Ablation and insulation performance of Al₂O₃/PI coatings deposited by wide-velocity range high-energy plasma spraying

Transient plasma ablation and molten aluminum contamination are major degradation modes of resin-based insulating supports in electromagnetic rail launchers. In this work, Al₂O₃/polyimide (PI) composite coatings with different thermoplastic/thermosetting PI ratios (TPI/TSPI = 10:0, 7:3, 5:5, and 3:7) were deposited on glass fiber reinforced epoxy resin substrates by wide-velocity-range high-energy plasma spraying. The TPI/TSPI ratio affected the powder thermal response, splat spreading, coating defects, and residual layer stability after ablation. PI73 showed a balanced coating structure and exhibited the lowest mass ablation rate of 0.024 g·s⁻¹ after 150 plasma ablation cycles, corresponding to a 78.9% reduction compared with the resin substrate, while retaining a flexural strength of 232.61 MPa. After molten aluminum erosion, the Al₂O₃/PI coating maintained volume resistivities above 10¹⁰ Ω·cm. Ablation and erosion results indicated that the retained Al₂O₃-rich framework, PI-derived carbonaceous products, and lamellar coating structure contributed to residual layer integrity and molten aluminum penetration resistance. The proposed regulation strategy for the ratio of thermoplastic to thermosetting resins offers a novel design route for protective coatings against synergistic erosion by high temperature and molten aluminum.