23rd International Colloquium Tribology - January 2022 403 Continuous wear measurements of diamond-like carbon (DLC) based on radioactive isotopes Manuel Zellhofer AC2T research GmbH, Wiener Neustadt, Austria Corresponding author: manuel.zellhofer@ac2t.at Martin Jech AC2T research GmbH, Wiener Neustadt, Austria Ewald Badisch AC2T research GmbH, Wiener Neustadt, Austria Ferenc Ditrói Institute of Nuclear Research of the Hungarian Academy of Sciences, Debrecen, Hungary Andreas Kuebler Robert Bosch GmbH Feuerbach, Germany Paul Heinz Mayrhofer Institute of Materials Science and Technology, Vienna University of Technology, Austria 1. Introduction Diamond-like carbon (DLC) coatings are being developed to protect engine parts from wear and create low-friction systems to save energy in applications. For combustion engines and electric cars, the thickness of DLC coatings is usually between 1 to 4 µm. Nevertheless, unintended failure of DLC coatings has been observed in many engine applications. In the automotive industry in particular, delamination processes caused by abrasive wear particles lead to failures [1]. However, it is not yet clear whether the delamination in the presence of abrasive particles is a result of a spontaneous burst of the coating or a continuous wear progress. Such an investigation requires a continuous wear measurement method to monitor wear. Radioactive tracer methods (especially the radioactive isotope concentration method - RIC) have proven to be effective indicators for monitoring the progress of wear, e.g. on metallic engine components [2]. Irradiation methods such as thin layer activation (TLA) are used for these wear measurement methods [3]. However, previous studies have shown that irradiation similar to the RIC method can affect the tribological behaviour of DLC [4]. Thus, at tribometer level, we investigate the application of the RIC method (influence of irradiation) to subsequently determine the wear behaviour of DLC coatings under abrasive particles. 2. Methods Within this study, two different tribocontacts were experimentally studied using tribometers in order to generate various loading conditions typical for automotive applications: an oscillating ball-on-plate contact (Universal Mechanical Tester - UMT), and an unidirectional journal-bearing contact (Sintered Bearing Tester - SLPG). The samples consisting of a ~2.3 µm thick DLC coating on a steel substrate (100Cr6) were provided by the partner. DLC was activated by 3 He irradiation to generate 7 Be isotopes [5]. As a result of the irradiation method, the steel substrate underneath the DLC coating was activated too. In the RIC wear measurements performed during the tribological experiment, the wear particles containing the isotopes are transported to a detector via the lubricant circuit, see Figure 1. A filter with a mesh size of 1 µm was placed in the detector to trap the wear particles in the measurable zone of the detector. Tribological tests performed under high contact stress in ball-on-plate configuration (UMT, DLC plate against 100Cr6 ball, 2750 MPa hertzian contact stress, 30 min, 45,000 cycles) were used to investigate the influence of the irradiation on the tribological behaviour of the DLC coating. For comparison of irradiated and original DLC, the worn areas (3 experiments respectively) were measured using a chromatic confocal profiler (Jr25, Nanovea). In addition, nanoindentation experiments and a structure analysis with transmission electron microscopy were 404 23rd International Colloquium Tribology - January 2022 Continuous wear measurements of diamond-like carbon (DLC) based on radioactive isotopes performed to evaluate the difference between irradiated and original DLC. To investigate the influence of abrasive particles, tribological tests were conducted well below the critical contact stress at which DLC fails (SLPG, DLC liner against X90CrMoV18 shaft, 275 MPa). Three experiments (SLPG-A to -C) were performed using different abrasive particle concentrations at a particle size < 1 µm collected from previous tests on engine test benches. Figure 1: Simplified representation of the RIC-circle containing detector with filter, pump, and tribo-contact. 3. Results and discussion 3.1 Influence of irradiation No significant difference was found between the irradiated and the original samples, see Table 1. In addition, no significant failure or delamination of the DLC was observed during the performed tribological tests at high contract stress. Table 1: Summarized results of material analyses and tribometrical investigations comparing irradiated and original DLC coating. Sample/ condition (material analyses) Irradiated unworn Original unworn Roughness Ra, µm 0.18 ± 0.02 0.18 ± 0.02 sp2-content, % 55 ± 2 55 ± 3 Hardness, GPa 20 ± 3 19 ± 3 Young´s modulus, GPa 175 ± 14 172 ± 12 Sample/ condition (tribometrical invest.) Irradiated and worn Original and worn DLC wear, 105 µm³ 4.4 ± 2.4 4.5 ± 2.1 COF 0.247 ± 0.011 0.245 ± 0.010 3.2 Influence of abrasive particles The wear of the DLC coating and the wear of the co-activated steel-substrate of experiment SLPG-A are displayed in Figure 2. The results show that DLC wear starts immediately, while steel-substrate wear is observed with a delay of ~3.5 hours (50,400 revolutions). A continuous slightly progressive increase of the DLC wear can be observed before the steel-substrate wear is detected, which leads to the assumption that the DLC coating is continuously worn in a mild abrasive wear regime. Test SLBG-B and C show similar results. Figure 2: DLC wear (blue up-triangles) and steel-substrate wear (green down-triangles) at a particle concentration of 7 × 10 6 particles per ml (experiment SLPG-A, sliding contact). Table 2: Summary of test duration, abrasive particle concentration, and total wear volume of experiment SLPG-A to C. Experiment Total test duration, hours (revolutions) Abrasive particle concentration, 10 6 p/ ml 1) Wear volume, 10 6 µm³ 2) SLPG-A 6 (86,400) 7 4.5 SLPG-B 5 (72,000) 4 3.3 SLPG-C 5 (72,000) 1 2.0 1 ) The abrasive particle concentration is given in particles/ ml (p/ ml) ± 50 %. 2 ) Wear volume as sum of DLC and substrate wear with a total uncertainty of ± 0.6 × 10 6 µm³. The results from the SLPG tests indicate that increased wear occurs at higher abrasive particle concentrations (measured with the particle measurement device [6]), see Table 2. Subsequently, in the presence of abrasive particles, which possibly originate from another tribo-contact, the wear increases significantly, which can lead to early failures. 23rd International Colloquium Tribology - January 2022 405 Continuous wear measurements of diamond-like carbon (DLC) based on radioactive isotopes 4. Conclusion The irradiation method specifically applied for RIC measurements within this study does not significantly affect the DLC structure and consequently the tribological behaviour. Therefore, this procedure is well suited for investigating the nanoscopic wear behaviour of DLC coatings. Throw the use of the RIC method a continuous wear behaviour was found in contrast to a spontaneous burst of the DLC coating. In addition, a correlation between the DLC wear and the abrasive particle concentration was indicated. During the tribotests performed under high contact stress the DLC coating did not fail. However, lower contact stresses and the presence of abrasive particles can become critical parameters limiting lifetime in automotive applications. 5. Acknowledgements This work was funded by the Austrian COMET Program (project K2 InTribology1, no. 872176). The work has been carried out within the TU Vienna (Austria) and the “Excellence Centre of Tribology” (AC2T research GmbH). TEM investigations were carried out using the USTEM facilities at the TU Vienna, Austria. References [1] T. Haque, D. Ertas, A. Ozekcin, H. W. Jin, and R. Srinivasan, “The role of abrasive particle size on the wear of diamond-like carbon coatings,” Wear, vol. 302, no. 1-2, pp. 882-889, Apr. 2013, doi: 10.1016/ j.wear.2013.01.080. [2] P. Brisset et al., “Radiotracer Technologies for Wear, Erosion and Corrosion Measurement,” 2020. [3] T. Wopelka et al., “Wear of different material pairings for the cylinder liner piston ring contact,” Industrial Lubrication and Tribology, vol. 70, no. 4, pp. 687-699, 2018, doi: 10.1108/ ILT-07-2017- 0218. [4] N. Zhang, L. Lin, B. Liu, G. Wu, W. Xu, and T. Peng, “Tribological properties improvement of H-DLC films through reconstruction of microstructure and surface morphology by low-energy helium ion irradiation,” Diamond and Related Materials, vol. 109, Nov. 2020. [5] F. Ditrói, S. Takh, F. Th-Khyi, and I. Mahunka, “Study of the nat-C(3He,2α) 7Be and 9Be(3He,αn) 7Be nuclear reactions and their applications for wear measurements,” 1995. [6] C. Haiden, “Dissertation Optical Microand Nanoparticle Characterization in Microfluidics,” 2016.