N2 and H2 plasma gasses' effects in reactive plasma spraying of Al2O3 powder

Reactive plasma spraying (RPS) is a promising technology for in-situ formation of aluminum nitride (AlN) coatings. In this study the alumina (Al2O3) powder was used as feedstock powder material upon fabrication of AlN coating in RPS process. The effect of plasma gasses (N2 and H2) on the fabrication process, the phase composition and the coating microstructure was investigated. Thick AlN/Al2O3 composite coating was successfully fabricated and the coating consists of cubic-AlN (c-AlN), α-Al2O3, Al5O6N and γ-Al2O3. The nitride content was improved with increasing the N2 gas flow rate. It is attributed to increasing the amount of the nitriding species surrounding the particles. Increasing the flow rate enhanced the particle velocity and therefore the thickness firstly increased. However, further increase reduced the particle residence time in the plasma and therefore decreased the deposition efficiency. The N2 gas flow rate did not affect so much on the phase transformation of the α-Al2O3 to the γ-Al2O3 phases. Furthermore, increasing the residence time (spray distance) of the particle under high N2 gas flow rate significantly improved the nitriding conversion and suppressed coating growth.On the other hand, the H2 plasma gas is required for the plasma reduction and nitriding of the Al2O3 particles in RPS process. Thus, in the absence of the H2 plasma gas, the AlN phase did not appear in the fabricated coating. However, increasing the H2 gas flow rate did not affect the nitriding conversion. The H2 gas and its flow rate significantly increased the particles' in-flight temperature gradually without any effect on the particles' in-flight velocity. Therefore, the H2 gas assists the particle melting and its phase transformation from the α-Al2O3 to the γ-Al2O3. Although, increasing the particles' temperature improved the coating thickness at short distance, it assists the particle vaporization and suppressed the thickness with increasing the flight time. Therefore, the H2 gas is required and low flow rate is recommended for RPS of Al2O3 powder.

► The Plasma gasses effect upon reactive plasma spraying of Al2O3 powder was investigated. ► The plasma gasses (N2 and H2) affected the coating composition, structure and particle behaviour. ► The N2 plasma gas enhanced the nitriding conversion (during flight and after deposition) and coating thickness. ► The N2 gas improved the particle in-flight velocity and did not affect its temperature. ► The H2 plasma gas is required for Al2O3 reduction, nitriding and low flow rate is recommended.