Non-conventional inverse-Stribeck-curve behaviour and other characteristics of DLC coatings in all lubrication regimes

► An unconventional inverse-Stribeck-curve behaviour was found for DLC contacts. ► Less viscous oils enable lower friction in DLC contacts under boundary-lubrication. ► In boundary regime, oil increases friction compared to non-lubricated contacts. ► In spite of increasing friction, oil is required in DLC contacts to reduce wear. ► Stribeck curve shape depends strongly on the type of materials in the contacts.