Mechanical properties of sintered NdFeB: fracture toughness, impact strength, flexural strength

The application of sintered neodymium iron boron permanent magnet is mainly to use its magnetic properties, but due to the different use environment and conditions, in addition to the requirements of magnetic properties, there are also requirements for mechanical properties and chemical properties. For example, some permanent magnets used in high-speed rotating machinery have to withstand large centrifugal force, or are used in a vibration environment, or subjected to extremely high acceleration (3g~5g), or are subjected to stress when the magnet is installed. It may peel off, fall off the edge, fall off the corner or crack, etc., so it is natural to have requirements on the mechanical properties of the magnet.

What are the mechanical properties of materials?

The mechanical properties of materials generally include strength, hardness, plasticity and toughness. These mechanical properties have different physical meanings.
Strength refers to the maximum ability of a material to resist damage from external forces. Strength is divided into different forms of external force:

• Tensile strength refers to the strength limit when the external force is tensile.
• Compressive strength refers to the strength limit when the external force is under pressure.
• Bending strength refers to the strength limit when the external force is perpendicular to the axis of the material and the material is bent after the action.

Hardness refers to the ability of a material to locally resist hard objects pressed into its surface. It is an index to compare the hardness of various materials. The higher the hardness, the stronger the metal’s ability to resist plastic deformation.
Plasticity refers to the ability of a solid substance to resist deformation under a certain external force. It is the ability of a material to permanently deform without being destroyed under the action of external force.
Toughness indicates the ability of a material to absorb energy during plastic deformation and fracture. The better the toughness, the lower the possibility of brittle fracture. In materials science and metallurgy, toughness refers to the resistance of a material to breaking when it is subjected to a force that causes it to deform. It is the ratio of energy to volume that the material can absorb before breaking.

Mechanical properties of sintered NdFeB

Sintered NdFeB has the highest magnetic energy product (BH)m, and its fracture toughness is still comparable to Sm₂Tm₁₇, SmCo₅, and ferrite permanent magnets, because they are all permanent magnet materials based on intermetallic compounds and are brittle materials. Bonded rare earth permanent magnet materials, Fe-Cr-Co, and magnetic steel have the best fracture toughness, but their magnetic energy product (BH)m is much lower than that of sintered NdFeB.

Brittle materials commonly use three indicators to describe the mechanical properties of materials:

• Fracture toughness usually reflects the strength of the material when cracks expand, and the unit is MPa·m^1/2. To test the fracture toughness of the material, a tensile testing machine, a stress sensor, an extensometer, a signal amplifying dynamic strain gauge, etc. should be used, and the sample should be made into a thin sheet.
• Impact strength (impact fracture toughness) reflects the energy absorbed by the material in the process of material fracture under the action of impact stress, in J/m². The measured value of impact strength is too sensitive to the size, shape, processing accuracy and test environment of the sample, and the dispersion of the measured value will be relatively large.
• The bending strength is the three-point bending method to measure the bending fracture strength of materials. Because the sample is easy to process and the measurement is simple, it is most commonly used to describe the mechanical properties of sintered NdFeB magnets.

Actility found the approximate fluctuation range of the mechanical properties of sintered NdFeB given by some scholars in different experiments. Because it is a brittle material, the test data has a large dispersion.

The sintered NdFeB permanent magnet material exhibits high strength and low toughness characteristics due to its own crystal structure. In addition, the following two factors will affect the flexural strength of sintered NdFeB, which is also a way to improve its strength .

• The Nd content has a certain influence on the strength of sintered NdFeB. The experimental results show that under certain conditions, the higher the Nd content, the higher the material strength.
• Adding other metal elements has a certain effect on the strength of sintered NdFeB. When a certain amount of Ti titanium, Nb niobium or Cu copper is added, the impact fracture toughness of the permanent magnet is improved; when a small amount of Co is added, the bending strength of the permanent magnet is improved.

The comprehensive mechanical properties of sintered NdFeB are not high enough, which is one of the important reasons that limit its application in wider fields. If the toughness of the product can be improved while the magnetic properties are improved or unchanged, the sintered NdFeB will play a greater role in military, aerospace and other fields, and enter a new period of development..

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