Deposition of mullite and Y₂O₃ coatings on the inner wall of a graphite tube via atmospheric long laminar plasma spraying (ALPS): Microstructural evolution and performance at 1500 °C

Ceramic coatings on the inner walls of small-diameter graphite tubes are important for the melting of reactive metals. However, depositing high-melting-point ceramic coatings inside such tubes remains challenging for conventional plasma spraying. In this study, a mullite/Y₂O₃ bilayer coating was deposited on the inner wall of a graphite tube with an inner diameter of 55 mm and a length of 240 mm by atmospheric long laminar plasma spraying (ALPS). The as-sprayed coating achieved 100% coverage over the evaluated axial lengths. Splat-scale characterization revealed that substrate temperature exerted competing influences on mullite splat spreading and interfacial bonding. After vacuum exposure at 1500 °C for 1 h, extensive macroscopic spallation was observed, and cracks and fracture zones were concentrated near the mullite/graphite interface. Pre-existing spraying defects and locally unbonded regions provided preferential paths for crack propagation, whereas thermal-mismatch stresses promoted crack propagation and coalescence. Thermodynamic analysis and previous studies support the possibility that carbothermal reduction occurred at the mullite/graphite interface under high vacuum, contributing to interfacial degradation.