Tribo-mechanical properties and adhesion behavior of DLC coatings sputtered onto 36NiCrMo16 produced by selective laser melting

The combination of selective laser melted nickel-chromium-molybdenum alloyed steel and diamond-like carbon (DLC) coatings is an aspiring approach to produce light-weight components with improved tribological properties. In order to assure the tribo-mechanical properties of the DLC coatings, good adhesion on the additively manufactured substrates under high loads is essential. 36NiCrMo16, produced by Selective Laser Melting (SLM) and wrought material techniques, served as substrate material for hydrogen-free a-C and hydrogenated a-C:H coatings with a chromium carbide interlayer. The structural and mechanical properties of 36NiCrMo16 were examined in order to consider their effects on the tribo-mechanical properties and adhesion of the DLC coatings deposited by magnetron sputtering. In x-ray diffraction analyses, retained austenite was identified for the 36NiCrMo16 substrate processed by SLM. This increased austenite content in the SLM built steel is attributed to the pre-heating temperature of 200 degrees C during fabrication. Additionally, a relative density of 99.98% was detected for the SLM manufactured tempering steel by using an x-ray microscope. In contrast to the conventional 36NiCrMo16, the SLM substrates exhibited a higher surface roughness, which is ascribed to the different phase composition and microstructure. In nanoindentation tests, the a-C and a-C:H coatings deposited onto 36NiCrMo16 exhibited a hardness of similar to 22 and similar to 19-20 GPa, respectively. In general, the DLC coatings revealed a good adhesion to the conventional and SLM 36NiCrMo16 in Rockwell C indentation tests. Local, limited spalling failures were identified for a-C and a-C:H on the SLM substrate, while no delamination was observed for the coatings on conventional steel. The differences in microstructure, such as the retained austenite and higher surface roughness, do appear to affect the adhesion of the DLC coatings. In tribometer tests against 100Cr6 counterparts, the DLC coatings significantly reduced the friction and increased the wear resistance of 36NiCrMo16. Therefore, the DLC coatings are promising to enhance the tribo-mechanical properties of SLM 36NiCrMo16 substrates, but it is crucial to consider the surface integrity of SLM steel on the adhesion behavior of DLC coatings.