Studies on Atmospheric Plasma Sprayed Mn1.0co1.9fe0.1o4 Coating on Crofer 22 Apu Interconnect for Solid Oxide Fuel Cells Applications

Abstract

Atmospheric plasma sprayed (APS) Mn1.0Co1.9Fe0.1O4 (MCF) coating is regarded as one of the excellent materials in mitigating Cr-evaporation in Crofer 22 APU ferritic stainless steel during high-temperature operation (> 600 °C) in solid oxide fuel cell (SOFC) conditions. The aim of the present study is to characterize the structural integrity using a correlative scratch indentation test, physio-thermal integrity using in-situ high temperature X-ray diffraction (HT-XRD), and thermal integrity using a long-term thermal oxidation test of MCF coated Crofer steel for SOFC applications. A network of micro-cracks and globular pores were seen in the cross-section analysis. The porosity of the as-sprayed MCF coating was 10.93 ± 1.323 %. XRD data revealed α-Fe as the major phase in as-received Crofer steel and CoO as the major phase in MCF coating. The micro-hardness measurements revealed strong metallic interlocking between the coating and substrate. The adhesion strength of MCF coating deposited on Crofer 22 APU ferritic steel was found to be in the range between 30 to 36 N, evaluated by scratch indentation test under progressive and constant loading conditions. Initial stage oxidation of Crofer 22 APU steel carried out in an in-situ HT-XRD stage at 950 °C and subsequent GD-OES characterization revealed the formation of two-layer oxides: Top layer spinel MnCr2O4 and fine-grained inner layer Cr2O3. The Cr2O3 formed initially led to the formation of MnCr2O4 spinel during the initial stage. The rapid diffusion of Mn through the fine-grained Cr2O3 layer results in an increased growth rate of MnCr2O4 spinel on the top of the fine-grained Cr2O3 layer. The thermal expansion mismatch of MCF coated Crofer steel interconnect has been investigated by in-situ HT-XRD from 25-900 °C. The results showed that the coefficient of thermal expansion of MCF coating was slightly higher than the steel substrate and showed no considerable thermal expansion mismatch as a function of temperature. The increase in lattice strain indicated the strain-induced phase transformation of MCF coating, supporting the phase transformation-induced self-healing phenomenon of MCF coating. The oxidation kinetics of plasma sprayed MCF spinel coating on the Crofer 22 APU substrate at 850 °C revealed phase transformation-induced crack healing and densification of the coating. MCF coating effectively acts as a Cr diffusion barrier and alters the kinetics of the two-layer oxide scale of the substrate. The UV-vis-NIR affirmed the reduction in band gap energy of the MCF coating, a beneficial effect to sustain the electrical conductivity at high temperatures.