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High Velocity Air Fuel (HVAF)

Overview

HVAF (High Velocity Air Fuel) thermal spray technology is similar to HVOF (High Velocity Oxy-Fuel) in that a fuel source is combusted to allow the propellant to accelerate the coating powder particles. However, HVAF diverges from HVOF in that it uses air as the oxygen source, eliminating the need for compressed bottle oxygen. The lower temperature burn of HVAF heats the sprayed powder particles less, thus controlling the thermal alteration of the coating. HVAF has the flexibility to generate HVOF-like coatings and coatings similar to those produced using cold spray. HVAF technology is well suited for tungsten carbide coatings on pump housings and axles for the oil and gas industries. HVAF is commonly used as a repair technology for the oil drilling and gas industries.

Coating Quality

The combustion temperature in air-fuel mixtures is typically 1000oC lower than in oxygen-fuel combustion. This lower temperature is ideal for the gradual heating of the feedstock particles of metals and cemented carbides to or slightly above the metals’ melting temperature. The initial oxygen content in the combustion gas mixture is 5-fold lower in the HVAF process compared to any HVOF process. Both factors prevent the oxidation of metals and the decomposition of carbides and their dissolution into the metal binder. These factors allow the retention of the original ductility of the powder feedstock material in the applied coating, even when the hardness of the cemented carbide coating exceeds 1600 HV300.

Acceleration Efficiency

The large diameter of the nozzle eliminates its length limitations, known to other guns. The spray particles can be accelerated up to the gas velocity.

Heating Efficiency

Heating of the spray particles in the combustion chamber occurs at high pressure, where heat transfer is the most efficient. The size of the chamber provides a long residence time for the spray particles to heat. Thus the energy transfer efficiency is increased noticeably compared to other high-velocity spraying methods, resulting in much higher spray rates. Heating efficiency is a primary reason for the reduced cost of the HVAF applied coatings.

Uniformity of Coating Quality

The diameter of the spray particle jet is several times smaller than the nozzle diameter. The influence of the nozzle walls on the spray particle velocity is negligible, providing even acceleration of the particles and improved consistency of the coating structure

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