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.
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.
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|