Alumina Coatings Formed by Electromagnetically Accelerated Plasma Spraying

Alumina (Al₂O₃) coatings on stainless steel substrates were fabricated by electromagnetically accelerated plasma spraying (EMAPS), which generated a high-velocity pulsed plasma jet of more than 2.5 km/s with a high peak pressure of 1 MPa. High amounts of α-Al₂O₃ crystalline phase in the raw powder was preserved in the EMAPS coatings where the ratio of α-Al₂O₃ to γ-Al₂O₃ was approximately 65:35, which is generally different from the α-Al₂O₃-rich coatings formed from α-Al₂O₃ raw powders using conventional spraying methods, such as plasma spraying, detonation gun spraying, and high-velocity oxygen-fuel (HVOF) spraying. The specific wear rate of the coating formed from 6.6 µm raw powder at a spray distance of 100 mm was 2.3 × 10^-6 mm³/Nm, which is comparable to that of sintered bulk α-Al₂O₃. Morphological features of the coatings and the crystal structures suggested that the plastic deformation by the high-velocity impact of highly viscous raw powders due to insufficient heating was dominant in the formation of dense coatings with highly structural ratios of α-Al₂O₃.