| In recent years, the domestic NdFeB magnets products in our country have been comparable to Japanese products in magnetic properties. However, there are still large gaps at the weather resistance of the magnet. The demand for automotive and cutting-edge technology of the magnet surface protection technology put forward higher requirements. Conventional surface protection techniques of magnet can not meet the requirements of corrosion resistance, mechanical properties and environmental profection for the high-end customers. In this work, cathodic electrodeposition, vacuum deposition and other environmental friendly ways were used for the preparation of new protective coatings. The relationship among the coating preparation, structure and performance were studied systematicly. Meanwhile, industrialization of this technique was promoted. The main research results are showing as follows:(1) The CeO2 nanoparticles enhanced epoxy resin composite coatings were prepared on the sintered NdFeB permanent magnets by cathodic electrophoretic deposition. The effects of CeO2 concentration on the micro structure and properties of the epoxy coatings were investigated by surface and cross-sectional morphologies observation, microhardness measurement, electrochemical test, neutral salt spray test and magnetic properties measurement. The results showed that the CeO2 particles were uniformly dispersed and embedded in the epoxy matrix when the CeO2 concentration was lower than 30 g/L., The amount of the nanoparticles embedded in epoxy matrix and the microhardness of the composite coatings increased with the increase CeO2 particle concentrations. At the same time, the neutral salt spray time of the coated samples prolonged to 480 h. The CeO2 nanoparticles can improve the microhardness and corrosion resistance of the epoxy coatings evidently without any deterioration on the magnetic properties of NdFeB substrates.(2) A Teflon (polytetrafluoroethylene) layer was coated on sintered NdFeB permanent magnet by dispersion spraying method for promoting the corrosion resistance property of the magnet. The results showed that the average microhardness of Teflon coating increased about 96.56% compared with epoxy resin coating. However, during curing process, poor melt flow and bigh melt viscosity of Teflon powder cause a lot of defects and pores in the coating, resulting in the rapid failure of Teflon coating. Thus, the corrosion resistance of Teflon coating is still slightly worse than that of traditional epoxy coating.(3) Zn and Al coated NdFeB specimens were prepared with different pretreating technologies, such as polishing, pickling (50 s), sandblasting and combined technology of sandblasting and pickling (5 s). The pretreating technologies possess obvious impact on the adhesive strength and anticorrosion property of Zn (or Al) coated NdFeB specimens. Compared with the pickling (50 s) pretreating technology, the combined pretreating technology of sandblasting and pickling (5 s) achieves the highest adhesive strength because of the pickling time was shorter and no loose layer produced on the substrate surface. Meanwhile, the Nd-rich phase in the grain boundary corners preferentially corroded resulting in the low electrochemical activity of the substrate surface. The Zn (or Al) coating can offer complete sacrificial anode protection due to its high electrochemical activity. Al coated NdFeB specimens with polishing pretreatments possess better corrosion resistance than that of samples with pickling, sandblasting and combined technology of sandblasting and pickling (5 s) because of high density of the Al coating. However, the industrial production process still lacks large-scale industrial production equipment of polishing. With lower damage to the substrate and the highest bonding strength between Zn (or Al) coating and substrate, combined technology of sandblasting and pickling (5 s) is a low damage, easy to industrial production and environmentally friendly pretreated technology before plating (of PVD).(4) An Al coating was achieved on NdFeB substrate via vacuum thermal evaporation method and the parameters of vacuum deposition process were optimizd. The results showed that the temperature of vacuum chamber possess no effect on the structure and corrosion resistance of Al coating. High temperature will lead to the increase of Al surface island structure in the film, resulting in the difficult control of Al coating thickness. Meanwhile, the Al coating on the surface of the substrate would adhesive with the Al layer deposited on the fixture. Therefore, the optimized temperature in vacuum champer is 50"C for Al coating deposition. The deposition rate of the Al film increases gradually with the increase of the temperature of the evaporation source. However, further extension of the deposition time, the Al film deposition rate decreases gradually. Trivalent chromium passivation treatment process was conducted for further improving the corrosion resistance of Al vacuum evaporation film. The original gaps between the Al thin films were filled by a passivation film. The NSS test oc passivated Al coating can reach 288 h, and the passivation process possess no influance on the structure and composition of the Al coating.(5) A dense Al coating was deposited on NdFeB substrate by plasma assisted physical vapor deposition method for enhancing the anticorrosion performances. The bias voltage during the deposition process was optimized according to the coating structures and anticorrosion performances. The density and flatness of Al coating increase with increasing the bias voltage, and the thickness decreases. The change of the coating density and thickness turns to be stable when the bias voltage reaches 1200 V. Also, the corrosion behaviors of the PA-PVD Al coatings in different mediums were studied. The self-corrosion potential of PA-PVD Al coatings keeps almost a constant in NaOH and HNO3 solutions with different concentrations. However, the self-corrosion current density decreases with the concentration of solutions. The self-corrosion potential shifts to negative direction and the self-corrosion current density decrease gradually when increasing the concentration of NaCl solution. Based on the corrosion behaviors of PA-PVD Al coatings in different mediums, the corrosion mechanisms are discussed.(6) The PA-PVD equipments were improved to achieve a uniform and compact PA-PVD-A1 coating on the surface of NdFeB magnet. The selfmade deplating solution can strip the Al coatings effectively. The NSS test, PCT test and moist heat test time of the PA-PVD-Al coating on the substrate can reach 120 h,168 h and 600 h, respectively. PA-PVD-Al coating possess excellent corrosion resistance than conventional metal platings. |
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