Effects of bias voltage on coating structures and anticorrosion performances of PA-PVD Al coated NdFeB magnets

Al coated NdFeB magnets were prepared by plasma-assisted-physical-vapor-deposition(PA-PVD)method fo r enhancing the corro sion resistance.Mo rphologies and structures were characterized by an Xray diffractometer and a scanning electron microscope.Corrosion behaviors of the samples with different bias voltages were studied by electrochemical methods and neutral salt spray test,respectively.The bias voltage during the deposition process was optimized according to the coating structures and corrosion resistances.The density and flatness of Al coating increase with increasing the bias voltage,and at the same time the thickness decreases.The coating density and thickness turn to be stable when bias voltage is 1800 V.Also,the corrosion behaviors of PA-PVD Al coatings in different media,such as NaOH,HNO_3 and NaCl solutions,were studied.The self-corrosion potentials(E_(corr)) of PA-PVD Al coatings almost keep constant in NaOH and HNO_3 solutions with diffe rent concentrations.However,the self-corrosion current densities(J_(corr)) decrease with the increasing concentrations.E_(corr) shifts to negative potential and J_(corr)decreases gradually when increasing the concentration of NaCl solution.The corrosion mechanisms of Al coatings are discussed based on the corrosion behaviors in different media.     

Preparation and characterization of sodium silicate/epoxy resin composite bonded Nd-Fe-B magnets with high performance

As an organic binder for bonded Nd-Fe-B magnets, epoxy resin(EP) has poor heat resistance but good moisture resistance, while sodium silicate(SS) has poor moisture absorption but better heat resistance and corrosion resistance. In order to improve high temperature stability and decrease moisture absorption of bonded Nd-Fe-B magnets, EP/SS composites were applied as the binder to prepare bonded Nd-Fe-B magnets. The magnetic properties, moisture absorption, corrosion resistance, compressive strength and microstructure of composite bonded magnets were investigated. The results show that EP/SS bonded magnets can obtain excellent magnetic properties at room temperature, and even useable magnetic properties a thigh temperature environments at 200°C. EP/SS composite binder effectively improves heat resistance and corrosion resistance of bonded Nd-Fe-B magnets, and reduces the hygroscopic properties. The molecule of sodium silicateis rigid and keeps it original shape at high temperature environments. In addition, SS in composite binder improves the mobility of the magnetic powders during the pre-pressing process, which makes the magnetic powders attain a more regular structure. These two factors will increase the mechanical properties. Moreover, sodium silicate in the composite binder can also cover the surfaces protecting the magnetic powders from oxidation and corrosion. EP in composite binder can cover SS surface to reduce the water absorption of SS as epoxy is a hydrophobic material. The EDX analysis shows that the composite binder has accumulated in the gaps of the magnet powders, which not only improves heat resistance and corrosion resistance, but also increases the mechanical properties. Therefore, EP/SS composite binder endows bonded Nd-Fe-B magnets excellent comprehensive properties.     

Microstructure improvement related coercivity enhancement for sintered NdFeB magnets after optimized additional heat treatment

The microstructure, especially the Nd-rich phase and the grain boundary, in sintered NdFeB magnets plays an important role in magnetic reversal and coercivity mechanism. To better understand the effects of the microstructure on the coercivity, we investigated the microstructure and properties improvements of a commercial sintered NdFeB magnet after optimized additional heat treatment. The coercivity is enhanced from 1399 to 1560 kA/m. This enhancement has been explained in terms of the evolution of the grain boundary structure, and the formation of continuous thin layers of Nd-rich phase is important for high coercivity. The micromagnetic simulation together with the numerical analysis based on the nucleation model suggest that the reversed magnetic domains nucleate mainly at the interface of multi-junctions of Nd₂Fe₁₄B grains with high stray fields during the demagnetization process. Both improved anisotropy fields at grain boundaries and reduced stray fields at multi-junction Nd-rich phases contribute to the coercivity enhancement. This work has importance in understanding the crucial microstructure parameters and enhancing the obtainable properties for sintered NdFeB magnets.     

Rare earth passivation and corrosion resistance of zinc coated NdFeB magnets

Rare earth passivation was conducted on Zn coated NdFeB magnets by chemical reaction to enhance the corrosion resistance performance.Morphologies,micro structures and compositions of different passivated coatings were studied by X-ray diffraction,field emission scanning electron microscopy,and X-ray photoelectron spectroscopy,respectively.The corrosion behavior was evaluated by electrochemical measurement and neutral salt spray test.The results show that the rare earth passivation can enhance the corrosion resistance of Zn coated NdFeB magnets.When the concentration of cerium nitrate is 5 g/L,the passivated specimens can achieve the longest NSS time of 360 h,which is 144 h longer than that of the pristine Zn/NdFeB magnets.The passivation layer on the Zn coating surface contributes to the enhancement of the magnets’corrosion resistance.     

Microstructure and corrosion resistance of sintered NdFeB magnet modified by intergranular additions of MgO and ZnO

Microstructure and corrosion resistance of sintered Nd15Dy1.2Fe77Al0.8B6 and Nd22Fe71B7 magnets modified by intergranular addition of MgO and ZnO were investigated. Both the remanence and sintering density of the magnets increased slightly with intergranular additions of MgO and ZnO. There was a remarkable increase in coercivity of Nd22Fe71B7 after addition. Besides, the effects on magnetic properties and an improved corrosion resistance were observed. Compared with the native magnets without addition, corrosion potential of the magnets with MgO and ZnO additives was more positive and the current density in the anodic branch of the polarization curve was reduced. Corrosion resistance resulting from autoclave testing （2×10^5 Pa of steam pressure, 120 ℃） showed that the corrosion rate of NdFeB magnets reduced with the increase of additive amount. Microstructure observation revealed that MgO and ZnO additives were incorporated into the intergranular phases in the magnets. With the introduction of MgO and ZnO, more intergranular phase with high oxygen content was formed while keeping the volume fraction of all the intergranular phases almost unchanged, which may contribute to improved corrosion resistance. Furthermore, addition of MgO and ZnO refined the grain size of Nd22Fe71B7.     

Multilayer ceramic coating for impeding corrosion of sintered NdFeB magnets

Sintered NdFeB magnets have complex microstructure that makes them susceptible to corrosion in active environments.The current paper evaluated the anticorrosion characteristics of multilayer titanium nitride ceramic coating applied through cathodic arc physical vapour deposition(CAPVD) for protection of sintered NdFeB permanent magnets.The performance of ceramic coating was compared to the electrodeposited nickel coating having a copper interlayer.Electrochemical impedance spectroscopy(EIS) and cyclic polar...     

Densification and anticorrosion performances of vacuum evaporated aluminium coatings on NdFeB magnets

Al coated NdFeB magnets obtained by vacuum evaporation technique were densified by high energy ball milling method.The surface morphology,metal composition and micro structure of the coatings were characterized by scanning electron microscopy,X-ray diffraction and X-ray photoelectron spectroscopy,respectively.The anticorrosive properties were investigated by potentiodynamic polarization curves and neutral salt spray test.The pores in the Al coatings of columnar crystals(Al) induced by the evaporation technique,were apparently filled in the following ball milling process,leading to the densification of Al coatings and the evident improvement of the anticorrosive performances.When treated with ball milling for 30 min,the sample achieves the best anticorrosive performances with the self-corrosion potential of-0.87 V,self-corrosion current density of 1.65 μA/cm~2 and the neutral salt spray(NSS) time of 144 h(red rust).The improvement of the anticorro sive performances of vacuum evaporated Al coating mainly lies in the densification effect of the coating,which depends on different loading conditions of ball milling process.     

Relationship between controllable preparation and microstructure of NdFeB sintered magnets

The double hard magnetic phase magnets with nominal compositions of Nd_30–xDy_xFe₆₉B₁(x=2, and 4) (wt.%) were prepared. The magnetic properties of the magnets were measured with a NIM-2000H hysteresigraph. The crystalline structures of the magnets were identified by X-ray diffraction (XRD). The Rietveld refinement was carried out using the FULLPROF software. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses were carried out in order to investigate the microstructure of the magnets. It showed that the magnets consisted mainly of Nd₂Fe₁₄B phase, and some Nd-rich phase. Two types of matrix-phase grains in dark grey and light grey were found in the magnets with x=2 and 4. The Dy content was obviously different in the two types of grains, which proved that the double hard magnetic phases (Dy-rich and Dy-lean phases) coexisted in the magnet. It revealed that the Nd-rich phases in junction regions had fcc structure, with the unit cell parameter of about 0.52–0.56 nm. The weak superlattice spots were found in the SAD patterns of the junction Nd-rich phases with large scale. The double hard magnetic phase structure seemed to improve the magnetic properties of NdFeB magnets with high coercivity, while decrease the consumption of Dy element, compared with the single alloy magnet.     

Anti-corrosion performance of Si-surface-alloying NdFeB magnets obtained with magnetron sputtering and thermal diffusion

Si alloying in the surface layer of NdFeB magnets was realized by thermal diffusion combined with magnetron sputtering. The surface composition, phase structure and morphology of NdFeB(S–Si) specimens were characterized by an X-ray diffractometer, an X-ray photoelectron spectrometer and a field emission scanning electron microscope, respectively. The corrosion resistance of bare NdFeB(S–Si) was analyzed by static full immersion corrosion test and electrochemical experiments. Effects of sputtering and thermal diffusion on the microstructure and corrosion resistance of the surface layer were studied. Results show that surface alloying layer can effectively improve the corrosion resistance of bare NdFeB with the optimized static total immersion corrosion test time in NdFeB(1S–Si)-800 of 36 h, which is much longer than that of the pristine NdFeB (less than 0.5 h). The E_corr of NdFeB(1S–Si)-800 positively shifts from −1.05 to −0.92 V, indicating that the corrosion tendency is obviously lower. The J_corr is 1.45 × 10⁻⁶ A/cm² which is 2 orders of magnitude lower than that of the pristine NdFeB (5.25× 10⁻⁴ A/cm²). The intergranular composite oxides existing in Nd-rich phase contribute to the enhancement of corrosion resistance of Si-surface-alloying NdFeB.     

烧结NdFeB磁体表面化学镀Ni-Cu-P合金及防腐性能

在烧结NdFeB磁体表面化学镀Ni-Cu-P以提高其耐腐蚀性能.研究了络合剂的质量浓度、镀液的pH值、施镀温度及金属离子配比[Cu²⁺]/[Ni²⁺]对沉积速度和镀层成分的影响.用扫描电镜(SEM)和能谱仪(EDAX)观察镀层形貌并分析镀层成分.测定Ni-Cu-P合金镀层在质量分数3.5%NaCl溶液中的极化曲线,并结合中性盐雾实验表征镀层的耐腐蚀性能.研究表明:烧结NdFeB永磁体经碱性超声波除油、酸洗活化后进行化学镀Ni-Cu-P,可得到结合力良好的合金镀层;随着镀液中金属离子配比[Cu²⁺]/[Ni²⁺]的增大,所得镀层从非晶向晶态转变,镀层中的磷含量先升高后降低,镀层表面变得平整、致密;化学镀Ni-Cu-P三元合金的耐腐蚀性能优于相同条件下所得到的Ni-P镀层,且从金属离子配比([Cu²⁺]/[Ni²⁺])为0.02的镀液中得到的镀层的耐腐蚀性能最强.     

Effect of silane coupling agents on flowability and compressibility of compound for bonded NdFeB magnet

Based on the difference of the Y-terminal functional group of the silane coupling agent (Y–Si(X)₃), four different silane coupling agents were employed to pretreat the surface of the NdFeB powders. The effects of silane coupling agents on the flowability and compressibility of compounds for preparing bonded NdFeB magnets were studied. It is indicated that compounds pretreated by silane coupling agents have weaker friction and meshing force. The apparent density is increased by 0.3 g/cm³ compared with the compound without silane coupling agent, and the radial crushing strength is significantly increased by about 3–4 times. In addition, the epoxy resin is more evenly distributed on the surface of the compounds treated by silane coupling agents observed by scanning electron microscopy, and some agglomerated particles are produced. Also, the compressibility of compounds with silane coupling agents is significantly improved due to the fact that hardening exponents are reduced. However, the addition of silane coupling agents has almost no effect on the magnetic properties of bonded magnets. The special energy was used to manifest the flowability of magnetic powder particles representing the macroscopic performance of the force between powder particles, providing a new direction for the study of the interface compatibility of two-phase or multiphase composite materials.     

Magnetic properties and magnetization mechanism of anisotropic NdFeB/SmFeN hybrid bonded magnets prepared with different coercivity NdFeB powders

Anisotropic NdFeB/SmFeN hybrid bonded magnets were prepared by warm compaction process under an orientation magnetic field of 22 kOe,mixing with anisotropic SmFeN powders in different addition and HDDR-NdFeB powders in different coercivity.With the addition of 20 wt% SmFeN,the density and remanence of hybrid magnets increase from 5.58 g/cm³,8.4 kGs to 6.02 g/cm³,9.0 kGs,respectively.And as the addition amount of SmFeN powders varies from 20 wt% to 40 wt%,the maximum energy ...     

Recovery of neodymium,dysprosium,and cobalt from NdFeB magnet leachate using an unsymmetrical dialkylphosphinic acid extractant,INET-3

Rare earths (REs) and Co are critical strategic resources.Their recovery is crucial for ensuring that their supplies are adequate and for reducing environmental pollution.End-of-life NdFeB magnets are important secondary RE sources.The separation of Dy,Nd,and Co from NdFeB leachate using a relatively new extractant (2,3-dimethylbutyl)(2,4,4'-trimethylpentyl)phosphinic acid (INET-3),was investigated in this study.The separation parameters,such as equilibrium pH,phase ratio,INET-3 concentration,and number of counter-current extraction stages,were optimised.The results show that INET-3 can separate Dy,Nd,and Co efficiently.For the preferential extraction of Dy from synthetic iron-free NdFeB leachate,the optimum equilibrium pH is 2.60.Under this condition,separation of Dy from the leachate requires four counter-current extraction stages at A/O (volume ratio of aqueous to organic)=1:2.This is demonstrated by a simulated four-stage counter-current extraction process.At equilibrium pH=2.52and A/O=1:2,98%of Dy is recovered with 95%purity.As to the subsequent separation of Nd from the leachate,nearly 100%is recovered with 99%purity through a simulated three-stage counter-current extraction process at equilibrium pH=5.33 and A/O=1:1.Approximately 95%of Co remains in the raffinate with 99.98%purity.     

Effect of nano-TiC dopant size on magnetic properties and corrosion resistance of hot-deformed NdFeB

Effects of dopant size on the magnetic properties and corrosion behavior of hot-deformed NdFeB with the intergranular additions of nano-TiC were investigated using X-ray diffraction (XRD), X-ray photoelectron spectrometer (XPS), scanning electron microscopy (SEM), electrochemical and magnetic measurement technologies. Additions of 50 nm-TiC, unlike 30 nm- and 40 nm-TiC, are found to increase the magnetic energy product and comprehensive magnetic properties, due in main to the improvement of magnet orientation by more strip-shaped Nd-rich phases. Nano-TiC dopants especially with a size of 40 nm can increase the electrochemical corrosion resistance of magnets. It is attributed to the hindrance of additives on the reaction channel formation and their barrier effect on the oxidations of Nd and Fe. For 40 nm-TiC doped magnet, the charge transfer resistance in electric double layer structure reaches 15000 Ω?cm² that is one order of magnitude higher than other cases, which is responsible for the lowest corrosion current density and accordingly the best anti-corrosion properties.     

放电等离子体烧结NdFeB永磁材料力学性能的研究

采用传统烧结技术和放电等离子体烧绱技术（SPS）制备相同成分的烧结NdFeB水磁合金。研究了所制备合金的力学性能和断裂行为。利用扫描电子显微镜观察了合金的显微组织和断口形貌；测量了合金的抗弯强度并对磁体的力学性能进行评价。结果表明：SPS NdFeB磁体的抗弯渔度较传统烧结体有显著的提高，前者a bb=402．25MPa，后者abb=278．97Mpa；SPS NdFeB磁体显微绀织主相晶粒细小均匀、富稀土相细小弥散且彼此隔断，断口观察发现了较明显的撕裂棱及一些解理断裂；而传统烧结磁体主相晶粒粗大、富稀土相粗大且彼此连接。沿晶断裂特征明显。     

Development of CeO2 nanorods reinforced electrodeposited nickel nanocomposite coating and its tribological and corrosion resistance properties

A simple electrodeposition technique was used to prepare Ni-CeO_2 nanorods composite coating(Ni-CeO_2 NRs) using Watt's nickel plating bath containing CeO_2 nanorods(NRs) as the reinforcement phase under optimized process conditions. The X-ray diffraction analysis(XRD) was used for the structural analysis of Ni-CeO_2 NRs composite coatings and their average crystalline size is ～22 nm for pure Ni and ～18 nm,respectively. The crystalline structure is fcc for the Ni-CeO_2 nanocomposite coatings. The surface morphology of the electrodeposited Ni-CeO_2 NRs composite coatings was analyzed by scanning electron microscopy(SEM). Microhardness of pure Ni and Ni-CeO_2 NRs composite coatings are found to be 253 HV and 824 HV, respectively. The inclusion of CeO_2 NRs increases the microhardness of Ni-CeO_2 NRs composite coatings. The corrosion resistance behavior of Ni-CeO_2 NRs composite coating was evaluated by Tafel polarization and AC impedance methods. It is revealed that CeO_2 NRs reinforced Ni matrix shows higher microhardness and corrosion resistance than existing reported electrodeposited pure Ni and CeO_2 nanoparticles reinforced Ni coatings.     

模压成形各向异性NdFeB粘结磁体性能研究

采用模压成形方法制备各向异性粘结NdFeB磁体,主要研究了粉末粒度以及取向磁场强度对粘结磁体磁性能和力学性能的影响.试验结果表明,随着磁粉粒度的减小,粘结磁体的剩磁有所增加,但矫顽力下降明显.随着取向磁场强度的增大,粘结磁体剩磁进一步提高,各向异性明显;粘结磁体密度及抗压强度随磁粉粒度的减小略有提高.经粒度配比后制备的粘结磁体获得了较高的磁性能和抗压强度,其B_r、H_(ci)及σ_(bc)分别为0.81T、828kA/m及204MPa.     

Structural characterization and corrosive property of Ni-P/CeO2composite coating

Electroless Ni-P/nano-CeO2 composite coating was prepared in acidic condition, and its microstructure and corrosive property were compared with its CeO2-free counterpart. Scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), X-ray diffraction spectrometer (XRD), and differential scanning calorimeter (DSC) were used to examine surface morphology and microstructure of the coating. Corrosive investigation was carried out in 3%NaCl+5%H2SO4 solution. The results showed that Ni-P coating had partial amorphous structure mixed with nanocrystals, whereas the Ni-P/CeO2 coating had perfect amorphous structure. In high temperature condition, Ni3P precipitation and Ni crystallization occurred in both coatings but at different temperatures, whereas the Ni-P/CeO2 coating had sintered phase of NiCe2O4 spinels. The anticorrosion property and passivity were improved in the CeO2-containing coating due to its less liability to undergo local-cell corrosion than its CeO2-free counterpart. During the co-deposition process, some Cen+ (n=3, 4) ions may be adsorbed to the metal/solution interface, hinder nickel's crystal-typed deposition and promote phosphorous deposition. The nano-CeO2 doping finally resulted in the coating' perfect amorphous structure and good anti-corrosive property.     

Study on Seawater Corrosion Resisting Properties ofAl-Mg-RE Alloy Gauze

TheAl-Mg-REalloygauzesarethenewtypesubstituteforplasticgauzesandlow-carb0nsteelgauzes.Atpresent,Alall0ygauesaremainlyAl-Mgall0ygauzesc0ntainingabout4%Mg.Thecharacteristicsofthismaterialare0fgoodstrength,butpoorplasticityandlowfin-ishedproduct.Morethantenh0mefact0riespro-duceAlalloygauzes,andthematerials0fthisgauzesareAl-MgorAl-Mg-Sialloys.TheAl-Mg-REalloygauzesdevelopedrecentlyimprovethepropertiesofplasticity,corrosionresistance,mechanicalpropertiesandmanufacturability.Thepropertyofcor…     

Improved corrosion resistance and thermal stability of sintered Nd-Fe-B magnets with holmium substitution

The effects of Ho substitution for Nd on the microstructure, corrosion resistance and thermal stability of the Nd-Fe-B magnets were investigated. The(Nd,Ho)-O phase was formed with increasing Ho substitution. The results of potentiodynamic polarization and highly accelerated stress test show improved corrosion resistance with increasing Ho substitution. The optimum mass loss 0.29 mg/cm~2 is achieved.Moreover, the average temperature coefficients for remanence and coercivity in the range of 25-150℃are both closer to zero, indicating improved thermal stability. The mechanisms for the improved corrosion resistance and thermal stability are discussed in relation to the microstructure featuring the(Nd,Ho)-O phase.