**7.5 Wire arc spraying**

Twin wire arc spraying is the most economical thermal spraying process. In this type of spraying process the heating and melting occur when two oppositely charged wires are fed together in a way that arc is generated at their intersection (**Figure 22**). Once struck, the arc continuously melts the wires, and compressed air blown directly behind the point of contact, atomizes and projects the molten droplets, which sticks to the substrate to form a coating. Arc fluctuations due to periodic removal of molten droplets from the electrode tips have strong effects on melting and coating properties such as porosity, microstructure and oxide content.

#### **7.6 Plasma spraying process**

Plasma spraying is a flexible and low-cost method to manufacture coating and bulk materials. The first idea of a plasma spray process was patented in 1909 in Germany, and the first structural plasma installation appeared in the 1960's, as the product of two American companies Plasmadyne and Union Carbide. A gas, usually argon, but occasionally including nitrogen, hydrogen, or helium, is allowed to flow between a tungsten cathode and a water-cooled copper anode. The cathode is placed in the cylindrical nozzle and the cylindrical nozzle is the anode. An electric arc is initiated between the two electrodes using a high frequency discharge and then

*Wire arc spray technology.*

**Figure 23.**

*Plasma spray coating technology.*

sustained using dc power. The plasma is generated by the ionization of gas by the arc. The feedstock materials injected through the gun nozzle into the plasma plume, where it is melted and propagated to the substrates (**Figure 23**) [2, 6, 8–12].
