Thermal exposure resistance and thermomechanical properties of ALPS SiC/Y₂O₃ coatings on graphite at 1500 °C

Graphite components are extensively utilized in the nuclear industry for the melting and casting of highly reactive metal alloys. To mitigate melt carburization, suppress carbide inclusion formation, ensure the reusability of graphite substrates, and minimize radioactive waste, the deposition of an effective graphite barrier coating (GBC) is imperative. Such coatings are required to withstand prolonged service temperatures up to 1500 °C. In this study, a novel atmospheric long laminar plasma spraying (ALPS) technique was employed to fabricate two systems: graphite/Y₂O₃ and SiC/Y₂O₃ coatings, which were subsequently subjected to thermal exposure at 1500 °C. Post-thermal exposure analysis revealed significant microstructural and mechanical evolution. The coating underwent densification. Concurrently, the hardness increased markedly from 3.24 GPa to 6.37 GPa, and the elastic modulus rose from 58.20 GPa to 132.52 GPa, indicating enhanced load-bearing capacity. However, thermal stresses arising from the coefficient of thermal expansion (CTE) mismatch induced vertical cracking that penetrated the coating thickness and interconnected with the pre-existing pore network. Additionally, horizontal cracks initiated at weakly bonded interfaces, culminating in large-area coating spallation. This elucidation of the Y₂O₃ coating failure mechanism provides a critical theoretical foundation for the design and fabrication of robust SiC/Y₂O₃ bilayer coatings on graphite substrates.