TLT: What is your background?
Rydel: I was always interested in science, mostly chemistry and physics, and how machines work. Therefore, when I went to university, materials engineering was an easy choice for me. At my university in Poland, there was, and still is, a strong electron microscopy group specializing in applying micro-analysis methods to materials science.³ That seemed to be a very cool and appealing branch of science to me. By the end of my master’s degree, I was allowed to join the group as an “apprentice” microscopist, and, due to kindness of all members of the group, I learned a lot in a relatively short period of time. When I started my doctorate on the influence of material on tribochemical performance of lubricating oils, it turned out that I could use a lot of my previous experience with microscopy. Although electron microscopy has proven to be less than perfect for what we were trying to do, I quickly learned atomic force microscopy and adapted it to the project’s needs. This is when I started to get involved with tribology. Unlike most tribologists, I had no background in mechanical engineering, but I realized I could bring a different, more materials- and micro-analysis-oriented perspective to the table. After finishing my doctorate, I continued working somewhere at the intersection between materials science and tribology, modeling the impact of non-metallic inclusions on fatigue performance of rolling element bearings. In parallel, I started consulting for Afton Chemical to help understand surface microstructure of novel materials used in industrial applications and its impact on tribological performance. Soon after that, I joined Afton as a full-time employee working on interesting projects requiring knowledge in both materials and tribology, such as understanding white etching cracks (WECs) in wind turbine gearboxes.

TLT: How long have you been working in the field of tribology?
Rydel: It is quite easy for me to pinpoint when my first actual contact with tribology occurred, as, before that, I was more focused on the microscopy side of materials science. I started learning about tribology at the beginning of my doctorate, so more than eight years ago. Hard to believe, as I still feel like a novice in this field, especially looking at the extent of knowledge of my colleagues at Afton, many of whom have been involved in this field for a couple of decades. I must say, by no surprise, I still feel most comfortable in the materials-related part of tribology and post-test microanalysis territory, as this is the field in which I have the most experience. Also, this is where I think I can make the most significant contributions to tribology.

TLT: What kind of tools do you use to study WEC?
Rydel: The most important thing for us is to be able to reproduce WECs in a well-controlled laboratory environment. This type of test also usually decreases the test duration allowing us to experiment more on factors influencing WECs, such as oil composition, mechanical parameters, etc. To generate WECs in the laboratory, we mostly use three-point rolling contact fatigue on the micropitting rig (MPR), as it allows great control of contact conditions under simplified geometry. This helps a lot, as WECs are already a complex enough problem. Simplifying geometry and mechanics of the system allows us to better understand the results we obtain. Often, as a component-scale test, we also use the FAG-FE8 rig. It uses a standard thrust bearing and is widely used in the industry to study WECs. Like the MPR, the test conditions are well controlled and allow easy detection of spalls caused by WECs, but due to slide/roll ratio (SRR) gradient across the contact line, results are more difficult to interpret and, as opposed to actual wind turbine gearbox bearings, the FE8 is purely thrust loaded.

Lab scale mechanical tests let us study the impact of different additives (and their combinations) on propensity of WECs formation, but we also actively work toward understanding the mechanism of WECs and ways in which additives affect this phenomenon. To achieve that, we employ a range of methods starting with small-scale tribometers to build correlations between the friction/wear properties of the oil and WECs.

Of course, we also use a wide range of analytical tools to examine physical and chemical changes in oil that occur during the test, as well as alterations in the material (for example, using electron microscopy) that may help us understand WECs mechanisms.


Where it all began for Rydel: The first analytical transmission electron microscope (TEM) that he learned to use and his best friend while writing his master’s thesis. Image courtesy of A. Gruszczynski, tem.agh.edu.pl.

TLT: Do lubricant additives affect WECs?
Rydel: Yes, definitely. We have confirmed that in studies we conducted in collaboration with the University of Southampton,^4-6 Argonne National Laboratory⁷ and SKF.⁸ In all these studies, we have observed different propensity for WECs formation depending on additives used. Certainly, WECs can be caused or accelerated by some specific surface- active additives. Other factors, such as rolling contact fatigue, surface stress or stray currents, are well known to be correlated with WECs,^{9, 10} but notice that also in these scenarios, the lubricant additive system may play an important secondary role. For example, conductivity of the oil is likely to have a large impact on WECs when stray currents are the root cause, whereas frictional properties of the oil will have a huge impact on WECs where surface shear stresses in the material are the main bad actor. Similarly, if we assume that hydrogen ingress into the steel is the mechanism of WECs, tribolayers deposited by additives would play a huge role, as some could form an effective barrier for hydrogen diffusion.

TLT: How are additives and WECs related? What is the mechanism?
Rydel: We don’t fully understand the relationship yet. I think the whole industry would like to know what the mechanism is. We have identified which additives contribute to WECs, but due to the complexity of the phenomenon, we do not have a comprehensive and fully verified theory yet. Nevertheless, I can tell you what my personal hypothesis is, and I will try to justify it.

In short: I think that WECs are caused by a combination of subsurface stresses and hydrogen embrittlement. My rationale behind this is as follows:
1. WECs occur at depths corresponding to maximum Hertzian shear stress and often nucleate at stress concentrators (such as non-metallic inclusions). Increasing load increases propensity for WECs. Therefore, WECs are most likely shear stress driven.
2. Some additives present in oil affect propensity for WECs. This effect cannot be explained by change in tribofilm morphology or friction, as often, “WECs” and “non-WECs” oil will show no differentiation in that aspect. In that case, the additive impact is most likely a purely chemical effect. If it is, then the chemical agent contributing to WECs must be able to diffuse into the high Hertzian shear stress zone. As maximum shear stress depth in the case of rolling element bearing is usually about tens to hundreds of micrometers below the surface, the chemical species must be able to cover that distance by diffusion within a reasonable time frame. The only species that could do so are interstitial atoms such as carbon, nitrogen or hydrogen. Carbon is abundant in bearing steels (as well as in hydrocarbon oil), and nitrogen is abundant in the atmosphere. Therefore, if it was one of these elements, we would see WECs much more often and would not note any impact of oil composition. On the other hand, hydrogen, or reactive species able to decompose into molecular hydrogen (for example, H2S), might be formed in lubricating oil in situ from certain additives. As hydrogen also is known to cause embrittlement in steels (and other metallic materials), it is the main suspect here.
3. A combination of these two effects (mechanical–shear stresses and chemical–hydrogen embrittlement) allows relatively easy explanation of many known root causes for WECs, such as presence of water (can react to produce hydrogen or cause hydrolysis of some additives), stray currents (may cause electrolysis), bad additive systems (potential source of reactive hydrogen), etc.

After developing this hypothesis for my own use, I have discovered that I am not the only person to come to similar conclusions.^{11, 12} Although it is reassuring that others also are thinking this way, more work is required to fully verify this hypothesis. 


Three-point rolling contact fatigue rig (micropitting rig) used for WECs testing.

TLT: Why are WECs still a problem for oil formulators? 
Rydel: This is a very interesting question—although I think that it requires some reframing. WECs are not as problematic as they were 10 years ago, when there was little understanding of the problem and contributing factors were not known. Now we can do laboratory screening tests, and formulators now know which components may cause WECs and should be avoided. Obviously, this limits the number of components available for formulating a gearbox additive system, but it is not anything new to formulators—they always must balance performance in many different areas at once. So yes, WECs are a problem in that sense, but additive industry is used to that type of problem, and formulators have already mastered optimizing performance by balancing different properties of the add-pack. On the other hand, WECs are a relatively new phenomenon, and, for that reason, testing procedures, as well as understanding of the failure mechanism, are not fully established yet. This is true, especially when comparing WECs to more “conventional” failure modes such as spalling or scuffing.

REFERENCES
1. Rydel, J. J., Vegter, R. H. and Rivera-Díaz-del-Castillo, P. E. J. (2016), “Tribochemistry of bearing steels: A new AFM method to study the material–tribofilm correlation,” Tribology International, 98, pp. 74-81.
2. Rydel, J. J., Pagkalis, K., Kadiric, A. and Rivera-Díaz-del-Castillo, P. E. J. (2017), “The correlation between ZDDP tribofilm morphology and the microstructure of steel,” Tribology International, 113, pp. 13-25.
3. www.tem.agh.edu.pl/
4. Richardson, A. D., Evans, M.‑H., Wang, L., Wood, R. J. K. and Ingram, M. (2018), “Thermal desorption analysis of hydrogen in non‑hydrogen‑charged rolling contact fatigue‑tested 100Cr6 Steel,” Tribology Letters, 66, Article number 4.
5. Richardson, A. D., Evans, M.‑H., Wang, L., Wood, R. J. K., Ingram, M. and Meuth, B. (2018), “The evolution of white etching cracks (WECs) in rolling contact fatigue-tested 100Cr6 steel,” Tribology Letters, 66, Article number 6.
6. Richardson, A. D., Evans, M. H., Wang, L., Ingram, M., Rowland, Z., Llanos, G. and Wood, R. J. K. (2019), “The effect of over-based calcium sulfonate detergent additives on white etching crack (WEC) formation in rolling contact fatigue tested 100Cr6 steel,” Tribology International, 133, pp. 246-262.
7. Gould, B., Demas, N. G., Pollard, G., Rydel, J. J., Ingram, M. and Greco, A. C. (2019), “The effect of lubricant composition on white etching crack failures,” Tribology Letters, 67, Article number 7.
8. Ruellan, A., Stadler, K., Rydel, J. J. and Ryan, H. (2020), “The influence of lubricant formulation on early thrust and radial bearing damage associated with white etching cracks,” Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 235 (5), pp. 1047-1059.
9. Evans, M. H. (2012), “White structure flaking (WSF) in wind turbine gearbox bearings: effects of ‘butterflies’ and white etching cracks (WECs),” Materials Science and Technology, 28 (1), pp. 3-22.
10. Evans, M. H. (2016), “An updated review: white etching cracks (WECs) and axial cracks in wind turbine gearbox bearings,” Materials Science and Technology, 32 (11), pp. 1133-1169.
11. Loos, J., Mangold, A., Blaß, T., Reiners, H., Suckfüll, T. and Goß, M. (2019), “Bearing currents as WEC-trigger in wind turbines,” CWD Conference.
12. Loos, J., Bergmann, I. and Goss, M. (2021), “Influence of high electrical currents on WEC formation in rolling bearings,” Tribology Transactions, 64 (4), pp. 708-720.

You can reach Kuba Rydel at kuba.rydel@aftonchemical.com.