24th International Colloquium Tribology - January 2024 85 Influence of Particles on DLC Coated Journal Bearings Alexander Hofer 1 , Manuel Zellhofer 1 , Thomas Wopelka 1 , Andreas Kübler 2 , Andreas Nevosad 1 and Martin Jech 1 1 AC2T research GmbH, Austria 2 Robert Bosch GmbH, Stuttgart, Germany 1. Introduction Diamond-like carbon (DLC) coatings provide low friction and superior hardness. Therefore, they are frequently used for wear prevention in crucial components such as journal bearings. Despite their robustness, DLC coatings can suffer critical damage such as delamination. This study focuses on a diesel-lubricated journal-bearing-like tribocontact, consisting of a DLC coated bearing shell with a steel-based shaft. Previous parameter studies, simulating the typical applied stress of this tribocontact in terms of pressure, temperature, and sliding velocity, did not reveal any critical wear conditions leading to the DLC delamination, but rather the presence of particles [1]. This study centers on a particle-focused investigation, examining this potentially critical parameter by thoroughly investigating the entire tribocontact in a close to application bench tests as well as in a model tribometer environment. The investigation of the influence of third body abrasives on DLC wear is not widespread. Haque et. al. [2] studied the influence of rather big sand particles (10-to 650-µm diameter) in a ring on block tribometer, with the result that the 10-µm particles lead to the highest wear increase in this setup. However, for the small lubricant gap in the journal bearing, lubricated with the high-quality diesel fuels, these particle sizes are too big for conclusions regarding this type of application. Therefore, investigations with smaller particle sizes were conducted. 2. Material and Methods The subject of the investigations in this work was a journal bearing-like tribocontact with a steel-based shaft and a DLC coated shell as counterpart. Wear of the components was measured continuously, employing the Radio-Isotope Concentration (RIC) method, which is based on radioactive tracer isotopes that are generated in the surface of the investigated specimen previously to the test with a particle accelerator in a so-called activation process. The first test series for the tribosystem was conducted in a test bench equipped with the whole aggregate containing the tribocontact of interest and applying the loading conditions of the real system. The goal of this test series was to determine the positions in the tribocontact where the wear process is initiated as well as the progress of wear. In this test series the shaft was activated as for DLC coatings there is only one tracer isotope available, which does not allow to distinguish wear from different positions. Therefore, the shaft was marked with two different radioactive tracer isotopes, 57 Co and 56 Co, in two different positions. The former was used to label the centre position of the shaft in axial direction while the latter was applied at the outer zones (see inlay in Figure 1). The second test series was carried out on a journal bearing tribometer with a unidirectional rotating motion which was connected to a closed lubricant circuit. This setup was applied to systematically investigate the influence of particles (that enter the tribocontact) on the wear behaviour and delamination of the DLC coating. The DLC coating was marked with 7Be as tracer isotope, which allows to directly measure the wear of the DLC coating. In the tribometer configuration, the shaft was pressed downwards onto the shell with a normal load of 200-N (resulted in ~30 MPa initial Hertzian pressure). The rotational speeds were varied perpetually in ramps between 0 and 240 rounds per minute (equivalent to 0 to 130-mm/ s sliding velocity) simulating boundary lubrication at start-stop conditions as well as hydrodynamic lubrication at high velocities. Diamond particles of 1, 6 and 9-µm diameter were introduced in a wide range of concentration in the lubricant circuit to study the particles’ influence on the wear behaviour. The DLC surfaces of selected test runs were investigated by scanning electron microscopy (SEM) for the analysis of wear tracks. 3. Results and discussion 3.1 Bench test During regular operation of the journal bearing, a typical running-in behaviour has been observed for the central zone for 100 minutes of the experiment, as shown in Figure 1. In some experiments, the outer zones showed a sudden tremendous increase of wear (blue curve) while wear increase of the central zone (red curve) was smaller by orders of magnitude (Figure 1). Figure 1: Wear depth curves of outer zones (Co-56, blue, primary y-axis) and central zone (Co-57, red, secondary y-axis) of bearing shaft (inlay top left corner). 86 24th International Colloquium Tribology - January 2024 Influence of Particles on DLC Coated Journal Bearings Figure 2: Offset (upper row) and wear rates (lower row) of DLC bearing bushes with addition of particle of different concentration and diameters. Upon conducting a thorough inspection of the components following the tests, it was discovered that wear particles were generated in another tribological contact near the end of the shaft. These particles were then transported into the journal bearing gap by the lubricant. This finding is in accordance with the RIC results showing a very steep increase for the outer zones followed by a flatter signal raise in the centre. Therefore, the incorporation of particles into the tribocontact as a significant wear mechanism for DLC was thoroughly examined in tribometer tests. 3.2 Tribometer tests Several tests were carried out with diamond particles of different diameters (1, 3, and 9-µm) and for reference without additional particles. Throughout the experiment, particles of increasing concentration (within the range between 10 3 to-10 9 ) were introduced into the lubricant circuit in discrete time steps of a few hours. The resulting wear behaviour was assessed by two quantities using the RIC method: Abrupt wear increases and following constant wear rates. Figure 2 shows the abrupt increase/ offset in the first phase after particle addition (upper row) and the constant wear rate during the last 40% of the duration of each particle concentration step (lower row). The 1-µm particles showed no influence on both quantities regardless of their concentration except for the-10 5 -ml -1 step, which is considered to be an outlier. For the 6-µm particles, both the offset and the wear rate increase significantly with admixture, with the values tending to rise at higher particle concentrations. For the 9-µm particle diameter, the offset shows similar behaviour as for the 6-µm species (note the values for 10 5 -ml -1 concentration). For the wear rate no significant increase could be observed compared to the diesel without added particles. Figure 3 shows a SEM picture after a wear test with the admixture of 9- µm particles. The image displays a distinct scratch in the DLC coating with an embedded particle. An Energy Dispersive X-Ray spectroscopy (EDX) measurement revealed a >99% (atomic percentage) concentration of carbon in the particle, which confirms that it is an added diamond particle and not a wear particle of, for example, the steel counterpart. Figure 3: SEM image after a test with addition of 9 µm particles. Thus, we conclude that there is a critical particle diameter between 1 and 6-µm for the investigated tribocontact above which sudden failure and increased wear rate of the DLC coating can occur. Furthermore, this behaviour also depends on the particle concentration, which can be explained by the fact that the introduction of particles into the lubrication gap is a statistical process and therefore more likely at higher concentrations. It was found that critical wear events can be provoked by the insertion of abrasive particles into the lubrication circuit. 4. Acknowledgement This work was funded by the Austrian COMET Program (project K2 InTribology1, no. 872176). The work has been carried out within the “Excellence Centre of Tribology” (AC2T research GmbH). References [1] Zellhofer, M., Jech, M., Wopelka, T., Hofer, A., Mayrhofer, P. H.,), Kuebler, A., Weckenmann, F., Experimental approach for investigating critical loading conditions leading to delamination of DLC coatings, 7th World Tribology Congress, WTC 2021, September 5-10, 2021, Lyon, France. [2] Haque, T., Ertas, D., Ozekcin, A., Jin, H.W., Srinivasan, R., 2013. The role of abrasive particle size on the wear of diamond-like carbon coatings. Wear 302, 882-889. https: / / doi.org/ 10.1016/ j.wear.2013.01.080