| With the booming development of the railway industry in China, the dynamic behavior and friction problem of rail-wheel system has very important effect on the traffic safety. Not only tribology problem of rail-wheel counterparts are the unsolved scientific question, it is also containing the significant meaning of industrial application. White etching layer (WEL) has been always considered as the material damage related to rolling contact fatigue. Due to WEL phenomenon observed randomly and occasionally in field, some of studies on WEL have been carried out with many arguments on its microstructure,phase transition mechanism and its effect on material fatigue. From the viewpoint of tribology and material science, in this thesis, the tribological-chemical behavior on the interfaces and the microstructure evolution near the worn surface of rail-wheel materials during the rolling contact friction have been investigated, to reveal the natural material characteristic of wear and fatigue damage, especially on the formation mechanism of white etching layer. The result of this thesis has an important significance and practical values on prolonging the service life of rail-wheel material used in the high-speed and heavy haul railways.In this thesis, rail-wheel rolling contact friction tests with different loads, rotational speeds and slippages were conducted on the experimental tester of MMS-2A, to investigate on the wear and damage behavior of the friction pair: U71MnG rail and ER8 wheel. Optical microscope(OM), scanning electron microscope (SEM), energy dispersive x-ray detector (EDX), X-ray photoelectric spectroscopy (XPS) and scanning white-light interferometry surface profile-meter were used to analyze worn scars and the chemical component distribution of worn surface. X-ray diffraction analyzer (XRD),Focus ion beam (FIB),Transmission electron microscope (TEM)and tribo-indenter in-situ nano-mechanical test system were used to character the chemical component and the microstructure of the white etching layer at the fixed sites. The main accomplished research contents and the chief conclusions are as followed.1. Parameters effect on the wear damage behavior during the process of rail-wheel rolling contact frictionBased on the friction counterpart of specimens made by the U71MnG rail and the ER8 wheel, the parameters (load, velocity, slippage) effects on the wear damage behavior of rail-wheel materials have investigated by considering wear loss, morphologies of worn surface,hardness and the microstructure evolution near the worn surface. The variation trend of hardness near the worn surface of rail-wheel material has been investigated under different loads,rotational speeds and slippages. Specifically, the effects of slippage on the wear and damage behavior during rolling contact friction have been investigated in detail. The result shows that the slippage parameter strongly effects on microstructural plastic deformation near the worn surface, and has significant influence on the wear mechanism transformation. As the slippage increasing, the wear mechanism is changed from oxidation wear to fatigue wear, accompanied with the gradually heavily abrasive wear.2. Study on the tribo-chemical behavior of rolling contact interfacesThe tribo-chemical behavior of rolling contact interfaces has hardly been reported. Quasi-rolling friction and rolling-sliding friction of rail-wheel material wear behaviors are greatly different. The smooth tribo-film including iron and iron oxides formed on the rail-wheel contact interfaces during rolling contact friction, which consists of Fe, FeO and Fe2O3. To investigate the tribo-chemical reaction mechanism of the rail-wheel rolling contact interfaces. The tribo-chemical reaction of the rail-wheel rolling contact interfaces is the tribo-oxidation reaction.Moreover, XPS results show that the ratios of different chemical atoms in these three zones(tribo-film zone, delamination zone, no-wear zone) interface, which indicated that the tribo-chemical behaviors are different. Specifically, the tribo-chemical reaction of no-wear zone tends to form the Fe2O3 rust, the tribo-film has the anti-oxidative ability, while the delamination zone can be oxidized to form a new tribo-film.3. Study on phase and microstructure evolution of layer near the worn surface -white etching layerIn term of the critical slippage, the tribologically transformed layer near worn surface can be formed with the phase and microstructural evolution,which usually can be called white etching layer.The microstructural morphology of white etching layer characterized by SEM indicated that the "white" appearance without the characteristic of pearlite is caused by the homogenous microstructure with the nano-scale size grains. White etching layer has the typical gradient structure with the gradual deformed microstructure. Specifically, the featureless microstructure near the shallow surface layer, it is contiguous to the granular characteristic microstructure and then deeply adjacent to the deformed pearlite microstructure. Meanwhile, the hardness at top surface layer is 12 GPa, and gradually decreases from the top surface to the inner matrix.The hardness of WEL is nearly three times than that of matrix. EPMA mappings of white etching layer show that the chemical elements distributions are inhomogeneous with local low content of carbon elements, which should result from the dissolve of cementite in pearlite. In addition, XRD pattern of WEL shows that the (110) peak of body-centered cubic (BCC) is broadened and shifted to the low angle, and the electron diffraction patterns of TEM show the polycrystalline rings in WEL zone. The results reveal that WEL consists of nanocrystalline ferrite, nanocrystalline body-centered tetragonal (BCT) martensite and a small part of broken cementite particles. Due to the absence of face-centered cubic (FCC) austenite, the mechanism formation of martensite in white etching layer can almost ignore thermal effects. Moreover, both the nano grain microstructure of WEL and the cementite dissolution that indicate the driving force to form the WEL and martensite in WEL is caused by the plastic deformation. Therefore,the formation mechanism of WEL is result of the plastic deformation mechanism during rolling contact friction. |
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