Investigations of White Etching Crack (WEC) Formation under Rolling Contact Fatigue

A premature failure mode in steel rolling element bearings called white structure flaking (WSF) occurs in wind turbine gearbox bearings. WSF can occur in as little as 6-24 months of operation. WSF is associated with microstructural alterations called white etching area (WEA) and white etching cracks (WECs) that form in the ~1 mm zone beneath the contact surface. The tribological drivers for WSF are contested. Suggestions of drivers are hydrogen release and diffusion into bearing steel (sourced from the lubricating oil or water contamination) and transient operating conditions, e.g., load, vibrations and slip (see review paper [1] for details and references). Hydrogen has previously been found to diffuse into steel during RCF tests at concentrations between about 0.14.2 ppm.1 For this reason, to replicate this diffusion of hydrogen into the steel during rolling contact fatigue (RCF), hydrogen charging on test specimens/ bearings has been used in various WSF studies. Hydrogen entering steel during rolling contact has been suggested to occur by two mechanisms: (1.) through surface cracks where water contamination or lubricant enters surface cracks, allowing local release of hydrogen ions by tribochemical reactions at crack tip nascent surfaces. (2.) through wear induced nascent surfaces where hydrogen is generated by decomposition of lubricants through catalytic reactions and tribochemical reactions of water.1 The initiation and propagation mechanisms of WSF are contested. Surface initiation at cracks and subsurface initiation by inclusions or impact events have been suggested.1 This group has applied serial sectioning to map subsurface wear volumes of bearings from service and large scale test rigs2 and test specimens from laboratory,3-5 which has allowed investigation of initiation/propagation mechanisms of WSF and thresholds of WSF formation. Extensive FIB/TEM studies to enhance understanding of butterfly crack and WEA formation mechanisms has also been conducted.6