Arc-PVD coating of powders in a microwave plasma fluidized bed

Fluidized bed reactors are widely used in chemical engineering, but recently there is also increasing interest for processing particulate advanced materials in fluidized bed reactors. Examples include heat treatment, plasma-assisted surface treatment, etching, and coating. In this paper, a novel process for deposition of thin metallic and metal-nitride coatings on particulate substrates is reported. The coating process takes place in an atmospheric pressure microwave plasma fluidized bed, with the novelty that, mixed with a substrate powder, a fine solid metal powder is used as "target", serving as a source of metal vapor. Interactions of fluidized metal particles with the strong electric field lead to plasma discharges. Due to this, the metal is being evaporated readily below its melting temperature, and being deposited on the surrounding surfaces, primarily on the substrate powder. Final composition of the coatings depends also on the process gas - evidence is presented that N₂ plasma yields metal nitrides (where applicable), whereas Ar/H₂ plasma treatment results with pure metallic coatings. In this paper, a study of coating of a micro diamond powder with Al, Si, and Ag, is presented.