Silicate Deposit Degradation of Engineered Coatings in Gas Turbines: Progress Toward Models and Materials Solutions

Abstract

Modern gas turbines rely on ceramic coatings to protect structural components along the hot gas path. These coatings are susceptible to accelerated degradation caused by silicate deposits formed when ingested environmental debris (dust, sand, ash) adheres to the coatings. This article reviews the current understanding of the deposit-induced failure mechanisms for zirconia-based thermal barrier coatings and silicate environmental barrier coatings. Details of the debris melting and crystallization behavior, the nature of the chemical reactions occurring between the deposits and coatings, and the implications for the thermocyclic durability of the coatings are described. Given the challenges posed in understanding how prospective coating materials and architectures will respond to a broad range of deposit compositions, it is proposed to develop an integrated framework linking thermochemical and thermomechanical models to predict coating durability. Initial progress toward developing this framework, and the requisite research needs, are discussed.

Reactions of molten silicate deposits with yttrium monosilicate

“…The reaction is enabled by inward transport of Ca 2+ from the melt, and inward or outward flow of SiO 2 for YMS (Equation ) or YDS (Equation ), respectively, through glass interpenetrating the apatite layer. In YDS, the transport paths are wide whereas in YMS they are exceedingly thin (<1 nm) . The apatite layer in YMS is therefore a more effective diffusion barrier, although it still allows sufficient transport for growth to continue at a low rate.…”

“…Twelve deposit compositions are considered, based on those characterized by Zaleski et al and shown in Table . The Ca:Si ratios vary from 0.14 to 0.75, covering the range of many volcanic ashes and engine deposits reported in the literature . The compositions are the same as those used in a previous study on YDS, thereby allowing direct comparisons between the behaviors of the two EBC candidates .…”

“…Molten deposits, generically known as “CMAS” after their primary oxide components of Ca, Mg, Al, and Si, also contain minor amounts of Fe, Ti, Na, and K oxides. Deposit compositions vary widely with geographic region and mineral source . Reactions between the molten deposit and the EBC gradually consume the coating at elevated temperature, producing phases that remain adhered to the coating and that exhibit a substantial CTE mismatch with the composite.…”

“…Consequently, reactions have direct impact on the robustness of the system against thermo mechanical damage. In many cases, the reaction processes are sensitive to deposit composition, making the problem even more challenging to address …”

“…Because SiO 2 is the only reactive melt constituent, the reaction in Equation () would not consume as much of the melt as that in Equation (). The progressive depletion of SiO 2 in the melt may trigger crystallization of phases bearing the remaining melt constituents . However, these scenarios have not been significantly explored.…”