fuel and oxygen are fed via a pre-mixing system and at
high pressure into a combustion chamber where they burn to
produce a hot, high pressure gas stream.
This is expanded through a laval type nozzle increasing the
gas velocity to around 1,500 m/sec and the pressure to slightly
above atmospheric. At this stage the powder can easily be
injected into the gas stream. This gas stream heats and accelerates
the powder particles within the confines of the secondary
nozzles so that they impact with tremendous energy upon the
substrate material. HVOF systems do not melt powders but only
soften them. Because less heat is imparted to the particles
and dwell times are very short, oxidation and decomposition
are minimal in HVOF coatings.