Permanent magnetic materials are also called “hard magnetic materials”. A material that maintains constant magnetism once magnetized. It has wide hysteresis loop, high coercivity and high remanence. In practice, permanent magnet materials work in the demagnetization part of the second quadrant of the hysteresis loop after deep magnetic saturation and magnetization. Common permanent magnet materials are divided into Al Ni Co permanent magnet alloy, Fe Cr co permanent magnet alloy, permanent ferrite, rare earth permanent magnet material and composite permanent magnet material, which are briefly described as follows:

(1) Al Ni Co permanent magnet alloy
With iron, nickel and aluminum as the main components, it also contains copper, cobalt, titanium and other elements. It has high remanence and low temperature coefficient and is magnetically stable. It is divided into cast alloy and powder sintered alloy. It was widely used in the 1930s and 1960s. Now it is mostly used in the instrument industry to manufacture magnetoelectric instruments, flow meters, micro motors, relays, etc.
(2) Fe Cr co permanent magnet alloy

With iron, chromium and cobalt as the main components, it also contains molybdenum and a small amount of titanium and silicon. It has good processability and can carry out cold and hot plastic deformation. Its magnetism is similar to that of Al Ni Co permanent magnet alloy, and its magnetic properties can be improved by plastic deformation and heat treatment. It is used to manufacture various small magnet elements with small cross-section and complex shape.

(3) Permanent ferrite
Mainly barium ferrite and strontium ferrite, with high resistivity and high coercivity, can be effectively used in atmospheric gap magnetic circuit, especially as permanent magnets for small generators and motors. Permanent magnet ferrite does not contain precious metals such as nickel and cobalt. It has rich sources of raw materials, simple process and low cost. It can replace al Ni Co permanent magnets to manufacture magnetic separators, magnetic thrust bearings, speakers, microwave devices, etc. However, its maximum magnetic energy product is low, its temperature stability is poor, its texture is brittle and fragile, and it is not resistant to impact and vibration. It is not suitable for measuring instruments and magnetic devices with precision requirements.

1. Ferrite is a non-metallic magnetic material, also known as magnetic ceramics. When we take apart the traditional radio, the horn magnet inside is ferrite.
2. The magnetic properties of ferrite are not high. At present, the magnetic energy product (one of the parameters to measure the performance of magnet) can only be slightly higher than 4mgoe. One of the greatest advantages of this material is its low price. At present, it is still widely used in many fields.
3. Ferrite is ceramic, so its machining performance is similar to that of ceramics. Ferrite magnets are formed by die and sintered. If machining is required, only simple grinding can be carried out. Because it is difficult to machine, most ferrite products have simple shape and large dimensional tolerance. The square shaped product is OK and can be ground. Circular, usually grinding only two planes. Other dimensional tolerances are given as a percentage of nominal dimensions.
4. Due to the wide application and low price of ferrite, many manufacturers will have ready-made rings, blocks and other products of conventional shape and size to choose from. Because ferrite is made of ceramic, there is basically no corrosion problem. The finished product does not need surface treatment or coating such as electroplating.

(4) Rare earth materials
They are mainly rare earth cobalt permanent magnet materials and Nd-Fe-B permanent magnet materials. The former is an intermetallic compound formed by rare earth elements cerium, praseodymium, lanthanum, neodymium and cobalt. Its magnetic energy product is 150 times that of carbon steel, 3 ~ 5 times that of Al Ni Co permanent magnet material and 8 ~ 10 times that of permanent ferrite. It has low temperature coefficient, stable magnetism and coercivity up to 800 Ka / m. It is mainly used for magnetic system of low-speed torque motor, starting motor, sensor, magnetic thrust bearing, etc. Nd-Fe-B permanent magnet material is the third generation of rare earth permanent magnet material. Its remanence, coercivity and maximum magnetic energy product are higher than the former, not fragile, has better mechanical properties and low alloy density, which is conducive to the lightness, thinness, miniaturization and ultra miniaturization of magnetic components. However, its high magnetic temperature coefficient limits its application.
(5) Composite material

It is composed of permanent magnetic material powder and plastic material as binder. Because it contains a certain proportion of binder, its magnetic properties are significantly lower than those of the corresponding magnetic materials without binder. Except for metal composite permanent magnet materials, other composite permanent magnet materials are limited by the heat resistance of the binder and the service temperature is low, generally not more than 150 ℃. However, the composite permanent magnet material has high dimensional accuracy, good mechanical properties, good performance uniformity of all parts of the magnet, and is easy to carry out radial orientation and multipole magnetization of the magnet. It is mainly used for manufacturing instruments, communication equipment, rotating machinery, magnetic therapy instruments and sporting goods.

Classification of permanent magnet materials

The first category is alloy permanent magnet materials, including rare earth permanent magnet materials (NdFeB Nd2Fe14B), samarium cobalt (SmCo) and aluminum nickel cobalt (alnico).
The second category is: ferrite permanent magnet materials.
According to different production processes, they are divided into sintered ferrite, bonded ferrite and injection molding ferrite. These three processes are divided into isotropic and anisotropic magnets according to the different orientation of magnetic crystals.
These are the main permanent magnet materials on the market, and some cannot be eliminated due to production process raw materials or cost reasons, such as Cu Ni Fe (copper nickel iron), Fe Co Mo (iron cobalt molybdenum), Fe-Co-V (iron cobalt vanadium) and MnBi (manganese bismuth).
It can be divided into the following three types according to different production processes:

(1) Sintered NdFeB – sintered NdFeB permanent magnet is smelted after air milling. It has high coercivity and high magnetic properties. Its maximum magnetic energy product (BHmax) is more than 10 times higher than that of ferrite. Its mechanical properties are also quite good. It can cut and process different shapes and drill holes. The maximum operating temperature of high-performance products can reach 200 ℃. Because its material content is easy to lead to corrosion, different coating treatments must be carried out on the surface according to different requirements. (such as zinc plating, nickel, environmental protection zinc, environmental protection nickel, nickel copper nickel, environmental protection nickel copper nickel, etc.). Very hard and brittle, high demagnetization resistance, high cost / performance ratio, not suitable for high operating temperature (> 200 ℃).

(2) Bonded NdFeB – bonded NdFeB magnet is a composite NdFeB permanent magnet made by uniformly mixing NdFeB powder with adhesives such as resin, plastic or low melting point metal, and then compression, extrusion or injection molding. The product can be formed at one time without secondary processing. It can be directly made into various complex shapes. Bonded NdFeB is magnetic in all directions and can be processed into NdFeB compression molds and injection molds. High precision, excellent magnetic properties, good corrosion resistance and good temperature stability.
(3) Injection molded neodymium iron boron – with high accuracy, it is easy to make thin-walled rings or thin magnets with complex anisotropic shapes
Sintered ferrite
The main raw materials of sintered ferrite include BaFe12O19 and SrFe12O19, which are divided into isotropic and anisotropic magnets according to the orientation of magnetic crystals. Because of its low price and moderate magnetic properties, it has become a widely used magnet. Ferrite magnet is made by ceramic process, and its texture is relatively hard. It is also a brittle material. Because ferrite magnet has good temperature resistance and low price, it has become a widely used permanent magnet.
Rubber magnetic

Rubber magnet is a kind of ferrite magnetic material series. It is made of bonded ferrite powder and synthetic rubber through extrusion molding, calendering molding, injection molding and other processes. It is a flexible, elastic and twisted magnet. It can be processed into strip, roll, sheet and various complex shapes. Rubber magnet is composed of magnetic powder (sro6fe2o3), polyethylene (CPE) and other additives (ebso. DOP), which is manufactured by extrusion and calendering. Rubber magnetic material can be same-sex or opposite sex. It is made of ferrite magnetic powder, CPE and some trace elements. It can be bent, twisted and rolled. It can be used without more machining. It can also trim the shape according to the required size. The rubber magnet can also be re PVC, back glue, UV oil, etc. according to customer requirements. Its magnetic energy product is between 0.60 and 1.50 mgoe. Application fields of rubber magnetic materials: refrigerator, information notice rack, fasteners for fixing objects to metal bodies for advertising, etc., and magnetic sheets for toys, teaching instruments, switches and sensors. It is mainly used in micro motor, refrigerator, disinfection cabinet, kitchen cabinet, toy, stationery, advertising and other industries.

Al Ni Co
Alnico is the first developed permanent magnet material. It is an alloy composed of aluminum, nickel, cobalt, iron and other trace metal elements. According to different production processes, it is divided into sintered alnico and cast alnico. The product shape is mostly round and square. Casting technology can be processed into different sizes and shapes; Compared with the casting process, the sintered products are limited to small size, the dimensional tolerance of the blanks produced is better than that of the cast products, and the magnetic properties are slightly lower than that of the cast products, but the machinability is better. Among permanent magnet materials, cast Al Ni Co permanent magnet has the lowest reversible temperature coefficient, and the working temperature can be as high as more than 600 ℃. Alnico permanent magnet products are widely used in various instruments and other application fields.
samarium cobalt
Samarium cobalt (SmCo) is divided into SmCo5 and Sm2Co17 according to different components, which are the first and second generation rare earth permanent magnet materials respectively. Due to the scarcity and high price of raw materials, its development is limited. SmCo magnet, as the second generation of rare earth permanent magnet, not only has high magnetic energy product (14-28mgoe) and reliable coercivity, but also shows good temperature characteristics in rare earth permanent magnet series. Compared with Nd-Fe-B, samarium cobalt is more suitable for working in high temperature environment (> 200 ℃).

Main properties of permanent magnet materials

1. Remanence induction intensity
After the permanent magnet material is magnetized to saturation in the external magnetic field, when the external magnetic field is zero, the magnetic induction intensity of the permanent magnet material. This index data is directly related to the air gap magnetic density in the motor. The higher the magnetic induction intensity, the higher the air gap magnetic density of the motor may be, and the main indexes of the motor such as torque constant and back EMF coefficient will reach the best value. Only when the value relationship between electric load and magnetic load of the motor is the most reasonable and the efficiency can reach the best.
2. Coercivity HC, (magnetic induction coercivity HCB)
In the case of saturation magnetization, the required reverse magnetic field strength when the residual magnetic induction br decreases to zero. This index is related to the anti demagnetization ability of the motor, i.e. overload multiple and air gap magnetic density. The greater the HC value, the stronger the anti demagnetization ability of the motor, the greater the overload multiple, and the stronger the adaptability to the strong demagnetization dynamic working environment. At the same time, the air gap magnetic density of the motor will also be improved.
3. Maximum magnetic energy product BHmax
The maximum value of magnetic field energy provided by permanent magnet material to the external magnetic circuit. This index is directly related to the amount of permanent magnet materials in the motor. The greater the BHmax, the greater the magnetic field energy provided by this permanent magnet material to the external magnetic circuit, that is, the less permanent magnet materials are used in the motor under the same power.
4. Intrinsic coercivity HCI
This index refers to the magnetic field strength value when the residual magnetization m drops to zero. When B = 0 on the demagnetization curve, the corresponding HCB value only indicates that the permanent magnet cannot provide energy to the external magnetic circuit at this time, and does not mean that the permanent magnet itself does not have energy. However, when m = 0, the corresponding HCI value indicates that the permanent magnet has really demagnetized and has no magnetic field energy storage. Although HCI is not directly related to the motor operating point, it is the real coercivity of permanent magnet materials, which represents the ability of permanent magnet materials to have magnetic field energy and resist demagnetization. The intrinsic coercivity is closely related to the temperature stability of permanent magnet materials. The higher the intrinsic coercivity, the higher the working temperature of permanent magnet materials.
5. Temperature coefficient α
Temperature is one of the main factors affecting the magnetic properties of permanent magnet materials. When the temperature changes by 1 ℃, the percentage of reversible change in magnetic properties is called the temperature coefficient of magnetic materials. The temperature coefficient can be divided into remanence induction temperature coefficient and coercivity temperature coefficient. This index has a great impact on the performance and stability of the motor. The higher the temperature coefficient, the greater the change of the index from cold state to hot state, which directly limits the service temperature range of the motor. It indirectly affects the power volume ratio of the motor.

Application of permanent magnet materials

Permanent magnet materials include ferrite permanent magnet, rare earth permanent magnet (rare earth cobalt, neodymium iron boron, etc.), aluminum nickel cobalt, iron chromium cobalt, aluminum iron and other materials, among which the most commonly used and used materials are ferrite permanent magnet and neodymium iron boron rare earth permanent magnet.
Although the comprehensive magnetic properties of ferrite permanent magnet in permanent magnet materials are low, compared with metal permanent magnet, ferrite permanent magnet has high resistivity, good stability, strong resistance to environmental changes, rich source of raw materials, high performance and price, mature process and no oxidation problem. Therefore, it is still the most ideal permanent magnet material in many application fields of permanent magnet materials. Since the mass production of ferrite permanent magnet in the 1950s, its development momentum is very rapid. At present, the output value is about 1.5 times that of rare earth permanent magnet. It is expected that it will still be the most widely used and most needed permanent magnet material for a long time in the future.
At the same time, ferrite permanent magnet and its application products are typical energy-saving, material saving, foreign exchange saving and export earning products. No matter from the perspective of resource utilization, or from the perspective of energy and application, its development prospect is very broad. The development of ferrite permanent magnet is of great significance to the development of China’s automobile, motorcycle, electronic information and other pillar industries of the national economy and foreign exchange earning through export, which is in line with the national industrial policy and planning. With the rapid development of electronic information technology, the market demand for high-performance ferrite permanent magnet at home and abroad is increasing. Therefore, it is necessary and promising to research, develop and produce high-performance ferrite permanent magnet materials.

Precautions for application of permanent magnet materials

1. The actual magnetic properties of permanent magnet materials are related to the specific manufacturing process of the manufacturer, and there is often a certain deviation between its value and the data specified in the standard. For permanent magnet materials of the same brand, there will be certain differences in magnetic properties between different factories or different batch numbers of the same factory. For the shape and size of the permanent magnet actually used in the motor, there will be some differences between its magnetic performance and the standard data.
In addition, the capacity of the magnetizer and the magnetization method will affect the uniformity of the magnetization state of the permanent magnet and the magnetic performance. Therefore, in order to improve the accuracy of motor design and calculation, it is necessary to ask the manufacturer for the measured demagnetization curve of the actual size of the batch of permanent magnets at room temperature and working temperature. If possible, it is better to sample and directly measure the demagnetization curve, which is more stable. For motors with high consistency requirements, permanent magnet materials need to be tested piece by piece.
2. The magnetic properties of permanent magnet materials are not only related to alloy composition and manufacturing process, but also related to magnetic field heat treatment process. The so-called magnetic field heat treatment is to apply an external magnetic field during the decomposition reaction of permanent magnet materials. After magnetic field heat treatment, the magnetic properties of permanent magnet materials are improved and directional. The paramagnetic field direction is the largest and the vertical magnetic field direction is the smallest, which is called anisotropy. For permanent magnetic materials without magnetic field heat treatment, the magnetic properties have no directionality, which is called isotropy. It should be noted that for anisotropic permanent magnets, the magnetic field direction during magnetization should be consistent with that during magnetic field heat treatment, otherwise the magnetic properties will be reduced.
3. The permanent magnet material shall rise from the room temperature to the maximum working temperature and be kept for a certain time before cooling to the room temperature. The open circuit magnetic flux is allowed to have an irreversible loss of no more than 5%. Therefore, in order to ensure the stable performance of the permanent magnet motor during operation and no obvious irreversible demagnetization, the magnetic stabilization treatment shall be carried out before use. The method is to raise the temperature of the magnetized permanent magnet material to the expected maximum working temperature and keep it warm for 2 to 4 hours, so as to eliminate this irreversible loss in advance.

Development history of permanent magnet materials

Overall development

With the development of society, magnets are more and more widely used. From high-tech products to the simplest packaging magnets, NdFeB strong magnets and ferrite magnets are the most widely used at present.
From the development history of permanent magnet materials, the magnetic energy product (BH) max (the physical quantity measuring the magnetic energy density stored by permanent magnets) of carbon steel used at the end of the 19th century is less than 1mgoe (megaohm), while the magnetic energy product of Nd-Fe-B permanent magnet materials produced in batches abroad has reached more than 50mgoe. Over the past century, the increase of remanence br of materials is very small, and the increase of energy product is due to the increase of coercivity HC. The improvement of coercivity is mainly due to the understanding of its essence, the discovery of high magnetocrystalline anisotropic compounds and the progress of preparation technology.
At the beginning of the 20th century, carbon steel, tungsten steel, chromium steel and cobalt steel were mainly used as permanent magnet materials. In the late 1930s, alnico permanent magnet materials were successfully developed, which made the large-scale application of permanent magnet materials possible. In the 1950s, the emergence of barium ferrite not only reduced the cost of permanent magnet, but also widened the application range of permanent magnet materials to the field of high frequency. In the 1960s, the emergence of rare earth cobalt permanent magnet opened up a new era for the application of permanent magnet.
In 1967, strnat of Dayton university successfully made SmCo5 permanent magnet by powder bonding method, marking the arrival of the era of rare earth permanent magnet. So far, rare earth permanent magnets have experienced the first generation of SmCo5, the second generation of precipitation hardening Sm2Co17, and developed into the third generation of Nd-Fe-B permanent magnet materials.
In addition, Cu Ni Fe, Fe Co Mo, Fe Co V, MnBi, a1mnc alloys have been used as permanent magnet materials in history. These alloys are rarely used in most occasions because of their low performance and low cost. Alnico, FeCrCo, PtcO and other alloys are also used in some special occasions. At present, BA. Sr ferrite is still the most used permanent magnet material, but many of its applications are gradually replaced by Nd-Fe-B materials. Moreover, at present, the output value of rare earth permanent magnet materials has greatly exceeded that of ferrite permanent magnet materials, and the production of rare earth permanent magnet materials has developed into a major industry.
Anyway, The development of permanent magnet materials has experienced several development stages. Before the 1950s, metal permanent magnets dominated the world. From the 1950s to the 1980s, it was the golden age of permanent ferrite. Since the 1990s, the rise of nanostructured magnetic materials has become a powerful competitor of ferrite. At present, looking for the next generation of permanent magnet materials with better performance is still the focus of the magnetic community. Nanocrystalline exchange coupled permanent magnet materials are the current focus The most promising material superconducting permanent magnet is another possible choice to use super strong permanent magnet at low temperature.

Rare earth permanent magnet material

Magnetism is one of the basic properties of matter, which was recognized about 3000 years ago. Magnetic materials can be divided into hard magnetic materials and soft magnetic materials. Hard magnetic materials refer to magnetic materials that are magnetized to saturation in the external magnetic field and can still maintain high remanence and provide a stable magnetic field after removing the external magnetic field, which is also called permanent magnetic materials. Using this characteristic, permanent magnet materials are widely used in many industries such as energy, information communication, transportation, computer, medical devices and so on. In today’s increasingly severe air pollution, especially in the trend of haze weather becoming the norm, the development of low-carbon economy has become the consensus of mankind. In recent years, permanent magnet materials have shown superior performance in the fields of energy-saving household appliances, hybrid electric vehicles / pure electric vehicles, wind power and hydropower generation, which has attracted more and more attention.

Development history of rare earth permanent magnet materials

The application and research of permanent magnet materials began at the end of the 19th century. With the deepening of people’s research on material magnetism and the improvement of various manufacturing technology levels, the research of permanent magnet materials mainly includes three stages: metal alloy magnet, ferrite magnetic material and rare earth permanent magnet material. Among them, although metal alloy magnets and ferrite magnetic materials have the advantages of low cost and rich raw materials, their maximum magnetic energy product (BH) max is generally less than 10mgoe and poor magnetism, so they are gradually replaced by rare earth permanent magnet materials.
Since its appearance in the early 1960s, after decades of development, three generations of rare earth permanent magnet materials with use value have been formed: the first generation of rare earth permanent magnet material (SmCo5), the second generation of rare earth permanent magnet material (Sm2Co17) and the third generation of rare earth permanent magnet material (Nd2Fe14B). As shown in the development history of rare earth permanent magnet materials.

Nanocomposite permanent magnet

After 30 years of development, the magnetic energy product of NdFeB permanent magnet material has been close to its theoretical value, which is difficult to be greatly improved. People are eager to find a magnetic compound with higher magnetic energy product, but it has not been found so far. Therefore, people began to develop nanocomposite permanent magnet materials in order to make maximum use of the intrinsic magnetic properties of existing magnetic materials. The first three generations of rare earth permanent magnet materials are mainly divided by their maximum magnetic energy product. From this point of view, the magnetic energy product can be used as a standard to measure the magnetic properties of materials. It is well known that the magnetic energy product depends on the saturation magnetization and anisotropic field of the material to a certain extent. However, in the first three generations of rare earth permanent magnet materials, the saturation magnetization and anisotropic field always change one another, which can not be combined. Nanocomposite permanent magnet materials are new permanent magnet materials developed under this background, which can have both these two intrinsic properties.

Nanocomposite permanent magnet

High coercivity and remanence are the basic requirements of permanent magnet materials. Although the coercivity of hard magnetic materials is large, the saturation is relatively low. In nanocomposite permanent magnet materials, the exchange coupling between the two phases is helpful to improve the magnetic properties of permanent magnet materials. The exchange coupling between the two phases and the remanence enhancement effect can be used to manufacture high-performance permanent magnet materials. In addition, because the soft magnetic phase is non rare earth phase, the amount of rare earth can be saved and the price of the alloy can be reduced.
In 1988, Dutch researchers coehoom et al. Obtained isotropic magnetic powder after crystallization heat treatment of nd4fe77.5b18.5 amorphous ribbon at different temperatures. It was found that the magnetic powder with low nd content had obvious remanence enhancement effect. Through the study of its structure, it was found that the magnetic powder with low nd content included hard magnetic Nd2Fe14B phase and soft magnetic Fe3B phase. Subsequent studies show that the exchange coupling between grains causes the remanence enhancement effect in these magnetic particles.
In 1991, Kneller et al. In Germany explained theoretically that the exchange coupling of two-phase grains can improve the magnetic energy product of materials.
In 1993, skomski and coey predicted the anisotropic nanocomposite magnet Sm2Fe17N from a theoretical point of view/ α- (Fe, CO) has a magnetic energy product of 1MJ / m3.
In terms of experiments, in 2005, J. Zhang et al. Inserted a partition layer Cu into the prepared SmCo / Fe film to prevent the contact and diffusion between Fe layer and SmCo layer during annealing heat treatment, which helps to maintain a better multilayer film structure. Its magnetic energy product is 32mgoe (255kj / mol), which is higher than the theoretical magnetic energy product of single-phase SmCo5.
In 2011, S. sawatzki and others in Germany thermally deposited epitaxially grown SmCo5 / Fe / SmCo5 three-layer films on MgO (110) substrates at high temperature, with a maximum magnetic energy product of 312kj / mol, 73% higher than the theoretical limit of 230kj / mol of SmCo5 hard magnetic phase.
On this basis, in 2012, S. sawatzki and others prepared epitaxially grown [SmCo5 / Fe] nsmco5 multilayers without changing the total thickness of hard and soft magnetic layers. When n = 2, the maximum magnetic energy product exceeded 400kj / m3. In the same year, Wei bincui et al. Found that after the nd rich phase diffused into the NdFeB magnetic layer, the contact between NdFeB grains was hindered and the coercivity was significantly improved. After that, they inserted a non-magnetic layer TA between the anisotropic NdFeB and FeCo, blocked the mutual diffusion of FeCo and nd rich phase while maintaining a better microstructure, and obtained the nanocomposite permanent magnet with the highest magnetic energy product so far, up to 486kj / m3. Although the magnetic energy product of nanocomposite permanent magnet obtained in the experiment has exceeded the maximum magnetic energy product obtained in the experiment of single-phase NdFeB material, there is still a large gap from its theoretical magnetic energy product of 1MJ / m3, indicating that it still has a large room for improvement.

Selection principle of permanent magnet materials

There are many kinds of permanent magnet materials, and the performance varies greatly. Therefore, when designing permanent magnet motor, we must first select the appropriate permanent magnet material varieties and specific performance indicators.

1. It shall be able to ensure that there is enough air gap magnetic field and specified motor performance index in the motor air gap.
2. The stability of magnetic properties shall be ensured under the specified environmental conditions, working temperature and service conditions.
3. Have good mechanical properties to facilitate processing and assembly.
4. Good economy and appropriate price.

Chemical protection technology of Nd-Fe-B magnet

The protection technology of NdFeB magnet is simply divided into chemical protection technology and physical protection technology. Chemical protection technology mainly includes electroplating and electroless plating of metal coating, conversion film of ceramic coating, spraying and electrophoresis of organic coating, etc. In production, electroplating process is most commonly used to prepare metal protective coating on the surface of Nd-Fe-B magnet workpiece.
Electroplating is a process in which the metal cations in the electroplating solution are reduced on the surface of the magnet by using the magnet workpiece as the cathode and using the external current to form a metal coating. The electroplating protection of sintered NdFeB magnet is mainly to improve the corrosion resistance of the magnet, and also to improve the surface mechanical properties and decoration. The advantages of electroplating include: relatively simple process, fast film-forming speed and easy mass production. Most of the plating species of electroplated metal layers used for the protection of steel and non-ferrous metal workpieces can be used for Nd-Fe-B magnets. The main plating species used for NdFeB magnet protection are Zn, Ni, Cu, Cr, Sn, Au, Ag, etc. Because NdFeB magnet has porous structure and active chemical properties, single-layer coating can not meet the high corrosion resistance requirements. Generally speaking, multi-layer composite coating can provide more effective protection for the magnet surface. At present, it is widely used in galvanizing, electroplating Ni Cu Ni, electroplating Ni Cu Ni + AG, electroplating Ni Cu Ni + Au, electroplating Ni Cu Ni + electrophoretic epoxy, etc.

Zinc has no magnetism, and as a protective coating, it has little effect on the magnetic properties of the magnet. Compared with nickel and copper, zinc plating is relatively cheap. The hardness of zinc is low and the internal stress of the coating is small. It is not suitable for protecting the easily worn Nd-Fe-B magnet workpiece. It is reported that zinc coating can protect the substrate by sacrificing anode when it is used for the protection of Nd-Fe-B magnet to form primary battery. The standard electrode potential of zinc coating is -0.762v. After studying the electrode potential of each component phase of NdFeB magnet, it can be concluded that zinc coating can not provide complete anode protection. In terms of practical application effect, the sacrificial protection effect of zinc coating on Nd-Fe-B magnet is not obvious. If the zinc coating is not treated, it will darken in the air, so passivation treatment is also required after galvanizing.

The standard electrode potential of nickel coating is -0.25v, which is more positive than that of Nd-Fe-B magnet. It is a cathode coating. Once the external electrolyte penetrates into the coating, it will accelerate the corrosion of the substrate, resulting in poor adhesion between the coating and the substrate, coating delamination, blistering and other defects. The density requirements of nickel coating are very high in application. Nickel plating on the surface of Nd-Fe-B magnet usually adopts multi-layer system such as Ni Cu Ni to reduce the porosity of the coating and improve the corrosion resistance of the coating. Relatively speaking, the cost of electroplating Ni Cu Ni is higher than that of electroplating zinc, but it is favored by users because of its excellent high temperature resistance, oxidation resistance, corrosion resistance, decorative performance and mechanical properties.
There are also copper, tin, gold, silver and other simple metal electroplated coatings that can be directly used for the protection of NdFeB magnets. At the same time, there are quite a number of alloy plating technologies that can also be used for the protection of NdFeB magnets, such as nickel phosphorus alloy, nickel boron alloy, zinc iron alloy, zinc nickel alloy and so on. For Nd-Fe-B magnets, Zn Ni alloy coating is a cathode type coating. After studying the stable potential [III] of Zn Ni alloy coating with different composition, it shows that when the Ni content is about 13%, the Zn Ni coating is γ It has high thermodynamic stability and corrosion resistance.
After years of production and use, it has been proved that the disadvantages of the electroplated protective coating for Nd-Fe-B magnet are also quite obvious: the porosity of the coating is large, the coating is not dense, and the shape depends on the tolerance. The coating will thicken at the corners of the workpiece due to the concentration of power lines in the electroplating process, so it is necessary to chamfer the corners of the magnet, and the deep hole samples cannot be plated; The electroplating process can damage the magnet matrix. In some severe occasions, the electroplating coating will crack, peel and fall off after long-term use, and the protective performance will be reduced; With the increasing awareness of environmental protection in China, the proportion of the cost of electroplating three wastes in the total cost of magnets has increased sharply.
Electroless nickel plating technology refers to the process of metal ion reduction deposition under the catalysis of the workpiece surface. Compared with electroplating, electroless plating process equipment is simple, does not need power supply and auxiliary electrode, and the coating thickness is uniform. It is especially suitable for surface plating of workpieces with complex shape, deep hole parts and inner wall of pipe fittings. The density and hardness of the coating are high. Electroless plating also has some disadvantages, such as poor coating thickness, few varieties that can be plated, relatively high process requirements and complex bath maintenance. Electroless plating mainly includes nickel plating, copper plating and silver plating. At present, electroless Ni-P alloy is used in the protection process of Nd-Fe-B magnet, and it is mostly used as additional protection of electroplating coating. Due to the large amount of hydrogen precipitation in the process of electroless nickel plating, it causes great damage to the matrix of Nd-Fe-B magnet, and makes the coating have high stress. The coating is easy to crack and peel in the process of use.
It is common to use conversion coating such as phosphating and passivation in iron and steel. A dense protective layer can also be formed on the surface of NdFeB magnet by traditional phosphating. The phosphorization of NdFeB magnet can increase the protection during transportation and improve the adhesion of viscose.
There are many kinds of organic coatings, most of which can be coated by spraying, brushing and electrophoresis. The organic coating has compact film formation and good barrier effect on salt spray and water vapor. Organic coating can be combined with Nd-Fe-B magnet electroplating technology to further improve the protection performance of magnet.

Preparation of samarium cobalt protective film

The formation of aging layer is related to the diffusion of oxygen. If the magnet used at high temperature is protected by surface treatment to isolate the diffusion of oxygen into the magnet, the attenuation of magnetic properties of samarium cobalt magnet can be greatly delayed. In addition, the mechanical properties of samarium cobalt magnet are brittle, especially Sm2Co17. It is easy to crack and lack angle in machining and use. The brittleness and reliability of samarium cobalt can be improved by means of electroplating and physical vapor deposition.

Source: China Permanent Magnet Manufacturer – www.rizinia.com

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