Fabrication of aluminum nitride coating by reactive RF plasma spraying

Abstract

Aluminum nitride (AlN) is one of the attractive ceramics applicable to the surface modification because of its excellent properties in chemical stability and thermal conductivity. Generally, thin AlN coatings are fabricated by CVD or PVD process. However, these processes cannot produce thick coatings, which might contribute to the structural parts, because of quite low deposition rate in these processes. Thick coatings can be easily given by thermal spray process. However, fabrication of AlN coating by conventional thermal spraying is fairly difficult due to decomposition of AlN at 2273K. In this research, AlN coating was fabricated by reactive Radio Frequency (RF) plasma spraying. Reactive plasma spray process, in which metal element reacts with surrounding active species in plasma, is effective for the spraying of non-oxide ceramics. RF plasma can offer the enough reaction chance to the elemental metal because both of its low flowing velocity and high energy. By increasing nitrogen content in plasma gas, AlN coating without pure Al phase was attained while the coating microstructure was heterogeneous, brittle and quite porous. By decreasing nitrogen content in plasma gas, on the other hand, Al/AlN composite coating with more homogeneous, less porous microstructure could be attained. Changing nitrogen fraction in plasma gas may be effective for controlling AlN content in Al/AlN composite coating.