There are five main types of thermal spraying process:

• Flame spraying
• Arc spraying
• Plasma spraying
• High Velocity Oxy Fuel (HVOF) spraying
• Detonation Gun spraying

The processes are explained briefly below:

Flame Spraying - uses the heat generated from combustion of a fuel gas (acetylene, propylene, propane, hydrogen) and oxygen mixture to heat the consumable, either a wire or powder. Compressed air/inert gas or aventuri effect, is used to propel the consumable towards the substrate. These coatings can exhibit low bond strength, high oxide content and high porosity. The process has a moderate spray rate and low cost.

Arc spraying - is the process with highest productivity. A DC electric arc is struck between two consumable wire electrodes which melt to form the spray material. Compressed gas is used to atomise the wires and propel the droplets towards the substrate. Higher bond strengths, and spray rates, plus lower porosity are achieved compared to flame spraying. However, it produces arc light, ozone and fume (like welding) which may cause difficulties in some situations.

Plasma spraying - uses a DC electric arc to form a high temperature plasma gas. This heats the powder consumable as it is fed into and around the plasma jet. Inert gas is fed continuously into the torch and expands rapidly when heated and ionised. This expansion gives rise to a very high velocity jet and particle speeds of typically 200 to 300m/s. Very good coatings of high melting point materials can be produced. VPS (Vacuum Plasma Spraying) or LPPS (Low Pressure Plasma Spraying) use a partial vacuum to give even higher quality coatings, but at much higher cost.

HVOF spraying - uses the heat and pressure generated from the combustion of a liquid or gas fuel mixed with oxygen. The mixture is combusted in a chamber where it heats and expands, forcing the exhaust gases out of the gun barrel at supersonic speeds. High particle velocities give very high bond strength, low porosity coatings, but at a higher cost than air plasma spraying.

Detonation Gun spraying - detonates a mixture of acetylene and oxygen to produce a shock wave that propels the powder consumable to the substrate at very high velocities. These coatings can be of very high quality, often providing the benchmark for other coating processes.

Source: TWI