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.

 

[rnd2]

IRstuff, I think you're on to something.
There could be at least two wear mechansims here.
1) Cotton essentially is cellulose fibre and some decomposition begins around 130 deg.C (266 deg.F) It really gets going around 200 deg. C (392 DEG.F). Now, as the cotton decomposes, it releases hydrocarbons, CO2 & H2O right at the sliding surfaces. None of these appear to be particularly corrosive to steel (H2O would be vapour) but the hydrocarbons may react with the protective metal oxide. At the same time because there is surface contact, the now carbon rich cotton line is able to present carbon to the steel surface. As carbon is soluble in steel the melting point of the steel will be lowered by any increase in carbon. All the while fresh cotton is sliding right on past and is clearing away debris so fresh surface is always exposed.

2) At high speed with sliding surfaces running dry, surface rubbing temperatures can go way past 200 deg. C and once past its comfort threshold, a weaker material will either melt (thermo-plastic synthetic fibre)or decompose (cotton fibre). When the fibre does not melt it is conceivable that carbon steel (and I'm talking about right at the mating surface)could reach its own plastic temperature. What's that now, 690 deg.C (1274 deg. F ?)It could reach that while the cotton is still mecahanically intact because fresh material is continuously being presented at high speed.The result of this would be to draw fine "fingers" or fine acicular protrusions of steel resulting with the rough texture that I described earlier.

 

[CdotS]

rnd2
I am in the process of analyzing the surface of a steel component that was exposed to cotton fibres. From the preliminary results, I have observed the following:
They do show some plastic protrusions due to deformation of the surface asperities by cotton fibres. I have also observed abrasion marks (grooves) generated by sharp hard surfaces, possibly sand particles that are present in cotton fibre feed. In fact, I have a photo showing a sand particle embedded on the wearing surface.

Any corrosion mechanism present will play a minor role. Abrasion and sliding (plastic deformation) seem to dominate.

I am not sure very sure about the concept of melting of cotton fibre and diffusion of carbon into steel at these speeds. At 200C (I am not sure about the interface temperature), there will be not noticeable happening in terms of diffusion.

This very similar to shaving blades that encounter human hair. Shaving blades do wear (maybe low speed, unless somebody is in a hurry!) while cutting human hair. I need read about the mechanisms of shaving blade wear. Another situation similar to this would be knives. They also seem to go after harder surfaces such as tungsten carbide-cobalt cermets or PVD based carbide coatings on steels.

 

[rnd2]

CdotS
At 200 C cotton will decompose and I agree if the surface temp of the steel was 200 C very little C would infuse into it. However, if the cotton travels fast enough and exerts sufficient tension the nett effect is high speed and compression around the steel part. It is possible to consider a few molecules of the otherwise unaffected cotton fibre to surface decompose at 200 C as well as micro parts of the steel surface to reach its plastic temp at the same point of time. This is possible because the steel part is "passive" to the moving cotton. While the cotton is able to cool as it passes the contact point, the steel surface cannot so is continuously heated with more cotton. Under these conditions, carbon could transfer from cotton to steel.

 

[rnd2]

CdotS
What I described in my earlier post certainly does not apply to shaving a beard! In this circumstance the wear is "normal" in that velocity and pressure are within both materials (steel & hair fibre) heh heh, comfort zone.
As far as cutting edges keeping their edge I think some hard materials like tungsten carbide and diamond hold their edge better or worse with different materials.
For example on one hand, tungsten carbides work great with steel yet fail miserably machining a simple composite such as resin filled with silica. On the other hand, diamond is not used for HS cutting steel yet lasts a very long time indeed cutting a composite heavily filled with silica. It is well worth trying different combinations to see what works best both in terms of materials that cut best against other materials as well as materials that resist other materials.
One suggestion for your cotton application is to hard chrome plate one of the parts and see how it does. Chrome oxide is very hard and chemically stable so may resist better than steel. I earlier suggested ceramic as a possible excellent wear surface but that may be difficult and expensive to implement.

 

[unclesyd]

Shaving a beard with the resulting in loss of cutting ability of the blade is mainly due to corrosion not wear.
The reason that WC2 isn’t good from some materials is that loosely consolidated small abrasive particles can rapidly remove the binder for the carbide.

Now back to cotton and wear.
The original machines handling cotton had very slow filament speeds and all the materials were plain steels or cast iron. As the speed of the threadline increased next came the hardenable steel and the ability to flame hardened cast iron. The next step in the progression was that someone realized that if the relative speed of the threadline vs guide could be decreased by the use of roller guides, spinning rings with guides, etc. These were used first where there was a relative high angle of attack of the threadline vs guide or change in direction of the threadline. These rolls were made from various materials, some of which were CI, Case Hardened steel, even basalt.

The evolution of a particular guide first used in cotton mills called a “balloon guide” started out as plain CS progressed to hardenable steel, then to case hardened, then to carburized and then to case hardened and carburized, then to 1095 CS which in turn was case hardened and carburized. With the introduction of hard chrome plating all steel textile guides were immediate candidates for plating.

With the advent of Nylon the guide problem became more acute. Unadorned Nylon fiber causes extreme wear on anything it crosses or touches. This problem was attacked by the use of the hard chrome plate, the used of ceramics mainly “AlSiMag”for all pin small pin (convergence) guides, as the process for improved larger and larger guides were made from ceramics. Next came the thermal sprays including the CrO, “Polymet”. Thermal spray also allowed the roll to be made from steel which in turn allowed another speed increase and a repeat of the wear problem. Today we have synthetic sapphire and diamond coating. To add to discussion of static electricity nylon fiber under the right conditions will become a VanDeGraaff generator.

Back to cotton, if the threadline speed is below a certain threshold it will not wear guides to any extent. But as stated above as the speed increases the contact stresses increase to a point where the application of a finish or lubricant will decrease or prevent wear on the same guide. Finishes were usually made behind closed doors and were proprietary information of the highest order. Finishes for cotton soon had became generic and available off the shelf. The finishes again went underground when they started treating the cotton fiber to enhance it’s properties. Some of these properties allowed an increase in speed of the threadline and the contest to combat wear of guides began anew.
These finishes presented other problems as some were corrosive to certain materials until dry. Some contained waxes, fats, and oils which each in it’s own way affected the choice of guide materials.

The wear of a steel guide is affected by the threadline material, guide material , cleanliness of thread line, speed of the threadline, contact pressure on the guide, whether it is lubricated or not. All of these are common to wear problems.

If the above wasn’t windy enough this will be.
I have in my hand an extremely fine, several thousandth’s, Cu wire plated with diamonds. This is analogous to the “Angel Hair” of storied fame. The diamond particles are so small that the wire feels smooth but will cut just about anything going. The secret in cutting with this wire is that it has to move to cut without wearing out. If you rotate the part the wire tends to wear.

Just the opposite.
In another wear situation we had a 2" dia shaft made from “Vasco Jet 1000" hardened to 60 RHc rotating at less than 100 RPM @ 600°F we had an insulator/mechanic wrapped a single strand of 1/16" SS tie wire around the shaft. The initial load on the wire was in the order of several pounds tension, but would have went to essenitally nothing quickly. After about 100 days an unscheduled outage allowed the shaft and wire to be exposed. The SS wire had cut a 5/16" deep by exactly 1/16" wide, using the wire as a feeler gauge, groove in the shaft. The precision of this groove would be hard to duplicate. The most amazing part of the event was that it took an electron microscope to see any effect on the SS wire. The apparent abrasive media, none was seen embedded in the wire, was a small amount of the expanded calcium silicate dust from the insulation. This event occurred on another machine that ran for one year but wasn’t cut, only a very slight polish. The difference was temperature. The shaft temperature in this case was approximately 150°F instead of 600°F.

There has been a tremendous amount of work done on wear of guides both in the natural and synthetic fiber business. I know a considerable amount is still considered proprietary but enough has been published to keep from reinventing the wheel. Sometimes one has to take the Edisonian approach to get a starting point then start making incremental improvements until a problem becomes manageable. This is especially true with wear and the textile industry.

 

[rnd2]

unclesyd
This is one of a number of interesting points you raised.
..."With the advent of Nylon the guide problem became more acute. Unadorned Nylon fiber causes extreme wear on anything it crosses or touches."....
I have not found this with virgin nylon. As a bearing material used wet, dry, with grease or with oil I have never found it naturally abrasive to steel. (I am not a proponent of nylon, just stating what are my observations.)
Although we occaisionally manufacture highly specialised bearing materials for the textile industry I cannot say I have a deep knowlege of that industry beyond the narrow field our products operate so I found your comments about nylon interesting. The fact you state with nylon threadline there is extreme wear gives weight to 25362's remarks about static electricity.
Nylon is an excellent insulator and in normal air will easily hold a static charge compared to cotton. Change a dry nylon shirt in the dark and see the sparks. Doesn't happen with cotton. Cotton is also an excellent electrical insulator but unlike nylon, moisture must be completely absent.
If the nylon line, travelling at high speed and charged with static electricity continuously discharged (earthed) against a metal wear part I imagine it would wear away that part in relatively short time.

 

[unclesyd]

In practice there is a difference in chemical composition of Nylon fiber and Nylon polymer used for bearing components. Both are made from the same starting nylon salt solution and each has a different additive package with fiber having the most initially. Some of the additives are metallic salts, again much more so in the fibers. Both products have silicone added as an antifoam agent.
These additive are known to affect the performance of the Nylon in subsequent processing.

Nylon fiber is very abrasive from the moment it changes from a liquid polymer to solid fiber, air quenched. At this point everything the fiber touches has to be wear resistant or turning roll guides plus the immediate application of a finish to enhance it’s processing qualities. The finish also protects the Nylon fiber from surface damage. Guides, surface finish of the guide, finish solutions on the fiber, and line speed are very critical to Nylon fiber as the fiber has to drawn, stretched or reduced in diameter, to be very useful. This draw ratio, based on length, can be from less than 1 to over 4. In every case the fiber or guide is moving relative to each other. Nylon fibers have a propensity to be more abrasive/agressive as the diameter decreases and the relative speed between the guide and threadline increases. There have been numerous studies, more unpublished than published, on this effect mainly dealing with what could you do to mitigate the damage. I seen Thermography, High Speed Photography, IR, Mechanical, and numerous other testing done on fiber thread lines. I’ve never seen much changed from what was known by empirically derived improvements.
An older process for Nylon used a modified machine from the cotton mills to do a secondary operation on Nylon. The only thing changed on the machine was that the guides were chrome plated, or changed to chrome plated or ceramic coated rollers.

Nylon molding resin polymer has to be treated in the same way except it's is water quenched due to it's heat capacity. Size reduction, dicing or chopping, is nearly always carried out underwater. This is true both for a batch process or continuos process material.
A 1/8" dia. Nylon molding resin extrusion running over a guide bar is very abrasive whereas Nylon pellets (1/8"x1/8") cut from the same bar aren't abrasive as they are convey by air both is SS and Al tubes, screened with SS wire mesh, and stored in Al silos with no problems. One of the external inspection points is to verify the grounding strap on the Al silos are functional.

The static electricity problems in our facility were handled by the normal grounding methods. plus the manufacturing area used conditioned air with a very high humidity to help keep the static down. Whether this help with the wear problems I can't say. The control of the air in the manufacturing area was job critical. The cotton mills that I’ve visited just keep it hot with the high humidity.

The worn parts we observed in our lab appeared to be mainly abrasive wear combined with polishing action. We never saw any surface effects that could be attributed to arc damage. For reasons I can’t divulge at the present time we were well aware of the surface effects of small arcs on metal. Nearly all additions of solid additives to molten Nylon by any method had a detrimental effect on the wear problem. There was an interesting effect noted in that there was some point in the additive concentration at which the effect of the additive became exponential as it pertains to wear. On some of the more agressive components the results were truly dramatic, the wear problem backed
into the molten state.


rnd,
You got me thinking that in my 42 years involved in and around Nylon and other synthetics I’ve only seen pure Nylon (66) one time in the research lab. It was in a very small quantity for use in deriving some physical properties. Nylon doesn’t behave very well without the additives.

 

[rnd2]

unclesyd,
I understand virgin nylon to be nylon product made from original resin, as against re-cycled.
I agree about additives. Mostly beneficial and neccessary to achieve a desired result.
We make a grade of bearing material that is re-enforced with woven nylon and is definetly not abrasive.

 

[unclesyd]

rnd2
We used a lot of your sleeve type bearings on our equipment.

Your mention of rework reminded me that we noticed a significant difference in the behavior of reworked vs virgin Nylon. When reworking local Nylon waste from our process we almost had to treat it as a different material based on several changes in physical parameters. One being it's ability to cause wear problems. We had to make several mechanical changes to the equipment for handling the rework. In fact it was essentially reworked on a separate line, FCM, Extruder Screw, and Dicer, due to the excessive wear problems. We only used reprocessed Nylon as compounding additive as it was extremely hard mechanically process as a fiber or monofilament.

As this Forum has pointed out the simple act of running almost any type thread line over a round guide can create a set of conditions that creates wear to the guide or damage the fiber filament.

I do know that having the tools of today, specifically the modeling and the basics of nanotechnology, I would like to get back and look at some things I noted in the macro world.

Though there are some specific conditions known to avoid, I've never seen one being able to offer a specific remedy for a every set of conditions. I think CdotS will have to find a solution to his specific conditions and then look back on the basic problem of wear.

It would be most interesting if all the work on wear by everyone in the fiber business was published as compendium, with links, to where it could be data-mined if nothing else.

 

[TapeDr]

UHMW Thermplastic Film Has 15 times the wear surface of steel. Unless there is something very sharp in the cotton fibers you will not be able to wear it out very easily. Of course that will depend on the heat generated. The coefficient of friction is much higher on the steel vs. the very low coefficient of friction on the UHMW film surface, which will not build up heat which is created by constant friction moving across a high surface energy substrate such as steel.

 

[25362]

The comment by TapeDr again brings to mind the subject of static charging. From what I've learnt from reading specialized books, UHMW PE and PTFE, as well as all conventional plastics, can be charged electrostatically whenever the conductivity is smaller than 10[sup]-9[/sup] S/cm and the relative humidity lower than ca. 70%, either by rubbing two surfaces against each other (triboelectric charging) or by contact of a surface with ionized air.

Static charging can indeed be diminished by incorporation of conducting fillers such as carbon black or metal powders (internal antistatics).

External antistatics reduce surface resistivities by increasing the polarity of the surface by application of humidity-absorbing additives or by reducing friction through lubricants or coating with PTFE or UHMW PE, as TapeDr said. However, external antistatics wear out and have to be renewed from time to time.

 

[rnd2]

Threadline speeds can be extremely fast, and where thread changes direction there is pressure placed on the bearing material. Conditions where high surface speed & pressure are present rapidly translate to conditions where high temperature and pressure exist and unfortunately that is no place for thermoplastic materials like UHMWPE.

 

[TapeDr]

Cdots issue is why does the steel surface wear when cotton fibre runs against it? We can agree that the steel surface is a high coefficient of friction which produces frictional resistance and will eventually wear. This is in direct proportion to the velocity or speed of the cotton material running or dragging across the steel surface and amount of constant force or pressure the cotton fibre has against the steel surface while in motion. Most would engineer this application where the wear surface posseses as low coefficient of friction or by a lubrication material that can be applied or coated to create the desired wear surface, paying special attention to possible high temperatures created by the shear speed of the cotton fibre line. I would disagree with rnd2 in that thermoplastics do have a place in higher temperature applications, including UHMWPE. With added polymers patent ingredients UHMWPE Special Formuation High Temperature UHMWPE can perform at a constant temperatures of 275F without melting or loosing its wear properties. Used extensively in chemical wash wear surface environments and many other harsh environments and high temperature applications. Straight UHMWPE skived or extruded) will not be able to handle 275F without the added components of oil-filled nylons, nylon 6, nylon mds, Acetol and Teflon®.

 

[Compositepro]

There is a very simple explanation for why soft materials wear away harder ones. The world is a dusty place and all surfaces will have some dust particles on them. Cotton fiber especially are grown in dirt, after all. These particles are abrasive and there is a tremendous amount of contact with a fiber guide. This is why most fiber guides are made of very hard alumina or other ceramic.

The process for lapping steel uses bronze or other soft metal with an abrasive slerry. The abrasive particles will get embedded into the soft metal and wear away the harder metal with litle wear to the soft metal.

 

[rnd2]

I can accept an explanation that soft materials wear away hard materials because they contain or support abrasive particles. However, it is not correct to say a soft material is therefore the cause of wear. In this specific instance, without doubt the wear mechanism is the hard abrasive material doing the material removal, not the soft copper or bronze material, which is merely the carrier. Substitute the soft copper or bronze with a water jet or even air jet to carry the abrasive particles and the abrasion continues.

 

[unclesyd]

You don't even have to have abrasives in water if the pressure is high enough. I think the threshold pressure for efficient cutting metals without abrasives is 77,000 psi, though 55,000 will do a pretty good job.
Abrasives just make the process economical, less wear and tear on the equipment.

 

[CdotS]

Hi All

I could not believe the growing interest in this subject after I hhave posted the first post so many weeks ago. I have done scanning electron micrscopic (SEM) work on some wornout steel components used in the textile fibre processing industry. Some of the mechanisms discussed previously by others are possibile reasons for wear.

Predominantly, abrasive wear in very minute scale (less than microns - nano scale perhaps)is one of the prime mechanisms (Polishing wear due to dust?).

Then there is plastic deformation of the surface asperities due to frictional forces. This si similar to UncleSyd's water cutting without abrasives. Just force on the metal to deform. In the same line, there is plastic delamiation due to repetetive application of force, resulting in laminar particle. Refer to N.P. Suh's delamination theory of wear. If yu are not familar with this theory, refer to a god text book on wear (e.g. book by Ian Hutchings).

I will post more when I do some more SEM work in the near future.

Thanks to all for keepig this thread alive and interesting.