Influence of pre-reduction on microstructure homogeneity and electrical properties of APS Mn1.5Co1.5O4 coatings for SOFC interconnects

Atmospheric plasma spray is employed to deposit Mn1.5Co1.5O4 (MCO) coatings onto T441 and porous 430L substrates respectively at room temperature and 400 °C. The coatings are then subjected to redox treatments to achieve a desirable gas tightness. The gas permeability, area specific resistance (ASR) and microstructure homogenization of MCO coatings are evaluated in different stages. The gas permeability of MCO coating deposited at 400 °C and exposed to redox treatments can be as low as ∼8.2 × 10−8 cm4 s−1 gf−1, which is essential for sufficient protection of metallic substrate against oxidation. The MCO-coated T441 without pre-reduction presents a nonuniform microstructure characterized by isolated precipitates rich in Co and dark matrix rich in Mn. In comparison, the MCO coating with 5 h pre-reduction presents a uniform and dense spinel structure. The ASR tests indicates that a homogeneous structure has contributed to an increase in the overall conductivity. While bare T441 shows a substantial increase in interfacial ASR during the 200 h test, MCO-coated T441 exhibites a high stability and a much lower ASR value of 13 mΩ cm2. Besides, the dense MCO coatings effectively inhibit the oxide scale growth and block outward diffusion of Cr during the oxidation tests. Cr-rich oxide scale formed at the MCO/T441 interface is less than 1.2 μm and no further migration of Cr was detected.