Lu2O3-SiO2-ZrO2 Coatings for Environmental Barrier Application by Solution Precursor Plasma Spraying and Influence of Precursor Chemistry

As environmental barrier coatings are subjected to thermal stress in gas turbine engines, the introduction of a secondary phase as zircon (ZrSiO₄) is likely to increase the stress resistance of Lu₂Si₂O₇ coatings generated by induction plasma spraying using liquid precursors. In a first step, precursor chemistry effect is investigated by the synthesis of ZrO₂-SiO₂ nanopowders by induction plasma nanopowder synthesis technique. Tetraethyl orthosilicate (TEOS) as silicon precursor and zirconium oxynitrate and zirconium ethoxide as zirconium precursors are mixed in ethanol and produce a mixture of tetragonal zirconia and amorphous silica nanoparticles. The use of zirconium ethoxide precursor results in zirconia particles with diameter below 50 nm because of exothermic thermal decomposition of the ethoxide and its high boiling point with respect to solvent, while larger particles are formed when zirconium oxynitrate is employed. The formation temperature of zircon from zirconia and silica oxides is found at 1425 °C. Second, coatings are synthesized in Lu₂O₃-ZrO₂-SiO₂ system. After heat treatment, the doping effect of lutetium on zirconia grains totally inhibits the zircon formation. Dense coatings are obtained with the use of zirconium ethoxide because denser particles with a homogeneous diameter distribution constitute the coating.