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Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen (2021). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021, White etching cracks (WECs) are a characteristic premature failure phenomenon found in components that experience very high cycle rolling contact fatigue (> 109 cycles). The issue is most prevalent in bearings but in a similar form also affects other applications such as rails. The most notable WEC occurrences are related to failures in gearbox bearings of wind turbines. Most of the bearings affected are made from high carbon bearing steel 100Cr6 and similar grades used in a bainitic condition. WECs manifest in microstructural alterations that are called white etching areas (WEAs). They are nanocrystalline ferritic regions located directly adjacent to the cracks and extend several tens nm to several µm around WECs. Their white appearance in optical microscopy, which is caused by increased etching resistance compared to the unaltered material, is the defining feature of WECs. The current understanding is that cracks in large scale bearings initiate below the surface in several hundred µm depths at non-metallic inclusions, which are remnants of the steel manufacturing process. Once cracks have formed and propagate, the shear loading also leads to reciprocating sliding movements of the crack surfaces. This leads to severe plastic deformation, which causes decomposition of the initial microstructure resulting in nanocrystalline WEAs. What is currently lacking is a detailed understanding of the alterations at the nm scale and, most importantly, a mechanism-based explanation of the premature nature of WEC related failures. The present thesis aims at taking steps in this direction by contributing detailed microstructural analyses that aid in understanding the nature of WECs. To this end, a wind turbine gearbox bearing that failed due to WECs is investigated. Characterization techniques are used covering length scales from several mm to near-atomic distances. Analyses of the crystallographic structure are conducted using scanning and transmission electron microscopy techniques. Compositional analyses are performed employing spectroscopic methods such as X-ray spectroscopy and atom probe tomography. The results obtained show that, contrary to the expectation, the composition of WEAs does not equal the nominal alloy composition. In most cases, depletion in carbon is observed instead. Additionally, nanosized pure carbon deposits are found in WEAs with comparatively large grain sizes (several hundreds of nm). These results demonstrate that significant elemental redistribution occurs inside WEAs. The presence of pure carbon is in so far significant, as it renders the possibility of carbon being present at the crack surfaces. A consequence of this could be reduced friction that results in accelerated crack propagation. Furthermore, it was found that the varying grain sizes inside WEAs are corresponding inversely to the local carbon content. High carbon contents up to around ten at% stabilize grain sizes down to less than 10 nm. No evidence of carbide formation was found inside WEAs. Carbon atoms partition between grain interior and grain boundaries, both increasing as the total local carbon content increases. The presented investigations furthermore support the assumption that WEAs are always present on both sides of a WEC, even if the appearance on the two sides of cracks is strongly asymmetrical. This asymmetrical distribution is construed as an indication that cracks are a precondition for the formation of WEA. Structure and chemistry at and around interfaces of WEAs and bainitic matrix indicate that plasticity takes place there. This finding indicates that transformation into WEA proceeds from these interfaces into the base material. Structure and chemistry at and around the WEA-matrix interfaces show that decomposition occurs in a narrow region with a width of several tens to a few hundred nm. This confined nature is probably a consequence of cyclic loading. The smooth morphology of WECs flanks as compared to ‘conventional’ faceted fatigue cracks and even WEC tips indicates that crack surface rubbing is causing a smoothing of asperities. However, crack surfaces are faceted on the nanoscale and still exhibit puzzle-like fits, which is a strong indication that WECs partially reweld during combined compressive and shear loading and reopen during unloading. This mechanism is thought to cause material transfer leading to lateral crack displacement., Published by RWTH Aachen University, Aachen