Wear and corrosion behaviour of HVOF WC–CoCr/CVD DLC hybrid coating systems deposited onto aluminium substrate

Hybrid coating systems consisting of a HVOF-sprayed WC–CoCr interlayer (75 μm or 125 μm thick) and a thin DLC-based top layer (obtained by electron cyclotron wave resonance–chemical vapour deposition: ECWR-CVD) were deposited onto AA6082-T6 substrates; their tribological and anti-corrosion performances were evaluated and compared to those of the DLC film deposited directly onto the Al alloy substrate without interlayer.In scratch adhesion testing, the interlayer delays the onset of cracking and spallation of the film, because it provides better mechanical support than the bare Al substrate. As soon as cracking begins, however, complete delamination of the films deposited on cermet interlayers occurs, revealing limited interface adhesion. Accordingly, in ball-on-disk wear tests, the films deposited on cermet interlayers can stand more severe contact conditions without cracking, but, as the contact pressure increases, most of the film delaminates.If no cracking occurs, the film is very effective at producing low wear and low friction, but it is not as effective at preventing penetration of corrosive agents, as shown by corrosion tests in aqueous environments, because it contains some small defects.

Research highlights
► HVOF-sprayed WC–CoCr as thick interlayer between DLC-based film and Al substrate.
► Scratch tests and ball-on-disk tests performed on films with and without interlayer.
► No interlayer: early film failure under contact conditions (cracking, buckling).
► Hard, stiff WC–CoCr interlayers improve the resistance to contact stresses.
► DLC-based films reduce friction and wear but do not protect against corrosion.