Technology  >  Surface Engineering  >  High Velocity Oxygen Fuel

Process Description
The HVOF (High Velocity Oxy-Fuel) process efficiently uses high kinetic energy and controlled thermal output to produce dense, low porosity coatings that exhibit high bond strengths, some of which exceed 83 MPa (12,000 PSI), low oxides and extremely fine as-sprayed finishes. The coatings have low residual internal stresses and therefore can be sprayed to a thickness not normally associated with dense, thermal sprayed coatings.

This process uses an oxygen-fuel mixture. Depending on user requirements, propylene, propane, hydrogen or natural gas may be used as the fuel in gas-fueled spray systems and kerosene as the fuel in liquid-fueled systems. The coating material, in powdered form, is fed axially through the gun, generally using nitrogen as a carrier gas. The fuel is thoroughly mixed with oxygen within the gun and the mixture is then ejected from a nozzle and ignited outside the gun. The ignited gases surround and uniformly heat the powdered spray material as it exits the gun and is propelled to the workpiece surface. As a result of the high kinetic energy transferred to the particles through the HVOF process, the coating material generally does not need to be fully melted. Instead, the powder particles are in a molten state and flatten plastically as they impact the workpiece surface. The resulting coatings have very predictable chemistries that are homogeneous and have a fine granular structure.

These coatings can survive harsh service conditions, particularly in wear and many corrosion applications, which greatly increase component service life. The smooth, as-sprayed surface, uniform chemistry and low porosity of the coating can be finished to very smooth surface profiles.

Features of the HVOF Spray Process:
    •   Produces very clean, hard and dense coatings with fine, homogeneous structures
    •   Coatings are tenaciously bonded to the substrate
    •   Low to compressive coating stress allows very thick coatings
    •   Excellent for wear and corrosion resistance
    •   Recommended for carbide coatings and often recommended for superalloy coatings
    •   Very thick coatings are possible
    •   Surface finishes are generally smooth and can be used in the as-sprayed condition
    •   Coatings can be ground and/or superfinshed
    •   Coating of complex geometries
    •   Easy masking of areas that should not be coated
    •   Process can be fully automated