Page 185 - 《摩擦学学报》2021年第5期

774 摩 擦 学 学 报 第 41 卷

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.
Key words: wheel-rail tribology; tribological transition structure (TTS); white etching layer (WEL); brown etching layer
(BEL); plastic deformation layer (PDL)

摩擦是抵抗两接触副表面在外力作用下发生相 光学显微镜下无明显特征的硬化层 [2-3] . 一般，铁基材
对运动或运动趋势的现象，广泛存在于各类机械运动 料的TTS呈白色，而有色金属有时呈“暗色” [4-5] . 根据
中，它常伴随着材料的去除、能量的耗散以及表层材 应用场合不同常被冠以白层(White layer, WL)、白色
[1]
料微观结构的转变 . 摩擦学转变结构(Tribologically 浸蚀层(White etching layer, WEL)、TTS等不同名称.
transformed structure, TTS) 是摩擦表面产生的重要特 白层的概念最早始于1912年由Stead在钢丝绳上发现

[6]
征组织，狭义的TTS通常认为是摩擦副表面形成的一 后提出 . 迄今，关于白层的研究虽不少，但尚未取得
种相对于基体不易浸蚀、具有超细晶粒、高硬且脆，在 重大突破，尤其在白层的形成机制、对摩擦的贡献等