Thermal spray coatings are 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 coatings 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.
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.
Two-Wire Electric Arc Spraying
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. 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.
In plasma spraying, a plasma torch is the primary mode of heating and applying the coating. Once the material, usually powder, has been melted, it is subsequently applied to the product in much the same manner as combustion flame spraying. Coatings can range in thickness from a few micrometers to several millimeters. Plasma spray coating raw material isn’t limited to powder, and includes metals and ceramics—the spray gun can apply a combination mixture evenly (both ceramic and metal) and results in a rapid application process. Additionally, plasma spray coating can be done in a wide array of conditions and is an adaptable process.
Vacuum Plasma Spraying
Like plasma spraying, vacuum plasma spraying is a low temperature process, but must be conducted inside a controlled environment, which not only sustains the vacuum but helps minimize damage to the material. Because a vacuum environment is controlled, it helps ensure a more precise application. Using different gas combinations to provide the needed pressure for spraying can also help manipulate the results. Vacuum plasma spraying is often used to coat automobile components, such as bumpers, dashboard parts, and door mirror housings. Additionally, the process can help pre-treat polyethylene moldings, enabling adhesion of water-based epoxy adhesives and other coatings.
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