Degradation mechanisms of air plasma sprayed free-standing yttria-stabilized zirconia thermal barrier coatings exposed to volcanic ash

In advanced gas turbine engines that operate in a dust-laden environment causing ingestion of siliceous debris into engines, thermal barrier coatings (TBCs) are highly susceptible to degradation by molten CMAS (calcium-magnesium alumino silicate) deposits. In this study, the degradation mechanisms other than the commonly reported thermomechanical damage are investigated with an emphasis on the thermochemical aspects of molten CMAS induced degradation of TBCs. Free-standing yttria stabilized zirconia (8YSZ) TBC specimens in contact with a model CMAS composition were subjected to isothermal heat treatment in air at temperatures ranging from 1200°C to 1350°C. Phase transformations and microstructural development were examined by using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Starting at 1250°C, the molten CMAS readily infiltrated and dissolved the YSZ coating followed by reprecipitation of zirconia with a different morphology and composition that depends on the local melt chemistry. Significant amount of Y2O3 depleted monoclinic ZrO2 phase evolved from CMAS melt that dissolved ť-ZrO2 was evident. Thus the mechanism of dissolution and reprecipitation due to molten CMAS damage resulted in destabilization of the YSZ with disruptive phase transformation (t’ f + m).

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Improving Thermal Shock Resistance of Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings by Laser Remelting

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.472-475.2502 ◽

2012 ◽

Vol 472-475 ◽

pp. 2502-2507

Author(s):

Zong Yin Duan ◽

Dong Sheng Wang

Keyword(s):

Thermal Shock ◽

Thermal Barrier Coatings ◽

Thermal Shock Resistance ◽

Yttria Stabilized Zirconia ◽

Thermal Barrier ◽

Laser Remelting ◽

Stabilized Zirconia ◽

Barrier Coatings ◽

Shock Resistance ◽

Plasma Sprayed

This paper deals with the microstructure and thermal shock behavior of laser remelting of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) deposited by plasma spraying. The microstructures of the coatings were analyzed by scanning electron microscopy (SEM). It was found that the as-sprayed ceramic coating had laminated structure with high porosity. However, the coating exhibited a dense lamellar-like layer with segment cracks on the remained plasma-sprayed porous layer. Thermal shock experiments for the two kinds of TBCs were performed by water quenching method. Testing result showed that the laser-remelted TBC had better thermal shock resistance than the as-sprayed one. The damage mode of the as-sprayed TBC was great-size whole spalling. In contract, the failure mechanism of the laser-remelted one was mainly local pelling. Segmented cracks of the top ceramic coatings caused by laser remelting improved the stress accommodation and were mainly attributed to the enhancement for thermal shock life of TBC.

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