High-velocity, oxy-fuel, (HVOF) devices are a subset of flame spray. There are two distinct differences between conventional flame spray and HVOF. HVOF utilizes confined combustion and an extended nozzle to heat and accelerate the powdered coating material. Typical HVOF devices operate at hypersonic gas velocities, i.e. greater than MACH 5. The extreme velocities provide kinetic energy which help produce coatings that are very dense and very well adhered in the as-sprayed condition.
In HVOF spraying process, the pressure in the combustion chamber of the spraying gun is increased to generate high-speed flame comparable to detonative combustion flame. The powder materials are feed and melted, or half-melted in the flame jet stream, then accelerated and continuously sprayed at supersonic speed.