A second type of thermal spray coating is Air Plasma Spraying (APS). In this process, an arc is created between an electrode and a spray nozzle, where pressurized inert gas is heated to extremely high temperatures, forming a plasma gas. Powder is then injected into the plasma, melting as it is propelled onto the substrate at high velocity. Plasma spraying is typically used for applying ceramics and materials that melt at elevated temperatures. The spray speed is around 727 feet per second, with an arc temperature exceeding 20,000°F, a plume temperature of about 4,000°F, and an inter-pass temperature of the part remaining below 300°F.
Plasma-applied ceramic coatings form a mechanical bond and are effective in combating corrosion and abrasion. Common materials used include titanium dioxide and chrome oxide. Titanium dioxide offers toughness and excellent corrosion resistance, while chrome oxide excels in both abrasion and severe corrosion resistance. Even if chrome oxide might not initially seem necessary, it can be the ideal coating choice if a previously selected carbide coating proves insufficient.
The Repair Process: Thermal Spray, Grinding, and Lapping
Grit blasting is used to remove heavy scale from parts. After the grit blast, an evaluation is performed to assess whether the part requires weld repair. Next, the coating is stripped down to the base metal in preparation for re-coating, effectively