Carbon-based coatings for dry sheet-metal working

This paper describes recent advances in the coating of tools for forming processes such as deep drawing or clinching of aluminium and stainless-steel sheets. Various carbon-based coatings, prepared by PECVD (RF and pulsed-DC mode) 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 exhibited 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 the 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 the aim of dry working is reached by these coatings. For forming of aluminium, only softer a-C:H coatings showed satisfying anti-galling properties. In particular, in the case of aluminium working with a high degree of form change, there is a need for coatings with further improved anti-galling properties. (C) 1998 Elsevier Science S.A.