Sintered NdFeB magnets
A neodymium magnet (also known as NdFeB or Neo magnet) is the most widely used type of rare-earth permanent magnet, from an alloy of neodymium, iron andboron to form the Nd2Fe14B tetragonal crystalline structure. Developed in 1982 by General Motors and Sumitomo Special Metals, neodymium magnets are the strongest type of permanent magnet commercially available. They are an essential part of a modern society and widely used in electric motors, loudspeakers and different type of sensors.
Manufacturing process
Sintered Nd-magnets are manufactured by a powder metallurgical process. The raw materials, Fe, Nd, B etc. are melted in a furnace and then cast into thin sheet by strip casting. The material is pulverized firstly by hydrogen decrepitation and then by jet milling under protetctive atmosphere. The powder is then oriented with external magnetic field and pressed into blocks or near net shape. Final properties and density are achivied in following sintering and heat treatments. The blocks are then cut to shape, surface treated against corrosion and magnetized.
Magnetic properties
As the importance of various magnetic properties differ between applications, there is large number of grades available (see table 1). The nomination of grades follow a Chinese Standard widely used in the industry. The Number after N gives the maximum energy product of the grade (in MGOe) and the letter code the maximum working temperature class of the grade. Properties given in the table are typical for each grade at the room temperature. More information, including demagnetisation curves in various temperatures, for each grade can be found in the datasheets (possibility to download). The same information is available in numerical form for magnetic modelling (request for info – e-mail).
Heavy Rare Earth Elements (HREE)
Pure Nd2Fe14B has relatively low coercivity and thus limited application temperatures. By alloying with heavy rare earth elements like Dysprosium (Dy) and Terbium (Tb) coercivity and application temperature can be increased to levels which makes it possible to use NdFeB magnets in electric motors etc. With inceasing Dy/Tb-content the coercivity increases and the remanence of the material decreases.
Grain boundary diffusion process
As the HREE Dy and Tb are relatively scarce one the goals in NdFeB R&D has been and is, to minimize the amount alloyed and still maintain the increased coercivity. Dy has been found to improve the coercivity by segregating on the grain boundaries and altering the grain boundary structure. By allowing Dy to concentrate on grain boundaries only, it is possible to achieve higher coercivity without negative effects on remanence.
HPMG has developed the grain boundary diffusion process and can offer grades with coercivity and remanence combination unattainable with traditional manufacturing process based on bulk alloying.
Physical properties
The typical physical properties of NdFeB are given in the table below. These values must not be understood as guaranteed, as the properties are not controlled in manufacture. NdFeB magnets are fragile and must never be used as load carrying elements in a design. Please note that the coefficient of thermal expansion is positive along the magnetizing axel and negative perpendicular to the magnetization.
As NdFeB magnets contains iron they corrode easily due to the high electrochemical activity of Nd and the high content of Iron. In low quality magnets the corrosion may happen by grain boundary corrosion, which very rapidly leads to pulverisation of the magnet and to catastrophical loss of mechanical integrity and magnetic properties. In properly manufactured magnets the process is similar to rusting – general corrosion which slowly covers the surface.
Moisture in normal atmosphere is potentially enough to cause an uncoated NdFeB magnet to show signs of corrosion. Marine environment is particularly effective at making NdFeB magnets corrode. As total magnetic failure is possible due to corrosion, an appropriate surface coating or finish is needed the protect the magnet.
There is a variety of surface finishes available, suitable for different corrosion environments and applications. Some, like passivation, are meant only to protect the surface during transport. Some, like epoxy coating, are meant to protect the magnets in demanding applications. In very difficult conditions it may be necessary to hermetically seal the magnets.
The standard coating is a triple layer plating of Nickel-Copper-Nickel (Ni-Cu-Ni). Unless requested otherwise, this Ni-Cu-Ni coating is always applied to the magnets. The Znic (Zn) coating is also wilely used due to its low cost. However, it can not be applied in the environment of pH<4 and pH>9.It should be noted that the level of protection given by any coating depends on the environment the magnet will be subjected to and how the coating is looked after. If the coating is scratched or broken, it will not offer protection from moisture.
Note:
1. The corrosion resistance of these coatings in different environment is influenced by the grade of magnets. The EH,AH grades always exhibit much higher corrosion resistance than that of N grade.
2. The corrosion resistance of these coatings in different environment is also influenced by the shape of magnets, e.g. chamfer, inner rings.
