Introduction The design of seals aims predominantly for service life and efficiency. The mineral oil-based and synthetic lubricants used are always associated with the risk of contamination, for example through leakage from gearboxes via seals, or via gear drives of conveyor systems, as well as through grease from bearings and pumps. Therefore, a minimum leakage and oil contamination is unavoidable. In addition, both dynamic and static seals for demanding fields of application are mostly based on fluoropolymers which will be affected by the possible ban of perfluorinated and polyfluorinated alkyl substances (PFAS). As with oil, wear of fluoropolymers can result in an undesirable contamination of the environment with wear particles. One solution being pursued is the use of water-based lubricated systems. However, due to the increased tendency of elastomers to build up a high level of adhesion without an oil-lubricated film, friction losses, e.g. on shafts and fatigue wear of the seal, must be avoided. For these applications, we investigate the influence of the choice of lubricant on the friction pairing in the form of friction changes and changes in the wear morphology and structure formation on the surface in model tests on sheet material. The wetting behavior of lubricants and base oils of greases is an essential information that can be used to correlate both the spreading behavior of the lubricant and the change in surface elasticity due to swelling by sorption. The surface elasticity and the spreading energy are the known characteristic values that determine the wave-like sliding and the tendency to form structure sizes of wear particles and crack structures. By investigating the tribological behavior of e.g. HNBR elastomers with various lubricating fluids, ranging from water-based mixtures to polyalkylene glycols and greases, the sensitive selection of advantageous systems and the mechanistically clear distinction between swelling and non-swelling systems becomes apparent. The adhesive friction of polymers arises from the continuous forming and breaking of adhesive bonds between the two gliding partners. This process was described by Schallamach as a thermally driven rate process. Schallamach’s model [1] for adhesive friction results in a coefficient of friction (COF) which initially grows monotonously with increasing gliding velocity. The COF reaches a maximum value at a specific gliding velocity and decreases if the gliding velocity is increased further. Since Schallamach’s approach is based on Science and Research 5 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0007 unikationswissenhe Sprachwissenent \ Altphilologie Kommunikationsistorische Sprachanagement \ Alttik \ Bauwesen \ schaft \ Tourismus ie \ Kulturwissenichte \ Anglistik \ \ BWL \ Wirtschaft The sensitive dependence of sealing elastomers on lubricants Christof Koplin, Bernadette Schlüter, Raimund Jaeger* submitted: 19.09.2024, accepted: 24.03.2025 (peer review) Presented at GfT Conference 2024 * Dr.-Ing. Christof Koplin Dr. Bernadette Schlüter Dr. Raimund Jaeger Fraunhofer-Institut für Werkstoffmechanik, Mikrotribologiezentrum µTC, Freiburg The design of elastomer seals for bearings and pumps requires the selection of the friction partner and lubricant. This study investigates the influence of swelling and spreading behavior on the tribological behavior of NBR systems. Interaction tendencies are described by the energies of interaction and spreading, which are determined by means of contact angle measurements. Intensive swelling was observed in elastomers with a high tendency to solve or interact. The elasticity and spreading energy are decisive for the formation of the elastic length of the adhesive contact. Highly deformable elastomers form an adhesive contact whose length influences the friction. If the elasticity of swollen elastomers is low, friction and wear increase. For elastomers with low lubricant absorption, the spreading tendency determines the friction. A characteristic speed shows at which a transition to forced wetting occurs. Polar lubricants as well as water-based lubricants show similar dependencies to dry systems, which still needs to be verified. For greases, the tendency to spreading and loosening was calculated for base oils, with non-spreading contacts predominating. In thermoplastic systems, the swelling tendency is dominant. Keywords seal, elastomer, lubricant, wear, friction, spreading, swelling, substitution, water-based Abstract amongst others, by Wu-Bavouzet et al. [8]. The effect of surface elasticity, interfacial energies, and viscosity on the formation of continuous or stick-slip dominated gliding was described. At low speeds, thermal fluctuations determine the formation of the adhesive bond and produce continuous gliding. At higher speeds, a minimum of the frictional force versus velocity occurs due to energetic equality of elastic strain and adhesion. High velocities lead to a cyclic tearing-off from the adhesively formed contact and a reduction of the pronounced contact surface. As a result, a slip movement is observed on the surface until a bonding or stick phase occurs again. The elastic length that is overcome until a slip phase occurs is described again by the ratio of interfacial energies and shear elasticity. The ratio of this length and the mean slip velocity is a characteristic time that is important for deriving further mechanisms. To do this, Wu-Bavouzet uses analytical approximations of the crack process of the adhesive interface. The main result is a model for the coupling of wavelength, crack length and contact length, wave velocity, release frequency and sliding velocity to the energy release rate for a gliding step at the interface. The descriptions were obtained for linear elasticity, but Wu-Bavouzet assigns universal validity to the obtained dependencies. Several other authors as well described the separation of the interface by spreading and as a function of viscosity. The adhesion and friction of bio-inspired nanoand microtextured liquid silicone rubber (LSR) surfaces produced by injection molding were analyzed [9]. Different methods were used to create anisotropic structures. The structures created in this way on a surface of LSR have been proven to have a tribological effect. Nanostructures can increase the adhesive friction of surfaces during sliding, increase the wetting tendency of liquids and decrease adhesive friction. Meso-structures increase the movement amplitude of stick-slip and decrease dry friction, absorb shear deformation at low velocities, reduce film formation during lubrication and increase friction. Overall, the structures enabled frictional anisotropy. When elastomer substrates were laser microstructured and subsequently coated with DLC, the coated samples showed a significantly reduced run-in friction compared to the uncoated ones. On NBR samples, a “tile patterned” substrate could efficiently suppress any further cracking of the DLC-coating except in the predisposed trenches. The structuring in the coated as well as in the uncoated state helped to lower the CoF and reduce wear. This effect partly wore off over time and was dependent on the surface structure [10]. Our aim is to understand the effects or sensitivity of the lubrication pairing with these elastomers and to investigate the effects of structures and coatings in demonstrative cases. Science and Research 6 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0007 an energy-based description of the slip rate for adhesive processes between the frictional partners, their surfaceand interfacial energies can give some insight into the influence of the lubricant on the adhesive friction. A good correlation was found for wetting and spreading parameter of lubricated self-pairing contacts for a wide range of lubricants [2, 3]. Quantities like the work of spreading W spreading and the work of solving W solving can be calculated with surfaceand interfacial energies [4]. The determination of surface and interfacial energies of the frictional partners is typically based on contact angle measurements. The work of adhesion can be determined, for instance, with the Young-Dupree equation by measuring the contact angle between lubricant and polymer. An alternative approach described by Owens is to determine the work of adhesion using appropriate averages of the polar and dispersive components of the surface free energy for the polymer and the cohesive energy of the liquid. The latter is also determined with contact angle measurements, but possibly using several other liquid-solid combinations than the one for which the work of adhesion is calculated. If there is a strong interaction between the lubricant and the polymer, both approaches can yield different results. The ratio of interfacial energy determined by Owen’s approach and by a contact angle measurement with the pairing of lubricant and polymer (denoted as W solving) is called the “interaction parameter”. An interaction parameter greater than unity indicates a strong interaction between polymer and lubricant. To sum up, if a dry contact of two material is formed the work of adhesion has to overcome if a liquid invades, wets, and separates the contact faces. The change in energy of these states is the spreading energy. Pairings of semicrystalline thermoplastic polymers with lubricants were studied to differentiate between tribological effects due to sorption and plasticization and those due to spreading. For mild loading in mixed lubricated conditions against technical steel surface, friction and wear properties seem to be primarily determined by the hardness-dependence of abrasive contact and less by adhesion or hysteretic mechanisms [5]. When base oils are transformed to greases using Li as thickener, the increased spreading tendency of the base oils results in an increased stiction. A possible explanation for this behavior is that an increased tendency of spreading of the base oil increases the bleeding tendency of the grease. The bleeding and recovery behavior of the thickener-rich layer was described by Zhang et. al [6]. As a result, a thickener with a degraded rheology is obtained, thus an increase in grease lubricated stiction can be found [7]. The basic mechanisms of friction of elastomers in contact with smooth surfaces was systematically studied, Methods Materials Thermally pressed plates of 2 mm thickness of different NBR elastomer were investigated (Table 1). The listed lubricants were chosen (Table 2). Tribological Testing Ball-on-3-plates tests with a 1.3505 steel ball (diameter ½”, R a = 0.15 µm: R z = 1.4 µm, R ku = 3.85, R Pc = 37/ cm) gliding on 3 elastomer plates of 2 mm thickness were used to carry out oscillatory tribological experiments [11]. In these experiments, a polished steel ball loaded with 2 N is brought into spherical contact. The plates are separated by 120° around the rotating axis and have an inclination of 45° to this axis. Therefore the normal force on each plate is 2 0.5 / 3 times the tribometer load. The formed contact line on the ball has half diameter of the ball itself. Using the normal load and distance to the axis the coefficient of friction can then be calculated from the measured frictional moment. The contact pressure is limited and defined by hyperelastic behavior of the elastomer in the range of 0.1 to 1 MPa, which was verified by the diameter of contact marks. The amplitude of the velocity and deflections can be varied over a wide range, starting at very low velocities (1.3 ×10 -4 mm/ s - 3.3 mm/ s). To achieve stable and reproducible system conditions, a testing sequence was applied to form a run-in situation. Conducting the oscillatory experiments in this range of sliding speeds (oscillatory mode) makes it possible to record friction data in the static friction and boundary-lubrication regimes. Using a data acquisition rate of 10 data points per decade, the loading torque was increased on a logarithmic scale at an oscillation frequency of 1 Hz. 3-ball-abrasion test with 1.4125 steel balls (diameter ½”, R a = 0.15 µm: R z = 1.4 µm, R ku = 3.85, R Pc = 37/ cm) gliding on elastomer plates of 2 mm thickness dry and with lubricant. 3-ball-abrasion test were used to induce surface fatigue in elastomers. Three rigidly mounted steel balls were loaded with 30 N and rotated in a circular track at a velocity of 56 mm/ s on the elastomer surface. After a wear track becomes visible, the wear can be quantified by measuring the weight loss of the specimen. Measurements were done for 0.5 h at room temperature and stable run-in conditions were achieved for the last 10 min. The run-in friction and wear rate are calculated for the last 10 min. Ring-on-disc tests with axial bearing rings INA AS1528 made of 1.3505 of technical roughness (diameter of track 12.5 mm, R a = 0.1 µm, R z = 0.8 µm, R Pc = 64/ cm) were used for gliding contact of elastomer rings glued on INA AS0821 with 9.2 mm - 21 mm diameter and 2 mm height. A load of 0.3 MPa at 148 RPM (0,14 m/ s) was used. Additional Tests und Analysis Contact Angle Measurements The sessile drop method placing 10 times 2 µL droplets on the samples was used by a contact angle measurement device produced by Data Physics GmbH (figure 1). Three standard liquids (water, ethylene glycol and diiodomethane) were used to characterize the unknown surface and interfacial energies. The droplet profile is recognized and recorded by the software of the instrument, and the contact angle is calculated automatically. Science and Research 7 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0007 NBR-H-A-NT-68 Goorex Automotive GmbH & Co. KG Westerrönfeld, Germany HNBR-2513-80 Festo SE & Co. KG Esslingen, Germany HNBR-DX107-4 NBR-804506.1 Freudenberg Weinheim, Germany NBR-802607.2 pPG: polar polyalkylenglycole = 30 mm 2 / s at 40 °C Klüber Lubrication München SE & Co. KG uPG: non-polar polyalkylenglycole = 30 mm 2 / s at 40 °C Ester: ester oil = 23.5 mm 2 / s at 40 °C PAO: polyalphaolefin = 30 mm 2 / s at 40 °C pPG Ligrease uPG Ligrease Ester Ligrease PAO Ligrease EG: ethylenglycole = 17 mPa.s at 25 °C Omnilab (Bremen, Germany) Gly: glycerol = 1412 mPa.s at 20 °C Table 1 Table 2 in the lubricants and stored in ovens at the temperature of interest and weighed again (here: 100 °C). Results and Discussion It is common knowledge that for dry contact of elastomers with a frictional partner, the elastic length of the interface is proportional to the ratio of the work of adhesion and the elasticity of the elastomer. The work of adhesion can be calculated as W 12 by the surface energy of each partner. For simplicity, the elasticity is replaced by the shore hardness. As a result, the wear rate was found to be increased for a longer elastic length (figure 2). An increase in the extent of surface fatigue by an increase of localization of shear load by increased elastic length is the driving force for an increased wear rate. This assumption was confirmed for the tested elastomers. For lubricated elastomers in boundary lubrication, the work of adhesion must be replaced by the Work of spreading, W spreading . For the system with a higher spreading tendency, spreading of lubricant or bleeding of base lubricant for grease is likely to occur. The hypothesis is that oil or bled base oil is sorbed and does swell the elastomer. Science and Research 8 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0007 Measurement of Shore Hardness. Indentations were performed with a Bareis HPE II and the value shore A was obtained. Mass Uptake To measure the uptake of the lubricants by the elastomers, elastomeric specimens with a thickness l of 2 mm were first pre-dried at a temperature of 100 °C and a gas pressure less than 3 mbar until a weight equilibrium was reached. The specimens were then weighed, immersed Figure 1: Contact angle of a sessile drop of liquid on different elastomers Figure 2: correlation of wear at dry gliding contact of different NBR elastomer with elastic length Consequently, the swelling is followed by a decrease of the elastomer elasticity and an increase of elastic length and friction. In case of grease, a sorption of base oil into the elastomer also degrades the grease viscosity. In both cases, the friction or stiction (static friction) and wear rate would increase if there were a high tendency to spread (W spreading < = 0 mN/ m) (figure 2). When choosing the onset of gliding motion and measuring stiction there is no effect of viscosity and a comparison of systems with different types of lubrication is possible. For polar lubricants as ethylene glycole, the spreading behavior is reduced and consequently friction rises. It is defined in literature that for a reduction in spreading W spreading is increasing. Being highly polar, W spreading for water is highest, and since the viscosity is very low, it is squeezed out of the contact. A squeezed-out contact is formed by a pure elastomer steel interface. For a spheric contact a transition velocity was calculated [9] for a transition between squeeze-out and forced wetting to be v = (l/ R) 1/ 3 W spreading / viscosity (l: contact length, and R: contact radius). For ethylene glycole and water, friction decreases by forced wetting by gliding velocities beyond the characteristic velocity. This velocity-dependent transition behavior of polar lubricants will lead to high breakaway moments and massive occurrence of stick-slip phenomena in applications. For the lubricated ring on disc tests, only friction can be measured (figure 4) but no wear data were obtained. HNBR-2514-80 is non-spreading in oil which is reflected in the linearly increasing friction values for the different oils. In contrast, NBR-H-A-NT-68 and oil is a spreading system, but its friction values are higher due to the increased solving tendency. Of these 4 systems, the higher friction of NBR-H-A-NT-68 with three oils compared to the fourth oil (PAO) is a result of the fact that friction increases due to an increased tendency to solve. In summary, it can be said that friction is determined by its ten- Science and Research 9 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0007 Figure 4: Correlation of run-in friction at lubricated contact of different elastomer using different lubricant types with spreading and solving energy Figure 3: Correlation of stiction at lubricated contact of elastomer using different lubricant types with spreading energy high tendency to spread increases the tendency to bleed out base oil from the pressurized contact, it is sorbed into the elastomer. The swelling goes along with a reduction in shore hardness A. A mass uptake of 14 % for pPG was followed by a reduction in hardness by 28 %. The swelling of the elastomer together with the possible reduction of the lubrication capability of the thickener richened phase changes the friction of the contact. If for a system (e.g. HNBR-2513-80) low spreading and no swelling is observed, then friction is decreased with increasing spreading tendency. Conclusion The design of elastomer seals for bearings and pumps should include an appropriate selection of the frictional partner and lubricant. For the lubrication of elastomers, a significant influence of the swelling and spreading behavior on the tribological behavior was found for the investigated NBR systems. Depending on these interaction tendencies, changes in friction and wear were obtained. Science and Research 10 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0007 dency to dissolve and, if not present, by its tendency to spread. In the abrasion test with lubricated steel balls, only the friction can be measured (figure 5), as no wear was detected here either. In these experiments, a different steel type with a slightly different surface energy was used, hence solving and spreading energies of the oils differ. The loading, lubrication and contact pressure is as well different from the ring on disc test, but the dependence on energies follows similar rules. The system NBR-H- A-NT-68 with uPG is non-spreading in oil but it is solving, and as a result the friction is obtained higher than for the 3 other systems, both for oil and for grease. For the other 3 oils or greases the contact with NBR-H-A- NT-68 is spreading. In contrast to oil lubrication, the wear could be measured for grease lubrication (figure 6). Here, the friction and wear of elastomers is sensitive to the interaction with the base oils. The tribological behavior is determined by the spreadingor swelling-tendency of the systems. When a Figure 6: Correlation of run-in wear for lubricated gliding contact of elastomers using 4 different oil-based greases with spreading energy and the lubricant uptake when stored at 100 °C Figure 5: Correlation of run-in friction of lubricated contacts for different elastomers using different lubricant types ( uPG) with spreading and solving energy × The interaction tendencies can be calculated by the energies W solving and W spreading which can be efficiently determined by contact angle measurements. For elastomers with a high solving tendency, intensive swelling was found. The shore hardness of swollen elastomer was measured in order to quantify the effect of lubricant absorption. The elasticity and the spreading energy are the main properties that are relevant for the formation of the elastic length of adhesive contact. Highly deformable elastomers form an adhesive contact with a specific length in the test before a slip process occurs. This can be described in a first approximation with an energetically based elastic length, for which the deformation energy and adhesion energy are in thermodynamic equilibrium. Accordingly, the proportion of friction increases with the dissipated adhesion energy and the elastic length. For the lower elasticity of swollen elastomers, the measured friction was immensely increased. Consequently, wear was also increased. If the elastomer absorbs little lubricant, the tendency to spread determines the friction. A high spreading tendency would result in a reduction in elastic length and therefore, a reduced friction and wear was found. Since the spreading energy is calculated for a static situation the description must be completed for a dynamic situation and the effect of lubricant viscosity. A characteristic velocity can be calculated for which a transition to forced wetting or squeeze-out occurs. One simple rule is that a constantly high friction occurs for non-spreading systems at velocities below the characteristic velocity. Polar lubricants are highly non-spreading. It can be assumed that the same dependence of friction and wear on the elastic length exists for these systems as for dry systems, but this still needs to be verified. For greases, the tendency to spread and solve was calculated for their base oils. For elastomers and grease lubrication, the measurable dependence on the spreading tendency is visible, as the base oils predominantly produce non-spreading contact. For thermoplastic systems, the tendency to swell is usually dominant, as these systems produce a predominantly spreading contact. References [1] Schallamach, A. A theory of dynamic rubber friction. Wear 1963, 6, 375-382, doi: 10.1016/ 0043-1648(63)90206-0. [2] Schertzer, M.; Iglesias, P. Meta-Analysis Comparing Wettability Parameters and the Effect of Wettability on Friction Coefficient in Lubrication. Lubricants 2018, 6, 70, doi: 10.3390/ lubricants6030070. [3] Kalin, M.; Polajnar, M. The Effect of Wetting and Surface Energy on the Friction and Slip in Oil-Lubricated Contacts. Tribol Lett 2013, 52, 185-194, doi: 10.1007/ s11249-013-0194-y. [4] Israelachvili j.n. Intermolecular and Surface Forces; Elsevier, 2011, ISBN 9780123751829. [5] Koplin, C.; Oehler, H.; Praß, O.; Schlüter, B.; Alig, I.; Jaeger, R. Wear and the Transition from Static to Mixed Lubricated Friction of Sorption or Spreading Dominated Metal-Thermoplastic Contacts. Lubricants 2022, 10, 93, doi: 10.3390/ lubricants10050093. [6] Zhang, S.; Klinghart, B.; Georg Jacobs, G.; von Goeldel, S.; König, F. Prediction of bleeding behavior and film thickness evolution in grease lubricated rolling contacts, Tribology International, Volume 193, 2024, doi: 10.1016/ j.triboint.2024.109369. [7] Koplin, C.; unpublished results [8] Wu-Bavouzet, F.; Clain-Burckbuchler, J.; Buguin, A.; Gennes†, P.-G. de; Brochard-Wyart, F. Stick-Slip: Wet Versus Dry. The Journal of Adhesion 2007, 83, 761-784, doi: 10.1080/ 00218460701586178. [9] Koplin, C.; Weißer, D.F.; Fromm, A.; Deckert, M.H. Stiction and Friction of Nanoand Microtextured Liquid Silicon Rubber Surface Formed by Injection Molding. Appl. Mech. 2022, 3, 1270-1287. https: / / doi.org/ 10.3390/ applmech3040073 [10] Vogel, S.; Brenner, A.; Schlüter, B.; Blug, B.; Kirsch, F.; Roo, T.v. Laser Structuring and DLC Coating of Elastomers for High Performance Applications. Materials 2022, 15, 3271. https: / / doi.org/ 10.3390/ ma15093271 [11] Koplin, C.; Abdel-Wahed, S.A.; Jaeger, R.; Scherge, M. The Transition from Static to Dynamic Boundary Friction of a Lubricated Spreading and a Non-Spreading Adhesive Contact by Macroscopic Oscillatory Tribometry. Lubricants 2019, 7, 6, doi: 10.3390/ lubricants7010006. Science and Research 11 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0007