Carbon-based coatings for dry sheet metal working

This paper describes recent advances on coating of tools for forming processes like deep drawing or clinching of aluminium and stainless steel sheets. Various carbon-based coatings, prepared by PECVD (RF and DC pulse mode (p-DC)) and by reactive DC Magnetron sputtering have been investigated with respect to hardness, abrasive wear resistance, adhesion properties and tribological behaviour under forming conditions. The softer and less wear resistant coatings exhibit the lowest adhesive wear and friction against aluminium and stainless steel counterparts. Application tests with coated tools were carried out under near production conditions. Correlation of mechanical test results with the film behaviour under service shows that for dry forming of stainless steel a high abrasive wear resistance of the hydrocarbon coating is necessary. No cold welding of the steel sheets on the tools was found, so that the aim of dry working is reached by these coatings. For forming of aluminium only softer a-C:H coatings show satisfying anti-galling properties. Especially in the case of aluminium working with a high degree of form change there is need for coatings with still improved anti-galling properties.