Diamond-Like Carbon (DLC) and AlCrN films onto Ti-6Al-4V substrates by Laser-Powder Bed Fusion (L-PBF): Effect of substrate heat treatment and surface finish

This paper focuses on the performance of thin-film coatings onto additively manufactured Ti-6Al-4V. Specifically, because metal parts obtained by laser-powder bed fusion (L-PBF) often require post-process heat treatments and surface finishing to meet end-user specifications, we studied how the resulting changes to mechanical strength and surface roughness affect the performance of films deposited by physical or plasma-enhanced chemical vapor deposition (PVD, PE-CVD). L-PBF Ti-6Al-4V substrates were heat-treated either below or above the β-transus and finished by grinding with different grit sizes, and then were coated with PVD AlCrN or a PE-CVD DLC-based film. Scratch adhesion on harder surfaces treated below the β-transus was higher with both coating types, whilst the substrate finishing had a negligible effect. Conversely, in ball-on-disc sliding wear tests, substrate roughness had a dominant effect: rough surfaces always resulted in earlier cracking and delamination of the coatings. Substrate hardness had a minor effect only with the AlCrN film. Moreover, the DLC-based films, because of the low-friction conditions they establish through a graphitized tribofilm as well as their higher H/E ratio, survived severe contact conditions better than the stiffer AlCrN. The results were interpreted in the light of the plasticity indices of the coated systems and their tribochemical interactions.