What Does High-Velocity Oxy-Fuel Spraying (HVOF) Mean?
High velocity oxy-fuel (HVOF) spraying is a thermal coating process involving the delivery of coating powder axially, into a pre-ignited mixture of oxygen and fuel, ejected through a nozzle. The material is then propelled at supersonic velocity toward the surface to be coated.
The study of the HVOF spraying process is useful for:
- Developing new cost-effective coating materials
- Providing low porosity coating with better corrosion resistance
- Indirectly controlling the damage and the risk of catastrophic failure due to corrosion
Corrosionpedia Explains High-Velocity Oxy-Fuel Spraying (HVOF)
High-velocity oxy-fuel spraying is a thermal technique of spraying surfaces of different shapes and sizes with effective anti-corrosion coating of low porosity in variable thickness. In every thermal spray process, pre-heated, melted materials are coated on the surface. The thermal spray technique can provide coatings of 20 microns to 5 mm thickness, at a faster deposition rate than other processes such as vapor deposition.
Coating materials can include:
These coating materials are fed into the spray gun in various forms such as powder or wire, in micron-size particles. Combustion of gas or liquid fuel is used as energy for HVOF spraying.
Coating quality can improve with high particle velocities. In classic (developed between 1910 and 1920), but still widely used, processes such as flame spraying and wire arc spraying, the particle velocities are generally low (< 150 m/s).
Coating effectiveness and quality are evaluated by monitoring:
- Surface porosity
- Oxide percentage
- Coat hardness
- Bonding strength
- Surface quality
The HVOF technique is used for a strong, dense coating that can enhance or salvage component lifespan. It is similar to the combustion powder spray technique. HVOF was developed to achieve very high particle velocity for achieving dense coating.
Different HVOF guns use different methods to achieve very high velocity. Typically there is a water-cooled combustion chamber with a large nozzle. Liquids such as kerosene or gases like acetylene or hydrogen can be used as fuels. The mixture of fuel and oxygen is forced into the chamber, and ignited to produce a high-pressure flame. As the flame enters the nozzle, the gas velocity increases tremendously. Coating powder is forced axially into the combustion chamber with extra-high pressure, or sometimes through the nozzle side, or in the center of the nozzle in different designs.
The coatings sprayed by HVOF process are more dense, stronger and have low residual stresses, thus enabling spray of thicker coatings. The higher kinetic energy of the powder particles hitting the substrate do not need the powder particles to be completely molten to provide a high quality HVOF spray coating. This is an additional advantage for special carbide cermet corrosion resistant coatings, this is where HVOF spray excels.