What is HVAF?
High Velocity Air Fuel (HVAF) – a thermal spray process characterized by a low combustion temperature (1,960-2,010°C | 3,560-3,650°F), high particle velocities (800 to over 1,000 m/s | 2,625-3,281 ft. /sec.), resulting in low-oxidized, ductile, non-porous high-bond carbide and metal coatings. Spray rate up to 500 g/min (66 lbs. /hr.) makes the process much faster, providing a significant advantage over HVOF.
The HVAF Gun is “a small jet engine”, combusting compressed air and LPG / propane/ propylene / MAPP fuel and generating a jet of metal particles with a velocity from 800 to over 1000 m/sec. Such particles form extremely dense and tough coatings. Our “signature” coatings are tungsten carbide coatings (WC12Co, WC-10Co-4Cr, etc.) which are non-permeable to gas and have hardness 1,400-1,600+ HV300. Regardless of their high hardness, these coatings are not brittle since decarburization or oxidation does not occur in the comparably low-temperature HVAF process.
HVAF Process Characteristics
- Lower propane flame temperature with air (HVAF) compared to pure oxygen (HVOF) – Oxygen: ~2820 ºC – Air: ~1970 ºC
- Higher particle velocity.
- Less oxidation • Lower porosity.
- High bond strength.
- Hard, wear resistant coatings.
The novel HVAF process enables deposition of dense, hard and wear resistant hard metal coatings with excellent technical properties.
The main benefits in HVAF are
- Lower flame temperature vs. HVOF.
- Higher retention of carbides producing excellent coating properties.
- High powder feed rates for high production rates.
Here in Mumbai, India we thermal spray coatings that exhibit high density and hardness, combined with excellent ductility. This high quality comes with the additional benefit of lower coating cost due to the efficient use of the systems’ combustion energy which results in high spray rates and deposits per pass four to eight times higher than our industry is limited to with conventional thermal spray equipment.
Oxidant and cooling gas: Compressed air, Optional oxygen injection in combustion air.
Fuel: propane, propylene, propane-butane, natural gas.
The quality and efficiency of the Kermetico family of coatings is the result of the relatively low combustion temperature of an air-fuel mixture combined with the axial injection of the feedstock through the long combustion chamber, where the low gas velocity provides sufficient time for the powder particles gentle heating. The materials’ acceleration occurs in the nozzle that has a large diameter. The large diameter of the nozzle is needed to pass through the volume of nitrogen present in the combustion air. It also guarantees that the nozzle walls do not interact with the particles, allowing the use of a nozzle that is as long as needed for feedstock acceleration, typically from 600 to over 1,000 m/s. The Ceramic Catalytic Insert is used in the combustion chamber to provide a wide range of stable combustion parameters.
Key Characteristics of HVAF Coatings
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. 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.
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 economic cost of the HVAF applied coatings.
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.
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.
Typical Applications of HVAF Coatings
Energy Paper Plastics Power Steel