High-temperature interaction of YSZ ceramic coatings and Fe,Ti-enriched CMAS-type deposits

Kurzfassung

The interaction of airborne mineral dusts and yttria-stabilized zirconia (YSZ) ceramic coatings is a key issue for turbine engines. In operation airborne inorganic particles stick to hot surfaces of YSZ-coated turbine components and typically form CMAS-type (CaO-MgO-Al2O3-SiO2) deposits. These deposits melt and easily infiltrate YSZ-coatings thus affecting coating performance and lifetime. However, the composition significantly influences the behavior of CMAS-type deposits. In particular the presence of Fe2O3 and TiO2 must be taken into account. Different types of YSZ-coatings (APS, EB-PVD) with artificial Fe- and/or Ti-rich CMAS-type deposits were annealed at temperatures between 1200 and 1300°C. It turns out that the presence of Fe and/or Ti along with a high CaO/SiO2 ratio leads to a solid-state reaction at the CMAS/YSZ interface. A kimzeyite-type calcia-zirconia garnet is formed at the expense of YSZ and Fe,Ti-containing CMAS. However, above the melting temperature of about 1280°C, the newly formed kimzeyite penetrates the YSZ coating producing new YSZ surfaces available for this ZrO2-consuming reaction. This may lead to progressive YSZ recession under cyclic conditions. Since peak surface temperatures of YSZ-coated components of gas turbine engines may exceed 1300°C, this process is considered critical for the durability of YSZ-coatings.