High-velocity oxygen fuel (HVOF) is a process to apply very dense, strongly adhered coatings. The way the HVOF process works is by liquid fuel and oxygen being fed into a combustion chamber where they are ignited, producing a hot high-pressure gas stream. This is then accelerated through a de-laval (converging/diverging) nozzle increasing the gas velocity to around 1500m/sec and the pressure to slightly below atmospheric. At this stage, the powder can easily be injected into the gas stream.
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.
Many Engineers will be familiar with the methods available to provide a barrier against corrosion and wear but how many are familiar with the HVOF process? The Metal Spray Industry has been established since the early 1900’s with process development leading us through Flame Spray, Arc Spray, Plasma Spray and now to HVOF.
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.
|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, Oil etc.
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.