Thermal spray coating is widely used in a variety of industrial applications. Molten powder or wire is heated either through oxy-fuel combustion or plasma—the flame of the spray device powers the heated mixture, and after even spray distribution onto a metal product, the mixture assumes its solid coating form. Thermal spray coating can serve an array of functions; commonly used to treat planes, they can also protect products from wear, temperature extremes, chemicals, and even protect buildings from external environmental conditions like rain and humidity. Although there are multiple methods and materials involved in thermal coating, they are distinguished by both heat source and the base material used for deposition. Combustion flame spraying, high velocity oxy-fuel spraying (HVOF), two-wire electric arc spraying, plasma spraying, and vacuum plasma spraying are several common coating application processes.
Combustion Flame Spraying
For applications that are not expected to weather extreme amounts of external stress, combustion flame spraying is a viable option. Combustion flame spraying results in a coating that isn’t strongly bonded to the product because the spraying mechanism is driven by a relatively low flame velocity and temperature—around 50M/s velocity and below 3000 degrees C. The flame is propelled by oxygen mixed with fuel, which also results in melting the mixture. Typically, combustion flame spraying uses powder or wire as the main coating mixture component. Because the process is relatively cost-effective and easy to apply, it is widely used in low-performance applications.
High Velocity Oxy-Fuel Spraying
HVOF is similar in theory to combustion flame spraying, but uses a different torch design that enables the flame to expand when the spray nozzle is activated. This causes a surge in acceleration, which in turn accelerates the mixture particles. When the mixture is released from the nozzle, the velocity of the mixture leads to a very thin and evenly applied coat. The final coating is well-adhered, strong, and dense. In fact, its hardness, corrosion resistance, and overall wear resistance is often superior to plasma spraying. Because of the low temperature of the torch flame, which both melts the powder and propels the coating’s deposition, the mixture is not suited to withstanding high temperatures.
This method of deposition relies on an arc-point formed by two electrically conductive wires. Where the wires meet, melting transpires. The “arc” is the heating force that enables melting and deposition, just as the combustion flame powers a combustion flame spray torch of thermal spray coating. After the metal wires meet and melt, the process depends on compressed air to spray the coating. The procedure cost-effective, and often uses zinc and aluminum as the base material for spray coating infrastructure applications. Both materials, when two-wire electric arc sprayed, provide strong corrosion resistance.