The article addresses effects of silicate deposit composition on reactions with yttrium monosilicate (YMS), a candidate environmental barrier coating for aero-engine components. Computed phase equilibria are used to predict the nature and relative proportions of reaction products and the extent of YMS consumption upon reaction with twelve deposits of varying composition at 1300°C. These predictions are compared with results of a corresponding experimental study on three exemplary deposits.Although the nature and sequence of reaction products formed (typically apatite and yttrium disilicate) depend on the Ca:Si ratio of the deposit, the degree of consumption of YMS at equilibrium is relatively insensitive to deposit composition and is predicted to proceed to a greater extent than that in yttrium disilicate. However, sluggish reaction kinetics associated with the formation of a thin apatite layer above the YMS prevents reactions from reaching their terminal equilibrium states within the experimental times investigated (250 hours). For deposit loadings of 18 mg/cm 2 (corresponding to a thickness of about 100 µm), the degree of consumption following 250 hours exposures is only about 10%-40% of the predicted terminal values, depending on deposit composition.
K E Y W O R D Schemical reactions, CMAS, Environmental barrier coating, yttrium silicate 2920 | SUMMERS Et al.
Environmental barrier coatings (EBCs) for use with SiC‐based composites in gas turbine engines may fail following reaction with molten silicate deposits. The processes involved may include dissolution of the EBC material into the deposit, reactions that produce new phases, and cracking or spallation upon cooling, the latter driven by thermal expansion mismatch between the reaction products and the underlying EBC and substrate. Here, we describe an integrated computational framework to simulate the processes and to predict the conditions leading to coating loss through reactive consumption and/or spallation. The framework integrates distinct models to determine: (a) the nature and quantity of phases resulting from dissolution and chemical reactions; (b) thermo‐physical properties of those phases; and (c) energy release rates for penetration cracking and spallation upon cooling. We demonstrate the use of the framework by computing critical deposit thicknesses for one specific EBC material (Y2Si2O7) as a function of deposit composition. In this system, the critical deposit thickness for typical coating thicknesses is dictated mainly by spallation, not by consumption, and may vary by orders of magnitude, depending on deposit composition and coating thickness and toughness. With respect to deposit composition, the key parameter governing coating failure is the Ca:Si ratio.
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