Tribological behaviour of DLC coatings for sheet metal forming tools

Sheet metal forming processes usually involve a sliding contact between the blank and tool surfaces, thus requiring the use of lubricants to prevent severe tribological conditions at the tool-workpiece interface. However, improper lubrication policies may have a negative impact on the environment, due to the use of unhealthy degreasing agents to wash the formed parts. In recent years, the introduction of the diamond like carbon (DLC) coatings has represented an attractive solution for the development of dry or nearly-dry forming processes. To be effective, these coatings must strongly bond to the dies substrate in order to grant resistance to the abrasive and adhesive wear. Although many approaches for evaluating the tribological performances of such coatings can be found in the existing literature, they often suffer from limitations in reproducing the interface conditions typical of the industrial processes. The objective of the present research work is to investigate the tribological behaviour of DLC coatings applied to sheet-forming tools under process conditions compared to the ones experienced by the traditional coatings used in industry. Experimental values of the friction coefficient have been determined by means of strip drawing tests and the coatings wear and adhesion to the substrate have been investigated by means of pin-on-disk and scratch tests.