This section provides information of the physical properties of Neodymium Iron Boron (NdFeB) magnets.
As already discussed in previous sections, the NdFeB magnet has various grades:- each grade has its own magnetic properties (relating to strength of magnetic field output and resistance to demagnetisation, maximum recommended operating temperature and temperature coefficients).
The grades have other physical properties which are similar between the grades. Below is an overview of these properties:-
Summary of Physical Properties of Neodymium Iron Boron, NdFeB, magnets
| Characteristic | Symbol | Unit | Value |
| Density | D | g/cc | 7.5 |
| Vickers Hardness | Hv | D.P.N | 570 |
| Compression Strength | C.S | N/mm2 | 780 |
| Coefficient of Thermal Expansion | C// | 10-6/°C | 7.5 |
| C^ | 10-6/°C | -0.1 | |
| Electrical Resistivity | r | µ Ω.cm | 150 |
| Temperature Coefficient of Resistivity | a | 10-4/°C | 2 |
| Electrical Conductivity | s | 106S/m | 0.667 |
| Thermal Conductivity | k | kCal/(m.h.°C) | 7.7 |
| Specific Heat Capacity | c | kCal/(kg.°C) | 0.12 |
| Tensile Strength | σUTS, or SU | kg/mm2 | 8 |
| Young's Modulus | l / E | 1011N/m2 | 1.6 |
| Flexural Strength | b | 10-12m2/N | 9.8 |
| Compressibility | s | 10-12m2/N | 9.8 |
| Rigidity | E.I | N/m2 | 0.64 |
| Poisson's Ratio | n | 0.24 | |
| Curie Temperature | Tc | °C | 310 |
Structural use of Neodymium Iron Boron, NdFeB, magnets
There is a risk of chipping or breaking the magnets because all magnets are inherently brittle. The Neo magnets are less brittle than SmCo. It is advised to not put magnets in conditions of mechanical stress e.g. in load bearing situations.
The Effects of Radiation on Neodymium Iron Boron, NdFeB, magnets
The NdFeB magnets may be demagnetised by radiation. The Neodymium Rare Earth magnets do not perform as well as SmCo Rare Earth magnets. E.W. Blackmore, (TRIUMF, 1985) and A.F. Zeller & J.A. Nolen (National Superconducting Cyclotron Laboratory, 09/87) demonstrated SmCo having a better performance, with Sm2Co17 offering 2-40 times better radiation resistance than NdFeB. Some NdFeB grade are demagnetised to half their maximum performance with a proton beam radiation of 4 x 106 rads and are completely demagnetised with a proton beam radiation of 7 x 107 rads. A rule of thumb is to select magnets with higher Hci values, designed to operate at high Pci and, where possible, to have radiation shielding protecting them when being subjected to any levels of radiation. The user of the magnets would need to test for effectiveness of the magnets as the magnet suppliers do not have the equipment to test for suitability of magnet grades for environments with raised levels of radiation.
Neodymium Iron Boron, NdFeB, magnets and corrosion resistance
The NdFeB magnets require a protective coating / surface finish to minimize the effects of corrosion. Iron within the structure can ‘rust’ which causes a permanent structural change in NdFeB which results in a permanent weakening of the magnetic performance – the worst case scenario is a total loss of magnetism.
A NdFeB magnet kept in dry conditions will not corrode and will retain its performance theoretically for ever (if not subjected to excessive heat, radiation or strong external magnetic fields). If the conditions are wet, it is recommended that alternative magnets be considered for use of that the magnet design try to protect the magnet from moisture (e.g. encasing, modified coatings such as zinc plus rubber, etc). The plating / surface finish should be hermetic for best corrosion protection – scratched or damaged surfaced may render the affected region more prone to corrosion. Marine environments (salt sprays, sea water) are particularly corrosive and far from ideal for NdFeB. In critical applications where corrosion and magnet failure are unacceptable, magnets such as ferrite or SmCo may be more suitable. Please note that any claims that a NdFeB magnet will not corrode is misleading. It is claimed that higher Hci magnets resist corrosion better although the empirical results are not so conclusive (a trend suggesting an improvement in corrosion resistance exists but it is not guaranteed). It is the application and the overall design that determines how well the magnet will perform in damp environments.
Table comparing main coating types
| COATING APPLIED | NICKEL | EPOXY RESIN | Ni + EPOXY | |||
| Electroless | Powder Spray Coating |
E-Coating | Nickel plating + Epoxy E-Coating |
|||
| Coating Thickness | Range (microns) | 12 to 25 | 25 to 40 | 20 to 40 | 15 to 25 | 25 to 40 |
| Homogeneity | Excellent | Good | Poor | Excellent | Good | |
| Effectiveness versus Magnet Size | Small (<20 grams) | Excellent | Good | Fair | Good | Good |
| Large (>20 grams) | Fair to Good | Good | Fair | Good | Good | |
| Hours before coating is likely to fail | Temp. & Humidity (60ºC, 95%RH) |
>2500 | >500 | >1500 | >2500 | |
| Temp. & Humidity (85ºC, 85%RH) |
>500 | >100 | >300 | >500 | ||
| Salt Spray (35ºC, 5% NaCl) |
>48 | <24 | >100 | >200 | ||
| Coating Colour | Silver | Silver | Black | Black | Black | |
| Heat Cycle | Fair | Fair | Fair | Fair | Fair | |
| Heat Resistance | Poor | Poor | Poor | Poor | Poor | |
| Collision Test | Fair | Fair | Fair | Fair | Fair | |
| Film to material adhesion test | Fair | Fair | Fair | Fair | Fair | |
| Glue adhesion test | Fair | Fair | Fair | Fair | Fair | |
| Tolerance accuracy | Excellent | Excellent | Fair | Fair | Fair to Poor | |
| Additional Remarks | 15-30 microns Ni-Cu-Ni Standard coating | Epoxy resins are not hermetic | Thickness buildup can be a problem | |||
