Influence of powder composition and manufacturing method on electrical and chromium barrier properties of atmospheric plasma sprayed spinel coatings prepared from MnCo₂O₄ and Mn₂CoO₄ + Co powders on Crofer 22 APU interconnectors

Protective coatings based on manganese cobalt oxide spinels are required in solid oxide fuel cells (SOFCs) to prevent the evaporation of volatile Cr(VI)-compounds from the metallic interconnectors and to minimize high temperature corrosion. Atmospheric plasma spraying (APS) was used to manufacture dense manganese cobalt oxide protective coatings on Crofer 22 APU substrates by employing two different spinel powders. The spray powders were MnCo₂O₄ and Mn₂CoO₄+Co (equivalence for Mn_1.5Co_1.5O₄). The Mn₂CoO₄+Co powder was prepared by agglomerating the oxide powder with fine metallic cobalt powder. The coated substrates were oxidized at 700 °C in air for 1000 h. During the high temperature oxidation, a four-point on-line measurement technique with a current density of 640 mA/cm² was simultaneously used for area specific resistance (ASR) studies. The coatings were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), an energy dispersive spectrometer (EDS) and Raman spectroscopy. The APS coatings had dense microstructure, which decreased the oxidation of the substrate. The MnCo₂O₄ coating showed indication of some Cr-migration, whereas Mn₂CoO₄+Co showed good Cr-barrier properties. The ASR test showed that APS coated Mn₂CoO₄+Co is a promising candidate material for SOFC interconnect applications.