HVOF vs. HVAF Coatings: Tungsten Carbide Quality Comparison
Kermetico Convertible HVAF & HVOF coating technology lets you define coating quality/cost ratio before you start the process.
|WC-10Co-4Cr Coating Features||Coating Hardness (HV300)||Coating Porosity (%)||Deposition Efficiency||Young’s Modulus|
|Good HVOF||1,050 - 1,250||< 0.8||45 - 55||300|
|HVAF Economy (E)||1,050 - 1,250||< 0.8||65+||400|
|HVAF Balanced (B)||1,250 - 1,350||< 0.5||48 - 58||450|
|HVAF Ultra (U)||1,350 - 1600+||< 0.3||36 - 42||450|
But what is the rationale behind those numbers?
Kermetico HVAF equipment uses the energy of gas combustion in air to spray powders. The combustion temperature in an HVAF gun is typically 1,000°C (1,830°F) lower than that in an HVOF coating system.
The Adiabatic Combustion Temperature of Fuels in Oxygen and Air (a=1, 20°C, 1 Bar)
This lower temperature is ideal for the gradual heating of the feedstock particles of metals and cemented carbides to or slightly above the metal’s melting temperature. The initial oxygen content in the combustion gas mixture is 5-fold lower in our HVAF process compared to any HVOF coating process. Both factors prevent the oxidation of metals and the decomposition of carbides.
Kermetico HVAF Gun Schematics
The ceramic catalytic insert is used in the combustion chamber to provide stable combustion. The material’s acceleration occurs in the nozzle that has a large diameter. The large diameter of the nozzle ensures that the particles do not interact with the nozzle walls, allowing the use of the longer nozzles for feedstock acceleration, typically from 800 to over 1,000 m/s (2,600-3,300 ft./sec.)
The quality and efficiency the Kermetico HVAF process is the result of the relatively low combustion temperature of an air-fuel mixture combined with the axial injection of the feedstock through the wide combustion chamber, where the low gas velocity provides sufficient time for the gentle heating of the powder particles.
These factors allow the retention of the original ductility of the powder feedstock in the applied coating, even when the hardness of the cemented carbide coating exceeds 1,600 HV300.
The result is higher hardness, higher ductility, higher corrosion, erosion and cavitation resistance of Kermetico HVAF coatings.
Please find below the results of a Schlumberger competitive coating evaluation conducted in 2010. The HVAF and HVOF coatings we are producing today is superior to the one evaluated in this test.
And Schlumberger is very selective in their choice of suppliers, so we were competing here with the best-of-the-breed HVOF and Detonation coatings.