Influence of Spray Parameters and Characteristics of Solutions on Microstructure and Phase Composition of Solution Precursor Atmospheric Plasma Sprayed (SPPS) Mn-Co Spinel Coating

Manganese-cobalt spinel coatings are being studied for use as a diffusion barrier on ferritic stainless steel interconnectors in order to prevent migration of harmful CrO₃ and Cr₂(OH)₂ compounds to the cathode triple phase barriers (TPB) in SOFC devices. At the TPB, these Cr-compounds reduce back to Cr₂O₃ and cause the degradation of the cathode. Therefore, a protective layer to prevent evaporation of chromium oxides and prevent aging of the SOFC device is required. A few studies are related to manufacturing these coatings by using atmospheric plasma spraying (APS) from spray dried Mn-Co spinel powder particles, but none of the studies are focused on spraying these coatings by using a solution of diluted manganese and cobalt nitrates. The objectives of using the solutions instead of the spray dried spinel powders are i) the length of production chain can be shortened, as the material synthesis and powder manufacturing phase can be cut out and; ii) thinner ceramic coating layers can be sprayed, in order to decrease the ohmic resistances. In this study manganese nitrate tetrahydrate (Mn(NO₃)₂•4H₂O) and cobalt nitrate hexahydrate (Co(NO₃)₂•6H₂O) were diluted to ethanol and deionized water. The solutions were sprayed by using Sulzer Metco F4-MB atmospheric plasma spraying system with a modified solution/suspension feeder. Various sets of spraying parameters were used in order to study the coating build up mechanisms. The coatings were sprayed on polished 1 mm thick ferritic stainless steel (Crofer 22 APU).  The as-sprayed coatings were characterized by using a field-emission scanning electron microscopy (FESEM) for structural analysis and an X-ray diffraction (XRD) for crystallographic analysis.