Microstructural characterization of the interaction between 8YPSZ (EB-PVD) thermal barrier coatings and a synthetic CAS

• Published in: Surface & coatings technology Vol. 239; pp. 41 - 48
• Main Authors: Vidal-Sétif, M.H., Rio, C., Boivin, D., Lavigne, O.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 25.01.2014; Elsevier
• Subjects: 8YPSZ; Applied sciences; CAS; Cross-disciplinary physics: materials science; rheology; Destabilization; EB-PVD; Electron back scattered diffraction (EBSD); Exact sciences and technology; Materials science; Metals. Metallurgy; Nonmetallic coatings; Physics; Production techniques; Surface treatment; Surface treatments; Thermal barrier coatings; Thermal barrier coatings; EB-PVD; 8YPSZ; CAS; Destabilization; Electron back scattered diffraction (EBSD); Electron diffraction; Electron beam; Surface treatments; Non metal coating; Thermal barriers; Physical vapor deposition; Microstructure
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2013.11.014

Thermal barrier coatings (TBC) in service are prone to the attack by molten calcium–magnesium aluminosilicates (CMAS) resulting from the ingestion of siliceous minerals (dust, sand, volcanic ash) by the aeroengines. As CMAS deposits observed on ex-service parts present complex and highly variable compositions, progress in the understanding of the interaction mechanism between CMAS and TBC has been mostly obtained during laboratory experiments using simplified CMAS compositions. This paper consists in a microstructural characterization of the isothermal chemical interaction between a synthetic calcium aluminosilicate (CAS) and a standard 8YPSZ EB-PVD coating deposited on alumina substrates. The chosen CAS is the lowest melting temperature eutectic (1170°C) in the ternary (CaO–Al2O3–SiO2) system and it is prepared as a glass. The evolution of the reaction zone morphology and chemistry is studied after 1200°C treatments from 15min to 100h. The CAS/TBC interaction is governed by the classical mechanism of dissolution/reprecipitation with formation of Y-lean zirconia and Ca2Zr(Y)Si4O12. Ca2Zr(Y)Si4O12 formation induces local variation in the CAS composition leading to CaAl2Si2O8 (anorthite) precipitation. Y2O3 depletion in the YSZ regions infiltrated by CAS is attested using X-ray compositional mapping and the association of EDS and EBSD clearly illustrates that the morphological transformation from porous initial YSZ to dense globular Y-lean YSZ is associated with the structural transformation from initial tetragonal t′ to tetragonal Y-lean zirconia transforming into monoclinic during cooling. The degradation of the TBC columns increases with the duration of the 1200°C heat-treatment. The interfacial Ca2Zr(Y)Si4O12 phase forms on an increasing amount of columns and is finally observed on all the columns. Interfacial phase formation is discussed and compared to other phase assemblages reported in the literature. •CAS/8YPSZ interaction is characterized after 1200°C treatments from 15min to 100h.•CAS is the lowest melting eutectic (1170°C) in the ternary (CaO–Al2O3–SiO2) system.•CAS/8YPSZ interaction obeys the classical mechanism of dissolution/reprecipitation.•Interfacial Y-lean zirconia as well as Ca2Zr(Y)Si4O12 and CaAl2Si2O8 are observed.•The degradation of the TBC columns increases with the 1200°C treatment duration.