High-Velocity Oxyfuel. (HVOF) coatings are presently used in many industrial applications because they develop very hard, wear resistant surfaces that have comparable performance requirements to those of chromium plating. HVOF spray is based on the detonation gun (D-gun) process developed. In HVOF coating application, an explosive gas mixture ignites in the barrel of the spray gun, which melts a powdered coating material and propels it (with a carrier gas) at supersonic speeds toward the substrate. The superior coating properties are a result of the high velocities that are reached in the process the higher the velocity, the greater the force of impact at the substrate, resulting in fewer voids in the coating and more desirable properties.
The High Velocity Oxy-Fuel (HVOF) process is one of the most popular thermal spray technologies and has been used in many industries due to its flexibility and the superior quality of coatings produced compared to other thermal spray techniques. It produces a coating of higher bond strength and higher hardness together with lower porosity than other thermal spray processes such as the plasma spray.
HVOF usually propane, propylene, MAPP, or hydrogen, is mixed with oxygen and burned in a chamber. In other cases, liquid kerosene may be used as a fuel and air as the oxidizer. The products of the combustion are allowed to expand through a nozzle where the gas velocities may become supersonic. Powder is introduced, usually axially, in the nozzle and is heated and accelerated. The powder is usually fully or partially melted and achieves velocities of up to about 550 m/s. Because the powder is exposed to the products of combustion, they may be melted in either an oxidizing or reducing environment, and significant oxidation of metallic and carbides is possible.
With appropriate equipment, operating parameters and choice of powder, coatings with high density and with bond strengths frequently exceeding 70 MPa (10,000 psi) can be achieved. Coating thicknesses are usually in the range of 0.05 to 0.50 mm, but substantially thicker coatings can occasionally be used when necessary with some materials.
HVOF processes can produce coatings of virtually any metallic or cermet material and, for some HVOF processes, most ceramics. Those few HVOF systems that use acetylene as a fuel are necessary to apply the highest-melting-point ceramics such as zirconia or some carbides. HVOF coatings have primarily been used for wear resistance to date, but their field of applications is expanding.
Generally, the higher-particle-velocity coating processes produce the densest and better bonded coatings, both cohesively (splat-to-splat) and adhesively (coating-to-substrate). Metallographically estimated porosities for detonation gun coatings and some HVOF coatings are less than 2%, whereas most plasma sprayed coating porosities are in the range of 5 to 15%. The porosities of flame sprayed coatings may exceed 15%.
Benefits of the HVOF coating:
- Reduced costs;
- Improved performance
- Improved electrical properties
- Enabling components to operate in higher/lower temperatures
- Enabling components to operate within harsh chemical environments
- Improved efficiency
- Improved life of mating components
The Thermal Spray Process
Particle Heating and Acceleration = Supply of Thermal and Kinetic Energy
Arrangement of a modern HP/HVOF Gun
Combustion Temperatur: 2800- 3150 °C
Gas Exit Velocity up to 2000 m/s
Particle Velocity up to 700 m/s
Low Porosity: 0.5 - 2 %
Large Thickness possible: above 1 mm
Low Average Coating Thickness: 0.08-0.1 mm
High Bond Strenght: above 70 MPa
Little Thermal Transformation
Internal Compressive Stresses
Little Preparation Expenditure
Reference: ASM Metals HandBook