited by air plasma spraying is investigated and compared with a typical 8YSZ coating at 1250 °C to unravel the underlying corrosion mechanism. The 8YSZ coating undergoes severe CMAS attack driven by dissolution-reprecipitation and intergranular corrosion. However, the 38YSZ and 55YSZ coatings can react with CMAS melt vigorously to form a dense reaction layer with CMAS-resistant apatite phase at the CMAS/coating interface, thereby resisting the CMAS infiltration. The apatite phase can be formed only when the yttria content in c-ZrO₂ precipitated from CMAS attains the critical value (∼20 at%). Moreover, compared to the 38YSZ coating, apart from the high yttria content, the low zirconia content in the 55YSZ coating also allows higher free Y³⁺ available to form apatite phase via inhibiting the formation of c-ZrO₂, thus facilitating the formation of more apatite phase.
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The wettability and thermal corrosion behavior of CMAS and CN (CMAS+NaVO₃) on high-entropy rare-earth zirconate (Gd_0.2Y_0.2Er_0.2Tm_0.2Yb_0.2)₂Zr₂O₇ (HEZ) at high temperature were studied and compared. The results reveal that HEZ is a promising TBCs material with good anti-CMAS performance. However, HEZ exhibits poor resistance to CN, and the reaction layer fails to effectively alleviate the melt penetration. Based on the calculation of OB (optical basicity) theory, the possibility that NaVO₃ can improve the reaction activity of CMAS is ruled out. It was determined that NaVO₃ depolymerizes the internal network structure of the CMAS, resulting in a lower melting point and viscosity, stronger wettability, and permeability of CN compared to CMAS, thus posing greater harmfulness. The influence of rare earth elements with different ionic radii on the formation of multicomponent RE-crystal was researched by crystal structure analysis and first-principles calculation, providing a theoretical basis for the reliability of the corrosion mechanism.
Deposition characteristics, sintering and CMAS corrosion resistance, and mechanical properties of thermal barrier coatings by atmospheric plasma spraying
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Atmospheric plasma spraying (APS) was used to fabricate 4.5 mol% Y₂O₃ stabilized ZrO₂ (YSZ) and 4.0 mol% Yb₂O₃ and 0.5 mol% Y₂O₃ co-stabilized ZrO₂ (YbYSZ) series coatings with different spraying parameters. This study presents the effects of particle size of feed powder and substrate temperature on the microstructure and performance of thermal barrier coatings. Single-pass and multi-pass experiments were conducted to explore the relationship between the splat and the coatings. The experimental results indicate that adjusting particle size of feed powders and deposition temperature can alter the splat behaviour and microstructure, improve anti-corrosion and anti-sintering abilities, and enhance the hardness and elastic modulus. Changes in parameters can cause changes in the microstructure of as-sprayed coatings. Lower substrate temperatures can lead to vertical cracks, finer powders can introduce more microdefects, and conventional sized powders can form larger pores. A coating deposited using a conventional feed powder at a substrate temperature of 800 °C shows excellent sintering resistance. A coating deposited using a fine feed powder on 800 °C substrate exhibits the best calcium‑magnesium-alumina-silicate (CMAS) corrosion resistance.
Improvement on corrosion resistance of molten calcium-magnesium-alumina-silicate (CMAS) by Yb<inf>2</inf>O<inf>3</inf>-Y<inf>2</inf>O<inf>3</inf> co-stabilized ZrO<inf>2</inf> ceramic pellets
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Five types of ceramic pellets with different doping contents of Yb₂O₃-Y₂O₃ co-stabilized ZrO₂ (YbYSZ) (x mol.% Yb₂O₃ - (4.5-x) mol.% Y₂O₃-ZrO₂; X = 0 (YSZ), 1, 2, 3, 4) were designed to investigate corrosion behavior under CMAS attack. The results show that the corrosion resistance of YbYSZ increased with increasing Yb₂O₃ content, which was attributed to the excellent phase stability and lower diffusion of Yb³⁺. More degradation with microstructural destruction was observed in the YSZ pellets, and less degradation was observed in YbYSZ with 4 mol.% Yb₂O₃ (YbYSZ-4). Further, YbYSZ-4 performs a larger contact angle than others, promoting CMAS resistance.
Integrating APS TBCs with a built-in Al<inf>2</inf>O<inf>3</inf> protection network for superior CMAS resistance
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Calcium–magnesium–alumina–silicate (CMAS) attack poses a great threat to the durability of thermal barrier coatings (TBCs). We introduce a novel Al₂O₃-modified TBC architecture to counter this threat. The architecture integrates an Al₂O₃ protection network into the microstructure of an atmospheric plasma sprayed (APS) TBC by drip coating method, which modifies the vulnerable infiltration pathway of CMAS melt and promotes the self-crystallization of CMAS at the infiltration front. Moreover, during sintering, the built-in Al₂O₃ can inhibit inter-splat crack healing. This innovative TBC design achieves robust CMAS resistance and strain tolerance, while also offering advantages during sintering.
Interaction laws of RE<inf>2</inf>O<inf>3</inf> and CMAS and rare earth selection criterions for RE-containing thermal barrier coatings against CMAS attack
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Thermal barrier coatings (TBCs) with higher rare earth (RE) contents are more resistant to CaO-MgO-Al₂O₃-SiO₂ (CMAS) attack due to their reaction to form apatite protective products. In this study, the reaction behavior between RE₂O₃ and CMAS was investigated and their interaction laws were clarified, based on which three criterions for selecting RE dopants for RE-containing TBC materials are proposed: 1) favorable reaction product types and strong protective products formation kinetics, 2) low RE³⁺ solubility in molten CMAS, and 3) low wettability of residual mixture. Sm₂O₃ and Gd₂O₃ are demonstrated as the most attractive dopants for RE-containing TBCs against CMAS attack.

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¹: On leave from Beijing Institute of Technology, Beijing 100081, PR China.

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Interaction between ceramic powder and molten calcia-magnesia-alumino-silicate (CMAS) glass, and its implication on CMAS-resistant thermal barrier coatings

Abstract

Graphical abstract

J. Eur. Ceram. Soc.

Surf. Coat. Technol.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Surf. Coat. Technol.

Acta Mater.

Surf. Coat. Technol.

Scr. Mater.

Acta Mater.

Surf. Coat. Technol.

J. Eur. Ceram. Soc.

Science

MRS Bull.