| Tungsten carbide (WC)-based powders are widely used in high-velocity oxygenfuel (HVOF) spraying to produce dense coatings with high hardness and excellentwear resistance. However, in order to achieve the best performance of the coating, theoptimization of the spraying process parameters is essential, which is especiallyimportant for the selection of the ratio between fuel and oxygen. In this work, fiveWC-12Co coatings were thermally sprayed by a Jet-Kote ΙΙΙ high velocity oxy-fuel(HVOF) system with a constant oxygen flow and various propylene flows (96L/min,106L/min,116L/min,126L/min and136L/min). The phase compositions,microstructure, abrasive and sliding wear performance and resistance toelectrochemical corrosion of the as-sprayed coatings were fully examined. The resultsindicated that：The degree of decarburization of WC gradually increased while the porositydecreased with the increasing propylene flow. The hardness and the fracture of theHVOF WC-12Co coatings firstly increased and then decreased with increasedpropylene flow. Moderate decarburization in HVOF-sprayed WC-12Co coatingsproduces beneficial effects on its abrasive and sliding resistance and electrochemicalcorrosion resistance as a result of a compact microstructure, a high hardness and highfracture toughness.WC decarburizes in the direct and indirect model during HVOF sprayingprocess，the decarburizing degree of WC-12Co coating increases with the temperatureof the flame and the residual time in the flame of sprayed powder. The inhomogeneitydistribution of bonding metal phase increases the decomposition of WC both on thedirect and indirect way. HVOF-sprayed WC-Co coatings still suffer fromdecomposition and decarburization during spraying process leading to generating ofdetrimental phases such as W2C, W and amorphous Co-W-C phase.The WC-12Co coatings with different degrees of decarburization exhibiteddifferent dominant abrasive and sliding wear mechanisms. With mordetated extent ofdecarburization, the wear mechanism of WC-12Co coating were extrusion of thebinder phase and removal by plastic deformation and fatigue, undermining of theparticles and subsequent particle pull-out, the wear rate of the coating is low. Withhigh degree of the decarburization, the wear mechanism of WC-12Co coating wasmicro-cutting, and its wear rate was high as a result of lacking of high hardness WC phase.WC-12Co coating were heat treated at the temperature of550℃，750℃，950℃and1150℃in vacuum heattreatment, respectively. The W2C and W phasedisappear and the other phases such as Co3W3C、Co6W6C and Co2W4C graduallyappear in the WC-12Co coating with the increase of heat treatment temperature. Thehardness of coating firstly increased with heat treatment temperature and thendecreased after the950℃. With less degree of decarburization extent, the bestabrasive wear resistance of the coating appeared at the tem perature of950℃. Withhigher degree, The WC-12Co coating obtained the best abrasive resistance after1150℃of heattreatment. The diffusion speed of element such as W, Fe and Co at theinterface between coating and substrate also increased with the heat treatmenttemperature, an obvious diffusion bond with the width of10μm generated at thecoating side and some Kirkendall voids with the size of1-10μm at the side ofsubstrate appeared at the temperature of1150℃. Moreover, a large amout of W2C andW phases in the as-sprayed coating can accelerate the generation of the Co2W4C phaseduring the process of the vacuum heatreatment, which can result in high hardness andabrasive resistance of the WC-12Co coating. |
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