Degradation of thermally sprayed Al2O3 coatings in reactor-grade liquid-sodium and its mitigation by laser treatment

Abstract

Laser treatment (LT) to enhance the performance of atmospheric plasma spray (APS) deposited Al2O3 thermal barrier coatings (TBCs) was attempted for applications in extreme liquid-sodium environments of a sodium-cooled fast reactor (SFR). Thermally sprayed sacrificial ceramic TBCs are being proposed and investigated for installation on the stainless steel (SS) 316LN core catcher assembly of future SFRs to combat the core disruptive accidents (CDAs), wherein the primary criterion to be satisfied by the TBC is its long-term compatibility with the liquid-sodium coolant over a whole reactor life. Sacrificial ceramic coatings are susceptible to liquid-metal induced degradation by reactor-grade sodium. This study provides new insights into the failure mechanisms of APS Al2O3 coatings in liquid-sodium environments. Subsequently, an attempt is made to improve the performance of coatings by a surface modification approach using a laser treatment of the TBC topcoat. APS coatings exposed to liquid-sodium failed by spallation and delamination of the incrementally deposited topcoat, which is attributed to the formation and growth stress of ternary Na–Al oxides between the lamellar layers of the APS coatings. Compared to the APS coatings, LT coatings successfully retarded the liquid-sodium ingression and thereby exhibited improved degradation resistance and structural stability in reactor simulated sodium environments. The enhanced performance of LT coatings is attributed to its carefully re-engineered architecture of the APS TBC that could beneficially control the kinetics of interaction with liquid-sodium.