Aluminum Nitride Coating Fabricated by Reactive Plasma Spraying of Al2O3

Aluminum nitride (AlN) is attractive material for electrical and electronic applications due to its outstanding properties. Plasma spraying is a versatile technique for producing abradable and protective ceramic coatings.  However, it was difficult to fabricate 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. The plasma spray processing of AlN coating through reactive plasma nitriding of Al₂O₃ powder in N₂/H₂ plasma was investigated in this study. 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 Al₂O₃ particles and the surrounding N₂/H₂ plasma. In order to investigate the in-flight reaction, the sprayed particles were collected into a water bath to maintain its particle features. It was clear from the microstructure and cross section observation of the collected powder that, the nitriding reaction started from the surface. During the coatings process, the sprayed particles were melted, spheroidized and reacted in the high temperature N₂/H₂ plasma and formed aluminum oxynitride (AlON) which have cubic structure. The particles collide, flatten, and rapidly solidified on a substrate surface. The AlON is easily nitride to c-AlN phase (same cubic symmetry). The c-AlN was formed by rapid solidification of AlON and plasma irradiation on the substrate. The high quenching rate of the plasma flame prevents AlN crystal growth to form the hexagonal phase. Furthermore, the effect of plasma gases and ambient gas on the nitriding process and coating microstructure was investigated to optimize the process.