Influence of the source of yttrium on the morphology of precipitated phases after interaction with a CaO-Al2O3-SiO2 melt at 1300°C

The durability of the ceramic coatings used to protect the turbine blades in the field of aeronautical engines is strongly linked to their ability to withstand corrosion by calcium-magnesium-aluminosilicates (i.e. CMAS), which can infiltrate their microstructure when liquid at high temperature. Protection can be provided thanks to the high reactivity of the ceramic coatings with CMAS leading to the fast precipitation of protective phases. Most ceramic coatings contain rare-earth elements (e.g. rare-earth monosilicates RE₂SiO₅ or rare-earth disilicates RE₂Si₂O₇), making them thus able to form oxyapatite after reaction with the silicate melts. The present works proposes to study the morphology of these precipitated phases depending on the nature of the rare-earth source. A model ternary melt CaO-Al₂O₃-SiO₂ has thus been put in contact at 1300 °C with different sources of yttrium: oxide Y₂O₃, monosilicate Y₂SiO₅ and disilicate Y₂Si₂O₇. The reactivity led to the precipitation of the two phases of interest (i.e. apatite Ca₂Y₈(SiO₄)₆O₂ and cyclosilicate Ca₃Y₂(Si₃O₉)₂). The use of these simplified systems allows to establish the precipitation mechanisms of the phases at equilibrium regarding previously established phase diagrams. Depending on the yttrium source, the chemical reactions involved local modifications of the melt composition, which strongly influenced the morphology of the precipitated phases at equilibrium. Specifically, it is demonstrated here that the release of SiO₂ in the melt when starting from yttrium disilicate involves a specific microstructure of the acicular apatite, which exhibits a core containing precipitated SiO₂.