Study On Process And Performance Of High Coercivity NdFeB Magnets By Magnetron Sputtering(Dy/Pr)-Zn Film

With the development and progress of society,sintered NdFeB magnets possessing both high magnetic energy product and high-coercivity become the research hotspot in the field of functional materials.Using the traditional dual-alloy method,simple reduction of dysprosium would lead to the decrease of coercivity,thermal stability and corrosion resistance of the sintered Nd Fe B magnets.Thus,it has become a major and urgent challenge to improve the utilization of dysprosium,and the coercivity,thermal stability and corrosion resistance of the magne,t and reduce the heavy rare earths simultaneously.In this study,DyZn/PrZn films were coated on the magnet surface via grain boundaries diffusion(GBD)by magnetron sputtering to prepare magnets with high comprehensive performance.The GBD technology can greatly improve the coercivity,thermal stability and corrosion resistance of the sintered NdFe B magnets while its remanence basically keeps invariable.The results show that the optimal heat treatment process of as-sintered DyZn-coated magnets is 850 oC×5 h+500 oC×2 h,which can effectively increase the coercivity.Compared with the as-sintered magnets,the coercivity increases 747.44 kA/m,but with only a slight reduction in remanence.Dy element diffuses along the grain boundaries from surface to interior,and the concentration of Dy decreases with the increase of diffusion distance.At the diffusion distance of about 1800 μm,the average Dy element content is 2.26 %.During the diffusion,Dy favors to substitute for Nd in the outer region of Nd2Fe14 B grains,and forms(Nd,Dy)2Fe14B hard magnetic phase which has higher magnetocrystalline anisotropy.The formation of continuous and smooth Nd-rich phase improves the demagnetization coupling interactionbetween the grains.Moreover,there is only a slight loss in remanence since Dy element is not diffused into the internal main phase grains.The optimal heat treatment process of as-sintered PrZn-coated magnets is 750 oC×3 h+500 oC×2 h.The coercivity increases 353.18 kA/m,the remanence and the maximum magnetic energy product keeps stable and the squareness remains good by using this GBD technology.The main reason for the improved coercivity of as-sintered PrZn-coated magnets is that the formation of continuous and smooth Nd-rich phase and optimal distribution of chemical composition improve the demagnetization coupling effect between the grains as well as the magnetization reversal nucleation field.The low-melting point and non-heavy rare earth PrZn film performed by the GBD process requires lower temperature and less time for the optimal heat treatment process,which can save the cost of raw materials and production.A comparative study about the effect of two different diffusion sources of DyZn and PrZn films on magnetic properties,thermal stability and corrosion resistance is conducted.It is shown that the coercivity and thermal stability of as-sintered DyZn-coated magnets are obviously higher than that of as-sintered PrZn-coated ones.At temperature ranging from 20 °C to 180 °C,the αBr and βHcj of the original magnets,as-sintered DyZn-coated magnets and as-sintered PrZn-coated magnets are-0.1188 %/°C and-0.5533 %/°C,-0.1051 %/°C and-0.4815 %/°C,-0.1180 %/°C and-0.5533 %/°C,respectively.The GBD process can improve the corrosion resistance of the magnets.It is noted that the corrosion resistance of as-sintered PrZn-coated magnets is equivalent to that of as-sintered DyZn-coated magnets.After 288 hours corrosion test under HAST environment,compared with the initial as-sintered magnets,the mass losses of as-sintered DyZn-coated magnets and as-sintered PrZn-coated magnets decrease by 89.69 % and 93.33 %,and the magnetic flux losses decrease by 51.08 % and 36.27 %,respectively.For DyZn or Pr Zn film diffused magnets,Dy-Zn rich or Pr-Zn rich phase with higher corrosion potential form at grain boundary regions,which optimizes the intergranular microstructures and improves the coercivity,and this is the main reason for the improvement of corrosion resistance and high temperature stability of the magnets.