Moving cracks form white etching areas during rolling contact fatigue in bearings
The unpredictable failure mechanism of white etching crack (WEC) formation in bearing steels urgently demands in-depth understanding of the underlying mechanisms in the microstructure. WECs and the associated white etching areas (WEAs) cause severe microstructure decay during rolling contact fatigue which drastically reduces their expected lifetime.
To improve, we thoroughly investigated a 100Cr6 wind turbine gearbox bearing after failure in service operation . We used post-mortem microstructure analysis to reconstruct the mechanisms that lead to the formation of WECs and WEAs. As it is hardly possible to observe the failure mechanism in-situ due to the highly complex loading conditions in bearings, we gathered a number of correlated indirect proofs that all point towards the same crack movement theory.
Based on our experimental findings from scanning (SEM) and transmission (TEM) electron microscopy as well as from atom probe tomography (APT), we propose that WECs do not only move forward in terms of conventional crack propagation but also move normal to their crack plane. During cyclic loading the white etching crack continuously changes its position and leaves behind a severely plastically deformed area consisting of ferritic nano-grains, i.e. the WEAs. The atomic-scale delocalization of the crack plane in a single loading cycle adds up to micron-sized WEAs during repetitive loading and unloading. After the initial formation of a fatigue crack around inclusions, crack face rubbing occurs during compressive loading segments. This leads to the formation of WEA by local severe plastic deformation in a confined zone around the crack. Importantly, it also leads to partial cohesion of the abutting crack faces and atomic-scale transport of material from one side of the crack to the other. As a result, WECs open at a slightly shifted position with respect to the prior crack location after each loading cycle. The resulting crack movement consistently explains the formation of crack networks with adjacent WEA and also the creation of the typical “butterfly” structure around non-metallic inclusions in the early state of failure.
 L. Morsdorf, D. Mayweg, Y. Li, A. Diederichs, D. Raabe, M. Herbig, Mater. Sci. Eng. A 771 (2020) 138659