HVOFHVOF, high velocity oxygen-fuel gas spraying, is used to transfer particles of material powder to the surface of the work at high velocity. The flame reaches supersonic speeds and the energy in the powder particles produces high bond strength. It is widely used in aircraft engine manufacture and repair and the electronic industry. Flows of oxygen and a fuel gas, hydrogen, acetylene or propylene are accurately monitored in the console, which also controls the powder feed rate and the handling equipment. Some HVOF guns use kerosene, a petroleum gas as the fuel. A pressurised hopper supplies dry process powder, metal, cermit or ceramic, with uniform particles to the HVOF gun. Oxygen is ignited in a chamber with the fuel to provide a high-energy combustion, which picks up the process powder and propels it onto the work. The velocity of the coating particles gives them the energy to bond with the substrate without excessive heating. Temperature in the parent material can be further controlled using cooling with air or carbon dioxide.
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