The actual service life of wind turbine gearboxes is often well below the desired 20 years. One of the prevalent failure modes in gearbox bearing raceways is white structure flaking (WSF) in as ...little as 6-24 months of operation by the formation of axial cracks and white etching cracks (WECs) with associated microstructural change called white etching areas (WEAs). Despite these failures having been observed for two decades in various industries, the drivers and mechanisms for their formation are still highly contested. Discussed in this review are methods for searching and analysing WECs, mechanisms for WEA microstructural change, WEC initiation and propagation theories, WSF formation drivers and finally technologies and processes offering resistance to WSF. This updated review serves as a recap, comprehensive update on findings, current focus areas and remaining challenges.
This paper is part of a Themed Issue on Recent developments in bearing steels.
The formation of white etching cracks (WECs) in steel rolling element bearings can lead to the premature rolling contact fatigue (RCF) failure mode called white structure flaking. Driving mechanisms ...are still debated but are proposed to be combinations of mechanical, tribochemical and electrical effects. A number of studies have been conducted to record and map WECs in RCF-tested samples and bearings failed from the field. For the first time, this study uses serial sectioning metallography techniques on non-hydrogen charged test samples over a range of test durations to capture the evolution of WEC formation from their initiation to final flaking. Clear evidence for subsurface initiation at non-metallic inclusions was observed at the early stages of WEC formation, and with increasing test duration the propagation of these cracks from the subsurface region to the contact surface eventually causing flaking. In addition, an increase in the amount of associated microstructural changes adjacent to the cracks is observed, this being indicative of the crack being a prerequisite of the microstructural alteration.
The actual service life of wind turbine gearboxes is often well below the desired 20 years. One of the prevalent failure modes in gearbox bearing raceways is white structure flaking (WSF) by the ...formation of butterflies and white etching cracks with associated microstructural change called white etching areas. Despite these failures having been observed for two decades in various industries, the detailed reasons and mechanisms for their formation are not fully understood. In this review, white etching area formation mechanisms are discussed, specifically grain refinement, and effects of carbon/carbide in a range of bearing steels of widely differing carbon content. The review also highlights the severe transient, cyclic loading and tribochemical operating conditions of gearbox bearings and explains how these may act as drivers to produce WSF. Much previous research has focused on the detrimental effects of hydrogen, but other work suggests that hydrogen is not the only cause for WSF. Possible methods for preventing WSF are discussed, with attention paid to special steels such as high chromium steels, low carbon stainless nitrogen alloy steels and carbonitrided steels. Beneficial compressive residual stresses, surface coatings and enhanced lubrication and additive packages are shown to offer degrees of prevention, although the mechanisms leading to improvements are not fully understood.
We demonstrate a quantum nanophotonics platform based on germanium-vacancy (GeV) color centers in fiber-coupled diamond nanophotonic waveguides. We show that GeV optical transitions have a high ...quantum efficiency and are nearly lifetime broadened in such nanophotonic structures. These properties yield an efficient interface between waveguide photons and a single GeV center without the use of a cavity or slow-light waveguide. As a result, a single GeV center reduces waveguide transmission by 18±1% on resonance in a single pass. We use a nanophotonic interferometer to perform homodyne detection of GeV resonance fluorescence. By probing the photon statistics of the output field, we demonstrate that the GeV-waveguide system is nonlinear at the single-photon level.
Premature wind turbine gearbox bearing failures in the form of white structure flaking (WSF) can occur in as little as 6–24 months of operation. WSF is not fully understood but is thought to be due ...to hydrogen release and diffusion into the bearing steel and/or transient operating conditions not fully understood. The initiation mechanisms of white etching cracks (WECs) are contested, where amongst others mechanisms, subsurface initiation at non-metallic inclusions (perhaps associated with extension of butterfly cracks) and surface crack initiation are cited. For the first time this study applies serial sectioning to map WEC networks in wind turbine gearbox bearings to elucidate WEC initiation mechanisms. A comparison is made between WEC data for inner rings of an industrial transient test gearbox bearing and a planet bearing that spalled in service. It is proposed that one mechanism of WEC formation in wind turbine gearbox bearings is due to subsurface WEC initiation from inclusions, either in a butterfly manner or non-butterfly manner; where these small WECs link together to form larger WEC networks, these eventually propagating to the surface resulting in WSF. Small size/length inclusions were found to be likely WEC initiators, therefore the data suggests that steel cleanliness standards analysing inclusion density (as opposed to maximum inclusion lengths) are more relevant in understanding butterfly/WEC initiation in wind turbine gearbox bearings. However standards used should be able to differentiate pure sulfides from sulfides+oxide encapsulations and record inclusions that are only a couple of mircometer’s in length/diameter.
Hydrogen diffusion during rolling contact fatigue (RCF) is considered a potential root cause or accelerator of white etching cracks (WECs) in wind turbine gearbox bearing steels. Hydrogen entry into ...the bearing steel during operation is thought to occur either through the contact surface itself or through cracks that breach the contact surface, in both cases by the decomposition of lubricant through catalytic reactions and/or tribochemical reactions of water. Thermal desorption analysis (TDA) using two experimental set-ups has been used to measure the hydrogen concentration in non-hydrogen-charged bearings over increasing RCF test durations for the first time. TDA on both instruments revealed that hydrogen diffused into the rolling elements, increasing concentrations being measured for longer test durations, with numerous WECs having formed. On the other hand, across all test durations, negligible concentrations of hydrogen were measured in the raceways, and correspondingly no WECs formed. Evidence for a relationship between hydrogen concentration and either the formation or the acceleration of WECs is shown in the rollers, as WECs increased in number and severity with increasing test duration. It is assumed that hydrogen diffusion occurred at wear-induced nascent surfaces or areas of heterogeneous/patchy tribofilm, since most WECs did not breach the contact surface, and those that did only had very small crack volumes for entry of lubricant to have occurred.
Butterflies are microscopic damage features forming at subsurface material imperfections induced during rolling contact fatigue (RCF) in rolling element bearings. Butterflies can lead to degradation ...of the load bearing capacity of the material by their associated cracks causing premature spalling failures. Recently, butterfly formation has been cited to be related to a premature failure mode in wind turbine gearbox bearings; white structure flaking (WSF). Butterflies consist of cracks with surrounding microstructural change called ‘white etching area’ (WEA) forming wings that revolve around their initiators. The formation mechanisms of butterflies in bearing steels have been studied over the last 50 years, but are still not fully understood. This paper presents a detailed microstructural analysis of a butterfly that has initiated from a void in standard 100Cr6 bearing steel under rolling contact fatigue on a laboratory two-roller test rig under transient operating conditions. Analysis was conducted using focused ion beam (FIB) tomography, 3D reconstruction and transmission electron microscopy (STEM/TEM) methods. FIB tomography revealed an extensive presence of voids/cavities immediately adjacent to the main crack on the non-WEA side and at the crack tip. This provides evidence for a void/cavity coalescence mechanism for the butterfly cracks formation. Spherical M3C carbide deformation and dissolution as part of the microstructural change in WEA were observed in both FIB and STEM/TEM analyses, where TEM analyses also revealed the formation of superfine nano-grains (3–15nm diameter) intersecting a dissolving spherical M3C carbide. This is evidence of the early formation of nano-grains associated with the WEA formation mechanism.
Many global environmental agendas, including halting biodiversity loss, reversing land degradation, and limiting climate change, depend upon retaining forests with high ecological integrity, yet the ...scale and degree of forest modification remain poorly quantified and mapped. By integrating data on observed and inferred human pressures and an index of lost connectivity, we generate a globally consistent, continuous index of forest condition as determined by the degree of anthropogenic modification. Globally, only 17.4 million km
of forest (40.5%) has high landscape-level integrity (mostly found in Canada, Russia, the Amazon, Central Africa, and New Guinea) and only 27% of this area is found in nationally designated protected areas. Of the forest inside protected areas, only 56% has high landscape-level integrity. Ambitious policies that prioritize the retention of forest integrity, especially in the most intact areas, are now urgently needed alongside current efforts aimed at halting deforestation and restoring the integrity of forests globally.
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