Why hard material wear when slid aginst soft one? 7

[CdotS]

When cotton fibre runs against steel, steel components wear out, e.g. steel or plastic components used in the textile industry. My question is " Why does a hard surface such as hardened steel wear when a soft material such as cotton fiber runs on it?"

I think cotton fiber is not abrasive. It is a natural fiber. Is cotton fiber corrosive? Are there any other dust or dirt particles mixed in the yarn? Please explain.

 

[IRstuff]

Why not? Even water erodes rocks.

Don't forget that you're running miles per day of cotton against a single spot on the metal.

TTFN

 

[CdotS]

IRstuff, thanks for your consideration and reply.

I am interested in the micromechanism/s of wear between cotton fiber/yarn and steel.

I agree with you on the fact that the same spot of steel is exposed to miles and miles of cotton fiber. I do not however understand why and how it happens.

Water erodes rock by many mechanisms. One of them is abrasive wear mechanism (cutting, plowing, chipping, etc.)due to erosion by the abrasives (stones/sand) present in the flow. There may be involvement of some kind of corrosive wear mechanism, depending on the pH and other chemical species present in water.

Could any body direct me to a good source of reference for this subject - wear of materials by fibers?

 

[IRstuff]

You might try posting this in the Sharp Tool Engineering forum: forum924, are there are some well-known blade sharpening approaches using soft materials such as leather, paper, etc.

TTFN

 

[25362]

Could static discharges on high voltages per mm[sup]2[/sup] be one cause ?

 

[IRstuff]

Against a conductive blade?

Leather is not well known as a high static generator in any case.

TTFN

 

[25362]

It is known that the transitory separation of surfaces (a good conductor from a poor conductor) by a very thin air film in a sliding process is supposed to cause arcing by a dielectric breakdown with an average duration of a few milliseconds.

It reminds me of the buildup of a static charge when placing two different materials in a tumble drier, or walking across a rug, or removing a sweater. The clinging of fabrics and sometimes the appearance of audible sparks are the result of static charge build-up.

This is only an hypothesis from applying some lateral thinking. As IRstuff insinuates I may be totally wrong.

 

[EdDanzer]

Have you checked the hardness of the cotton fiber? Even though it feels soft, the hardness, coefficient of friction, and number of contact areas may cause the material erosion. There probably is some level of dirt in the fiber, and yes it can be corrosive to metal if moist. Just lay a wet rag on clean steel and check later.

 

[aviat]

If the softer material is moving faster than the harder material, the harder material will be removed. This is why you can polish diamonds. Some polishing wheels are made of cloth.
From Marks' Standard Handbook for Mechanical Engineers "The nature of the polishing process has been debated for a long time. Two mechanisms appear to play a role: One is finescale abrasion, and the other is melting of surface layers. In addition to removal of material by abrasive particles, the high temperatures generated because of friction soften the asperities of the surface of the workpiece, resulting in a smeared surface layer. Furthermore, chemical reactions may also take place in polishing whereby surface irregularities are removed by chemical attack."

The rate of removal would depend on the RPM of roller and the diameter. The amount of dirt in cotton will affect rate.

 

[unclesyd]

As we produced synthetic fibers the problem with wear was several orders of magnitude greater than with cotton and pursuant to same information posted by aviat, we had to attack the problem from several different angles.

We used hard chrome plating on hardened metal guides, ceramic guides, sapphire inserts, and ceramic coatings on larger guides and rollers.

As we had both mechanisms mentioned at work at the same time one of the most significant methods of preventing wear was have the thread line dance or move around on a guide or roll. This didn’t do much for the abrasive wear but help with the heating problem.

It get very complicated where a threadline crosses a guide when you take into account the thread material, addiatives, finishes, and guide material.

 

[CdotS]

Thanks to all of you for your valuable posts. My job now is to get evidence from the worn steel surfaces for any or all the mechanisms discussed (no particular order).
1. Abrasion of steel by cotton itself
2. Abrasion of steel by dirt, sand and other plant materials present in cotton fibers
3. Melting of steel surface asperties due to frictional heating
4. Softening of steel surface asperties due to frictional heating
5. Plastcic deformation of asperities (smoothening)
6. Discharge of static electricity through arcing, causing arc craters
7. Corrosion of steel by cotton fiber, especially when humid or wet
8. Any other theory?

My question to EdDanzer:
How can I measure hardness of cotton fiber? I can get hardness values for synthetic materials from plastics handbooks.

I do sincerely appreciate your thought-provoking posts.

 

[EdDanzer]

Cdots,
Try finding a collage that does MEMS work. In the thin film industry they appear to have methods of testing the hardness of small areas.
Microscopic viewing of the fiber will also allow you to size and number of points on the fibers, these could be penetrating the soft areas and voids in the surface of the metal, pulling small pieces of metal from the surface, then abrading the rest of the surface more.
The corrosion may be accelerated by additional static electricity, particularly for some elements in the metal being contacted. If you stain the metal to show the grain structure, take a several 100X picture, then run the fiber over it and take another micrograph, you may see where the erosion starts.

 

[TapeDr]

The simple answer is high coefficient of friction. Steel is not a wear surface. Rub on it long enough with any fiber (soft or hard) and you will eventually see wear. A thermo-plastic UHMW Polyethylene will outwear steel 15/1 and is a very low coefficient of friction. The film is skived in 3 mil. and up and is available with a pressure sensitive adhesive. The meat and packing industry lines all of their shutes and stainless steel trays with UHMW to keep the product from hanging up and everything slides across the surface and cleans up easily. It is also FDA approved for direct food contact. If you can, consider lining the surface of the steel with a UHMW film tape. We produce this in 24" in wide rolls in black and clear with a permanent adhesive and liner for bonding and mounting.

 

[CdotS]

TaperDr, can you use this film on knife edges?

 

[TapeDr]

Cdots,
You can but it needs to be in block form, similar to guide rails or cutting board type of sheet. There a few variations of the UHMW. I was talking about a skived film in the previous post. The knife edges will cut into the UHMW, film or solid form, and it all depends on the force that is used. We produce in both solid form skived film. The black verson has black carbon fiber added to the UHMW prior to manufacturing. It is also known as Poor Mans Teflon as it is a viable low cost alternative to PTFE where temp and and a superior wear surface is desired.

 

[bosha]

I had the same question about three years ago, but it was how does a seal cut into and wear a shaft. The closest answer I could find that made any sense was that the softer material (seal or cotton in your case) would wear away the surface by removing the oxides from the surface, and could possibly be picking up the oxides and become abrasive. But I'm convinced that oxide removal is the reason why.

 

[rnd2]

"If the softer material is moving faster than the harder material, the harder material will be removed. This is why you can polish diamonds. Some polishing wheels are made of cloth."
This is an amazing statement that appears to turn the world of material removal on its head.
A "soft" carbon steel part is turned at high speed against a static HARD tungsten cutting tip and the SOFT CS part wears away the HARD tungsten edge?
In your dreams.
A tungsten cutting edge will eventually dull over but not before a hell of a lot of steel is removed.
Cotton is NOT naturally abrasive. However it is a very strong material. Just as importantly it will carbonise before it melts. Therefore, under the "wrong" conditions it is known to wear carbon and stainless steel significantly. The wear process occurs as the surface of the material (in this case cotton)breaks down as it is pyrolized against the steel sliding surface. This is caused by continuous intense localised heat and pressure. However, the cotton that wears away is continuously replaced with fresh material so appears not to suffer. The surface of the affected steel part appears rough like a file. As far as coating a steel part affected this way, any attempt to remedy the problem using UHMWPE or similar thermo-plastic is a complete waste of time.
My suggestion would be to consider ceramics.

 

[Boomerang]

This article from Peter Min may be of assistance. Thank Uncle Syd for finding it a while back.

http://www.solorb.com/dat-heads/digests/V4.100/D100

 

[25362]

To boomerang, the article is valuable in summarizing various types of wear. However, it doesn't mention all of the wear mechanisms listed by CdotS.

 

[IRstuff]

The soft material in this case is always under some tension and there would be a measureable coefficient of friction.

That shows that there are atomic interactions, since the wear would otherwise result in a smooth surface.

It may be arguable that any friction must result in wear.

TTFN