Tailored modifications of amorphous carbon based coatings for dry deep drawing

Abstract

Avoiding negative effects of conventional lubrication in forming operations stimulates research on lubricant-free sheet metal forming. Without lubricants, the direct contact between tool and workpiece leads to intensive interaction in terms of increasing friction and wear. Especially, when forming aluminum alloys, adhesion is detected as main wear mechanism propagating already at the initial contact between tool steel and sheet material. Thus, surface modifications need to be developed to substitute the functions of conventional lubricants. Former studies revealed that amorphous carbon based coatings improved the tribological conditions. Depending on the specific coating type adhesion and friction could be significantly reduced. Besides achieving a lower level of friction, a control of material flow is necessary to improve the forming results in deep drawing processes. Thus, there is a need for contact zones with higher and lower friction to locally accelerate or restrict the flow of the sheet material. In order to achieve these tailored tribological conditions a broad range of surface modifications needs to be developed and evaluated in a process-like environment. The tribological behavior is affected by coating properties and structure. By varying gas flow and ratio for a-C:H coatings the relationship between deposition parameters and coating properties are investigated. A further approach to realize a broader range of friction coefficients is the laser based texturing of coated tools. Additionally, a laser based heat treatment is investigated to control the sp2/sp3 ratio of ta-C coatings to affect friction and wear mechanisms. The tribological behavior of the investigated surface modifications is analyzed in ring-on-disc and flat strip drawing tests. A surface characterization is performed to identify about the wear mechanisms. Based on the modifications of a-C:H and ta-C coatings a correlation between chemical and mechanical properties and the tribological behavior in contact with zinc-coated deep drawing steel and aluminum alloys is found.