The formation of white etching layer (WEL) on rails and wheels is extensively studied in the last decades. The name is based on the appearance under light optical microscopy after etching with Nital, however, there are numerous variations observed in terms of microstructural characteristics and the formation is still under discussion. The formation process is commonly described as severe plastic deformation and/or thermal loading. WELs are often accompanied by so-called brown etching layers (BEL). The combination of WELs and BELs is also subsumed under the term "stratified surface layers". BELs and their formation process have been investigated in detail in recent studies. Stratified surface layers have been observed as sites for crack initiation. Thus, understanding the failure mechanism of stratified layers is of high relevance, as these layers may contribute to maintenance-relevant rolling contact fatigue (RCF) damage patterns.

Within this study, rail wheels that have been in service for approximately 200,000 km are investigated with respect to the formation of stratified surface layers, crack initiation, and fatigue crack propagation (Figure 1a). In parallel, stratified surface layers for two different rail wheel grade materials (ER7 and ER9) are created and tested in the lab under reproducible conditions by laser surface treatment using a diode laser system and by applying a rolling contact loading in a twin disc test rig.

Firstly, a heat-treated wheel material disc with a comparable microstructure to a new rail wheel is mechanically deformed by the twin disc tribometer. Secondly, thermal loadings are applied by two consecutive laser treatments with defined energy input, producing a stratified surface layer. Finally, twin disc tests are done, and the failure mechanisms of the stratified surface layers are investigated by detailed optical and scanning electron microscopy.

Results indicate that RCF cracks initiating at the surface of the WEL are mostly shallow, propagating parallel to the surface and leading to spalling. The most critical crack initiation and growth can be detected in the transition regions of the plastically deformed wheel material and the stratified surface layer. RCF cracks either propagate until the WEL-BEL interface or, under certain conditions, propagate into the BEL with a change in direction.

The work gives an insight in the failure mechanisms of stratified surface layers on rail wheel materials and proposes a promising methodology to imitate reproducible stratified surface layers under defined loading conditions helpful for future studies.