Research On Construction Technology And Protective Mechanism Of Functional Coatings On The Surface Of Sintered Nd-Fe-B Magnets

In recent years,China has continuously made breakthroughs in the magnetic properties of sintered Nd-Fe-B permanent magnets.However,research on magnet protection lags behind the development of permanent magnet materials,to a certain extent this restrict the scope of use of sintered Nd-Fe-B magnet products.High end manufacturing industries such as new energy vehicles have put forward higher requirements for the mechanical properties,corrosion resistance,and environmental friendliness of magnets.Traditional surface protection technologies for magnets can no longer fully meet the market’s demand for the quality and diversity of permanent magnets.Therefore,developing new environmentally friendly and highperformance protective coatings remains an important research direction for sintered Nd-Fe-B permanent magnets.On the basis of the research on the pre-treatment process of sintered NdFe-B substrate,this paper constructs different functional protective coatings on the surface of magnets using various environmentally friendly vacuum coating technologies such as magnetron sputtering,multi arc ion plating,and chemical vapor deposition.The relationship between the microstructure and performance of the coating is systematically studied,aiming to provide new ideas for the development of high-quality protective coating technology on sintered Nd-Fe-B magnet surfaces.The main research contents and conclusions are as follows:(1)Four different processes,including pickling,sandblasting,mirror polishing,and vibration polishing,were used to pretreat the sintered Nd-Fe-B substrate before vacuum coating.The effects of pre-treatment processes on the microstructure and properties of the magnet surface and coatings were systematically studied.The results show that sandblasting and pickling pre-treatment processes cause significant damage to the substrate,and pickling processes cause particularly severe corrosion of RE-rich phases at grain boundaries of magnets.After depositing Alcoatings on different pretreated substrates,the coating on the surface of the substrate pretreated by mirror polishing is densest,with the best corrosion resistance,followed by the sample polished by vibration polishing.Due to the difficulty in achieving mirror polishing technology in industrial substrate pretreatment production,this article for the first time developed a new vibration polishing pretreatment process for vacuum coating of sintered Nd-Fe-B magnets.Although the roughness of the sample after vibration polishing is slightly higher than that of the mirror polished sample,it has the characteristics of better film/substrate bonding force,less damage to the substrate,strong environmental protection,and excellent comprehensive performance.The vibration polishing pre-treatment process is a low-damage,environmentally friendly,and easy to industrialize high-quality sintered Nd-Fe-B magnet vacuum coating pre-treatment process.(2)The corrosion mechanism of magnetron sputtered Alcoating on the surface of sintered Nd-Fe-B magnets was studied,and it was proposed a corrosion model about Al/Nd Fe-B magnets in 3.5wt.% NaCl solution.The corrosion process is divided into four stages,and the role of the Alcoating varies in different corrosion stages.The paper compares the microstructure and protective performance of Alfilm layers prepared by multi arc ion plating and magnetron sputtering.This paper also explored the protective performance improvement techniques of Alcoatings: passivation and Parylene sealing.Research has found that the multi arc ion plating Alcoating is relatively rough and has many defects in the coating resulting in weaker protective effects than the sputtering Alcoating.Passivation can reduce the gaps between grains in the coating,thus improving the density of the coating,and achieving a significant improvement in the anti-corrosion performance of the Alcoating.After passivation,Alcoating of 5.5 μm can withstand neutral salt spray test for up to 360 hours.The combination of Alcoating and Parylene polymer produces a synergistic effect,and the sintered Nd-Fe-B magnet with Al/Parylene coating can withstand a neutral salt spray resistance test for over 720 hours.(3)Hard TiN coatings were prepared on the surface of sintered Nd-Fe-B magnets using multi arc ion plating and magnetron sputtering,respectively.The phase composition,surface properties,and mechanical properties of these two coatings were systematically analyzed and compared.Research has shown that although the multi arc plating TiN coating has the same phase composition and similar surface properties as the sputtered TiN coating,there are significant differences in the microstructure of the two.As a result,the microhardness,film/substrate bonding force,and wear resistance of the multi arc plating TiN coating are superior to those of the sputtering TiN coating.This paper also explored the improvement effect of Al/TiN composite coating and TiAlN coating on the performance of TiN coating.Research has found that the bonding force between the coating and substrate of Al/TiN coating is much greater than that of a single-layer TiN coating,due to the presence of a base Allayer.The hardness and wear resistance of TiAlN coating are also improved compared to single-layer TiN coating.In addition,the corrosion resistance of both has been improved.However,overall,the corrosion resistance of the hard coatings in this experiment is weaker than that of the Alcoating.To further improve its corrosion resistance,it is also necessary to increase the thickness of the coating or develop relevant technology to improve the anti-corrosion performance of the coating.