Aluminum Nitride Coating Fabricated by Reactive Plasma Spraying of Al₂O₃

Reactive plasma spraying (RPS) has been considered as a promising technique for in situ formation of aluminum nitride (AlN) based thick coatings. This study investigated the reactive plasma spraying of AlN coating with using Al₂O₃ powder and N₂/H₂ plasma. It was possible to fabricate a cubic- AlN (c-AlN) based coating. The phase composition of the coating consists of c-AlN, α-Al₂O₃, Al₅O₆N and γ-Al₂O₃. Understanding the nitriding process during coating deposition is essential to control the process and improve the coating quality. The nitriding process was performed by spraying, collecting the particles into a water bath (to maintain its particle features) and observing their microstructures and cross sections. During the coating process, the sprayed particles were melted, spheroidized and nitrided in the N₂/H₂ plasma to form the cubic aluminum oxynitride (Al₅O₆N). The particles collided, flattened, and rapidly solidified on the substrate surface. The Al₅O₆N is easily transformed to c-AlN phase (same cubic symmetry) by continuous reaction through plasma environment. Improving the specific surface area by using smaller particle sizes enhances the surface nitriding reaction and improves the nitriding conversion. Furthermore, using AlN additives enhances the nitride content in the coatings. It was possible to fabricate thick and uniform coatings with high AlN content by spraying fine Al₂O₃/AlN mixture. Furthermore, the N₂ gas flow rate improved the nitriding conversion and the coating thickness.