24th International Colloquium Tribology - January 2024 75 Effect of Phosphonium Ionic Liquid as Lubricant Additive in Gear Oil against White Etching Areas Formation in Bearing Steel Linto Davis and P. Ramkumar * Advanced Tribology Research Lab (ATRL) Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, India, 600036 * Corresponding author: ramkumar@iitm.ac.in 1. Introduction Wind energy is a renewable source of energy and a viable alternative to the current restricted supply of fossil-fuel-generated electricity. While there is a strong push to employ wind energy, there is a disadvantage in terms of the expense of maintaining wind turbines, notably the gearboxes. Bearing components frequently fail prematurely owing to contact fatigue due to microstructural decay, resulting in costly repairs and turbine downtime. These premature bearing failures are allied with the formation of white etching areas and white etching cracks in the bearing subsurface [1, 2]. Wind turbine operations are disrupted by premature bearing damage, which adds significant maintenance charges in the form of bearing up-tower replacement. Therefore, it is critical to advance research that will allow the creation of bearing components and lubricants less prone to white etching areas (WEAs)/ white etching cracks (WECs) failure. Bearing manufacturers and tribologists have made significant efforts in recent years to understand the causes of premature bearing failures and overcome this issue. To attain this purpose, several approaches can be adopted. Considering the industrial challenge of optimising the tribological behaviour of material pairs, one of the most promising ways is the production of high-performance gearbox oils with new additives. In the last decade, ionic liquids (ILs) have attracted attention in the tribology community as potential lubricants and lubricants additives for challenging contacts. The researchers have investigated two approaches to the use of ILs in lubrication: as pure lubricants or base stocks, which has the disadvantage of being more expensive than hydrocarbon oils, and as lubri-cant additives , which is a cheaper solution despite the low solubility of ILs in non-polar hydrocarbon oils. This work investigates the performance of phosphonium phosphinate IL additive in poly alpha olefin (PAO) against WEAs formation under dynamic loading and boundary lubrication regime. Further, the performance is compared with the base stock PAO based on the metallographic inspection and diffusible hydrogen measurement. 2. Experimental Methodology 2.1 Test materials The phosphonium cation-based ionic liquid, trihexyltetradecylphosphoniumbis (2,4,4-trimethylpentyl) phosphinate ([P6,6,6,14][(iC8)2PO2] CAS No: 465527-59-7, supplied by Sigma Aldrich is used in this work as lubricant additive in PAO 6. The AISI 52100 bearing balls (diameter 10 mm) and AISI 52100 washers were used as the counter surface. Posttest analysis were carried out on the test worn pin samples. 2.2 Dynamic load PoD tests The WEAs replication tests for the current research were performed in a dynamic load PoD tribometer. The dynamic PoD is created so that the compressive force and severe slippage (± 200%) of the bearing ball sample occur at the point of contact. The schematic diagram of the dynamic load PoD tribometers is shown in Figure 1. The test parameters are shown in Table 1. Figure 1: Schematic diagram of dynamic load PoD tribometer Table 1: Test parameters Parameter Value Maximum contact pressure 2.0 GPa Loading frequency 4.5 Hz Sliding velocity 0.2 m/ s Film parameter (λ) 0.2 Following the dynamic PoD tests, the tested bearing steel ball was sectioned through the wear scar into equal hemispherical halves and then subjected to polishing and nital etching. Further, the samples were analysed using optical microscopy, SEM and EDS for WEAs presence. For the validation of WEAs, microhardness tests were carried out on the WEAs and the surrounding matrix. Consequently, the tests are repeated to measure the amount of diffusible hydrogen ingressed into the samples. The tests were carried out with PAO oil till the formation of WEAs in the bearing steel samples. Afterwards, tests were repeated with optimised PAO+IL as the lubricant. Some of the preliminary results are discussed in the following section. 76 24th International Colloquium Tribology - January 2024 Effect of Phosphonium Ionic Liquid as Lubricant Additive in Gear Oil against White Etching Areas Formation in Bearing Steel 3. Result and Discussion 3.1 Subsurface metallographic analysis Figure 2 (a) shows the subsurface microstructure cut sectioned AISI 52100 steel ball sample in as-received condition. The microstructure consists of uniformly dispersed globular-shaped cementites in the tempered martensite matrix formed by the quenching of steel from the austenitisation temperature. Figure 2: (a) SEM image of the bearing steel subsurface microstructure in as-received condition showing uniformly spread spherically shaped cementites, (b) Test 1 SEM image: irregularly shaped WEA (c) and (d) OM image of Test 1: showing WEA Experiments were conducted using AISI 52100 steel on the dynamic load PoD tribometer until the formation of WEAs. After several trial-and-error tests followed by extensive metallographic analysis, the WEAs formation was observed in AISI 52100 steel pin samples after 13 hours (Test 1), corresponding to 2.1 × 10 5 load cycles. The microstructural analysis of the tested sample showed long irregular WEAs spreads more than 200 μm long, as shown in Figures 2 (b-d) OM and SEM images. The tests were repeated for the same duration with PAO+IL (Test 2) as the lubricant with the same test parameters. However, there was no microstructural alteration observed in the sample subsurface. Wear scar analyses and diffusible hydrogen testing were carried out to comprehend the delay in WEAs formation, while PAO+IL was used as the lubricant. 3.2 Wear scar and diffusible hydrogen analysis The amount of diffusible hydrogen content in the PAO+IL tested sample was less compared to the sample tested with PAO for the same test duration, as shown in Table 2. In addition, there was no formation of WEAs observed in the metallographic analysis. The presence of phosphorus content was observed in the EDS analysis and Raman analysis (not shown), as shown in Figure 3. Table 2: Test results Test No Number of load cycles (in million) Lubricant Diffusible hydrogen content (ppm) Observations 1 0.216 PAO 0.19 WEAs formation 2* 0.216 PAO+IL 0.05 No microstructure alteration * More tests havet to be conducted for the confirmation, and till the formation of WEAs Figure 3: SEM and EDS analysis result showing the presence of phosphorus in the tribofilm. 4. Conclusions The preliminary outcomes of this study are: • Diffusible hydrogen content in the PAO+IL tested sample is seen to be lesser than that of the PAO base stock. • The WEAs formation in the steel subsurface is delayed when PAO+IL is used as the lubricant. References [1] Richardson A D, Evans M H, Wang L, Wood R J K, Ingram M, Meuth B. The Evolution of White Etching Cracks (WECs) in Rolling Contact Fatigue-Tested 100Cr6 Steel. Tribol Lett 2018; 66: 1-23. https: / / doi. org/ 10.1007/ s11249-017-0946-1 [2] Evans M H. White structure flaking (WSF) in wind turbine gearbox bearings: Effects of “butterflies” and white etching cracks (WECs). Mater Sci Technol 2012; 28: 3-22. https: / / doi.org/ 10.1179/ 02670831 1X13135950699254