23rd International Colloquium Tribology - January 2022 67 Formation of White Etching Crack under sliding, boundary lubrication and additional current passage using modern lubricant compositions Daniel Cornel RWTH Aachen University, Institute for Machine Elements and Sytems Engineering, Schinkelstrasse 10, 52062 Corresponding author: daniel.cornel@imse.rwth-aachen.de Florian Steinweg RWTH Aachen University, Institute for Materials Applications in Mechanical Engineering, Augustinerbach 4, 52062 Francisco Gutiérrez Guzmán RWTH Aachen University, Institute for Machine Elements and Sytems Engineering, Schinkelstrasse 10, 52062 Georg Jacobs RWTH Aachen University, Institute for Machine Elements and Sytems Engineering, Schinkelstrasse 10, 52062 Adrian Mikitisin RWTH Aachen University, Central Facility for Electron Microscopy, Mies-van-der-Rohe Straße 39, 52074 Aachen, Germany Summary This contribution investigates the influence of modern lubricant formulations on the formation of White Etching Areas/ White Etching Cracks (WEA/ WEC). The main variation parameters are surface acting additives, especially extreme pressure-/ anti wear additives and corrosion inhibitors. Experiments using a thrust bearing and a radial bearing test rig indicate that modern lubricant formulations minimize WEA/ WEC formation risk under proven WEA/ WEC conditions such as high-sliding under boundary lubrication. In contrast, full fluid lubrication with additional current passage impressed on the aforementioned formulations can lead to WEA/ WEC. 1. Introduction Increasing power densities of drivetrains and drivetrains components also result in increasing demands on the associated lubricants and machine elements. Therefore, a wide range of testing methods is used to evaluate the performance of tribological systems. The wear and fatigue performance of a lubricant in rolling contacts can be, for example, evaluated using standarizered tests, such as the FZG Pitting test or the FAG FE8 Wear test. Among others, lubricants can also be evaluated regarding elastomer compatibility or foaming behaviour. The development of standardized tests for further damage patterns, such as white etching areas/ white etching cracks (WEA/ WEC), still requires a further understanding of the relying tribological mechanisms. In this context, the test guideline according to FVA 707 V forms the basis for the assessment of the tribological system “thrust bearings” regarding the risk of WEA/ WEC formation [1]. However, understanding the additive’s contribution to this type of damage and the influence of further parameters, such as current passage, needs to be further investigated. Lubricants consist of base oil and additives, with additional solubilizers being used depending on the base oil. The corresponding percentages, especially of the additives, vary depending on the aimed application, e.g. gearbox and engine oils in automotive applications have a much higher additive content or different additives than gearbox oils for wind applications. The influence of the oil formulation on the formation of WEA/ WEC has been discussed in a large number of studies [2, 3, 4, 5]. The majority of published research focuses on fully formulated oils or model oils, using WEA/ WEC-promoting additives. Especially the well-known Zinc-dialkyl-dithiophosphate (ZnDDP) and over based calcium sulphonate (Ob CaS) have been investigated regarding their influence on the formation of WEA/ WEC. However, these are additives that are no longer used in modern wind oil formulations. 68 23rd International Colloquium Tribology - January 2022 Formation of White Etching Crack under sliding, boundary lubrication and additional current passage using modern lubricant compositions Therefore, this contribution focuses on the influence of modern, i.e. without metal-containing additives, formulated lubricant formulations on the formation of WEA/ WEC. The main variation parameters are surface acting additives, especially extreme pressure-/ anti wear additives and corrosion inhibitors. Furthermore, each formulation also contains the same set of fixed additives, such as antioxidants, metal deactivators and defoamers. The investigations are carried out on component level using boundary lubrication and high sliding on the one hand and on the other hand full fluid lubrication with additional electrical loads. 2. Techniques and experimental methods In contrast to previous studies, no known WEA/ WEC critical oils are used here. All experiments have in common the usage of Polyalphaolefin (PAO) with Trimethylol propane tricaprylate (TMTC) as a solubilizer and a set of fixed additives. The base oil was blended with a viscosity grade of ISO VG 100 (ν 40°C ≈ 100 mm²/ s) and antioxidants, metal deactivators (Tolutriazol) and defoamers (Silicone) as fixed additives. Additives of the group’s extreme pressure/ anti-wear (EP/ AW) and corrosion inhibitors (CI) react significantly with the bearing surface and therefore influence the reaction layer formation and thus affect multiple tribological properties such as friction and permeability, e.g. to hydrogen. In the context of the presented investigations, sulphurised olefin (EP/ AW) and alkyl succinic acid half ester (CI) are used. The base oil formulation together with the fixed additives is later referred to as blend “0”. Adding the EP/ AW and CI additives mentioned before is referred to as blend “1”. Within the research project (c.f. Chapter 5), 14 further lubricant formulations were tested using for example dithiophosphate, aminphosphate and triphenyl phosphate as EP/ AW Additves and imidazoline, amid and neutral calcium sulphonate as CI. The rolling contact tests were performed using a thrust bearing and radial bearing test rig. The thrust bearing test rig (FE8) uses cylindrical thrust bearings of type 81212, and the radial bearing test rig (RBT) uses angular ball bearings of type 7206. Testing is carried out until either 500 h are passed or a vibration level, usually caused by spalling, surpasses a set threshold. Regarding the operating conditions, both test rigs have shown a lubricant sensitivity in the past and are operated under known WEA/ WEC critical conditions. While the investigations on the FE8 focus on boundary lubrication conditions [1], the RBT test are conducted in full fluid lubrication under additional current passage. Test bearings on both test rigs are operated at a bearing temperature of 100 °C and Hertzian pressure of ≈ 2.1 GPa. All further operating conditions are shown below (Table 1). Table 1: Test conditions Test Test rig Oil blend J s [A/ mm²] SRR [%] λ [-] 1 FE8 0 0 0 ± 12 0.5 2 FE8 1 0 ± 12 0.5 3 RBT 0 0.3 ≈ 0 ± 13* 2.3 4 RBT 1 ≈ 0 ± 13* 2.3 J s : Apparent bearing current density; SRR: Slide roll ratio; λ: Specific film thickness * Calculated for similar operating conditions in Arnaud Ruellan 2014 3. Results The test results, focussing on blend 0 and 1, are presented in Table 2. While Test 1 reached undamaged 500 hours, test 2 resulted in a premature failure after 109 h. Test 3 and 4 have reached 500 hours but with significant damage, due to an incorrect threshold setting. Regarding the 14 additional FE8 tests mentioned in chapter 2, no WEA/ WEC could be confirmed throught the conducted metallographic investigations. Table 2: Test results Test Test rig Runtime [h] Metallographic results 1 FE8 500 No damage 2 FE8 109 Pitting (Shaft washer) 3 RBT 500 Spalling & WEC (Outer ring) 4 RBT 500 Melted surface (Outer ring) To observe microstructural alterations and crack networks, metallographic investigations were conducted on all tests. After preparation, samples were etched in 3 % nital solution. The cross-sections of tests 1 and 2 (FE8) did not exhibit any WEA/ WEC. In contrast, test 3 (RBT) displayed WEAWEC under the outer ring’s raceway (OR, c.f. Figure 1). The inner ring (IR) & OR from test 4 exhibited a frosted surface attributed to the electrical discharges. The OR’s axial cross-section revealed melted layers on the OR’ surface, which appear white in light optical microscope (LOM, c.f. Figure 2). 23rd International Colloquium Tribology - January 2022 69 Formation of White Etching Crack under sliding, boundary lubrication and additional current passage using modern lubricant compositions Figure 1: Micrograph of test 3’s outer ring (OR). WEA/ WEC appeared in the axial cross-section. Figure 2: Micrograph of test 4’s outer ring (OR). The raceway surface showed melted layers in the axial cross-section. 4. Conclusion Initial results on the thrust bearing test rig, under proven WEA/ WEC inducing loads (see also FVA 707 V and VI), indicate that modern formulations minimize the risk of WEA/ WEC formation under boundary lubrication conditions with high-sliding. As so far, no WEA/ WEC-related failures could be generated in FE8 tests with the used formulations. In contrast those modern formulations lead to WEC or melted surfaces in tests on the RBT under full fluid lubrication and additional current passage. Further investigations are needed to determine if the results are solely related to the formulation and therefore to single additives. 5. Acknowledgments The research project FVA 707 VI (IGF-Nr. 20881) is supported by the Federal Ministry of Economic Affairs and Energy (BMWi). The authors would like to thank the FVA as well as the participating member companies for the support and the helpful advice. References [1] M. Linzmayer et al.: Interlaboratory comparison: Round robin test for the damage reproduction of white etching crack in cylindrical roller thrust bearings, Wear 480-481 (2021) 203925. https: / / doi. org/ 10.1016/ j.wear.2021.203925 [2] Gould, B. et al.: The Effect of Lubricant Composition on White Etching Crack Failures, Tribology Letters 67 (2019). https: / / doi.org/ 10.1007/ s11249- 018-1106-y [3] Richardson, A.D. et al.: The effect of over-based calcium sulfonate deter-gent additives on white etching crack (WEC) formation, Tribology International 133 (2019) 246-262. https: / / doi.org/ 10.1016/ j.triboint.2019.01.005. [4] Franke, J. et al.: Influence of Oil Formulation on White Etching Crack Formation Depending on WEC Main Mechanism, International Colloquium Tribology 2020, Technische Akademie Esslingen. [5] Manieri, F. et al.: The origins of white etching cracks and their significance to rolling bearing failures, International Journal of Fatigue 120 (2019) 107-133. https: / / doi.org/ 10.1016/ j.ijfatigue.2018.10.023.