HVOF Spray Process (High Velocity Oxygen Fuel)
The HVOF thermal spray process uses the combustion of gases, such as propane, propylene, hydrogen, or a liquid fuel such as kerosene. Fuel and oxygen mix and atomize within the combustion arc under conditions that monitor the correct made and pressure.
The process creates a very high velocity which is used to propel the particles at near supersonic speeds before impact into the substrate. One of the basic rules of spraying is that high combustion pressure = high gas velocity, high particle velocity and resulting high coating quality.
One of the key benefits of this system’s high velocity is the extremely high coating density and low oxide content. The low oxides are due partly to the speed of the particles spending less time within the heat source and partly due to the lower flame temperature (around 3000˚ cel) of the heat source compared with the alternative processes.
The very high kinetic energy of the particles striking the substrate do not require the particles to be fully molten to form high quality HVOF coating. This is certainly another advantage for the carbide cermet type coatings and is where this process excels.
Some HVOF coatings can be sprayed very thick due to the exceptionally high velocities producing coatings in compression instead of tension.
HVOF coatings are used in applications requiring the highest density and strength not found in most other thermal spray processes
Benefits
- 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; and
- Improved life of mating components
Application & Materials
Thermal spray coating is an attractive technique as it offers a wide choice of materials and processes that have a reduced impact on the environment when compared to conventional plating processes.
HVOF coating materials available for thermal spray coating include metals, alloys, ceramics, plastics, and composites.
Advantages of HVOF spraying over other thermal spray processes primarily relate to improved coating quality, such as:
- Higher density (lower porosity) due to greater particle impact velocities
- Higher strength bond to the underlying substrate and improved cohesive strength within the coating
- Lower oxide content due to less in-flight exposure time
- Retention of powder chemistry due to reduced time at temperature
- Smoother as-sprayed surface due to higher impact velocities and smaller powder sizes
- Better wear resistance due to harder, tougher coatings
- Higher hardness due to less degradation of carbide phases
- Improved corrosion protection due to less through thickness porosity
- Thicker coating due to less residual stresses