High Velocity Air Fuel (HVAF) Thermal Spray System

USTS has chosen the High Velocity Air Fuel (HVAF) thermal spray system from Uniquecoat Technologies LLC because this technology produces the highest quality cemented carbide coatings.

Wear resistant coatings containing a high percentage tungsten carbide (WC 80-90%) cannot be applied by traditional welding procedures. There is insufficient matrix (Cobalt or Nickel) to wet-in during welding to form a metallurgical bond to the substrate. Such a high carbide content material, in the form of a fine powder (2-5 micron), requires a high velocity spray system for application. Essentially, the WC particles impact the substrate at supersonic speeds which creates a strong, dense mechanical bond.

The main benefits in HVAF include:
  • Lower flame temperature compared with HVOF
  • Higher retention of carbides producing excellent coating properties
  • High powder feed rates for high production rates

Historically, these coatings have been applied using a High Velocity Oxygen Fuel (HVOF) thermal spray system.

A mixture of pure oxygen and fuel (kerosene) ignites in the combustion chamber creating a high velocity jet. Powder is accelerated by this exhaust jet and impacts the substrate. A downside of using pure oxygen as accelerant is the high temperature (2900° C) and the potential to decarburize the powder. WC is converted at high temp + 02 to W2C which is hard, yet brittle and tends to fracture. Another concern is the elevated combustion temperature can vaporize the matrix metals reducing their ability to bond powder particles.

The HVAF process uses compressed air (+/-21% Oxygen) and propane.

This combustion temperature is 1000° C lower than HVOF while velocity is 200-400 m/sec higher. Lower temperature, less oxygen and higher velocity results in a tougher, less brittle and more wear resistant coating.

Spray Method Combustion Temp °C Particle Vel (m/sec) Adhesion (PSI) Oxide Content (%) Porosity Spray Rate (kg/h) Typical Thick (mm) G65 Vol Loss mm3
HVOF 2950 500-800 >10,000 1-5 1-2 2-8 0.1-2 3.5-4.5
HVAF 1900 600-1200 >10,000 =feedstock 0-0.2 2-30 0.1-12 2.2-2.7