Hard magnetic materials (such as strong Neodymium-Iron-Boron magnets) possess two distinct characteristics: first, they can be strongly magnetized when subjected to an external magnetic field; and second, they exhibit hysteresis—meaning that even after the external magnetic field is removed, the hard magnetic material retains its magnetized state. The relationship between the magnetic induction *B* and the magnetic field strength *H* for a hard magnetic material; this curve is known as the hysteresis loop.
As the magnetic field is reversed and gradually decreases from O to –Hc, the magnetic induction B vanishes. This indicates that to eliminate residual magnetism, a reverse magnetic field must be applied. Hc is termed the *coercivity*; its magnitude reflects the magnetic material's ability to retain its state of residual magnetism. The purple line segment is referred to as the *demagnetization curve*.
The internal magnetic induction generated within a permanent magnet material after it has been magnetized by an external magnetic field is termed the *intrinsic magnetic induction* (Bi), also known as the *magnetic polarization* (J). The curve describing the relationship between the intrinsic magnetic induction Bi (or J) and the magnetic field strength H serves to characterize the intrinsic magnetic properties of the permanent magnet material; it is called the *intrinsic demagnetization curve*, or simply the *intrinsic curve*.
On the intrinsic demagnetization curve, the magnetic field strength corresponding to the point where the magnetic polarization J equals zero is termed the *intrinsic coercivity* (Hcj).
Bare sintered NdFeB magnets are prone to oxidation and corrosion when exposed to air. When NdFeB magnets undergo prolonged handling or storage—particularly when the subsequent surface treatment method has not yet been determined—a phosphating process is typically employed to provide a simple, temporary form of corrosion protection. The phosphating process for magnet surfaces involves the following steps: degreasing → water rinsing → acid pickling → water rinsing → surface conditioning → phosphating treatment → sealing and drying. Currently, the phosphating process primarily utilizes commercially available phosphating solutions. After phosphating, the products exhibit a uniform color and a clean surface; they can then be vacuum-sealed, significantly extending their shelf life. This storage method offers superior performance compared to traditional methods such as oil immersion or simple oil coating.
Electrophoretic coating involves immersing a component in an aqueous electrophoretic bath solution. An anode and a cathode are simultaneously inserted into the bath, and a direct current (DC) is applied across the two electrodes to induce an electrochemical reaction. This reaction causes water-soluble coating materials—typically high-molecular-weight resins, such as epoxy resin—to deposit uniformly onto the component, forming a corrosion-resistant layer composed of resin particles (essentially, a polymeric protective coating). Electrophoretic coatings not only exhibit excellent adhesion to the porous surfaces of magnets but also demonstrate strong resistance to corrosion from salt spray, acids, and alkalis. While their overall anti-corrosion performance is outstanding, their resistance to humid heat environments is relatively poor.
Parylene (chemically known as poly-para-xylylene) is a protective polymeric material applied via vapor-phase deposition under vacuum conditions. The exceptional penetrating power of Parylene's active molecules allows them to form a transparent, insulating coating—free of pinholes and uniform in thickness—within, beneath, and around the component. This provides the component with a complete, high-quality protective layer that shields it against attacks from acids, alkalis, salt spray, mold, and various corrosive gases. The combination of Parylene's unique deposition process and its superior performance enables it to provide a seamless, comprehensive coating—without any weak points—even for small and ultra-small magnetic components. Such coated magnets can withstand immersion in hydrochloric acid for over 10 days without corroding. Consequently, Parylene is currently widely adopted internationally as the preferred insulating and protective coating for a vast array of small and ultra-small magnetic materials.
Dimensional tolerance—often referred to simply as "tolerance"—denotes the permissible range of variation in a part's dimensions during machining operations. For magnetic materials, a certain degree of dimensional variation is considered permissible. The magnitude of the tolerance is defined as the absolute difference between the maximum limit dimension and the minimum limit dimension, or alternatively, as the difference between the upper deviation and the lower deviation.
Form and position tolerance—generally referred to as geometric tolerance—encompasses both form tolerance and position tolerance. Every mechanical part is composed of points, lines, and surfaces; these constituent elements are collectively termed "features." Following machining, the actual features of a part invariably exhibit deviations relative to their ideal counterparts; these deviations include both form errors and position errors. Since such errors can impact the functionality of mechanical products, appropriate tolerances must be specified during the design phase and clearly indicated on engineering drawings using standardized symbols.
The salt spray test is an environmental test primarily utilizing specialized equipment to create an artificially simulated salt spray environment, thereby assessing the corrosion resistance of products or metal materials. It is categorized into two types: neutral salt spray and acidic salt spray. The distinction lies in the differing applicable standards and test methodologies; these are also referred to as the "NSS" and "CASS" tests, respectively. For sintered NdFeB magnets, the Neutral Salt Spray Test is conducted in accordance with national standards using a continuous spray method. The test conditions are as follows: a temperature of 35°C ± 2°C; a 5% ± 1% NaCl solution (by mass fraction); and a pH value of 6.5–7.2 for the collected salt spray condensate. The orientation of the specimen significantly influences the test results; therefore, the specimen surface is positioned within the salt spray chamber at an inclination angle of 45° ± 5°.
The Damp Heat Test for sintered NdFeB magnets is a testing methodology designed to evaluate—in an accelerated manner—a sample's resistance to degradation caused by damp heat. During this test, the sample is subjected to high, unsaturated vapor pressure over an extended period. The test conditions are: a temperature of 85°C ± 2°C and a relative humidity of 85% ± 5%. The water used for humidification must be either distilled water or deionized water. The severity level is designated as Level 1, corresponding to a test duration of 168 hours.
The High-Pressure Accelerated Aging Test is commonly referred to as the "Pressure Cooker Test" or "Saturated Steam Test." Its primary objective is to place the item under test within a rigorous environment characterized by high temperature, saturated humidity, and elevated pressure, thereby assessing the sample's ability to withstand conditions of high humidity.
For sintered NdFeB magnets, the High-Pressure Accelerated Aging Test involves placing the specimen inside a specialized high-pressure aging chamber containing distilled water or deionized water with an electrical resistivity greater than 1.0 MΩ·cm.
Hardness refers to a material's localized resistance to indentation by a harder object pressing against its surface. It serves as a metric for comparing the relative "softness" or "hardness" of various materials; a higher hardness value indicates that the metal possesses a greater capacity to resist plastic deformation.
Strength refers to a material's maximum capacity to withstand the destructive forces exerted by external loads. Strength is classified according to the form of applied external force as follows:
Tensile Strength (or Tensile Limit): The strength limit when the external force is a tensile force.
Compressive Strength: The strength limit when the external force is a compressive force.
Flexural Strength: The strength limit when the external force is perpendicular to the material's axis and causes the material to bend upon application.
Generally, this property reflects a material's strength in the presence of propagating cracks; its unit is MPa·m¹/². Testing for fracture toughness requires equipment such as tensile testing machines, stress sensors, extensometers, and dynamic strain gauges with signal amplification capabilities; additionally, the test specimens must be prepared in the form of thin sheets.
This property reflects the energy absorbed by a material during the fracture process when subjected to impact stress; its unit is J/m². Measured values of impact strength are highly sensitive to factors such as the specimen's dimensions, shape, machining precision, and testing environment, resulting in a relatively high degree of data dispersion.
This property measures a material's resistance to fracture under bending stress using the three-point bending method. Due to the ease of specimen preparation and the simplicity of the measurement procedure, this parameter is most frequently used to characterize the mechanical properties of sintered NdFeB magnets.
