Heat Source
Fuel (Liquid or Gas) and Oxygen Flame
Material
Powder (Metal)
Transfer
Via the Flame
Process
The Hypersonic Spray Process / High Velocity Oxygen Fuel (HVOF) is a process to apply very dense, strongly adhered coatings. A fuel (commonly kerosene or hydrogen) is mixed with oxygen and ignited in combustion chamber. The combustion gases are accelerated through a nozzle. Powder is introduced into the gas stream where it softens and gathers speed before coating onto the sprayed surface. These coatings are commonly used as a hard chrome replacement process and produce very dense, hard wearing coatings. Metallisation offer two variants of HVOF Pistol Control Consoles to operate either liquid fuel or gas fuel pistols.
The gas stream heats and accelerates the powder particles to around twice the speed of sound, simultaneously softening them. They impact onto the sprayed surface with tremendous energy to form a very dense, strongly adhered coating.
There is always a demand for more wear and corrosion resistant surfaces and for these reasons High Velocity Oxygen Fuel (HVOF) spray systems are becoming increasingly popular. Unlike the other methods of metal spraying, where the feed stock is melted and projected onto the substrate, the HVOF process simply softens the powder before projecting it. This is because less heat is imparted to the particles and the dwell times are very short, oxidation and decomposition are minimal in a HVOF coating. The main difference which provides the superior quality, is offered by the use of a combustion chamber and accelerating nozzles that produce very fast (approx 1500m/sec) particle velocity that results in high impact energy and hence reduced porosity levels as when compared with other metal spray processes.
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| HVOF Flame with shock diamonds producing the necessary speed for high impact deposits. |
The use of low level porosity coatings combined with hard wearing, corrosion resistant, conductive and high bond strengths allow the process to be used in many applications for a variety of industries including Paper, Pump, Aerospace, Mining and Oil.
The coating density for most metallic coatings will be more than 99.5% of the theoretical density, micro-hardness in excess of 1300 HV300 are commonplace and the bond strengths are beyond the normal values measurable by the ASTM 633 test.
