Diamond-Like Carbon (DLC) films are employed to improve the sliding wear performance of tribosystems, including those used in the automotive and aerospace sectors. Recently, Al- and Ti- based additively manufactured materials have gained increasing interest in these sectors due to their high strength/density ratios. However, their relatively low hardness affects their wear resistance unless coatings are applied. DLC-based films offer a potential solution, but the softness of these substrates could represent an issue. Indeed, when the substrate is more elastically and plastically compliant than the film, the latter could experience excessive deformation, leading to failure.
In this work, we studied the effect of substrate hardness on the adhesion and wear behaviour of PVD and PA-CVD DLC-based coatings deposited onto AlSi10Mg and AlSi7Mg alloys produced via Laser Powder Bed Fusion (L-PBF) and subjected to different heat treatments. The unique microstructure of L-PBF materials indeed allows non-conventional heat treatments that result in a wide range of achievable properties. Specifically, we considered substrates in four different conditions: as-built, directly aged, solution-treated, and T6 (solutionized and aged). As the hardness of the substrate decreased due to increasing reorganization and coarsening of the fine cellular network of eutectic Si, the fraction of delaminated coating area in the Rockwell indentation test increased, and the delamination load in the scratch test decreased. In the ball-on-disc test, the DLC top layer followed the plastic deformation experienced by all substrates under the contact stress. In contrast, the WC/C intermediate layer cracked and caused localized film spallation, with greater severity on softer substrates. The additional stress concentration caused by the random presence of open surface pores on L-PBF substrates promoted larger spallation or delamination whenever the wear trace passed through one of them. After the T6 treatment, the extensive reduction in hardness led to systematic coating delamination.