| Metallic carbide, as a kind of hard phase, are used to improve the wear resistance of metal materials. Among them, chromium carbide have been paid more and more attention for its superior performance in high melting point, high hardness and excellent tribological properties. Electrical Discharge Coating Technology(EDC) is a new surface treatment technology developing on the basis of the traditional Electrical Discharge Machining Technology (EDM), and it makes use of heat pulsed discharge to melt tool electrode material and decomposition of the liquid medium and the workpiece so as to have complex physical and chemical reaction to form the strengthening coatings. Therefore, it can improve the surface properties of the workpiece. Previous studies focused on the preparation and properties of cermet coatings such as TiC while research on Cr-C surface modified layers by Electrical Discharge Coating Technology (EDC) is rarely reported from related literatures. The article studies the key process parameters of pulse width, pulse interval and peak current in order to investigate the influence of the Cr-C surface modified layer quality by orthogonal experiment. Optimize the discharge parameters based on orthogonal experiment, prepare the tool electrode by semi-sintered powder metallurgy methods and the Cr-C surface modified layers on the 45 steel substrate using the best process parameters.Organizational characteristics, phase structure and chemical composition of the modified layers have been analyzed and characterized by metalloscope, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The Vickers hardness tester, coating adhesion scratch tester, and CFT-I tribometer are used to investigate the mechanical properties and sliding wear characteristics of modified layers. Study on sliding wear characteristics of the modified layers at room temperature and high temperature and comparing with the modified layers manufactured by pure chromium powder tool electrode by semi-sintered powder metallurgy methods. The differences of wear properties between two modified layers prepared by two different composition tool electrodes have been investigated. The main conclusions show as follow:1、By analyzing the orthogonal experiment,the primary and secondary order of each factor of surface roughness, the surface hardness and wear resistance on preparation of surface modified layers has respectively been obtained. Optimal combination of parameters, which was obtained by Multiple Indicator balance method of orthogonal experiment, is the peak current 9A, pulse duration 8μs and pulse interval 100μs.2、Under the optimal parameter, the modified layers prepared by EDC mainly composed of Cr7C3 phase, as the same time, it also contains small amounts of Cr and Fe-Cr phase. Selected area electron diffraction patterns of Cr7C3 phases exhibit a range of bright discrete concentric diffraction rings, showing characteristics of a fine polycrystalline organizations and Cr7C3 grain is nanoscale size. The surface of the modified layers was smooth, the average thickness is about 20μm, the highest hardness is 1550HV and the hardness decreases from the surface to the substrate. Modified layers and the substrate exsited element diffusion and showed the metallurgical bonding state, the binding force up to 93.5N.3、By Electrical Discharge Coating Technology(EDC), prepare the Cr-C modified layers(45 steel as the substrate material) by two tool electrodes of different composition and compare the friction and wear with two modified layers. The results show as follow: (1) The COF of modified layer A keeps steady when the applied load is 30N. The wear mechanism is slight abrasive wear. However, the COF of modified B depicts a growing tendency with time. Local exfoliation presents on the surface of grinding crack. When the load rises to 50N, both of the COF and abrasion loss of layer A and B increase with the growing of load but the growing rate of layer A is obviously lower than layer B. The wear mechanism of modified layer A becomes plastic deformation and abrasive wear. Meanwhile, the wear mechanism of modified layer B is severe abrasive wear combing with partial exfoliation.(2)The COF of the two Cr-C modified layers obviously influenced by temperature.The results show that the COF of the modified layers drops with the increasing temperature. Modified layer A has the lowest COF at 600℃,only 0.10. The variation of the COF has a connection with the wear behaviour at high temperature of two Cr-C modified layers. For the modified layer A, slight abrasive wear is the main wear mode of modified layers at 300℃ while corrosion wear caused by the oxidation and mildly adhesive wear are the main wear modes at 450℃. Corrosion wear caused by the oxidation as well as deformation and loss of the oxidation film are the main wear modes at 600℃. For the modified layer B, abrasive wear is the main wear mode of modified layers at 300℃ while corrosion wear caused by the oxidation is the main wear modes at 450℃; Corrosion wear caused by the oxidation and adhesive wear are the main wear modes at 600℃. |
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