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

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 degrees 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 degrees C treatments from 15 min to 100 h. 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 degrees 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. (C) 2013 Elsevier B.V. All rights reserved.