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HVOF/AF

High Velocity Oxy Fuel spraying (HVOF) is basically a high velocity flame spray process and in principle a small rocket engine.

In the early 1980's Browning and Witfield, using rocket engine technologies, introduced a unique method of spraying metal powders - HVOF. The process utilizes a combination of oxygen with various fuel gases (some systems using gas, some liquid) including hydrogen, propane, propylene, hydrogen, LPG, natural gas and even kerosene. In the combustion chamber, burning by-products (typical combustion between 5000ºF and 6000ºF) are expanded and expelled outward through an orifice where at very high velocities. Typically 9 vivid "shock diamonds" can be seen originating at the nozzle exit when the powder feeder is off.

HVOF powders are injected axially into the expanding hot gases where they are propelled forward, heated and accelerated onto a surface to form a coating. Gas velocities exceeding 7000 fps (4772 mph) have been reported with temperatures approaching 2,300°C (4,172°F) depending on the fuel gas used. The coupling of inertially driven/highly plasticized particles can achieve coatings approaching that of theoretical density.

 

HVOF

Disadvantages include low deposition rates and in-flight the oxidation of particles. Future efforts will focus on applying thick coatings and improvements in processes control including in-flight transit time and exposure to atmospheric oxygen.

Variation of this process include Wire HVOF and High Velocity Air Fuel (HVAF), where Air is used in place of Oxygen.