HVOF technology generally used for WC coating application sometimes generates material oxidation and decomposition of WC feedstock materials during in-process HVOF deposition.
It is directly related to the high combustion temperature which is more than 3000ºC as well as the presence of oxygen in HVOF combustion products. This usually results in coating embrittlement and further reduction of service life in the condition of abrasive and erosive wear.
For tungsten carbide-based powders, particle velocity exceeds 700-900 m/s, while particle surface temperature remains about 100˚c below the melting point of the metallic binder, allowing HVAF equipment to operate in a “solid-particle” spray mode. The latter is possible due to the relatively low combustion temperature of air-gaseous fuel mixtures at 3.5 – 4.5 bar chamber pressure.
The result is a formation of coatings with extremely low oxygen content as well as little, if any, thermal deterioration of carbides. Improved toughness, cohesion and low residual stresses in the HVAF sprayed layers permit routine deposition of thick carbide coatings, the superfinishing of coatings to an optical mirror, as well as result in noticeable improvement of fatigue resistance of the coatings.
HVAF combustion temperature below 2000°C, the surface temperature of cobalt and chromium alloy particles usually remains near their melting point.
A characteristic feature of the HVAF process is that particles are heated gently and not fused during spraying.
Uniformly high hardness and ductility, low stress and porosity result in a substantial increase in the service life of parts protected with HVAF WC-Co coatings in comparison with HVOF tungsten carbide versions.
|WCCo 88/12 Powder Properties||Wt.%|
|Total Carbon||5.3 – 5.4|
Typical Applications of WCCo coatings in India
|Apparent metallographic porosity||<0.1|
|Bond strength of tungsten carbide coating to carbon||PSI 12,000+|
|hardness range, HV300||1150-1600|
|hardness deviation from a target value, HV300||±60|
|typical as-sprayed roughness Ra, microns||3|
|maximum coating thickness, mm||3|
|the maximum working temperature of the coating, °C||510|