Advancement of a nanolaminated TiHfN/CrN PVD tool coating by a nano-structured CrN top layer in interaction with a biodegradable lubricant for green metal forming

After many essential applications e.g. in the field of machining, PVD tool coatings (physical vapour deposition) materially gain importance for metal forming processes. Coated metal forming tools may enormously reduce tool and work piece wear, form capacities or deliver special functions formerly provided by lubricants and often toxic additives. As main properties, the coatings have to feature a very high adhesion to the substrate, a high hardness and adequate oxidation stability. To meet these requirements a nanolaminated TiHfN/CrN tool coating was developed using the arc ion plating technique. After a first development step the coating delivered the highest possible hardness in combination with the lowest possible Young's modulus. These basic properties, in combination with the ability to prevent the formation of cracks inside the coating structure, already led to outstanding performances in several metal forming processes. As a second step, this work reports the development of an additional nano-structured CrN top layer which is used for the reduction of the friction coefficient by providing a lubrication supporting functional surface. To prove the developed coating's tribological performance application oriented tests were carried out, e.g. pin-on-disc, regarding the coating's behaviour in interaction with a biodegradable synthetic ester as lubricant. Results of cup backward extrusion processes using coated punch tools are presented including measurements of the forming capacities.