Neodymium surface prep 1

[MAC3382]

I have been reading the posts for the past few months and you all offer a wealth of information. I now find myself needing some advice about bonding a permanent magnet material.


I need to bond some sintered NdFeB magnet material to a 416 SS shaft. I have read quite a lot about corrosion with NdFeB and I am curious as to the surface preparation. I am going to paralyne coat the entire assembly, but I am concerned with the bondline junction between the magnets and the shaft. No vendor will commit to a specification or method and much of my research conflicts. I would appreciate any suggestions or insight into this matter.

Thx

 

[MagMike]

I have a random jumble of thoughts/questions:
Were you planning on applying the Parylene coating after assembly, or were you going to coat the individual parts before assembly?

If you were planning on coating the individual parts with Parylene before assembly: I don't recommend that. Parylene has a coefficient of friction close to Teflon and is very difficult to bond to. It is an excellent coating for corrosion protection, but it is tough for adhesives to stick to it.

If you were planning on coating the assembly afterwards, check with a Parylene coater first. I may be wrong, but I don't think one can Parylene coat something with adhesives.

It shouldn't be surprising the an adhesive vendor will not commit to a specification. There are so many variables invovled with adhesives. Surface prep and the method of application (both of which the adhesive vendor can not control) play huge roles.

If you want maximum bonding strength and the application is not too harsh: Bond uncoated NdFeB to the 416 SS. You could maybe do a spray coat afterwards to provide some corrosion protection. With the appropriate adhesive, there will be minimal corrosion in the bond line over time.

It's very important that you deal with a reputable supplier of NdFeB. There are plenty of low cost suppliers out there, but they'll offer NdFeB with high oxygen content which are more prone to rust. It's worth paying a little extra to get better quality magnets.

If the application is somewhat harsh (i.e., high humidity, water, etc), then you'll have to trade off some adhesive strength for corrosion protection. Most NdFeB magnets are plated with nickel which provides excellent corrosion protection and bonds reasonably well with most adhesives. I've spec'd out zinc plated magnets with good success. The zinc provides a nice reactive surface that'll bond well with many adhesives.

 

[stick1]

We have had good success with Ni-Cu-Ni plating over NdFeB magnets. We then bond the magnet to 6063-T5 aluminum with Loctite 326 prepped with Loctite 7649 primer. If the surfaces are properly cleaned, and adhesive correctly applied, the joint will withstand a lot of stress. We have thoroughly tested the bond, and have had only a few failures after bonding ?80,000 magnets.

 

[MAC3382]

Thank you for the replies:

Additional Information:
-The NdFeB vendor is from Germany and they are "high-end".
-The magnet vendor will not help specify the surface pre-treatment. They have some proprietary phosphate dip and will only apply it in Germany. This does not help me prior to assembly in the US.
-I want to paralyne coat the finished assembly (shaft and magnets).
-The multi-layer electrolytic Ni-Cu-Ni seems to have somewhat poor surface adhesion.
-The end product will be used in a vacuum, hence the paralyne.

I am concerned with corrosion between the shaft and the magnet bonding surface. Should I use acetone, alcohol, grit blast, some passivation I have not yet discovered ?????

I would appreciate any help!

Thanks

 

[MagMike]

Hello Mac3382

Since you'll be bonding uncoated NdFeB, I'd recommend using an appropriate degreaser (most adhesive companies sell one in a spray can) and then clean the bonding surfaces with masking tape (we use 3M Tartan 200 - it does a good job picking up dirt and leaves no residue) just prior to bonding.

Some people recommend grit blasting the surfaces, the theory is that it'll produce pockets, or "hooks" in the substrate for the adhesive to lock onto. Sometimes this helps, sometimes it doesn't make a difference. Trial and error is the only way to be sure.

Since your application is in a vacuum, there should be minimal corrosion on the magnet bonding surface, especially since you'll be using a reputable vendor.

Please confirm that your Parylene coater can coat the assembly after its bonded together. I've never heard of that.

 

[EdStainless]

I presume that you will be assembling un-magnetized magnets.
I would suggest a light blasting of the stainless. Don't reuse the media. I used to glass bead blast in a once-through system. It keeps things cleaner.
What adhesive? There are a lot that will work. Most will result in a bond that is stronger than the magnets. Each adhesive will have an optimum film thickness. Controling that is important.
I like using tape to clean the magnets. Not much else really works.


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Rust never sleeps
Neither should your protection

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[IRstuff]

One comment about Ni coatings. We've had, in the past, stress cracking of magnet assemblies from a relatively thin Ni coating. Electroless Ni can be deposited in a variety of stress conditions. You'd need to anneal the finished parts and temp cycle to make sure that the coatings don't have too much stress.

TTFN

 

[MagMike]

Hello IRstuff,

Interesting to hear about the stress cracking! I hope it wasn't too big of a headache to fix.

I only wanted to mention that most nickel platings on NdFeB magnets are electrolytically deposited (not electroless). One can get electroless Nickel plated magnets, but one has to go out of their way to specify it. If a drawing just says Nickel plating, 99 times out of 100 you'll get electolytically deposited Nickel.

 

[IRstuff]

I can't swear that it was electroless; that's just from vague memory.

The root cause analysis took almost 6 months, and was quite painful indeed, since we had to build up the magnets, coat, and then test. Turned out that the supplier liked to deposit "shiny" Ni, which supposedly has higher stress.

TTFN

 

[EdStainless]

I have seen some 'great'(read disaster) electro-plated RE magnets. You do know how much hydrogen these materials will absorb, don't you? Some of this doesn't show for weeks or months.
We used electroless Ni. You need to watch thickness (too thick is bad) and P levels (too much is bad). We liked a nice matte finish, no shine. A bit of low temp bake after plating can help.

All in all, I would skip the plating and use the para' coating.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm