Assessment of a multifuncional tribological coating (nitride+DLC) deposited on grey cast iron in a mixed lubrication regime

The performance of coatings for tribological applications is strongly associated with substrate properties and the presence of lubricants between the contacting faces. In this study, plasma nitriding followed by the application of a hydrogenated Diamond-like Carbon (DLC) film (a-C:H) was coated onto grey cast iron (GCI) substrates using a single thermal cycle. Four distinct reciprocating cylinder-plane configurations (horizontal cylinder sliding without rolling against a plane) were used: DLC coated plane on a GCI cylinder, a DLC coated cylinder vs. a GCI plane, a DLC coated cylinder vs. a DLC coated plane and an uncoated cylinder GCI against an uncoated plane GCI. These configurations were tested in dry conditions and using low viscosity oil (ISO5) lubricated reciprocating sliding tests. The respective friction coefficients and wear rates were measured. The wear tracks were analyzed using white light interferometry (WLI) to evaluate the surface topography and wear rates. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and micro Raman spectroscopy (RS) were carried out to identify the wear mechanisms. For dry tests, wear debris on the contact and DLC surfaces favour the formation of tribolayers that define the wear rates. Furthermore, regardless of the horizontal cylinder surface (coated or uncoated), the presence of DLC on the plane surface increases the dry friction coefficient. For lubricated tests, the tribolayers that formed on the metallic surfaces played a key role in governing the wear rate.