1 Introduction and state of the art This article examines whether tribologically optimized high-performance polymers can replace non-ferrous metal alloys in oil-hydraulic tribological contacts. In the first chapter these contacts are classified and delimited, followed by a presentation of currently used materials and the current state of research on polymers in hydraulic contacts. Building on this, methods for characterizing selected polymers are presented, followed by associated test results, that are discussed in the context of the research question afterwards. 1.1 Tribological systems in oil-hydraulic machines Hydraulic drives are used when large forces and torques or high power in a limited space are needed, which mechanical or electro-mechanical solutions cannot achieve [1]. Hydraulic drives are suitable for that, because power is transmitted by a fluid, allowing force and torque transmission via surface and volume ratios without solid surface contact [1]. Therefore, the highest tribological loads in hydraulic systems occur at the input and output, where hydraulic and mechanical energy are converted into each other. Rotary machines typically used for this experience higher tribological loads than linear machines. Figure 1 classifies the tribological loads of various rotary hydraulic machine types based on data from Wegner [2], Gärtner [3], and Pietrzyk [4], supplemented in an overview by Czichos [5]. As some rotary machines operate under variable load and speed, only the upper limit of typical loads is clearly defined in Figure 1. Hydraulic vane machines are less frequently used, and gear machines are tribologically similar to gearboxes. Thus, afterwards the focus is on piston pumps, whose tribological contacts are unique for hydraulics. A basic distinction is made between radial, axial and in-line piston machines [1]. As their tribological contacts do not differ fundamentally, the axial piston machine (Figure 2), the most common in industry and research, is used here as case study. The operating principle of an axial piston machine is explained in detail by Ivantysinova [6]. The machine functions as both a pump and a motor. In pump mode, oil is drawn in and expelled by an oscillating piston movement, generated by the rotation of the cylinder block and the pistons’ support on the swash plate via slippers. The three key tribological contacts are the piston-bushing, cylinder block-valve plate, and slipper-swashplate contact [6]. They share the following characteristics: Science and Research 5 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0002 Swelling, wear and property changes of high-performance polymers in oil-hydraulic tribological contacts Felix Schlegel, Marius Hofmeister, Dino Osmanovic, Katharina Schmitz * submitted: 23.08.2024 accepted: 6.02.2025 (peer review) Presented at GfT Conference 2024 In oil-hydraulic tribological contacts, hard-soft material pairings of steel and non-ferrous metal are usually used, whose main component copper is often alloyed with lead. These materials have significant disadvantages in terms of oil ageing and toxicity, which is why research is being carried out into alternatives. This article examines whether modern high-performance polymers can replace conventional non-ferrous metal alloys in oil-hydraulic contacts. Experimental studies on the media compatibility of various PEEK, PAI, PI and POM-H polymers in mineral and ester oil are discussed. In addition, the friction and wear behavior of PEEK and PAI is quantified by tribometer tests under typical oil-hydraulic conditions. Keywords hydraulics, alternative materials, media compatibility, wear, polymer Abstract * Felix Schlegel, M.Sc. (corresponding author) Orcid-ID: https: / / orcid.org/ 0009-0004-5501-3754 Marius Hofmeister, M.Sc. Orcid-ID: https: / / orcid.org/ 0000-0002-6672-5800 Katharina Schmitz Univ.-Prof. Dr.-Ing. Orcid-ID: https: / / orcid.org/ 0000-0002-1454-8267 Institut für fluidtechnische Antriebe und Systeme der RWTH Aachen University Campus-Boulevard 30, 52074 Aachen Dino Osmanovic B.Sc. For the following discussion, the slipper-swashplate contact was chosen as a representative example. The comparatively complex slipper kinematics are discussed in detail by Manring [8] and are briefly outlined here. The slippers are connected to the pistons via ball joints, providing three degrees of rotational freedom, allowing them to follow the swash plate under the piston load. Due to centrifugal forces, friction and hydrodynamics, the slippers tilt along the path tangent and radius and rotate around their axis, creating an uneven gap with an average height of 2 - 20 µm [7, 8]. The main slipper load is the piston pressure force, oscillating with the change of suction and delivery pressure at twice the pump’s rotation frequency. Additionally, inertia, friction and centrifugal forces occur [8]. To reduce the slippers nominal load, usually several kilonewton, the slippers are hydrostatically relieved by feeding oil from the piston chambers into the lubrication gaps through bores in the pistons and slippers (Figure 2 left). It is possible to adjust the contact’s remaining normal force by the area ratio of the piston cross-section to the compensated slip- Science and Research 6 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0002 • High normal forces due to support of hydraulic pressure (> 50 kN in large pumps) • High relative speeds compared to plain bearings (up to 20 m ⁄ s) • Comparatively large conformal surface contacts • Elasto-hydrodynamic lubricated (EHL) contacts with high deformation • Lubrication with hydraulic fluid under full immersion with comparatively large quantity of fluid, that is permanently filtered and cooled • Typical oil temperatures of approx. 40 - 80 °C • Pronounced load and speed-dependent contact pressure distribution due to deformation and complex kinematics [7] • Hydrostatic relief and hydrodynamic pressure build up relative velocity [m ] 1 10 100 1000 0.1 0.01 0.001 0.01 0.1 1 10 100 1000 10000 hertzian surface pressure [N/ mm 2 ] turbine ring seals valve guides slide ring seals aerodynamic spring bearings joint bearings gearboxes ball bearings sliding bearings brakes/ clutches piston rings friction gearboxes piston pumps vane pumps gear pumps Figure 1: Classification of oil-hydraulic tribological systems (based on [5]) suction line delivery line slipper piston cylinder block valve plate drive shaft swashplate bushing hydrostatic relief volume flow compensated slipper area sealing land Figure 2: Structure of an axial piston pump per area. With usual relief degrees of 80 -105 % [6, 8], contact pressures of approx. 0.01 - 3 MPa typically result at relative speeds of up to 20 m/ s. Slippers are usually designed to maintain a remaining normal force to close the lubrication gap and prevent floating, which can cause severe edge wear and high leakage losses. This load adjustment, along with lubrication and cooling from hydrostatic relief, provides the basis for exploring polymers in this unusual application. A key challenge will be the normal force adjustment to polymers while preventing floating. 1.2 Materials for oil-hydraulic tribological systems In oil-hydraulic tribological contacts, self-adjusting hard-soft pairings of non-ferrous metal and hardened steel are commonly used. These allow the softer contact partner to run-in, embed particles, and reduce adhesion risk. Typical used non-ferrous metals are special brass alloys (e.g. CuZn37Mn3Al2PbSi), gunmetal (e.g. CuSn7ZnPb), and bronze alloys (e.g. CuPb15Sn). Special brass alloys, containing 20 -30 m% zinc, tin, silicon, about 10 m% additional elements like aluminum, manganese, nickel, and iron, as well as 0.1 - 2 m% lead, are widely used [9]. A popular example is CuZn40Al2Mn2Si, known as Aeterna 3838. Lead improves grain refinement, machinability, corrosion resistance, and emergency running but is toxic to humans and the environment, classified as such under CLP and REACH regulations. Lead-free copper alloys like CuZn30Al2Mn22Ni1FeSiSn (OF2299) are available but not widely accepted by component manufacturers [10]. Furthermore, copper catalyzes oil aging and increasing acid content with oil age can cause copper corrosion and component damage [11]. That’s why, polymers may be an alternative to avoid these issues. In a “disc-on-disc” experiment with HLP46, on the tribometer described in chapter 2.2, at 40 °C of fluid temperature, 3 MPa nominal pressure, 15 m/ s relative speed and 216 km wear distance, the sliding friction- (f dyn ) and wear coefficients (K W ) for special brass alloys were measured by the authors (Table 1). 1.3 State of research of polymers in oil-hydraulic machines Polymers are typically classified based on mechanical properties and maximum operating temperature as standard, technical and high-performance polymers [12]. Previous Research on polymers in water-hydraulic piston pumps, e.g. by Rokala [13], Schoemacker [14], Li et al. [15, 16], Guan et al. [17] and Ni et al. [18], indicates, that only high-performance polymers, particularly PEEK, are suitable for hydraulic tribological contacts. Therefore, the authors selected PEEK natural, PEEK-CF30 (30 vol% carbon fibers), PEEK HPV (10 vol% carbon fibers, 10 vol% graphite, 10 vol% PTFE), PAI natural, PI natural and POM-H to investigate their suitability in oil-hydraulic contacts. These polymers have different structures. POM-H and PEEK are semi-crystalline thermoplastics, while PI and PAI are amorphous thermoplastics, also known as “pseudo thermoplastics”, due to their decomposition temperature being lower than their re-melting temperature. Amorphous thermoplastics have random arranged polymer chains, whereas semi-crystalline thermoplastics form regularly crystalline areas. The spaces between these chain structures allow fluid molecules, additives and fillers to be incorporated. Thus, thermoplastics are generally swellable and soluble [12]. Extensive databases for these polymers are available from manufacturers and research, which are not reviewed here. The density ρ, friction and wear properties of the selected polymers, based on ISO 7148-2-A-dr-F4-U4 dry “pin-on-disk” tribometer experiments are shown in Table 2 [19-24]. To the best of the authors’ knowledge, there is no data for characterizing the friction and wear behavior of these polymers under typical oil-hydraulic operating conditions, measured in experiments with comparable geometries for piston pumps. Moreover, the impact of hydraulic fluids on the properties of these polymers is not well understood. This article aims to address these gaps by presenting insertion and tribometer tests of the selected polymers to evaluate their suitability for oil-hydraulic tribological contacts. Science and Research 7 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0002 Material (0.5 m/ s / 12.5 m/ s ) [ / ] [ mm 3 N m ⁄ ] CuZn40Al2Mn2Si 0.14 / 0.032 0.95 ∙ 10 − 10 CuZn30Al2Mn2Ni1FeSiSn 0.14 / 0.05 0.98 ∙ 10 − 10 Table 1: Sliding frictionand wear coefficient of special brass alloys Material [ / ] [ mm 3 N m ⁄ ] [g/ cm 3 ] -limit [ MPa ∙ m s ⁄ ] PEEK [19] 0.3-0.5 9,33 ∙ 10 − 6 1.31 0.33 PEEK-CF30 [20] 0.2 − 0.3 6.66 ∙ 10 − 7 1.40 - PEEK-HPV [21] 0.15 − 0.25 6.66 ∙ 10 − 7 1.45 0.66 PAI [22] 0.35 − 0.6 1.66 ∙ 10 − 6 1.41 0.32 PI [23] 0.25 − 0.5 1.0 ∙ 10 − 6 1.50 2.9 POM-H [24] 0.2 − 0.3 2.66 ∙ 10 − 6 1.50 0.26 Table 2: Dry sliding frictionand wear coefficient of different polymers on steel ing on the density ρ r of the sample liquid and the gravitational constant g (eqn. 1). eqn. 1 The pressure ∆p acts on the bottom surface area A 0 and leads to an additional weight force F g which is proportional to the measured mass m V (eqn. 2). eqn. 2 The volume of the sample material V s equals the change of liquid level multiplied by the difference of A 0 and the cross-section area A 1 of the support (eqn. 3). eqn. 3 2.2 Tribometer tests - friction and wear To investigate friction and wear, a specialized “Slipperon-Disc” Tribometer, shown in Figure 4 was developed, to observe a simplified slipper-swashplate contact at nominal contact pressures of up to 30 MPa and relative velocities of up to 15 m/ s. Five abstracted slippers, each with only one ring surface, with an outer diameter of 16 ± 0.05 mm and an inner diameter of 8 ± 0.05 mm are mounted in a slipper holder at a mean frictional radius of 40 mm. To mitigate tolerances, the slippers were surface grinded and lapped together when installed. There is no hydrostatic relief. Instead, the test is conducted fully immersed in approx. 0.7 l of HLP46 at a controlled fluid temperature of 40 °C. = r g g g = 0 = r 0 g s = ( 0 1 ) = 0 1 0 r Science and Research 8 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0002 2 Methodology and experimental setup The accuracy and the standard deviation of the used measurement instruments in their actual lineup, is given by Table 3. The instruments for the insertion test and the optical measurements were used at 23 °C ± 2 °C. A separate precision scale in a clean room classified as DIN EN ISO 14644-1 class 7/ 8 was used to determine wear masses. Optical inspections of test objects were conducted at x50 magnification, while roughness measurements were performed using a focus variation microscope at 100x magnification. 2.1 Insertion tests - media compatibility To investigate media compatibility, two fluids were chosen following DIN 51524-2: a mineral oil-based hydraulic fluid HLP46 (ρ: 0.878 g/ cm3) and a synthetic ester oil HEES46 (ρ: 0.922 g/ cm 3 ). According to DIN EN ISO 175: 2011-3 and DIN ISO 1817: 2016-11, 3 samples of each of the 6 polymers were fully immersed in 75 ml of fluid at 40 °C ± 2 °C. The samples sizes were Ø 18 x 5 mm, differing from the standard. After cleaning with isopropanol, hardness (ISO 48-4), mass, volume and optical appearance were assessed over 42 days at intervals specified in DIN EN ISO 175: 2011-3. Figure 3 illustrates the setup used for volume determination. The measurement setup comprises a vessel on a precision scale and a support anchored on the ground on one side, extending into the vessel on the other. Initially, the vessel is filled with a reference liquid of known density, and the scale is zeroed. Next, the sample material is placed on the support, causing the liquid level to rise (∆h) and thereby increasing the hydrostatic pressure ∆p, depend- Measured Value Measurements per Sample Accuracy/ Accuracy Class Standard Deviation 4 0.02 mg 0.03 mg 3 0.01 mg 0.016 mg H 5 0.24 Micro-Shore D continuously 0.2 % (HBM) continuously 0.03 % (Bosche) n 1024 ppr. ± 0.5 % - Roughness 5 0.01 μm 0.002 μm Measured Value Measurements per Sample Accuracy/ Accuracy Class Standard Deviation 4 0.02 mg 0.03 mg 3 0.01 mg 0.016 mg H 5 / 0.24 Micro-Shore D continuously 0.2 % (HBM) continuously 0.03 % (Bosche) n 1024 ppr. ± 0.5 % - Roughness 5 0.01 μm 0.002 μm Table 3: Key data of the used measurement instruments 1 2 g g A 1 A 0 vessel support ∆h Figure 3: Measurement setup for the determination of sample volume Load application is facilitated via an air bearing, connected to a force sensor. Relative velocity is determined by the rotational speed of then drive shaft, while friction torque is measured by a force sensor through the support of the rotor holder. The measurement of wear masses is sensitive to the slipper cleaning process, hence an ultrasonic cleaner with isopropanol is used. It was observed that the cleaning process is robust, but particles stored in the polymer due to contaminated test fluid can significantly affect measurements. The particle load in the test fluid was classified as 18/ 15/ 11 according to ISO 4406: 1999. Furthermore, the running surfaces of the slippers were optically examined at 50x magnification to exclude larger particles before weighing. Wear coefficients are calculated using eqn.4 incorporating the measured test time t, the nominal contact pressure p C , the relative velocity v r , the polymer density ρ, the contact surface area A C , and the wear mass m W according to Czichos [5]. eqn. 4 3 Experimental results This section first shows the experimental results of the insertion tests and then the results of the friction and wear tests, followed by a discussion and comparison in section 4. 3.1 Insertion test results The results of the insertion tests for HLP46 are shown in Figure 5 and the results for HEES46 in Figure 6. Polymer density changes were calculated from mass and vo- = lume measurements. Microscopic examinations at 20x, 50x, and 100x magnifications showed no color or morphological changes of the polymers for both fluids. For all polymers, volume increases in HLP at a maximum of 0.71 % while density decreases at a maximum of 0.89 %. These effects are more pronounced with higher initial polymer density. The PEEK derivatives therefore show the lowest swelling. The swelling of both filled and unfilled polymers is almost the same. Since the fluid’s density is lower than that of the polymers, HLP46 is likely stored in the polymer matrix. Over the 42-day measurement period, a decreasing gradient is observed, but saturation is not yet reached. The hardness increases by maximum approx. 1.3 % due to fluid storage. In HEES46, a maximum volume decrease of 0.66 % and a maximum density increase of 0.33 % are observed. Thereby PEEK shows the least shrinkage. PEEK filled with 30 % carbon fibers shrinks slightly less and has a lower density increase than unfilled PEEK. This suggests additives are released or the polymer itself dissolves. Despite the opposite change in volume, there is also a hardness increase of a maximum of about 1.6 % in HEES46. 3.2 Tribometer test results Based on the insertion tests and wear coefficients from the Table 2, PEEK and PAI in HLP46 were selected for tribometer testing. Before measuring friction and wear, the influence of surface treatment was investigated for natural PEEK, as shown in Figure 7. Before the tribometer tests, a run-in process was conducted until the roughness of the contact partners stabilized. In all experiments material transfer to the mating surface was observable. For a grinded pairing (1), the Science and Research 9 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0002 external cooling heat sink momentum support with force sensor air bearing normal force application + force sensor drive test fluid chamber temperature sensor angle compensation stator disk rotor slipper holder abstracted slipper conical press fitting F U n U T U speed sensor Figure 4: Structure of the “Slipper-on-Disk” Tribometer Science and Research 10 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0002 81 82 83 84 H [Micro-Shore D] -0.2 0 0.2 0.4 0.6 0.8 Volume Change [%] -1 -0.8 -0.6 -0.4 -0.2 0 0.2 Density Change [%] 0 10 20 30 40 Duration of Insertion [days] 81 82 83 84 H [Micro-Shore D] 0 10 20 30 40 Duration of Insertion [days] -0.2 0 0.2 0.4 0.6 0.8 Volume Change [%] 0 10 20 30 40 Duration of Insertion [days] -1 -0.8 -0.6 -0.4 -0.2 0 0.2 Density Change [%] Figure 5: Insertion test results in HLP46 mineral oil 81 82 83 84 H [Micro-Shore D] -0.8 -0.6 -0.4 -0.2 0 0.2 Volume Change [%] -0.4 -0.2 0 0.2 0.4 Density Change [%] 0 10 20 30 40 Duration of Insertion [days] 81 82 83 84 H [Micro-Shore D] 0 10 20 30 40 Duration of Insertion [days] -0.8 -0.6 -0.4 -0.2 0 0.2 Volume Change [%] 0 10 20 30 40 Duration of Insertion [days] -0.4 -0.2 0 0.2 0.4 Density Change [%] Figure 6: Insertion test results in HEES46 ester oil Ra=0.1514 Rz=1.0037 Ra=0.1546 Rz=0.8069 x50 x50 Disk grinded Slipper grinded (1) Disk grinded Slipper lapped (2) Ra=0.2671 Rz=1.2016 Ra=0.1322 Rz=0.7042 x50 x50 x50 x50 x50 x50 Ra=0.0868 Rz=0.4388 Ra=0.1763 Rz=0.9699 Ra=0.0655 Rz=0.3432 Ra=0.1398 Rz=0.7035 Disk lapped Slipper lapped Disk lapped Slipper grinded (3) (4) (1) (2) (3) (4) 0 2 4 6 8 10 K W [mm 3 / Nm] 10 8 6 4 2 0 x10 -8 K W [mm 3 / Nm] Figure 7: Wear depending on the surface treatment at 1 MPa and 5 m/ s contact overheats, resulting in an increased wear coefficient. In this test series, low roughness parameters and anisotropy due to lapping can reduce the wear coefficient by up to 81.8 % compared with grinded surfaces. Thereby a better finish of the harder mating surface has a greater impact. Figure 8 shows measurements of frictional coefficients for lapped contact partners. All polymers exhibit a typical Stribeck-curve, indicating significant hydrodynamics in the test. This results in very low friction at high speeds, with little variation between the polymers. However, even at low relative speeds, the friction coefficients are much lower than in Table 2. Fiber reinforcement in PEEK-CF30 slightly increases friction compared to natural PEEK in contrast to the dry “pin-on-disk” test, while the lowest friction values are achieved with graphite and PTFE reinforcement in PEEK-HPV. All polymers show decreasing friction with increasing load, which appears to follow a saturation curve. Figure 9 shows the wear coefficients of the polymers, while Figure 10 shows the corresponding slipper surfaces. Sorted by contact pressure, the tests were conducted in the order: 1.5, 0.5, 2.5, 1, 2, and 3 MPa. Due to the preceded run-in and the given sequence of tests, in Figure 9, the hydrodynamic effect also visible in Figure 8 is recognizable. For all polymers, the wear coefficient significantly decreases up to a relative speed of 7.5 m/ s and then remains roughly constant. PEEK natural has the lowest wear coefficient. For all PEEK polymers, the wear coefficient are two orders of magnitude better, than in Table 2. The surfaces show no signs of adhesion or overheating, with even abrasion traces. Also, on the disks surface no signs of overheating are recogniz- Science and Research 11 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0002 PEEK PEEK-HPV PEEK-CF30 PAI x x x x 0.023 0.017 0.025 0.022 x 0.139 x 0.149 x 0.086 x 0.117 0.5 0.5 p C , T=40°C Figure 8: Frictional coefficient depending on relative velocity and contact pressure x10 -8 x10 -8 x10 -8 x10 -8 PEEK PEEK-HPV PEEK-CF30 PAI 7.02·10 -9 8.01·10 -9 8.13·10 -9 3.59·10 -8 45 31.25 20 11.25 5 1.25 15 12.5 10 7.5 5 2.5 3 2.5 2 1.5 1 0.5 pv [MPa·m/ s] v r [m/ s] p C [MPa] 45 31.25 20 11.25 5 1.25 15 12.5 10 7.5 5 2.5 3 2.5 2 1.5 1 0.5 pv [MPa·m/ s] v r [m/ s] p C [MPa] Figure 9: Wear coefficients depending on the pv-product 4.2 Tribometer tests With the aid of the limiting pv-product of minimum 150 MPa m/ s in short term, with adequate cooling, and the wear coefficients of down to 8.01·10 -9 mm 3 / Nm, it can be shown, that polymers can generally be used in oil-hydraulic tribological contacts. The measured frictional coefficients of the selected polymers are up to 38.5 % lower for lowand up to 46.8 % lower for high speeds, than those of special brass alloys, which can significantly contribute to increase efficiency. However, the measured wear coefficients suggest that without changes to contact geometry and hydrostatic relief, long-term lifetime issues may arise, as they are approx. a factor 81 worse than those of special brass. Nevertheless, the results indicate that improved lubrication and cooling through hydrostatic relief could further reduce wear coefficients, despite hydrostatic pressure increases the absolute surface stress, influencing wear mechanisms. Li demonstrates this for an “pin-on-disc” tribometer and an axial piston pump under water lubrication [17]. To fully evaluate the potential of polymers in oil-hydraulic contacts, thus wear must be tested in a hydrostatically relieved contact. Due to the high contact deformation, this must be conducted on a real, geometrically adapted slipper to avoid local excess pressure. 5 Conclusion and outlook This publication shows that PEEK, PAI, and PI are suitable for use in mineral oil based on insertion tests. The Science and Research 12 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0002 able, despite significant material transfer. In fiber-reinforced variants, a graphite solid lubricant layer is visible, which breaks down at higher loads without increasing the wear coefficient. To determine the limiting pv-product, the pressure was increased by 0.25 MPa steps for 30 seconds each, starting from 3 MPa at 15 m/ s. In a dry test at 0.5 MPa and 2.5 m/ s, this duration was sufficient to wear the slippers down to their holder. In contrast, in the oil-lubricated tests, up to 150 MPa·m/ s no significantly increased wear could be detected. It is therefore concluded that the polymers can withstand this load, at least in the short term, with sufficient cooling. 4 Discussion 4.1 Insertion tests All tested polymers swell slightly under HLP46, with PEEK proving to be particularly resistant. This must be considered when designing the hydrostatic relief, as even less than one percent swelling is significant for precise contact balancing. The swelling saturation point still needs determination. An approach for application is to use polymers, that have already reached swelling saturation for precise contact manufacturing. Under these conditions, all tested polymers seem suitable for HLP46. However, since additives may be released or the polymer itself may dissolve in HEES46, more pronounced property changes are expected despite similar hardness changes. Long-term degradation under HEES46 should be monitored, as use in this fluid might be problematic. Ra=0.1240 Rz=0.6242 Ra=0.1359 Rz=0.7097 Ra=0.1376 Rz=0.7070 Ra=0.1300 Rz=0.6649 Ra=0.1417 Rz=0.6961 Ra=0.1802 Rz=1.0325 Ra=0.1259 Rz=0.6869 Ra=0.1337 Rz=0.7075 Ra=0.1362 Rz=0.7105 Ra=0.1760 Rz=0.8365 Ra=0.1587 Rz=0.8206 Ra=0.1285 Rz=0.6124 pv- Stage Polymer after finish PEEK PEEK−CF30 PEEK−HPV PAI x50 x50 x50 x50 x50 x50 x50 x50 x50 x50 x50 x50 Figure 10: Surface structures during wear tests limiting pv-product demonstrates that these polymers can withstand the loads in oil-hydraulic tribological contacts. They have significantly lower friction than special brass but higher wear. Further measurements on hydrostatically relieved contacts are needed for a final evaluation of their potential. Acknowledgement This publication was created as part of the research project “Validation of a simulation methodology for the design of additively manufactured polymer slippers for oil-hydraulic piston machines” (funding code SCHM 3289/ 18-1) in cooperation with the Exzellenzcluster 2186 “The Fuel Science Center”, both funded by the German Research Foundation DFG. The authors would like to thank the DFG for this support. References [1] Findeisen D., Helduser S., Ölhydraulik Handbuch der hydraulischen Antriebe und Steuerungen, 6 th Edition, Springer, Berlin, 2015, ISBN: 978-3-642-54908-3 [2] Wegner S., Experimental and simulative investigation of the cylinder block/ valve plate contact in axial piston machines, Doctoral Thesis, RWTH Aachen, Shaker, Reihe Fluidtechnik. D108, 2020, DOI: 10.18154/ RWTH-2021- 03796 [3] Gärtner M., Verlustanalyse am Kolben-Buchse-Kontakt von Axialkolbenpumpen in Schrägscheibenbauweise, Doctoral Thesis, RWTH Aachen, Shaker, Reihe Fluidtechnik. D97, 2019, ISBN: 978-3-8440-7182-5 [4] Pietrzyk T., Entwicklung einer Hochdrehzahl-Innenzahnradpumpe für die Elektrifizierung mobiler Anwendungen am Beispiel einer autarken dezentralen elektro-hydraulischen Achse, Doctoral Thesis, RWTH Aachen, Shaker, Reihe Fluidtechnik. D109, 2021, DOI: 10.18154/ RWTH- 2022-00767 [5] Czichos H., Habig K.-H., Tribologie-Handbuch Tribometrie, Tribomaterialien, Tribotechnik, 5 th Edition, Springer Vieweg, Berlin, 2020, ISBN: 978-3-658-29483-0 [6] Ivantysynova M., Ivantysyn J. Hydrostatic Pumps and Motors: Principles, Design, Performance, Modeling, Analysis, Control and Testing, 1 st Edition, Tech Books International, 2003, ISBN: 81-88305-08-1 [7] Schlegel F., Merkel A., Schmitz K., Experimental and Simulative Investigation of a Partially Hydrostatic Relieved Contact in Variable Speed Axial Piston Machines, Tribologie und Schmierungstechnik, 70/ 4-5, 2023, DOI: 10.24053/ TuS-2023-0023 [8] Manring N. D., Fluid Power Pumps & Motors Analysis, Design, and Control, 1 st Edition, McGraw-Hill Education, New York, 2013, ISBN: 978-0-07-181220-7 [9] Reetz B., Münch T., Challenges of novel lead-free Alloys in Hydraulics, Proceedings of the 12 th International Fluid Power Conference Vol1-Sym1, pp.17-25, Technische Universität Dresden, Dresden, 2020, DOI: 10.25368/ 2020.6 [10] Holzer A., Reetz B., Münch T., Schmitz K., Experimentelle Untersuchung zum Einlaufverhalten bleifreier Sondermessinglegierungen, Tribologie und Schmierungstechnik, 69/ 22-1, 2022, DOI: 10.24053/ TuS-2022-0003 [11] Blanke H.-J. et al., Expert Praxislexikon Tribologie Plus: 2010 Begriffe für Studium und Beruf, 2 nd Edition, Expert Verlag, 2000, ISBN: 3-8169-06915 [12] George S. C. et. al, Tribology of polymers, polymer composites and polymer nanocomposites, Elsevier Series in Tribology and Surface Engineering, 1 st Edition Elsevier, Amsterdam, 2023, ISBN: 978-0-323-90748-4 [13] Rokala M., Analysis of Slipper Structures in Water Hydraulic Axial Piston Pumps, Doctoral Thesis, Tampere University of Technology, Pub. 1055, 2012 [14] Schoemaker F., Murrenhoff H., Piston Slippers for Robust Water Hydraulic Pumps, Proceedings of the 11 th International Fluid Power Conference Vol1, pp.236-247, RWTH Aachen University, Aachen, 2018, DOI: 10.18154/ RWTH-2018-224460 [15] Li D. et al, Failure analysis on the loose closure of the slipper ball-socket pair in a water hydraulic axial piston pump, Engineering Failure Analysis, Vol. 155/ 107718, 2024, DOI: 10.1016/ j.engfailanal.2023.107718 [16] Li D. et al, The Difference in Tribological Characteristics between CFRPEEK and Stainless Steel under Water Lubrication in Friction Testing Machine and Axial Piston Pump, Lubricants, 11(4): 158, 2023, DOI: 10.3390/ lubricants11040158 [17] Guan Z. et al., Friction and Wear characteristics of CF/ PEEK against 431 stainless steel under high hydrostatic pressure water lubrication, Materials & Design, Vol. 196/ 109057, 2020, DOI: 10.1016/ j.matdes.2020.109057 [18] Nie S., Lou F., Yin F., Tribological Performance of CF- PEEK Sliding against 17-4PH Stainless Steel with Various Cermet Coatings for Water Hydraulic Piston Pump Application, Coatings, Vol. 9(7)/ 436, 2019, DOI: 10.3390/ coating9070436 [19-24] Mitsubishi Chemical Advanced Material Group, Product Datasheet of: 19 - Ketron 1000 PEEK, rev. 14.02.2024; 20 - Ketron CA30 PEEK, rev. 25.06.2024; 21 - Ketron HPV PEEK, rev. 22.11.2023; 22 - Duratron T4503 PAI, rev. 08.05.2024; 23 - Duratron DF7040G PI, rev. 19.01.2024; 24 - Ertacetal H-TF POM-H, rev. 25.06.2024 Science and Research 13 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0002