Adhesion of DLC-based coatings on LPBF Al-Si-Mg alloys: influence of substrate hardness

The microstructure and mechanical properties of Laser-Powder Bed Fusion (LPBF)-manufactured Al alloys can be tailored through heat treatments. To study how this affects the mechanical support offered to DLC-based thin films, we studied the adhesion of a DLC-based coating onto LPBF AlSi7Mg and AlSi10Mg alloys in four different conditions: as-built, directly aged, solution-treated, and T6 (solutionized and aged). Notably, the solution-treated substrates were harder than the T6 ones after coating due to precipitation during the deposition process itself, whilst the T6 substrates experienced over-aging. The fraction of delaminated coating area in the Rockwell indentation test increased and the delamination load in the scratch test decreased when the heat treatment reduced the hardness of the substrate and altered the eutectic Si network produced by the LPBF process. During ball-on-disc sliding wear tests, all substrates, including the as-built ones, deformed plastically under the contact stress. The DLC top layer, with its high H/E ratio, could follow such deformation, but the underlying W-C:H intermediate layer cracked and caused localized spallation of the film, with increased severity on softer substrates. If the wear track passed through an open pore on the LPBF substrate, the additional stress concentration produced an even larger spallation or, with the directly aged or solutionized substrates, a complete delamination. Thus, a softened substrate, coupled with the typical defects of LPBF materials, worsened the repeatability of the sliding behaviour. After the T6 treatment, further reduction in hardness caused the film to delaminate systematically under the chosen test conditions.