Mitigation of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings

Gledhill, Andrew D.; Reddy, Kolan Madhav; Drexler, Julie M.; Shinoda, Kentaro; Sampath, Sanjay; Padture, Nitin P.

doi:10.1016/j.msea.2011.06.041

Cited by 111 publications

(68 citation statements)

References 30 publications

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“…But it undergoes significant sintering above 1200 º C, which is not desirable [4]. Furthermore, YSZ is susceptible to CMAS infiltration, which results in loss of strain tolerance and change in near surface mechanical properties leading to early coating spallation [5][6][7][8][9][10][11][12][13].…”

Section: Introductionmentioning

confidence: 99%

Thermal conductivity and thermal cyclic fatigue of multilayered Gd2Zr2O7/YSZ thermal barrier coatings processed by suspension plasma spray

Mahade

Curry

Björklund

et al. 2015

Surface and Coatings Technology

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Section: Introductionmentioning

confidence: 99%

Thermal conductivity and thermal cyclic fatigue of multilayered Gd2Zr2O7/YSZ thermal barrier coatings processed by suspension plasma spray

Mahade

Curry

Björklund

et al. 2015

Surface and Coatings Technology

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“…As reported by Gledhill, et al 14 and Drexler et al, 16 the corrosion mechanism of CMAS on pyrochlore materials involves the melting of the CMAS glass, wetting of molten glass on the pyrochlore phase, dissolving of the pyrochlore oxides into glass melts, crystallization of newly formed Gd-apatite and other phases. Thus, the compositions of CMAS variants have a notable effect on each step of the reaction process.…”

Section: Resultsmentioning

confidence: 79%

“…4 Since Gd 2 Zr 2 O 7 is a promising alternative TBC material, the influence of CMAS on its performance has been of interest. [12][13][14][15][16] Molten CMAS wets the surface, penetrates the pores, and reacts with the pyrochlore material resulting in the establishment of a compact layer composed of a lanthanide-apatite phase, usually in a rod-like shape, and a cubic fluorite phase in a globular form, which is conducive to the suppression of further CMAS permeation. 17 The extent of degradation strongly depends on temperature and the composition of CMAS glass.…”

mentioning

confidence: 99%

Effect of CMAS Composition on Hot Corrosion Behavior of Gadolinium Zirconate Thermal Barrier Coating Materials

Wen

Fergus

2017

J. Electrochem. Soc.

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The resistance of synthesized pyrochlore-type Gd 2 Zr 2 O 7 bulk specimens to four calcium-magnesium aluminosilicate (CMAS) compositions at different temperatures was investigated. The reaction products were identified by X-ray diffraction and penetration depths were examined using scanning electron microscopy. A dense reaction layer is comprised mainly of Ca 2 Gd 8 (SiO 4 ) 6 O 2 and a cubic fluorite phase formed during the CMAS attack, and some unreacted CMAS was found in a transition layer below the reaction layer. The overall infiltration depth changed slightly with temperature, however, the thickness of the reaction layer and the morphology of the transition layer varied distinctly with temperature. The sintered sample underwent the most severe degradation by the CaO-lean CMAS, whereas the effect of CaSO 4 and CaCO 3 was not significant. Moreover, the Gd content of the ZrO 2 -based cubic fluorite phase depends on the temperature and the molar ratio of Ca:Si in the CMAS. The efficiency of gas turbine engines increases with increasing operating temperature. Thermal barrier coatings (TBCs) act as thermal insulators for metallic fans and vanes, which allows for a higher operating temperature and thus increases the thrust-to-weight ratio of the turbine.1 Yttria-stabilized zirconia (YSZ) with the intermediate t'-tetragonal phase has been widely used in aero engines for decades owing to its excellent mechanical and chemical properties during thermal cycling.2 However, YSZ cannot meet the requirements of modern turbines, because issues like delamination and spallation due to the phase transformation accompanied with volume changes and poor chemical resistance to molten salts at elevated temperature are detrimental to the performance and shorten the lifetimes of jet engines. [3][4][5][6][7] Conversely, the rare earth zirconates, especially gadolinium zirconate (Gd 2 Zr 2 O 7 ) with the pyrochlore structure, which exhibit relatively low thermal expansion coefficient, high-temperature phase stability, and low thermal conductivity, are of great potential. 8,9 In general, TBC materials can be deposited using atmospheric plasma spraying (APS) on the stationary components or electron beam physical vapor deposition (EB-PVD) on the rotating parts of the turbine. The corresponding specific lamellar or columnar structure of APS or EB-PVD deposited coatings with porosities of 10-25% benefits mitigation of stresses and decreases thermal conductivity. 10,11 However, the intersplat pores or intercolumn gaps are the main paths for the infiltration of molten salts during service. The molten salts, which come from sand, dust, volcanic ash and even impurities in the fuel, are mainly composed of calcium-magnesium-alumino-silicate (CMAS).4 Since Gd 2 Zr 2 O 7 is a promising alternative TBC material, the influence of CMAS on its performance has been of interest. [12][13][14][15][16] Molten CMAS wets the surface, penetrates the pores, and reacts with the pyrochlore material resulting in the establishment of a compact layer composed of a ...

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“…Besides, little channels perpendicular to the cracks were formed because CMAS melt reacted and dissolved the solid matrix (marked by a diamond in Fig. 5) [24]. Moreover, a yttria reduction was observed in the solid matrix (from 7-8 wt.% in as-sprayed coating to 6 wt.% in attacked coating) as well as a significant presence of yttria (around 5 wt.%) in the CMAS melt zone, concluding that an yttria transport or lixiviation from the solid matrix (bright zones) toward the CMAS melt (dark zones) was observed [7].…”

Section: Description Of the Cmas Attackmentioning

confidence: 99%

Molten salt attack on multilayer and functionally-graded YSZ coatings

Carpio

Salvador

Borrell

et al. 2018

Ceramics International

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Thermal barrier coatings have been extensively studied in the last years in order to increase the operational temperature of the current gas turbines as well as to improve the coating lifetime. Many coating characteristics must be met to achieve these requirements (low thermal conductivity, high thermal fatigue resistance…); therefore, complex systems have been engineered for these purposes. One of the possibilities to optimise the different properties deals with the design of multilayer or functionally-graded coatings where various types of microstructures with different characteristics are combined.One of the most important cause of gas turbines degradation relates to the attack of different type of particles which are suspended in the atmosphere (sand, fly ash…). These solid particles are molten at the operational temperatures and then, the molten salts chemically react with the coating. For this reason, the present research was focused on this type of attack.In the present work, the molten salt attack of various YSZ coatings with multilayer and functionally-graded design was addressed. Two different type of microstructures were specifically combined for this design: the APS coating microstructure obtained from conventional (microstructured) powder and a bimodal structure with nanozones obtained from nanostructured feedstock. Besides, different salts were used to simulate different attack environments (desert sand and volcanic fly ash). Findings show that nanozones act as barrier against the penetration of molten salts toward deeper layer. However, a layer formed by nanozones can detach when the salt attack is too aggressive. Hence, functionally-graded coatings, where two types of microstructures are combined through the whole coating, become ideal to diminish the molten salt attack.

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Mitigation of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings

Cited by 111 publications

References 30 publications

Thermal conductivity and thermal cyclic fatigue of multilayered Gd2Zr2O7/YSZ thermal barrier coatings processed by suspension plasma spray

Thermal conductivity and thermal cyclic fatigue of multilayered Gd2Zr2O7/YSZ thermal barrier coatings processed by suspension plasma spray

Effect of CMAS Composition on Hot Corrosion Behavior of Gadolinium Zirconate Thermal Barrier Coating Materials

Molten salt attack on multilayer and functionally-graded YSZ coatings

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