Sintered Neodymium (NdFeB) magnets, known for their high magnetic energy density and excellent operational characteristics, are widely used in various fields including household appliances, transportation, healthcare, and aerospace. To objectively assess the suitability and quality of these magnets, various tests are conducted, generally categorized into three aspects: appearance and physical properties, magnetic properties, and auxiliary performance tests.
Testing Appearance, Dimensions, and Physical Properties
The appearance, dimensions, and physical properties are fundamental for the use of NdFeB products. Good products must not only meet magnetic performance requirements but also satisfy various practical dimensions, coatings, and mechanical properties to meet basic assembly and different environmental usage requirements. The product dimensions and tolerances not only affect the assembly effect but also impact the magnetic performance consistency deviation and the stability and energy consumption of motor operation. Below are the testing items and other details for appearance, dimensions, and physical property testing:
Testing Items | Testing Instruments | Testing Indicators | Influencing Factors | Testing Purpose |
Dimensions | Calipers, Micrometers, Go/No Go Gauges, Projector, CCD Vision Detector | Length, Width, Thickness, Perpendicularity, Parallelism, Radius, Chamfer, etc. | Machining equipment, precision of testing tools/instruments, calibration, and staff skills | Ensure product assembly characteristics and quality performance consistency |
Appearance | Naked Eye Observation, Comparison Card, Tapping or Developer, etc. | Pinhole, Chipped Corner, Scratches, Hidden Cracks, etc. | Material formula, production process, machining process, etc. | Reduce product corrosion performance, risk of mechanical failure, etc. |
Coating Adhesion | Mechanical Testing Machine or Scratch Testing | Degree and force of coating peeling, proportion of peeling in scratched area | Pre-treatment before electroplating and electroplating process, type of coating | Ensure coating corrosion resistance |
Mechanical Property | Mechanical Testing Machine | Tensile Resistance, Compressive Resistance, etc. | Material formula, production process, machining process, product specifications, etc. | Reduce the risk of product cracking during use |
Corrosion Resistance | Salt Spray Test Chamber, High Pressure Accelerated Aging Test Chamber | Weight Loss, Salt Spray, etc. | Electroplating process, coating thickness, environmental conditions, etc. | Reduce the risk of product corrosion in harsh environments |
Magnetic Performance Testing
The magnetic performance of NdFeB is a major requirement for its use. Only products that strictly meet magnetic performance requirements can fully exert their effects in the required fields for stable application. In terms of magnetic performance, early attention was focused on residual magnetism (Br) and intrinsic coercivity (Hcj), but now more and more users are beginning to pay attention to magnetic moment and thermal demagnetization. In fact, the main use of magnetic steel is its two important characteristics: the magnetic energy density within the unit volume and the magnetic stability at high temperatures. Evaluating with magnetic moment and thermal demagnetization is more reasonable, and the magnetic steel screened out by the thermal demagnetization index will undoubtedly have better service reliability and higher cost performance. Below are the testing items and other details for Magnetic Performance Testing:
Testing Items | Testing Instruments | Testing Indicators | Influencing Factors | Testing Purpose |
Magnetic Performance | Permanent Magnet Measuring Instruments (e.g., Hirst) | Room/High Temp Br, Hcb, Hcj, Hk/Hcj, (BH)max, Temperature Coefficients for Br and Hcj, Inflection Point Recovery Permeability, etc. | Material formula, production process, machining precision, instrument accuracy, testing environment, etc. | Verify that various performance indicators meet product usage requirements, etc. |
Surface Magnetism | Surface Magnetic Distribution Measuring Instruments, Tesla Meters, etc. | Central Surface Magnetism or Specific Area Magnetism Size, Distribution Uniformity, etc. | Remanence, product dimensions, measurement methods, etc. | Verify the magnetic field size in specific areas of the product. |
Magnetic Moment | Magnetic Moment Tester, Fluxmeter, etc. | Magnitude of Magnetic Moment | Remanence, product dimensions, etc. | Accurately reflect the magnitude of the product’s magnetic performance. |
Thermal Demagnetization | Fluxmeter, Oven, etc. | Magnetic Flux, Percentage of Magnetic Loss | Material components like Co, NdGd, etc., product Pc value, Hcj, Hk/Hcj, etc. | More accurately reflect the magnetic stability of the product in high-temperature environments |
Auxiliary Testing
The auxiliary testing of NdFeB is mainly to inspect whether the product performance achieved during the production process meets expectations and to verify whether the production process is defective. In the event of abnormal situations or new product development, product composition and appearance are analyzed and confirmed. Important tests such as salt spray tests and aging tests will be introduced one by one.
Testing Items | Testing Instruments | Testing Indicators | Influencing Factors | Testing Purpose |
Density | Solid Density Meter | Archimedes drainage method for calculating weight to volume ratio | Bulk density | Material composition, sintering process, preparation process |
Carbon & Sulfur Content | Infrared Carbon-Sulfur Analyzer | High-temperature combustion of carbon and sulfur in the sample to detect oxides | C, S content | Raw material preparation process |
Oxygen, Nitrogen & Hydrogen Content | Oxygen, Nitrogen, Hydrogen Tester | Sample high-temperature conversion to produce CO, CO2, N2, H2 for detection | O, N, H content | Raw material preparation process |
Composition | Plasma Emission Spectrometer | Detection of electromagnetic wave energy released by atomic ionization transitions, corresponding to different characteristic spectra | Elements such as Pr, Nd, Fe, Cu, B, etc. | Formula design, raw material composition, production process, testing accuracy |
Microstructure | Scanning Electron Microscope, Energy Spectrometer | Analysis of sample surface characteristics through secondary electrons, backscattered electrons, and characteristic X-ray signals | Grain appearance, size, and microscopic composition | Sample preparation, microscope resolution, accuracy |