High-Velocity Thermal Spraying: HVOF and HVAF
HVOF (High-Velocity Oxy-Fuel) and HVAF (High-Velocity Air-Fuel) are advanced high-velocity thermal spray technologies. They use the kinetic energy of a supersonic jet to produce ultra-dense coatings with exceptionally high bond strength.
How HVOF and HVAF Work
- Pressurised combustion: High-pressure gases are continuously supplied to the spray gun’s enclosed combustion chamber.
- HVOF: Uses a gaseous or liquid fuel—propane, propylene, hydrogen, or kerosene—and pure oxygen.
- HVAF: Uses fuel gas and compressed air.
- Supersonic acceleration: The combustion gases expand through a Laval nozzle, reaching supersonic velocities of 1,000–2,000 m/s.
- Powder injection: Fine powder is fed into the nozzle, where the particles are heated to a plastic state, often without melting completely.
- Impact and deformation: The particles strike the substrate at extremely high velocity. Their immense kinetic energy produces a strong bond and an exceptionally dense coating with virtually no porosity.
Key Differences Between HVOF and HVAF
- Flame temperature: HVOF operates at higher temperatures—up to 3,000°C—due to the use of oxygen. HVAF temperatures are lower, reaching approximately 2,000°C, because air contains around 78% nitrogen, which cools the flame.
- Particle velocity: HVAF produces higher particle velocities, compensating for the lower temperature through greater impact energy.
- Powder oxidation: Material oxidation is greatly reduced in HVAF due to its lower operating temperature and the presence of nitrogen in the process air. HVAF coatings are therefore cleaner and harder.
- Operating costs: HVAF is considerably less expensive to operate because it does not require costly industrial-grade oxygen.
Advantages
- Exceptional bond strength: Adhesion strength exceeds 70–80 MPa and is often greater than the limit of the adhesive used in pull-off testing.
- Minimal porosity: Coating density can reach 99.5% or more.
- Low thermal stress: The coatings develop compressive residual stresses, allowing thick layers to be applied with a low risk of delamination.
- Retention of phase composition: Carbides, such as WC-Co, resist degradation and decomposition during spraying.
Disadvantages and Limitations
- High equipment costs: The process requires sophisticated gas-control systems, powerful robotic manipulators, and sound-insulated booths.
- Noise: Sound pressure levels from the supersonic exhaust can reach 130–140 dB.
- Geometric limitations: Effective deposition generally requires a spray angle of 45–90° to the surface. Treating the internal diameters of pipes is extremely difficult.
Applications
- Oil and gas: Protecting ball valves, gate valves, and drilling tools against hydro-abrasive wear.
- Hydropower: Coating hydraulic turbine blades to protect them against cavitation.
- Metallurgy: Coating rolling-mill rolls to protect them against severe abrasive wear.
- Aerospace: Replacing environmentally hazardous hard-chrome electroplating on landing gear and engine shafts.