Synthesis and Characterization of Plasma Sprayed High-Entropy Oxide Coatings

In this study, four ceramic materials (OM- 1, 2, 3 & 4) with five principal cations were systematically designed to achieve high-entropy oxides (HEOs) for thermal barrier coating (TBC) applications. Oerlikon Metco’s thermodynamics model based Rapid Alloy Development (RAD^TM) technology was utilized to inform on composition. The HEO design approach was targeted for coatings to achieve single phase in their as-sprayed form, with lower thermal conductivity (against conventional 7YSZ APS coatings), and a very good phase stability at high temperatures. The materials were synthesized as thermal spray powders via an agglomerated and sintering process and were deposited as coatings via atmospheric plasma spray. Due to the high enthalpy associated with plasma spray process, maintaining a desired phase in a multi-oxide thermal spray powders and coatings remains a challenge for future design of TBCs. However, two of the coating materials OM-3 and OM-4 were able to achieve single phase composition, with moderate to high configurational entropy (1.3-1.6R). X-ray diffraction of the thermally aged coatings (250 hours, 1250°C) further demonstrated the high temperature phase-stability of OM-3 and OM-4. The above two coatings also possessed a lower thermal conductivity at room temperature against 7YSZ coating. Interestingly, the measurements on thermal expansion coefficient were found to be far from the rule-of-mixture based estimates- suggesting the uniqueness of HEOs properties.