A computational modeling framework for reaction and failure of environmental barrier coatings under silicate deposits

Abstract

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.

Developments in Thermodynamic Models of Deposit-Induced Corrosion of High-Temperature Coatings

“…A number of studies have used the FactSage or Thermo-Calc databases to predict either the melting behavior of specific individual deposits or across broad ranges of potential debris compositions. 2,12,13,17,39,90 The latter has proven useful in the identification of ranges of representative deposits corresponding to the limits of the expected extent of deposit melting. These are typically simple point equilibrium, step (vs temperature), or mapping type calculations that are readily performed with various software packages described in Sect.…”

“…For instance, conversion of yttrium and ytterbium disilicate to apatite generates tensile stress and channel cracking upon cooling due to the increased CTE mismatch between the SiC substrates and apatite. 17,49,94 Melt infiltration and subsequent stiffening of the coating are not the major drivers for thermomechanical failure of EBCs, but may contribute to blistering-type failure associated with combined reactions and grain boundary penetration. 95,96 The typical reaction progression for EBCs exposed to CMFAS deposits is shown in Fig.…”

“…Subsequent integration with mechanics models facilitates the prediction of scenarios in which coating failure would be primarily thermochemical (i.e., consumption of the coating) or thermochemical (i.e., due to thermal stress induced cracking). 17 The same modeling approach was applied to investigate the reactivity of YMS with a variety of representative deposit compositions. 48 The results suggest a different reaction progression: CaO-rich melts still react with the coating material to form apatite, while CaO-lean (and SiO 2 -rich) melts tend to form YDS as a reaction product.…”

“…More than two decades of study on the degradation of thermal and environmental barrier coating (TBC and EBC) materials by silicious CMFAS* deposits have advanced the understanding of the coupled thermo-chemical and -mechanical mechanisms leading to accelerated coating failure. 2,[9][10][11][12][13][14][15][16][17] The majority of this work has focused on rare earth (RE) zirconate, silicate, and aluminate materials owing to their advantageous property combinations for use as protective coatings. Infiltration of the molten deposit into porous or segmented coating microstructures reduces the coating compliance.…”

“…To overcome this limitation, recent efforts have focused on elucidating the underlying phase equilibria and thermodynamic properties for key subsystems. [41][42][43][44][45][46] The intent is that computational thermodynamics models based on these insights could be used alongside thermophysical property and fracture mechanics models in an integrated framework to predict coating performance, 2,12,13,17 in the spirit of integrated computational materials engineering (ICME). 47 Such a framework (Fig.…”