Thermal spray is a group of coating processes in which melted (or heated) materials are sprayed onto a prepared surface. The coating material or "feedstock" is heated by electrical (plasma or arc) or chemical means (combustion flame).
Thermal spray coatings can be thick (thickness range from 20 micrometers to several mm).
Thermal Spray Coating materials for thermal spray includes metals, alloys, ceramics, plastics and composites. They are fed in powder or wire form, heated to a molten or semi-molten state and accelerated towards substrates in the form of micrometer-size particles. Combustion or electrical arc discharge is usually used as the source of energy for thermal spraying. Resulting coatings are made by the accumulation of numerous sprayed particles. The surface may not heat up significantly, allowing the coating of flammable substances.
Thermal Spray Coating quality is usually assessed by measuring its porosity, oxide content, macro and micro-hardness, bond strength and surface roughness. Generally, the coating quality increases with increasing particle velocities.
Several variations of thermal spray are distinguished:
In classical (developed between 1910 and 1920) but still widely used processes such as flame spraying and wire arc spraying, the particle velocities are generally low (< 150 m/s), and raw materials must be molten to be deposited. Plasma spraying, developed in the 1970s, uses a high-temperature plasma jet generated by arc discharge with typical temperatures >15000 K, which makes it possible to spray refractory materials such as oxides, molybdenum, etc. HVAF and HVOF has gas velocities greater than the speed of sound.
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Definition of Thermal Spray: Thermal Spray can be defined as:
Group of processes where a material is heated (electric or combustion), atomized, and propelled unto a prepared substrate forming a coating. Here is a simple Thermal Spray Process Diagram