Thermal spraying generally refers to the use of heat to melt a substance, then spread the droplets across a surface to strengthen and protect it. Within this classification, however, there are several different processes, each with their own distinct features.
Flame spraying
Flame spraying involves oxygen being applied to fuel, such as propane, propylene, hydrogen or acetylene. The heat from the combustion turns powder or wire into liquid and uses compressed air or the venturi effect to direct the spray towards the substrate. It is a low cost method with moderate spray speed, low bond strength, high porosity and high oxide content.
Plasma spraying
Plasma spraying involves electricity, but it is used to form plasma gas. It is the gas that fuels the torch, which sprays at very high speeds and works well, even for materials with high melting points. It can, however, be costly. You can learn more about thermal plasma spray at www.poeton.co.uk/surface-treatments/thermal-metal-sprays/plasma-coatings/.
Arc spraying
Two electrodes with an arc of electricity between them are used to melt the wire, then compressed air is the method of propulsion towards the substrate. High spray speed and bond strength are advantages, but the arc light and resulting fumes can be hazardous, and may require additional safety measures.
High velocity oxy fuel, or HVOF, spraying
Liquid or gas is combined with oxygen in a contained chamber. The pressure and heat of the resulting combustion cause the mixture to expand until it is forced through the nozzle into an extremely high velocity spray that can reach supersonic speeds. This leads to particularly strong bonds and low porosity.
Detonation gun spraying
Another extremely high velocity option that achieves supersonic speeds when producing high quality coatings, the detonation gun explodes a combination of oxygen and acetylene so the consumable power is carried out on the resulting shockwave.
Each thermal spraying process has its own characteristics, including differing heat sources and speed of particle spray as well as probable cost. This will impact the quality of your final product, so it is important to understand the distinctions.