Reactive Plasma Spraying of Fine Al2O3/AlN Feedstock Powder

Reactive plasma spraying (RPS) is a promising technology for in situ formation of aluminum nitride (AlN) coatings. Recently, AlN-based coatings were fabricated by RPS of alumina (Al2O3) powder in N-2/H-2 thermal plasma. This study investigated the feasibility of RPS of a fine Al2O3/AlN mixture and the influence of the plasma gases (N-2, H-2) on the nitriding conversion, and coating microstructure and properties. Thick AlN/Al2O3 coatings with high nitride content were successfully fabricated. The coatings consist of h-AlN, c-AlN, Al5O6N, gamma-Al2O3, and a small amount of alpha-Al2O3. Use of fine particles enhanced the nitriding conversion and the melting tendency by increasing the surface area. Furthermore, the AlN additive improved the AlN content in the coatings. Increasing the N-2 gas flow rate improved the nitride content and complete crystal growth to the h-AlN phase, and enhanced the coating thickness. On the other hand, though the H-2 gas is required for plasma nitriding of the Al2O3 particles, increasing its flow rate decreased the nitride content and the coating thickness. Remarkable influence of the plasma gases on the coating composition, microstructure, and properties was observed during RPS of the fine particles.