Wear surfaces / Surface Hardness? 1

[wchowe]

I have a question regarding surface hardness. I have a rotary actuator that pushes a flat plate against a stop. During the pushing a small amount of sliding takes place, due to the design. I am seeing wear after about 100k cycles. I currently have both pieces as hard coat anodize on aluminum. So they are both the same hardness. What materials or platings would provide a longer life without visable wear?
In general is it better to have one material harder than the other? by how much?
Thanks,
Bill

 

[tomwalz]

Harder is one way - lubricity (coefficient of friction)is another

 

[Metalguy]

In general it isn't advisable to have both sliding surfaces made from the same metal. They are more likely to weld the tiny high point together (adhesive wear, greatly simplified). It's also not a simple difference in hardness relationship-large diffs. in hardness don't necessairly mean good wear rest.

Alum. against alum. is usually especially bad-I think the hard anod. is saving you.

Kenvlach can provide good info on electroless Ni coating-dunno how it works w/ alum, but he will.

 

[CoryPad]

What type of anodizing are you using now? Newer methods based on micro-arc oxidation may be better for you. Two companies to investigate are Microplasmic and Keronite:

http://www.microplasmic.com

http://www.keronite.com

You also may want to consider Diamond-Like Coatings. One DLC supplier I can recommend is Diamonex. You can find more information at:

http://www.diamonex.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[wchowe]

We are currently specifying a Clear Hard Coat Anodize per ASTM-A-8625 .001 to .002 thk if I remember correctly. I know there are other surface treatments I could use. I was more interested, in wether one surface should be softer than the other or not? and would a lubricant type surface for one surface provide longer wear protection?
Thanks,
Bill

 

[Metalguy]

If you can change one of the parts to one of the "soft but slippery" SS like Nitronics 60, it *may* work well-if plain/corrosive water isn't involved. In that case you might well get dissimilar metal corrosion.

In any case, you'd need to test it well first.

 

[kenvlach]

Are there any considerations which prevent use of a spray-on
lubricant? If you cannot use conventional lubricants, then I have 2 alternatives:

One approach would be to have the hard anodize (unsealed) impregnated/coated with a PTFE-containing dryfilm lubricant.
There are 3 grades, from simple dip + air dry to a baked on coating with phenolic (or other thermosetting resin) binder.
See MIL-A-63576A, which can be downloaded from the DoD's ASSIST site:

http://assist2.daps.dla.mil/eAccess/index.cfm?ident_number=31464

Another approach would be electroless nickel containing co-deposited PTFE particles:
"Composite Coatings: These types of coatings are specifically designed to enhance or improve the wear resistance properties for numerous applications. Co-deposits are chosen for frictional wear conditions and also heavy load wear conditions. Depending upon the type of co-deposit, composite coatings offer a wide variety of options for meeting specific demands.

Niklad Nilon: A unique Teflon dispersion blend that provides a 18 to 26 % v/v nickel Teflon co-deposit. The system produces the highest Teflon co-deposit, which is favored for frictional, low load wear environments.

Elnic 121 TF: A Teflon dispersion blend that provides a 10 to 20 % v/v nickel Teflon co-deposit. The advantage of lower Teflon co-deposits is the excellent coating performance under frictional wear and heavier loads. This condition is more representative in most applications today.

Niklad Dispersant: Utilized to enhance the co-deposition of diamonds, boron nitride, silicon carbide or other similar particles into an electroless nickel phosphorus deposit matrix."
The above info is from MacDermid;

http://www.macindustrialproducts.com/enproducts.html

other EN suppliers like Atotech have similar products.

 

[Celebur]

Regarding the hard/soft issue, if there is any likelihood for abrasive particles to get between the plate and the stop, then I would suggest that both should be hard. The reason is that such particles would tend to imbed in the softer item and then abrade the harder one at accelerated rates.

If this is not a problem, then you're free to try different combinations with consideration for wear-resistance, lubricity, and anti-galling (hard/soft combinations are often used precisely to avoid galling because the materials will be dissimilar).

 

[Metalguy]

Celebur,

With hard particles present, the use of one soft surface allows the particles to imbed easily, and they don't usually score the harder surface-eg, journal bearings.

 

[wchowe]

Thanks to all for the information. If I understand everyone, it appears that a softer bearing material, PTFE or somethign loaded with Teflon,will provide a longer lasting solution than two hard surfaces. Given that the load is light. Wet lubricants are not an option for me. Cost wise, does anyone have any idea which solutions (PTFE, Teflon impregnate, or others) are more economical than the others?

Thanks,
Bill

 

[Mark127]

I agree with the previous responses to your post. Hardness is not the only factor to consider regarding wear resistance, friction coefficient is also inextricably linked. I would consider coating the two surfaces with a low fiction coating. Diamond-like carbon deposited by PVD or PACVD has been mentioned and this is the most widely used coating where wear and friction reduction are important performance related factors. However I would consider MoST™ a patented hard, low fiction, wear resistant coating. In addition low friction coatings tend not to suffer from adhesive wear when both surfaces are in contact.

More information regarding MoST™ and carbon based coatings is available from

http://www.teercoatings.co.uk

PVD theory is available from

http://www.pvd-coatings.co.uk

 

[Metalman01]

The wear can occur by either adhesive, abrasive, or subsurface fatigue. Since the materials are the same I doubt if abrasive wear is an issue unless foreign particles are be introduced by the environment. This can be check for easily. The number of cycles are pretty low for fatigue unless the contact stresses are quite high. But again this can easily be determined. The most likely candidate is adhesive wear since the two materials are similar. Changing one of the materials could have a big impact on the wear. I would suggest a telfon coated or teflon impregnated Ni plating. It should not be too expensive and it will reduce the friction between the sliding components.

 

[funnelguy]

I'd try TFE hardcoating or look into mechanical application of a dry lube such as lamellar tungsten. Diamond will be expensive and may not hold up well. The aluminum substrate may not have sufficient local strength to adequately support the amorphous diamond. Just a guess, your mileage may vary.

 

[wchowe]

LOL, my mileage may vary!
Thanks all for the input. I'm going with A anodized Hardcoat Rc65 on one surface and a Polyond ( Teflon impregnated Nickel) Rc55/60 on the other. I'll post the results when I know.
Thanks Again
Bill