G. Bolelli, L. Lusvarghi, University of Modena and Reggio Emilia, Modena, Italy; M. Barletta, University of Rome "Tor Vergata", Roma, Italy
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.
Summary: The subject of the present study is the characterisation of the microstructure, the wear resistance and the corrosion behaviour of WC-CoCr cermet coatings, deposited by HVOF-spraying onto an aluminium alloy substrate. Specifically, different coatings were produced by performing a different number of torch scans in front of the substrate: from 2 to 5 scans were performed, in order to produce coatings’ thicknesses in the 50 μm - 150 μm range.
The number of scans remarkably affects all of the coatings' properties. Coatings indeed become denser as the number of torch scans increases. For a better interpretation of this phenomenon, the Focused Ion Beam (FIB) technique was employed in order to investigate single WC-CoCr splats, sprayed onto both aluminium and steel surfaces.
As a consequence of such densification, the hardness, the sliding wear resistance (as determined by ball-on-disk tests) and the protectiveness against corrosion increase with increasing number of torch scans. The largest improvement occurs when passing from 2 to 3 torch scans.
Comparison to some reference anodized films indicates that WC-CoCr coatings are much more resistant against wear and against cyclic impact conditions. Three torch scans seem enough to produce a coating having excellent resistance against dry sliding. By contrast, the cermet coatings are not as protective against corrosion as anodized films, because of some interconnected porosity.
