Abstract
Thermal barrier coatings (TBCs) of magnesium aluminate spinel (MgAl2O4) and yttria (Y2O3) were investigated for long term compatibility with molten sodium for applications in sacrificial core catcher of Future Fast Reactors (FFRs). Thermal barrier ceramic coatings were deposited over austenitic grade stainless steel SS 316LN (flat and cylindrical samples), with an intermediate NiCrAlY bondcoat, using Atmospheric Plasma Spray (APS) technique. The coated samples were exposed to high purity molten sodium at 400 °C under an inert argon atmosphere for 500 h and 1000 h. Sodium exposed samples were subjected to post-cleaning by vacuum distillation followed by chemical cleaning for effective removal of sodium from the specimens. Visual examinations revealed discoloration in the Na exposed samples. Microstructural characterization of Na exposed samples revealed densification in the ceramic topcoats. X-ray diffraction analysis on the topcoats did not reveal any phase change or formation of any corrosion product upon interaction with sodium. Tensile adhesion test on cylindrical samples, as per ASTM C633, revealed ~62% and ~33% drop in the bonding strength after 1000 h of sodium exposure for spinel and yttria coatings, respectively. The mechanical properties were correlated to the microstructural characteristics of the TBC system before and after Na exposure.
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