Abstract
Adhesion and hardness of Diamond-Like Carbon films are improved by nitriding of the steel substrate prior to PVD deposition. Since the mechanical properties of the nitrided steel layer are not homogeneous, i.e. a significant hardness decrease is observed in the upper nitrided layer close to the surface, an outer surface layer of ~ 15 μm is removed prior to the film deposition. In the present work, a 316L stainless steel substrate is nitrided in a cyanide–cyanate solution at 570 °C during 3 h. The coated system involved the deposition of a hydrogenated, amorphous carbon (a-C:H) solid lubricant of ~ 2 μm including a chromium carbide interlayer. The comparison between the hardness behavior of the DLC/steel and the DLC/nitrided steel systems reveals the existence of a very important hardness gap, which highlights the benefit of the nitriding treatment prior to coating deposition. In addition, the microhardness-depth profile is determined from a load–depth curve, by applying a simple hardness model. The predicted change in hardness is found to be in a very good agreement with the experimental profile, which allows the hardness determination both in the white layer and in the diffusion zone over ~ 30 μm in total depth. However, only the composite hardness modeling allows the accurate determination of the intrinsic hardness of the film.
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