Written on October 21, 2010 at 12:19 am by Power Tool Team

Reach Carbide Rotary

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Reach Carbide Rotary
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Looking for a high-performance ceramic coating?

A High-velocity Oxyfuel (HVOF) thermal spray process may be what you need.

HVOF spraying has been developed principally to produce high-quality ceramic coatings with superior properties to conventional flame spray processes. It is a dry process that can also produce a dense metallic coating whose desired physical properties are equal to or surpass those of hard chrome plating (HCP).

How does it work?

The HVOF process typically uses oxygen and hydrogen (although other fuels can also be used) as the fuel gases to produce coatings of the highest quality.

Mixing of the gases occurs inside the gun and the externally-ignited flame reaches a temperature of about 2700ºC.

The combustion process melts a metal-containing powder that is continually fed into the gun using a carrier gas (argon) and propels it at velocities approaching 550 metres/second towards the surface of the part to be coated.

Even so, this result in a lower average particle temperature compared to plasma spray.

Despite this lower average particle temperature, high coating densities are still achieved through the impact velocity which deforms particles that may not have been well melted. Some particle heating can occur on impact which further aids in producing a dense coating.

Because the high kinetic energy of particles striking the substrate surface does not require the particles to be fully molten in order to form a high quality ceramic coating which means it can be an advantage for the carbide cermet type coatings. This is where the process really excels.

HVOF coatings are very dense, strong and show low residual tensile stress or in some cases compressive stress, which enable very much thicker coatings to be applied than previously possible with other processes.

HVOF coatings are used in applications requiring the highest density and strength not found in most other thermal spray processes.

Metal powders are available in many compositions including WC/Co, WC/Co/Cr and Ni/Cr/Fe

Precise control of all spraying parameters, powder feed rates and the use of a robot to precisely position the gun, enables high quality metallic and ceramic coatings to be repeated on a large variety of different components.

The properties of HVOF coatings depend on a number of parameters. These include the part surface preparation, composition, morphology, size distribution and feed rated of the powder. Also to be taken into consideration is the precise control of gas flows, relative torch-to-part motion, stand-off angle of deposition and part temperature.

What are the benefits of this type of coating?

High bond strength (83 MPa - 12000 p.s.i.)

High density, low porosity

Low oxide content

High hardness (DPH 500 1000 - 1300 for WC/Co)

Fine as-sprayed texture, able to be ground or lapped to a fine finish (0.1-0.2 microns)

Excellent resistance to wear by abrasion, erosion or fretting

The only waste stream produced by HVOF is from the capture of the overspray. Current users either use a water-curtain filter system or a dry, high-efficiency particulate air (HEPA) filter.

Although a line-of-sight process, it is possible to coat bores; the extent down the bore being generally equal to the diameter of the bore.

Added part cooling may be required where small or thin plate geometry is encountered