What is High-Velocity Oxyfuel Spray ?
( HVOF )

High-Velocity Oxyfuel Spray
( HVOF )

The principle for HVOF is that high-volume combustible gases are fed into a combustion chamber. The combustion takes place internally at very high chamber pressures. It is then fed into a long confining nozzle or barrel through which the combusting gases exit the device generating a supersonic gas jet with very high particle speeds. The sheer volume of gas flow, coupled with the high temperature of combustion, creates gas velocities in the 1525 to 1825 m/s (5000 to 6000 ft/s) range at the nozzle exit. The combustion jet temperatures ranges between 2500 to 3100 °C (4500 to 5600 °F). High-velocity oxyfuel gases used include hydrogen, propylene, propane, acetylene, and kerosene etc.

The process results in extremely dense, well bonded coatings, making it attractive for many applications. The high gas velocity generated by HVOF, much higher than in conventional flame or plasma spray, has been shown to increase particle velocity, with a corresponding increase in coating density and coating adhesion.

Generally, HVOF processes are used to deposit dense, hard cermet coatings of WC/Co or Cr2C3/NiCr. Many uses have been found, however, for HVOF sprayed metallic coatings. MCrAlY coatings are being applied to aircraft turbine blades by oxykerosene HVOF spray.
Components of HVOF System
Coating Characteristics
High-velocity oxyfuel coating properties are reported to be comparable to those of detonation gun coatings, particularly for carbide and oxide coatings. High Velocity Oxyfuel sprayed coating density, adhesion, and oxide contents also compare favourably with high-energy, plasma sprayed coatings. Porosity is typically reported at less than 1%. Compositional analysis of HVOF WC/Co coatings have shown that, compared to plasma spray, only a small amount of WC decomposition (to W2C) occurs, preserving the intrinsic high hardness values of the material.
Typical coatings