Volatilization of metal powders in plasma sprays

Ideally, plasma spraying of metal powders must take place within a narrow processing “window” where the particles become fully molten before they hit the substrate, but are not overheated to the point that substantial volatilization occurs. Metal evaporation in flight results in a decrease in the deposition efficiency. In addiiton, the emission of vapors leads to the formation of metal and oxide fumes that are undesirable from the viewpoints of both resource conservation and environmental control. This study examines the vaporization and fume formation in the plasma spraying of iron powders of different size ranges. The experimental part involves the determination of the population (number density) of metal atoms at different cross sections along the trajectory of the plasma jet, and the collection of the submicronic particles resulting from vapor condensation. The experimental results are compared with the projections of a mathematical model that computes the gas/particle velocity and temperature fields within the jet envelope, projects the rate of heat/mass transfer at the surface of individual particles, and determines the rate of volatilization that results in the formation of metal and metal oxide fumes. This paper was presented at the International Thermal Spray Conference sponsored by the ASM Thermal Spray Society, the DVS-German Welding Society and the IIW International Institute of Welding, May 8–11, 2000 in Montreal, Canada.