Rod Flame Spray represents cost effective and efficient alternatives to powder flame, plasma or HVOF processes for producing metallic and ceramic coatings due to the lower cost of feedstock materials, the lower gas consumption, the high spray rate and the high deposition efficiency. Furthermore, the handling, storage and operation using rods are somewhat easier and simpler than by using powder feedstock materials.
Hard materials like ceramics can be processed in form of rods. Flame spraying rods are usually manufactured by pressing a mixture of the ceramic powder, a glass type binder on the basis of SiO2 and an organic binder into the final shape. These pre-shaped rods have to be sintered at high temperatures of about 1300°C to attain the required mechanical stability for handling. Using such feedstock material in especially designed spray guns, ceramic coatings can be produced, which appear as an suitable alternative to plasma sprayed coatings.
Rod Flame Spray process involves the continuous combustion of a fuel gas within an air-cooled nozzle. The hot gases are released through the nozzle cap and further accelerated by a supplementary high-velocity air stream. The rod are fed axially and centrally into the flame and melt at the tip. The droplets, which are stripped off from the molten wire tip, are accelerated in the process gas stream. By fragmentation during the early stages of flight, the droplet size can be further reduced. In HVCW Spraying, the velocity of the gas stream leaving the nozzle is supersonic, as indicated by a series of shock diamonds visible in the flame. By spraying metallic materials, particles can reach a velocity about twice or three times higher than in conventional wire spraying. Nevertheless, the particle velocities are still only about half of that obtained in the HVOF processes using powders as feedstock.