Wear and Corrosion Behaviour of HVOF-Sprayed WC-CoCr Coatings on Al Alloys

The sliding wear resistance, impact resistance and corrosion protectiveness of HVOF-sprayed WC-CoCr cermet coatings deposited onto an aluminium alloy substrate were investigated. In particular, coatings’ thicknesses in the 50 μm – 150 μm range were produced by varying the number of torch scans.

The results showed that this variation significantly affects all of the coatings’ properties; indeed, as the number of torch scans increases, coatings become not only thicker, but also significantly denser, as noted by scanning electronic microscopy (SEM) + image analysis investigations. This phenomenon is related both to the peening action exerted by the incoming particles on the previously deposited layers and to the mechanisms of splat formation: in order to investigate the latter process, single WC-CoCr splats sprayed onto aluminium surfaces were observed and sectioned by the FIB technique.

Because of such densification, the hardness (tested by Vickers microindentation), the resistance against dry sliding wear (ball-on-disk test) and the protectiveness against corrosion (assessed by electrochemical impedance spectroscopy and by free corrosion tests over a 1-week period) increase as more torch scans are performed. The largest improvement occurs when passing from 2 to 3 torch scans.

These coatings were also compared to two different state-of-the-art anodized films: the cermet layers are much more resistant against sliding wear, and can better stand cyclic impact conditions. Three torch scans seem enough to produce a coating having excellent resistance against dry sliding. By contrast, the cermet coatings offer less protection against corrosion than anodized films, because of some interconnected porosity.