23rd International Colloquium Tribology - January 2022 43 Vacuum tribology of superhard ta-C coatings Dr. Volker Weihnacht Fraunhofer IWS, Dresden, Germany Corresponding author: volker.weihnacht@iws.fraunhofer.de Lars Lorenz TU Dresden, Institute of Manufacturing Science and Engineering, Dresden, Germany Fabian Härtwig TU Dresden, Institute of Manufacturing Science and Engineering, Dresden, Germany Stefan Makowski Fraunhofer IWS, Dresden, Germany Frank Kaulfuß Fraunhofer IWS, Dresden, Germany 1. Introduction Diamond-like amorphous carbon (DLC) coatings attract a great deal of interest due to their exceptional mechanical and tribological properties. Especially the superhard ta-C coatings show superior behaviour with extraordinary low wear rates combined with low friction under lubricated and even under unlubricated conditions in ambient air. In the absence of humidity, especially in a vacuum, ta-C coatings tend to exhibit unfavourable friction behaviour. In this contribution, a systematic vacuum-tribological study on ta-C coatings with variation of air pressure, counter body material and doping of ta-C with other elements is carried out. The aim was, to see why ta-C coatings lose their superior tribological performance in the lack of atmosphere and if there are possibilities to improve the behaviour while retaining the general advantages of ta-C. 2. Experiments The ta-C coatings were produced using the Laser-Arc process [1]. An approximately 0.1 µm thick Cr layer was used as an intermediate adhesion layer. The thickness of the ta-C coating was about 3 µm. In order to eliminate the deposition-related defects from the coating, a mechanical polishing was subsequently carried out. In addition to the pure ta-C coatings, softer a-C coatings and doped (t)a-C: X coatings (X = B, Cu, Fe, Mo and Si) were also produced. The doping was about 5 at% and was achieved via laser arc evaporation of graphite composite targets. The coatings were applied to 18 mm x 13 mm x 3 mm flat specimens of 100Cr6. As counter bodies in tribological experiments balls with Ø 10 mm were used. Beside experimental series with uncoated 100Cr6 steel balls, also balls made from Brass, Bronze, Copper, Al2O3, SiC and ta-C coated steel were used. A TETRA BASALT-C UHVT-14 ultra-high vacuum tribometer was used for tribological investigations both in oscillation and rotation mode. The parameters for oscillation mode were 5 N normal load, 3 mm stroke, 0.5 Hz frequency. For rotation experiments, 5 N normal load and 3 mm/ s sliding speed were applied. 3. Results 3.1 Variation of humidity The influence of humidity on friction and wear of ta-C vs. steel was measured using oscillation-mode tribometry. 44 23rd International Colloquium Tribology - January 2022 Vacuum tribology of superhard ta-C coatings Fig. 1: Wear track on ta-C (left) and abrasion on the 100Cr6 steel ball counter body (right) after 2000 cycles in oscillation tribometer at different atmospheres. The wear results (Fig. 1) show that both the wear track on the ta-C coating and the abrasion from the steel ball increase with each reduction in air pressure. In normal air (1000 mbar), neither a wear mark on the ta-C coating nor abrasion on the steel ball can be detected. Already at a reduction of the air pressure to 1/ 100, there are significant amounts of wear and also an obvious tribochemical change on the tribocontact of the steel ball. Fig. 2: Friction coefficient of ta-C vs. 100 Cr6 as a function of air pressure in an oscillation-mode vacuum tribometer. Fig. 2 shows the mean friction coefficients as a function of air pressure. As already found in wear results, also for friction a systematic deterioration with decreasing pressure can be observed. 3.2 Variation of counter body material In another series, the influence of counter body material on friction and wear behavior of ta-C coatings was tested. These tests were carried out in rotation mode in the vacuum tribometer at a pressure of about 3x10-7 mbar. Fig. 3: Friction coefficient of ta-C vs. different counter-body materials (balls) in an rotation-mode vacuum tribometer at about 3x10-7 mbar. From the results in Fig. 3 it can be seen, that the friction force can be significantly reduced by using counterparts different from steel. The highest friction is observed for a ta-C coated steel ball, i.e. a ta-C/ ta-C pairing, followed by a ta-C/ steel contact. A significant reduction is observed for Alumina, Bronze, and particularly for Brass. 4. Summary The results show that the tribological behaviour of ta-C in vacuum has to be considered in a very differentiated way and is not an intrinsic property of ta-C. It depends strongly from atmospheric pressure and from counter body material. Friction results are achieved that differ by up to an order of magnitude and wear rates over several orders of magnitude. For example, while the contact of ta-C/ steel in high vacuum is associated with a coefficient of friction of about 1, the coefficient of friction decreases to even ultra-low friction values for ta-C in contact to Brass. This is only a small selection of a large number of results with different variations of atmosphere, counter body and doping of ta-C coatings in this study. The presentation goes into detail about the various dependencies. In addition to the friction and wear results, individual analytical results on the friction contacts are also presented. References [1] F. Kaulfuss et al., “Effect of Energy and Temperature on Tetrahedral Amorphous Carbon Coatings Deposited by Filtered Laser-Arc,” MDPI Materials (Basel, Switzerland), vol. 14, no. 9, 2021.