Thermal spraying

Thermal spraying is a surface finishing process in which a protective layer is applied to aluminium at high temperature. It results in high wear resistance. Metals, ceramics and plastics, in powder or wire form, are used as coating materials.

This type of coating is particularly necessary when aluminium components are subjected to exacting conditions and where additional resistance to wear and corrosion is required. Thermally sprayed aluminium components are used, for example, for machine parts or in car making.

The surface to be treated is first cleaned and roughened by sandblasting to ensure good adherence of the coating. The spray material is then melted in a combustion chamber – the temperature varies according to the process used – and then sprayed onto the surface of the aluminium under pressure using a carrier gas.

The most important thermal spraying processes are flame spraying, plasma spraying and electric-arc spraying. Besides differences in the temperatures used, the processes also differ in the type of carrier gas and the method used to melt the coating material, as well as in the properties of the coatings they produce.

In flame spraying, the spray material is melted using an oxy-acetylene flame at temperatures of 1750-3100 °C and then applied using compressed air. This process can only be used, however, to produce coatings with limited adhesion.

Metallic coating materials in the form of two consumable wire electrodes are used with electric-arc spraying. The two wire electrodes, with an applied electrical potential, are brought together so that a controlled arc is produced and the wires melt at temperatures of around 4000 °C. The molten metal is atomised and the particles are propelled onto the aluminium using compressed air or an inert gas. This process produces coatings that are very thick and have very good adherence.

With plasma spraying, the coating material in powder form is introduced into a plasma stream, comprising ionised gases, melted between a tungsten cathode and a copper anode and then sprayed at high speed onto the aluminium surface. The impinging plasma beam has a temperature of up to 25 000 °C and the resultant coating is very highly adherent.