24th International Colloquium Tribology - January 2024 109 Effect of Atmospheric Composition on the Friction and Wear of Cobalt-Based Alloys at Elevated Temperatures Tobias König 1* , Philipp Daum 2 , Dominik Kürten 1 , Andreas Kailer 1 , Martin Dienwiebel 2 1 Fraunhofer Institute for Mechanics of Materials IWM, Microtribology Center mTC, Wöhlerstr. 11, 79108, Freiburg, Germany 2 Institute for Applied Materials - Reliablity and Microstructure at the Karlsruhe Institute of Technology, Straße Am Forum 7, 76131, Karlsruhe, Germany * Corresponding author: tobias.koenig@iwm.fraunhofer.de 1. Today’s challenges Today`s increasing efficiency and emission requirements of internal combustion engines lead to higher combustion temperatures and pressures and require complex exhaust gas systems with multi-stage turbochargers and exhaust gas recirculation. Due to the high control effort in these exhaust systems, many different actuators were used, such as exhaust gas flaps or valves, VTG guide vanes, EGR flaps, brake flaps or wastegates in turbochargers. All these actuators include plain bearings, which are thermally loaded up to 800-°C and exposed to the exhaust gas atmosphere. Most of the tribological material pairings were tested and analysed under lab air atmosphere to investigate their wear behaviour and predict their lifetime. In general, a severe wear section at lower temperatures due to high abrasion or adhesion and a wear and friction reduction based on a glaze-layer formation was observed [1, 2]. But the surrounding atmosphere and the related tribochemical reactions play a major role in the wear behaviour especially for unlubricated tribosystems at elevated temperatures, shown by [3-5]. The aim of this study was to identify the atmospheric effect on the wear and friction of a widely used cobalt-based material combination at high temperatures. A reciprocating model test was used, combining a modified friction measurement with a line contact configuration and an atmospheric admission. 2. Tribological testing The tested material pairing consists of a cast cylinder of Tribaloy ® T400 rubbing on a steel plate (1.4713) coated with a 200-µm thick HVOF sprayed Tribaloy ® T800. An OPTIMOL SRV IV ® test rig with a specially developed cylinder-on-plate contact configuration was used to perform the reciprocating wear tests. A detailed description of the test method can be found in [5]. The Calculation of the energetic coefficient of friction was applied, a proved method for the high friction of unlubricated systems, which was developed by [6].To investigate the atmospheric effects, wear experiments were done in lab air and a CO 2 -N 2 -O 2 atmosphere with 5-vol.% O 2 and 15.2-vol.%-CO 2 , simulating exhaust gas. The tribological tests were performed with a normal force of 50-N, a stroke length of 1.6-mm, a frequency of 15-Hz at constant temperatures between RT and 800-°C for two hours. The wear measurement was conducted with a confocal microscope. Further analysis of the worn samples were conducted with a SEM, Raman spectroscope and XPS. 3. Important Results The measured friction behaviours (Fig. 1) of the tested atmospheres show up to 300-°C significant differences. In air an almost constant level around 0.6 was observed, whereas in the CO 2 -N 2 -O 2 atmosphere higher values up to 0.9 were measured. Above 300-°C both atmospheres show an increasing friction tendency up to 600-°C and at higher temperatures comparatively low values of 0.35. Fig. 1: Friction behaviours in different atmospheres Comparable to the friction, the wear of this material pairing (Fig. 2) also shows atmospheric differences up to 300-°C. In air the wear occurs mainly on the cylinder and shows minimal values for RT and 100-°C. In contrast, the wear in the CO 2 -N 2 -O 2 atmosphere takes place mainly at the plate. At 300-°C the cylinder determines the wear behaviour in both atmospheres, between 400-600-°C mainly the plate is worn out and at higher temperatures only a minimal amount of wear was measured. Fig. 2: Wear behaviours in different atmospheres 110 24th International Colloquium Tribology - January 2024 Effect of Atmospheric Composition on the Friction and Wear of Cobalt-Based Alloys at Elevated Temperatures For the identification of different tribological wear mechanisms due to atmospheric variations and their impact on the wear scar formation selected plates were analysed with confocal microscopy. The optical images of the surface and the topography is shown in Fig. 3. Fig. 3: Wear scars of plates tested at 200-°C In air the plate shows slight abrasive grooves, which can only be seen by high magnifications. Moreover, the porous structure of the HVOF-coating can clearly be seen also in the wear track, where debris sporadically adheres. Contrary, the plate tested in CO 2 -N 2 -O atmosphere shows larger voids in the wear scar, flaking out of the porous coating, but also high amount of adhering debris. At the corresponding position on the cylinder opposite the voids, large material transfer and adhering debris can be found. With XPS analyses a formation of metallic carbides in the debris was detected. Following this, the mainly abrasive wear of the cylinder in air is replaced by adhesive wear of the coating and adhesive debris formation in CO 2 -N 2 -O atmosphere. Between 400-600-°C, where no difference of the wear was documented, adhering debris on the cylinder result in a highly abrasively worn coating. At higher temperatures than 600- °C, a wear and friction reducing glaze layer formation takes place in both atmospheres. This layer is formed by oxidized wear particles, which were compacted and sintered by the high tribological pressures and temperatures in the contact zone. Raman analyses reveal a glaze layer composition out of Co 3 O 4 for 700 and 800-°C, that corresponds to the results of [2]. Following this, the high friction measured at 600-°C is constituted by an incomplete oxidation of the debris and their lower oxidation states. 4. Findings In summary, it can be stated that an exhaust gas-like atmosphere is highly affecting the tribological behaviour of the investigated material pairing (Fig 4). Lower oxidation rates in CO 2 -N 2 -O atmosphere led to a change from abrasion to adhesion at temperatures up to 300-°C. Due to the weaker stability and porous structure of the HVOF-T800-coating, large material flaking occurs and adheres on the cast cylinder out of Tribaloy ® T400. Moreover, tribochemical formed carbide debris groove the coating. However, the dominant tribological mechanisms at high temperatures were not affected by the different atmospheres in this study. It is concluded that the temperatures compensate lower oxygen amount and result in oxidic debris, forming a wear and friction reducing glaze layer. Finally, this research confirms the necessity to test and investigate tribological systems in their application-oriented environment, especially for HT-tribology. Fig. 4: Wear behaviours in different atmospheres References [1] Rynio C., Hattendorf H., Klöwer J. et al. (2014) The evolution of tribolayers during high temperature sliding wear. Wear 315: 1-10. [2] Dreano A., Fouvry S., Guillonneau G. (2020) Understanding and formalization of the fretting-wear behavior of a cobalt-based alloy at high temperature. Wear 452-453: 203297. [3] Velkavrh I., Ausserer F., Klien S. et al. (2016) The influence of temperature on friction and wear of unlubricated steel/ steel contacts in different gaseous atmospheres. Tribology International 98: 155-171. [4] Rahman M. S., Ding J., Beheshti A. et al. (2019) Helium Tribology of Inconel 617 at Elevated Temperatures up to 950- °C: Parametric Study. Nuclear Science and Engineering 193: 998-1012. [5] König T., Kimpel T., Kürten D. et al. (2023) Influence of atmospheres on the friction and wear of cast iron against chromium plated steel at high temperatures. Wear 522: 204695. [6] Fouvry S., Duó P., Perruchaut P. (2004) A quantitative approach of Ti-6Al-4V fretting damage: friction, wear and crack nucleation. Wear 257: 916-929-929.