What are Disc Magnets Made Of? The Complete Guide

Disc magnets, also known as circular or cylindrical magnets, are incredibly useful for a variety of applications. But what exactly are these strong magnets made out of?

In this comprehensive guide, as a professional disc magnets manufacturer, we’ll cover everything you need to know about the composition and manufacturing of modern disc magnets.

An Overview of Disc Magnet Materials

The most common disc magnets today are made from neodymium iron boron (NdFeB). Also known as neodymium magnets or neo magnets, NdFeB offers the highest magnetic strength per unit volume of any permanent magnet material.

NdFeB disc magnets are composed of neodymium, iron, and boron as their key ingredients. Other elements like dysprosium, praseodymium, cobalt, nickel, niobium, and aluminum may be added in trace amounts to enhance specific properties.

For example, dysprosium and praseodymium help improve corrosion resistance. Cobalt enhances the Curie temperature. Nickel and copper coatings protect against oxidation.

Samarium cobalt (SmCo) is another key rare earth magnet material used for disc magnets when high-temperature stability is needed. SmCo offers excellent resistance to demagnetization, although their magnetic strength is lower compared to NdFeB.

There are also ceramic or ferrite magnets, which are composed of strontium carbonate and iron oxide compounds. While less powerful than rare earth magnets, ceramic disc magnets have the advantages of low cost and corrosion resistance.

Manufacturing Process of NdFeB Disc Magnets

Manufacturing powerful NdFeB magnets involves carefully controlling a complex, multi-step precision process:

1. Alloy formation

Pure neodymium, iron, boron, and any other elements are first melted together into an alloy blend using induction melting furnaces. The liquid alloy mixture is then cast into ingots.

2. Decrepitation and jet milling

The brittle alloy ingots are decrepitated using hydrogen gas to break them down into a magnetic powder. This powder is then finely milled in a jet mill to an average particle size around 3 microns.

3. Pressing

The NdFeB powder is pressed into a die mold using extreme axial, transverse, or isostatic pressure methods. An alignment magnetic field is applied to orient the powder particles.

4. Sintering

The compressed “green” magnet body is sintered at temperatures near melting point, causing the particles to fuse together. The magnet shrinks and densifies during this process.

5. Annealing and quenching

The sintered magnet is rapidly heated, soaked, and cooled (quenched) to form the optimal metallurgical microstructure and crystal alignment.

6. Machining

The brittle sintered magnet block requires diamond-cutting tools to slice and grind it down to the final dimensions. Holes and other complex shapes may be wire cut using EDM machines.

7. Coating and plating

A protective coating like nickel-copper-nickel is applied to shield against corrosion agents. Other functional coatings can be added as well.

8. Magnetization

The finished magnet receives a strong saturating pulse in a magnetizer to orient the magnetic domains for optimal magnetic performance.

And those are the key steps for fabricating modern neodymium disc magnets! Next let’s examine the manufacturing process for bonded and ceramic magnets.

How Bonded Disc Magnets Are Made

Bonded magnets take a different approach to production:

1. Jet milling

Like sintered magnets, bonded magnets start with decrepitated NdFeB alloy that gets finely milled into a magnetic powder. A binding polymer is added to the jet mill to homogenize the blend.

2. Blending with polymer

The powder is further mixed with a plastic or elastomeric binding polymer, along with lubricants and curing agents. The goal is to evenly coat each powder particle.

3. Pressing and curing

The feedstock blend is pressed into the disc magnet shape using compression or injection molding. It cures at elevated temperature, causing the polymer to set around the powder particles.

4. Magnetizing

The cured bonded magnet body can be magnetized just like a sintered part. However, due to the non-magnetic binder taking up volume, the magnetic strength is lower per the same size.

5. Coating (optional)

A protective coating may be applied after magnetization to boost corrosion and scratch resistance.

So in summary, bonded disc magnets substitute the sintering phase with a distributed polymer matrix to hold the magnetic particles together.

The binder allows more complex shapes but limits the thermal and magnetic performance. Now let’s look at conventional ceramic magnet production.

Ceramic Disc Magnet Composition and Fabrication

1. Powder preparation

Disc magnets start with powder preparation. Iron oxide is reduced from an iron precursor and ball milled to a fine particle size of around 2 microns. After deoxidizing and drying, the resulting iron powder exhibits soft ferromagnetic behavior.

Strontium carbonate powder is also weighed and milled to match the chosen stoichiometry.

2. Mixing and calcining

The iron and strontium carbonate powders are thoroughly mixed to ensure homogeneity throughout the batch. The mixed powder undergoes calcination at 900-1000°C to remove volatile compounds like CO2 and adjust powder surface chemistry.

3. Compaction

Following ball milling and binder additions, the ceramic powder is compacted into disc magnet green compacts using an applied magnetic field much like their NdFeB counterparts. Pressure around 100-300 MPa is required to achieve necessary green density.

4. Sintering

The compacted ferrite discs then undergo an extended firing around 1200°C in a highly oxidizing kiln atmosphere to fully densify yet avoid reducing the ferric oxide content.

5. Machining and testing

The sintered ferrite magnets may be ground to tolerance before final property checks like magnetic strength and resistivity. No further magnetization is done since the ferrite is already magnetized from the compaction stage.

So in summary, the conventional powder metallurgy approach allows economical manufacturing of ceramic ferrite magnets, taking advantage of simple raw precursors, uncomplicated processing, and no heavy rare earths.

Choosing The Right Disc Magnet Composition

Now that we’ve surveyed the manufacturing processes behind the major disc magnet types, how do you choose the best material for your application?

Here is a quick comparison chart covering the relative magnetic strength, temperature limits, corrosion resistance, and cost for the common disc magnet materials:

Neodymium (NdFeB) disc magnets offer the strongest magnetic performance per unit volume by a large margin but have vulnerability to oxidation and loss of magnetism above 80°C.

Samarium cobalt (SmCo) trades off some of the magnetic strength of neodymium for far greater temperature resistance and corrosion resistance, along with higher raw material costs.

Bonded neodymium takes the unmatched performance of NdFeB powders but combines them in an easy-to-shape polymer matrix that lowers the maximum service temperature.

Finally, ceramic (ferrite) magnets composed of barium or strontium ferrite have the lowest magnetic output yet best corrosion resistance and thermal properties, along with very low costs.

Key Takeaways on Disc Magnet Composition

That wraps up our deep look at what modern disc magnets are made of and how the key fabrication processes work. Here are some of the key takeaways:

• Disc magnets mainly consist of neodymium iron boron (NdFeB), samarium cobalt (SmCo), bonded NdFeB, or ceramic/ferrite as the magnetic material.
• Neodymium magnets offer the highest strength but have lower heat and corrosion resistance. SmCo trades some magnetic performance for radically better thermal stability and corrosion impedance.
• Bonded magnets provide intermediate magnetic strength and convenient formation but lower temperature limits. Ceramic/ferrite magnets have the lowest outputs with outstanding thermal and chemical durability.
• Manufacturing techniques like induction melting, jet milling, pressing, sintering, machining, coating, and magnetizing transform these alloys into precision disc magnets.
• Choosing the optimal disc magnet composition depends on your requirements for magnetic strength, operating temperature range, environmental resistance, mechanical needs, geometrical constraints, and budget considerations among application-specific factors.

Hopefully this guide gave you keen insights into the materials science and engineering of modern disc magnets. Let me know in the comments if you have any other questions!