Solid‐State CMAS Corrosion of an EB‐PVD YSZ Coated Turbine Blade: Zr 4+ Partitioning and Phase Evolution

The corrosion of an ex‐service 7‐ YSZ coated high‐pressure turbine blade by Si ‐undersaturated, Fe‐ , Ti ‐rich CMAS ( FT ‐ CMAS ) was investigated. The constituents of the FT ‐ CMAS deposit and its synthetic laboratory counterpart are identified by means of analytical SEM and XRD . FT ‐ CMAS mainly consists of rhönite‐, melilite‐, and clinopyroxene‐type solid solutions. FT ‐ CMAS reacts with the 7‐ YSZ thermal barrier coating to a continuous layer of a Ca , Zr ‐rich garnet phase also known as kimzeyite. The microstructural analysis indicates that kimzeyite has formed via solid‐state reaction. The formation of kimzeyite was studied in laboratory using YSZ / FT ‐ CMAS diffusion couples and powder compacts. Experiments provide strong evidence that kimzeyite is forming predominantly from YSZ and melilite, whereas clinopyroxene and especially rhönite appear relative stable versus YSZ . The preferred kimzeyite formation from melilite and YSZ is explained by the availability of sixfold coordinated lattice sites for Zr 4+ on melilite decomposition and subsequent garnet crystallization.