Characterization of the tribological behaviour of tool surfaces depending on higher contact pressures

During deep drawing, different contact pressures between the sheet and the tool surface occur. Under the blankholder the contact pressure is relatively low, can be analytically calculated and adjusted. Generally, the highest contact pressures result at the drawing radius, which can be calculated using finite element methods. Besides of the material combination of the tool and the sheet, the surface structures, the intermediate medium and the relative speed, the contact pressures have a significant influence on the tribological system. Strip drawing tests offer the possibility to characterize tribological systems for sheet metal forming under variation of the contact pressure. In this work, the tribological behaviour of three different tool surfaces was investigated: hot-working steel, aluminium bronze and metal matrix composite (MMC). The MMC-surface had a supporting plateau out of hard particles with a depression of 3 micrometre. The tests were carried out with and without lubrication by applying different contact pressures up to 20 MPa. Using hot-working steel in dry-sliding resulted in friction coefficients at times higher than 1 even at low surface pressures of 4 MPa. It was shown that the friction force was successively increased each time of strip drawing. This was an indication for significant progressive wear in dry sliding, whereby the first strip draw led to a smooth forcepath curve comparable to lubricated sliding and the subsequent experiments showed pronounced peaks in the force-path curves. By contrast to the tribological behavior of the hot-working steel, the friction coefficients in dry sliding using aluminium bronze and the MMC surface were significantly lower. By increasing the surface pressure by a factor of five the friction coefficients in lubricated strip drawing were decreased by up to 8% and in dry sliding the friction coefficients were increased by up to 63%.