Flame Spraying (FS) of Thin Coatings
Flame Spraying (FS) is a conventional thermal spray process that uses the heat generated by the combustion of a gas mixture to melt and apply protective coatings.
How the Process Works
- Gas supply: Fuel gas—acetylene, propane, MAPP gas, or hydrogen—and oxygen are supplied to the spray gun.
- Ignition: The mixture is ignited at the nozzle outlet, producing a focused flame with temperatures of up to 3,000°C.
- Feedstock supply: The coating material is continuously fed into the flame in powder, wire, or rod form.
- Melting and atomization: The material melts and is atomized into droplets by a stream of compressed air or process gas.
- Particle transfer: The gas stream accelerates the droplets and carries them toward the prepared surface of the component.
- Solidification: Upon impact, the particles deform and solidify, forming the coating’s characteristic lamellar structure.
Advantages of Flame Spraying
- Cost-effectiveness: Low equipment and consumable costs.
- Mobility: Compact equipment that can be easily used on-site—for example, on bridges and storage tanks.
- Ease of operation: Basic applications do not require complex automation or highly skilled operators.
- High productivity: Large surface areas can be treated quickly.
- Flexibility: Rapid changeover between wires of different diameters and powder feedstocks.
Disadvantages and Limitations
- Low particle velocity: The relatively low droplet velocity results in lower coating density.
- Moderate adhesion: Bond strength is lower than with APS, HVOF, or detonation spraying.
- High porosity: Typical coating porosity ranges from 5% to 15%.
- Temperature limitations: The process is unsuitable for pure high-melting-point ceramics, such as zirconium oxide.
- Risk of overheating: An incorrect spray distance may cause excessive heating of the component.
Applications
- Corrosion protection: Spraying zinc and aluminium onto steel structures to create protective metallic coatings.
- Shaft repair: Restoring shaft journals and bearing seats using steel wire feedstock.
- Decorative coatings: Applying brass, bronze, or copper to interior design elements.
- Basic wear protection: Applying Babbitt alloy coatings to plain bearings.