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 (N-2 and H-2) 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), alpha-Al2O3, Al5O6N and gamma-Al2O3. The nitride content was improved with increasing the N-2 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 N-2 gas flow rate did not affect so much on the phase transformation of the alpha-Al2O3 to the gamma-Al2O3 phases. Furthermore, increasing the residence time (spray distance) of the particle under high N-2 gas flow rate significantly improved the nitriding conversion and suppressed coating growth. On the other hand, the H-2 plasma gas is required for the plasma reduction and nitriding of the Al2O3 particles in RPS process. Thus, in the absence of the H-2 plasma gas, the AlN phase did not appear in the fabricated coating. However, increasing the H-2 gas flow rate did not affect the nitriding conversion. The H-2 gas and its flow rate significantly increased the particles' in-flight temperature gradually without any effect on the particles' in-flight velocity. Therefore, the H-2 gas assists the particle melting and its phase transformation from the alpha-Al2O3 to the gamma-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 H-2 gas is required and low flow rate is recommended for RPS of Al2O3 powder. (C) 2012 Elsevier B.V. All rights reserved.