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 cell stacks, a good chemical, microstructural and phase stability for the protective coating are required in both the oxidizing and reducing environments. In this work, MnCo₂O₄ coatings of approximately 150 μm were deposited onto porous Ni50Cr50-Al₂O₃ substrate by atmospheric plasma spray (APS) process. 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. Then the tested coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD results elucidated that the tested coating had a high structural stability on the upper layer, while presented a reducing microstructure on the substrate side. The surface morphology of 100 hours tested coating indicated that the spinel granules still arranged closely with small particle size of ∼250 nm and no obvious grain enlargement was observed. According to the cross-section, the upper layer kept stable and dense. While at the underneath region, the microstructure presented to be rather porous. However, the resistance presented a decreasing trend with the extension of exposure duration. After exposure for 95 h, the ASR decreased to 18.5 mΩ² although a substantial Cr diffusion from the substrate was detected.