Experimental investigations of scuffing of chromium nitride and diamond-like carbon protective coatings and AISI 52100 steel

Hard nitride and diamond-like carbon (DLC) protective coatings currently used in fuel-lubricated mechanical assemblies will have to accommodate greater temperature, load, and pressure requirements to prevent scuffing wear enabling future engines to operate efficiently with various lower lubricity fuels. In this study, load progression tribological experiments were conducted to investigate the scuffing resistance of three coatings, a chromium nitride (CrN) coating, DLC coating, and DLC coating with a CrN underlayer in decane and ethanol fuel environments. The results were compared to the baseline material, hardened AISI 52100 steel. Multiple characterization techniques, including progressive load scratch testing, optical microscopy, optical profilometry, nanoindentation, scanning electron spectroscopy, and energy dispersive spectroscopy were used to uncover the mechanisms responsible for their performance. The DLC coating delaminated in both fuel environments, exposing the steel substrate underneath which subsequently scuffed in the decane fuel environment. In contrast, the CrN coating remained adhered in both fuel environments, effectively shielding the steel substrate from scuffing and wear. The multi-layer coating, composed of the DLC coating with the CrN underlayer, maintained protection of the steel substrate in both fuel environments while also reducing the friction and wear of the sliding surfaces. In the ethanol environment, however, the DLC layer was worn away, leaving the CrN underlayer to protect the steel substrate. These results indicate that CrN is a promising candidate for protecting fuel-lubricated components. Moreover, incorporating a CrN underlayer with DLC coatings significantly enhances their friction and wear-reducing characteristics, making them suitable for applications involving ethanol-based fuels.