Tetrahedral Amorphous Carbon ta-C Films Research Articles

Deep drawing process without lubrication – an adapted tool for a stable, economic and environmentally friendly process

Abstract In today’s industry, waste prevention and an efficient use of resources are becoming more important due to economic and environmental requirements. Especially in forming processes such as deep drawing, a reduced use of lubricants is highly promising for saving resources and reducing production costs. Ultimately, the highest savings can be realised by a complete lubricant free deep drawing process - namely dry deep drawing. In this paper, a combined approach consisting of a new tool design for deep drawing in lubricant free applications and development of a protective tool coating is presented. The combination of macro and micro structuring of the tools and the protective coating ensure a stable process with a sufficiently wide process window under dry conditions. Since the friction in the flange area and die radius has the largest contribution to the drawing force, these parts of the tool will be adapted for the new tool design. In order to decrease the level of friction force in flange area for a given friction coefficient, the integral of the contact pressure over the contact area has to be reduced. This is achieved by macro structuring of the tool. The absence of lubricants in drawing die radius has to be compensated by tetrahedral amorphous carbon ta-C films, which combine low friction coefficients and anti-adhesive behaviour with a high hardness and wear resistance. The ta-C films can additionally be micro structured by Direct Laser Interference Patterning (DLIP) to further improve their tribological performance.

Low friction ta-C films with hydrogen reservoirs

Ž. 3 Diamond-like tetrahedral amorphous carbon ta-C films, which contain no hydrogen and high content of sp bonds have a Ž. very high hardness and elastic modulus up to 600 GPa . They have also excellent tribological properties in atmospheres that Ž. contain humidity 50% RH or also when submerged in water. However, in dry atmospheres the co-efficient of sliding friction against bearing steel, for instance, increases to values of over 0.7. In this paper ta-C films were modified by adding hydrogen to the films. The ta-C films were deposited by using non-filtered pulsed vacuum arc deposition in the ambient pressure of hydrogen and hydrocarbon gases. A novel layered structure containing titanium layers with hydrogen and carbon have also been grown by using filtered metal arc in combination with the pulsed carbon arc. The hydrogen content and the composition of the films were Ž. measured by using time-of-flight elastic recoil detection analysis TOF-ERDA . The hardness of the films was measured by nanoindentation. The tribological performance of the coatings were determined by using pin-on-disk tests. The counter part Ž. material was steel AISI 52100 , the normal load applied 5 N and the sliding velocity 0.02 ms. In the modified films the hydrogen content was up to 16 at.% in ta-C and up to approximately 50 at.% in the titanium layers. A co-efficient of friction of approximately 0.2 at dry atmosphere was measured for hydrogen-containing ta-C and also for layered structures where a hydrogen-containing Ti layer was buried under a hydrogen free ta-C layer. The results indicate that the lowered co-efficient of friction is a result of hydrogen transport to the contact surfaces with the aid of surface diffusion through the pinholes of the ta-C layer. 2001 Elsevier Science B.V. All rights reserved.