A study on influence of lubricant chemistry on the formation of white etching cracks under sliding/rolling conditions

Abstract

Bearing failures due to White Etching Cracks (WEC) pose an ongoing challenge and present significant economic burdens for bearing manufacturers and users. WEC have been reported to provoke bearing failures often as early as 5 %  10 % of their estimated bearing life according to ISO 281. Despite efforts in research over decades in both academia and industry, a complete understanding of the root cause still escapes comprehension. This is likely due to the multidimensional drivers of this type of failure including material composition, contact mechanics, lubricant chemistry, and electrical influences. Existing Research indicates that the occurrence of WEC significantly depends on the lubricant chemistry. The complex formulations of lubricants used in this research complicates the analysis on interactions amongst lubricant constituents and their roles in WEC development. This study thus focuses on investigating the influence of simplified lubricant chemistry on WEC formation using an FE8 bearing test rig. Advanced techniques including scanning electron microscopy, energy dispersive X-ray analysis, secondary neutral mass spectrometry, time of flight secondary ion mass spectrometry and infrared reflection spectrometry have been used to analyse the surface and the sub-surface of the bearing components. To address the multidimensional drivers of WEC, the torque sensor on the test rig as well as mechanical modelling have been used to understand the impacts of a range of parameters involved in the experiment. The chemistry of the low reference lubricants has been significantly simplified, and WEC have been reproduced on an FE8 test rig using only PAO as a base oil and zinc dialkyldithiophosphate as an additive. Tests on the influence of lubricant chemistry have revealed that dicyclohexylamine, a common substance in the bearing industry and used as a corrosion inhibitor and gear oil additive, significantly accelerates WEC formation. Prior to the occurrence of WEC , nanopores with sizes in between 20 nm – 100 nm have been observed at depths of 80 µm and 140 µm underneath optical alterations on the bearing surface without any cracks or white etching area formation. Computational modelling by using the Tanaka-Mura equation showed that these pores significantly reduce the number of load cycles required for crack initiation. This suggests that pores lead to crack initiation under further continuous cyclic loading. This finding has led to the question of how lubricant chemistry can cause the formation of pores in the subsurface. The torque sensing results reveal that the torque responses during the early stage of the testing are increased by more than 30 % when bearings were lubricated by the “low reference” oil, leading to WEC formation, attributed to a patchy tribofilm formed on the bearing surface. In contrast, the ‘high reference’ oil PAO lead to a smooth conversion layer found as oxides. Hydrogen, identified by secondary neutral mass spectrometry is found to be present within the patchy tribofilm lubricated by the low reference oil. The presence of hydrogen within the bearing washers is not proven. The observation of local surface hardening on bearing washers prior to the emergence of WEC has led to an attempt to computationally investigate their effect. The results from the modelling of a hardened surface layer show that surface hardening can lead to an increase of subsurface stresses due to overrolling of these areas, which suggests a potential cause for pore formation. The influence of the FE8 test rig oil supply hose materials on WEC formation has been investigated for the first time following the observation of unexpected experimental results. Standard materials used in the FE8 rig and also similarly in various applications in the field were demonstrated to influence WEC development in combination with a specific lubricant chemistry. This result emphasizes the importance to widen the view not only to the tribocontact, but to the whole tribosystem. Based on the results of this study, necessary properties of additive chemistry are suggested to combat WEC formation under the prevalent conditions, and preliminary investigation on countermeasures has been conducted.

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ORCID for Ling Wang: orcid.org/0000-0002-2894-6784

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