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Can soft materials scratch hard materials?

  1. Jan 22, 2014 #1
    We all know that hard materials scratch soft materials, and the effectiveness is related to the difference in their Moh's hardness level.

    My question is whether some minute amount of abrasion occurs on the harder material that is scratching the soft material?

    If so, is there any mathematical relationship to the amount of abrasion to the soft material? (I am thinking of it might be inversely related to the difference in Moh hardness, then probably raised to an exponent.)

    My curiosity came about as I recently bought a stylus for my smartphone. The stylus (Retrak) is not rubber tipped, instead it has a cloth tip which is impregnated with nano-sized silver particles. Silver has Moh's hardness of 3, whereas the smartphone has glass with hardness of 7. So, the stylus is softer than the glass, but could abrasion to the glass still arise over repeated use?
    Last edited: Jan 22, 2014
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  3. Jan 23, 2014 #2


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    Good question. I can't answer it, but I'd guess that it is indeed possible to scratch the harder material with enough time/material.
  4. Jan 23, 2014 #3
    i think the answer is that every two contact areas that are intouch wear out with time , the only difference is how fast each of them breaks down and that is related to the material hardness level.

    I think we all know some areas were a situation like you described occurs.
    High performance auto brakes for example , the brake disc is a very rugged and strong metal , the pads are much softer, they do have to be changed over time but at the end you can also see wear on the brake disc itself.
    rubber tires and road.Asphalt is harder than tires yet eventually the road breaks down with time.Ofcourse there are other factors at play here like weather conditions etc but friction between two surfaces sill is the main one.
    I think not only the hardness of the materials involved is important but also the force which is used upon them , car brakes and all kinds of friction situations involve a lot of force being exerted on the friction surfaces , in other words they are being hard pushed , you dont press on your phone with all the strength you have right? :D
    So I would say that in your situation the friction is negligible.

    I too can't give you calculations , but I pretty certain that the enclosure wont scrub your glass , assuming people use a phone for no more than 5 years these days and even that is a long time for some.So don't worry nothing bad will happen.
  5. Jan 23, 2014 #4

    Vanadium 50

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    What could be softer than water? But look at stones in a river, and how smooth they become over enough time.
  6. Jan 23, 2014 #5


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    Thats not because water is soft.its the friction between the water and the rock which breaks the tiny bits on the rock.So as the bits are removed,the rock becomes smoother
  7. Jan 23, 2014 #6
    yes but the hardness level of water is so tiny that when we wash our car with it we dont scratch the paint.
    the rock too isn't scratched but with enough water flow and time the effect is similar to that of a very smooth sandpaper.
    So the point is that everything wears and tears down with time if it expierences friction , even the most hard and stiff stuff around. Like diamond on a diamond abrasive cut disc etc.
  8. Jan 23, 2014 #7


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    I'm not sure you can call water soft or hard since it isn't a solid.

    Per wiki: http://en.wikipedia.org/wiki/Hardness

  9. Jan 23, 2014 #8

    I think water against rock is different. Water in a stream is propelling small rocks to roll about, causing rocks to rub against other rocks. So, the rocks are wearing each other.

    Likewise, a waterjet often incorporates an abrasive grit material within the fluid. The water acts as the propellent. The abrasive grit serves to score and cut the base material.


    So what about soft materials rubbing against hard materials. I have speculated that the following mechanism is arising, causing the soft material to occasional abrade the harder material:

    On a microscopic scale, there are ridges and valleys to the surface of both materials. When the two materials are pushed and slide against each other, the ridges and valleys of one materials interlock against the complementary valleys and ridges of the other. This is illustrated by noting how the teeth of the cutting edge of a saw blade can interlock into an opposing saw blade.

    When abrasion is occurring between two materials, a shearing force develops across root of the ridges (base of teeth on a saw). The ridge of one material will shear off when its shearing resistance is weaker than the ridge of the other material. The shearing force is calculated as the product of the shearing stress and the shearing area.

    When the shearing area of two materials are the same (two similar saw blades), then the weaker material will yield to the strong material during abrasion - Moh's hardness is solely relevent here.

    However, consider the case where the length of the teeth (ridge length) of the soft material is longer than those of the harder material. The following example data quantifies the material's parameters: ultimate shearing stress (vu), the length of the surface roughness (teeth length - L), the width of the surface roughness (t). The ultimate shearing force (V) is calculated.

    V = vu*(L*t)

    vu.s = 1 MPa
    L.s = 2 mm
    t.s = 2 mm
    Vu.s = 4 N

    vu.h = 2 MPa
    L.h = 1 mm
    t.h = 1 mm
    Vu.h = 2 N

    Vu.s > Vu.h

    In this example, the softer material will have a higher ultimate shearing resistance (force, not stress) than that of the harder material.

    This calculation and result is correct only in materials where the compressive strength is much greater than the shearing strength. Otherwise, the corners of the ridges (teeth) on the softer material will be compressed and thereby amplify the internal shearing stress. Consequently, the softer corners will fractured off (rounded edges).

    _____ ......... ______ ......... _____ ........ _____...... SOFT MATERIAL
    ........|_____|.........|_____|.........|_____|............. ---> slide rightward

    ._ ............... _ .............. _ ............... _ ............. HARD MATERIAL
    | |_________| |_________| |_________| |____...... <--- slide leftward
    Last edited: Jan 23, 2014
  10. Jan 23, 2014 #9

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    Look at the objections you've given for water - things like "there's friction" and "it's moving". How exactly is it different from a cloth rubbing some glass?
  11. Jan 23, 2014 #10


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    Of course soft things can scratch hard things. It's just harder to do so because if you press a soft thing against a hard thing, the soft thing gives away, preventing you from applying more force. But, given enough energy, you can damage anything with anything else.
  12. Jan 23, 2014 #11


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    But smoothing is the opposite of scratching. :wink:

    And wood is harder than water.
  13. Jan 24, 2014 #12


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    They are not abraded by the water itself, they are abraded by tumbling over each other and being sandblasted by suspended sediment like in a rock tumbler. Same goes for wind erosion.

    Through friction, no, a softer object cannot abrade a harder one. But through impact, a soft object can break a harder one.
  14. Jan 24, 2014 #13
    Russ is correct. Over history, many diamonds have been destroyed by looters who mistakenly believed that you could not destroy a real diamond with a hammer. They would hit the gem with a hammer and it would shatter. Gem cutters use a steel chisel and wooden mallet to fracture diamonds along their cleavage lines. It doesn't take much force.

    Many housewives have broken their diamond rings simply by knocking them against a porcelain basin.
  15. Jan 25, 2014 #14
    So you're saying that if I would use a smooth cloth to rub a diamond or steel I wouldn't cause some wear or change in the surface , assuming the action would go on long enough.?

    I can tell you a real life example why I ask this question.My grandmother has some very old soviet or maybe even older stainless steel scissors, Now I have done some mechanical work with them and nothing has happened not a single scratch because they are very very hard.
    But the places were she has held them for years with her fingers, while cutting textile, have been worn out, one can clearly see that , I assume it has happened because after decades of use the fingers and skin has rubbed the metal surface and sxome friction or whatnot has happened , I have also seen some other examples were a very very smooth and soft surface has caused considerable wear to a very hard surface over the years.

    the effect on the scissors looks exactly the same as to the effect a very smooth sandpaper would have on a similar piece os metal.So how come soft things don't leave marks on much harder things?
    I am probably asking this to Russ?
  16. Jan 25, 2014 #15
    I would say that soft materials most definately scratch hard materials. A difference is that the effects, such as the depth of scratch, are greater on the softer materials.
  17. Jan 25, 2014 #16


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    Every contact results in surface changes to both materials. It is just the relative wear rate that is different.
    What is hardness? It is a relative scale of things that scratch each other more or less.

    There is also surface chemistry at play. Iron scissors will be polished by the hands of the user. The users skin will be replaced but the iron can only rust. That rust is harder than iron, but it is not well attached, so it will be polished from the surface every time the scissors are used. Repetition of surface chemistry results in wear of a hard material by a softer material.

    A wooden bearing block carrying a steel shaft, and lubricated with fat, will last for many years. But the steel shaft will wear relatively quickly because the lubrication will trap windblown grit and rust that will embed in the softer wood. The steel shaft is worn by the embedded contamination, not by the wood. It seems paradoxical at first, but because the abrasive particles embed in the softer material, it is the harder material that wears first.

    The subject of wear is part of the field called tribology.
  18. Jan 25, 2014 #17


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    What about polishing hard surfaces with a piece of cloth? Isn't it a proof soft materials can abrade the hard ones? (Edit: OK, I see it was already mentioned).

    I remember watching a video about design of a very fast yarn spinning machines. There was a place where the fibers touched the surface, and the surface was getting worn out in time, no matter how hard it was. Video was about a new design, in which fibers were twisted by the air vertex, without contact with any surface.
  19. Jan 25, 2014 #18
    Borek, you described wire draw and how to avoid it as it is possible to scratch very hard, atoms held in place tightly, with very soft materials. You can increase the energy for tearing the tight bonds apart by making the soft materials move across it fast with force. That is what wire draw is where metal can be cut by a string by pulling a string across the metal with force.
  20. Jan 25, 2014 #19


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    That just confirms answer to the original question is "yes".
  21. Jan 25, 2014 #20

    OK, so I just did a little experiment in my garage. I used a Dremel tool with a few new cotton brushes on various materials. Each time, I started with a clean brush that was free of any grit. I spun it on various materials: steel and wood. The surface area of the cotton brush (4mm x 20mm diameter) was about 15 times larger than the area of the test material (4mm x 4mm).


    I ran the experiment on the wooden shaft of the shovel. Quickly (5 secs) the wood area got hot (burning coloration on face of wood) and became grooved.

    I suspect that the wood material is not harder than the cotton. They are both made of cellulose and lignin, although the wood is more densely packed than the cotton brush. Clearly the micro surface of the wood was being burnt, as evident by both smell and coloration. This process probably weakens the cell wall of the cellulose and lignin - which provides the strength.


    The experiment was repeated on steel that was a scratched face of a garden shovel. Running the Dremel tool for a few seconds (5 secs), there was no polishing evident on the steel. However, after about 20 seconds the small test area began to shine (polished). I noticed that the steel area was very hot and the cotton brush (circular) was accumulating metal grit from the steel test area.

    Once the steel was sufficiently polished, there rate of steel removal seemed to diminish. This was judged by the the amount of residue developing on the cotton brush. Once polished, the brush did not get any darker with grit. This suggests to me that the originally scratched steel was easy to polish because the cotton fibers were able to snag the micro ridges. Once all the ridges were removed (polished state), then the fibers had nothing further to grab on to. Thus, further polishing subsided.

    Perhaps the hard steel material is getting over-heated from the polishing method (friction) while the soft material (circular cotton brush) isn't. The brush has a much larger surface area and its speedy movement through the air allows for better heat dissipation. Steel has a weaker strength when heated. Probably the softer cotton fibers are tearing off micro ridges of steel flakes from the previously roughened steel material (shovel). Once enough residue is removed and impregnated into the face of the cotton brush, then a second process is arising. Eventually steel grit speeds up the abrading process as now the dirty brush rubs against the steel plate.


    I was delighted by the quickness of the polishing/grinding ability of the cotton on the test materials. I suspect a different process is happening here where a soft material can polish a hard material.

    This trivial experiment might validate my hypothesis that soft materials can polish hard materials by mechanically removing surface roughness on the hard material.

    Note, polishing is perhaps opposite to scratching. In my vocabulary: polishing is the removal of surface ridges, whereas scratching is the creation of surface valleys.

    This experiment did not test if a soft material can scratch (make valleys) onto the surface of a hard material.
    Last edited: Jan 25, 2014
  22. Jan 25, 2014 #21
    i think one thing is clear , every two objects , physical objects that interact tend to wear or change there structure or surface with time.
  23. Jan 25, 2014 #22
    I think the question has been fully and properly answered. However, I would like to add a bit of what I think is interesting information. Gemologists would warn you not to confuse hardness with toughness. Diamonds are the hardest of all gem stones, but are not very tough. They are easily fractured along their cleavage planes. Corondum gems (oriental rubies and sapphires) and the second hardest gemstones, but also the toughest. It is extremely difficult to shatter them. Jade is not very hard at all, and is easily carved. However, it is very tough and does not break easily. This makes it ideal for elaborate and very thin carvings.
  24. Jan 26, 2014 #23


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    I see the removal of ridges as grinding, scraping or lapping, while “polishing” is the removal of surface contamination to reveal the reflective material surface below.

    If the surface contamination is abrasive then polishing with a soft cloth may scratch the base material with the contaminant.

    If the surface contamination is a chemical product of the base material reacting with the environment, then polishing will erode the weathered material and there will be a progressive loss of base material due to repetition of surface corrosion.

    Where the contamination is tightly bound to the surface, polishing will require a fine abrasive, which can cause many fine scratches in the surface.
  25. Jan 27, 2014 #24


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    Soft materials will definitely affect hard materials, i insist on affect as scratching is a bit much.
    Mainly the surface atoms, molecules or grains are not held as strongly as in the bulk of a material so even hard materials can be chipped of. (of course small chips)
    Ultra fine Polishing looks like atomic scratching as the energy requirements of polishing tend to be similar to the specified materials boil off energy. Usually the ultra-fine polishing mediums are not necessarily as hard as one might expect (rust is a nice one, 5 on Mohs).
    Diamond is with most Carbon atoms bonded to other 4 Carbons, yet the surface atoms do not have that luxury and are bonded only to 2 or 3 atoms, this does not make it necessarily a weaker bond as other effects can come into play but it enhances its reactivity. Ex. diamond will quickly abrade when milling Iron/steel without cooling. Carbon reacts chemically with iron. This is not uncommon in wear, it will look like abrasion but it could be a reaction.
    On the same note wood which is quite soft abrades iron/steel quickly (The wood-milling industry prefers diamond in a nonferrous mesh for speed cutting wood).

    Of course speed is important and temperature too but for cotton and steel, it is the steel that is harder at cottons max temperature (cotton begins to dehydrate around 180-200 deg C). So at best your polishing is just the "boiling off" abrading of some of the irons grains or surface atoms.
  26. Jan 27, 2014 #25


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    I suppose then it depends on how you define "scratch". It seems to me the laymens definition is very broad. But in materials science, "wire drawing" has a different name because it is a different process. Breaking something with kinetic energy is not the same as wearing it with friction.

    In any case, I'm not familiar with using cloth to polish glass, again except using a definition that really just means cleaning. That said, I remember in school polishing hard metals with a buffing wheel and buffing compound: which contained hard materials for abrasion.

    The OP is asking whether the stylus he bought will scratch his cell phone screen. The answer is no.
    Last edited: Jan 27, 2014
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