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PVD Vacuum coating of NdFeB permanent magnet material

Nov 08, 2018

PVD Vacuum coating of NdFeB permanent magnet material


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NdFeB is a rare earth permanent magnetic material developed in the 1980s, which is small in size, light in weight and has excellent magnetic properties. It is widely used in electronic information, metallurgy, communication industry, medicine and other fields. Modern science and technology and information industry is integrated, lightweight, intelligent direction, such as new energy, energy conservation and environmental protection industry is higher and higher demand for the performance of the rare earth permanent magnet materials, such as frequency conversion air conditioning compressor, industrial energy conservation and new energy vehicles driven motor with magnetic steel must have a high coercive force, high magnetic energy product, high consistency, high corrosion resistance and other characteristics, it is a huge challenge to the traditional magnetic steel manufacturing industry, at the same time, the corrosion resistance of ndfeb permanent magnet material put forward higher requirements.

 

At present, the methods to improve the corrosion resistance of NbFeB permanent magnetic material include adding alloy elements and additional protective coating, but the main method is to add protective coating (metal coating, organic coating and composite coating). The protective coating can prevent the contact between the corrosion phase and the substrate and thus slow down the corrosion of the magnet. Electroplating, electroplating, physical vapor deposition, etc. Electroplating protection technology is widely applied to the protection of NdFeB permanent magnet material due to its low technical threshold, mature process and low price. NdFeB permanent magnet is mainly made from sintered powder metallurgy industry, porous surface, in the process of electroplating or electroless plating, acidic or alkaline electrolyte aqueous solution will inevitably remain in NdFeB matrix pore, seriously affecting the quality of the protective coating, NdFeB matrix can not reach the expected service life, and electroplating and chemical plating wastewater discharge.

 

It also pollutes the environment. Therefore, in recent years, domestic and foreign researchers have been working to develop instead of plating surface protection technology research, physical vapor deposition (PVD) technology as a kind of environmental friendly technology, has the characteristics of what many other technology does not have, by controlling the process parameters can be grain of small, uniform thickness, film adhesion excellent plating; At the same time, PVD is a dry plating technology, which can avoid the defects of coating brittle due to acid or alkaline electrolyte solution residues in the magnetic pores and hydrogen absorption during plating. However, the surface treatment of Nd-FeB PVD is limited by mass production cost and some factors.

 

In this paper, various PVD technologies applied to NdFeB permanent magnet materials at home and abroad are summarized, and the basic principles, characteristics and research status of these technologies are described. Relevant pretreatment and post-treatment processes of physical gas deposition applied to NdFeB permanent magnetic material were also summarized, and corresponding analysis was conducted to provide reference for relevant workers.

 

1. PVD protection technology

 

NdFeB permanent magnet material generally operates under certain temperature and medium conditions, and it is required to maintain the integrity of its external size and magnetic performance in the long term. When the NdFeB material corrodes, partial surface area will cause damage of composition and structure, which will cause the magnetic property to decline, thus affecting its practical application. The PVD technology can effectively solve this problem by depositing protective coating on NdFeB surface. The coating prepared by PVD technology has good stability, high bonding force and high density. In addition, during PVD plating, the thickness of the coating affected by the side Angle of the magnetic workpiece is much lower than that of electroplating and electroless plating, and there is no pollution in the preparation process. Moreover,PVD technology can obtain a wide range of coating types (such as Al, Ti/Al, Al/Al2O3, TiN, etc.), which is a promising NdFeB surface protection technology. At present, the PVD techniques commonly used for surface treatment of NdFeB permanent magnetic materials at home and abroad mainly include evaporation, magnetron sputtering and ion plating. The following is an overview of the three technologies from the basic principles and the research status at home and abroad.

 

1.1 the evaporation plating

 

The process of Vapor deposition is to put the workpiece into a vacuum chamber and heat it in a certain way. This technology has the advantages of simple equipment and easy control of the process, but the film layer obtained by general thermal evaporation is relatively rough, with poor adhesion strength, and it is easy to form thick columnar crystal structure, and the corrosion liquid can easily pass through the film layer to corrode the NdFeB substrate material. At present, the evaporation plating technology in NdFeB permanent magnet materials for surface protective treatment reported less, abroad have a small amount of evaporation of Ion assisted deposition (Ion vapor deposi - tion, IVD) preparation of aluminum membrane technology. IVD technology refers to adding negative bias pressure on the workpiece above the evaporation source and generating glow discharge around the workpiece. In the evaporation coating process, when the evaporated metal vapor atoms pass through the glow area, some metal atoms are ionized into metal ions, and the accelerated metal ions or atoms move to the workpiece surface to form a film. The metal coating prepared by this technique has the advantages of good density, high binding degree with substrate, rapid deposition rate, etc. Therefore, this technique can be applied to the corrosion protection of NdFeB permanent magnetic material.

 

1.2 magnetron sputtering technology

 

Magnetron sputtering technology is the technology of deposition of film on the workpiece after the argon ions generated by glow discharge sputtering the target atoms. Magnetron sputtering coating is characterized by low deposition temperature, uniform and controllable film composition, no change in the surface finish of the substrate, and good adhesion to the substrate. It can be applied to the surface protection of NdFeB permanent magnetic material.

 

MaoSD et al. deposited Al films on NdFeB using dc magnetron sputtering technology to obtain Al films with columnar crystal structure, as shown in FIG. 1(a). The results show that the magnetron sputtering aluminum plating improves the corrosion resistance of the magnet. As the micro pores between the aluminum coated columnar crystals run through the membrane, the corrosion solution will reach the matrix through these pores when the materials with Al protective coating corrode. MaoSD and adopted such as Ion beam assisted magnetron sputtering (Ion - beam - assisteddeposition, IBAD) method of preparation of the Al film in the surface of NdFeB, as shown in figure 1 (b), it can be seen that the columnar crystal structure, membrane layer is more uniform and dense, results show that after 240 h neutral salt fog test, the pure Al magnetron sputtering film surface appear a large area of red rust, but only a small amount of preparation of the Al film IBAD surface red rust, its corrosion resistant performance is improved obviously, this is mainly due to the IBAD - Al film layer and the oxide film has more. MaoSD et al. prepared Al/Al2O3 multilayer films by plasma assisted magnetron sputtering.

 

Li jinlong et al. deposited AlN/Al multilayer films on the surface of NdFeB using dc magnetron sputtering technology. The study showed that the AlN/Al films deposited on the surface of NdFeB were denser and had the best corrosion resistance when the nitrogen argon partial pressure was 1:1. The salt spray corrosion resistance of AlN/Al multilayer films was significantly better than that of single-layer Al films, which not only did not destroy the magnetic energy of NbFeB, but also slightly increased its magnetic energy. Ti/Al multilayer films were deposited on the surface of sintered NdFeB magnets by xie tingting et al.. Studies show that Ti/Al multilayer films have denser surface than single Al films, and Ti layer interrupts the growth of the columnar crystal structure of Al layer. Its self-corrosion current is nearly 2 orders of magnitude smaller than that of pure Al thin film, and it has higher corrosion resistance and destructive strength.

 

1.3 ion plating

 

Ion plating technology is based on the vacuum evaporation plating, and plasma activation, in the heart of the inert gas glow discharge steam ionization of membrane material and the basal bombardment and coating. In addition to the advantages of vacuum evaporation and sputtering, ion plating technology combines glow discharge, plasma technology and vacuum evaporation coating technology. Besides, it also has the advantages of rapid deposition, strong adhesion of film layer, good diffraction and extensive coating materials.

 

In Japan, ion aluminum plating technology has been widely used in the NdFeB material of SPM(surface magnetic body), IPM(internal magnet) motor and electric vehicle.In the 1990s, the preparation of Al membranes on NdFeB surface by ion plating was reported in China.Xie Fa frequently using ion plating technology on the permanent magnetic material such as 8.5 microns thick aluminum plating film, found a 5% increase over the coercive force, remanence and the maximum magnetic energy product changes by 21.8% and 2.1% respectively, and the membrane layer and matrix between permanent magnet material has good bonding strength, this is due to the high energy ion bombardment on the surface of magnets and atom, caused a certain degree of ion implantation;The salt fog test showed that the salt fog resistance time of the permanent magnetic material plated with 8.5 irmal layer reached 168h.AAli et al. prepared TiN ceramic coating on NdFeB by using cathode arc ion plating technology, which can improve the corrosion resistance of NdFeB without affecting the magnetic properties of the magnet itself.Du jun et al. prepared ZrN/TiN coating on the surface of NdFeB magnet by arc ion plating method. The cross-section morphology indicated that the coating was relatively compact with obvious multi-layer structure, and there was an obvious transition layer between the coating and the substrate, which was conducive to the improvement of the binding force between the coating and the substrate.The results show that the ZrN/TiN coating can not only reduce the corrosion rate of NdFeB magnets by 2 orders of magnitude, but also improve the wear resistance of the magnet.In the field of stomatology,NdFeB permanent magnetic material is used for human magnetic orthodontic treatment due to its high coercivity, high remanence and high magnetic energy accumulation, but it has poor corrosion resistance and cannot be used for a long term in the oral environment, which limits its application.The deposition of TiN coating on the surface of NdFeB permanent magnetic material by ion plating can improve the corrosion resistance of NdFeB permanent magnetic material in the oral environment.

 

2. Pretreatment process

 

NdFeB has a large number of loose pores on the surface of permanent magnet material, which is affected by the early processing techniques such as mechanical processing, as well as residual oil, dust and other substances on the surface, causing difficulties in PVD surface treatment. The conventional PVD pretreatment technique is not completely suitable for NdFeB surface cleaning. This is because in the process of using electrolyte water solution such as metal cleaning agent to clean the magnetic surface dirt, if these treatment solutions remain in the pores, it will cause poor adhesion of the coating layer, the coating is easy to spalling. Furthermore, the grain boundary of nd-feb permanent magnetic material is rich in Nd phase. If the previous treatment process is not appropriate, intercrystalline corrosion will also occur, which will seriously shorten the service life of the magnet. Therefore, the pretreatment process is the key to improve the coating adhesion and corrosion resistance.

 

At present, researches on the preplating treatment of NdFeB permanent magnet materials are more frequent, most of which are electroplating and chemical plating. The author believes that this is because the surface treatment of NdFeB PVD is in the initial stage temporarily, and relevant PVD pretreatment technology is also less studied. However, there are many pretreatment processes for electroplating and electroless plating. Some good pretreatment processes for electroplating and electroless plating can be used for reference in the pretreatment of Nd-FeBPVD. Common processes of NdFeB permanent magnet material preplating include sanding, polishing, oil removal, rust removal, hole sealing, activation, etc.

 

Sandpaper grinding and polishing treatment is a conventional preplating treatment method, which is suitable for the treatment of NdFeB materials with small batch shape rules, and not suitable for the pretreatment of bulk NdFeB materials. Hole sealing is a method to soak the pore-sealing agent into the micro hole of the workpiece and then solidify it into a solid. The sealing hole can effectively prevent the acid and alkali from infiltrating into the pores of the NdFeB material in the process of oil and rust removal, and avoid the internal and external corrosion caused by the magnet. At present, the main methods of hole sealing are as follows :(1) soaking zinc stearate, heating zinc stearate to the molten state, then putting the sample into it, taking out and cooling after 20min, and solidifying the hole in the magnetic pore; (2) when the pores are sealed by boiling water, the NdFeB sample is put into boiling de-ionized water and boiled for 3-5min. Water is sucked into the internal pores of the magnet through capillary action. (3) dip the sample into pore-sealing agent and put it into the vacuum kettle for 10-15 min. After taking out, wash the sample at a certain temperature and solidify in the curing medium. Wang xin et al. found that the sealing hole can significantly improve the adhesion of the film base and the corrosion resistance of the magnet. Xiao xiangding et al. compared the effect of the magnetic sealing effect of organic infiltration rapid solidification agent and inorganic water glass pore-sealing agent on the corrosion resistance of the magnet through experiments, and determined that the fast solidification organic impregnation agent was the appropriate NdFeB pore-sealing agent. NdFeB magnets must be dried after sealing treatment to reduce solution residues. The PVD protective coating was applied to NdFeB after the seal hole was dried. This method is effective.

 

NdFeB permanent magnet material should avoid corrosion by highly acidic or alkaline cleaning agents in the process of oil and rust removal. Zhou qi et al. showed that neodymium in Cl- and NdFeB permanent magnet materials reacted strongly, so hydrochloric acid was contraindicated during pickling and rust removal. At the same time, in the solution of derusting and oil removal, substances with complexing ability and corrosion inhibitor are added to prevent the oxidation of neodymium and overcorrosion of the matrix. Rao hou et al. studied different oil removal techniques before NdFeB nickel plating, and the results showed that Na3PO4 and Na2CO3 solutions were first used for chemical oil removal, and then electric oil removal was the most effective, while metal cleaner was the least effective. Nd of NdFeB is a very active metal. If anodic oil removal is carried out, the substrate surface is easy to be oxidized and dissolved, resulting in over-corrosion. Therefore, in the oil removal process, it is better to use the cathode for oil removal. JingChen et al. used anodic electrolytic etching to remove the oxide film on the magnetic surface when electrodeposited al-mn coating on NdFeB. This method can not only effectively remove the oxide film on the magnetic surface, but also greatly improve the binding force between the coating and the substrate. In addition, ultrasonic assisted cleaning has a good effect on the treatment of NdFeB before plating. Li xiaodong studied the cleaning process of magnetic materials and believed that the combination of high-frequency and low-frequency ultrasonic cleaning can significantly improve the cleanliness of cleaning workpieces.

 

Dry sand blasting is an effective method to remove rust corrosion products and oxide scale on the working surface. With high efficiency, high mechanical degree and good rust removal quality, it is suitable for the surface rust removal of NdFeB and other powder metallurgical materials. The surface roughness of the matrix after sandblasting can improve the binding force between the film and the matrix. Han wensheng et al. studied different preplating processes on NdFeB surface, and replaced traditional alkaline oil removal and pickling rust removal by baking oil removal and dry sand blasting. The study showed that this pretreatment before anhydrous plating can improve the adhesion between coating and substrate and obtain crystalline fine, smooth and dense coating. It is important to note that as a result of NdFeB permanent magnet materials containing active rare earth neodymium, sand in the air after soon form a layer of oxide film, after drying treatment oxidation further, if you don't remove the layer of oxide film, can affect the quality of the coating, cause bad combination between substrate and coating, the author thinks that, when the PVD coating, can adopt the method of high-energy ion bombardment in furnace to remove oxide on the surface of the NdFeB.

 

3. Post-processing technology

 

After coating PVD with a protective coating, an effective posttreatment process can further improve the corrosion resistance of the coating, thus meeting the service requirements of NdFeB permanent magnetic material in a harsh environment with high temperature and strong corrosivity, and extend its working life. Common post-plating treatments include shot peening, vacuum heat treatment, chemical conversion, etc.

 

Tang zhihui et al. studied the effect of shot peening on the micro-morphology and corrosion resistance of ion aluminized coating. Sun bao-yu et al. used dc magnetron sputtering technology to conduct vacuum heat treatment on the magnetic material of Al film after aluminum plating on the surface of NdFeB magnet. The results show that the NdFeB permanent magnet material Al plating film after 650 , 10 min after heat treatment, the Al film layer and the NdFeB substrate in metallurgical bonding interface, the improved the film adhesion, maintain the integrity of the coating, to further improve the corrosion resistance of NdFeB permanent magnet materials. Sun bao-yu et al. prepared DyAl alloy film on the surface of the sintered NdFeB magnet, conducted vacuum diffusion infiltration and aging treatment on the coating samples, and the study showed that Dy and Al element diffused into the surface substrate, and the intrinsic coercivity of the magnet increased Hcj, heat resistance and corrosion resistance. Xie fazhen et al. applied aluminum plating on NdFeB permanent magnetic material and chromate conversion treatment to improve the salt spray corrosion resistance of the magnet by one time.

 

4. Endnotes

 

Improving the corrosion resistance of NdFeB permanent magnet material is a systematic project, which needs to be comprehensively studied from several aspects, such as pre-plating process, plating process and post-plating process. Although PVD is a promising surface protection technique for NdFeB, further improvements are needed in the following aspects.

 

(1) adopting a single membrane layer is not a good solution to the problem of poor corrosion resistance of NdFeB permanent magnetic material, and a multi-technology composite preparation method should be developed to obtain multi-layer films. It is worth noting that the magnetic energy of the NdFeB matrix can not be damaged by the multi-layered films which improve the corrosion resistance of the NdFeB permanent magnetic material.

(2) as the magnetic protection requires equal plating on all surfaces of the NdFeB workpiece, the three-dimensional rotation of the NdFeB magnet should be solved in the preparation of PVD to ensure the film quality consistency.

(3) when PVD is used to prepare protective coating, different barrel structures are designed for NdFeB products of different shapes to increase the number of furnaces as much as possible, which is conducive to reducing the cost of large-scale production of PVD technology and improving its market competitiveness, so as to replace the existing electroplating and chemical plating technologies that are heavy on environment and resources.

(4) more pre-treatment and post-treatment processes suitable for large-scale production of PVD technology have been developed to give full play to the corrosion resistance of the protective coating on the basis of ensuring the integrity of the magnetic material.

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