Synthesis of Cubic Aluminum Nitride Coating from Al₂O₃ Powder in Reactive Plasma Spray Process

Abstract

Plasma spraying is a versatile technique for producing abradable and protective ceramic coatings. However, it was difficult to fabricate aluminium nitride (AlN) coating by conventional plasma spray processes. It is due to the thermal decomposition of the feedstock AlN powder during spraying without a stable melting phase. Reactive plasma spraying (RPS) has been considered as a promising technology for in-situ formation of AlN thermally sprayed coatings. This study investigated the feasibility of reactive plasma spraying of Al₂O₃ powder in N₂/H₂ plasma upon fabrication of AlN coating. It was possible to fabricate a cubic-AlN (c-AlN)/Al₂O₃ composite coating and the fabricated coating consists of c-AlN, α-Al₂O₃, Al₅O₆N and γ-Al₂O₃. Furthermore, the AlN content was improved with increasing the flight time (spray distance), due to increasing the reaction time between the Al₂O₃ particles and the surrounding N₂/H₂ plasma. It was possible to fabricate coating contains about 97% of c-AlN. However, it was difficult to clarify the in-flight reaction during the coating process, due to losing the particles shape and features after colliding and flattening on the substrate surface. The sprayed particles were collected into a water bath to maintain its particle features in order to investigate the in-flight reaction. It was clear from the microstructure and the cross section observation of the collected particles that, the nitriding reaction started from the surface. During the coating process, the sprayed particles were melted, spheroidized and reacted in the high temperature N₂/H₂ plasma and formed aluminum oxynitride (Al₅O₆N) which have cubic structure. The particles collided, flattened, and rapidly solidified on a substrate surface. The Al₅O₆N is easily converted to c-AlN phase via continuous nitriding (both have the same cubic symmetry: cubic and closely packed) during the rapid solidification and plasma irradiation on the substrate. The high quenching rate of the plasma flame prevents the AlN crystal growth to form the hexagonal phase. Therefore, it was possible to fabricate c-AlN/Al₂O₃ composite coatings through reactive plasma nitriding of Al₂O₃ powder and the nitriding process of the Al₂O₃ particle as well as the formation process of c-AlN phase were investigated.