Phase Transformation-Induced Changes in Microstructure and Residual Stresses in Thermally Sprayed MnCoFeO 4 Protective C
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Phase Transformation-Induced Changes in Microstructure and Residual Stresses in Thermally Sprayed MnCoFeO4 Protective Coatings Hyoung Chul Back1 • Jens Gibmeier1 • Robert Vaßen2
Submitted: 8 November 2019 / in revised form: 21 January 2020 The Author(s) 2020
Abstract The contribution comprises the investigation of the microstructure and residual stresses in thermally sprayed Mn1.0Co1.9Fe0.1O4.0 (MCF) protective coatings for interconnectors of SOFC stacks, deposited on ferritic steel Crofer 22 APU via atmospheric plasma spraying (APS). The coatings are designated to prevent Cr evaporation during high operation temperature of the SOFCs. The local microstructure, pore distributions and pore shapes, phase fractions, micro-hardness, Youngs’ modulus and residual stresses through the coating thickness were characterized in as-sprayed state and compared with longtime (10-100 h) heat-treated samples (700 and 850 C). The results show that the long-term thermal aging treatment causes a successive high sintering of the coatings characterized by a reduction in pore density, by phase transformation from the metastable rock salt structure that gradually transformed to a spinel structure and by a slight relaxation of the processinduced tensile residual stresses in the coating. For SOFC application of the MCF coating, this indicates an improvement in the coatings integrity. During operation, a self-repair proceeds leading to dense and gas-proof coatings, while the mechanical properties are mainly retained.
This article is part of a special topical focus in the Journal of Thermal Spray Technology on Advanced Residual Stress Analysis in Thermal Spray and Cold Spray Processes. & Jens Gibmeier [email protected] 1
Institute for Applied Materials, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
2
Institute of Energy (IEK-1), Forschungszentrum Ju¨lich GmbH, Ju¨lich, Germany
Keywords atmospherically plasma spraying MCF residual stress determination—testing solid oxide fuel cells (SOFCs)—applications
Introduction Solid oxide fuel cells (SOFC) are attractive candidates for energy conversion systems for an effective way of producing electrical energy directly from chemical energy. Due to the limited output power, single cells can be arranged into stack system and interconnectors are used to link the individual cells. High-chromium-containing ferritic stainless steels are widely used for interconnectors due to their good electrical conductivity, chemical and mechanical stability, suitable thermal expansion behavior and low cost. However, during high operation temperature these materials form chromia-containing oxide scales on the surface and these scales tend to evaporate especially under water vapor environment. Due to chromium migration into the cathode side, the cell performance can suffer from degradation (Ref 1-3). Protective layers can be applied between interconnector and cathode to suppress the evaporation of Cr from chromium-containing oxide scales, hence to enhance the long-term perform
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