high velocity oxy/fuel spraying (HVOF), a high
velocity flame spray process (1)
In the early 1980's Browning and Witfield, using rocket engine
technologies, introduced a unique method of spraying metal powders.
The technique was referred to as High Velocity Oxy-Fuel (HVOF).
The process utilizes a combination of oxygen with various fuel
gases including hydrogen, propane, propylene, hydrogen and even
kerosene. In the combustion chamber, burning by-products are expanded
and expelled outward through an orifice where at very high velocities.
Often times they produce "shock diamonds" exiting the
spray gun as in the graphic below.
Powders to be sprayed via HVOF 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 Mach
1 have been reported with temperatures approaching 2,300°C
(4,172°F). The coupling of inertially driven/highly plasticized
particles can achieve coatings approaching that of theoretical
density. 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.
