Objective Tribologically transformed structure (TTS) is an important characteristic structure, which is generated by the friction surface. More narrowly conceived, the TTS is usually regarded as a hardened layer formed on the surface of the tribo-pairs, which is a hardened layer with ultra-fine grains, high hardness and brittleness, and no obvious characteristics under optical microscopy, but not easily etched relative to the substrate. The TTS layer is also a typical damage characteristic induced by the rolling contact process of wheel-rail system. Its occurrence, development and evolution process have an important influence on the failure behaviors of wheel-rail interface, including surface wear, delamination, squat-type defect, fatigue crack initiation and propagation in the near surface. In the field of railway, white and brown layers are common tribologically transformed structure, which usually appear at the wheel-rail contact interface and involve changes in the microstructure of materials, and widely exist in various operating railway lines. Therefore, it is very important to carry out the research on the TTS layer to further reveal the wheel-rail interface damage.
Methods This work expounded on the typical characteristics and microstructure of the TTS, including white etching layer (WEL), brown etching layer (BEL) and plastic deformation layer (PDL), at the wheel-rail interface were analyzed from tribological perspective, and the differences and commonalities of the WEL and BEL were compared. The arguments and disagreements on the formation mechanism (mainly including the thermal action phase transition mechanism, plastic deformation mechanism, and thermal-mechanical mechanism) and current research status of the TTS at wheel-rail interface are discussed in detail. The mechanical behavior of the TTS layer was analyzed based on the results of key indexes, such as fracture toughness, hardness and elastic modulus. In addition, some innovative analytical and testing methods used by domestic and foreign scholars in recent years to characterize the TTS are also introduced. The influence of wheel-rail operating conditions on the TTS and the main contribution of the TTS induced to wheel-rail interface damage are summarized. Finally, the importance of the TTS in wheel-rail tribological research is reaffirmed and the development trend for the research of the TTS is also proposed.
Results The TTS at wheel-rail interface is an inevitable product in the wheel-rail service process, which is mainly composed of the WEL, BEL and PDL layers. It has a profound influence on surface wear, delamination, squat-type defect, fatigue crack initiation and propagation on the wheel-rail surface. At present, scholars at domestic and foreign still have many arguments and disagreements on the formation mechanism and action mechanism for the TTS. For the TTS, especially BEL, their occurrence, development and evolution process on the microstructure still need to be further discussed. The results show that the hardness of wheel/rail surface or TTS layer generally decreases with the increase of depth, forming a typical gradient structure. However, some researchers have different views on the variation of WEL or BEL single-layer hardness with depth, which may be related to the different railway operating conditions in different countries and regions. For example, the heavy-haul railways are mainly distributed in the United States and Australia, while the high-speed railways are located primarily in the Japan and Western Europe. The operating conditions (braking, axle load, creep, service environment, etc.) in wheel-rail rolling contact process have an important influence on the occurrence and development of the TTS. It has been proved that the initiation and propagation of microcracks in wheel and rail are closely related to the TTS, and the microcracks nucleation is usually caused by the brittle fracture of the TTS layer. The failure mode of wheel-rail interface is dominated by crack propagation behavior. Firstly, the crack initiates at the WEL edge and propagates along the WEL boundary. Secondly, the crack propagates vertically into the matrix along the core position of the WEL.
Conclusion At present, the understanding of the TTS at the wheel-rail interface is not sufficient and the further research is necessary. Therefore, the relevant researches for the TTS should be pay more attention by scholars. Focusing on the TTS research should become an important direction for wheel-rail researchers. There is no doubt that friction heat is an inevitable phenomenon during the friction process and plastic deformation is an important feature during the wear process, both of them play a key role in the formation of TTS. On this basis, we propose that it is necessary to understand the tribo-chemical effect and energy dissipation law between the friction interfaces during wheel-rail rolling contact. Meanwhile, we need to take the synergistic effects of tribo-chemical effect, plastic deformation and their respective contributions to the formation process of the TTS into consideration. With the innovation of research ideas and the progress of test technologies, new breakthroughs are expected to be made in the formation mechanism and action mechanism of the TTS in the future, which has important guiding significance for effectively controlling the failure of wheel-rail interface, extending the service life of wheel-rail and the safe and reliable operation of trains.
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