Introduction The radial-shaft-seal (RSS) is a widely used machine element, e.g. in gearboxes to seal lubricants against the environment. For this purpose the RSS, the shaft surface, and the sealed fluid as well as the operation conditions (sliding speed, temperature, pressure, etc.) form a tribological system - the radial shaft sealing system (Figure 2). In terms of sustainability and cost efficiency, high expectations are set on the reliability and service life of such radial shaft sealing systems. Therefore, downtimes due to maintenance or even leakages must be avoided. In particular, the complex inter- Aus Wissenschaft und Forschung 26 Tribologie + Schmierungstechnik · 70. Jahrgang · 4-5/ 2023 DOI 10.24053/ TuS-2023-0020 µ-Mechanical characterization of tribologically stressed elastomer surfaces with respect to radial shaft sealing systems Christian Wilbs, Matthias Adler, Daniel Frölich, Alisa Bellon, Nicole Schuster, Jasmin Menzel, Emely Bopp* Dieser Beitrag wurde im Rahmen der 64. Tribologie-Fachtagung 2023 der Gesellschaft für Tribologie (GfT) eingereicht. In diesem Beitrag wird die neue μ-mechanische Charakterisierungsmethode und ihre Anwendung zur Analyse von Radial-Wellendichtringen (RWDR) nach dynamischen Schmierstoff-Elastomer-Verträglichkeitsprüfungen vorgestellt. Dazu werden das Messverfahren und die Messgenauigkeit erörtert sowie die Kennwerte, die sich aus der gemessenen Kraftwegkurve ableiten lassen. Darüber hinaus, wird gezeigt, wie diese Kennwerte eine genaue Bewertung des RWDR-Materialzustands hinsichtlich harter Ablagerungen, Verhärtung und Erweichung ermöglichen. Zusammenfassend wird die Korrelation zwischen den visuell-haptischen Befunden und den Ergebnissen der neu entwickelten Charakterisierungsmethode aufgezeigt, die die Aussagekraft dynamischer Dichtungsprüfungen erhöhen wird. Schlüsselwörter dynamische Dichtung, Prüfung, Schmierstoff-Elastomer-Verträglichkeit, Radialwellendichtring, mikromechanische Charakterisierung, Dichtungstechnik, Simmerring, Öldichtung, Flüssigkeitsfreigabeprüfung, LNP This paper introduces the new µ-mechanical characterization method and its application for the post-test radial shaft seal (RSS) analysis of dynamic lubricantelastomer compatibility tests. Therefore, the measurement procedure and accuracy will be discussed as well as the features which can be drawn from the measured force-displacement curve. Furthermore, it will be shown, how these features allow a precise evaluation of the seal material condition regarding hard deposits, hardening and softening. This will be summarized by showing the correlation of the visualhaptic elastomer assessment and the results of the newly developed characterization method which will increase the informative value of dynamic lubricantelastomer tests. Keywords dynamic seal, testing, lubricant elastomer compatibility, radial shaft seal, micro mechanical characterization, sealing technology, Simmerring, oil seal, fluid approval testing, LNP Kurzfassung Abstract * M.Sc. Christian Wilbs 1 Dr.-Ing. Matthias Adler 1 Dr.-Ing. Daniel Frölich 1 Orcid-ID: https: / / orcid.org/ 0000-0002-2707-948X M.Sc. Alisa Bellon 1 Dr.-Ing. Nicole Schuster 2 Orcid-ID: https: / / orcid.org/ 0000-0002-9766-5548 Dr.-Ing. Jasmin Menzel 2 B.Sc. Emely Bopp 1 1 Freudenberg FST GmbH, Höhnerweg 2-4, 69469 Weinheim 2 Freudenberg Technology Innovation SE & Co. KG, Höhnerweg 2-4, 69469 Weinheim TuS_4_2023.qxp_TuS_4_2023 20.09.23 09: 16 Seite 26 actions between the sealed lubricant and the radial shaft seal have a major influence on the long-term reliability and lifetime of a radial shaft sealing system. It is there- Aus Wissenschaft und Forschung 27 Tribologie + Schmierungstechnik · 70. Jahrgang · 4-5/ 2023 DOI 10.24053/ TuS-2023-0020 Figure 1: Worm-gearbox with leakage due to lubricant-elastomer incompatibility Figure 2: Radial-Shaft-Sealing System [1] Figure 4: Lubricant-elastomer interaction Figure 3: Reverse pumping effect [2] fore essential to ensure lubricant-elastomer compatibility under dynamic tribological stress. The lubricant elastomer compatibility can be verified by dynamic tests in accordance with established industry test specifications. Due to the tribological stress of the lubricant and seal material within the sealing gap (contact zone) physical and chemical interactions between lubricant and elastomer develop (see Figure 4). Depending on whether the compatibility between lubricant and elastomer is given or not the intensity of interactions is different and has impact on the properties of the lubricant and elastomer. Especially the near surface material properties (µ-mechanical material properties within the first 60 µm) of the sealing edge are crucial for a well performing reverse pumping effect (see Figure 3) and therefore the performance of the radial shaft sealing system. TuS_4_2023.qxp_TuS_4_2023 20.09.23 09: 16 Seite 27 essential for the dynamic sealing function and at the same time the most difficult to rate. Because all three characteristics are evaluated not only through visual inspection, but also by haptic feedback, which indicates whether the sealing edge is harder or softer than before (pre-test) or if there is a hard, brittle layer covering the sealing edge. Figure 5 shows a post-test sealing edge of a NBR seal without any indication of a significant change of the material property, indicating a good lubricant elastomer compatibility. All other results have been within the limits too. In comparison, the sealing edge in Figure 6 is covered by a dark, brittle, hard layer - hard deposits - indication a lubricant incompatibility. The dynamic test results show that 3 out of 3 seals leaked after only 120 h and the shaft wear is above 10 µm. This comparison uses two extreme examples - highly compatible (Figure 5) vs. highly incompatible (Figure 6) lubricants which makes it easy to differentiate. However, for the purpose of development and approval tests, it is crucial to distinguish the condition of the seal material across a range of possibilities, ensuring repeatability and reproducibility even within these extreme variations. Therefore, it is necessary to move from a visual-haptic feedback rating with a categorial scale to an objective measurement method of continuous values. µ-Mechanical Material Characterization of RSS The new tactile seal characterization method - “µ-mechanical seal characterization” - is realized by using a micro indentation tester named “LNP ® nano touch” with a high precision force-displacement sensor (force resolution: 0.6 mN). Due to the very small probe tip radius of R = 40 µm it is possible to measure directly on the wear band, even of narrow wear band widths of 100 - 150 µm (see Figure 7, detailed view). Aus Wissenschaft und Forschung 28 Tribologie + Schmierungstechnik · 70. Jahrgang · 4-5/ 2023 DOI 10.24053/ TuS-2023-0020 RSS Analysis and Characteristics The post-test analysis of the RSS is an essential process to determine the material condition and how the lubricant affects the seal under tribological stress. Besides leakage, seal wear width, shaft wear depth, radial loadand interference change it is of great importance to consider the near surface material condition of the sealing edge. While the seal and shaft wear, the radial load and interference are measurable continuous values the seal material condition, is rated based on visual-haptic feedback of the material by seal experts with years of practical experience. The material condition rating scale has five categories of change: without (= 1), little (= 2), moderate (= 3), heavy (= 4) and very heavy (= 5). This scale is applied to nine post-test seal characteristics to describe the intensity of the seal condition change, see Table 1. While grooving and hollow wear, in addition to the wear band width, are describing the form of the seal wear, the other characteristics (deposits, cracking, blistering, hardening and softening) allow a conclusion regarding the physical and chemical interaction influencing the seal material properties. Even though that all characteristics are important to be considered, hard deposits, hardening and softening are Characteristic Method Grooving Visual Hollow wear Visual Contact discoloration Visual Cracking Visual* Blistering Visual* Soft deposits Visual-haptic Hard deposits Visual-haptic Hardening Visual-haptic Softening Visual-haptic *) in some cases, also haptic feedback Table 1: Post-test seal condition characteristics Figure 5: NBR seal (test no. 3) after 768 h dynamic test showing no damage of the sealing edge due to incompatibility Figure 6: NBR seal (test no. 2) after 768 h dynamic test showing hard deposits at the sealing edge TuS_4_2023.qxp_TuS_4_2023 20.09.23 09: 16 Seite 28 ment Value Unit type Sphere (Radius) radius 40 µm 60 µm 5 µm/ s 0 s 0 s 5 µm/ s 0 s v-Control Linear - Characteristic Unit ! "# Maximum force mN $ Hysteresis - % Stiffness mN/ µm & Homogeneity - In Figure 7 is shown the measurement setup with the clamped seal and a detailed view of the probe tip positioned at the wear band. The measurement results in a force-displacement curve (Figure 8) showing the force F on the vertical axis in mN and the displacement s on the horizontal axis in µm. The color differentiates the force when the probe is moving in the seal F in (black line) and reverse F rev (red line). To ensure comparability of the results it is necessary to use the same measurement parameters (see Table 2). However, with the force-displacement curve alone it is not possible to characterize a seal properly. Thus, features need to be calculated from the curve which then allow the characterization of the seal material properties, in particular: hard deposits, hardening and softening. Figure 9 shows the calculated features from the forcedisplacement curve in blue. F max is the maximum measured force at the maximum penetration depth sM. The Hysteresis H is the relative area enclosed by the F in and F rev -curve related to the area under the F in -curve. From a linear regression of the force-displacement curve at a discretization grid of 5 µm the slope corresponds to the stiffness of the seal material. Another interesting feature is the homogeneity b which describes the rate between the surface stiffness and the bulk stiffness. The surface stiffness is considered as the mean stiffness at a displacement s from 0 to 15 µm and the bulk stiffness as the mean stiffness at a displacement s from 50 to 60 µm. Table 3 summarizes the most important features calculated from the force-displacement curve for a proper post-test µ-mechanical seal material characterization. Aus Wissenschaft und Forschung 29 Tribologie + Schmierungstechnik · 70. Jahrgang · 4-5/ 2023 DOI 10.24053/ TuS-2023-0020 Measurement Value Unit Probe-tip type Sphere (Radius) - Probe-tip radius 40 µm sM 60 µm vM 5 µm/ s tM 0 s sL 0 s vL 5 µm/ s tL 0 s Type v-Control Linear - Table 2: Measurement parameters Characteristic Unit ! "# Maximum force mN $ Hysteresis - % Stiffness mN/ µm & Homogeneity - Table 3: µ-Mechanical features for seal characterization Figure 7: µ-Mechanical measurement setup Figure 9: µ-Mechanical characteristics calculated from the force-displacement curve Figure 8: Force-displacement curve RSS Characterization Map and Interpretation Finally, Figure 10 shows the RSS Characterization Map including the maximum force F max in mN on the horizontal axis, the hysteresis H in % on the vertical axis and the homogeneity b as colour code. TuS_4_2023.qxp_TuS_4_2023 20.09.23 09: 16 Seite 29 Figure 11 the seal condition and stiffness curves of test 1 to 3 are shown. Seal 2 and 3 of test 1 show a typical stiffness curve for hardened seals, a constant progression at a high stiffness level (bulk stiffness approx. 3 mN/ µm). Sometimes the material at the surface is even more hardened as the stiffness curve of seal 1 shows. However, the stiffness curve of seals with hard deposits show a significant drop to a lower level of stiffness (bulk stiffness approx. 1.6 mN/ µm) after the first 15 -20 µm (layer thickness). The stiffness curves of test 3 are slowly rising with increasing displacement s and leveling out at a bulk stiffness of around 1 mN/ µm indicating no material degradation and therefore a good lubricant elastomer compatibility. Summary and Outlook The new µ-mechanical seal characterization method realized by using the micro indentation tester from LNP ® and the here presented features drawn from the force-displacement curve, are the basis to successively update the state-of-the-art visual-haptic seal assessment Aus Wissenschaft und Forschung 30 Tribologie + Schmierungstechnik · 70. Jahrgang · 4-5/ 2023 DOI 10.24053/ TuS-2023-0020 The Characterization Map includes only results of dynamic tests with seals out of 72 NBR 902, but from tests with different test conditions (e.g. fluid, pressure, duration, sliding speed, etc.). The two examples, test no. 2 and 3, were tested at the same test conditions but with different fluids and test no. 1 additionally at a higher duration time and pressurization. The general trend in data shows with increasing maximal force F max a fast rise of the hysteresis H and the homogeneity b (green to red) before it is leveling out at high F max values. This curve progression correlates with the seal material degradation intensity due to lubricant elastomer incompatibility. Table 4 shows the post-test material condition rating results of the visual-haptic feedback method and the corresponding results from the micro mechanical characterization. The test results clearly show the difference between hardening and hard deposits. Since hardening is mainly correlated to high F max values and a bulk stiffness S greater than approx. 1.8 mN/ µm while hard deposits correlate with a high hysteresis H, inhomogeneity b greater than 1.1 and relatively small F max values. Often the seal material underneath hard deposits is softened as well. In Figure 10: RSS Characterization Map Test no. Damage Rating (-) Fmax (mN) Hysteresis (%) Stiffness bulk (mN/ µm) Homogeneity (-) 1 Hardening 5 280 54 2 Hard deposits Softening 5 4 99 78 3 non 61 38 Table 4: Post-test material condition rating results TuS_4_2023.qxp_TuS_4_2023 20.09.23 09: 16 Seite 30 method. Features such as the maximum force F max , hysteresis H, homogeneity b and the stiffness curve S allow a characterization of the seal material properties change and thus an evaluation of the lubricant elastomer compatibility. This characterization and evaluation are based on highly precise measurements which will increase the significance of dynamic lubricant elastomer compatibility tests even more. Other interesting outcomes of this analysis are: 1. The new µ-mechanical characterization method accurately reflects the results from the visual-haptic feedback method and the dynamic test results. 2. With increasing seal material degradation due to incompatibility, the precise µ-mechanical measurement shows an increased, up to 4 times higher, variation of the results. This suggests that the variation is mainly caused by unstable tribological conditions within the contact due to an incompatibility. Increasing the experience with the new µ-mechanical characterization method will allow to narrow down the exact feature combinations and limits for a complete characterization. Also, the development of further features will provide the possibility to characterize additional characteristics e.g. in terms of seal wear [3] or permanent deformation. Furthermore, it is necessary to extend the analysis to other elastomer compounds such as FKM’s and ACM’s. Literature [1] Freudenberg FST [2] Bauer F., Federvorgespannte-Elastomer-Radial-Wellendichtungen: Grundlagen der Tribologie & Dichtungstechnik, Funktion und Schadensanalyse, Springer Vieweg, 2021 [3] Alt K., Hüttinger A., Wöppermann M., Automatisierung und Standardisierung der Analyse von tribologisch geschädigten Radialwellendichtringen 64. Tribologie-Fachtagung, 2023 Aus Wissenschaft und Forschung 31 Tribologie + Schmierungstechnik · 70. Jahrgang · 4-5/ 2023 DOI 10.24053/ TuS-2023-0020 Figure 11: Seal wear band and stiffness curve TuS_4_2023.qxp_TuS_4_2023 20.09.23 09: 16 Seite 31