HVOF-sprayed WC-CoCr coatings on Al alloy: Effect of the coating thickness on the tribological properties

The microstructure, the micromechanical properties, the wear behaviour and the impact resistance of WC-CoCr cermet coatings, deposited onto an aluminium alloy substrate by High Velocity Oxygen-Fuel (HVOF) flame-spraying, were examined as a function of the coating thickness, which was varied between 50 μm and 150 μm by performing different numbers of scans of the HVOF torch in front of the substrate. The coatings became denser and significantly harder as the number of torch scans increased: the analysis of single WC-CoCr splats by combined SEM and Focused Ion Beam (FIB) techniques enabled the interpretation of the mechanisms underlying this phenomenon. In accordance to such densification, the sliding wear resistance increased with the number of torch scans, as abrasive grooving and brittle failure mechanisms were progressively suppressed. The resistance to cyclic impact was also enhanced. In comparison to anodised films, the WC-CoCr coatings appeared much more resistant against wear and cyclic impact; specifically, three torch scans seem enough to produce a coating having suitable characteristics.