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What is a High Velocity Oxygen Fuel (HVOF) Coating?

   

High Velocity Oxygen Fuel (HVOF) coating is a thermal spray coating process, which is used to improve or restore a component's surface (properties or geometry). This surface engineering technique better enables life extension of equipment by increasing erosion and wear resistance, and corrosion protection.

HVOF spraying was developed in the 1980s and is a subset of thermal spraying. HVOF spraying works by mixing liquid fuel and oxygen, which is fed into a combustion chamber and ignited. The resultant gas has an extremely high temperature and pressure, which is ejected through a nozzle at supersonic speeds. To the high velocity gas stream, powder is injected, which partially melts. The stream of hot gas and powder is directed towards the surface to be coated. The resulting dense coating has low porosity and high bond strength [1], providing many benefits such as corrosion resistance.

What are the Advantages of Using a HVOF Coating?

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

What are the Disadvantages of Using a HVOF Coating?

The disadvantages associated with HVOF spraying include:

  • HVOF sprayed coatings can be extremely complex, with their properties and microstructure depending upon numerous processing variables
  • Powder sizes are restricted to a range of about 5 - 60µm, with a need for narrow size distributions
  • HVOF spraying requires experienced, qualified personnel to ensure safe operation and to achieve consistent coating quality
  • As with all the thermal spraying processes, particular health and safety issues should be addressed. HVOF spraying usually needs to be undertaken in a specialised thermal spray booth, with suitable sound attenuation and dust extraction facilities
  • HVOF equipment requires more investment than other thermal spraying processes, for example flame and arc spraying
  • Manual operation of an HVOF spray gun is not recommended and automated manipulation of the gun is usually needed
  • Deposition of coatings is difficult or impossible to achieve on to internal surfaces of small cylindrical components, or other restricted access surfaces, because HVOF spraying needs line of sight to the surface and a spray distance of 150-300 mm

Applications and Materials

High velocity oxy-fuel (HVOF) spraying is primarily used to apply high quality coatings of cermets such as WC/Co and WC/Co/Cr. Coating thickness is usually in the range 0.1-2 mm. Characteristic features of HVOF sprayed coatings include their low level of porosity at about 1-2vol% and low oxide content in the range 1-2wt%. The bond strength often exceeds 80MPa. These coatings can be used across a spectrum of engineering and manufacturing applications to enhance the surface properties of components. This allows preparation of cost effective, high performance parts made from low cost or lightweight materials with a functional surface.

Commonly deposited coating materials include:

  • Cermets (e.g. WC/Co, WC/Co/Cr, Cr 32/NiCr, NiCrSiBC)
  • Ceramics (e.g. Cr 23, Al 23, ZrO 2)
  • Metal alloys (e.g. steels, nickel, chromium and cobalt alloys including NiCrSiB and MCrAlYs)
  • Pure metals (e.g. Ni, Cu, Al, Mo, Ti)
  • Polymers (e.g. polyester, nylon)
  • Composites (e.g. Ni-graphite)

HVOF spraying can produce coatings for:

  • Wear protection
  • Low friction surfaces
  • Corrosion protection
  • High temperature oxidation resistance
  • Electrical insulation
  • Repair and restoration of damaged components

How Can TWI Help?

TWI has a long history of working with its Members, across a range of industry sectors, on High Velocity Oxygen Fuel Coatings. Contact us to learn more.

References

  1. Kuroda S, Kawakita J, Watanabe M, Katanoda H. Warm spraying-a novel coating process based on high-velocity impact of solid particles. Sci Technol Adv Mater. 2008;9(3):033002. Published 2008 Sep 10. doi:10.1088/1468-6996/9/3/033002

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