Interaction of air plasma-sprayed spinel and yttria coatings with ultra-high temperature ceramic-metal hot-melt

Air Plasma Sprayed (APS) ceramic coatings are conventionally used for protecting metallic parts of hot section components in aero-and land-based gas turbines. Owing to their capability to withstand very high temperatures high specific heat capacity, and thermal shock resistance, such APS coatings are also investigated for the po-tential to protect the core catcher in nuclear reactors during the high-temperature core-meltdown accidents resulting in corium hot-melt. In this study, candidate plasma-sprayed spinel and yttria coatings on SS 316LN substrates were subjected to an ultra-high temperature (UHT) ceramic-metal hot-melt at-2500 degrees C. Uncoated steel substrates showed melting, whereas coated substrates were found un-attacked at the expense of degradation of the coatings such as surface melting, topcoat sintering, columnar grain growth and thermal shock cracks. The evolution of interfacial fusion was observed in the case of yttria coatings due to the formation of primary YAG and Al2O3-YAG eutectic at the interface. Spinel coatings with a comparatively lower thermal conductivity could generate a higher Delta T across the topcoat thickness with limited grain growth in the substrate. Evidence of local melting at the interface and evolution of temperature gradient between the hot-melt and substrate are comprehensively illustrated. Out of the two coatings tested, spinel was found to be more protective to the steel substrate.