Influence of Fine Powder Feedstock (-10+2 µm) on the HVOF Spraying Behavior, Coating Morphology, and Properties of WC-CoCr 86-10-4

The use of fine powder feedstock can extend the feasibility and scope of HVOF coatings to new fields of applications. Especially in the purpose of near-net-shape coatings they facilitate short spraying distances, homogeneous layer morphologies, and smooth coating surfaces. However, the small particle sizes also lead to several challenges. One major issue is the in-flight behavior which is distinctly affected by the low mass and the relatively large surface of the particles.

In this paper the in-flight and coating characteristics were investigated employing WC‑CoCr 86-10-4 (‑10+2 µm). It was determined that the fine powder feedstock shows a high sensitivity to the gas flow, velocity, and temperature of the spray jet.  Because of their low mass inertia, their velocity, for example, is actually influenced by local pressure nodes (shock diamonds) in the supersonic flow. Additionally, the relatively large surface of the particles facilitates partial overheating and degradation.

Despite this sensitivity to the spray jet the morphological and mechanical properties of the sprayed layer are less affected. In fact, the coatings feature superior surface roughness, porosity, hardness and wear resistance.