The Microstructure Stability of Atmospheric Plasma-Sprayed MnCo₂O₄ Coating Under Dual-Atmosphere (H₂/Air) Exposure

Based on the specific structure of tubular solid oxide fuel cells, good chemical, microstructural, and phase stabilities for the protective coating are required in both the oxidizing and reducing environments. In this work, MnCo₂O₄ coatings were deposited onto porous Ni50Cr50-Al₂O₃ substrate by atmospheric plasma spray. The coated samples were tested at 800 °C with the coating exposed in air environment and the substrate in H₂ environment. Reducing and pre-oxidizing treatments were performed prior to the stability test. The microstructural stability, phase composition, and electrical properties of the tested coatings were investigated. The surface morphology exhibited an excellent surface stability, and no obvious crystal coarsening was observed. With enhancement of the testing duration, the area-specific resistance presented a decreasing trend attributed to increase in the contact interface and densification of the upper layer. The cross-section views presented a dense upper layer and a relatively porous bottom layer. The x-ray diffraction results also indicated a single MnCo₂O₄ phase in the upper layer exposed to air environment and a reduced phase structure in the bottom layer from the substrate side. The evolution mechanism between the oxidation frontier and the reduction interface was then discussed.