Science and Research 40 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0016 Introduction: brake fluid and challenges noise and wear Passenger cars and light commercial vehicles are equipped with hydraulic brake systems. A hydraulic medium transmits and converts the braking force from the brake pedal in the brake master cylinder into hydraulic pressure, which is routed via brake lines to the wheel brakes and there converted back into braking force. Vehicle-dynamics control systems such as the antilock braking system ALC or the electronic stability program ESP can additionally modulate the hydraulic pressures at each wheel to prevent the wheels from locking through pressure reductions or to stabilize the handling through additional pressure build-ups. Brake fluids must comply with stringent requirements to ensure reliable brake-system operation. Their properties are defined in various standard requirements which are very similar in terms of content (SAE J1703 [1], SAE J1704 [2], FMVSS 116 [3], ISO 4925 [4], JIS K2233 [5]). Exclusively for the vehicle operating medium brake fluid, the performance requirements of FMVSS 116 (Federal Motor Vehicle Safety Standard) became mandatory in the USA and often serve as a universal reference. In this standard, the US Department of Transportation (DOT) - comparable with other standards - has defined specific ratings for salient properties [7]. A list of criteria enables material compatibility (e.g. corrosion limits for metals, swelling limits of rubber parts (in case of polyglycol based brake fluids, EPDM is the preferred rubber material),…) and provide force/ pressure transfer function over a large application range (viscosity & boiling point requirements defining the brake fluid classes,…). Improvements in brake fluid standardization to avoid noise & wear problems Michael Hilden, Gerd Dornhöfer, Harald Dietl* Presented at the GfT Conference 2024 Brake fluids offer large temperature ranges of incompressible fluidity for dynamic braking performance. Modern brake fluids combine increased boiling points in “dry” (new fluid) and “wet” (used fluid after few years in vehicle) conditions with reduced cold viscosity to achieve minimized vapor lock risks with more dynamic pressure build up performances. However, reduced lubrication performance risks have been observed within the development of first modern brake fluids. Since sealing rings in braking systems are “lubricated” by brake fluid and form tribo-systems, lubrication performance of the brake fluid is essential in vehicle application and is currently integrated into brake fluid standardization. Automated driving and increasing noise requests due to electrification will further increase tribological requests for the pressure medium. In order to avoid future wear and noise problems, it is essential to develop and validate appropriate test methods to assess and prescribe lubrication performance of brake fluids. Within the project TriNoWe - funded by the Federal Ministry for Economic Affairs and Energy (BMWi) - the impact parameters, stress conditions and their dependencies based on a deep tribo-system analysis were identified to develop appropriate test methods. A valid noise lab test method - provoking the product behavior in a lab test - was developed as DIN 51834-5 and was already introduced as pass/ fail into international brake fluid standards. For wear, an appropriate test method based on identical test specimen and comparable tribometer use (but with incline setting, high temperature and longer load run time with load increasing starts and stops) - applicable after performed noise test - was developed as DIN 51834-6. It is in release phase by finished round robin tests to be also introduced into brake fluid standardization soon. Keywords Brake fluid, test method, lubrication, noise, stick-slip, friction, SRV, wear, surface conditioning Abstract * Dr.rer.nat. Michael Hilden Robert Bosch GmbH Robert Bosch Allee 1, D-74232 Abstatt Dr.rer.nat. Gerd Dornhöfer Robert Bosch GmbH Robert-Bosch-Campus 1, D-71272 Renningen Dr.rer.nat. Harald A. Dietl BASF SE Carl-Bosch-Strasse 38, D-67056 Ludwigshafen am Rhein The applications do also contain rubber (EPDM) sealing rings for moved metal pistons, e.g. in the master cylinder of the booster or in the pump elements of the electronic stability program unit modulating the brake pressure. However, no criteria for lubrication are contained in the brake fluid standards (beside a stroking test in FMVSS 116 not applied in other standards due to poor availability of old test specimen). Challenge noise Few brake fluids are known to provoke noise when applying the brake pedal by generating stick-slip oscillations. Such noise challenges typically disappear, if the brake fluid is changed to another type. Thus, brake fluid suppliers try to modify such brake fluids in order to avoid the noise challenges typically by performing noise tests in selected products already within fluid formulation design. For electrical vehicles without masking combustion engine, the noise level in the vehicle is reduced and the driver will have more awareness and notice noise challenges more disturbing. Thus, appropriate brake fluids have to avoid noise issues in field (see upper line in Figure 1). Challenge wear Modern brake systems shift the brake force application from the master cylinder (partly-) automatically to the electronic stability program, e.g. if sensors detect the need for vehicle deceleration. In brake fluid performance tests of a new modern brake fluid with an increased load scenario, wear of sealing rings of the pump elements was detected by different brake component suppliers in product tests. In order to avoid such wear issues in field, a certain minimum lubricity quality is required for the brake fluids - but currently not contained in the criteria list of the standards (see lower line in Figure 1). Targets of SAE & ISO TF brake fluid lubrication and public funded project TriNoWe The trends of electrification and automated driving underline the need for a minimum lubricity requirement of brake fluids. In order to assess this lubricity product-independent, appropriate lab tests have to be developed and validated in order to introduce them into the brake fluid specifications. Two active international committees for brake fluid standardization, SAE and ISO, did agree to integrate such criteria and did from the joint SAE & ISO task force brake fluid lubrication defining exactly these tasks in 2017. Many partners of the SAE & ISO task force did initiate the public funded project TriNoWe to focus on the tasks. Figure 2 presents the TriNoWe tasks on high flight level: For the two challenges noise (upper line) and wear (lower line), the product behavior (left block) has to be rebuild by simplification and abstractions (middle block) by basic lab tests moving rubber (in brake fluid applications EPDM (Ethylen-Propylen-Dien-rubber) against metal bodies (right block). Finally, product challenges have to be provoked in simplified lab tests. Therefore, the project TriNoWe -publically funded by BMWiidentified the impact parameters, stress conditions and their relationships based on a deep tribo-system analysis. Appropriate test bodies have been identified or developed for the lab tests, see right hand side of Figure 2. Science and Research 41 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0016 Figure 1: Challenges noise and wear with provoking trends to define fluid lubrication targets Science and Research 42 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0016 Noise lab test DIN 51834-5 Figure 3 shows basics of the developed noise lab test DIN 51834-5. In the upper line center, the test body scheme is shown as metal ball and rubber (EPDM) disc contact in the tribometer setting. On the right hand side, a noisy fluid test is shown and on the left hand side an ok and not noisy fluid test of the reference fluid RF ISO defined in ISO 4926 [6], identically applied as RM 66-07 by SAE as reference fluid (RF ISO = RM 66-07). Only difference in the test and pictures is the test fluid itself (no picture difference visible), but audible noises occur that are also measurable and assessable by stick-slip oscillations of the friction coefficient on the right hand side. Thus, within the developed setting, the system noise behavior is provoked in the tribometer test, measurable by stick slip oscillations of the coefficient of friction µ(t). Consistent to the audible noise with fluid RF 31, the stick-slip effect is also noticeable within the measured friction force signal (or friction value µ(t)) as severe oscillations in case of stick-slip as shown in Figure 4. In the left diagrams, where two cycles of the tribometer test for RF 31 (red, upper line) are compared to RF ISO (green, lower line). Oscillations occuring in µ(t) are mathematically evaluated by subtracting the filtered data obtaining Δ as remaining oscillations and determine their deviation as an appropriate measure σ(Δ). Figure 5 shows the high repeatability of the test results in a statistical study performed by Bosch: Three operators did test 10 test fluids (5 fluids RF ISO and 5 fluids RF 31 = RF G in double repetition, i.e. 10 ok sample tests and 10 not ok sample tests per operator). The high reproducibility is finally possible by selection of high quality metal ball surfaces (G5), valid EPDM sample Figure 2: Tasks of TriNoWe: develop valid lab tests for noise and wear reflecting product results Figure 3: Differentiation between not noisy and noisy fluid in the tribometer with identical set up y p μ( ) Science and Research 43 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0016 Figure 4: Measured differentiation between RF 31 (upper line) and RF ISO (lower line) and mathematical evaluation method σ(Δ) (left measurement, middle method, right complete test run results in 15 different working points) Figure 5: Bosch performed a statistical study of noise test DIN 51834-5 Figure 6: Results of R2TN: Round Robin Test Noise (averages of 7 international labs with sigma on y-axis as valid pass/ fail criterion) within a component endurance test at 100 °C, applied after observed wear with a selected brake fluid. The wear pattern of the sealing ring is measured by the change of the inner sealing diameter referenced in an optical diagram between original inner ring diameter and outer metal piston diameter. This test method was applied as component test at Bosch to assess the lubrication behavior of the brake fluids with respect to wear, but it takes a few weeks run time and high efforts and is product based. Figure 8 explains an essential difference between product and tribometer tests observed during TriNoWe: in the product, the sealing ring separates the fluid-side from the air-side and the wear is typically starting from the air-side. Thus, the root cause for the wear is combined with the missing availability of lubricating brake fluid in the contact. Science and Research 44 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0016 (DIK recipe according to Appendix A of DIN 51834-5) and its surface roughening as PDR, where a sharp metal tool harmonizes the EPDM surface of the test discs in dry condition before the fluid test. A round robin test noise (R2TN) was performed within the SAE&ISO TF brake fluid lubrication in collaboration with DIN 51834. The valid differentiation between noisy fluids (red points with high sigma and increased static friction coefficient (SFC, standard tribometer output) and not noisy fluids (green points with low sigma values) below the pass/ fail criterion 0,005 was confirmed by seven international lab results (see Figure 6). Wear lab test DIN 51834-6 Figure 7 explains the Bosch procedure to assess the wear behavior of brake fluids by applying a defined load Figure 8: Difference between tribometer and product wear tests Figure 7: Component test procedure for wear at Bosch, applied as assessment for brake fluids However, in the tribometer “point contact” without real sealing behavior, the availability of brake fluid is always given and never limited. Thus, the dominant wear mechanism is not rebuilt sufficiently within this test system. Modifications of the tribometer test towards component tests behavior were required. Bosch did propose to rebuild the separation between fluidand air-side in the tribometer by applying the test in incline settings of the tribometer with many starts and stops. Severe wear pattern could be provoked in such test settings already after short test times. Furthermore, the beginning of “lack of lubrication” can be measured in the tribometer by a jump in the friction force. Thus, the applied load cycles until this jump happens could be applied as simplified assessment criteria for wear. Figure 9 shows the valid and required enrichments of the tribometer test and the valid fluid differentiation. Essentially for the success was the development of an enhanced test bath as required adapter and a precise introduction of only 0.4 mL test fluid amount (yielding half EPDM disc covered in new test fluid bath). It also contains the important detail, that also further suppliers tested the wear reference fluid MTG with strong wear in their component tests. Figure 10 compares the measured friction data and wear profile of the EPDM discs. In the upper graph, the measured friction values jump between 0 (result delivered during breaks) and measured friction value resulting in a black area. The next graph below shows the 200 s averaged friction values as profile line, with its derivate in the graph below to detect a friction jump when exceed- Science and Research 45 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0016 Figure 10: Valid differentiation between wear ok reference fluid (left hand side) and wear reference MTG (right hand side) with friction jump detected already after 2.232 load cycles and significant wear depth on the EPDM test specimen Figure 9: Enrichment of tribometer test to rebuild wear behavior: incline position and start / stop integration on between wear and no wear fluids of 7 international labs with the results of the statistical study in the background in bright colors. Finally, system challenges noise and wear can be provoked by appropriate lab tests with transferable test fluid results as summarized in Figure 13. Figure 14 compares the test criteria and the repeatability of the noise (upper) and wear test (lower line) and the already achieved status for standardization on the left hand side. Since identical test specimen are applied for noise and wear test (both with identical EPDM preconditioning by “PDR”), it is valid to first perform the noise test and Science and Research 46 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0016 ing a slope limit after a certain number of load cycles to be evaluated during the test run. The wear profile has to be measured as cross section line across the EPDM disc over the center of the PDR window enabling a valid and reproducible measurement of the wear depth d wear as assessment size for pass/ fail criterion. Figure 11 shows the high reproducibility of the results by a statistical study performed by Bosch. Also for the wear test a high reproducibility was achieved: all 30/ 30 tests of ok reference RF ISO do not show wear without friction jump and all 30/ 30 of wear reference MTG do show wear with a friction jump and Figure 12 summarized the round robin test results with valid differentiati- Figure 12: Results of R2TW-2 Round Robin Test Wear (closed 08/ 2024) Figure 11: Analysis of statistical study of wear test DIN 51834-6 afterwards the wear test (cleaning and valid refilling of fluid amount preserved). Test specimen as DIK reference rubber and standard 10 mm diameter metal balls with G5 surface quality are defined in the standards DIN 51834-5 and -6 and are available long term without concerns and challenges of the test specimen of stroking test in FMVSS 116. Finally, the targets of the pfp TriNoWe and of SAE & ISO are reached and fulfilled. Conclusions The project TriNoWE developed a tribometric test procedure in order to assess the potential of brake fluids to generate friction-induced noises in EPDM-metal contacts by determining the potential fluctuations in the frictional response in a wide range of operating conditions. An appropriate EPDM surface conditioning approach is essential to map lab test and product results and to obtain a valid and robust test procedure disclosed as DIN 51834-5. The noise test is finished and already introduced into SAE & ISO brake fluid standards. It succeeds in provoking stick slip noise of product noisy fluids in a defined lab test procedure. The wear test DIN 51834-6 is in closure after performed statistical study and round robin test wear R2TW. Since Science and Research 47 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0016 Figure 14: Comparison of noise and wear test reproducibility and status of integration into brake fluid standards of SAE & ISO Figure 13: Success of develop valid lab tests for noise and wear DIN 51834-5 & -6 reflecting product results [6] ISO 4926: Road vehicles - Hydraulic braking systems - glycol-based reference fluid. [7] Hilden, M., Dietl, H.: Brake fluids, page 322-325 in: Kraftfahrtechnisches Taschenbuch, 2019, Springer Verlag, ISBN 978-3-658-23583-3 [8] Hilden, M., Robert Bosch GmbH: Future mobility and driving assistance: How to deal with new challenges for brake fluids? Evolution in Motion, BASF’s Fuel and Lubes Conference 2018, October 16-17, 2018. [9] Hilden, M.: Results & follow up of the pfp TriNoWe - methods to assess lubrication performance of brake fluids, Maschinenbau-Kolloquium Hochschule Mannheim, 17.05.2021 (online) [10] Hilden, M.: Public funded project TriNoWe: Development of NOise & WEar standard tests for brake fluids, Vortrag bei der Jahrestagung der Gesellschaft für Tribologie e.V., 09/ 2021, pp.49/ 1-49/ 13, ISBN978-3-9817451-6-0 [11] Hilden, M. and Dietl, H.: Improvements in brake fluid standardization to avoid noise & wear, Vortrag bei ATZ Chassis.tech plus 2022, 06.07.2022, München. Veröffentlichung im Tagungsband, ISBN 978-3-662-70347-2 Science and Research 48 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0016 wear is a much more severe failure mode compared to noise, it should be expected that a comparable reproducible test method for wear will also be introduced into SAE & ISO brake fluid standards as requested already in 2017 by charging the SAE & ISO TF brake fluid lubrication to develop the two appropriate lab test method for standard introduction. References [1] SAE J1703: Motor Vehicle Brake Fluid. [2] SAE J1704: Motor Vehicle Brake Fluid Based Upon Glycols, Glycol Ethers and the Corresponding Borates. [3] FMVSS 116: Federal Motor Vehicle Standard No. 116: Motor Vehicle Brake Fluids. [4] ISO 4925: Road vehicles - Specification of non-petroleum-base brake fluids for hydraulic systems. [5] JIS K2233: Non-petroleum base motor vehicle brake fluids.