What is the Thermal Spray HVOF process?
High Velocity Oxygen Fuel (HVOF) coating is a thermal spray process in which a fuel and oxygen are mixed, fed into a combustion chamber, and ignited. The gas produced in the combustion chamber has an extremely high temperature and pressure and is ejected through a nozzle at supersonic speeds. Powder is injected into the high velocity gas stream and propelled at the substrate to be coated. The result is a coating that has a low porosity and a high bond strength to the substrate material, which provides it with wear and / or corrosion resistance.
In the HVOF spraying process, the high velocity of the particles produces kinetic energy, which is more important than the temperature. One of the benefits of the HVOF spraying process is that extremely dense and hard coatings can be produced at relatively low temperatures.
HVOF Spraying Materials
In order to achieve optimal coating and an extension of equipment life, a variety of materials can be used for the spraying process. Some commonly used materials include:
- Tungsten Carbide: HVOF Tungsten Carbide coatings are extremely dense, well bonded and can be applied to a high thickness. HVOF Tungsten Carbide materials are recommended for applications that are subject to wear.
- Tungsten Carbide-Cobalt-Chrome: HVOF Tungsten Carbide-Cobalt-Chrome coatings are more wear resistant in most applications than Tungsten Carbide coatings and are more corrosion resistant because of the Chromium content. HVOF Tungsten Carbide-Cobalt-Chrome is recommended for applications with abrasive or sliding wear and are exposed to some corrosion. HVOF Tungsten Carbide-Cobalt-Chrome coatings are an excellent alternative to hard chromium plating.
- Tungsten Carbide-Nickel Superalloy: Spraying a material with HVOF Tungsten Carbide-Nickel Superalloy produces a well bonded coating with a high level of wear resistance to abrasive grains, hard surfaces, and fretting. These coatings are ideal for hydraulic rods, ball valves, drilling, and saltwater applications.
- Chromium Carbide: HVOF Chromium Carbide coatings have high microhardness and are well bonded. These coatings are ideal for providing resistance to solid particle erosion, abrasion, and tribocorrosion at elevated temperatures.
- Stainless Steel: Spraying a material with HVOF Stainless Steel produces a dense and well bonded lower cost coating. These coatings are used for protection against corrosion, cavitation, and low temperature particle erosion in salvage and repair applications.
Benefits of the HVOF process
HVOF coatings are perfect for protecting parts that are susceptible to damage from wear and corrosion. Spraying aircraft landing gear with HVOF coatings is a prime example of how this process can be used. The landing gear hydraulic cylinder is subjected to a high amount of tensile stress from flexing and sliding wear as the hydraulic cylinder retracts to absorb shock when an aircraft lands. Spraying parts with an HVOF coating protects them and offers wear and corrosion resistance superior to chromium plating. This is just one of the many benefits of the HVOF process when compared to other thermal spray processes. Others include:
- High density
- High bond strength
- Improved hardness and toughness
- Higher coating thickness capability than other thermal spraying processes
- Increased corrosion resistance
- Smoother as-sprayed and ground surface finishes when compared to other thermal spraying processes
TAFA Model 5220 HP/HVOF Gun
The TAFA Model 5220 is a kerosene-fueled HP/HVOF gun that is used to achieve extremely high coating densities. The unique design of this gun allows it to produce particle velocities of 3,300 to 3,900 fps, which is significantly higher than any other gaseous-fueled HVOF device on the market today.
Model 5220 features:
- Efficient combustion and particle heating
- Converging/diverging nozzle at combustion chamber exit produces supersonic gas velocity
- Efficient gun cooling
- Multiple gun configurations with multiple barrel lengths
- Radial, low-pressure powder injection after combustion chamber
- Superior coating integrity and uniformity
- Thick coating capability
- Less sensitive to powder particle size than other HVOF systems
- Longer, less-sensitive thermal spray distance