SiCNO coatings with carbon content tuned by annealing atmosphere for SOFC interconnect applications: oxidation behavior and electrical response

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In this study, SiCNO coatings with tunable carbon content were derived from a single preceramic polysilazane and deposited on AISI 441 steel. The carbon content was controlled through the annealing atmosphere. The coatings were annealed in argon, in argon followed by cooling in air, and entirely in air. As a result, three coatings with different carbon concentrations were obtained. XPS together with Raman spectroscopy confirmed that the annealing atmosphere altered the amount and nature of the free carbon phase (FCP). ToF-SIMS revealed the outward diffusion of manganese and chromium notwithstanding the annealing conditions. During oxidation at 800 °C for 100 h in air, the coatings inhibited the net mass gain of steel by 85–98% relative to AISI 441, although they did not act as perfect diffusion barriers, as evidenced by the formation of oxide scales above the coatings. The area specific resistance of the oxidized coatings ranged from 0.13 to 0.82 Ω ∙ cm 2 , compared with 0.09 Ω ∙ cm 2 for bare AISI 441, and thus remained too high for SOFC interconnect applications consistent with the discontinuous, non-percolating FCP network. The annealing atmosphere modified the relative contributions of silica, and SiON/SiOC/SiOx environments within the coating, thereby affecting metal transport and the resulting oxide-scale composition. The coating annealed in argon developed a scale containing mainly (Mn,Cr) 3 O 4 and Cr 2 O 3 , whereas the coatings exposed to air during annealing or cooling formed manganese-rich oxides and silicates together with smaller amounts of (Mn,Cr) 3 O 4 and Cr 2 O 3 . Overall, the annealing atmosphere strongly controlled carbon structure, metal transport, and oxidation-product formation.